Optical Fiber and Optical Fiber Communication Systems

1.1 HISTORICAL PERSPECTIVE

In any communication system, signal from one end is transmitted via a transmitter and received at other end by a matched receiver through a low loss medium. Similarly, the data and various information from different users are transmitted form one end of the transmitter and received at the other end using low loss medium as shown in figure 1.1

Fig. 1: Block Diagram Explaining Optical Fiber Signal Transmission Process

Telephone signals are transmitted through copper wire at frequencies up to 4 KHz. Higher frequencies are transmitted through atmosphere from ground to satellite and then back to ground. Broadband communication at super high frequencies (10 ¹⁴ Hz) is done over optical fiber cables which are satisfactory working at 1300 nm, 1550 nm and in future it will go to higher wavelengths around 2550 nm.

In the electromagnetic spectrum infrared region (0.7–100mm) lies between microwave frequency and visible spectrum, (0.32 – 0.7mm). First generation optical fiber communication was designed at 820 nm but after 1980, the optical fiber communication is done at 1300 nm and 1550 nm. The 1550 nm range provides the minimum attenuation of light signals over long distances but this wavelength has much larger dispersion.

At present, most of the systems, which are operational in the world, are based on 1550 nm laser wavelength in the near infrared region. This wavelength, 1550 nm is selected after studying the fiber attenuation with respect to wavelength from 300 nm to 2000 nm. It is found that fiber attenuation is 0.5 dB/km for 1300 nm and it is around 0.2 dB/km for 1550 nm wavelength. So, optical fiber transmitter, optical receiver and fiber cable suitable for 1300nm and 1550 nm are successfully developed and are now operating worldwide. Further low attenuation losses are seen in fibers around 2.55 mm. research work is going on so that attenuation losses of the order of 0.001 dB/km may be achieved on longer wavelengths of optical fiber cables.

Advantages of Optical Fiber

1.2 Advantages of optical fiber

The advantages of optical fiber communication with respect to copper wire systems are:

1. Broad bandwidth

2. Immunity to electromagnetic interference

3. Low attenuation loss over long distances

4. Signal security and no cross talk

5. Light weight and small diameter cables

6. Electrical insulator

7. Low cost for long distance communication.

Broad bandwidth

Broadband communication is very much possible over fiber optical cables which means that audio signal, video signal, microwave signal, text and data from computers can be modulated over light carrier wave and demodulated by optical receiver at the other end. A diagram depicting the concepts of optical fiber communication is shown in Figure 1.2.

Fig. 2: Figure Showing Optical Fiber Communication Concept

It is possible to transmit around 3,00,000 two ways voice signals or 90,000 TV channels over one optical fiber. Now a days it is possible to transmit and receive data and information up to 10¹² bits per seconds, by using Wavelength Division Multiplexing (WDM) technique. The research work is going on to increase the information capacity on optical fiber communication upto 40 tera bits per second (Tb/s), which will meet the demand of large information capacity.

Immunity to electromagnetic interference

These cables are carrying the information over light waves which are traveling in the fibres due to properties of the fiber materials. There are no pick up of signals from vehicles, lighting and other sources and power cables adjacent to fiber cables, even there is no effect of electromagnetic pulse generated by nuclear devices. So, electromagnetic interference with the light wave traveling inside the optical fiber cables does not exit.
Low attenuation loss over long distances

There are various optical windows in the optical fiber cable at which the attenuation loss is found to be comparatively low and so transmitter and receiver devices are developed and used in this low attenuation region. Due to low attenuation of 0.2 dB/Km in optical fiber cables, it is possible to achieve reliable long distance communication efficiently over information capacity rate of Tb/s.

Signal security and no cross talk

Since optical fibers are coated with plastic layers to give it further mechanical strength along with other coatings, the light signal passing through these fibers does not leak out of fiber and so these fibers are very safe for security reasons. The signals cannot be tapped due to opaque jacket around the fiber cable. The cross talk from one fiber to another fiber is also not possible due to opaque coatings and total internal reflection of light traveling within the fiber cable itself.

Light weight and small diameter cables

The diameter of fiber is in the range of few microns and the fibers are further coated with plastics and so these fibers are of very lightweight and have small size due to thin diameter. Even with 12 core fiber cable in which 6 fibers are packaged together for long distance communication, the weight and size is very small so it is easy to lay fiber optical cables whereas copper cable laying due to heavy weight is time consuming and expensive.

Electrical insulator

Optical fibers are made and drawn from silica glass, which is nonconductor of electricity, and so there are no ground loops and leakage of any type of current. Optical fibers are thus laid down along with high voltage cables on the electricity polls due to its electrical insulator behavior.

Low cost for long distance communication

Since optical fibers are made from silica glass, which is found in abundance on earth, the cost of optical fibers is low with respect to copper cable due to lower cost of silica glass. The cost on fibers is incurred for making extremely pure materials out of earth silica and then drawing the cables, packaging and testing the cables. The cost of optical fiber cable is coming down day by day due to its large-scale use in long distance communication on earth’s surface and underneath the sea.

The fiber transmitter can be either LED (Light Emitting Diode) or laser source. High intensity Light Emitting Diode (LED) is used as a transmitter for fiber optical communication in LAN application and in data processing. LED light carrier wave is modulated by data/audio and video signals and the message/signal is transmitted either on multimode fiber for low cost system or on single mode fiber for slightly costly system. Both can provide reliable operation. In long haul operation the system works without repeater on single mode fiber up to 80 km.

Classificaiton of Fiber Cables

1.3 CLASSIFICATION OF FIBER CABLES

The classification of fiber cable is based on various technical parameters and its end application. Following parameters must be defined for classification of fiber cable:

The cable count, cable type and span length.

The transmission characteristics tell about the type of fiber.

1. Single mode or multimode

2. Wavelength of operation (850 nm 1300 nm or 1550nm)

3. Maximum bandwidth in (MHz- km) for different wavelengths

4. Length of optical fiber cable in bundle along with weight.

Based on above information, it is possible to classify the cable for indoor and outdoor or offshore and inshore application. Once the wavelength of operation is decided, type and nature of fiber can be finalized. For networking in LAN, multimode fiber cable is sufficient but for long haul application. Single mode fiber (mono mode fiber) will be the choice due to distortion free communication.

Loose buffer and tight buffer

There are two major types of buffers:

Loose buffer

Tight buffer

Loose buffer

A loose buffer’s inner diameter is much larger than a fiber’s outer (coating) diameter. Two major advantages from this design are perfect fiber isolation from mechanical forces (within given range) and protection from moisture. The first advantage is due to mechanical dead zone. A force imposed on a buffer does not affect the fiber until this force becomes large enough to straighten the fiber inside the buffer. A loose buffer can be easily filled with a water-blocking gel, which provides its second advantage. In addition, a loose buffer can accommodate several fibers, thus reducing the cost of the cable. On the other hand, this type of cable cannot be installed vertically and its end preparation for connectorisation (splicing and termination) is labor-intensive. Also loose buffer structure protects the breakage of fiber due climate variations. Consequently, the loose buffer type of cable is used mostly in outdoor installations because it provides stable and reliable transmission over a wide range of temperatures, mechanical stress, and other environmental conditions.

