Лабораторная работа

Коммуникация, связь, радиоэлектроника и цифровые приборы

Causes of attenuation extinction in fibers Signal damping in line channel is an important factor which should be taken into account in designing any communication system. There are some points in fiber-optic transmission systems FOTS where losses arise: in point of inputting signal into fibers straight in fibers in splices...



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Laboratory work № 3



Learning causes of light attenuation in optical cable’s fibers and measuring power damping according to methods of break and insertion loss.

2 key positions

2.1 Causes of attenuation extinction in fibers

Signal damping in line channel is an important factor which should be taken into account in designing any communication system. Input power must be more than some minimum value for any receiver in order to provide necessary transmission quality. Propagation medium loss therefore limits total length of any communication line. There are some points in fiber-optic transmission systems (FOTS) where losses arise: in point of inputting signal into fibers, straight in fibers, in splices (out of stations) and in connectors (at stations).

Let’s examine causes of loss in fibers for wavelength of 0,5…1,6 microns (1 micron = 10-6 m). There are fibers with low loss and efficient receivers and transmitters being employed in this range of spectrum which is used for most of FOTSs. Fibers with low loss for other wavelengths don’t exist yet.

Fibers for communications are made of quartz glass. This material provides low loss, can be stretched to long fibers enables changing transmission density to provide refractive index drop of core and cladding. Multimode step-index fibers are made of quartz glass or plastic whereas multimode gradient-index fibers and singlemode fibers are made of quartz glass only.

Quartz glass, which fibers are made of, consists of molecules of molten silicon dioxide SiO2, which have different spatial orientation at different point of the material (amorphous substance). It in essence differs from crystal structure (for example such natural mineral as rock crystal), where atoms, which it consists of, occupy fixed positions in space and the structure periodically repeats itself. To alter refractive index other materials are added to the glass. Doping usually is performed by using titanium, thallium, germanium, boron and other chemical elements. The basis for producing fibers is glass with high content of SiO2 which enables to form fibers with low loss in case high chemical purity is obtained. Power loss in quartz fibers arises in consequence of absorption, scattering and geometrical defects. Let’s look over these kinds of losses.

Absorption. Even the purest quartz glass absorbs light in certain spectral regions. It is so called characteristic absorption which is natural property of glass. High characteristic absorption appears in ultraviolet spectrum (UV) i.e. for short waves. Absorption arises in consequence of redistribution of energy which is transmitted along fiber to electron transitions. The peak of loss is in the UV region. The loss lowers with wavelength approaching visible spectrum. UV absorption is away from the spectral region, where  FOTSs operate, therefore it influence on the total loss is insignificant. UV absorption is demonstrated in Figure 2.1.

Peaks of characterisitc absorption also appear in infrared (IF) spectrum. Absorption peaks between 7 and 12 microns for quartz glass are distant from the the operation spectrum of FOTSs. Infrared loss is conditioned by oscillation of chemical compound of Si with О. Temperature influence makes atoms permanently oscillate so that particles Si-O expand-shrink. The oscillations has resonance frequency in infrared spectrum. As it is demonstrated in Figure 2.1 short-wavelength limit of this absorption approaches the operation spectrum of VOTSs. Infrared absorption inserts low loss in the higher spectrum region (about 1.6 microns) which is used in FOTSs. In fact, this loss makes it impossible to use quartz fibers for longer wavelength.

It can be concluded that characteristic loss is usually low and insignificant in the spectrum where VOTSs operate but the same loss makes employing fiber systems both in UV range less than 0.4 microns and infrared range more than 1.6 microns impossible.

Dirts cause the highest loss in any real fiber. Two classes of dirt are rather adverse; they are ions transition metals and ions of hydroxyl groups (OH).

Metallic dirts such as Fe, Cu, Со, V, Ni, Mn and Cr absorb in the range which is used in transmission systems and shouldn’t exceed quantity of a few particles a milliard to provide attenuation coefficient of fibers less than 20 dB/km. To understand mechanism of losing energy in metals inner electron shells of atoms should be examined. When atom absorbs light quanta, its electrons transit from uncompleted low order electronic shell (low energy state) to an electronic shell of higher order (high-energy state). Energy imparted to electrons is extracted from incident light. Allowed transition energy meets quanta which frequencies coincide with ones in the operation spectrum of VOTSs.

