Transmission media laboratories - OptoLab

FAKULTA ELEKTROTECHNIKY A KOMUNIKAČNÍCH TECHNOLOGIÍ

VYSOKÉ UČENÍ TECHNICKÉ V BRNĚ

Transmission media – laboratories

Author:

prof. Ing. Miloslav Filka, CSc.

Komplexní inovace studijních programů a zvyšování kvality výuky na FEKT VUT v Brně OP VK CZ.1.07/2.2.00/28.0193

2 FEKT VUT v Brně

Content

1 INTRODUCTION.............................................................................................................. 8

1.1 MEASUREMENT OF TRANSMITTING QUANTITIES ........................................................... 10 1.2 MUTUAL CONVERSIONS OF NP – DB UNITS AND TRANSMISSION QUANTITIES. ............. 11

2 LOOP RESISTANCE MEASUREMENT & UNBALANCE OF RESISTANCE ..... 12

2.1 MEASUREMENT OF THE LOOP RESISTANCE AND UNBALANCE OF RESISTANCE. .............. 12

3 LOCALIZATION OF CABLE FAULTS ...................................................................... 15

3.1 INSTRUCTIONS: .............................................................................................................. 15 3.2 MEASURING PROCEDURE: .............................................................................................. 15

3.2.1 Measurement of loop resistance rab ......................................................... 15 3.2.2 Murray’s method ....................................................................................... 15 3.2.3 Varley’s method ......................................................................................... 15 3.2.4 Balancing the bridge ................................................................................. 15

3.2.5 Calculation of the resistance to failure: .................................................... 16

4 IMPULSE METHOD OF MEASURING ...................................................................... 18

4.1 INSTRUCTIONS: .............................................................................................................. 18 4.2 INTRODUCTION:............................................................................................................. 18

5 LOCATION OF THE FIBRE OPTIC CABLES USING ELECTRONIC

MARKERS AND GPS. .................................................................................................... 22

5.1 TARGETS OF LABORATORY EXERCISE ........................................................................... 22 5.2 INSTRUCTIONS FOR THE LESSON: .................................................................................. 22

5.3 THEORETICAL INTRODUCTION....................................................................................... 22 5.4 PRINCIPLE OF LOCALISATION......................................................................................... 22 5.5 DESCRIPTION OF THE MARKER LOCATOR DYNATEL 1420 EMS-ID ............................... 23

5.6 SIGNAL RESPONSE OF THE MARKER ............................................................................... 24 5.7 ELECTRONIC MARKS – EMS MARKERS .......................................................................... 25

5.8 THE TYPES OF MARKERS ................................................................................................ 25 5.8.1 Ball marker ................................................................................................ 25 5.8.2 Mini marker ............................................................................................... 26 5.8.3 Near surface marker .................................................................................. 26

5.8.4 Full range marker ..................................................................................... 26

5.8.5 Disc marker ............................................................................................... 27

5.9 DESCRIPTION OF THE NAVIGATION USING MOBILE PHONE NOKIA 700 AND PROGRAM

OPENWIG ..................................................................................................................... 27 5.9.1 Starting of navigation ................................................................................ 27

5.10 PROCESSING OF THE LABORATORY EXERCISE ................................................................ 28 5.11 LOCALISATION OF THE MARKER .................................................................................... 28

5.12 DATA READING FROM ID MARKER ................................................................................. 28 5.13 DATA RECORD INTO ID MARKER ................................................................................... 29 5.14 MEASUREMENT OF THE DEPTH OF THE MARKER ............................................................ 30 5.15 LOCALISATION OF MORE MARKERS ............................................................................... 31 5.16 FORMULATION OF THE LABORATORY EXERCISE ............................................................ 31

5.17 USED DEVICE ................................................................................................................. 33

5.18 CONCLUSION: ................................................................................................................ 33

6 LAN STRUCTURED CABLING TESTING ................................................................ 34

Transmission media – laboratories 3

6.1 THEORETICAL INTRODUCTION ....................................................................................... 34 6.1.1 T568A and T568B Specifications ............................................................... 34

6.2 NEXT ............................................................................................................................ 35 6.3 FEXT ............................................................................................................................ 35

6.4 PSNEXT ....................................................................................................................... 36 6.5 ELFEXT ....................................................................................................................... 36 6.6 PSELFEXT ................................................................................................................... 36 6.7 RETURN LOSS ................................................................................................................. 36 6.8 ATTENUATION ............................................................................................................... 37

6.9 LENGTH ......................................................................................................................... 37 6.10 PROPAGATION DELAY ................................................................................................... 37 6.11 ACR .............................................................................................................................. 38

6.12 PS ACR ......................................................................................................................... 38 6.13 DELAY SKEW ................................................................................................................. 38

7 TRANSMISSION PROPERTIES OF OPTICAL FIBRES ........................................ 39

8 MEASURING METHODS IN OPTICAL COMMUNICATIONS ............................ 41

8.1 METHODS OF OPTICAL FIBRE EXCITATION FOR MEASURING PURPOSES ........................... 42

9 MEASUREMENT OF BACKSCATTER ..................................................................... 54

10 MEASURING OF ATTENUATION BY OTDR .......................................................... 60

10.1 INSTRUCTION .................................................................................................................. 60

10.2 INTRODUCTION ............................................................................................................... 60

11 SPLICING OF OPTICAL FIBERS AND MEASURING OF THE ATTENUATION65

11.1 INSTRUCTIONS ............................................................................................................... 65 11.2 INTRODUCTION .............................................................................................................. 65

11.3 SPLICING PROCEDURE .................................................................................................... 68 11.4 SPLICING PROCES ON A S122C ....................................................................................... 71

11.5 MEASURING OF ATTENUATION ....................................................................................... 71

12 OPTICAL WIRELESS TRANSMITTION IN LABORATORY ............................... 73

12.1 MODIFICATION THE RONJA IN LABORATORY ................................................................. 73

12.2 MEASURING DEVICE ...................................................................................................... 73 12.3 MODIFICATION OF THE TRANSMITTER ............................................................................ 75 12.4 VOLTMETER AT THE RECEIVER ...................................................................................... 75

12.5 MEASURING PROGRAM IN COMPUTER ............................................................................ 75 12.6 MECHANICAL CONSTRUCTION ....................................................................................... 76

13 EDFA MEASUREMENT ............................................................................................... 78

13.1 ASSIGNMENT ................................................................................................................. 78

13.2 THEORETICAL INTRODUCTION ....................................................................................... 78 13.2.1 Optical source LS 420 ................................................................................ 83 13.2.2 Optical Power Meter PM 420 .................................................................... 83 13.2.3 SFT-TAP coupler ....................................................................................... 84

13.3 AMPLIFIER EDFA CLA-P(B)-01F ................................................................................. 84

13.4 INSTRUCTIONS RS232.................................................................................................... 85 13.5 PROCEDURE ................................................................................................................... 85

13.5.1 Working-out of protocol ............................................................................. 86 13.6 CONCLUSION ................................................................................................................. 87

4 FEKT VUT v Brně

14 MEASUREMENT OF CHROMATIC DISPERSION ................................................. 88

14.1 METHOD OF PHASE SHIFT AND DIFFERENTIAL PHASE SHIFT ........................................... 88 14.2 METHOD OF DELAYED PULSES IN THE TIME DOMAIN ..................................................... 88 14.3 MEASUREMENT OF POLARIZATION MODE DISPERSION (PMD) ....................................... 89

14.3.1 Method of scanning the wavelength .......................................................... 90 14.3.2 Method of POTDR ..................................................................................... 91

15 REFERENCES ................................................................................................................. 94


List of figures

FIG. 1: CONNECTION FOR MEASURING BY THE MURRAY‟S METHOD. ......................................... 16 FIG. 2: CONNECTION FOR MEASURING BY THE METHOD. ............................................................ 17 FIG. 3: LINE WITH SHORT CIRCUIT. ............................................................................................ 19 FIG. 4: LINE WITH OPEN CIRCUIT. ............................................................................................... 19

FIG. 5: CAPACITY OF THE LINE. .................................................................................................. 20 FIG. 6: INDUCTANCE ON THE LINE. ............................................................................................. 20 FIG. 7: MULTIPLE REFLECTION OF THE SAME PULSE................................................................... 21 FIG. 8: THE PRINCIPLE OF MARKER LOCALISATION. ................................................................... 23 FIG. 9: DESCRIPTION OF THE FRONT PANEL. ............................................................................... 24

FIG. 10: LOCALISATION OF THE MARKER AND ITS INDICATION. ................................................. 24 FIG. 11: MINI MARKER. .............................................................................................................. 26

FIG. 12: NEAR SURFACE MARKER. ............................................................................................. 26 FIG. 13: FULL RANGE MARKER. ................................................................................................. 27 FIG. 14: DISC MARKER. .............................................................................................................. 27 FIG. 15: LOCALISATION OF THE MARKER. .................................................................................. 28 FIG. 16: LOCALISATION OF THE MARKER. .................................................................................. 29

FIG. 17: DATA RECORD INTO THE MARKER. ............................................................................... 29

FIG. 18: DATA RECORD INTO THE MARKER – PROCEDURE. ......................................................... 30 FIG. 19: DEPTH OF THE MARKER. ............................................................................................... 30 FIG. 20: DEPTH OF THE MARKER. ............................................................................................... 31

FIG. 21: MAP OF LOCALISATION AREAS. .................................................................................... 32

FIG. 22: T568A VS. T568B. ....................................................................................................... 34

FIG. 23: MEASUREMENT OF NEXT. ........................................................................................... 35 FIG. 24: MEASUREMENT OF FEXT. ........................................................................................... 35

FIG. 25: MEASUREMENT OF PS NEXT....................................................................................... 36 FIG. 26: MEASUREMENT OF RETURN LOSS. ............................................................................... 37 FIG. 27: MEASUREMENT OF ATTENUATION. ............................................................................... 37

FIG. 28: MEASUREMENT OF ACR. ............................................................................................. 38 FIG. 29: MEASUREMENT OF DELAY SKEW. ................................................................................. 38

FIG. 30: MULTIMODE STEP-INDEX FIBRE. ................................................................................... 39 FIG. 31: MULTIMODE FIBRE WITH VARYING REFRACTIVE INDEX. ............................................... 39 FIG. 32: SINGLE-MODE STEP-INDEX FIBRE. ................................................................................ 40 FIG. 33: EXAMPLES OF DIFFERENT REFRACTIVE INDEX PROFILES. .............................................. 40

FIG. 34: COUPLERS A) CLASSICAL, B) FIBRE COUPLER. ............................................................. 43 FIG. 35: MODE FILTER. .............................................................................................................. 43

FIG. 36: EFFICIENCY OF MODE FILTERING. ................................................................................. 44 FIG. 37: MECHANICAL MODE SCRAMBLERS. .............................................................................. 45 FIG. 38: FIBRE-TYPE MODE SCRAMBLERS. ................................................................................. 45 FIG. 39: STABILIZED OPTICAL SOURCE CIRCUIT. ........................................................................ 46 FIG. 40: MEASUREMENT OF OPTICAL POWER. ............................................................................ 48

FIG. 41: BLOCK DIAGRAM OF OPTICAL POWER METER. .............................................................. 48 FIG. 42: MEASUREMENT OF ATTENUATION BY CUT-BACK METHOD. .......................................... 49 FIG. 43: MEASUREMENT OF ATTENUATION BY INSERTION LOSS METHOD. ................................. 50 FIG. 44: OPERATIONAL MEASUREMENT OF ATTENUATION, USING INSERTION LOSS METHOD. .... 51 FIG. 45: MEASUREMENT OF ATTENUATION IN A SPLICE (FIBRE VALUES ARE KNOWN). ............... 52

FIG. 46: MEASUREMENT OF ATTENUATION IN A SPLICE (FIBRE VALUES ARE NOT KNOWN). ....... 53 FIG. 47: ELEMENT OF OPTICAL FIBRE. ........................................................................................ 55

6 FEKT VUT v Brně

FIG. 48: BLOCK DIAGRAM OF PULSE REFLECTOMETER. ............................................................. 56 FIG. 49: WAVEFORM OF RECEIVED BACKSCATTER POWER SHOWN ON A DISPLAY. .................... 58 FIG. 50: MEASUREMENT PROTOCOL OF OTDR REFLECTOMETER. ............................................. 59 FIG. 51: BLOCK DIAGRAM OF OTDR. ........................................................................................ 61

FIG. 52: IDEAL CURVE OF BACKSCATTER FOR LONGITUDINALLY HOMOGENEOUS FIBER. ........... 62 FIG. 53: TYPICAL EXAMPLES OF POSSIBLE FAILURES IN BACKSCATTER CURVE. ......................... 63 FIG. 54: DETAILS OF SPLICER. ................................................................................................... 66 FIG. 55: LCD DISPLAY WITH DETAILS. ...................................................................................... 66 FIG. 56: CONTROL BUTTENS OF EQUIPMENT. ............................................................................. 67

FIG. 57: EXAMPLE OF USING. ..................................................................................................... 68 FIG. 58: ONE OF THE EQUIPMENTS WHICH IS USED. ................................................................... 68 FIG. 59: EQUIPMENT AND MANUAL FOR THE STRIPPED FIBER. ................................................... 68

FIG. 60: EQUIPMENT FOR THE CLEAVE THE BARE FIBER. ........................................................... 69 FIG. 61: INSERTING OF FIBER. .................................................................................................... 69 FIG. 62: LCD MONITOR WITH DETAILS OF PROCES. ................................................................... 70 FIG. 63: SEQUENCE OF SPLICING PROCES. .................................................................................. 71

FIG. 64: FINALIZATION OF THE SPLICING PROCES. ..................................................................... 71 FIG. 65: GENERAL SCHEME OF MEASURING OF ATTENUATION. .................................................. 72 FIG. 66: WIRING DIAGRAMMEASURING DEVICE TO LABORATORY NEEDS. ................................. 74 FIG. 67: EXAMPLE OF DATA IN HYPERTERMINAL WINDOW........................................................ 76

FIG. 68: OVERALL LOOK TO ASSEMBLED MECHANICAL CONSTRUCTION. ................................... 77 FIG. 69: PRINCIPLE OF OPTICAL EDFA AMPLIFIER. ................................................................... 79

FIG. 70: OPTICAL RAMAN AMPLIFIER. ....................................................................................... 79 FIG. 71: WDM – WAVELENGTH DIVISION MULTIPLEXER. .......................................................... 80

FIG. 72: OLS 806 IN “RING APPLICATION”. ............................................................................... 81 FIG. 73: SPECTRUM OF WAVELENGTH DIVISION MULTIPLEXER. ................................................. 82

FIG. 74: IMPLEMENTATION AND CONNECTION OF WDM IN DT NETWORK. ............................... 82 FIG. 75: SCHEME OF THE WORKPLACE CONFIGURATION. ........................................................... 83 FIG. 76: BLOCK DIAGRAM OF THE SPLITTER. ............................................................................. 84

FIG. 77: METHOD OF PHASE SHIFT. ............................................................................................ 88 FIG. 78: METHOD OF DELAYED PULSES. .................................................................................... 88

FIG. 79: METHOD OF DELAYED PULSE, WITH A CASCADE OF BRAGG GRATINGS. ....................... 89

FIG. 80: WAVEFORM OF CHROMATIC DISPERSION. .................................................................... 89 FIG. 81: MEASUREMENT OF PMD BY INTERFEROMETRIC METHOD. ........................................... 89

FIG. 82: EXAMPLE OF PMD PLOT OF OPTICAL FIBRE, OBTAINED BY INTERFEROMETRIC METHOD.