Tight buffer

A tight buffer’s inner diameter is equal to the fiber’s coating diameter. Its primary advantage is its ability to keep the cable operational despite a break in the fiber. Since a buffer holds a fiber firmly, a small separation of the fiber ends won’t interrupt the service completely, although it will definitely degrade signal quality. That is why the military was the first customer and still is the largest for this type of fiber cable. A tight buffer is rugged, allowing a smaller bend radius. Since each buffer contains only one fiber and there is no gel to be removed, it is easy to prepare this cable for connectorisation. Cables having a tight buffer can be installed vertically. In general, tight buffer cables are more sensitive to temperature, mechanical stress etc.

Types of Optical Fibres

Different types of OPTICAL FIBERS

In order to plan the use of optical fibers for a variety of communication applications it is necessary to consider the various optical fibers currently available.

The following is a summary of the dominant optical fiber types with an indication of their general characteristics. The performance characteristics of the various fiber types discussed vary considerably depending upon the materials used in the fabrication process and the preparation technique involved. The values quoted are based upon both manufacturers’ and suppliers’ data, and practical descriptions for commercially available fibers, presented in a general form rather than for specific fibers. Hence in some cases the fibers may appear to have somewhat poorer performance characteristics than those stated for the equivalent fiber types produced by the best possible techniques and in the best possible conditions. It is interesting to note, however, that although the high performance values quoted were generally for fibers produced and tested in the laboratory, the performance characteristics of commercially available fibers in many cases are now quite close to these values. This factor is indicative of the improvements made over recent years in the fiber materials preparation and fabrication technologies.

This section therefore reflects the relative maturity of the technology associated with the production of both multi component and silica glass fibers. In particular, high performance silica-based fibers for operation in three major wavelength regions (0.8 to 0 9, 1.3 and 1.55/mm) are now widely commercially available. Moreover, complex refractive index profile single-mode fibers, including dispersion modified fibers and polarization maintaining fibers, are also commercially available and in the former case are starting to find system application within communications. Nevertheless, in this section we concentrate on the conventional circularly symmetric step index design, which remains at present the major single-mode fiber provision within telecommunications.

Multimode Step Index Fibers

MULTIMODE STEP INDEX FIBERS

Multimode step index fibers may be fabricated from either multi component glass -compounds or doped silica. These fibers can have reasonably large core diameters large numerical apertures to facilitate efficient coupling to incoherent light Structure

Core diameter: 50 to 400 mm

Cladding diameter: 125 to 500 mm

Buffer jacket diameter: 250 to 1000 mm

Numerical aperture: 0.16 to 0.5. Performance characteristics

Attenuation: 2.6 to 50 dBkm^-1 at a wavelength of 0.85 mm, limited by absorption or scattering. The wide variation in attenuation is due to the large differences both within and between the two overall preparation methods (melting and deposition). It is observed that the multicomponent glass fiber has an attenuation of around 40 dBkm^-1 at a wavelength of 0.85 mm, whereas the doped silica fiber has an attenuation of less than 5dBkm^-1 at a similar wavelength. Furthermore, at a wavelength of 1.3mm losses are reduced to around 0.4 dB/ km.

Bandwidth: 6 to 50 MHz km.

Applications: These fibers are best suited for short-haul, limited bandwidth and relatively low cost applications.

Application – These fibers are best suited for short haul , limited band width and low cost applications

Performance characteristics

Bandwidth: 6 to 50 MHz km.

Applications: These fibers are best suited for short-haul, limited bandwidth and relatively low cost applications.

Multimode graded index fibers

These fibers have a graded index profile and may also be fabricated using multicompoment glasses or doped silica. The performance characteristics of these fibers are better than that of the multimode step index fibers due to index grading and low attenuation.

Structure

Core diameter: 30 to 100 mm.

Cladding diameter: 100 to 150 mm,

Buffer jacket diameter: 250 to 1000 mm

Numerical aperture: 0.2 to 0.3

Although the above general parameters encompass most of currently available multimode graded index fibers, in particular the following major groups have now emerged:

1. 50/125 mm (core-cladding) diameter fibers with typical numerical apertures between 0.20 and 0.24. These fibers were originally developed and standardized by the CCITT (Recommendation G. 651) for telecommunication applications at wavelengths of 0.85 and 1.3 mm but now they are mainly utilized within data links and local area networks (LANs).

2. 62.5 / 125 mm (core-cladding) diameter fibers with typical numerical apertures between 0.26 and 0.29. Although these fibers were developed for longer distance subscriber loop applications at operating wavelengths of 0.85 and 1.3 mm, they are now mainly used within LANs

3. 85 / 125 mm (core/cladding) diameter fibers with typical numerical apertures 0.26 and 0.30. These fibers were developed for operation at wavelengths of 0.85 and 1.3 mm in short-haul systems and LANs.

4. 100/125/mm (core-cladding) diameter fibers with a numerical aperture of 0.29. These fibers were developed to provide high coupling efficiency to LEDs at a wavelength of 0.85 mm in low cost, short distance applications. They can, however, be utilized at the 1.3 mm operating wavelength and have therefore also found application within LANs.

Performance characteristics

Attenuation: 2 to 10 dB/km at a wavelength of 0.85 mm. Average losses of around 0.4 and 0.25 dBkm^-1can be obtained at wavelengths of 1.3 and 1.55 mm respectively

Bandwidth: 300 MHz km to 3 GHz km.

Applications: These fibers are best suited for short-haul, short to medium haul high bandwidth applications using incoherent and coherent multimode sources (i.e. LEDs and injection lasers respectively).

Single Mode Fibers

SINGLE-MODE FIBERS

Single-mode fibers can have either a step index or graded index profile. The benefits of using a graded index profile are to provide dispersion modified single-mode fibers. The more sophisticated single-mode fiber structures used to produce polarization-maintaining fibers. These fibers are quite expensive at present and thus they are not generally utilized within optical fiber communication systems. Therefore at present, commercially available single-mode fibers are still usually step index. They are high quality fibers for wideband, long-haul transmission and are generally fabricated from doped silica (silica-clad silica) in order to reduce attenuation.

Although single-mode fibers have small core diameters to allow single-mode propagation, the cladding diameter must be at least ten times the core diameter to avoid losses from the evanescent field. Hence with a buffer jacket to provide protection and strength.

Structure

Core diameter 5 to 10 /mmmm, typically around 8.5 mm

Cladding diameter: generally 125 mm

Buffer jacket diameter: 250 to 1000 mm

Numerical aperture: 0.08 to 0.15, usually around 0.10.

Performance characteristics

2 to 5 dB km^-1 with a scattering limit of around 1 dB km^-1 at a wavelength of 0.85 mm. in addition, average losses of 0.35 and 0.21 dB/km at wavelengths of 1.3 and 1.55 mm can be obtained in a manufacturing environment.

Bandwith : Greater than 500 MHz km. in theory the bandwith is limited by waveguide and material dispersion to approximately 40 GHz km at a wavelength of 0.85 mm. however, practical bandwidths in excess of 10 GHz km are obtained at a wavelength of 1.3 mm.

Applications : These fibers are ideally suited for high bandwidth very long- haul applications using single–mode injection laser sources.

PLASTIC CLAD FIBERS

Plastic clad fibers are multimode and have either a step index or a graded index profile. They have a plastic cladding (often a silicone rubber) and a glass core which is frequently silica. (i.e plastic clad silica – PCS fibers).the PCS fibers exhibit lower radiation – induced losses than silica – clad silica fibers and therefore, have an improved performance in certain environments. Plastic – clad fibers are generally slightly cheaper than the corresponding glass fibers, but usually have more limited performance characteristics.

