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Saturday, 28 September 2019

Optical Fiber Communication System Block Diagram

General Light Wave System:


Communication systems that use high carrier frequencies in the near IR region of visible spectrum are called optical communication systems or general light wave systems. Light wave system that employs optical fibre as channel for information transmission is called ‘fibre Optics Communication Systems’. The technology to which light is propagated through very fine cylindrical hair like transparent fibres is called fibre optics. Optical communication system or light wave system can be broadly classified based on the nature of the communication channel into two.

1.   Unguided Systems:

The optical beam emitted by the transmitter or optical signal propagates through air or vacuum. It is less suitable for broadcasting applications since optical beams spreads out mainly in the forward direction. Hence require accurate pointing between transmitter and receiver.

2.   Guided Systems:

The optical beam emitted by the transmitter remains spatially confined.

Eg: Transmission using Optical Fibres.

Basic Optical Fiber Communication System

Information Source provides the input electrical signal.
Optical Fiber Communication System Block Diagram
Electrical Transmitter contains electrical stage which drives an optical source to give modulation of light wave carrier.

Optical Source provides electrical to optical conversion can be LED’s on laser. Requirements are

1. High output power
2. High linearity
3. Narrow spectral width
4. High modulation rate
5. Temperature stability
6. Long life time



Optical fibres used as transmission medium to compensate for losses during transmission repeatrs or optical amplifier can be used at regular intervals. Required characteristics are low dispersion, lower fibre non linearity, low attenuation, high optical signal to noise ratio, large repeater span.

Optical detector detects and convert optical signal to proportional electrical signal.
Eg: Photo diodes, Photo transistors etc

Requirements are sensitive at operating wavelength, requirements are sensitive at operating wavelength, low power consumption and operating voltage, fast response active area match fibre parameter, temperature stability small size and cost capability of internal gain, low noise.

Repeaters and Optical Amplifier:

To compensate for signal degradation in long distance convert optical signal to electrical signal, restores the signal used then converting back to optical signal for further transmission. This method increases cost, complexity and reduces operational bandwidth. Optical amplifiers simply amplify the optical signal. They provide improved SNR due to all optical domain operation.

Fibre Couplers and Fibre Connectors are used to distribute light from main fibre into one or more branches of fibres and to convert one fibre with another.

Advantages of Optical Fibre Communication:

Wider Bandwidth
Small Size and weight
Electrical Isolation
Immunity to interference and cross stock (free from EMI, RFI, EMP)
Signal Security
Low transmission loss
Flexibility
System Reliability

Disadvantages of Optical Fibre Communication:

Cost
Signal distribution
Difficulty in installation and maintenance
Sensitivity

Fibre Birefringence

Modes propagate with different phase velocities in an optical fiber. The difference between their effective refractive indices is called fibre birefringence (βf).
βf = nx - ny

Where nx = Effective refractive index of x mode
ny = Effective refractive index of y mode

Fibre Beat Length:

When light is injected to the fibre such that both nodes are excited one mode will be delayed in phase relative to the other mode as they propagate through the fibre. When the phase difference is an integral multiple of 2π the two modes will beat at that point, and the input polarization state is reproduced. The length over which this beat occurs is called the fibre beat length (LB).

LB = 2π/(βxβy) = λ/ (nx - ny) = λ/βf

Where βx and βy = Propagation constants of two orthogonal polarization modes.

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