90

FIG. 83: METHOD OF SCANNING THE WAVELENGTH. ................................................................. 90 FIG. 84: MEASUREMENT OF PMD BY THE METHOD OF DOP ANALYSIS. .................................... 91


List of tables

TAB. 1: THE DIAMETER OF THE CU CORE AND SPECIFIC RESISTENCE. ........................................ 14 TAB. 2: CALCULATION OF THE RESULTED UNBALANCES. ........................................................... 14 TAB. 3: THE SPEED OF THE MEASURING PULSE. .......................................................................... 20 TAB. 4: TABLE OF THE BALL MARKER TYPES AS TO ITS USE. ...................................................... 25

TAB. 5: EXAMPLE OF THE TABLE FOR RECORDS OF MARKER INFORMATION. .............................. 31 TAB. 6: EXAMPLE OF THE TABLE FOR RECORDS OF ID MARKER INFORMATION. ......................... 32 TAB. 7: DATA TO BE RECORDED INTO THE MARKER. .................................................................. 32 TAB. 8: DESCRIPTIONS OF BUTTONS. ......................................................................................... 67 TAB. 9: TABLE OF COMMAND FOR SETTINGS. ............................................................................. 85

TAB. 10: SETTING OF HYPERTERMINAL. .................................................................................... 86

TAB. 11: THE TABLE OF MEASURED VALUES. ............................................................................. 86

8 FEKT VUT v Brně

1 Introduction

Introduction for Laboratory Training, Security of Labour, Transmitting Quantities:

The laboratory of transmitting media is in the building of FEKT, Technick 12, C5/52

The student is obliged to change his shoes by his own slippers. His/her shoes coat will be

laid under the hatrack which may be used for coats too.

The credit, scored by reached points will be given after completion of all measuring tasks

and hand-over of protocols.

The security in the laboratory of “Transmitting media” is in conformity with the exam of

security, passed through in the first year of study. Respecting the work with fibre optics there

is necessary to be very careful for fragments of fibres. The fragments may stick into the

finger, eye or may be breath in. There is imperative to collect all fragments into the “fragment

box”. Despite there are not used power lasers in the laboratory, there is not recommended to

watch output connectors directly while the retina of your eye may be endangered. (Attention –

infrared band!)

There is essential to become familiar with transmitting quantities.

Transmitting quantities are able to express transmitting features of the telecommunication

equipment by evaluation of two values of specific physical unit. The main transmitting

quantities used in telecommunications are attenuation (loss), gain and level are the ratio

quantity using unit decibel – dB.

Decibel is dimensionless unit of transmission, based on the decimal logarithm.

The unit of dB doesn‟t belong into the SI system.

The loss (attenuation) is transmitting quantity expressing the ratio of the power (voltage)

in the input to the power (voltage) in the output of the transmitting system.

The gain is transmitting quantity expressing the ratio of the power (voltage) in the output

to the power (voltage) in the input of the transmitting system.

The level is transmitting quantity expressing the ratio of the powers (effective values of

voltages) in two arbitrary points of transmitting system.

- P1, U1, …. quantities in the input of the transmitting system,

- P2, U2, …. quantities in the output of the transmitting system,

- Px, Ux, …. quantities in the arbitrary point of the transmitting system compared always

in the numerator.

Name of

quatity Symbol of quantity Definition

Mark

of

unit

Power loss a | |

| | dB

Voltage loss a | |

| | dB


Power gain z | |

| | dB

Voltage gain z | |

| | dB

Absolute power

level um

| |

| |

| |

| |

dBm

Absolute

voltage level uu

| |

| | dBm

Relative gain

level ur

| |

| | dBm

Relative

voltage level uru

| |

| | dBm

- P0,= 1 mW, U0 ... basic relative quantities, where U0 = 0,775 V

- The symbols of units dBm, dBu, dBr, dBru are determined for usage in tables and

graphs first of all.

Absolute level of power is transmitting quantity expressing the ratio Px/ Po, where Px,is

the power (complex or resistive) in arbitrary point of specific system and Po is basic power,

equal 1 mVA or 1 mW defining absolute zero level of power. Absolute level of power may be

expressed in units of the transmission (the mark dBm).

Absolute level of voltage is transmitting quantity expressing the ratio Ux/ Uo, where Ux

performs the effective value of voltage in arbitrary point of specific system and Uo is

reference value of voltage, equal to 0,775 V. Relative level of voltage is defined by gaining

the power 1 mW on the real resistance 600 Ω may be expressed in units of the transmission

(the mark dBm).

.

Absolute voltage level may be expressed in units of transmission (mark dBu)

Relative power level is transmitting quantity, expressing the ratio Px/P1, where Px is the

power in the defined point of system and Px is the power in the point chosen as the input of

the transmitting system. Relative power level is expressed in the units of transmission (mark

dBr).

Relative level of voltage is transmitting quantity expressing the ratio Ux/ U1, where Ux

performs the effective value of the voltage in arbitrary point of specific system and U1 is

effective value of voltage in the arbitrary point chosen as the input of the transmitting system.

Relative voltage level is expressed in units of the transmission (the mark dBr).

Apart of quantities mentioned above are used another quantities expressing transmitting

characteristics of the circuits, elements or couplings. There is possible to express them by a

10 FEKT VUT v Brně

ratio of equal quantities. They are named as loss (attenuation) and may be described also in

decibel (operational loss, reflection loss, crosstalk loss, connection loss etc.

The transmission quantities defining characteristics of disturbing effects in the output or

receiver point of the transmitting system are also described in decibel (signal /noise ratio,

equal level of crosstalk e.g.)

There are used following characteristics for detailed expression of various levels:

dBm0: absolute power level related to the point of relative level zero,

dBmp: absolute power level measured by psophometrically

dBm0p: absolute power level measured by psophometrically related to the point

of relative level zero

The point of relative level zero is the reference point of transmission, which is usually

identified with transmitting point of four-wire circuit (channel). For the point with the relative

level zero may be written:

| | | | | | | |,

where

P1 is the power in the point of relative level zero,

U, is the voltage in the point of relative level zero

Absolute level of power related to the point of relative level zero is defined numerically

by difference of absolute and relative power level in the defined point (x) in decibel. The

relative level is related to the point of relative level zero.

| |

| |

| |

Absolute level of power measured psophometrically and related to the point of relative

level zero is the level of noise power in defined band of transmission weighted by the filter in

accordance with the ITU recommendation; expressed numerically is defined by the difference

of the absolute and relative noise level in the defined point (x) in decibel. Relative noise level

is related to the point of relative level zero. The probability of noise amplitude distribution is

drawing to the law of normal distribution.

1.1 Measurement of Transmitting Quantities

The measurement of transmitting quantities may be described as an activity comparing

two values of the equal physical phenomenon.

The measurement is based on the effective value of the alternative voltage.

The basic measuring method for measurement of transmitting quantities is performed by

the measurement of absolute level of signal or noise.

Level meter – dB meter is calibrated in absolute voltage levels expressed in decibels.

Zero point of the dB meter scale is relative level zero.


The power level meter is calibrated in dBm. It points the value of absolute voltage level

directly enlarged by coefficient Δz expressing the ratio of the 600 Ω to the impedance for the

measurement of |Zx|, value of which is:

| |.

Psophometer – the meter of the noise is calibrated in dBm. It points directly the value of

the absolute noise level.

Its frequency band as well as amplitudes of their noise elements are limited by the filter

with amplitude characteristic of which express the sensitivity of human ear together with

electro-acoustic converter. The scale of the psophometer expresses also the objective value of

the power (voltage) of the noise and the subjective estimation of the disturbance rate of the

average listener.

The meter of power level related to the point of relative level zero is calibrated in dBm0;

it points directly the amount of specific power level reduced by the amount of relative level in

this point.

1.2 Mutual Conversions of Np – dB Units and Transmission Quantities.

The conversion of decibel to Neper units and vice versa is linear conversion of the

decimal logarithm with the base 10 to the natural logarithm with the base e = 2,71828

following formulas:

| |

| |

| |

| |

| |

| |

| |

| |

where M1, M2 are modules of the conversion with the values M1 = 2,30259 = ln 10 and

M2 = 0,43429 = log e

Examples:

1. The voltage of input of the line was measured as U1 = 60 V on the impedance of

Z1 = 600Ω. At the end of line was measured current I2 = 10 mA. What is the loss of the line?

2. The loss of line is 18 dB. At the end is the impedance Z1 = 600Ω and the current is

I2 = 10 mA. There was measured the voltage U11 = 49 V in the distance 1 km from the

beginning of line. Calculate the length of the line!

12 FEKT VUT v Brně

2 Loop Resistance Measurement & Unbalance of

Resistance

Assignment:

Accomplish the measurement of the loop resistance of pairs (I, II) and the unbalance of

their resistance inside the quad!

b) Theoretical introduction:

The aim of measuring of telecommunication cables is to determine values of electrical

characteristics of assembled cable line. Cable line is terminated on the clamps of headend for

the purpose of various measurements. Acceptance tests are provided after completing of

assembly.

The results of measurements are completed in protocols.

The needed values are determined in accordance with the ITU-T Recommednations.

The test of continuity

The test of continuity confirms if the tested wire is alongside whole cable line well

connected as well as uninterrupted and if the sequence of individual wires is identical in both

headends of the cable line section.

2.1 Measurement of the loop resistance and unbalance of resistance.

This measurement verifies the resistance of the wire loop to be documented for the

calculation of the unbalances inside the quads. This measurement is provided by the cable

bridge.

There are measured five values or the loop resistance of individual wires of the quad,

which are on the far end mutually connected. There are measured following loop resistances:

Resistance unbalance of the first pair is defined by subtraction of values of 2nd

and 3rd

measurement:

( ) ( )

Resistance unbalance of the second pair is defined by subtraction of values of 3rd

and 4th

measurement:

( ) ( )

Resistance unbalance of the combined (phantom) circuit is defined by subtraction of

values of 1st and 5

th measurement:

( ) ( )

The results of measurements for all cable elements are recorded into measuring protocol

”Measurement of Loop Resistance & Unbalance of Resistance”.


In case of only single pair or unpaired number of pairs the individual wire of arbitrary

element, which is assigned as the wire c in the protocol. Then are measured only loop

resistances:

and resistance unbalance is defined by subtraction of 2nd

and 3rd

measurement:

( ) ( )

The resistance measured by direct current of individual wires is dependent to the

temperature in accordance with the formula:

( )

where Rt …. resistance of the wire [Ω] by the temperature t [oC]

R20 .. resistance of the wire [Ω] by the temperature t = 20 oC

ɑCu …. temperature coeficient of resistance [1/oC], for copper is 0,00393 1/

oC

t ..... temperature of wire [oC]

The ratio:

( ),

The temperature recount constants for resistance are plotted in the Table (by the

laboratory task) for t temperatures of copper cores of wires insulated by air – paper spanned

between –5 and +30 oC related to the temperatures +20

oC; +15

oC; +10

oC.

The specific resistance of the wire core in accordance with the formula:

where R ….. specific resistance of the core [Ω/km]

Rs …. resistance of the measured loop [Ω]

l …. length of the line [km].

Calculated values of the specific resistance (always maximal value for each type of line

– different coiling, diameter of wires etc. – are recorded into measuring protocol to the

column “Specific wire”.

Specific resistance of the copper core without resistance of Pupin coiles should not be

over values of the Table 2.1 by the temperature 20 oC.

There is necessary to add resistance measured by direct current for coiled elements.

Eventual overrun of the tabled values shall be justified in the measurement report, e.g. by the

higher specific resistance of used core material or higher temperature than 20 oC during

measuring etc.

Maximal permissible resistance unbalance among wire cores of each pair or combined

circuit for sections of the length over 35 km may be calculated by the multiplication of

following values by the length of the cable in km for cores:

14 FEKT VUT v Brně

Tab. 1: The diameter of the Cu core and specific resistence.

Diameter of the Cu

core Specific resistance

[mm] [Ω/km]

0,9 28,47

1,2 15,95

1,3 13,64

a) up to Ǿ 1 mm 0,04 [Ω/km]

b) over Ǿ 1 mm 0,027 [Ω/km]

For cable sections shorter than 35 km the resistance unbalance shall be for cores

a) up to Ǿ 1 mm less than 1,5 Ω

b) over Ǿ 1 mm less than 1 Ω

c) The procedure:

Use the manual of the measuring bridge

Measure in order with enclosed Tab. 1

Measurement of the loop resistance and resistance unbalance

Cable …….…….…….…….……. Measured from: ………….…….

Section …..…….…….…….……. Measuring device ………..…….

Length ….. …….…….…….……. Measured by ……………..…….

Tab. 2: Calculation of the resulted unbalances.

Quad/ Pair

Loop resistance *Ω+ Unbalance [Ω]

Specific

resistance

[Ω/km]

Notes

a + b a + c b + c b + d c + d a – b c – d


3 Localization of cable faults

3.1 Instructions:

1. Familiarize with the cable bridge M1T 450.

2. Focus simulated fault (short circuit) on the line of length l = 500 m.

3. For focusing use and verify the accuracy of two methods:

MURRAY‟s METHOD

VARLEY„s METHOD

4. Compare the results.

3.2 Measuring procedure:

For measurements, use the following connection:

- Goog wire – RED – connect to terminal X1.

- Bad wire – BLUE – connect to terminal X2.

- Ground – WHITE – connect to terminal Z

3.2.1 Measurement of loop resistance rab

Function switch (left of the measuring instrument)put to the position „Rx“

The value is obtained by subtracting the value of resistance of the decade after

balancing the bridge, and multiplying by the scale.

3.2.2 Murray’s method

Function switch (left of the measuring instrument)put to the position „M“

Range switch (right of the measuring instrument put to the position) „M“

RANGES IN THE GREEN FIELD!!!

3.2.3 Varley’s method

Function switch (left of the measuring instrument)put to the position „V“

Range switch (right of the measuring instrument put to the position) „Rx V“

RANGES IN THE BLUE FIELD!!!

3.2.4 Balancing the bridge

Switch on the device – put the switch in the upper right corner to the position „ZAP“

and the side switch put to the position „10 V=“.

Gradualy increase the sensitivity of 1 to 5 and search for the best range and

simultaneously adjust the decade resistor to the full balance of bridge (the bridge will show

"0" in the middle of gaude).

16 FEKT VUT v Brně

3.2.5 Calculation of the resistance to failure:

Murray’s method:

For Murray‟s method is used connection shown on fig. 1. The measured and set values

M and R are applied in the relation for calculating the fault resistance:

Where is the value of loop resistance (measured in 1.), R is the value of resistance

subtracted on a decade and M je is the value of set range.

Murray‟s method (Fig. 1) is used to localization a cable faults on the far end.

Fig. 1: Connection for measuring by the Murray‟s method.

Varley’s method:

For Varley‟s method is used connection shown on fig. 2. The measured and set values

M and R are applied in the relation for calculating the fault resistance:


Where is the value of loop resistance (measured in 1.),

is the value set on range

RxV and V is the value of resistance subtracted on a decade.

Varley‟s method (Fig. 2) is used to localization a cable faults on the near end..