Causes of Signal Degradation

1.4 Causes of signal degradation in optical fibers

ATTENUATION

Attenuation of a signal as it propagates along a fiber is an important consideration in the design of an optical communication system, since it plays a major role in determining the maximum transmission distance between a transmitter and a receiver or an in line amplifier. The basic attenuation mechanisms in a fiber are absorption, scattering, and radiative losses of the optical energy. Absorption is related to the fiber material, whereas scattering is associated both with the fiber material and with structural imperfections in the optical waveguide. Attenuation owing to radiative effects originates from perturbations (both microscopic and macroscopic) of the fiber geometry.

ABSORPTION

Absorption is caused by three different mechanisms:

Absorption by atomic defects in the glass composition.
Extrinsic absorption by impurity atoms in the glass material.
Intrinsic absorption by the basic constituent atoms o the fiber material.
Atomic defects are imperfections in the atomic structure of the fiber material. Examples are missing molecules, high – density clusters of atom groups, or oxygen defects in the glass structure. Usually, absorption losses arising from these defects are negligible compared with intrinsic and impurity absorption effects. However, they can be significant if the fiber is exposed to ionizing radiation, as might occur in a nuclear reactor environment, in medical radiation therapies, in space missions that pass through the earth’s Van Allen belts, or in accelerator instrumentation. In such applications, high radiation doses may be accumulated over several years.

Radiation damages a material by changing its internal structure. The damage effects depend on the energy of the ionizing particles or rays.

The basic response of a fiber to ionizing radiation is an increase in attenuation owing to the creation of atomic defects, or attenuation centers, that absorb optical energy. The higher the radiation level, the larger the attenuation.

The dominant absorption factor in fibers prepared by the direct – melt method is the presence of impurities in the fiber material. Impurity absorption results predominantly from transition metal ions, such as iron, chromium, cobalt, and copper, and from OH (water) ions. The transition metal impurities which are present in the starting materials used for direct – melt fibers range between 1 and10 parts per billion (ppb), causing losses from 1 to 10 dB/km. The impurity levels in vapor-phase deposition processes are usually one to two orders of magnitude lower. Impurity absorption losses occur either because of electronic transitions between the energy levels associated with the incompletely filled inner sub shell of these ions or because of charge transitions from one ion to another. The absorption peaks of the various transition metal impurities tend to be broad, and several peaks may overlap, which further broadens the absorption region.

The peaks and valleys in the attenuation curve resulted in the designation of various “transmission windows” to optical fibers. By reducing the residual OH content of fibers to around 1 ppb, standard commercially available single – mode fibers have nominal attenuations of 0.5 dB/km in the 1300 –nm window and 0.3 dB/km in the 1550-nm window. An effectively complete elimination of water molecules from the fiber results in low loss in the fiber.

Intrinsic absorption is associated with the basic fiber material (e.g., pure SiO₂) and is the principal physical factor that defines the transparency window of a material over a specified spectral region. It occurs when the material is in a perfect state with no density variations, impurities, material inhomogeneties, and so on. Intrinsic absorption thus sets the fundamental lower limit on absorption for any particular material.

Intrinsic absorption results from electronic absorption bands in the ultra violet region and from atomic vibration bands in the near- infrared region. The electronic absorption bands are associated with the band gaps of the amorphous glass materials. Absorption occurs when a photon interacts with an electron in the ultraviolet region of the electron absorption bands of both amorphous and crystalline materials

SCATTERING LOSSES

Scattering losses in glass arise from microscopic variations in the material density, from compositional fluctuations, and from structural inhomogeneties or defects occurring during fiber manufacture; glass is composed of a randomly connected network of molecules. Such a structure naturally contains regions in which the molecular density is either higher or lower than the average density in the glass. In addition, since glass is made up of several oxides, such as S_iO₂, G_eO₂ and P₂O₅compositional fluctuations can occur. These two effects give rise to refractive –index variations, which occur within the glass over distances that are small, compared with the wavelength. These index variations cause a Rayleigh –type scattering of the light.

Structural inhomogeneties and defects created during fiber fabrication can also cause scattering of light out of the fiber. These defects may be in the form of trapped gas bubbles, unreacted starting materials, and crystallized regions in the glass. In general, manufacturing methods that have evolved have minimized these extrinsic effects to minimum where scattering that results from them is negligible compared with the intrinsic Rayeigh scattering . Since Rayleigh scattering follows a characteristic l^-4 dependence, it decreases dramatically with increasing wavelength, for wavelengths below about 1 mm it is the dominant loss mechanisms in a fiber. At wavelengths longer than 1 mm, infrared absorption effects tend to dominate optical signal attenuation.

Combing the infrared, ultraviolet, and scattering losses, for multimode fibers and for single –mode fibers. It is found that the losses of multi –mode fibers are generally higher than those of single –mode fibers.

BENDING LOSSES

Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature. Fibers can be subject to two types of bends: (a) macroscopic bends having radii that are large compared with the fiber diameter, for example, such as those that occur when a fiber cable turns a corner, and (b) random microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables.

The large – curvature radiation losses, are known as macro bending losses or simply bending losses. For slight bends the excess loss is extremely small and is essentially unobservable. As the radius of curvature decreases, the loss increases exponentially until at a certain critical radius the curvature loss becomes observable. If the bend radius is made a bit smaller once this threshold point has been reached, the losses suddenly become extremely large.

Another form of radiation loss in optical waveguides results from mode coupling caused by random microbends of the optical fiber. Microbends are repetitive small –scale fluctuations in the radius of curvature of the fiber axis. They are caused with in the fiber by nonuniformities in the manufacturing of the fiber or by nouniform lateral pressures created during the cabling of the fiber. The latter effect is often referred to as cabling or packaging losses. An increase in attenuation results from microbending because the fiber curvature causes repetitive coupling of energy between the guided modes and the leaky or nonguided modes in the fiber.
One method of minimizing microbending losses is by extruding a compressible jacket over the fiber. When external forces are applied over the fiber the jacket will be deformed but the fiber will tend to stay relatively straight.

SIGNAL DISTORTION IN OPTICAL FIBER

An optical signal becomes increasingly distorted as it travels along a fiber. This distortion is a consequence of intramodal dispersion and intermodal dispersion effects. These dispersion effects can be explained by examining the behaviour of the group velocities of the guided modes, where the group velocity is the speed at which energy in a particular mode travels along the fiber.

Intramodal dispersion or chromatic dispersion is pulse spreading that occurs within a single mode. The spreading arises from the finite spectral emission width of an optical source. This phenomenon is also known as group velocity dispersion (GVD), since the dispersion is a result of the group velocity being a function of the wavelength. Because intramodal dispersion depends on the wavelength, its effect on signal distortion increases with the spectral width of the optical source. This spectral width is the band of wavelengths over which the source emits light. For light-emitting diodes (LEDs) the rms spectral width is approximately 5 percent of a central wavelength. For example, if the peak emission wavelength of an LED source is 850 nm, a typical source spectral, width would be 40 nm; that is, the source emits most of its optical power in the 830-to-870-nm wavelength band. Laser diode optical sources have much narrower spectral widths, with typical values being 1-2 nm for multimode lasers and 0.01 nm for single-mode lasers.

The two main causes of intramodal dispersion are as follows:

Material dispersion, which arises from the variation of the refractive index of the core material as a function of wavelength. (Material dispersion is some times referred to as chromatic dispersion, since this is the same effect by which a prism spreads out a spectrum.) This causes a wavelength dependence of the group velocity of any given mode; that is, pulse spreading occurs even when different wavelengths follow the same path.