For all ordinary purposes the most important dope to be minimized is the one due to ions of hydroxyl groups (OH). The loss is caused by resonance oscillation of OH groups, which is analogous to absorption process in SiO compound. Atoms of oxygen O and hydrogen H oscillate as a result of thermal excitation. Resonance frequency of oscillations corresponds with wavelength of 2.73 microns. Although the peak of resonance absorption 2.73 microns is out of spectral band used in the communication systems, subharmonics of this resonance belong to VOTSs’ range. The highest loss takes place at wavelength of 1.73, 1.23, 0.95 microns in case there are OH ions in fiber glass. OH absorption peak is shown in the spectral characteristic of loss in fiber in Figure 2.1. To obtain results given in Figure it’s necessary to limit OH content up to a few units a million.

In quartz glass manufacturing special measures are taken in order to provide low content of OH ions in fibers. Dry fibers contain low OH level whereas damp fibers content much more the ions. OH absorption peak belongs to region of low intrinsic loss of fibers. Therefore when constructing VOTS wavelengths with minimum OH absorption loss are chosen*.

Rayleigh scattering. Molecules move chaotically in fused glass when fibers are being stretched from it. High temperature keep them moving. Melt (liquid state) scarcely cool down to solid state when molecules’ movement stops. On reaching solid state random positions (orientations) of quartz molecules are “frozen” in fiber glass. It leads to local changes in material density and as a result to variation of local refractive index of glass. The variation can be interpreted as microscopical objects which cause scattering in uniform material. Objects’ sizes (sizes of centers of scattering) are much less than signal wavelength (about 1 micron) which is transmitted by fiber. Light beam traversing such formations loses some energy which scatters by these objects (Figure 2.2).

Such kind of loss is known as Rayleigh scattering. It occurs when light propagates through medium which includes obstacles smaller than wavelength. According to Rayleigh law such scattering is proportional to -4. It is therefore rather substantial at short wavelengths. Scattering loss depending on wavelength is shown in the picture 2.1.

There is also another cause of scattering loss. In case fiber material consists of a few oxides variations of component levels are possible. It can happen due to nonideal chemical mixing of different components of glass. In this rate virtual composition of glass becomes nonuniform. It in turn causes microscopic difference in refractive index of glass, which leads to Rayleigh loss proportional to λ-4.

Light scattering restricts using short wavelengths of signal to be transmitted in fibers. If wavelength is less than 0.8 microns, then loss caused by this effect is the main obstacle in propagation of light at long range. Scattering loss lowers when increasing wavelength. This effect leads to use of wavelengths longer than 0.8 microns. In fact, if fiber loss ever reach value less than 0.08 dB/km, then operating wavelength needs to be longer than 2 microns. In this case fibers need to be made of not quartz but other materials.

Kinds of loss mentioned above are considered as intrinsic loss). It can’t be lowered by any methods of manufacturing of fibers. The loss can only be changed by changing composition of glass. Scattering loss caused by the effects mentioned above is the physical minimum which loss of quartz glass can’t be lower than.

Material irregularity also randomly arises in glass when manufacturing and causes scattering loss. Nonideal mixing and dissolution of chemical components of glass cause obstacles appearing in fiber core. Errors made in production of fibers lead to defects of surface (unevenness) at the fiber-cladding boundary.

In this case scattering objects measures larger than wavelength of optical signal. In contrast to Rayleigh scattering the loss caused by large scattering objects doesn’t depend on wavelength. It can be lowered by improving technologies of fiber manufacturing.

Geometrical defects. Bends of fibers cause additional loss. Two classes of bends exist. They are microscopic bends and macroscopic ones.

Macroscopic bends are considered as large-scale bends. For example they appears when spooling fibers (coiling cable) or in case cable run bends. For instance fiber which outer diameter is 125 microns can be bended with radius up to 25 mm only in order not to cause high additional loss. Obviously fibers won’t be broken with even smaller bending radius. Thus a fiber won’t be damaged (broken) if bending radius is 10 mm only. This example illustrates certain flexibility of quartz fibers and indicates possibility of using them in situations when fibers are frequently bended.  