Fig. 2: Connection for measuring by the method.

18 FEKT VUT v Brně

4 Impulse method of measuring

4.1 Instructions:

1) Familiarize with the impulse locator of faults MEGGER TDR1000/2.

2) Find the appropriate time of duration of pulse for wiring with length l = 500 m.

3) For submitted samples of wiring measure distance and the character of the fault (short

circuit, disconnection).

4) Connect the supplied capacitors and resistors, and characterize the measured changes.

4.2 Introduction:

Impulse measurement technique is used to quick localization of failure (broken wire or

short circuit), especially in line with the spread of high speed (structured cabling, coaxial

cables). They are based on the fact that the voltage pulse propagating along line is partially

reflected at the point where the rapid impedance transition is. Short circuit or disconnection

causes complete reflection and the progressive impulse is reflected back to the source. This is

used for targeting faults.

Impulse methods are fast and comfortable, but it is not possible to measure broken

isolation of the cable. This is possible only in the case, when the impedance of the cable is

changed so much, that it cause reflection of the pulse.

Impulse sighting device converts measuremnt of the distance (to the point of failure)

to measuremnt of the time. Real distance is calculate from the velocity of the transmitted

pulse and from the half of time which elapses between sending and receiving the pulse.

To achieve maximum accuracy of focus is need to be established as accurately as

possible the impulse propagation velocity of the line. If this speed is measured, we use

reflections from inhomogeneity, whose distance from the point of measurement is known

(end of line is open, short circuit, connectors, etc.), Fig. 3 and Fig. 4. The speed of the v/2 is

calculated from the length of the line la to the known inhomogeneity and from the time that

elapses between sending the pulse and receiving the reflected pulse by the relation:

Distance to the place of the failure is calculated by the relation:

where tx is the propagation time corresponding to the distance lx. If velocity is not

known or cannot be determined from the reflection of the known inhomogeneity, we use

informative data from tab..


Fig. 3: Line with short circuit.

Fig. 4: Line with open circuit.

20 FEKT VUT v Brně

Tab. 3: The speed of the measuring pulse.

speed of spread

v/2 (m/µs)

wireless line 146

plastic cables 100

cable with air-paper izolation of wires with metal shield 112–118

A sudden increase in operational capacity of pair respectively in capacity of the line-

ground is caused either by water intrusion into the cable coating or by distortion of the cable.

The shape of the pulse is reflected in .

Fig. 5: Capacity of the line.

If the change of longitudinal inductance is occurred in the line, the shape of reflected

pulse is as in Fig. 6. This reflect is caused by the couplings, or the transition to a line with

lower operating capacity.

Fig. 6: Inductance on the line.


Multiple reflection of the same pulse (Fig. 7) occurs when the output impedance of the

device is not adapted to the wave impedance of the line. Pulse reflected from the point of

failure at the beginning of the line partially reflects back and causes not only their own views

and views of other secondary pulses at times 2tx, 3tx, etc.

Fig. 7: Multiple reflection of the same pulse.

If the place of defects is near the beginning or end of line, reading is complicated by the

presence of the transmitted pulse or pulse reflected from the end of line.

22 FEKT VUT v Brně

5 Location of the Fibre Optic Cables Using Electronic

Markers and GPS.

5.1 Targets of Laboratory Exercise

Learning of fibre-optics cables location using EMS markers supported by the GPS

technology. 3M marker locator Dynatel 1420 EMS iD produced and patented by 3M

company. Learning of the program OpenWIG supported by the mobile phone Nokia 700.

5.2 Instructions for the Lesson:

1) Determine initial point of the line entered by the lecturer using mobile phone

Nokia 700 and the program OpenWIG. Let the GPS switched on during your

passage along located line. It will support the navigation. GPS will generate the

code by reaching of target point, which is to be recorded into protocol!

2) Utilise 3M location system during passage which is able to localise EMS

markers. Each route is equipped with various types of markers for

telecommunication, power cables and water ducts!

3) Locate markers indicating the route of optical cable as well as other lines and

draw them into the map!

4) The figures of individual gains, signal response and depth of laid markers!

5) Record information loaded in iD markers if they are applied!

6) In case of no iD marker applied, locate the marker close to the entrance of

Technická 10 building and try to program gained information into the marker.

Data to be used for programming are in the Table titled: Data to be loaded into

the marker.

5.3 Theoretical Introduction

There is necessary to use special techniques for the localisation of fibre-optic cables due

to their special properties. This cable has no metallic elements and therefore the generating of

any magnetic or electric field for localisation is not available. The information is transmitted

by electrically neutral photons. There are several methods utilisable for localisation of these

dielectric cables: By-laying metallic wire alongside the cable route, the non-dielectric optical

cable using metallic wire inside or EMS markers utilising resonant circuit. This circuit is

initialised through the locator and therefore the markers need not any power supply. EMS

markers are delivered in various shapes and colours in accordance with their application.

5.4 Principle of localisation

The locator Dynatel 3M is used for transmitting of HF signal into the marker, which is

buried underground. Marker reflects the signal back to the locator, which indicates

acoustically or graphically the position of the marker.


Fig. 8: The principle of marker localisation.

5.5 Description of the marker locator Dynatel 1420 EMS-iD

Basic control of the locator is possible by functional keys under its display. There are four

SW keys. The function of the key is displayed over the key. These functions are variable

depending on the operational regime of the locator.

Description of the functional panel of the marker locator Dynatel 1420 EMS-iD:

1. Switch receiver sensitivity.

2. Trace mode for locating markers – setup entries OK.

3. Menu – configuration of the device: clock, language, depth units and marker data.

4. Choice of the display backlight.

5. Icon of the loudspeaker volume.

6. Icon of the battery level.

7. Magnitude of the marker signal – reading of receiver signal.

8. Column indicator – graphical display of the received signal.

9. Gain value – on/off.

10. Volume of the receiver – off, low, medium, high.

11. Choice of the display contrast.

12. Choice of the indicates sensitivity of receiver.

13. Soft keys of the device, menu orientation, choices.

24 FEKT VUT v Brně

Fig. 9: Description of the front panel.

5.6 Signal response of the marker

As to the reached signal response from the marker is possible to define exactly its

position. The position of the locator against position of the marker is seen in the Fig. 10

together with its signal response.

Fig. 10: Localisation of the marker and its indication.


5.7 Electronic marks – EMS markers

The passive LC circuit is laid inside the marker. Markers perform passive antennas

without internal power sources to be charged. The marker locator transmits electromagnetic

field, which excites the chip being touched by the locator, which transmits the information of

its position. Outer covering of the marker is made from the polythene shell. They are

delivered in various types and colours specific for their usage.

The markers may be also provided with the iD accessory. This marker is labelled with

information, where iD code is typed. This code performs a number of 10 digits and a special

chip inside the marker together with the resonant LC circuit. The chip is determined for the

record of additional information concerning the line as about the owner, date of the origin,

applied technologies etc. This chip needs no power supply too. The information is loaded into

chip using locator of EMS markers.

5.8 The types of markers

Markers are differentiated during the manufacturing not only by the colour, but also in

accordance with the depth and mode of their burying.

5.8.1 Ball marker

This marker performs the ball of diameter about 11 cm. There is possible to bury its into

the depth of 150 cm. It is used in the narrow trenches. Ball marker is constructed in such a

way to be able to balance horizontal position of the chip not regarding its actual position after

burying. Inside is the mixture of the polypropylenglycol and water, in which flows the case of

LC circuit. Ball marker is produced in various colour modifications in accordance with its

usage.

Tab. 4: Table of the ball marker types as to its use.

Type of the marker Function Colour

Telecommunications Cable routes, extensions,

joints, bends, covers, crossing

Power supply

Cable routes, extensions, joints, bends, covers,

crossing, buried transformers, Loops, street

light

Cable TV Cable routes, extensions,

joints, bends, optical fibers

26 FEKT VUT v Brně

Water ducts

Routes of tubes, valves,

distribution system, Tube

crossings, adapters, cleaning

outputs

5.8.2 Mini marker

This marker is of special construction enabling possible burying into the depth up to

180 cm also in the hard approachable soil. It is of the spoken form with diameter about 20 cm.

This form is helpful for the burying (see Fig. 11).

Fig. 11: Mini marker.

5.8.3 Near surface marker

This type of marker is used for optical cable line tracing under roadways or badly

accessible surface (Fig. 12). There is possible to bury its directly into asphalt or concrete. It

can be buried into the depth up to 60 cm. Near surface marker is of the cylindrical form of the

length 15 cm.

Fig. 12: Near surface marker.

5.8.4 Full range marker

This marker is utilised as a protection against damaging (Fig. 13) of the optical cable by

digging. It may be buried into the depth up to 240 cm and its diameter is 38 cm.


Fig. 13: Full range marker.

5.8.5 Disc marker

Disc marker (Fig. 14) is not laid under the surface; it is designed for laying on the badly

accessible places into the dense vegetation, on the fillings etc.

Fig. 14: Disc marker.

5.9 Description of the navigation using mobile phone Nokia 700 and

program OpenWIG

GPS navigation is used for the navigation to the outgoing point of simulated

engineering network and also for the tracing of the route. It provides additional information

about areas where the markers are applied and prevents the unwished abandoning of the trace.

There is necessary to consider certain inaccuracy of GPS.

5.9.1 Starting of navigation

Switch on mobile phone Nokia 700,

Execute the program OpenWIG, which is on the main display,

Permit the usage of data of the application Position by the program OpenWIG,

Chose Start,

Permit the administration of user data,

Choose the trace entered by the lecturer (TraceA.gwc, TraceB.gwc, TraceC.gwc)

Follow the instructions from the display, for the control of the OpenWIG program use

the displayed soft keys (pointer, menu)

Record the code generated by the program by the tracing the route.

28 FEKT VUT v Brně

Don‟t load the trace,

After exercise is completed, check the battery and if it is well, switch off the mobile

phone; consult the lecturer in opposite case.

5.10 Processing of the laboratory exercise

The manual for the locator Dynatel 1420 EMS-iD is available in the workplace. Study

the device before action outside and check the signal response of ball markers in laboratory.

Check following functions: Localisation mode of one marker, the data reading from the iD

marker, localisation mode of two markers and the measuring of the depth of marker. Check

also the work with mobile phone Nokia 700, in which the program for GPS localisation is

installed. Start up the application whereigo and become familiar with its operation. Load the

trace, which will be told by the lecturer. After these steps is possible to leave laboratory to the

terrain. Area for the localisation is located in front of the entrance of the building Technická

10 - dean's office of the Faculty of electro and communication technologies.

5.11 Localisation of the marker

There is necessary to choose the type of localised marker. You may choose several

types of them, but you will use in your exercise first of all telecommunications markers

labelled TEL. These markers are orange. Next are utilised markers for power supply. These

markers are coloured red and labelled PWR. Your locator is switched on for choose of

engineer route. Push button Locate and next button Markr and you will choose type of trace.

By localisation of single marker the mark Markr 2 shall be in position OFF. You lower the

gain by pushbuttons Gain Adjust until this moment, when the column indicator is opened.

After identification of the marker for assigned engineer network will be this column indicator

closed, acoustic signal will be single tone and the maximal signal is displayed.

Fig. 15: Localisation of the marker.

5.12 Data reading from iD marker

Check the reading of information from iD marker. If your route has not iD marker, you

will find them in the grass by the entrance to the Technická 10 - dean's office of the Faculty of

electro and communication technologies. There is necessary to push button Locate for data

reading, choose cyclically Marker 1 and choose the type of engineer network (see in Fig. 16).

The mark Marker 2 is switched off – OFF. Pus button Read and loaded data from iD will be


red. Record this data into the table. Using pointers is possible to go through the menu. Data

from marker are loaded also in the locator, where it is possible to find them in Read history as

well as in the file named equally. There is loaded date and time of reading, information

concerning the owner eg. etc.

Fig. 16: Localisation of the marker.

5.13 Data record into iD marker

Thanks to the functionality of information record is possible to load important

information, these may be useful for next localisation or by the trouble shooting of optical or

another route. Check the data recording into the marker in front of the entrance of the building

Technická 10. Marker is to be found in the localisation mode of locator, it means Markr 1 Tel

and Markr 2 OFF. Push button Menu and next Write mode after marker is found. You may go

through menu using pointers up and down and you will choose the template BPMR and then

confirm by the pushbutton Veiw/Edit. Information of the marker will be displayed. Next you

will go through all information and paste them in accordance with the table: Data to be

recorded into the marker. Push button Modify and choose the User entry in the modification

window. Insert your login into the field Company and the number of the route, which you

went through into the field Job. Let rest settings unchanged. Next push the button OK.

Confirm it by push button Write marker finally after programming all data. There is necessary

to choose the type of marker, which the data are programmed into. There is necessary to hold

locator directly over the marker after uploading of all information and push Start write. Next

is the confirmation of steady blocking of data in the marker. You choose No Fig. 17 for data

will stay rewriteable.

In case of data blocking in the marker, your credit evaluation for this exercise will

be lowered and you are obliged to notify it to the lecturer.

Fig. 17: Data record into the marker.

30 FEKT VUT v Brně

Fig. 18: Data record into the marker – procedure.

5.14 Measurement of the depth of the marker

There is necessary to keep the point of locater closely on the surface over the measured

marker. Then push the button Depth. The receiver will undergo “Looking for iD Marker(s)”

=> “No iD Marker Found” and next “Calculating signal, please wait”,,. Next is displayed

order to lift up the locator up to inch (15 cm) (see in Fig. 19 and Fig. 20). Next after locator is

elevated push the button Depth again. Then is displayed the depth of the marker in several

moments. Pushing button Locate you will return into the regime localisation of the marker.

Fig. 19: Depth of the marker.


Fig. 20: Depth of the marker.

5.15 Localisation of more markers

This regime is necessary to choose by soft key Marker 2. Then it is necessary to choose

the needed type of engineer network. Using buttons Gain Adjust the gain is lowered until the

column indicator is open. After the marker is detected the indicator will be closed. We try to

reach the largest amplification of signal. After detection of the first marker we will push the

button of the type of another detected engineer network as PWR only eg. Locator is switched

into the regime of single marker localisation. For return to the regime of two markers

localisation is necessary to push soft key button Marker 2.

5.16 Formulation of the laboratory exercise

Use Tab. 5 for the records of gain, response and depth of the marker.

Tab. 5: Example of the table for records of marker information.

Marker – type (TEL, PWR …)

Signal response [dB]

Gain [dB]

Depth [cm] Notice

32 FEKT VUT v Brně

Map (Fig. 21)of localisation areas in the facilities of the FEKT VUT Brno.

Fig. 21: Map of localisation areas.

Following table (Tab. 6) is for the recording of iD markers information.

Tab. 6: Example of the table for records of iD marker information.

Marker – type (TEL, PWR …)

Signal response [dB]

Gain [dB]

Depth [cm] Data

Company

Job

Location

Description

Company

Job

Location

Description

Company

Job

Location

Description

Company

Job

Location

Description

Next table (Tab. 7) is for data uploaded into iD marker.

Tab. 7: Data to be recorded into the marker.

Upload of data into iD ball marker.

Company Your login

Job Number of the route

Location PARK

Description BPRM


5.17 Used device

Locator Dynatel 1420 EMS-iD, 3M, v.č. 09470018

5.18 Conclusion:

The aim of exercise is the processing of protocol, in which the measured values of

markers, code generated by GPS and drawing of approximate route of simulated optical or

power cable or water duct.

