OPTICAL FIBER COMMUNICATION IN INDIAN RAILWAY

INVERTIS UNIVERSITY

OPTICAL FIBRE COMMUNNICATION IN INDIAN RAILWAY

presented By: SANJU VERMA

CONTENTS About the ORGANISATION history Optical Fiber Basic Physics of OFC Specification of OFC in INDIAN RAILWAY Principle of Total Internal Reflection Types of Fibers Jointing and Termination of OFC Measurement and system testing Losses in Optical fiber Advantages Drawbacks Applications conclusion

ABOUT THE ORGANISATION Rail Tel Corporation of India Limited is a Government

Department that comes under the Ministry of Railways.

The Corporation was formed in Sept 2000 with the objectives to create nationwide Broadband Telecom and Multimedia Network in all parts of the country, to modernize Train Control Operation and Safety System of Indian Railways.

Rail Tel has created state of the art multimedia telecom network using SDH/DWDM based transmission systems.

Railway communication provides uninterrupted motion of trains. Due to faster means of communication there is increase in the efficiency and greater control

HISTORY 1870: John Tyndall U.K demonstrated the phenomenon of light guiding in a thin water jet.

1880: Sir Alexander Graham Bell invented the “PHOTOPHONE”. He used sunlight reflected from thin voice modulated mirror to carry conversation.

1960: Advent of LASER as a coherent optical source.

1966: Kao and Hockman discovered that high loss in fibers was due to impurities in the material used.

1966: Mr. Kao gave thought of communicating using fiber with loss up to 1000 dB/km.

1970: M/s Corning glass works; USA made fiber giving loss of 20dB/km.

HISTORY 1974: Modified chemical vapour Deposition (MCVP) process developed by M/s Sumitomo electric & NTT/JAPAN.

1976: First generation systems multimode graded index fiber 850nm (wavelength), GaAs laser / LEDs as a source & PIN/APD as detector.

1980: Second generation systems multimode fiber at 1300 nm single mode fiber.

1990: Development of longer wavelength fibers, improved version of LASER, APDs for better performance and to support 565 M bits/sec & 2 G bits/sec.

OPTICAL FIBRE

Optical fiber is generally made of glass & is made into very thin fibers or hair size.

OPTICAL SOURCES AND DETECTOR An optical fiber communication system consists of

transmitter, which converts the multiplexed electrical signal into an optical signal. A source of light launches the optical signal through a coupler into the fiber. The fiber carries this signal to the receiver, where another coupler couples the light from the fiber to the detector. The transmitter uses either a LASER DIODE or LIGH EMITTED DIODE (LED) for electrical to optical conversion. The receiver uses either a PIN diode or an AVALANCHE DIODE (APD) for electrical conversion.

STRUCTURE OF OPTICAL FIBER CORE – Thin glass center of the fiber

where the light travels . CLADDING – Outer optical material

surrounding the core that reflects the light back in to the core.

BUFFER COATING – Plastic coating that protects the fiber from damage and moisture.

SPECIFICATIONS OF OFC USED IN INDIAN RAILWAY In Indian Railways, 24 FIBER ARMOURED Optical Fiber

Cable. General Requirement Of Cable -The cable shall consist

of 24 mono mode fiber and shall be suitable for direct underground burial as well as mechanized laying in the duct.

Service Condition Optical Fiber cable shall be able to withstand the following environmental conditions.

Ambient temperature 0 to + 550 C Storage temperature -200 C to + 700 C The overall diameter of the cable shall not be more than

20 mm and uniform throughout the length from top to end.

Fiber& Unit Identification Fibers The 6 loose tubes have the following colors :1-Blue,2-Orange,3-Green,4-Brown,5-Slate,6-White.

PRINCIPLE OF TOTAL INTERNAL REFLECTION Angle of incidence is greater than

critical angle ,the light is totally reflected back in to the incidence higher refractive index medium , a process known as TIR.