2. Waveguide dispersion, which occurs because a single-mode fiber confines only about 80 percent of the optical power to the core. Dispersion thus arises, since the 20 percent of the light propagating in the cladding travels faster than the light confined to the core. The amount of waveguide dispersion depends on the fiber design, since the modal propagation constant is a function the optical fiber dimension relative to the wavelength.

3. The other factor giving rise to pulse spreading is intermodal delay, which is a result of each mode having a different value of the group velocity at a single frequency. Of these three, waveguide dispersion usually can be ignored in multimode fibers. However, this effect is significant in single-mode fibers. The full effects of these three distortion mechanisms are seldom observed in practice, since they tend to be limited by other factors, such as nonideal index profiles, optical power-launching conditions (different amounts of optical power launched into the various modes), nonuniform mode attenuation, and mode mixing in the fiber and in splices; and by statistical variations in these effects along the fiber.

1.5 CCITT RECOMMENDATIONS G652 FOR 1300 nm SINGLE MODE OPTICAL FIBRE

CCITT has issued recommendations for single mode fibre which has dispersion wavelength of which is optimized for used in 1400 nm wavelength. This covers only the salient points related to single mode 1300 nm fibre in BSNL, which is used for the present link engineering. The recommendations are as under:

Fiber Characteristics

1.5.1 Fiber Characteristics :

1. Mode Field Diameter :

The nominal value of mode field diameter (MFD) at 1300 nm shall be within the range of 9 to

m. the MFD deviation should not exceed the limits of

10% of the nominal value.

It may be noted that fibre performance required for any given application is a function of essential fibre and system parameters, i.e, mode field diameters, the cut –off wave length, total dispersion, system operating wave length and bit rate /frequency of operation.

2. Cladding diameter :

The recommended nominal value of cladding diameter is 125

m, cladding deviation should not be exceed the limits

2.4%.

Mode field concentricity error.

The recommended mode field concentricity error at 1300 nm should not exceed 1

Cladding non circularity should be less than 2%

Cut off wave length:

Two types of cut off wavelength can be distinguished

(a) Cut of wavelength c of primary coated fibre.

(b) Cut off wavelength cc of a cabled fibre in deployment condition.

“In general the recommended value of cc. c. c is recommended less than 1280 nm, and greater than 1100nm. cc should be less than 1270 nm. However, out of these two wavelengths, normally only one is specified.

1.5.2 Fibre Properties:

Fibre material substances of which fibres are made may be indicated.

1. Protective Material :

The physical and chemical properties of the a material used for the fibre primary coating and the best way to remove it should be indicated?

2. Attenuation Coefficient :

Attenuation should be les than 1 db per km. however, now the values in the range of 0.3 to 0.4 db per Km. have been achieved

3. Chromatic dispersion coefficient :

The maximum chromatic dispersion coefficient should be specified by the allowed range of zero dispersion wave length between min = 1295 nm and max = 1322 nm.

Table -1 gives the maximum chromatic dispersion coefficient in different wave length ranges. For high capacity 565 Mb/s or above or long length systems, narrower range may need to be specified.

Table -1

Wavelength Maximum chromatic dispersion

(nm) coefficient

1285 -1330 3.5

1270 -1340 6

SPECIFICATION OF OPTICAL FIBRE CABLE (STANDARDIZED BY DOT) DOT FOLLOWS STANDARDS AS RECOMMENDED BY CCITT NO. G 652

1.0 FIBRE :

1.1 Core – 5-10 Microns

– Wave length cut off 1125 -1280

for 1300 nm.

– Material Si. 02/ Ge 02

1.2 Cladding 125 Microns (overall Diam)

1.3 Dispersion Max.3 ps/Km. nm at 1300 nm &

18 ps/Km. nm at 1550 nm.

1.4 Attenuation better than 0.5 db/ Km

1.5 Primary Coating 250 Microns UV Cured Acrylate

1.6 Secondary 0.9 mm Nylon PE Jelly

coating, if filled tube

not slotted

line construction.

2.0 Central strength Fibre Reinforced

Plastic (FRP)

3.0 Core covering Longitudinally applied Layer of

Non –hygroscopic Dielectric

Material, non –adhesive

to secondary coating.

4.0 Moisture Barrier Non metallic Polythylene sheet or of high

Weather resistant compound, free from

Pinholes &other defects.

5.0 Polyethylene Sheath Tough, weather resistant high

molecular weight polyethylene circular, free from pin –hole & joints etc.

6.0 Nylon outer sheath Nylon -12 as protective sheath

(0.7 mm thickness) against termite & partially against rodent.

7.0 Strength to with – 3x 9.8 W. Newtons, where W is

stand a Load weight of O/F cable per KM in Kg.

8.0 MAX Strain allowed 0.25%

In fibre

9.0 Max Attenuation Permissible

0.02 dB from that

variation of nominal 20⁰C to 60⁰C

10.0 Flexibility Maximum bending radius allowed 24d where d is diameter of optical fibre cable.

11.0 Compressive Stress 1600 N between two Plates of 50 x 50 mm Maximum for 60 seconds

12.0 Cable drum lengths 2 KM

10%

13.0 Cable ends – One end fitted with wire grip.

– Other end sealed with cap.

Pleasiochronous Optical Fiber System

Chapter 2

CHARACTERISTICS OF 140 Mb/s (HFCL) PLESIOCHRONOUS

OPTICAL FIBER SYSTEM

The plesiochronous network

According to ITU-T standards, corresponding signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate

The plesiochronous digital hierarchy

The primary multiplex group of 24 or 30 channels is used as a building block for larger numbers of channels in higher-order multiplex systems. At each level in the hierarchy, several bit streams, known as tributaries, are combined by a multiplexer. The output from a multiplexer may serve as a tributary to a multiplexer at the next higher level in the hierarchy, or it may be sent directly over a line or radio link.

If the inputs to a multiplexer are synchronous, i.e. they have the same bit rate and are in phase, they can be interleaved by taking a bit or a group of bits from each in turn. This can be done by a switch that samples each input under the control of the multiplex clock.There are two main methods of interleaving digital signals: bit interleaving and word interleaving. In bit interleaving, one bit is taken from each tributary in turn. If there are N input signals, each with a rate of f, bit/s, then the combined rate will be Nf, bit/s and each element of the combined signal will have a duration equal to 1/N of an input digit. In word interleaving, groups of bits are taken from each tributary in turn and this involves the use of storage at each input to hold the bits waiting to be sampled. Since bit interleaving is simpler, it was chosen for the PDH.

There are three incompatible sets of standards for plesiochronous digital multiplexing, centred on Europe. North America and Japan. The European standards are based on the 30-channel primary multiplex and the North American and Japanese standards on the 24-channel primary multiplex. The European hierarchy is shown below in the figure 2.1

These systems all use bit interleaving. The frame length is the same “as for the primary multiplex, i.e. 125 ms, since this is determined by the basic channel sampling rate of 8 kHz. However, when N tributaries are combined, the number of digits contained in the higher-order frame is greater than N times the number of digits in the tributary frame. This is because it is necessary to add extra ‘overhead’ digits for two reasons.

The first reason is frame alignment. A higher-order demultiplexer must recognize the start of each frame in order to route subsequent received digits to the correct outgoing tributaries, just as a primary demultiplexer must route received digits to the correct outgoing channels. The same technique is employed. A unique code is sent as a frame-alignment word (FAW), which is recognized by the demultiplexer and used to maintain its operation in synchronism with the incoming signal. The European hierarchy uses a block FAW at the start of each frame, but the other hierarchies use distributed FAWs.