34 FEKT VUT v Brně

6 LAN structured cabling testing

Make the following tests for the two enclosed (S) UTP cables. Specify the type of cable.

Test results for both cables evaluate and compare each other.

Make the following tests (SINGLE TEST, the manual from page 28):

Wire Map, NEXT, PS NEXT, ELFEXT, PS ELFEXT, REMOTE ELFEXT PS, PS

ELFEXT PLOT SCREEN, Return Loss, Attenuation, ACR, PS ACR, Length, Delay Skew,

Propagation Delay.

6.1 Theoretical introduction

Measurements for the correct functionality of structured cabling is essential. Precision

instruments can measure installed components and determine whether they met all the

requirements defined in international standards to ensure reliable operation of the

applications. In the case of CAT5e and CAT6 are measured following main parameters:

Wire Map

This parameter controls the correct termination of pairs is the patch panel, including the

shielding in STP cabling. It also checks the signal along the cable,

In problem:

• check the connection of the wires

6.1.1 T568A and T568B Specifications

T568A T568B

1 white and green 1 white and orange

2 green 2 orange

3 white and orange 3 white and green

4 blue 4 blue

5 white blue 5 white-blue

6 orange 6 green

7 white and brown 7 white and brown

8 brown 8 brown

Fig. 22: T568A vs. T568B.


6.2 NEXT

Near End Cross Talk (Fig. 23) is a value that expresses how much signal gets from one

pair to another pair. Measurement of crosstalk at the near end is from the same end of the

cable as the source. All combinations of pairs within one cable are measured - ie 12-36, 12-

45, 12-78, 36-45, 36- 78, and 45-78 - on both ends.

Fig. 23: Measurement of NEXT.

In problem:

determine at which end of the cable shows NEXT error

check the maximum allowable conductor bifurcation chambers - that is 13 mm.

a frequent source of problems in cross-talk can also be the connectors or

couplings

6.3 FEXT

Far End Cross Talk (Fig. 24) expresses the crosstalk signal from one pair to another pair

measured at the far end. It is the same as NEXT, the parameter with the only difference being

that in the case FEXT is measured at different ends of the cable. Again, all combinations of

measured pairs within one cable - ie 12-36, 12-45, 12-78, 36-45, 36-78, 45-78. FEXT is an

important basis for parameter ELFEXT.

Fig. 24: Measurement of FEXT.

36 FEKT VUT v Brně

6.4 PSNEXT

Power Sum NEXT (Fig. 25) is the theoretical value calculated from the previously

measured NEXT. PSNEXT parameter is particularly important for protocols that are used to

transmit all four pairs (eg Gigabit Ethernet). Power sum near end crosstalk expresses how

much signal in one cable gets from three pairs to the remaining fourth pair. Source of signal

and crosstalk measurements is done on the same end of the cable.

Fig. 25: Measurement of PS NEXT.

6.5 ELFEXT

EqualLevel Far End Cross Talk is better when transferring data than FEXT parameter.

Crosstalk inside the cable is reduced with increased attenuation. As in the ACR it is a

theoretical parameter (ie not measured but calculated from other previously measured values)

ELFEXT [dB] = FEXT [dB] – A [dB]. Far end crosstalk FEXT is reduced by the attenuation.

6.6 PSELFEXT

Power Sum ELFEXT is calculated from the values ELFEXT. As PSNEXT this

parameter is important for protocols that use the signal all four pairs. PSELFEXT shows how

much signal in the same cable gets from three pairs to the remaining pair. Source of signal

and crosstalk measurements carried out on opposite ends of the cable.

6.7 Return loss

Return Loss (Fig. 26) shows the reflected signal due to the different impedance. Due to

these imbalances, the part of the energy goes back to the transmitter, which can cause the

signal noise.


Fig. 26: Measurement of Return Loss.

6.8 Attenuation

Attenuation shows the difference between the input signal and the signal at the end of

the wire. This is mainly due to resistance of the wire and is usually larger for higher

frequencies. Attenuation also increases with decreasing the diameter of the cable.

Fig. 27: Measurement of attenuation.

6.9 Length

There is a direct correlation between length and attenuation (ie the longer cable has the

higher attenuation). Measuring instruments used to measure the length of the TDR (Time

Domain Reflectometry), which means that the cable pulse is sent to the remote unit to bounce

back and is then recorded the time at which the pulse travels the whole track. Based on NRC

(Nominal Velocity of Propagation = percentage ratio of the speed signal cable to the speed of

light in vacuum) is then calculated by measuring the length of the segment. It is important to

realize that this is a length of twisted pairs (the electrical length), not "untangled" cable (so-

called physical length). To 85 m can be a difference between the electrical and physical length

to 5 m depending on the twisting of each pair.

6.10 Propagation Delay

This value expresses the signal delay from one end of the cable to the other. Typical

signal delay for category 5e cable is about 5 ns to 1 m, the limit is 5.7 ns to 1 m - which is

570 ns to 100 m PropagationDelay also serves as the basis for determining the value

DelaySkew.

38 FEKT VUT v Brně

6.11 ACR

Attenuation (Fig. 28) to Crosstalk Ratio is a theoretical parameter (ie not measured but

is derived from two already measured values), which expresses the difference between NEXT

and attenuation: ACR [dB] = NEXT [dB] - A [dB]. If the level of attenuation meets or

approaches the level of crosstalk, signal is lost. The interval between NEXT and attenuation

must be at least 10 dB.

Fig. 28: Measurement of ACR.

6.12 PS ACR

This parameter is calculated from the value PSNEXT and attenuation. PSACR (f) =

PSNEXT (f) – Attenuation (f).

6.13 Delay Skew

DelaySkew (Fig. 29) determines the signal delay difference between the fastest and

slowest pair. The parameter affects DelaySkew - (1) pairs of different length, (2) differences

in the material (resistance, impedance, etc.) (3) the effect of ambient noise. If the difference is

too large, may be an incorrect interpretation of data in the active element. As with PSNEXT

and PSELFEXT DelaySkew parameter is critical to the protocols used to transmit signals all

four pairs.

Fig. 29: Measurement of delay skew.


7 Transmission properties of optical fibres

The transmission properties of optical fibres depend in the first place on the type of

fibre design. In this respect, three types of fibre are distinguished:

multimode fibres with constant refractive index of the core and step refractive

index of the jacket; these fibres are simple to manufacture and handle, and of

a comparatively simple design but their drawback is greater attenuation and

dispersion, and small transmission capacity. They feature large core and jacket

diameters. An example of this type of fibre can be seen in Fig. 30.

Fig. 30: Multimode step-index fibre.

Some characteristics of this type of fibre: Dj = 50-200 µm, Dp = 120-300 µm,

dispersion 50 ns∙km-1

, attenuation 5-20 dB∙km-1

, and bandwidth 60 MHz.

Fibres of this type are mostly used in short-haul links, in particular for automation

purposes, short data transmissions, local networks, etc.

Multimode fibres with varying refractive index in transversal section of the core, which

have feature lower dispersion, lower attenuation, somewhat more complicated manufacture

and thus more complicated fibre design and splicing. The fibre has been standardized

according to the ITU-T recommendation: Dj = 50 µm, Dp = 125 µm. An illustration of the

refractive index pattern is given in Fig. 31.

Fig. 31: Multimode fibre with varying refractive index.

Some selected characteristics of this type of fibre are: dispersion at 0.85 µm is

1 ns∙km-1

, attenuation 2.5-5 dB∙km-1

, transmitted bandwidth 600 MHz.

In view of the above parameters, this type of fibre is particularly suited for

telecommunication purposes, namely for short-haul links.

40 FEKT VUT v Brně

Single-mode fibres with constant refractive index of the core and step refractive index

of the jacket, which feature low dispersion, very low attenuation, and large transmission

capacity. They mainly find application in long-haul transmissions. In this case there is only

one mode propagating in the fibre, in the direction of the axis. To be able to achieve this state

it is necessary to reduce the core diameter to a value equal to only a few light wavelengths.

The diameter range is Dj = 7-9 µm, Dp = 125 µm, as shown in Fig. 32.

Fig. 32: Single-mode step-index fibre.

Fibre characteristics: dispersion ca. 0.3 ns∙km-1, attenuation below 0.2 dB∙km

-1 at

a wavelength of 1.55 µm, and a bandwidth of 10 GHz.

In cases when the refractive index changes stepwise, the term layered lightguides is

often used. In these cases the transmission is based on the principle of total refraction at the

core-jacket interface. In the second type, the lightguide with continuously varying refractive

index (so-called gradient-index lightguide), the path of the ray has the form of elliptic or

circular helix.

Since the transmission properties of optical fibres depend on the pattern of refractive

index distribution, various manufacturers apply further variants of different refractive index

profiles.

Frequently occurring are two-layer lightguides and gradient-index lightguides with the

curve of refractive index close to the parabolic curve. As will be given below, a more

complicated pattern of refractive index can yield a shift in the dispersion characteristic, etc..

Fig. 33: Examples of different refractive index profiles.


8 Measuring methods in optical communications

The development of optoelectronic telecommunication systems has entailed the

development of new measuring methods and instruments for the measurement of the

parameters of fibres, cables and other optoelectronic elements. At first glance it might seem

that the measuring methods are analogous to the methods used for metallic lines but in fact

they are markedly different. Take, for example, the measurement of attenuation: the name is

the same in both cases, the theoretical foundation is the same but the approach is completely

different. The difference in measuring methods is given by the specific properties and

behaviour of light.

Leading manufacturers of optoelectronics currently offer a whole range of measuring

devices that feature simple operation and rapid measurement. The instruments are in most

cases provided with standardized connectors for optical radiation input and output.

For the area of optical measurement numerous recommendations have been worked out

in IEC, ITU-T and DIN, others are in the stage of preparation and a lot has still to be done, in

metrology in particular.

Optical measuring methods can be divided from various points of view; in the following

we will stick to this division:

Measurement on optical fibres;

optical measurement,

mechanical measurement.

Transmission measurement on optical fibres:

measurement of attenuation,

measurement of dispersion,

measurement of polarization mode dispersion,

measurement of backscatter;

measurement of bandwidth.

Measurement of optoelectronic components.

Special measuring methods.

Measuring instruments.

Another division can be from the viewpoint of application: for example, measuring

methods for the manufacture of optical fibres, measuring methods for the manufacture of

optical cables, measuring methods for the installation of cables, and measuring methods for

operational measurement.

From the viewpoint of building an optical track the following measurement must be

taken into account:

preparatory measurement (prior to acceptance inspection and prior to

installation),

measurement during cable installation,

measurement on an installed route,

final measurement (acceptance inspection after installation),

measurement during device installation,

regular check measurement of operation,

localization of possible failures on optical track,

check measurement of time stability of devices,

permanent check of the working of the whole optical system,

42 FEKT VUT v Brně

experimental measurement.

Most of the measurement consists in establishing the parameters of optical fibre and the

properties of optoelectronic modules of the system. The measurement set includes

a permanent check of the working of the system, which consists in electrical measurement of

error rate and its evaluation.

8.1 Methods of optical fibre excitation for measuring purposes

Since the lightguide is never ideally straight and cylindrical but exhibits random bends

and cross-section ellipticity, rays propagating in it are also liable to random changes. New

modes can be formed on fibre inhomogeneities that were not present in the mode distribution

before this inhomogeneity and, on the contrary, modes can disappear (by emission into the

jacket or by absorption) that till then participated in the transmission. This event is known as

mode overflow, the so-called mode conversion.

After a certain passage through optical fibre (of several units to hundreds of metres)

there is a certain balance in the distribution of power into individual modes, so-called stable

mode distribution. This state is highly desirable for the conditions of correct measurement of

optical fibre parameters.

When this requirement is not respected the measurement can, due to the existence of

differential attenuation, carry an error that completely distorts the results. When measuring

close to the source in particular, the mode distribution changes and does not guarantee

measurement reproducibility.

Conditions of stable mode distribution also for verification on short sections can be

approximated by using special devices inserted between the source and the fibre being

measured. It is possible to use:

couplers,

mode scramblers,

mode filters.

Couplers are used for specific coupling of optical power to the fibre being measured, i.e.

with exactly determined numerical aperture and with the required size of beam trace. The

couplers can be of the classical type, i.e. made up of lenses and diaphragms or of the fibre

type, with a fibre of suitable parameters.

In the mode scrambler heavy decoupling (mixing) of modes takes place, which leads to

a homogeneous distribution of power on its output.

The mode filter is used to remove undesirable higher modes before the input to the unit

being measured.

Coupler realizations for a fibre of 50 μm in diameter and theoretical numerical aperture

NA = 0.2, inclusive of the coupling condition, are standardized in IEC. According to this

recommendation, the mode distribution can be considered stable when after the passage of

radiation through a fibre 2 m long the beam half-width measured in the near region is 26 ± 2

μm and the numerical half-aperture measured in the far region is NA = 0.11 ± 0.02.

According to the American association EIA, excitation approximates well the stable mode

distribution when the diameter of excitation beam is 70% ± 5% of the core diameter of the

fibre being measured, and the numerical aperture is 70% ± 5% of its numerical aperture. For

the fibre under consideration, the diameter of the beam trace should be 35 μm and its

numerical aperture 0.14. The above examples of coupler are illustrated in Fig. 34.


Fig. 34: Couplers a) classical, b) fibre coupler.

Another approach consists in overexciting the measured fibre by an optical beam, which

means that the fibre is being excited by a beam whose diameter and numerical aperture are

larger than the diameter and numerical aperture of the fibre being measured. For the

telecommunication gradient fibre 50/125 μm the excitation beam should (according to IEC)

have a diameter larger than 140 μm and a numerical aperture larger than 0.3. The excitation

beam axis must be identical to the fibre axis. To remove the higher modes and to approximate

stable mode distribution a mode filter is connected to the input of the fibre being measured.

The filter can be obtained by winding the fibre onto a smooth cylinder (mandrel wrap filter).

For the 50/125 μm fibres a filter is recommended that consists of 5 fibre turns on a smooth

cylinder of 18 to 22 mm in diameter (see Fig. 35). Using such a mode filter a measuring

accuracy of ± 0.05 dB . km-1

can be obtained.

Fig. 35: Mode filter.

44 FEKT VUT v Brně

Perfect stable mode distribution can be obtained in such a way that a 1000 m section of

so-called dummy fibre (pre-fibre) is inserted between the transmitter and the fibre being

measured, as shown in Fig. 36.

Fig. 36: Efficiency of mode filtering.

The filter quality can be assessed using the filter efficiency criterion ΔΘ according to

the relation

%,1002

12

Θ

ΘΘΘ

where Θ1 is the angle of a beam exiting a fibre element 1 km long,

Θ2 is the angle of a beam exiting a fibre 2 m long, coupled to a mode filter

(see Fig. 36).