TYPES OF FIBERS There are two general categories of

optical fiber in use today, multimode fiber and single-mode fiber.

Single mode fiber only one mode is propagated.

Multimode fiber , has a larger core than single-mode fiber. It gets its name from the fact that numerous modes, or light rays, can be carried simultaneously through the waveguide.

.

BASIS OF REFRACTIVE INDEX Step index fiber - there is a uniform

index of refraction throughout the core; Thus there is a step in the refractive index where the core and cladding interface.

Single mode step index fiber cable has low inter mode dispersion where as, multimode step index fiber cable has more dispersion.

.

GRATED INDEX FIBER An optical fiber whose core has a refractive

index that decreases with increasing radial distance from the optical axis of the fiber.

Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fiber. The most common refractive index profile for a graded-index fiber is very nearly parabolic.

The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion.

. Multi-mode optical fiber can be built with either graded index or step index . The advantage of the multi-mode graded index compared to the multi-mode step index is the considerable decrease in modal dispersion . Modal dispersion can be further decreased by selecting a smaller core size (less than 5-10μm) and forming a single mode step index fiber.

Modal dispersion is a distortion mechanism occurring in multimode fibers in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes.

JOINTING AND TERMINATION OF OFC There are two methods for jointing OFC SPLICING CONNECTORS

.

CONNECTORS Connector facilitates re – mate able

connections. Connectors are used in where flexibility is

required in routing an optical signal from lasers to receivers.

It consists of four parts Ferrule Connector body Cable Coupling device

MEASUREMENT AND SYSTEM TESTING

LOSSES IN OPTICAL FIBER ATTENUATION ABSORPTION DISPERSION BEND LOSS SCATTERING LINK BUDGET POWER BUDGET

ATTENUATION Means loss of light energy as the light

pulse travels from one end of the cable to other is known signal loss or fiber loss .

It also decides the number of repeaters required between transmitter and receiver.

It is directly proportional to the length of the cable.

ABSORPTION Absorption is a major cause of signal loss in an

optical fiber. It is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. In optical fibers it is explained by three factors:

Imperfections in the atomic structure of the fiber material

The intrinsic or basic fiber-material properties The extrinsic (presence of impurities) fiber-

material properties Imperfections in the atomic structure induce this

assimilation process by the presence of missing molecules or oxygen defects. It is also induced by the diffusion of hydrogen molecules into the glass fiber.

DISPERSION As an optical signal travels along the fiber, it becomes

increasingly distorted. This distortion is a sequence of intermodal and intra modal dispersion.

Two types: 1. Intermodal Dispersion 2. Intra modal Dispersion

Intermodal Dispersion: Pulse broadening due to intermodal dispersion results from the propagation delay differences between modes within a multimode fiber.

Intra modal Dispersion: It is the pulse spreading that occurs within a single mode. Its of two types Material Dispersion and Waveguide Dispersion .

. Material Dispersion: Also known as spectral dispersion or chromatic dispersion. Results because of variation due to Refractive Index of core as a function of wavelength, because of which pulse spreading occurs even when different wavelengths follow the same path.

Waveguide Dispersion: Whenever any optical signal is passed through the optical fiber, practically 80% of optical power is confined to core & rest 20% optical power into cladding

BEND LOSS The loss which exists when an optical fiber

undergoes bending is called bending losses. There are two types of bending -

Macroscopic bending- In which complete fiber undergoes bends which causes certain modes not to be reflected and therefore causes loss to the cladding.

Microscopic Bending- Either the core or cladding undergoes slight bends at its surface. It causes light to be reflected at angles when there is no further reflection.

SCATTERING LOSS It occurs due to microscopic variations in the

material density, compositional fluctuations, structural in homogeneities and manufacturing defects.