The second reason for adding extra digits to the frame is to perform the process known as justification. This process is to enable the multiplexer and demultiplexer to maintain correct operation, although the input signals of the tributaries entering the multiplexer may drift relative to each other. If an input tributary is slow, a dummy digit (i.e a justification digit is added to maintain the correct output digit rate. If the input tributary speeds up, no justification digit is added. These justification digits must be removed by the demultiplexer, in order to send the correct sequence of signal digits to the output tributary. Consequently, further additional digits, called justification service digits, must be added to the frame for the multiplexer to signal to the demultiplexer whether a justification digit has been used for each tributary.

The term ‘justification’ originated in the printing trade. Since different lines of print on a page contain unequal numbers of letters, the printer inserts additional spaces to ensure that all the lines on a page are of equal length. Word processors can also perform justification.

When bit interleaving is used, bits for a particular channel occur in different bytes of a higher-order frame. In order to separate one channel from the aggregate bit steam, a total demultiplexing process is required. This results in the ‘multiplexing mountain’ shown in Figure

GENERAL system Description

The ML33 Equipment can be configured as 34 Mbits/s or 140Mbits/s OPTIMUX, OLTE or OPTICAL REGENERATOR link on single-mode optical fibres by means of 2nd window (1300nm) O.F. line Interface.

The ML33 equipment can be outfitted with different types of common & optional units to obtain the required network configurations, fig. 1-1 (a) and fig. 1-1 (b) shows some of the standard configurations for OPTIMUX, Multiplexer & Regeneator respectively.

The equipment sub-rack has 12 slots, out of which 8 slots are dedicated for various optional modules and 4 slots for common units.

The equipment is housed in a sub rack which can be fitted in to 19″ rack of size 600 x 300 x 2200 (W x D x H).

The rack can accommodate a maximum of four sub racks.

Optional Unit

These units are listed below and their functions are summarized.

2/34Mbits/s Muldex Units, which multiplexes / demultiplexes up to sixteen 2048kbit / s tributary streams into one 34368kbit / s HDB3 stream, and vice versa, The 34368kbit / s aggregate signal is transmitted and received in duplicate, i.e. over a main line and a stand-by line. For this function the equipment uses 4 mux/demux cards
34/140Mbits/s Muldex Units, which multiplexes / demultiplexes up to four 34368kbit /s HDB3 tributary streams into one 139264kbit/s CMI signal, and vice versa. The 132964kbit / s aggregate signal is transmitted and received in duplicate, i.e. over a main line and stand -by line.
140Mbits/s O.F. Line Terminal Unit, which is a 2nd window (1300±20nm) interface, is able to transmit / receive a 139264kbit / s line signal overmodulated with a 128kbit /s data channel comprising of 64kbit /s service telephone channel and a 4800 Baud auxiliary data channel.
Remote loop back unit, which loops back the 140Mb / s stream at slave and is activated from Master.
In Regenerator configuration, it receives 140Mb optical signal and separate out over modulation of 128 Kbit/s and then amplify and over modulate again for retransmission.
34Mbit/s Of Line Terminal Unit, which is a 2nd window (1300nm) interface, is able to transmit and receive a 34368Kbit/ s line signal . modulated with 128kbit Is data channel comprising of 64kbit Service telephone channel and a 4800 Baud auxiliary data channel.
In Regenerator configuration, it receives the 34Mb optical signal and separate out over modulation of 128 Kbit I s and then amplify and over modulate again for retransmission.

Common Units

These units are always present in the equipment sub-rack and are located in fixed position. The units are listed below and their functions are summarized-

Control and alarms Processor Unit which allows to configure the equipment and display the alarm conditions by means of a Hand-Held Programming Terminal (PCD) connected to a front panel interface.
Service Channel Interface Unit, which allows the communications between two terminal exchanges in a point to point link.

-40Vdc to -60Vdc Converter Unit, which delivers the required voltages (±5Vdc) to the circuits with input from the Exchange Batteries

-48 to -60Vdc. .

All the configuration must include one Control and Alarms Processor Unit and one or two Converter Unit for single or duplicated power supply.

Optical protection

An optical protection device is provided in the 140 Mbit/s O.F. Line Terminal Unit in order to assure the automatic switching-off of the two laser diodes of a section, in case of an optical fiber interruption (CCITT Recommendation 958).

The recovery of the optical power (switching–on of the two laser diodes) can be carried out, either manually by means of a push button on the 140 Mbit/s O.F. Line Terminal Unit, or via a software command selected by means of the Hand-Held Programming Terminal (PCD).

The unit is also fitted internally with devices, which enable inhibition of the optical protection circuit in order to permit measurements of the emitted optical power. This condition is indicated by the lighting-up of a yellow LED on the 140 Mbit/s O.F. Line Terminal Unit of local and remote stations.

Alarms

Following alarm conditions are indicated.

Visual indications (LED) on the units where the alarm conditions have been detected (for internal alarms) and on the Control and Alarm Processor Unit.
Issue of earth contacts to the appropriate sub – rack connector.
Displaying of the alarms on the Hand – Held Programming Terminal (PCD) connected to the Control and Alarm Processor Unit.

The system detects and summarizes the following alarm conditions.

Function Alarms (relevant to the functions performed by the equipment).
System Alarms (relevant to failures detected by the equipment but not resulting from failures of the equipment itself).

Indication Alarms (relevant to failures detected at the remote stations).
Configuration Alarms (relevant to incorrect outfitting of units in the subrack).

The “Rack Alarm” is indicated by the lighting-up of a red LED at the top of the cabinet. The Function, System and Indication Alarms are available for remote transmission.

By means of a push button control, the rack alarm can be cancelled and stored (switching-off of the red LED at the top of the cabinet and lighting-up of the adjacent yellow LED).

The alarms are processed in accordance with specific criteria in order to generate the following information.

Auxiliary facilities

In case of configuration with O.F. Line Terminal Unit, the following auxiliary facility is available.

· Service Channel Interface Unit, STC for telephone calls between the operators at the terminal or intermediate stations,

With STC) it is possible to transmit/receive speech and ringing over any one among a eight different channels. STC has the following characteristics

at the local station, the speech and ring signals are conveyed over the channel selected by means of a dial of the associated telephone set to a remote stations:

at the remote station, the incoming speech and ring signal art activated automatically over the same channel selected at the local station. At the remote station, the speech and Service Telephone Channel recognize its own assigned identification number;

From any station, it is also possible to carry out omnibus calling towards al network stations at the same time.

The connection between local and remote operators is of point-to-point type and established by a ringing signal which activates a buzzer in the remote station.

Configuration of the units present in each system.
Display of the equipment configuration.
Display of the alarm conditions.
Special functions
The activation of laser in the nominal power O.L.T. Units where it had been deactivated. Activation of test points.

Supervision and Control System Interface

The Equipment contains a Interface Subunit, which realizes the connection is. n and Control and Alarm Processor Unit It also allows for the supervision of the units contained in the ML33 Equipment.

FUNCTIONS

Management of dialog with Supervision
Polling of Control and Alarm Processor Unit and storage of ML33 Equipment status
Programming of Control and Alarm Processor Unit in the subrack
TYPE OF UNIT INTERESTED

The alarms of each unit are grouped together to activate an earth contact relevant to the alarmed card. A red LED on each unit indicates the presence of internal alarms.

ALARM LOCATION

§ Internal Alarms (INT).

§ External Alarms (EXT).

§ Indication Alarms, from other equipments (IND).

The relative earth contacts and the visual indications on the Control and Alarm Processor Unit are also available.