Mode scramblers can also act as mode filters: because of the coupling of higher modes

in mode scramblers, they act as filters. Examples of a mechanical scrambler and a serpentine

scrambler (the fibre is intertwined with seven cylinders of 1 cm in diameter, their centres are

spaced 1.3 cm) are given in Fig. 37. Mode scramblers made from different types of fibre are

shown in Fig. 38. The first of them, made up of different bits of fibre, is frequently used in

practice. The assembly is usually composed of ca. 1 m of SI fibre, a GI fibre of the same

length, and a third, SI fibre of 1 m in length. After the mode scrambler in the above

composition a certain fibre section (ca. 500 m) is connected to stimulate stable mode

distribution. Only an output prepared in this way can be connected to the fibre being

measured. In the other case given in Fig. 38, a section (ca. 2 m) of poor-quality fibre of

different dimensions and inhomogeneities is used for mode scrambling.


Fig. 37: Mechanical mode scramblers.

Fig. 38: Fibre-type mode scramblers.

The results of attenuation measurement depend, to a considerable extent, on excitation

conditions of the fibres being measured. With the development of optoelectronics, its specific

parts, instruments, and measuring methods this dependence gradually decreases.

46 FEKT VUT v Brně

Couplers that need to be adjusted are of no use for practical purposes; they are better

suited to laboratory measurement. On the other hand, however, instruments designed for field

measurement require much attention and great care in repeated measurement. The reason lies

in the special effect of losses in the splices (connectors) of the measuring apparatus on the

fibre being measured. While in electrical measurement the possibility of losses in connectors

and splices can usually be neglected, in optical measurement these losses play an important

role. Because of the difficulty of setting accurately the same measuring conditions, the

definition and measurement of the input (output) power are very difficult. Losses in optical

connectors are comparable with the losses in tens to hundreds of metres of optical fibre.

Moreover, even in types of the highest quality there are certain fluctuations in losses in

repeated installation, which are difficult to define generally.

In the first place, they are losses due to connector design tolerances, in particular axial

misalignment, imperfect contact of fibre edges, and faulty fibre cleavage. In practice this

means the impossibility to measure with a greater accuracy than these uncertain losses in

connectors. Since connectors are deployed on the transmitting as well as the receiving side,

we are faced here with a fundamental limitation of the potential measuring accuracy (above

all the measurement of optical powers). For these reasons the measurement of optical fibres is

conducted at both ends and the average value is then established.

Formation of optical flux

Laser diodes (LD) or light-emitting diodes (LED) are used as stabilized sources. The

output level of LD depends on temperature and reflected light. To remove changes caused by

this dependence the output level is monitored and, using the feedback loop, the output level is

stabilized.

A stabilized source operating on the 1.3 μm wavelength requires the surrounding

temperature to be strictly stabilized. This stabilization is provided using a feedback circuit and

the Peltier cooling element, see Fig. 39.

Fig. 39: Stabilized optical source circuit.

The temperature characteristic of the output level of stabilized optical source with LD has

in the temperature range 5 - 50 ºC a deviation of less than 0.05 dB.


At a constant temperature, LEDs have a stable output level for a long time and with

extremely high reliability. But output level is very sensitive to effects of surrounding

temperature. If a surrounding temperature sensor with a diode for temperature compensation

is used, adequate stability can be obtained. The temperature characteristic of the output level

of stabilized optical source with LED has in the temperature range 5 – 50 ºC a deviation of

less than 0.5 dB for the 0.85 μm and 1.3 μm bands.

The wavelength of the spectral width of light emitted by stabilized source must also be

stabilized. For the sake of measurement accuracy the emitted average wavelength must be

around 5 nm.

Some instruments are equipped with temperature control, which eliminates changes in

wavelength oscillation caused by changes in surrounding temperature. The light-emitting

elements used are LED. Due to their high temperature stability they are also suitable for the

measurement of transmission bandwidth.

The source of visible light consists of a He-Ne laser and an optical fibre connector. The

connector is designed such that it forms a clearly visible optical flux in optical fibre. The

mutual position of the He-Ne laser tube and the connector is stable, and to couple the optical

flux to the optical fibre a lens is used most frequently.

The radiation emitted from the optical fibre connector into space is immediately

diffused in order to protect workers against any health risk. For increased safety a protective

device is provided, which interrupts the emission of light when the optical fibre connector is

removed.

Sources of visible light are used in testing the optical fibre from the viewpoint of

damage and for the identification of individual fibres.

Optical detectors

The optical receiver detector is usually formed by the avalanche photodiode or the PIN

photodiode. The choice of sensor material (Si or Ge) depends on the required region of the

measurement wavelength:

Si material 0.5-1.1 μm

Ge material 1.1-1.6 μm

PIN diodes are used for highly sensitive detectors:

PIN Si, input power -90 dBm,

PIN Ge, input power -75 dBm.

APD for current measurement of higher levels:

APD Si, input power -60 dBm,

APD Ge, input power -40 dBm.

The range of levels measured is up to +10 dBm.

48 FEKT VUT v Brně

Measurement of optical power

The measurement of optical power is one of the basic types in the area of optical

measurement. This measurement is performed via converting the power of optical beam,

which is emitted directly by different optical sources or emanates from optical fibre, to

electric signal by means of an optoelectronic (O/E) converter.

An optical power meter consists of three parts: indicator, sensor, and adaptor, see Fig.

40. The adaptor adapts the light flux from a source or fibre such that it is best adapted to the

sensor dimension, and thus maximum optical power can be delivered to the sensor. The

sensor converts optical power to electric power. The indicator serves to display the electric

signal on a screen.

Fig. 40: Measurement of optical power.

These parts of the optical power meter can be exchangeable so that the device can be

used for measuring the required wavelength range of the power being measured, and for

various types of signal reception.

The measurement enables establishing

operation of opto/electronic converters and modules (or transmitter and receiver),

losses in fibres (fibre attenuation),

losses in the splices of parts of optical route,

function of passive optical elements (attenuation networks, connectors, optical

switching arrays).

The measurement is conducted on the respective wavelength, which can be set in

advance. The choice depends on the electro/optical and opto/electronic converters used, which

are fundamental elements of sources and receivers.

The block diagram of an optical power meter is given in Fig. 41. To increase the

receiver sensitivity, an optical signal chopper with subsequent synchronous detection is used

in addition to the generally known blocks (CU stand for control unit). The sensitivity is thus

increased by ca. 20 dB.

Fig. 41: Block diagram of optical power meter.


Measurement of attenuation

Attenuation represents the basic and most important transmission parameter and is an

overall measure of optical power losses in the optical signal propagation through the fibre.

Using the well-known definition, the attenuation of optical fibre between two points (1, 2 –

input, output) is determined from the relation

1

2

10 log dB ,P

AP

where P1, P2 are the optical powers (W) for wavelength λ.

In the case of stable mode distribution in the fibre, specific fibre attenuation can be

defined for wavelength λ

-1dB.km ,

A

where (km) is the distance between point 1 and point 2.

The measurement of attenuation is mostly performed only for discrete wavelengths of

850 nm, 1300 nm, 1310 nm or 1550 nm. The spectral characteristic of attenuation is mainly

important to fibre manufacturers.

IEC recommends three methods for the measurement of fibre attenuation:

- cut-back method,

- insertion loss method,

- backscattering method.

Because of its high sensitivity, the cut-back method is recommended as a reference

method (although it is a destructive method). After coupling the optical power from

a stabilized optical source (with connected internal or external transmit unit T.J. – coupler,

filter) to the measured fibre (Fig. 42) of length l, the power is measured at point 2 at the end

of the fibre (power meter). With the coupling conditions unchanged, the fibre is cleaved ca.

2 m from the beginning (at point l) and output P1 is measured. Attenuation and specific

attenuation of the fibre are calculated using relations. An accuracy of 0.01 dB . km-1

can be

achieved by this method.

Fig. 42: Measurement of attenuation by cut-back method.

The insertion loss method also requires measuring in two steps. This is an operational

method and is particularly suitable in the case of connectored fibres and cables. In the first

50 FEKT VUT v Brně

place, the measuring equipment must be calibrated via interconnecting the source and the

detector (see Fig. 43). After measuring we obtain the value of power P1. The fibre being

measured is then connected between the optical transmitter and the power meter, and the

value of power P2 is obtained. Attenuation and specific attenuation of the fibre are again

determined using relations. In this case, the attenuation measured consists of fibre attenuation

and attenuation of the splice of the fibre being measured. In the measurement of connectored

cables, the measurement precision is a function of the connector used and is usually worse

than 0.2 dB.

Fig. 43: Measurement of attenuation by insertion loss method.


Fig. 44: Operational measurement of attenuation, using insertion loss method.

The method is used in practice also in such a way that on each side of the route (A, B)

both the source (transmitter) and the power meter (receiver) are located. The method proceeds

in four steps: two measurements are first made on each side via connecting the power meters

to sources (calibration), and two measurements with the fibre connected in both directions.

A simplified fundamental schematic is given in Fig. 44. The four values of optical power

obtained, P11, P12, P21, P22, are used to calculate the operational attenuation of fibre according

to the relation

2211

2112

PP

PPA

The specific attenuation can be calculated by substituting previous relations.

If in the above method we want to establish losses in the splices, we use for calibration

fibres of 2 to 3 m in length and of the same kind as the fibre being measured. On the

assumption that the coupling losses of connecting the reference cable and the cable being

measured are the same and the attenuation of reference cable can be neglected, then the

attenuation obtained in this way corresponds to losses in the cable alone.

The backscattering method (the principle is described below) for the measurement of

fibre attenuation is based on a completely different principle. In the two preceding methods

optical power was measured after passage through the fibre while in this method the time

dependence of backscattered optical power P (or the level) during pulse propagation in the

fibre is evaluated, which yields information on the quality of the whole fibre in dependence

on its length (see Fig. 45). From this dependence we can establish the attenuation of

a homogeneous section of fibre according to the relation:

dBlog52

1

P

PA

52 FEKT VUT v Brně

Fig. 45: Measurement of attenuation in a splice (fibre values are known).

The meter, an optical reflectometer, is much more complicated than the power meter

and, consequently, also more expensive. However, using a display and recorder, the method

provides information not only about fibre attenuation but also about the fibre quality (failures,

defects) along the whole of its length.

The measurement of attenuation in a splice belongs to important measurements in the

construction of optical routes. Immediately after fusing (or otherwise joining) sections of

optical fibre it is absolutely necessary to measure and check the quality of the splice made. If

the splice is found to be of poor quality, the splice must be made again.

The measurement of attenuation in a splice is laborious and time-consuming, and

requires much care. There are two ways how to proceed in practice:

using the manufacturer‟s parameters of the fibres being joined,

without knowing the attenuation of the fibres being joined.

In the first case, the sections of the two fibres to be joined by splice S are from the

viewpoint of attenuation known from the measurement protocols provided by the

manufacturer. Prior to making the splice S, the level P1 is measured on the side of the first

fibre section (see Fig. 45). Subsequent to making the splice S, the level P2 is measured on the

output of the section being joined. The splice attenuation is determined from the relation

dB221S aPPA ,

where a2 dB is the attenuation from factory protocols for a specific length and

transmission wavelength.

In the second case, when the attenuation of the sections being joined is not known, the

procedure is as follows. Prior to fusing, the level P1 is measured on the output of the first

section. A provisional splice PS is made (see Fig. 46) and the level is measured on the output

of the spliced section P2. After the provisional splice, in the direction from the input, the fibre

is interrupted and the level P3 is measured. The definite splice DS is made and the value of

power level P4 is found. Attenuation V2 of the section being joined is then

2 3 2 dB .A P P

Attenuation in the definite splice is

DS 1 4 2 dB .A P P A


Fig. 46: Measurement of attenuation in a splice (fibre values are not known).

In the measurement of connector attenuation a special reference fibre of 2-3 m in length

is used. It is connected between the transmitter and the receiver, and the level of optical power

P1 is measured. The reference fibre is replaced with a fibre obtained by joining two short

sections by a connector (the fibre must be of the same length and type as the reference fibre).

The level P2 is measured and the connector attenuation is established from the relation

K 1 2 dB .A P P

54 FEKT VUT v Brně

9 Measurement of backscatter

The backscattering method is an effective means of diagnosing optical fibres. The

method provides a detailed picture of the attenuation and possible fluctuations in geometrical

and physical parameters along the fibre, inclusive of failure localization.

The method is based on evaluating the time dependence of backscattered power of

a narrow optical pulse coupled to the fibre. Backscattered light detected at its input comes

from the Fresnel reflections from refractive index discontinuity and from Rayleigh‟s

scattering on microscopic fluctuations of glass refractive index. The amount of backscattered

light is directly proportional to the passing optical power. Changing the intensity of

backscattered light enables measuring the fibre attenuation.

The fibre length can be established from the time delay of the reflection from the rear

end of fiber with respect to the reflection from the input end.

An analysis of backscattered light provides a picture of the fibre homogeneity

(inhomogeneity), and allows monitoring whether the mode distribution has become stable.

Great advantages of the method are its non-destructive nature and the possibility of

measuring from one fibre end.

To get a mathematical representation of backscatter signal it is necessary to perform an

analysis of light energy on an element of optical fibre, then to consider light scattering on a

length, then to determine the power returning back to the fibre, and finally to define the

relation for backscatter as a function of time.

Consider an optical pulse of energy E0 sent at time t = 0 from the fibre beginning x = 0. At

a distance x from fibre beginning the radiant energy of pulse Ei(x) will be

i O

0

exp ,

w

E x E d

where is the attenuation. For a certain constant value it will hold

i O e .

xE x E

Now consider scattering in x, x + dx, then

w

d O d

0

d x x exp d d ,E E x

where d (x) is the scattering coefficient at point x. Only part of this energy can

propagate in opposite direction in the fibre (S(x)). Thus

w

p O d

o

d x exp dE E S x x d x.

When viewed from the input side, it holds


w x

O d

o o

d x exp d d d ,E E S x x x

where is the attenuation of reverse direction. In the case that ,

then

2 α x

O dd x e dE E S x.

Symbols for the calculation are given in Fig. 47.

Fig. 47: Element of optical fibre.

By interchanging the variables E and x we obtain the dependence of power on time

O OE P t.

It holds

sk2 . ,x v t

skd d ,2

vx t

where Δ t is the pulse width, and vsk is the group velocity.

Then we get

.eS5,0t tvskdO

vtPP

It is clear from the result that backscatter power is dependent on input power Po, pulse

width Δt and on the fibre parameters S and αd (αd are losses due to Raileigh‟s scattering per

unit of length).

The coefficient of backscattering S for a fibre with step refractive index is calculated

according to the relation

2

2

1

3,

8S

n

for a fibre with parabolic refractive index

56 FEKT VUT v Brně

2

2

1

,4

Sn

where

2 2

1 2 .n n

The resultant exponential curvature depends on the size of attenuation and on group

velocity.

When choosing the pulse width it is necessary to accept a compromise between the

requirement for photodiode sensitivity and the resolution power. With increasing pulse length

the maximum measurable total attenuation increases but the accuracy of length measurement

decreases. The power of reflected radiation is ca. 50 to 60 dB lower than a passing optical

pulse and this makes heavy demands on the device sensitivity.

For reasons given above, the design of optical reflectometer is continuously being

perfected. Principles known from information theory (problems of signal and noise reception)

are applied such as correlation methods and frequency synthesis methods. Also, memories are

employed, in which certain values of attenuation of reflected pulses are stored and their

average values are evaluated (this reduces measurement errors caused by receiver noise). A

more detailed discussion of these methods would be beyond the scope of this publication; this

is rather a matter for specialists in the area of measurement in telecommunications.