A. Linear Scattering - It has of 3 types are following Rayleigh Scattering losses ,Mie Scattering Losses and Waveguide Scattering Losses

B. Non-linear Scattering –It has of two types are following Stimulated BRILLOUIN Scattering(SBS) and Stimulated Raman Scattering(SRS)

LINEAR SCATTERING Rayleigh Scattering Losses: These losses are

due to microscopic variation in the material of the fiber.

Unequal distribution of molecular densities or atomic densities leads to Rayleigh Scattering losses .

Glass is made up of several acids like SiO2, P2O5,etc. compositions, fluctuations can occur because of these several oxides which rise to Rayleigh scattering losses .

. Mie Scattering Losses: These losses results from the compositional fluctuations , structural in homogeneous, defects created during fiber fabrications, causes the light to scatter outside the fiber.

Waveguide Scattering Losses: It is a result of variation in the core diameter, imperfections of the core cladding interface, change in RI of either core or cladding.

NON-LINEAR SCATTERING SBS Scattering: STIMULATED BRILLOUIN SCATTERING

(SBS) may be regarded as the modulation of light through thermal molecular vibrations within the fiber.

P b =4.4x10-3 d2 λ2 α dB v watts ; where, λ= operating wavelength μm ,d= fiber core diameter μm ,v = source bandwidth in GHz

SRS Scattering : Stimulated Raman Scattering is similar to SBS except that high frequency optical phonon rather than acoustic phonon is generated in scattering processes.

P b =5.9x10-2 d2 λα dB watts ; Phonon: Collective excitation in a periodic arrangement of atoms or molecules in solid.

ADVANTAGES  Bandwidth – Fiber optic cables have a much

greater bandwidth than metal cables.  With the high performance single mode cable used by telephone industries for long distance telecommunication.

 Low Power Loss - This allows for longer transmission distances.  In comparison to copper; in a network, the longest recommended copper distance is 100m while with fiber, it is 2000m. 

 Interference - Fiber optic cables are immune to electromagnetic interference. 

 Size - In comparison to copper, a fiber optic cable has nearly 4.5 times as much capacity as the wire cable has and a cross sectional area that is 30 times less. 

.  Weight - Fiber optic cables are much thinner and

lighter than metal wires.  They also occupy less space with cables of the same information capacity.  Lighter weight makes fiber easier to install. 

 Safety - Since the fiber is a dielectric, it does not present a spark hazard. 

 Security - Optical fibers are difficult to tap.  As they do not radiate electromagnetic energy, emissions cannot be intercepted.  Fiber is the most secure medium available for carrying sensitive data. 

 Flexibility - An optical fiber has greater tensile strength than copper or steel fibers of the same diameter.  It is flexible, bends easily and resists most corrosive elements . 

DRAWBACKS  Cost - Cables are expensive to install

but last longer than copper cables.   Transmission - transmission on optical

fiber requires repeating at distance intervals.

 Protection - Optical fiber require more protection around the cable compared to copper cables.

APPLICATIONS CATV(cable television) services are supplied via a fiber

optic network to an optical node, which converts and distributes the electrical signal to subscribers via a coaxial cable connection. CATV applications utilizes both single mode and multimode signals within different areas of the network. Single Mode fiber is used to distribute signal from the central office to optical nodes, where it can be converted to multimode.

Data transmission fiber optics, simply put, is the sending and receiving of data from point-to-point via a network. It ranges from very simple cables connecting servers or storage arrays inside a network or telecommunications system, to large multi-fiber distribution cables supporting intra-building connectivity and beyond

CONCLUSION A huge amount of development can be made by

making further research and work on fiber optics . We need it for a faster and more sophisticated infrastructure which would be the prime demand of the ever growing population of tomorrow. At present there are many optical fiber communication links throughout the world without using OPTICAL SOLITON.

When we introduce OPTICAL SOLITON as light pulses through the fibers, we can achieve high quality telecommunication at a lower cost. We can expect a great revolution in optical fiber communication within a few years by means of SOLITON.

SOLITON in a fiber optic system are described by the MANAKOV EQUATION.

THANK YOU