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Optical Fiber communication system and Application of optical fiber communication system
A technology used for transmitting information (data, speech, music, etc.) from one place to another place that uses light as the carrier of information through Optical Fiber is called Optical Fiber communication.
The simple block diagram of the Optical Fiber communication system is shown in the figure. The optical communication system consists of three main parts:
- Optical transmitter.
- Communication channel.
- Optical receiver.
The information to be transmitted is converted into an electrical signal using a transmitter (i.e. microphone). The electrical signal modulates the intensity of light from the optical source like LED or laser diode using an electro-optic modulator or acousto – optic modulator.
The modulated light signal is sent through the optical fiber to the optical receiver. The received signal is then converted into an electrical signal by the optical receiver using an optical detector like a p-i-n photodetector or avalanche photodetector.
Optical Transmitter
The optical transmitter converts an electrical signal to an optical signal and feeds this optical signal to the communication channel (i.e. optical fiber). The block diagram of the optical transmitter is shown in the figure.
The optical transmitter consists of an optical source, modulator, and channel coupler. Semiconductor laser or light-emitting diodes (LCD) are used as optical sources.
The optical signal is produced by modulating the carrier wave (~100Thz). The coupler is a device that focuses the modulated optical signal onto the entrance plane of the optical fiber. Thus, coupler acts as an optical lens.
Communication channel
The communication channel is a link between the optical transmitter and the optical receiver. In the optical communication system, Optical Fibre acts as a communication channel.
Optical Fiber
Optical Fibre is very thin ( Of the order of the thickness of human hair), the Long and flexible substance of cylindrical shape. Usually, Optical Fiber is made of silica glass.
The silica glass fibre consists of a central core of a very small diameter surrounded by a cladding made of glass of slightly lower refractive index than the refractive index of the core. The core-cladding system is enclosed in a protective jacket made of plastic or polymer.
In an optical fibre, light is transmitted from one end of the fibre to the another and of the fibre by the phenomenon of total internal reflection(T I R).
Types of optical fiber:
- Step-index fibre.
- Graded-index fibre.
1. Step-index fiber
In step-index fiber refractive index of the core is uniform throughout the length of the fiber but its value undergoes an abrupt or steps change at the core-cladding interface.
Step-index fiber is further divided into two categories depending on the mode (path) of propagation of light in the fiber:
- Single-mode step-index Optical Fibre.
- Multimode step-index Optical Fibre.
Single-mode step-index Optical Fiber
The diameter of the central core of single-mode step-index Optical Fibre is so small that light travels through the fibre only in one pass as shown in the figure.
However, this type of fibre is very weak and it is very difficult to join the ends of the fibre together. The refractive index of the core of the single-mode step-index optical fibre is much greater than the refractive index of the cladding.
Multimode step-index Optical Fiber
Multimode step-index fibre is similar to single-mode step-index fibre except the central core is much larger with multimode (multipath) propagation of light through the fibre.
The diameter of the core of multimode step-index Optical Fibre is very large as compared to the diameter of the core of the single-mode step-index Optical Fibre. Therefore, a large amount of light can enter this type of fibre and propagate in multimode through it.
The lights rays striking the core-cladding interface at an angle greater than the critical angle propagate through the core in a zigzag manner, continuously after reflecting off the core-cladding interface.
The light rays striking the core-cladding interface at an angle less than the critical angle enter the cladding and are lost. Therefore, there is a loss in the industry of light in the multimode step-index fiber. The paths followed by propagating light rays in multimode step-index Optical Fibre are shown in the figure.
2. Graded-index Optical Fiber
The refractive index of the central core of graded-index Optical Fibre is non-uniform. The value of the refractive index of the core is maximum at the centre of the four and it is value decrease gradually toward the outer and of the core.
The propagation of light rays through the graded-index Optical Fibre is shown in the figure.
When light enters the optical diagonally, they travel toward the less dense medium from the more dense medium. ( i.e. from the centre of the core to Outer part of the core). As a result, the light rays are continuously refracted and consequently, they suffer branding from their parts.
As the light rays propagate through the core of the fibre, they travel different distances. the Ray of light propagating in the outermost area of the core of the fibre, they travel different distances.
The Ray of light propagating in the outermost area of the core of the fiber travel a greater distance than the Ray of light propagating near the center of the core.
The speed of light is inversely proportional to the refractive index { v = c/n } Where n is the refractive index of the material of the core and C is a velocity of light. Therefore, the speed of light ray travelling near the centre of the core.