The fundamental principle and method of backscatter measurement are obvious from

the block diagram shown in Fig. 48. For the measurement itself it is sufficient to connect the

fibre being measured to the input connector of the reflectometer.

Fig. 48: Block diagram of pulse reflectometer.


The reflectometer is composed of a source of optical pulses, an optical system for

splitting the optical beam, and an optical receiver with evaluation of the time interval between

the sent pulse and the reflected pulse.

The source is a semiconductor laser with a minimum power of 5-15 mW at 10 ns pulse

width. The optical system with lenses and semi-transmissive mirror serves to couple optical

power to the fibre and deliver reflected light back to the detector so that the ratio is 1:1; 50%

of optical power flowing through the optical splitter is lost for the measurement.

The relations between direct and reflected radiation in the fibre have already been

defined by a derived relation (9.21). To make the method clear, let us assume that the fibre

exhibits a serious failure or is interrupted; the power of light PR reflected from the failure at

a distance ℓ from the fibre beginning will be

R O ,2

R kP t P

where t is the time interval between the sent pulse and the reflected pulse, Po is the

power of transmitter pulse, R is the surface reflectance, k are losses due to the optical system

inclusive of beam splitting, 2α (ℓ) is the average of losses in forward and reverse direction.

For the accuracy of localizing the failure the reflectance of the surface under

examination, which is very different for different damages, is of great significance. It follows

from the Fresnel relations that the maximum reflection is for perpendicular light incidence,

and it holds

,

221

221

nn

nn

where n1 is the effective refractive index, and n2 is the refractive index of the

surroundings.

For the typical core refractive index n1 = 1.5 and n2 = 0 (the case of air), which is

a situation occurring in the ideal case, the result is 0.04, which corresponds to a back

reflection of 4% of energy. In practice, however, the fibre fracture surface may be rough,

cracked and surrounded with water (n2 = 1.33) so that a reflection of 0.1 to 1% of energy can

mostly be reckoned with.

During the return passage the pulse is directed by the divider to the photodetector,

where it is converted to electric pulse. The input and the output pulse are displayed

simultaneously on the screen and the time interval between them is subtracted. Distance to

failure is determined from the relation:

1 1

,2 2

t c t c

n n

where t is the time difference between the sent pulse and the returned pulse, Δt is the

widening of reflected pulse due to fibre dispersion, c is the velocity of light in vacuum, and n1

is the core refractive index.

Finally, to express the total dependence of detected power of backscatter P(t) on time

delay t or on the corresponding distance from the fibre end we calculate fibre attenuation

from two points. The total fibre attenuation with the boundaries 1 and 2 is established from

the relation

58 FEKT VUT v Brně

2 1 2 1

2 12 1

1

.c 2

n P n P n P n Pa

t tn

The meter is usually equipped with an amplifier with logarithmic characteristic, which

enables displaying sections with constant optical power losses as straight lines, whose

gradient of line is a quality indicator of these losses. Any discontinuity (e.g. attenuation in

splices and connectors) shows as a sudden drop in the characteristic being measured.

An example of the waveform of received power of backscatter from optical route. In the

example (Fig. 49), the reflection of light on fibre input and on fibre interruption is evident.

From the decreases in (losses of) attenuation on splices, attenuation can be read directly in dB

if a calibrated scale is used. The line gradients give directly a picture of the size of attenuation

in the fibre.

Fig. 49: Waveform of received backscatter power shown on a display.

An illustration of the result on the display and the on the print-out of a fully automated

reflectometer is shown in Fig. 50.


Fig. 50: Measurement protocol of OTDR reflectometer.

The reflectometer used was EXFO FTB300. A ballast fibre (L = 500 m) was connected

between points 2 and 3. Points 4, 5 and 6 are splices along the route, where cable transfer was

carried out. Mark 7 terminates the cable in RSU. Part of the meter is a table giving the

attenuation values at the route points given above. The measurement was conducted on the

1310 nm wavelength.

The speed of measuring by this method is evident; the print-out gives immediately the

values of splice attenuation, specific attenuation and operational attenuation of individual

lengths, and, last but not least, some important values of the measurement protocol such as

refractive index, length of measured fibres, time of measurement, etc.

The accuracy of the measurement itself is also noteworthy when compared with similar

measurements known for metallic lines. The high accuracy is due to the fact that pulses

modulated to the carrier frequency of optical radiation are used in the measurement. The pulse

frequency bandwidth is much smaller than the carrier frequency. The optical pulse is then

deformed to a lesser degree and this enable higher measurement accuracy. For example, for a

length of 70 km and a pulse width of 10 ns the accuracy of localizing the fibre fracture is

better than ± 10 m.

The above backscatter method of measurement is a precise method frequently used in

practice. Using this method the following measurements can be conducted:

measurement of optical power losses of the route (i.e. in the fibre, splices and

connectors),

measurement (localization) of failures and damage in fibres (currently the only method

for this kind of measurement),

measurement of the lengths of individual route sections.

60 FEKT VUT v Brně

10 Measuring of attenuation by OTDR

10.1 Instruction

1) Familiarize with the principle of measuring method OTDR (Optical

Time Domain Reflectometery).

2) Familiarize with measuring device EXFO FTB-400.

3) Using the reflectometer measure and draw the curve of backscatter

for the two wavelengths.

4) Measure in auto mode, then set two values of the pulse width.

5) Check the fiber attenuation for the two wavelengths.

6) Check the length of fiber.

10.2 Introduction

Currently the most used device for mounting and operating in many measurement

parameters, and optical cable routes is optical reflectometer. By this device you can measure

the length of the fiber, its homogeneity, attenuation of splices, connectors and fiber optic

connectors also allows to locate the fault. An optical reflectometer uses a backscatter method.

An OTDR injects a series of optical pulses into the fiber under test. It also extracts,

from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back

from points along the fiber. (This is equivalent to the way that an electronic time-domain

reflectometer measures reflections caused by changes in the impedance of the cable under

test.) The strength of the return pulses is measured and integrated as a function of time, and is

plotted as a function of fiber length.

An OTDR may be used for estimating the fiber's length and overall attenuation,

including splice and mated-connector losses. It may also be used to locate faults, such as

breaks, and to measure optical return loss. To measure the attenuation of multiple fibers, it is

advisable to test from each end and then average the results, however this considerable extra

work is contrary to the common claim that testing can be performed from only one end of the

fiber.

In addition to required specialized optics and electronics, OTDRs have significant

computing ability and a graphical display, so they may provide significant test automation.

However, proper instrument operation and interpretation of an OTDR trace still requires

special technical training and experience.

OTDRs are commonly used to characterize the loss and length of fibers as they go from

initial manufacture, through to cabling, warehousing while wound on a drum, installation and

then splicing. The last application of installation testing is more challenging, since this can be

over extremely long distances, or multiple splices spaced at short distances, or fibers with

different optical characteristics joined together. OTDR test results are often carefully stored in


case of later fiber failure or warranty claims. Fiber failures can be very expensive, both in

terms of the direct cost of repair, and consequential loss of service.

OTDRs are also commonly used for fault finding on installed systems. In this case,

reference to the installation OTDR trace is very useful, to determine where changes have

occurred. Use of an OTDR for fault finding may require an experienced operator who is able

to correctly judge the appropriate instrument settings to locate a problem accurately. This is

particularly so in cases involving long distance, closely spaced splices or connectors, or

PONs.

OTDRs are available with a variety of fiber types and wavelengths, to match common

applications. In general, OTDR testing at longer wavelengths, such as 1550 nm or 1625 nm,

can be used to identify fiber attenuation caused by fiber problems, as opposed to the more

common splice or connector losses.

Block diagram of OTDR can be seen in Fig. 51.

Fig. 51: Block diagram of OTDR.

The optical dynamic range of an OTDR is limited by a combination of optical pulse

output power, optical pulse width, input sensitivity, and signal integration time. Higher optical

pulse output power, and better input sensitivity, combine directly to improve measuring

range, and are usually fixed features of a particular instrument. However optical pulse width

and signal integration time are user adjustable, and require trade-offs which make them

application specific.

A longer laser pulse improves dynamic range and attenuation measurement resolution at

the expense of distance resolution. For example, using a long pulse length, it may possible to

measure attenuation over a distance of more than 100 km, however in this case an optical

event may appear to be over 1 km long. This scenario is useful for overall characterization of

a link, but would be of much less use when trying to locate faults. A short pulse length will

improve distance resolution of optical events, but will also reduce measuring range and

attenuation measurement resolution. The "apparent measurement length" of an optical event is

62 FEKT VUT v Brně

referred to as the "dead zone". The theoretical interaction of pulse width and dead zone can be

summarized as follows:

Pulse length Event dead zone

1 ns 0.15 m (theoretically)

10 ns 1.5 m (theoretically)

100 ns 15 m

1 µs 150 m

10 µs 1.5 km

100 µ 15 km

The OTDR "dead zone" is a topic of much interest to users. Dead zone is classified in

two ways. Firstly, an "Event Dead Zone" is related to a reflective discrete optical event. In

this situation, the measured dead zone will depend on a combination of the pulse length (see

table), and the size of the reflection. Secondly, an "Attenuation Dead Zone" is related to a

non-reflective event. In this situation, the measured dead zone will depend on a combination

of the pulse length.

Backscatter curve for longitudinal homogeneous fiber is shown in Fig. 52. At the

beginning and end can be seen reflection of the Fresnel from the front input and output

fibers. This reflection is approximately three orders of magnitude larger than the amplitude

of the reflected signal.

Fig. 52: Ideal curve of backscatter for longitudinally homogeneous fiber.


Measuring of the attenuation of the real optical fibers and optical lines are usually not

meet the ideal course of the backscatter curve. Real course of these curves are distorted in

various ways. These distortions can be caused by changes along the fiber attenuation,

inhomogeneity in the route, fluctuations of waveguide structure or incorrect measurements

mode.

Typical examples of possible failures in backscatter curve are shown in Figure 3.

Fig. 53: Typical examples of possible failures in backscatter curve.

Typical examples of possible failures in backscatter curve are shown in Fig. 53.

Reflection from the input fiber interface with a marked dead zone.

1) In this section is measured fiber homogeneous and backscatter curve has

a constant slope. In this section is the attenuation constant.

2) Local increase in attenuation. This increase may be caused by splice.

3) Sharp peak, which arises due to Fresnel reflection at the connector coupling of

two fibers or fiber defects.

4) Apparent amplification occurs when in the route is a fiber section with a larger

diameter of mode field.

5) Multiple reflections can occur an incorrect choice of the linear range.

6) Ripple of the curve is usually caused by a measuring device, fluctuations

of waveguide structure or polarization effects.

7) Changing the slope of the curve. The cause of this effect may be changing

attenuation along

64 FEKT VUT v Brně

8) the fiber or continuous change of diameter mode field.

9) Reflection from the fiber end. This reflection is not always evident.


11 Splicing of optical fibers and measuring of the

attenuation

11.1 Instructions

1) Familiarize with the fiber cleaver and hand-held fusion splicer.

2) Stripp the ends of the two attached fibers, clean by izopropylalkohol, cleave and then

splice it.

3) Restore the function of primary protection.

4) Measure the attenuation of carried splice.

11.2 Introduction

Splicing of the optical fibers is one of the most used methods of connections of optical

fibers.

In the laboratory is available splicer FITEL S122.

With its low profile and IP-52 rated super rugged body, the FITEL S122 series fusion

splicer offers speedy operation in every splicing field, FTTx, LAN, backbone or long-haul

installations.

Splicer S122 is used for making reliable connections of optic fibers with low

attenuation. It is equipped with software for all common single and multimode fiber with a

standard cladding diameter of 125 microns and coatings from 250 to 900 micrometers. Splicer

has a sytem PAS (Profile Alignment System) for extremely low attenuation of splice,

independent of the operator.

Depending on the splicing process (see specification Fig. 54) and cleaning of fibers

offers high-precision positioning to coat of optical fiber, optimization of each splicing process

through automatic control of splicing time AFC, evaluation of attenuation and fully

automatic splicing process by pressing a single key.

PAS system is used for positioning the core to the core of fibers and automatic control

of splicing time. This system is supplemented by a video image evaluation L-PAS. Using two

cameras and magnifying optics detects the position and quality of the of fibers ends. At

present it is the most used technology of optical fibers centering.

Splicer machines with technology PAS (Profile Alignment System) used for centering

the fibers an active mechanism, which centered on the core of optical fiber with a minimum

deviation. Centering of the fibers takes place in three axes (3-D technology) and the resulting

centering is controlled by video image in two axes, which captures the optical lenses and

evaluated in the microprocessor.

66 FEKT VUT v Brně

Fig. 54: Details of splicer.

LCD (display see Fig. 55) shows the fiber simultaneously in two different views (plane

X and Y).

Fig. 55: LCD display with details.


The control panel (see Fig. 56 and details in Tab. 8: Descriptions of buttons.) consists of a set

of buttons that have the following functions:

Fig. 56: Control buttens of equipment.

Tab. 8: Descriptions of buttons.

Button Name Main function

Start Start/Pase/Restart of splicing process.

Function 1 Select the function (s) shown (s) in the right bottom

corner of the LCD.

Function 2 Select the function (s) shown (s) in the left bottom

corner of the LCD.

Up Move up, increasing the value.

Down Move down, decreasing the value.

Left Move left, switch the view X/Y.

Right Move right, switch the view X/Y.

Heat Switch on/off heating.

Power Switch on/off.

Through the USB it is possible to send data (parameters of splices) from the splicer to

PC or printer.

68 FEKT VUT v Brně

11.3 Splicing procedure

WARNING: The smallest possible values of attenuation can be achieved only when the

ends of fibers are carefully prepared for splicing.

1) Slide the protection (Fig. 57) of splice either to the left or to the right fiber.

Fig. 57: Example of using.

2) Stripp (Fig. 58) about 30 mm of fiber protection in each end of fiber.

Fig. 58: One of the equipments which is used.

3) Clean stripped (Fig. 59) fiber by izopropylalkohol and cleaning tissue.

Fig. 59: Equipment and manual for the stripped fiber.


4) Cleave the bare fiber (Fig. 60) 10 mm of bare fiber will protrude from the protection

fiber.

Fig. 60: Equipment for the cleave the bare fiber.

Do not clean the bare fiber after it has been cleaved.

Do not touch the bare end of the fiber with any surface.

We recommend using safety glasses.

5) Inserting of fiber

Open the front cover.

Insert the fiber to the “V” groove (Fig. 61). Make sure that the end of the bare fiber

does not touch anything.

Make sure that the bare fiber is positioned correctly in the V-grooves. If not, remove

the fiber and set it again.

Repeat it for second fiber.

Close the front cover. READY screen will be displayed.

Fig. 61: Inserting of fiber.

70 FEKT VUT v Brně

Do not insert the tip ends of the fibers through the V-groove.

!

Warning

If you will insert the bare fibers to the V-grooves,do not break

them about V-groove or another part of the splicer.

Broken fiber could get into your eyes!

6) Splicing

Make sure that READY is displayed on the screen.

Push for start of splicing cycle.

S122 performs the following functions automatically. For interruption splicer

during one of these functions, press On display will be shown „PAUSE To

restart the operation, press again

On LCD monitor will be shown right and left end of the fiber.

There will be cleaning discharge to clean the fibers end.

The fibers are configured with a gap of about 30 micron between the ends.

Axis shift is checked and the state of fiber cleaves. Electrodes will discharge.

