94648

ANALYSIS OF OPTICAL LINE CHANNEL BY REFLECTOMETRY

Лабораторная работа

Коммуникация, связь, радиоэлектроника и цифровые приборы

All measuring procedures are based on metering reflected and skattered radiation which propagates backwards when high-power optical pulses pass a fiber in forward direction. Monitoring pulses from laser diode are inputted into fibers. These are so called Fresnel reflection signals formed due to reflection at macrocracks and microcracks...

Английский

2015-09-15

313 KB

0 чел.

Laboratory work № 4

ANALYSIS OF OPTICAL LINE CHANNEL BY REFLECTOMETRY

1 PURPOSE OF WORK

Learning pulse location methods which are used for measuring parameters of fibers of optical communication cables.

2 KEY POSITIONS

2.1 Physical phenomena

All measuring procedures are based on metering reflected and skattered radiation which propagates backwards when high-power optical pulses pass a fiber in forward direction.

Monitoring pulses from laser diode are inputted into fibers. Following signals return:

- Signals reflected from large obstacles which measure more than emission wavelength. These are so called Fresnel reflection signals formed due to reflection at macrocracks and microcracks, fiber junctures, input and output ends of fibers, bends of fibers, which cause warping index profile of glass;

- signals skattered backwards owing to microscopic fluctuations of index profile (density nonuniformities of material and composition).

There are directions of radiation propagation in volume of shown in Figure 2.1. 1 is core; 2 is fiber cladding. Backscattered signals are usually by 20..25 dB lesser than reflected signals.

2.2 Realization of back-scattering method

A source of high-power monitoring optical pulses and a wide-band optical receiver (OR), which contains a registrator of received signals (ROS), are joined to the input end of a fiber by using an optical splitter. Due to OR there are diagrams of reflected and back-scattered signals got as a function of line length, which is known as reflectogram. A reflectogram illustrates power dependences of reflected signals PR back-scattered ones PS on line length х (or propagation time t). If a time space between monitoring and reflected pulses t is determined, than distance to reflection (scattering) point can be calculated:

(2.1)

Where  is light speed in fiber core with maximal reflective index n; с is light velocity in free space; propagation of an optical pulse in forward and backward directions is taken into account by dividing the expression by 2. To analyse fibers according to pulse methods there is a flow chart is shown in Figure 2.2.

Figure 2.2 – Flow chart of an instrument to analyse fibes according to pulse methods

Where 1 is a generator of short electric pulses; 2 is a source of high-power optical pulses (laser diode); 3 is a light splitter; 4 is an analysed fiber; 5 is a optical receiver; 6 is a signal processing module (SPM); 7 is a registrator (for example oscillograph, display, plotter, etc); 8 is an adjuster; 9 is a synchronizing circuit.

Such an instrument enables detecting reflected signals as well as backscattered ones. Scattering is a change in propagation direction of light rays or photons at obstacles in the core which are smaller than emission wavelength.

If there is not SPM in an instrument’s circuit, than the instrument is called meter of point of break because it permits registering only high-power Fresnel signals reflected from large reflective obstacles. Typical waveform of signals registered by such an apparatus is shown in Figure 2.3 where 1 and 4 are signals reflected from input and output ends of a fiber, 2 and 3 are signals reflected from microcracks or those places where optical connectors are.

There a few shortcomings of this meter such as

  1.  Impossibility of revealing nonreflective obstacles;
  2.  Insensibility to backscattered signals;
  3.  Fresnel reflection dependence on characteristics of surface of chip (smooth or rough surface) of a fiber.

Instruments that permit registering Fresnel signals as well as backscattered waves are called reflectometers. They always contain signal processing modules which permit considerably increase sensitivity of a receiver (maximize signal-to-noise ratio at the input of a registor) and enhance wanted backscattered signals which power levels are much lower than reflected signals’ ones.

Figure 2.3 – Typical reflectogram of reflected signal

There you can see general view of a curve which shows time distribution of backscattering power in Figure 2.4. In the general case it is similar to exponent as a function of time (length), which slope determines power loss factor of optical radiation in fibers. The difference between the real curve and a drop-down exponent are caused by defects of a fiber. For example any inclusions (impurities) which are bigger than emission wavelength cause backscattering boost, what leads to turning up power  2 on a reflectogram. Jump 3 is caused by fiber defects such as welding spots so on. In case optical connectors are used or low-quality splices have been completed, then reflection and return loss jumps appear. At that rate, if backward radiation in the first of two spliced fibers is lower than in the second one, than positive power step 5 appears. High-power pulses 1 and 6 are caused by reflection from respectively input and out put fiber ends.