ALARM PRIORITY

1. - Urgent Alarm (URG).
  - Not-Urgent Alarm (NURG).

The relative earth contacts and the visual indication on the Control and Alarm Processor Unit are also available.

Power supply

The ML33 equipment is fed with two separate secondary battery voltages of -48V (- 40 Vdc to -60Vdc)

A separate -48 V supply is also necessary for the rack alarm storage even during power supply failure.

Equipment programmability and controllability

In order to program and control the equipment effectively it is divided into as many functional groups as are necessary to set up different links. Each functional group is referred to as System and can consist of a single unit or some units connected together with a view for achieving the required functions.

By means of a suitable Hand-Held Programming Terminal ( PCD) connected to the front panel interface of the Control and Alarm Processor Unit. the operator can activate a series of commands which make it possible to carry out the following operations.

Configuration of the systems present in the sub-rack.

Technical Specifications

Power supply

Secondary power supply from -40 Vdc to -60 Vdc battery (duplicated )

Equipment consumptions (*)

Common parts (Alarms) £ 2 W

Common Parts (Alarms and Auxiliary Facilities) £ 3 W

2/34 Mbit / s Muldex Unit £ 7 W

34/ 140 Mbit / s Muldex Unit £ 7 W

140 Mbit / s O.F.Line terminal Unit £ 6 W

34 Mbit s O.F. Line Terminal Unit £ 6 W

Note (*) The total consumption of the equipment is obtained by adding the Common Parts consumptions and the Units consumption.

Mechanical characteristics

Dimensions

Cabinet height 2200 mm

Width 600 mm

Depth 300mm

Sub – rack Height 450mm

Width 532mm

2/ 34 Mbit /s Muldex Unit height 233mm

34/140 Mbit/ s Muldex Unit width 40.6 mm

O.F Line Terminal Unit depth 220mm

Control & Alarm Processor height 233mm

Unit width 30.5mm

Power Supply Unit depth 220mm

-40 to -60 Vdc height 100mm

Power supply Unit width 50.8mm

depth 220mm

Weights

– Cabinet 60Kg

– Sub – rack 7 Kg

– 2/34Mbit /s Muldex Unit 0.71 Kg

– 34/140 Mbit /s O.F. Line terminal Unit 0.94 Kg

– Control & Alarm Proc.Unit 0.65 Kg

– -40 to -60 vdc Power 1.16 Kg

– Supply Unit

– Service Channel Interf. Unit 0.20 Kg

Functional characteristics

Multiplexing frames

2/8/34Mbits /s Multiplexing

The 2Mbit/s tributaries are multiplied to 8 Mbit/s streams according to CCITT recommendation G.742

The 8Mbit/s tributaries are multiplexed to 34 Mbit/s streams according to CCITT Recommendation G.751

34/140Mbit/s Multiplexing

The 34Mbit/s tributaries are multiplexed to 150mMbit/s streams according to CCITT Recommendation G.751

Digital Interfaces

2048Kbit/s Interface

Bit rate 2048kbit /s

Tolerance 50ppm

Impedance 120 balanced

Type of cable Screened Cable

Input and output signal code HDB3

Pulse peak voltage 3.0 Vp 10%, max attenuation 6dB at 1024kHz, trend af

No –pulse peak voltage 0.3V

Overshoot of pulse amplitude < 20%

(1)

Pulse width of half amplitude 244ns 25ns

Tributary outgoing jitter <0.05 UIpp in the 18 Hz to 100 kHz band

<0.25 UIpp in the 0 to 100kHz band

NOTE (1) For pulse of nominal amplitude and width.

34368 Kbit/s Interface

Bit rate 34368kbit /s

Tolerance 20ppm

Impedance 75 balanced

Type of cable Coaxial Cable

Return loss <18dB in the 680 kHz to 3.4 MHz band

<20dB in the 3.4MHz to 51MHz band

Input signal code HDB3

Output signal code HDB3

Wave form in accordance to the mask as shown in

Fig.4-8

Pulse peak voltage 1 Vp / unbal., max. attenuation

12dB at 17184kHz, trend af

Acceptable incoming sinusoidal jitter within the mask as shown in Fig. 4-9

Common parts outgoing jitter <0.05 UIpp in the 100 Hz to 800 kHz band

(2/34Mbit /s Muldex Unit)

Tributary outgoing jitter <0.05 UIpp in the 10kHz to 800 kHz band

(34/140Mbit/s Muldedx Unit ) <0.03 UIpp in the 0 to 800 kHz band

13926Kbit/s Interface

Bit rate 139264kbit /s

Tolerance 15ppm

Input /output Impedance 75 un balanced

InputReturn loss <15dB in the 7 MHz to210 MHz band

Input signal code CMI

Output signal code CMI

Wave form in accordance to the mask in Fig.4-11

and 4-12

Pulse peak voltage 1 Vp / unbal., max. attenuation

12dB at 70MHz, trend af

Pulse width of half amplitude 7.18ns

Acceptable incoming within the mask as shown in Fig.4-13

Outgoing jitter from multiplex

ing circuits (Tx section of

34/140 Mbit/s Muldex) <0.05 UIpp in the band 200Hz to 3500

kHz band.

Outgoing jitter from 140 Mbit/s

O.F. Line Terminal Unit <0.05 UIpp in the band 10kHz to 3500

kHz band.

Telephone service channel interface

Equivalent -4dBm/ 600 1dB measured at 1020 Hz

Noise £50dBm

Total distortion above limits of mask as shown in Figure 4-16

Impedance on sectioning

point of telephone 375 Ohms bal.

Input level 0 dBm

Output level -4dBm 1dB

Return loss ³14dBm in the 300 Hz to 3400 Hz band.

The PCD Terminal

The PCD Terminal, shown in Figure 2-1, is a hand-held apparatus used for programming, checking and displaying information concerning the MuldexUnit or the Control and Alarm Processing Unit of the ML33 Equipment.

The PCD terminal consists of three fundamental parts: Display, Keyboard and Connector Cable.

The display is a liquid-crystal type with four 16-character lines; the keyboard is provided with 32 keys.

Functions of PCD

FUNCTIONS

The PCD Terminal manages the software (programme) stored in the microprocessors of the above-mentioned units. The programme allows three main options, i.e. Alarm Procedure, Alarm Configuration, Procedure and Special Function procedure, which are dealt in the next chapter.

The software handling all the functions is stored in the microprocessor of Control and Alarm Processing Unit. In the PCD Terminal are contained the circuits for the operation of the key board and display.

The functions of PCD are divided into four main branches each dealing with one of the following procedures:

· Alarms

· Configuration

· Special functions

· Network Configuration Menu

Functions selectable by the PCD

The PCD is the user interface with the Control and Alarm Processing Unit. The software of PCD is controlled by the options present in the menu and performs the following functions:

Alarm Procedure:

– active alarm display;

– supervisory status display;

Configuration Procedure

– programming and verification of the interchangeable and common units ;

– verification and programming of supervisory maintenance;

Special Functions Procedure

By means of proper command sequences following operations are performed:

– status verification:

– programming and displaying the laser recovery sequence of the

Optical Fiber Line Terminal Units.

Networking

By means of proper command sequences following operations are performed:

– station address verification & controlled station verification

– telecommands verification; choice of station

– station address set-up

– telecommands activation; choice of station.