Inspection of splice is performed.

It is performed estimate attenuation is displayed on the LCD monitor, as shown in

Fig. 62.

Fig. 62: LCD monitor with details of proces.


11.4 Splicing proces on a S122C

For details see the following sequence Fig. 63.

Inserting of fibers Check Discharge Estimate attenuation

Fig. 63: Sequence of splicing proces.

When the splice is done we can heat the protection. Move the protection on stripped part

of fiber (through the splice) and put the protection to the heating part of splicer machine as

shown on Fig. 64. Push the button .

Fig. 64: Finalization of the splicing proces.

11.5 Measuring of attenuation

The following Fig. 65 shown the general scheme of measuring of attenuation.

72 FEKT VUT v Brně

Fig. 65: General scheme of measuring of attenuation.

1) For measuring the attenuation of splice will be used measuring kit Optokon

LS(PM)420. Read the instructions for detailed measuring equipment and familiarize

with the measuring device.

!

Warning

Be sure to clean! In particular, the purity of the connectors.

2) For measuring will be used reference method. At first, do the splice. After that, switch

on the light source (LS) and power meter (PM). For measuring use wavelength of

1550 nm. Connect the connectors to the LS and PM. Set new reference on PM (REF -

CONFIRM). Switch off PM and LS. Do not disconnect connectors from PM and LS.

!

Warning

Never disconnect connectors from PM and LS during the

measurement.

3) Cut the fiber next to the first splice (about 20 cm next to the splice). Then make the

new splice.

4) Switch on the LS and PM. Switch the wavelength on LS to 1550 nm. PM shows the

value of attenuation of splice.

5) Compare the measured value of attenuation with the estimate value from splicer

machine. The value of attenuation should be about 0,05 dB.


12 Optical wireless transmittion in laboratory

This chapter describes modification of cheap free technology point-to-point FSO

from project Ronja (Reasonable Optical Near Joint Access), which is used in optical networks

laboratory in department of Telecommunications, Technical University of Brno. The Ronja

is optoelectronics device, which uses narrow light beam as a transmission channel

in atmosphere. This beam is crated throw lens system. Purpose of device is wireless

connection of two separate computer networks with transfer speed of 10 Mbps. Maximum

distance to communicate is 900 meters and must be in line of sight. FSO Ronja is constructed

from three main devices, transmitter, receiver and interface. The transmitter contains LED

(light emitting diode) for transmitting data and the receiver contains PIN photodiode

with very short switching time as a detector. Ronja communicates in full-duplex (allows

communication in both directions simulatelously). The interface alters signal levels and

impedance for optical transmitting. It generates signal at 1 MHz, which is needed to foolproof

function the Ronja with interferences such as sunlight or another shining source, which should

influence connection.

12.1 Modification the Ronja in Laboratory

Several devices were added to the Ronja for purpose of acquaint students

with the functionality and capabilities of this FSO. Also was created own unique mechanical

construction.

12.2 Measuring device

This device is design for laboratory needs. It task is to measure RSSI voltage at both

receivers and send their values to PC via USB or RS232 port. Regulation of current

transmitting diode is from PC via this measuring device. Measuring device can turn off entire

Ronja device via keyboard in PC. There is special plug with relay inside and it is connected to

USB power supply, so if the PC is shutting down this plug disconnect power supply to entire

device and the Ronja. Measuring device controls microcontroller Atmega8 from Atmel.

Wiring diagram measuring device is at Fig. 66 on next page.

74 FEKT VUT v Brně

Fig. 66: Wiring diagrammeasuring device to laboratory needs.


12.3 Modification of the transmitter

The Ronja is designed to work at distance from 135 to 900 meters, but consoles are

in laboratory only few meters from each other. So it needs to reduce intensity of transmitter

light beam to simulate longer link. That‟s why there is added NPN transistor serially to

transmitting LED in the transmitter. The transistor is connected between cathode transmitting

LED and ground. The transistor reduces current in transmitting LED via increasing resistance

in base of the transistor, so light beam from LED is weaker. This regulation of current can be

makes electronically from PC via resistors R17, R18, R23 – R27 in the measuring device or

via potentiometer at the cover of the transmitter pipe.

12.4 Voltmeter at the receiver

At the cover of the receiver pipe is fit voltmeter to measures and displays actual value

of RSSI. There is used single-line LCD display with backlight. The voltmeter controls

microcontroller Attiny26 from Atmel. With this voltmeter is targeting both end of the Ronja

much more comfortable.

12.5 Measuring program in computer

Program Hyperterminal is used to communicate with the measuring device. This

program is part of each operating system from Microsoft. Hyperterminal job is to monitor and

record RSSI values from both receivers, which are send from the measuring device. Every

second Hyperterminal shows measured RSSI from both receivers. Data should log, so that it

is possible to create graph RSSI values in Excel from longer period of time.

Regulation of current one transmitting diode is represented via keys 1 to 7, where

number the most left has the most significant affect to limitation current and number the most

right has the least significant affect. Every number can have value 1 or 0, where 1 increase

light beam. Thought this program you should turn on and off power supply to the Ronja. If is

press the button “z”,

Ronja has power supply, button “v” cut off power supply. This has purpose to saving

energy, when is no measuring at the Ronja. Example of Hyperterminal window is at Fig. 67

on the next page.

76 FEKT VUT v Brně

Fig. 67: Example of data in Hyperterminal window.

12.6 Mechanical construction

There is 100 mm lens in the front part of each pipe. Laser cutting metal plates

are supposed to fit transmitter and receiver to the center of pipe, where these devices move

inside pipes to reach focal distance of the lens.

The receiver setups in pipe only once, so construction of metal plates are simple. But

metal plates for the transmitter are much more complicated, because transmitter can move in

pipe via spinning the handle on the screw rod. This is cause change of diameter the light beam

from the transmitter. Under each pipe there is very fine targeting system, which is assemble

from two screw rods. Vertical screw rod has purpose to support pipe and vertical advance of

the pipe, while horizontal screw rod is for horizontal advance of the pipe.

For tight fix of the pipe on its position there are always two nuts to counter with each

other. Two pipes with targeting system are assembled to console, which is screwed to wall.


Every iron part from this console is galvanized to the best protection against rust. Overall look

to assembled mechanical construction is photographed on Fig. 68.

Fig. 68: Overall look to assembled mechanical construction.

Modification the Ronja expanded possibilities for different kinds of measurements

at the Ronja and improved working with the entire device. The Ronja will very well serve

students in the lessons in optical network laboratory. Where they will try to work with

the Ronja, its behavior in the limit conditions and affect function of Ronja by various external

influences. Students will try complex problems of wireless optical transmission in practice.

78 FEKT VUT v Brně

13 EDFA Measurement

13.1 Assignment

1) Become familiar with the amplifier EDFA CLA-P(B)-01F!

2) Measure the powers of wavelengths in the input of the optical amplifier!

3) Measure the output powers by various values of gain, output gain and the

current of pumping diode!

4) Evaluate the measured values with the values in the output of EDFA amplifier!

13.2 Theoretical Introduction

Optical amplifiers

Optical amplifiers are used with advantage in wavelength division multiplexing

systems. Unlike repeaters, they enable restoring luminous flux in the fibre without the

necessity of converting it to electric form. These amplifiers are universal elements that

amplify both analogous and digital signals of arbitrary transmission speed.

Optical fibre amplifiers EDFA (Erbium Doped Fibre Amplification)

A simplified block diagram of the EDFA amplifier is given in Chyba! Nenalezen zdroj

dkazů.Fig. 69. The amplifier is formed by the so-called laser pump and a special optical fibre,

which is doped with rare-earth elements (erbium, et al.). Due to the laser pump emission (of

980 nm or 1480 nm wavelength) coupled to a special fibre of several metres in length, atoms

of doped element are excited to a higher energy level. The energy obtained from the laser

pump radiation is thus temporarily stored in the atoms. This energy is released due to the

presence of the signal being transmitted, whose energy calls forth stimulated emission of

radiation, which is of the same wavelength and phase as the signal transmitted. This amplifies

the transmitted optical signal. Optical fibre amplifiers enable increasing the signal level by as

much as 50 dB (one channel, C – band). Via internal arrangement of the amplifier a wide

range of the amplified band can be obtained and thus the signal can be amplified in the C and

L bands simultaneously. Various possibilities of deployment in an optical transmission system

result from the principle of EDFA function. Basically, the amplifiers can be applied in four

manners:

Booster – which is located right after the optical transmitter; it serves to amplify its

signal to a maximum level that can be coupled to the fibre. It must be capable of

accommodating a relatively large input signal from optical transmitter.

In-line amplifier – which is located on the optical fibre route; it amplifies a small input

signal to a maximum output signal.

Pre-amplifier - serves to amplify very low signal levels to a level that is sufficient for a

correct functioning of optical amplifier at the end of transmission route. The

requirement put on the pre-amplifier is to have a minimum internal noise.


Compensation of losses in optical networks (CATV) – in optical community antenna

television the reduction of signal level is primarily due to the requirement to divide the

optical signal into several fibres. Before being divided the signal is amplified by

means of EDFA such that the same signal level is obtained in output fibres as in the

original fibre.

These amplifiers are manufactured as single-channel EDFA amplifiers, WDM

amplifiers, and CATV amplifiers.

Fig. 69: Principle of optical EDFA amplifier.

Optical Raman amplifiers

The Raman type of amplifiers is used to amplify an optical signal. It is practically just

a laser source of radiation connected to the optical route. To amplify the optical signal the

Raman scattering on particles of the waveguide material is used. With this scattering there is,

among other things, a shift of energy from the lower wavelengths (wavelength of Raman

pump radiation) to higher wavelengths (wavelengths of the signal being transmitted) and

consequently an amplification of the signal. A simplified connection diagram is shown in Fig.

70.

Fig. 70: Optical Raman amplifier.

80 FEKT VUT v Brně

The amplification of optical signal thus takes place directly in the transmission route

fibre. No special fibre is necessary here; any communication fibre can be used. Amplification

values obtained with this type of amplifier are not as high as with EDFA. The signal level can

be increased by ca. 15-20 dB.

The Raman amplifier is located at the end of optical transmission fibre and radiation

from the laser pump propagates against the signal being amplified. This amplifier can be used

to amplify an arbitrary wavelength, provided the appropriate wavelength of laser pump is

chosen (e.g. 1450 nm for the 1550 nm band). The EDFA amplifier and the Raman amplifier

can with advantage be combined. However, these amplifiers also amplify distortion and thus

in the case of long routes it is necessary to connect the classical repeater to restore the signal.

Example of four-wavelength multiplexer - is a frequently demanded multiplexer for

rapidly increasing the network capacity at a relatively low cost. It is based on the principle of

cascading interference filters with 8 nm spacing (see Fig. 71).

Fig. 71: WDM – wavelength division multiplexer.

Example of dense wavelength division multiplexer (DWDM) – leading manufacturers of

the DWDM device include Lucent Technologies, Alcatel, Nortel, NEC, and others. Currently

marketed DWDM have 16, 20, 40, 60 and up to 100 spectral channels.

The Wave Star OLS 806 device by Lucent Technologies will be given as an example. It

employs 16 wavelengths, spanning attenuation 33 dB, which corresponds to a distance of 120

km without amplification (non-zero dispersion fibre True wave is assumed). The device can

be used with advantage when creating ring topologies, as can be seen in Fig. 72Chyba!

Nenalezen zdroj odkazů..

Effects influencing the quality of multiplex transmissions

For a good quality transmission it is necessary to meet the respective limits, which are

verified by measuring and which include:

the average wavelength, which must meet the respective standards; accurate

measurement must be assured with respect to temperature changes, laser instability,

and back reflections,

the bandwidth must also meet the criteria of spectral characteristics,


the insertion loss must provide the most favourable transmission conditions,

in DWDM installations, the cross-talk must satisfy the cross-talk quantities between

neighbouring wavelengths, as was earlier the case with metallic conductors,

Fig. 72: OLS 806 in “ring application”.

the back reflection may be different in individual channels and the values need to be

kept within tolerances, mainly with a view to system stability,

the peak power of individual channels must satisfy the respective tolerances,

the type of fibre used is a separate and very important consideration.

An example of the spectral characteristics of four-wavelength multiplexer is shown in .

In the case of new installation, when the deployment of DWDM system is assumed, the

technical parameters can be selected to meet the operator‟s requirement. This principle is

currently implemented in the construction of transport networks of many operators, using the

so-called “Telehouses”.

A substantially more difficult situation exists as regards the future exploitation of

already installed fibres for the needs of DWDM.

In the first place, earlier fibres (ITU G.652) are not ideal for DWDM. Relatively large

chromatic dispersion in the 1550 nm band limits the link range and compensation is necessary

for longer routes.

82 FEKT VUT v Brně

Fig. 73: Spectrum of wavelength division multiplexer.

All the effects given above influence transmission and they must be taken into

consideration in the design of networks.

Application potentials of wavelength division multiplexers in academic computer

network of universities

One of the first applications is the deployment of two-wavelength multiplexer in the

experimental network of the Department of Telecommunications (DT). The system

demonstrates its functionality and enables the measurement of elements. The system was used

to demonstrate its application in the supervision for optical cables. Its connection is shown in

Fig. 74.

Fig. 74: Implementation and connection of WDM in DT network.


Scheme of the workplace configuration:

Input

2% TAPcoupler

Insulator

Optical aplifier

Insulator0.1% TAPcoupler

Output

Back reflection Output monitor

Laser pump

Fig. 75: Scheme of the workplace configuration.

The workplace consists of optical source LS 420, two TAP Couplers, amplifier EDFA

CLA-P(B)-01F and optical power meter PM 420.

13.2.1 Optical source LS 420

Optical source LS 420 is conform to necessary technical requirements for

operational devices. Rechargeable battery secures long operational time with the minimal

lifetime period of 5 years. These sources offer the choice among six operational wavelengths

of 650, 850, 1310, 1490, 1550 and 1625 nm. It works with CW modulation, which enables to

create up to seven combinations of wavelengths. These sources are applicable in

measurements of optical fibres as well as testing fibre continuity.

13.2.2 Optical Power Meter PM 420

This device is designed for the measurement of absolute or relative optical power in

optical networks. Internal memory is available in this measuring equipment, which enables

saving as well as data uploading up to 512 measurements including number of fibre,

wavelength and other parameters. Internal SW enables memory download and generation of

test reports.

84 FEKT VUT v Brně

13.2.3 SFT-TAP coupler

This splitter is designed for monitoring in optical networks by operation. It enables

bidirectional or unidirectional monitoring of optical fibre on whole span of CWDM

wavelengths. Splitting ratio may be chosen from 1:99 up to 10:90. It is independent of

protocol as well as bit rate.

SFT-TAP-AT

AT

B

Fig. 76: Block diagram of the splitter.

13.3 Amplifier EDFA CLA-P(B)-01F

This amplifier is of the family low-nose, high-powered amplifiers designed for the

realisation of the “key-solution” in optical networks. It consists of two EDFA amplifiers –

pre-amplifier Pre AMP and power amplifier Booster.

Characteristic features:

Low noise,

Broad wave span,

Low energy consumption,

Microcontroller enabling remote control,

Interface RS232, Ethernet, USB, optionally GSM/GPRS and WiFi,

Uni channel and multi channel,

LED signalling.