Figure 2.4 – General curve of backscattered power distribution

Reflectometers enable: 1 – analysis of radiation loss along fibers; 2 – determination of nature and location reflecting obstacles as well as non-reflecting (absorbing) ones; 3 – measurement of power loss at joints; 4 – determination of complete attenuation of radiation in fibers; 5 – obtainment of information about stability at some fiber sections.

Back-scattering method (BSM) due to its universal potentialities considerably excels other methods. It has become predominant and nowadays it is the main method of taking measurements in manufacturing optical cables, building, assembling and exploiting fiber optics transmission lines (FOTL). The significant attainment of the method is possibility of taking measurements just in a field when only one end of a fiber is accessible.

2.3 Principles of back-scattering method

Glass naturally is disordered structure where microscopic density fluctuation may be observed as well as local changes in material composition. Each change leads to fluctuation of refractive index in microscopic areas which are smaller than emission wavelength (Rayleigh scattering). Scattering by obstacles which are equal or slightly stronger than the wavelength is called Mie scattering.

Power of light flux after passing through the fiber x metres long exponentially lower from Ро to Рі, as it is shown in Figure 2.5. There  is a scattering area which cause backward radiation at any point of fiber core.

Figure 2.5 – Origin of back-scattered radiation

Mathematically it can be expressed by the formula:

P(x)=P0·e-αx=P0·e(-α+α)                                                   (2.2)

Where is attenuation coefficient which can be separated into absorption factor а and light scattering coefficient s. Measurement units of , а and s are 1/km, (if to multiply them by 0,23 one will get them in dB/km).

Radiated power varies by a value of  over interval . For  <<   following ratio is valid:

,               (2.3)

Where the first member corresponds with loss due to absorption αа and the second corresponds with loss due to light scattering  αs over an interval х.

Backward radiation constitute some part of scattered radiation (which is “captured” by fiber aperture); this part is determined by backward scattering factor S.

        for gradient fibers                                     (2.4)

        for step-index multimode fibers                 (2.5)

         for singlemode fibers                                (2.6)

Where n1 is maximal refractive index of fiber core, NA is numerical aperture of a fiber. Parameter S can be expressed in relative units: s =10 S, dB.

Expressions (2.4)...(2.6) are correct in case light scattering is isotropic so that it is the same in all directions. In fact, intensity of Rayleigh backscattering is higher and Mie backscatter is lower than forward scattered radiation. For usual parameters of fibers we can tabulate following findings.

Table 2.1 – Typical parameters of fibers

Class of fiber

NA

S

s, dB

Step-index

1,5

0,2

– 21,8

Gradient-index

1,5

0,2

– 2,5

Singlemode

1,5

0,1

– 30,1

Measurement error over the length depends on monitoring pulse duration is determined as

                                                                                               (2.7)

Where ng = n1(1) is group-refractive index of fiber core, which takes account of dispersion of core’s dispersion. Interval is numerically equal to length resolution of a reflectometer. For example if  = 100 ns, then we would get = 20 m.

Backscattered signal which returned the sending end of fiber:

                       , W.                  (2.8)

Where the first multiplier (in brackets) conforms to all scattered on the interval х radiation, the second conforms to part of backscattered radiation (2.4), (2.5) or (2.6), the third one conforms to backward radiation flux.

One of the most important parameter is dynamic range of measurement Q, which is determined by emission power of laser diode Рep and sensitivity of ROM Ро:

, dB.                                    (2.9)

If  Рep = 1 W; Р0 = 10-9 W, then power budget is equal to 90 dB. Dynamic range of measurement (DRM) of reflectometer is consumed by  following “losses”:

– caused by light attenuation when propagating forward and backward the fiber 2;

due to scattering аs;

– inserted by optical splitter due to unmatched ends of fibers (input-output loss) аio

In a whole, power balance:

Q = 2аmeas+ аs+ aio.                                          (2.10)

Backscattered loss is calculated as ratio of power backscattered over the interval to light power passed through the cross-section of fiber at the distance x from its beginning:

             (2.11)

Maximal value of backscattered radiation (x = 0) for pulse duration of = 100 ns ( = 20 m) is shown in the Table 2.2.

Table 2.2 – Parameters of backscattered signal

OF

, nm

,   dB/km

S, dB/km

NA

Ps(x = 0)/Po

Gradient-index

850

2,5

2,17

0,2

Multimode

1300

0,5

0,43

0,2

Одномодовое

1500

0,25

0,22

0,1

According to (2.11) and taking account of the formulae (2.5) and (2.7) one can derive an expression

,                         (2.12)

Thus if NA = 0,2;  = 100 ns; s = 0,5 1/km; n1=1,5, then from expression (2.12) аs = 42 dB. To provide reception of pulses 100 ns long ROM need to have bandpass of F  1/2 = 5 MHz.

