The outlines of an optical communication system are given as below. The sound signal from different subscribers are multiplexed and converted into digital form by the ADC/ENCODER. The signal is then modulated using a pulse modulation technique and given by Electrical to optical (E-O) converter. This device produces modulated optical signals. These signals are transmitted through an optical fibre to the desired destination.
Optical fibres are used for transmission of optical signals in the same manner as coaxial cables for radio wave transmission. The main advantage of using optical frequencies as the carrier is that a very high density information transmission becomes possible at these frequencies. In addition to this, the optical fibres also have extremely low loss of nearly 0.2 db/Vm, which permits larger repeater spacing. Other advantages of optical fibres are their much smaller size, light weight as compared to coaxial cables. They require much less duct space and transportation cost. Since they are made of
di-electronic materials they are immune to electromagnetic interference,short-circuits/ground loops and are free from cross-talks.
Block diagram of an Optical Communication System
They are more tolerant to hostile temperature environments and are very much cost effective, as compared to other transmission media for a large volume of information traffic.
The above illustration shows the geometry of an optical fibre. The fibre consists of a central core made of silica glass or poly-styrene/Perspex. In a silica fibre, the cladding is also of silica glass, but its refractive index is lower than the refractive index of the core. In the poly-styrene /Perspex cable, the cladding is also of the same plastic material. Silica fibres have a better performance at higher cladding. Silica fibre has a better performance at higher bit rates but their cost is high. The plastic fibres have higher attenuation and low temperature withstanding capability and can be used for short distance transmission.
The core has a diameter of 4 - 100 m and cladding is about 100 - 200 m. The standard overall diameter of the optical fibre is 125 m.
A light ray injected through the air-core interface of the core will undergo total internal reflection, if it is incident at an angle '' smaller than the critical angel c for that interface,
Where, c is the critical angle and given as n1 and n2 are the
refractive indices of the core and cladding respectively.
At the receiver end, an optical detector is employed to convert optical signals to electrical form. These signals are amplified and de-modulated, de-coded/de-multiplexed. The resultant digital signals are converted into analogue form by the Digital to Analog converter and given to the subscribers.
The progress in optical fire communication has been so fast that developments within a short period may be classified into three generations.
The first generation of optical communication employed 8 - 0.9 m light signals produced by light emitting diodes (LED) or laser diodes (LD) and multimode fibres. Avalanche and PIN diodes were used as photo-detectors. This system required a repeater spacing of 10 km at data rates of 100 Mbps. The Ga Al As/Ga As and Si were used as semiconductor materials for the light source.
The second generation of optical fibre communication employed longer wavelengths of 1.3 and bandwidths of 100 Gbps/km as compared to 1 G.b.ps/km for the first generation and repeater spacing of about 100 km at 100 Mbps data transmission rate. The semiconductor materials used for light source at this wavelength are Ga. In As P/In Ge in large capacity system, single mode fibre transmission is employed using laser diode (LD). For small capacity compact systems, multimode fibres with laser diodes or light emitting diodes are employed.
The third generation optical fibre communication employs wavelength of 1.5 - 1.65 m and single mode fibres. The semiconductor material used for light source is same as used in second generation. The multimode laser diode gives a bandwidth of 5 Gbps/Km, whereas the integrated dynamic single mode (DSM) laser diodes give a bandwidth of 185 Gbps or higher giving a further increase of repeater spacing.
Among the important application of optical fibre system are:
International communication
Inter-city communication
Inter-exchange communication
Data links
Domestic communication
Plant and traffic control
Defense application like communication, night vision, thermal imaging, laser radar
m and cladding is about 100 - 200 
' smaller than the critical angel 
c is the critical angle and given as
n1 and n2 are the 
m and single mode fibres. The semiconductor material used for light source is same as used in second generation. The multimode laser diode gives a bandwidth of 5 Gbps/Km, whereas the integrated dynamic single mode (DSM) laser diodes give a bandwidth of 185 Gbps or higher giving a further increase of repeater spacing.