The amplifier may be switched among following modes:

AGC – Automatic Gain Control,

APC – Automatic Power Control,

ACC - Automatic pumping Current Control.

The control is based on Linux. Remote control CLA proceeds via SSH or SNMP.

Critical reports are sent by e-mail. There are supervised all important parameters as input and

output power, the current of laser diodes and others.

The control is based on Linux. Remote control CLA proceeds via SSH or SNMP.

Critical reports are sent by e-mail. There are supervised all important parameters as input and

output power, the current of laser diodes and others.

13.4 Instructions RS232

The instruction consists of the name of instruction, one or more disjunctive marks,

which isolate chosen parameters and of terminating mark: ACC [1,2] value . Important

instructions are in the Tab. 9.

Tab. 9: Table of command for settings.

AM Apmlifier mode G, P, C, O

AGC Gain control 1, 2, value

APC Power control 1, 2, value

ACC Circuit of pump control 1, 2, value

Example: AGC 1 18. The gain for level 1 in the mode AGC OADM will be set on 18

dB.

Example of the instruction for report statement about adjustment of individual

parameters: AGC

Response: AGC 1: 30 dB

AGC 2: 17 dB

13.5 Procedure

Become familiar to the amplifier EDFA CLA-P(B)-01F. Connect it into CLI (Command

Line Interface). In case of direct connection of monitor and keyboard to the amplifier, there is

possible to survey and edit CLI directly. In case of direct connection of amplifier to the

computer via serial port, let connect to CLI by hyperterminal.

86 FEKT VUT v Brně

For more details see in Tab. 10.

Tab. 10: Setting of Hyperterminal.

Parameter Setting Unit

Baud Rate 9 600 Baud

Data Bits 8 Bits

Parity None

Stop Bits 1 bit

Flow control None

Determine its actual setting in the modes AGC, APC and ACC by using instructions.

Connect optical source LS 420 to measured configuration. Attach power meter PM 420

to the monitoring output of the splitter SFT-TAP. Set the combination of wavelengths 1300,

1550 and 1650 nm. Read absolute and relative powers of optical signal on the display for

individual wavelengths. Add measured values into the table. Then disconnect the measuring

device from the input splitter.

Connect measuring device to the monitoring output of output splitter. Change modes of

amplifier among AGC, APC and ACC using instructions. Set the parameters of amplifier in

individual modes and read values of the optical power in the display. Fill in measured values

into the Tab. 11.

13.5.1 Working-out of protocol

Tab. 11: The table of measured values.

Measurement of input power of wavelengths

Wavelength Relative power Absolute power

nm dBm dBm

1 300

1 550

1 650

Measurement of output powers

AGC APC ACC

Adjusted

gain

Measured

gain

Adjusted

output

power

Measured

gain

Adjusted

current

Measured

gain

dB dBm mW dBm mA dBm


13.6 Conclusion

After all measuremnt you need to do a protocol.

88 FEKT VUT v Brně

14 Measurement of chromatic dispersion

14.1 Method of phase shift and differential phase shift

By the ITU-T G.650 recommendation, the method of phase shift is given as the

reference method for measuring the chromatic dispersion of optical fibres. A modulated

radiation source of several wavelengths is used for the measurement. At the receiver side the

instrument used for the detection of the test signal being received is an instrument for phase

measurement such as the vector-voltmeter. The output phase measured is compared with the

input phase of the signal and their difference is used to determine the change in the signal

phase after the passage through the optical cable route being measured. A disadvantage of this

method can be seen in the necessity to use a different fibre in the cable as the reference route

in Fig. 77Chyba! Nenalezen zdroj odkazů., via which information about the input phase is

transmitted from the transmitter to the receiver.

Fig. 77: Method of phase shift.

14.2 Method of delayed pulses in the time domain

This method consists in transmitting optical pulses in/on different wavelengths but with

a precisely determined pulse magnitude and spacing. A comparison of the spacing of input

pulses with that of the pulses received on the output is used to determine the delay due to

chromatic dispersion. The connection which is similar to that in the preceding methods but

without reference fibre is in Fig. 78.

Fig. 78: Method of delayed pulses.

Fig. 79 gives an example of the connection of generator of optical pulses with given

time spacing. A cascade of Bragg gratings serves as the monochromator. The pulse generator

modulates the radiation of a wide-spectrum source such as LED diode. From the coming

pulse the diode reflects components of selected wavelengths with certain time spacing back

into the route being measured. The base is a cascade of Bragg gratings, which is formed by

different gratings with sections of optical fibre between the gratings. Each of these gratings

reflects radiation of different wavelength. The result of this connection is that a sequence of

pulses of different wavelengths with given time spacing comes into the measured fibre of the

route. After the passage through the route the time spacing of pulses changes due to the effect

of chromatic dispersion. By comparing the spacing on the input with that on the output of the

route being measured the values of delay due to chromatic dispersion are established.


Fig. 79: Method of delayed pulse, with a cascade of Bragg gratings.

An example of the resultant measurement of optical route using a chromatic dispersion

meter is given in Fig. 80.

Fig. 80: Waveform of chromatic dispersion.

14.3 Measurement of polarization mode dispersion (PMD)

The interferometric method of measuring PMD is based on the interference (wave

addition) of low-coherence (coherence – spectral purity) of optical radiation. The block

representation of the method is shown in Fig. 81. An interferometer is placed on the output of

the optical route being measured, which separates the radiation into two branches. A fix

mirror is in one branch and a movable mirror in the other branch. The movable mirror

changes the phase shift between received signals of the two branches and, with the aid of

interference, the delay due to PMD is shown on the detector.

Fig. 81: Measurement of PMD by interferometric method.

90 FEKT VUT v Brně

A typical example of the plot obtained by this method is given in Fig. 82.

Fig. 82: Example of PMD plot of optical fibre, obtained by interferometric method.

In the bottom of the figure the interferogram shows the correlation functions of two

mutually perpendicular polarization planes. The pronounced peak is the autocorrelation

function of the measuring signal itself, which depends on the shape of its spectrum.

This method is sometimes referred to as TINTY (Traditional Interferometry Analysis);

the more recent method GINTY (General Interferometric Analysis) suppresses the effect of

autocorrelation peak. In this method the resultant signal, which contains optical radiation from

both branches of the interferometer, is again divided by polarization light into two mutually

perpendicularly polarized components, with each of them incident on a separate detector.

Interference occurs on the detectors and the two correlation components are expressed. By

subtracting the interferogram we obtain the mutual correlation, and by adding up we obtain

the pure autocorrelation. This method enables measuring also routes with EDFA amplifiers. It

is a quick method, it is not necessary to measure individual route sections separately.

14.3.1 Method of scanning the wavelength

The measurement of PMD by the method of scanning the wavelength is based on the

principle of measuring the optical power passing along the measured optical route in

dependence on the wavelength. The block representation is given in Fig. 83. The radiation

source can be a tunable laser or a wide-spectrum LED diode.

Fig. 83: Method of scanning the wavelength.


Compared with the preceding method, this one is slower and the fibre is susceptible to

vibration.

14.3.2 Method of POTDR

This method of measuring combines the measurement of PMD with the method of

optical reflectometry. This means that it enables measuring the whole route and determining a

possible critical section with increased PMD value, which can be subsequently replaced and

thus the prescribed values of PMD are achieved (POTDR – Polarization Optical Time

Domain Reflectometry).

The POTDR method makes partial use of the classical OTDR method of backscatter

measurement. It operates on a similar principle but the POTDR method differs in that the

reflectogram is evaluated via polarization. The principle of the method: we try to transmit into

the route fibre a measuring signal in the form of a train of pulses and from the backscattered

radiation (effect of Rayleigh‟s backscatter) we read information about the PMD of individual

sites on the route fibre. The dependence of the PMD of route fibre can be expressed by the

relation

PMD ,h

where β is the double refraction in the fibre (ps·km-1

), i.e. the difference in the

propagation speed of the two polarization modes mentioned, l is the fibre length, and h gives

the coupling length at which there is a significant change in the axis (shape) of the double

refraction in the fibre, which leads to a marked exchange of energy between the polarization

modes. PMD increases with the magnitude of double refraction in the fibre, with the fibre

length and with the coupling length. With increasing length of the fibre and thus a smaller

energy exchange between the two modes, which propagate at different speeds, PMD will play

a greater role. For the longitudinal analysis of PMD we need to obtain from the backscattered

radiation from the fibre also information about its local double refraction and coupling length.

To establish this information we send short pulses of polarized optical radiation into the fibre.

This purpose is served by the DOP (Degree of Polarization) method, which establishes the

results from backscattered radiation. We monitor the degree of polarization. The schematic of

this connection is shown in Fig. 84. The radiation source is a DFB laser of a very narrow

spectrum, which is different from the current OTDR meters. It is used here in order to prevent

signal depolarization in the fibre because the signal might propagate via several wavelengths.

In this case, double refraction of the fibre would cause for different wavelengths different

changes in the state of polarization SOP and thus depolarize the signal. Such a (undesirable)

mechanism of depolarization must be suppressed by narrow-spectrum radiation source.

Polarized output radiation from the DFB laser is coupled to the fibre being measured. For

backscattered radiation from individual sites of the route fibre the DOP is analyzed using

a polarimeter and an OTDR detector.

Fig. 84: Measurement of PMD by the method of DOP analysis.

92 FEKT VUT v Brně

A strong double refraction in fibre β brings about a quick rotation of polarization state,

which leads to the depolarization of radiation within the measuring pulse, thus contributing to

a reduction of the degree of polarization. A weak double refraction of fibre β will result in

a high DOP measured, and vice versa. But the DOP will also depend on intermodal coupling

(coupling length h). With some simplification, the situation can thus be divided into three

groups:

1. fibres with weak double refraction (small β) – the DOP will be high (up to 1),

irrespective of intermodal coupling. In practice, these are optical fibres with

a small PMD value,

2. fibres with a strong double refraction and strong intermodal coupling (large β and

short coupling length h) – the DOP will be low due to the strong double refraction

(for a backscattered signal it will approximate the value 1/3) and will change

rapidly due to the strong intermodal coupling. In practice, these are optical fibres

with average PMD values,

3. fibres with a strong double refraction and weak intermodal coupling (large β and

long coupling length h) – here, in addition to β and h, it also depends on the

mutual position of SOP of polarization and on the shape of double refraction in

the fibre. The DOP can then fluctuate between low and high values but it will

change only slowly. In practice these are optical fibres with high PMD values.

It follows from the above that not only the DOP value itself but also the speed of DOP

change is important. The measuring instrument then performs an analysis of the results

measured. Because of the considerable speed of DOP change it is first necessary to determine

the average DOP value from several tens of samples. The measuring instrument then performs

measurement for two states of input polarization, which yields two measurement results: DOP

and DOPC (complementary), from which the DOPGEO parameter is calculated using the

relation

,DOPDOPDOP 2

C

2

GEO

which provides information about the real DOP value of radiation backscattered from

a given section of route fibre. The measuring instrument has a parameter hDOP for

monitoring the speed of DOP changes. This parameter is equal to the fibre length along which

the DOP changes markedly – the quicker the DOP changes, the smaller the hDOP parameter.

From an analysis of the DOP parameter of individual sections of optical fibre of route the

following can be concluded:

- on sections with high DOPGEO value the PMD value will be low because the there

is a small double refraction of fibre here,

- on sections with varying or low DOPGEO value and thus with a possible larger

double refraction of fibre, and on the assumption that the hDOP parameter is

small, then the PMD value will be low because there is a strong intermodal

coupling in the fibre,

- average, then the PMD value will be average,

- large, then the PMD value will be high because there is a weak intermodal

coupling in the fibre.

After the evaluation of the values measured, the polarization reflectogram (POTDR)

will display several measurement results and graphs, above all the POTDR reflectogram,

where individual cable sections and route sites can be followed. The longitudinal resolution


power of this method is of the order of hundreds of metres; routes of tens of km in length can

be measured. In the POTDR reflectogram display, sites with low, increased and high PMD

values are presented in colour.

The second graphical representation that can be displayed is the waveform of the hDOP

function, which is yet another important curve of the overall evaluation of PMD. In addition

to the hDOP function, horizontal straight lines are also displayed, which mark the limit values

of PMD. These limit values can be set by the users themselves.

The third (and most important) graphical representation is the DOP curve. In one

graphical display several parameters can be seen in different colours – the DOP, DOPC and

DOPGEO curves. The DOPGEO curve has the greatest informative value regarding the actual

state of PMD.

The last possibility is the graphical representation of the curves (for both input states of

POTDR polarization) of normalized Stokes parameters S1 to S3.

On top of all this, information about splice and connector sites can, of course, also be

read from the final measurement protocol.

It is necessary to stress that the measurement of POTDR does not replace the total

absolute values of PMD delay, which are measured using one of the interferometric methods,

it only complements them.

94 FEKT VUT v Brně

15 References

[1] FILKA, Miloslav. Optoelectronics: for telecommunications and informatics. Dallas:

OPTOKON CO., LTD., 2009, 398 s. ISBN 978-0-615-33185-0.

[2] KRESS, B a Patrick MEYRUEIS. Applied digital optics: from micro-optics to

nanophotonics. Chichester, U.K.: Wiley, 2009, xx, 617 p. ISBN 04-700-2263-9.

[3] Fiber optics handbook: fiber, devices, and systems for optical communications. Editor

Michael Bass, Eric W Van Stryland. New York: McGraw-Hill Professional, 2001, 1 sv.

McGraw-Hill telecom engineering. ISBN 00-713-8623-8.

[4] RAZAVI, Behzad. Design of integrated circuits for optical communications: fiber,

devices, and systems for optical communications. Editor Michael Bass, Eric W Van

Stryland. Boston: McGraw-Hill, 2003, xiv, 370 s. McGraw-Hill telecom engineering.

ISBN 00-728-2258-9.

[5] YEH, Cavour. The essence of dielectric waveguides: fiber, devices, and systems for

optical communications. Editor Michael Bass, Eric W Van Stryland. New York:

Springer, 2008, p. cm. McGraw-Hill telecom engineering. ISBN 03-873-0929-2.

[6] ERSOY, Okan K. Diffraction, fourier optics and imaging. New Jersey: John Wiley,

2007, xvi, 413 s. ISBN 04-712-3816-3.

[7] IIZUKA, Keigo. Elements of Photonics, Volume II For Fiber and Integrated Optics.

Volume II. New York, N.Y: Wiley-Interscience, 2002, xvi, 413 s. ISBN 04-712-2137-6.

[8] IGA, Kenʾichi a Y KOKUBUN. Encyclopedic handbook of integrated optics. Boca

Raton, FL: CRC Press, 2006, xvi, 507 p. ISBN 08-247-2425-9.

[9] IIZUKA, Keigo. Engineering optics. 3rd ed. New York: Springer, c2008, xx, 525 p.

ISBN 9780387757247-.

[10] AGRAWAL, Govind P. Fiber-optic communication systems. 3rd ed. New York: Wiley-

Interscience, 2002. ISBN 04-712-2114-7.

[11] OKAMOTO, Katsunari. Fundamentals of optical waveguides. 2nd ed. Burlington:

Academic Press, 2006, xvi, 561 s. ISBN 978-0-12-525096-2.

Date post:	06-Feb-2022
Category:	Documents
Upload:	others
View:	1 times
Download:	0 times

Transmission media laboratories - OptoLab

Documents