The frame format of the 24-channel system is shown in Figure 2.12. The basic frame consists of 193 bits; thus, the digit rate is 193 x 8kbit/s = 1.544 Mbit/s. The first bit is used for framing and is called the F bit; the others form 24 8-bit time-slots for speech channels. On odd-numbered frames, the F bit takes on the alternating pattern ‘1,0,1,0,—’, which is the pattern for frame alignment. This is a distributed frame-alignment signal as opposed to the block alignment signal used in the 30-channel system. The even frames carry the pattern ‘0,0,1,1,1,0,…’, which defines a 12-frame multiframe. On frames 6 and 12 of the multiframe, bit D8 of each channel time-slot is used for signalling for that channel. This process of bit stealing causes a small degradation in quantizing distortion, which is none the less considered acceptable.

In a transmission network which has not been designed for synchronous
operation, the inputs to a digital multiplexer will not generally be exactly synchronous.

Although they have the same nominal bit rate, they commonly originate from different

crystal oscillators and can vary within the clock tolerance. They are said to be
plesiochronous

2.2 140 Mbps HFCL Optical Fiber System

Networks are becoming fully digital, operating synchronously, using high-capacity optical-fiber transmission systems and time-division switching. It is advantageous for the multiplexers used in these networks to be compatible with the switches used at the network nodes, i.e. they should be synchronous rather than plesiochronous. In 1990. the CCITT denned a new multiplex hierarchy, known as the synchronous digital hierarchy (SDH). In the USA this is called the synchronous optical network (SONET), since the muldexes use optical interfaces. The SDH uses a digit rate of 155.52 Mbit/s and multiples of this by factors of 4n, e.g. 622.08 Mbit/s and 2488.32 Mbit/s, giving the hierarchy shown in Figure 2.17. Any of the existing CCITT plesiochronous rates up to 140 Mbit/s can be multiplexed into the SDH common transport rate of 155.52 Mbit/s. The SDH also includes management channels, which have a standard format for network-management messages.[20]

The basic SDH signal, called the synchronous transport module at level 1 (STM-l) is shown in Figure 2.18(a). This has nine equal segments, with ‘overhead’ bytes at the start of each. The remaining bytes contain a mixture of traffic and overheads, depending on the type of traffic carried. The total length is 2430 bytes, with each

4. splicing and testing of ITU G652 cable

The ITU-T G.652 fiber is also known as standard SMF and is the most commonly deployed fiber. This fiber has a simple step-index structure and is optimized for operation in the 1310-nm band. It has a zero-dispersion wavelength at 1310 nm and can also operate in the 1550-nm band, but it is not optimized for this region. The typical chromatic dispersion at 1550 nm is high at 17 ps/nm-km. Dispersion compensation must be employed for high-bit-rate applications. The attenuation parameter for G.652 fiber is typically 0.2 dB/km at 1550 nm, and the PMD parameter is less than 0.1 ps/ km.

For the present optical fiber link engineering the ITU – G 652 optical fiber cable is used. ie single mode non-dispersion shifted optical fiber is used. The fiber is spliced using X76 (Siemens make) fusion splicer available at RTTC, Thiruvananthapuram. The X76 fusion splicer is suitable for producing reliable low-loss splices of optical fibers. It can be used for single mode and multimode fibers with cladding diameter of 125mm and coating diameters of 250-400mm. Also it has 2 CCD cameras for video image evaluation which enables 3-dimensional fiber alignment, end-face quality inspection and contamination- detection

4.1 Steps in cable splicing

First the outer jacket and the inner jacket of the cable are removed using optical fiber cable stripper. The buffer tube of the stripped cable is cut using buffer stripper so that individual fibers can be identified using the colour code The different optical fibers are cleaned using ethyl alcohol to remove the petroleum jelly and other contaminations

Fig. 3: An Image OF Cable, Buffer And Fiber Stripper

Fig. 4: An Image of Optical Fiber

4.2 Color code

Fiber number	color
1	Blue
2	Orange
3	Green
4	Brown
5	Slate
6	White
7	Red
8	Black
9	Yellow
10	Violet
11	Rose
12	Natural

Preparing and Inserting the Fibers

4.3 PREPARING and inserting the fibers

Strip the coating from the fiber over a length of 50mm

Clean the fiber-end with alcohol

Cleave uncoated fiber end with a suitable diamond cleaver to cleave a length of 10 mm

Insert the fiber into the V grooves of the respective side so that the fiber end is located

between the electrode tips.

Now the fiber is spliced (by pressing the splice button) by sending 14mA current for

nearly 1 second.

Now the spliced fiber is subjected to tensile strength testing with the help of splicing

machine.

The spliced point is then protected using a thermo-shrinkable sleeve.

The sleeve is heated up to 80⁰C so that it permanently covers the splice point to provide

enough mechanical strength to the splice point.

Fig. 5

For the present study 40Km of optical fiber cable is spliced and both the ends of the fiber is terminated with FC/PC connector. For this both ends of the spliced cable is spliced with pig tail An Image Of Spliced Cable With Pig Tail

Fig. 6: An Image of Spliced Cable with Pig Tail

For 40 Km of cable 40/2 +1 = 21 splices are done for single fiber. For this study, 4 fibers are spliced in the above cable. The spliced cable is tested with OTDR to check the individual splice losses.

Fig. 7: An Image of OTDR

And those splice points which showed a splice loss of > 0.1 dB/ splice are cut opened and again spliced till the splice loss falls within the BSNL standard value of 0.1dB.or less. Then the total spliced cable is subjected to power- loss measurements using standard optical source operating at 1310 and 1550nm and optical power meter as shown below. And the result obtained is given in table below.

Wavelength nm	Transmitter- power	Receive- power	Loss	Loss db/km	Standard Limit db/Km
1310 1550	-3dBm -2dBm	13dBm 12dBm	10dB 9dB	0.25 0.225	0.25 0.25

Fig. 8: Block Diagram Showing Power Loss Measurment

Figure 4: power loss measurement

Here as per BSNL standard the loss per km should be £ 0.25dB. Thus the above cable satisfies the above criteria so that this cable can be used to design PDH as well as SDH optical fiber link.

140MBPS Optical Fiber System

5. INSTALLATION of 140 mBPS oftical fiber system

The STM-1 ADM equipment available at RTTC Trivandrum is in the rack sub rack form. The sub rack is first fitted to the rack and the power supply is extended to the sub rack connection field. The power supply is extended from the RTTC power plant and its standard value is -48 ± 2V. Before connecting the power supply to the sub rack its value is verified using multimeter. The metallic part of the sub rack is connected to the RTTC ring earth, which has an earth resistance value of 0.5 ?.

The ADM-1 sub rack has 3 identical mother boards and each mother board has 6 slots. Thus altogether 18 slots are available in a sub rack. A single sub rack can be configured in different ways. For this present link design the mother board is equipped In the 1.5 mother board configuration i.e. one and half portion of the mother board is equipped with cards/ modules. The ADM-1 equipment has three types of cards they are

1) OEO card

2) TEX 1 card

3) PS card

The OEO card is also called as aggregate card its name stands for optical- electrical- optical card. Its main function is optical to electrical and electrical to optical conversion. This card houses the optical source and the optical detector. The ADM card has two ports labeled as S1 and S2 and each port has separate TX. and Rx. Points. The second port also has separate Tx. and RX. Points. The OEO card can also cater for 21 E1s

TEX-1 card stands for tributary extension card it is a mux/De mux card. One TEX-1 card can cater for 21 E1 s. For an ADM-1 system it has 2 such cards. The TEX cards can be installed in any of the following slots 5,6,11,12,17 and 18.