Bending loss is not the only adverse factor for fibers. Curving fibers leads to lowering their strength. Fiber strength depends on microscopic defects (cracks), which appear on its surface. If a fiber is subject to buckling (or moisture effect), then mentioned cracks will grow and after some time has passed they will lower fiber strength. Thus bending load can cause premature damage of fibers. Minimum bending radius of 25 mm for fibers 125 microns over guarantee low bending and tensing losses.

Bending loss can be explained using Figure 2.3 where a directed ray being incident on the core-cladding interface at an angle of 1 > cr is shown. cr is a critical angle (remind you that the angle is counted out with respect to the incidence normal). In this rate total reflection of ray takes place. The same ray falls on the bended fiber at another angle 2, which can be smaller than the critical one (the value of 2  falling with decreasing bending radius R). The angle 2 becomes smaller than critical one at certain bending radius so that phenomenon of total reflection vanishes and the ray is refracted into the cladding and it is guided by the fiber no longer (i.e. loss of energy). Higher-order modes (which angle of incidence can be close to critical value) run the loss more than lower-order modes.

Microscopic bends appear when pacing fibers into a cable. Load on fibers in producing cables leads to slight axial displacements (microbends), which are randomly placed along the fiber. Microbends cause energy exchange between propagating modes so that some part of energy is lost in the fiber.

Total attenuation. Summation of losses caused by all described above effects with exception of the ones due to process of cable production leads to attenuation spectrum curve which is shown in Figure 2.1 as solid line. The curve indicates that for fibers of quartz/quartz type (Q/Q) low-loss wave region is limited by Rayleigh scattering in short-wave region and by infrared absorption in long-wave one. The low-loss region of spectrum where transmission of energy in quartz fibers is the most effective is shown in Figure 2.4.

Quartz fibers are made of pure or alloyed glass. There are shown attenuation spectrum of multimode gradient-index fibers with the core 50 microns over and single-mode step-index fibers with mode spot 5 microns over at a wavelength of 1310 nm in Figure 2.5.

Relatively low loss in quartz fibers in the range from 800 to 900 nm allows the region to be used in short transmission lines. This region is also called first frequency window. In the range from 1300 to 1600 nm loss in quartz fibers is lower. This region is divided into two parts by OH absorption peak, which is located slightly lower than 1400 nm. There are second window round the wavelength of 1300 nm and third frequency window round the 1550 nm. Typical parameters of Q/Q fibers are given in the Table 2.1.

Table 2.1 – Parameters of Q/Q fibers

Type of fibers

Wavelength, nm

Diameter of the core 2а, microns

Numerical aperture NA

Attenuation coefficient, dB/km

Type of emission source
































Note. LED – light-emitting diode, LD – laser diode.

2.2 Measurement of light attenuation in fibers  

Measurement of light attenuation in fibers can be performed in different ways according to the circumstances. The simplest method is to measure output power at the far end of the spooled fiber by using power meter. Then the fiber is cut at the distance of 1..3 metres away from the sending end (in which energy of the emission source is inputted) and one repeats measurements of power at the end of the fiber. Attenuation coefficient of the fiber (in dB/km) is equal to difference of these powers divided by the length (in km) of spooled fiber.

The method is called cutting method and it can be performed in case both ends of a fiber are easy of access. However, it is not possible to carry it out for fibers in a cable which ends are wide apart. In this case optical reflectometer is commonly used.

2.2.1 Cutting method

The method is based on comparing power of optical radiation at the output end of an entire fiber with power at the output end of the short section of the fiber ( 1 m away from the sending end) obtained by cutting of the fiber. When measuring it is necessary to provide stability of power which is inputted into the fiber and permanent mode structure of the radiation.

A block diagram of the equipment for measuring according to cutting method is shown in Figure 2.6. There  1 is an emission source (ES) with the wavelength of ; 2 is an input device (micromanipulator); 3 is a mode mixer; 4 is a cladding mode suppressor; 5 is a fiber being measured; 6 is a optical power measurer (receiving optical module - ROM) with  a large-sized photodiode for registration the entire cone of light giving out of the fiber end.