In fact, losses caused by attenuation of light propagating forward and backward determine maximal length of fiber which parameters can be measured according to BSM. They are actually dynamic range of measurements.   

.                                    (2.13)

If аs = 42 dB; аio = 8 dB, then аmeas= 20 dB, in case Q = 90 dB. Reflectometer with such a range enables for example metering parameters of a fiber 4 km long with attenuation factor up to 5 dB/km.

If in order to improve length resolution one reduce pulse duration to 10 ns ( = 2 m), then attenuation of backscattered signal in reference to (2.12) will heighten up to 54 dB whereas dynamic range will decrease to 14 dB. In this case bandpass of ROM will increase by 10 times

MHz.

Production of wideband ROM with high sensitivity and wide dynamic range is a complicated task which makes it difficult to improve length resolution of reflectometers.

When measuring by reflectometer it is necessary to avoid parasitic Fresnel reflection and in the first turn reflection from the input end of fiber being observed. Reflected-to-backscattered radiation power ratio

,

Where R is reflection factor.

 If R = 0,04 (4 %) (that takes place when signal reflects from some fracture or shear of a fiber, which are perpendicular to its axis), then backscattered signal is smaller than reflected one by 1000 times or by 30 dB. Even though reflection factor is R = 0,01 (1%), then reflected signal will be hundred times weaker than scattered one. Such a reflected signal may lead to overload of receiver.

According to backscattering curve (Figure 2.5) one can calculate attenuation factor of a fiber at the set wavelength. To do that it is necessary to determine power loss over the interval between х1 and х2.

, dB,

And then determine attenuation coefficient

, dB/km.

Accuracy of attenuation factor determination depends on measurement errors such as backscattered signal power Р1 and Р2; position of points х1 and х2. The closer measured section is to the input end of a fiber the higher accuracy of attenuation factor determination. Usually they choose the section no farther than 1 km as well as no closer than 200…300 m (dead area) from the input end of a fiber; there considerable effects occur due to unsteady mode structure of optical radiation inputted into a fiber.

The most difficult task for reflectometer to solve is low-level detection. There are various method of its solution:

- increase of power inputted into a fiber;

- shortening of monitoring pulses duration;

- emission wavelength (monitoring signals) is chosen in the set spectral window of quartz glass;

- optical splitters are produced with low loss when signal is inputted into fibers and scattered radiation is outputted back to ROM;

- they use ROM with high sensitivity and wide dynamic range;

- they assume special measures in order to avoid reflection of signal from input fiber end by using for example index-matching fluid.

SPM plays significant part for reflectometer. It implements storing (integration) of number of responses to 100...1000 monitoring pulses. Scattered signals add arithmetically whereas noise components add mean-squarely what provides considerable increase of noise-to-signal ratio and enables receiving weak backscattered signals.

3 TEST questions

3.1 Explain physical phenomena of reflection and scattering of radiation.

3.2 Explain purpose of units given in the structure chart of the device to analyse fibers according to pulse method.

3.3 Give an explanation of how it is possible to calculate distance to big refractive obstacles in fibers.

3.4 How can you estimate attenuation factor with the backscattered curve? (Figure 2.5)?

3.5 Which fiber characteristics and parameters can be determined with a reflectometer?

3.6 Which is the difference in signal levels between signal reflected from even, perpendicular to fiber’s axis cleavage and backscattered one?

3.6 What is dynamical range of reflectometers and what is it spent on?

3.7 How can you calculate minimally possible length of a fiber which parameters are measured with reflectometer?

3.8 How is it possible to determine length resolution of a reflectometer (accuracy of location of obstacles)?

3.9 Describe all methods of optimization of reflectometer’s characteristics you know.

4 home task

4.1 Learn principles of the pulse methods to measure parameters of fibers.

4.2 Determine parameters of  a reflectometer which operates at three frequency windows of quartz glass.

Value of scattering coefficient  and attenuation one  of the fiber are given in the Table 4.1. According to the expression (2.12) calculate backscattering loss s for three values of  and  which are given in the Table 4.1. Set N, τ to be equal N = 0,2; τ = 100 ns and choose n1, which is in the Table 4.2 in accordance with your own variant. Variant’s number corresponds with the last numeral in your mark book.

Table 4.1 — Values of s and  

, μs

0,85

1,3

1,55

s,1/km

0,250

0,045

0,025

, dB/km

4,0

1,0

0,5

Таблица 4.2 — Значение n1

Variant

0

1

2

3

4

5

6

7

8

9

n1  nq

1.470

1.473

1.475

1.478

1.480

1.483

1.485

1.487

1.490

1.493

Then according to the formula (2.13) calculate dynamic range of measurement performed by a refelectormeter using attenuation factor given in the Table 4.1 in case Q = 90 dB and = 8 dB.