Hence, all the Ray of light propagating through the graded-index Optical Fibre takes almost the same time to travel the length of the fiber.
Optical Receiver
Optical receiver Converts optical signal received at the output of the communication channel ( optical fibre) back into the original electrical signal. It consists of a coupler, photodetector and a modulator. The block diagram of the optical receiver is shown in figure
The channel coupler focusses the received modulated optical signal onto the photodetector. Semiconductor photodiode like positive intrinsic negative ( p-i-n ) photodiode and Avalanche photodiode is used as a photodetector. The photodetector converts the optical signal into an electrical signal.
Positive intrinsic negative (p-i-n) photodiode
It consists of a p+ and n- regions separated by an intrinsic (i) region which is lightly doped n-type semiconductor. The schematic diagram of the p-i-n photodiode is shown in the figure.
For normal operation of the p-i-n photodiode, it is significantly high reverse biased so that i-region is fully depleted of charge Carriers. The optical signal enters the device through the p+ region as shown in the figure.
The thickness of a p+ region is very small so that there is minimum absorption of the incident optical signal in this region.
When the incident optical signal (i.e photon) enter the i-region, electron-hole pairs are produced in this region by the photoexcitation process.
The thickness of a region is much greater than the thickness of p+ -region n+ -region so that most of the incident photons are absorbed in this region.
The electron-hole pairs in i – region is produced if the energy of the incident optical signal (incident Photon) is greater than or equal to the forbidden energy gap or band energy gap of the material of i – region.
The holes and electrons produced in i – region drift in opposite directions under the influence of the large electric field due to high reverse bias. The diffused holes in the intrinsic region enter the p+ – region which contributes to the external photoelectric current in the external circuit.
Avalanche photo diode
Avalanche photodiode is operated under reverse Biased condition where the electron-hole pair produced by the optical signal incident on the device can undergo Avalanche multiplication processes.
It consists of a p+ -i-p-n structure, Where n+ -layer is used as a substrate. When the optical signal enters the i-region through the p+region, electrons-holes pairs are generated in this region by the process of photoexcitation.
These carriers are drifted into a high electric field region, called the Avalanche reason. The holes are drifted toward p+ region. These electrons entered into the avalanche region gain sufficient kinetic energy to initiate the impact ionization mechanism which generates secondary electron-hole pair.
Each electron and hole of The generated electron-hole pair May make the multiple numbers of ionizing Collision resulting in a large number of electron and hole during their travel in the Highfield avalanche region.
These carriers (electron and hole) constitute an electric signal that is detected in an external circuit.
The sensitivity and responsibility of an Avalanche photodiode are high as compared to that of the p-i-n photodiode.
7 application of optical fiber communication system
Communication through Optical Fibre has a number of the application over conventional metallic transmission media like conductors in telephony.
1. Wider bandwidth and large information capacity.
The information-carrying capacity of a transmission medium is directly proportional to the carrier frequency used in transmitting a signal.
The optical carrier frequency is in the range of 1013Hz to 1015 While radio waves frequency is about 106 Hz and microwave frequency is about 1010 Hz.
Thus, Optical Fibre communication system provides much higher bandwidth than conventional Radio Communication. Optical Fibre communication is capable of transmitting large data or information over one optical fibre cable.
2. Avoid cross-talk.
Optical Fibers are made of glass or plastic, which are electrically electrical insulators. Therefore, Optical Fibers do not pick up an Electromagnetic wave outside the fiber.
The changing field of e.m. waves are the cause of crosstalk in the metallic conductor. However, there is no crosstalk in optical fiber.
3. There is no static interference
A static noise due to electromagnetic interference caused by lightning, electric motors, relays, power cables, fluorescent light, and other electrical noise sources distort the transmitted signal in the metallic conductor used in communication.
However, such distortion is not present in optical fiber communication as Optical Fibre is a non-conductor of electrical currents.
4. Not affected by environmental changes.
Optical Fibre cables are more resistant to environmental extremes including weather variation than metallic tables. Thus, communication through Optical fiber cable is not affected by environmental extremes.
5. Signal security.
Transmitted through Optical fibre cable does not radiate. As a result, the signal cannot be tapped from the fibre cable. Hence, Optical Fibre communication provides 100% signal security.
6. Low transmission loss.
There is considerable less signal loss in optical fibre than in the metallic cable. Therefore, transmission loss in optical fibre communication is very small.
7. Durable and reliable.
Optical Fibre cables have a long life and more reliable than metallic cables because they have a higher tolerance to changes in environmental conditions and are not affected by corrosive material (liquid and gases).