By the use of an input device micrometrical shift of the fiber sending end can be performed in three mutually perpendicular directions in order to provide maximum of the power inputted into the fiber.

Mode mixer is necessary for exciting in a fiber radiation with the mode structure which corresponds to permanent mode distribution. To control it method of measuring numerical aperture can be used. The main factor of permanency of mode distribution is identity of power distribution at the far-field region of a mode mixer and a fiber.


Cladding mode suppressor removes modes propagating through the fiber cladding. To control suppressing cladding modes method of measuring intensity at near-field region (i.e. at a region where the modes are absent) and at the far end of the fiber and at the output end of the suppressor is used.

Measurement procedure. Remove waterblocking and armoring coverings from the both ends of a cable at a distance of 0,5..1 m. Clean the fibers of hydrophobic adsorbent with a spirit-wet napkin. Primary protective covering and silicone lacquer are also removed from the fibers. By using a shearing device one can chip the fiber ends and control quality of the butt with a 8-power microscope. Fix One of the fiber ends in the micromanipulator while the other end connect to a receiving optical module. Then carefully move the fiber input end in three directions in order to attain maximum power registered by ROM. Note down the obtained value of power (or power level) at the output of the measured fiber Р2, W (р2, dB). Don’t change conditions of inputting radiation into the fiber, cut it at a distance of 0,5 m away from its beginning. Connect ROM to the output of the short fiber section (which was formed by cutting the fiber) and measure power Р1, W or power level р1, dB. Calculate attenuation of the light power

And attenuation coefficient at the wavelength

= А/(l2l1), dB/km,

Where l1 and l2 are respectively the lengths of the short section (l1 = 0,5 m) and the whole fiber in km. To improve accuracy a few measurements are to be performed (no less than 3) as well as fiber ends are to be swapped (i.e. direction of light propagation is changed) and then mean values of А and and measurement errors А и  are calculated.

2.2.2 Method of insertion loss

Measurement of cables with connectors are performed according to method of insertion loss, which is a modification of cutting method. A block diagram of the equipment for measuring attenuation according to the method is shown in the picture 2.7, where 1 is an emission source; 2 is an input device; 3 is a mode mixer; 4 is an auxiliary fiber 1..3 m long; 5 is an optical connector; 6 is a fiber of the cable being measured; 7 is a ROM.

Requirements to the parameters of the elements and devices in Figure 2.7 are the same as for cutting method. Optical connectors installed at the measured and auxiliary fibers need to have determined values of the insertion losses.

Measurement procedure. Assemble the model, by using the ROM register the power of optical radiation at the output of the measured fiber р2, dB. Remove the measured fiber. Join the auxiliary fiber with ROM using an optical connector and register power level р1, dB, at the output end of the auxiliary fiber. Damping and attenuation coefficient of the measured fiber can determined according to the expressions:

А = p1(dB) – p2 (dB)  aao, dB,

= А/l, dB/km,

There aao is attenuation of the optical connector 5 - 5, dB. Measurements are to be done no less than 3 times swapping  fiber ends (changing light propagation direction), then mean values of  А and as well as measurement errors А and  are calculated.


3.1 Which basic types of materials and impurities are used for producing fibers?

3.2 Explain process of absorption light by material of a fiber. Which kinds of absorption take place in quartz fibers?

3.3 Explain process of light scattering in fibers. What is Rayleigh scattering and how it is possible to reduce this effect?

