Study on Transmission of Audio Signal using Laser Communication System

Abstract

Laser communication systems are wireless connections through the atmosphere and it works on the principle of "Amplitude Modulation" process. In the process of amplitude modulation, the amplitude of the carrier is varied according to the instantaneous amplitude of the input modulating signal. In this system carrier signal corresponds to laser beam, amplitude stands for intensity of laser beam and input signal refers to audio signal. Hence, the intensity of the laser beam is varied according to the instantaneous value of audio signal and the same is sensed by the optical sensor at the receiver. In present investigation laser communication system consists of two sections Transmitter and Receiver. At the transmitter audio signal is coupled to the laser light by using a transistor operating in common collector mode. Since, in common collector mode transistor acts as Impedance matching device, therefore intensity of the laser beam changes in proportion to audio signal strength. At the receiver this varying intensity is sensed by solar cell. The photo transistor is biased and connected to the input of audio amplifier (LM386) which drives the loud speaker. In present study frequency response and gain of received signal have been studied by varying the medium (air, fresh water, salty water and muddy water), its density and the distance between transmitter and receiver.