Comparison of Different Transmission Media

Topics: Optical fiber, Fiber-optic communication, Laser Pages: 31 (9321 words) Published: June 21, 2013
1.1 Communication
Communication is thus a process by which we assign and convey meaning in an attempt to create shared understanding. This process requires a vast repertoire of skills in intrapersonal and interpersonal processing, listening, observing, speaking, questioning, analyzing, and evaluating. It is through communication that collaboration and cooperation occur. It is the process of transferring information or message like voice, video, text, data, picture, etc. from one distance to another. Communication is a technique/method by which two or more entities exchange their information. The function of communication system is to convey the signal from the information source over the transmission medium to the destination.

1.1.1 Basic operation of communication system
Communication systems convert information into a format appropriate for the communication link. Analog communication systems convert (modulate) analog information signals into modulated signals, which are also analog. Digital communication systems convert information in the form of bits into digital signals. Any communication system is composed of the following basic components, like source encoder, transmitter, channel, receiver, source decoder etc. Source encoder Converts the source or input message into an analog signal or bits called the message signal. Transmitter Converts the message signal into a modulated signal in a format suitable for transmission over the channel.

Channel Bridges the distance between the transmitter and receiver and introduces distortion and random noise. As signal propagates through the channel, it gets attenuated due to transmission loss and distortion due to various nonlinear effect and interference. Receiver Receiver extracts the original message signal or bits from the channel output signal. Source Decoder Converts the message signal or bits back into the format of the original message. In general the information carrying capacity should be high so that it can meet the requirement of the user. The information carrying capacity is closely related to the bandwidth supported by the communication system. The greater the bandwidth, the higher is the information carrying capacity of the communication system. The system should be such that the information can be carried at an affordable cost and the quality of the message received at the destination is sufficiently free from noise and interference. In this respect optical fiber communication system has been developed.

1.1.2 Optical Sources
In most optical communication systems, semiconductor light sources are used to convert electrical signals into light. Optical sources for wireless transmission must be compatible to overcome the atmospheric effects and they should be such that one can easily modulate the light directly at high data rates. Generally either LASERs or LEDs are used in optical communication systems.

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Light Emitting Diode (LED) Light emitting diodes (LEDs) used in optical communication system are the same as visual display LEDs except that they operate in the infra-red region and with many times higher intensity of emission. When the p-n junction is forward biased, photon emission takes place due to recombination of electron-hole pair. The wavelength of emission will depend on the energy gap. Laser Laser stands for “light amplification by stimulating emission of radiation”. Compared to LED, a laser has wider bandwidth, higher power output, higher modulation efficiency, narrower spectral linewidth and narrower emission pattern. Laser sources are much brighter than LEDs.




h12 (in phase)




Figure 1.1: The three keys transition process involved in laser action

Figure 1.1: The three keys transition process involved in laser action. The open circle represents the initial state of the electron and the heavy dot represents the final state. Incident photons are...
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