3.4 Which geometrical defects take place in fibers, why do they appear and how do they influence on fiber attenuation?

3.5 What are frequency windows of quartz fibers and how would you explain their formation?

3.6 Why is the attenuation factor of single-mode fibers less than multimode ones?

3.7 Why does attenuation coefficient of a fiber increase after the fiber was packed into an optical cable?

3.8 Explain causes of losing energy at microbends and macrobends of fibers.

3.9 Which methods of measuring fiber attenuation do you know?

3.10 When are cutting method and method of insertion loss used for measuring attenuation?

3.11 Why are a mode mixer, a mode suppressor (Figure 2.6, 2.7) used and what physical processes take place there?

4 home task

Attenuation of a fiber is caused by natural power damping αn and additional  loss αad, which appears when packing fibers into a cable. Thus attenuation of the cable 

                                       α=αnad.                                                   (4.1)

Natural power damping

                                         αrsmairOH                                             (4.2)

where αrs, αma, αir та αон are loss components caused by Rayleigh scattering (rs), material absorption (ma), infrared absorption (ir), OH absorption (OH). When calculating components of αn it is convenient to use approximation formulae

    αrs=(6,3·10114)·(1+215Δ),     (4.3)

                        αma=2,55·10-3 exp(4,63·103/λ),                 (4.4)

                        αir=7,81·1011 exp(-4,85·104/λ),    (4.5)

There Δ=(n1-n2)/n1,  where n1 and n2 are refractive indices of respectively fiber core and cladding; λ is substituted into a formula in nm; the results are in dB/km. It has been found experimentally that


For example if  (a single-mode fiber), then αrs varies between 0,34 and 0,17 dB/km and αmlvaries from 0,9 to 0,05 dB/km when increasing wavelength  from 1300 to 1500 nm.

According to the formulae (4.1)…(4...4) calculate attenuation coefficient of a quartz fiber at wavelengths of 850, 1300, 1500 nm using n1 and n2 which are given in the Table 4.1.

Таблица 4.1 – Basic data for doing homework

of variant































Note. The number of variant corresponds with the last numeral of your mark-book.

Chart the spectrum of fiber attenuation (Figure 2.4) and plot values of n you have determined.

Prepare the report for performing the work.

5 Laboratory task

5.1 Learn description of the laboratory equipmet.

5.2 Learn principles of operation of the laboratory equipment for measuring fiber attenuation coefficient.

5.3 Makeready the laboratory equipment to measurements according to cutting method  under the block diagram shown in Figure 2.6.

5.4 Prepare patterns of an optical cable (fiber) to measurements? Prepare fiber ends by using the shearing device.

5.5 Measure attenuation of the fibers three times, calculate theirs attenuation coefficients and measurement errors.

5.6 Compare theoretical and experimental results at the wavelength of 850 nm and draw a conclusion.

5.7 Prepare the laboratory equipment to measurements according to method of insertion loss under the block diagram which is shown in Figure 2.7.

5.8 Measure attenuation of each fiber pattern with connectors three times, calculate theirs attenuation factors and measurement errors.

6 Equipment

6.1 Configuration 

Standard attenuation meter for multimode fibers ИФ-193 performed as four separate blocks (Figure 6.1): 1 is a test-signal generator with laser emission source; 2 is a micrometric manipulator, which enables moving fiber end relative to the emission source in three direction ( x, y, z); 3 is a fixed carriage, which enables docking output end of a fiber with a photodiode of optical power meter; 4 is a photodiode of ROM with a numerical display, which indicates attenuation in dB.

The test signal generator is designed for pumping a laser source with operative wavelength of 850 nm and formation optical test signal power of 0.2 mW. Optical receiver with a large-area photodiode serves for reception, gain and indication of attenuation (dB) of signal with the wavelength of 850 nm, which has passed the fiber. The micrometrical manipulator is necessary for joining the input end of the test fiber to the emission source. The fixed  carriage is intended for joining the output end of the fiber to the photodiode of the optical power meter.

6.2 Principles of operation of the equipment

The principle of attenuation measurement by the use of the device ИФ-193 consists in determination of fiber attenuation by making measurements of radiation power level twice. Thus insertion attenuation А (dB) is determined as a difference of values of power attenuation (in dB) measured for random-length test fiber and short fiber which was obtained after cutting the fiber at a distance of l = 0,5 m away from its beginning.

