Optical Communication System:-
An Optical fiber transmission link comprises the elements shown in the given figure.
The key sections are
- A transmitter consisting of a light source and its associated drive circuitry.
- A cable offering mechanical and environmental protection to the optical fibers contained inside it.
- A receiver consisting of a Photo detector plus amplification and signal restoring circuitry.
Additional components include optical connectors, splicers, couplers (or) beam splitters and repeaters.
optical fiber is one of the most important elements in an optical fiber link. The cable may contain copper wires for powering repeaters which are needed for periodically amplifying and reshaping the signal in long distance communication.
The cable generally contains several cylindrical hair-thin glass fibers, each of which is an independent communication channel.
Similar to copper cables, the installation of optical fiber cables can be either aerial, in ducts, under sea (or) buried directly in the ground.
As a result of installation and (or) manufacturing limitations, individual cable lengths will range from several hundred meters to several kilo meters for long distance applications.
The real size and cable weight determines the actual length of a single cable section.
Cable in ducts—- shorter length
Aerial/ buried applications—– longer lengths.
The complete long-distance transmission line is formed by splicing (or) connecting together these cable sections.
In optical fibers attenuation is a function of wave length .
In early stages of technology, optical fibers were used in
First window: (800nm-900nm) wave length.
Later on optical fibers are used in the long-wave length region.
Long-wave length region (1100-1600) nm
Second window-centered around 1300nm.
Third window- centered around 1550nm.
Once the fiber cable is installed a light source (which is dynamically compatible with the fiber cores) is used to launch power into the fiber.
The electric i/p signal is either analog (or) digital form the Transmitter circuit converts this electric signal to an optical signal.
Optical source is a square-law device. In (800-900) nm region the light source is made up of
Ga Al As and in long distance region (1100nm-1600nm) In Ga AsP is the alloy used.
after an optical signal (light) has been launched into the fiber, it will be attenuated and distorted with increasing distance because of scattering, absorption and dispersion mechanisms in the wave guide.
The attenuated and distorted , modulated optical power emerging from the fiber end will be detected by photo diode (or) photo detector.
Photo detector converts optical power into electrical signal (it also uses a square-law).
photo detectors are PIN diodes, Avalanche photo diodes and the type of material it is made up of is In Ga As.
further the electrical signal will be amplified and restored.
therefore the design of the receiver is more complex than that of transmitter.