The PS module can be installed in any one of the following slots i.e. slots 1,2,7,8,13,14. The PS module is a DC to DC converter which converts the -48 V DC into + 5V ane

Fig. 9: An Image of Installed Fiber Optical System In slot of CPU

The commonly used optical fiber PDH systems are 8 Mbps, 34Mbps, and 140Mbps. For the present optical link engineering the 140mbps HFCL (Himachal Fueristic Corporation Ltd) optical fiber system available at RTTC Thiruvananthapuram is used, the card details of which are shown in figure.

PDH system	Operating wave length	No: of E1s
8 34 140	1310nm 1310nm 1310nm	4 16 64

The system essentially has 5 MUX / DEMUX cards Out of these one is a 34 to 140 Mbps MUX card and four cards are 2-34 Mbps MUX cards. The 8 Mbps level is skipped. So the system is also called as SKIP MUX.. Another important card is OLT card which converts the incoming electrical signal in to optical signal and back. The OLT card in the Trans-direction (.Transmitter- direction) has a Laser Diode operating at 1310 nm. And in the receive- direction it has a photodiode which converts the optical signal in to electrical ones.

The other cards are Power Supply card (PSU), Alarm Processor Control card (APC) and Service Channel Interface (SCI) card. These are auxiliary cards.

The system has an ALS function (Auto Laser Shut down) ie, whenever a break occurs in the cable, the laser will be automatically shutting down. For

testing the system the ALS function is disabled and the OLT Tx port is directly connected to the optical power meter. The wavelength of 1310nm is selected and the Transmitter power of the 140Mbps is measured. Then the trans-fiber (Tx) is connected to a variable attenuator. Its value is initially set to a high value say 40 dB. The other end of the attenuator is connected to the receiver. And the attenuator is decreased slowly till BER of 10^-6 alarm is extinguished.(Fig 6)

Fig. 10: Table of Card Details of PDH System

Now the receive fiber is removed and it is directly connected to the optical power meter as shown in the figure.

Fig. 11:

And now the received optical power is measured. This is the minimum power required at the receiver for its proper working without alarm This is called the ‘Sensitivity’ of the receiver. Again the receive fiber is connected to the receiver and the attenuator is decreased till the alarm again appears. This is the maximum input power that the receiver can handle safely. The of optical power within which the system can work safely is called Dynamic range.. The readings obtained in the above experiment are tabulated below.

Trans-power	– 3dBm
Minimim power at the receiver (for its proper working without alarm)	-34dBm
The maximum input power at receiver the without alarm (for its proper working without alarm)	8dBm

Receiver sensitivity is -34dBm

Dynamic range = -34dbm – -8 = 26 dB

ie,. The receiver will be working normally from – 8 dBm to -34dBm over a range of 26dB loss.

ie,. The O.F.system can be installed at any point in the route where the received power is

between -8dBm and -34 dBm.

Total loss offered by the cable at 1310 nm = 40 x 0.25 =10 dB.

Maximum permissible splice loss = 0.1 x 21 –= 2.1 dB.

But the actual measured total splice loss = 2dB

Total loss = 10 + 2 = 12dB

Also in the OF link two FC/ PC adaptors are involved at both the ends.

According to BSNL standard the permissible connector loss is 0.5dB/ connector.

Total connector loss = 1dB

Normally in the BSNL link engineering design a margin of 5 to 6dB will be added to the total loss so as to contribute to the expected future losses like ageing of the component, future cable faults and future up-gradation or shifting of the systems, etc.

Total loss = cable loss + splice loss + connector loss + margin (5dB) = 17 dB

The output power available at the receiver when this system is connected to the above spliced cable is -3 -17 = -20 dB

ie,. If we are installing the system as such it will be working properly for a cable length of 40Km. And as per BSNL standard also the system can be safely installed at any that the system is operating on 1310 nm using G652 cable (single-mode cable). distance less than 80 km.

Rise Time Budget

7.2. Rise- Time budget

The power budget analysis ensures that sufficient power is available through out the link to meet the application. Rise time budget ensures that the link is able to operate for a given data rate at specified BER. All the components in the link must operate fast enough to meet the band width or rise time requirements.

The rise time of the light source is specified by the manufacturer. The typical values of the rise time for MLM laser is 0.1-1.0 ns. The chosen fibre must have low pulse spreading to achieve longer transmission distance. Finally the receiver rise time should also be as low as possible.

Component	Rise time	Worst case value
MLM Laser diode (t_s)	0.1-1.0 ns	1 ns
G 652 fibre (t_f)	< 3.5 ps/km	0.05
APD receiver (t_r)	< 0.14 ns	0.14

Table-9 The rise time data for source cable and receiver

The rise time data for source, cable and receiver supplied by the manufacturer is shown in the above table-9

The total system rise time is given by Ts= Root ( t_s²+t_f²+_tr²)

For 80 Km, t_f = 3.5 X80 = 0.28ns

Ts= Root ( 1+0.28²+0_.14²) = 1.048 ns

The system band width = 0.7/Ts

= 0.7/1.048 ns = 667.9 Mbps

This rise time budget shows that the selected components can be used to design a SDH, STM-1 link because the bit rate of STM-1 is 140 Mbps

In BSNL the optical fiber transmission systems are classified as Short-haul, Long-haul, Very long-haul, and Ultra long-haul. The distance range of the above systems are shown in the table below

The optical fiber link designed using 140 Mbps O.F .system belongs to Long-haul. And according to ITU-T classification the system belongs to the class L.1. Here the ‘L’ shows that it is a long-haul system and the ‘1’shows that the system is operating on 1310 nm using G652 cable (single-mode cable).

Here the system is connected using the already spliced cable with an attenuator of 10 dB in the receive direction which is equivalent to operating the system in the full range of 80 km.

Fig. 12:

The system is verified using DTA for no alarm condition which shows the designed link is working satisfactorily.

To judge the quality of the link the E1 stream of the system is subjected to ITU-G821 performance analysis.

6. performance measurements.

Transmission errors are common in communication systems due to different reasons like noise, interference, inter-modulation, echoes, signal fading, equipment limitations, etc.

Though optical fiber medium is considered to be the best medium, we know that practical media cannot be hundred percent error free.

Furthermore, the tolerance of the disturbance depends on the type of service carried by the circuit. To check the quality of the system as well as the medium ITU-T in its G821 guidelines recommends a set of tests to be taken. In BSNL a system is declared as commissioned if and only if the system survives these tests. The performance parameters to be tested are :

(1)Bit Error Ratio.(BER)

Bit-Error-Ratio is defined as the ratio of the number of bits received in

error to the total number of bits transmitted in a specified time

interval.

(2) Error Seconds(ES)

A second with at least one anomaly or defect is called Error Second.

(3) Severely Errored Seconds(SES)

Severely Errored Seconds is defined as the errored seconds with BER

greater than or equal to 10^-3

(4) Degraded Minutes(DM)

Degraded Minutes is a group of 60 consecutive seconds after excluding SES, with a BER of 10^-6 or worse. Hence a DM will have at least 5 errors, assuming a data rate of 64 kbit/s.

(5)Available Seconds(AS)

The measure of percentage of time for which the circuit is available for use in an error free condition is called Available Seconds.

(6)Unavailable Seconds(US)

If the error activity continues at an excessive level for a significant period of time ( say 10 seconds or more )then the circuit is considered to be unavailable. Un available Seconds is a measure of percentage of time the circuit is not available for use.

The 140 Mbps optical fiber link designed for the study is then subjected to G821 analysis for three hours continuously to ascertain the quality and stability of the designed link. The readings obtained are tabulated below.