When measuring attenuation the test signal from ES is inputted into fiber by the use of the positioner 2. The signal passes the fiber and it is received by the photodiode, which is placed on the fixed carriage 2. Photodiode coverts optical signal into electric one which is gained, then the numerical display indicates power level of the optical signal (dB). Then one cuts the fiber and measures power of optical radiation at the output end of the short section of the fiber (0,5 m long). Difference between two measured values is the attenuation of optical power in the fiber. Thus 

А = А1А2, dB      (6.1)

where А is attenuation of the fiber being measured; А1 is reading of the display (dB) for the fiber l1 long; А2 is reading of the display (dB) for the fiber l2 long.

Attenuation coefficient

, dB/km     (6.2)

where l1 is length of the test fiber, km; l2 is length of short section of the fiber, km (l2 = 0,0005 km).

6.3 Technique of performing measurements 

6.3.1. Place an optical fiber with prepared ends into the groove of the micromanipulator so that its input end was at a distance of 0,5 m from the ES.

6.3.2. Other (output) end of the fiber place into the grove of the fixed carriage 3 close to the curtain behind which there is a photodiode.

  6.3.3. Alternately using screws of vertical, transversal, longitudinal shift of the micromanipulator 2 adjust the fiber relative to the ES achieving minimal indication in dB (maximal power) on the optical receiver’s display.

6.3.4. Take the reading A1 on the display. Then scissor off the fiber at a distance of 0,5 m from ES and not changing the joint of the fiber with the ES place output end of the short fiber section according to the requirements mentioned above and take reading A2 of the numerical display. Fading and attenuation factor of the test fiber you can determine according to formulae (6.1) and (6.2).

6.3.5. Under instructions of the lecturer measure attenuation and calculate attenuation coefficient of other fibers.

6.3.6. Under instructions of the lecturer determine attenuation of fibers of an optical cable with connectors according to the method of insertion loss having assembled the metering equipment in accordance with the structure chart in the picture 2.7.

7 Content of the report

7.1 Results of calculation of natural attenuation coefficient of a quartz fiber according to paragraph 4.

7.2 Spectrum of quartz fiber’s attenuation coefficient (Figure 2.5).

7.3 Structure chart for measuring attenuation factor of fibers according to the cutting method (Figure 2.6).

7.4 Flow chart of the laboratory equipment (Figure 6.1).

7.5 Table with the results of fiber attenuation measurements and attenuation factor calculation at the wavelength of 850 nm.

7.6 Results of measurements of fiber attenuation according to the method of insertion loss.

7.7 Conclusions from the laboratory work and comparative analysis of the results which were obtained in measuring attenuation at the wavelength of 850 nm with typical ones, which are given in the Table 2.1.


8.1 Корнейчук В. И. Измерение параметров компонентов и устройств ВОСП: Учебное пособие, Одесса, Изд-Ий. центр УГАС, 2000 г. - С. 36-50.

8.2 Корнейчук В. И., Панфилов И. П. Волоконно-Оптические системы передачи: Учебник, Одесса, «Печать»», 2001, С. 129-138.






 0,5    0,7   1  1,5   10

Infrared absorption peak

Absorption peak caused by ions  ОН 

Scattering loss

UV absorption

Wavelength, microns

ttenuation coefficient, dB/km

Total loss

Figure 2.1 – Spectrum of refractive index of quartz fiber alloyed by germanium

Figure 2.2 – Illustration of Rayleigh scattering




Figure 2.3 – Illustration of light radiation at a bend of a fiber. R is bending radius 


800       1000     1200     1400    1600

Wavelength, nm

First window

Second window

Third wimdow

Total absorption loss

Rayleigh scattering

OH absorption peak








Attenuation coefficient

Figure 2.4 – Attenuation coefficient spectrum of quartz fibers

800      1000    1200    1400    1600

Wavelength, nm








Attenuation coefficient, dB/km

Figure 2.5 – Spectrum of attenuation coefficient spectrum of single-mode (SM) and multimode (MM) fibers










Figure2.6Block diagram of the equipment for measuring fiber attenuation according to cutting method










Figure 2.7 – Structure chart of the equipment for measuring attenuation according to the method of insertion loss







Tested fiber

Spot of cut

0,5 m

Figure 6.1Flow chart of ИФ-193 device

* In 1988  Lucent Technologies company developed and patented Allwave fiber which doesn’t have OH absorption peak at a wavelength of λ=1.37 microns.


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