Analyse obtained results and explain why parameters of the reflectometer change in lengthening radiation wavelength.

4.3 In case break location meter  is used determine distance to a reflecting obstacle in a step-index fiber. Time interval between monitoring and reflected pulses is t = 25 μs. Refractive coefficient of the fiber core is in the Table 4.2.

4.4 Determine length resolution of the reflectometer according to the results obtained in previous paragraph. Monitoring pulse duration is = 50 ns and total dispersion of fiber is 10 ns/km.

5 equipment

As a laboratory model break location meter  is used.

Its mechanism is based on probing lightguide by series of short optical pulses and measuring signals reflected from obstacles and backscattered ones called signals of Fresnel reflection and Rayleigh scattering respectively.

As a result of processing these signals reflectogram of the fiber probed is displayed.

Vertical axis corresponds to relative fiber attenuation (in dB per scale mark) at log scale whereas horizontal one – fiber length which is proportional to signal’s propagation time.

Control elements, trim and connection

Control elements and connection 

Purpose

Origin datum 

1

2

3

Switch

„СЕТЬ

Buttons 

„пуск

Switch on the device

Measure from the beginning (from О)

Pull down 

Push respective button 

„ПО МАРКЕРУ”

Knob

„МАРКЕР”

Perform repeated measutements (use with the knob „МАРКЕР”           )

Shift of marker

Push respective button

Push respective button

Buttons

dB”

„дБ/дел

Losses

To fix vertical scale 1 dB/mark or 4 dB/mark

Push respective button

Push respective button

Buttons „М

„Масштаб м/дел

Length measurement

To set horizontal scale (from 2,5 to

400 m/mark)

Push respective button

Set the switch

„Усреднение”

To choose quantity of measurements 24; 28; 212 according to which the final result is determined

Push respective button

Duration

To set duration of monitoring pulses 50; 200; 400 ns

Push respective button

Knobs

„яркость

„фокус

Brightness control

Sharpness control

Preparation for making measurements.

1 Connect to the instrument’s optical output cable being measured (the one might consist of two sections joined by an optical connector).

2 Turn on the instrument pushing the switch „СЕТЬ”.

3 Set horizontal scale according to length of the fiber being measured.

4 Set necessary quantity of measurements 24, 28, 212. Take into account that maximum possible cable attenuation you can measure using maximum duration of the monitoring pulses and number of measurements, which in its turn lessens resolution of the instrument.

5 To make measurements faster it is much better to use lesser number of pulses (24 or 28). Shortening monitoring pulses increase resolution of the instruments which makes it possible to locate reflective obstacles in the fiber more accurate.

6 After pushing the button „ОТ НАЧАЛА” you will see the curve of reflected and scattered signals on the display.

7 By using the button „ПО МАРКЕРЕ”  and the knob „МАРКЕР” you can analyse any cable section in details. To do it, you have to use the knob „МАРКЕР” and set vertical line of the marker at that section of the curve you need to, then using the scale knob (m/mark) scale it up. After it push the button „ПО МАРКЕРЕ”. Remember that if refractive index is equal to 1.499 than time slot of 11 μs corresponds with cable length of 100 m.

8 Remember there is some “dead area”. You can therefore analyse cable parameters only at some distance from the beginning.

Length of the “dead area” is less than double duration of the monitoring pulse (in meters).

If pulse duration is 1 μs than dead area is 200 m; if 0.5 μs – 100 m; if 50 ns – 10 m.

If you need to analyse the beginning of some cable, than you have to connect an additional section of lightguide which is longer than “dead area”.

Addition:

Remember that when you use an instrument working at a wavelength of 1.3 μm, connectors’ tip need to be moistened by immersion liquid (solution of pure glycerine).


. . .     .   . . . . . . . . . . . . .  . . . . . . . . . . .

. . .  

Backscattering

Рассе

ние

вперед

Cladding scattering

Cladding scattering

 

adiation input

вход

Radiation output

2

1

Figure 2 1 Direction of radiation propagation 

9

z

Y

x

2

1

3

7

6

4

5

8

t, μs

Р

1

2

3

4

t

0

0,5

0,5

1

2

3

2

4

5

2

6

P

t

0

 x

х, km

t, μs

PS,PR

х

х

х

Р1

                        Р2

                                       Р3

        Р, W

Ро

    o              2 t1                    2 t2                           2 t3                        t, μs

    

     

     

по

маркеру

Градиент.

световод

50 / 125

1,3 м

400

200

50

212

28

24

Длитель

ность

Усред

нение

2,5

4000

Яркость

Фокус

Определитель

места обрыва

Маркер

от

начала

dB

M

1

4

Дистанция/Затухание

Масштаб м/дел

20 mW max

Пуск

dB/дел


 

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