Jain V.K - Free Space Optical Communication

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The first experiment of transmitting signal over the atmosphere was conducted by Alexander Graham Bell in 1880. He used sunlight as a carrier to transmit voice signal over a distance of about a few feet. However, the experiment was not successful due to inconsistent nature of the carrier. Later, in the 1960s, Theodore H. Maiman discovered the first working laser at Hughes Research Laboratories, Malibu, California. From this point onward, the fortune of FSO has changed. Various experiments were conducted in military and space laboratories to demonstrate FSO link. In the 1970s, the Air Force sponsored a program known as Space Flight Test System (SFTS) to establish satellite to ground link at Air Force ground station, New Mexico. The program was later renamed as Airborne Flight Test System . This program achieved its first success in the 1980s where a data rate of 1Gbps was demonstrated from aircraft to ground station. After this, a flurry of demonstrations were recorded during the 1980s and 1990s. They include Laser Cross-Link Subsystem (LCS), Boost Surveillance and Tracking System (BSTS), Follow-On Early Warning System (BSTS), and many more [], respectively.

Very large-scale development is carried out by NASA in the USA, Indian Space Research Organisation (ISRO) in India, European Space Agency (ESA) in Europe, and National Space Development Agency (NSDA) in Japan. Demonstrations have established a full duplex FSO link with high data rates between various onboard space stations and ground stations, inter-satellite, etc. with improved reliability and 100% availability. Besides FSO uplink/downlink, extensive research is carried out for FSO terrestrial links, i.e., link between two buildings to establish local area network segment that will provide last mile connectivity to the users (Fig.).

FSO communication is well suited for densely populated urban areas where digging of roads is cumbersome. Terrestrial FSO links can be used either for short range (few meters) or long range (tens of km). Short-range links provide high-speed connectivity to end users by interconnecting local area network segments that are housed in building separated within the campus or different building of the company. Long-range FSO communication links extend up to existing metropolitan area fiber rings or to connect new networks. These links do not reach the end user but they extend their services to core infrastructure. FSO communication system can also be deployed within a building, and it is termed as indoor wireless optical communication ( WOC ) system . This short-range indoor WOC system is a futuristic technology and is gaining attention these days with the advancement of technology involving portable devices, e.g., laptops, personal digital assistants, portable telephonic devices, etc.

Indoor wireless optical communication links provide a flexible interconnection within a building where setting up a physical wired connection is cumbersome. It consists of lasers or light-emitting diodes as transmitter and photodetectors as the receiver. These devices along with their drive circuits are much cheaper as compared to radio-frequency equipment or existing copper cables. Further, indoor WOC is inherently secure technology since the optical signals do not penetrate walls unlike electromagnetic waves which can cause interference and thus provides a high degree of security against eavesdropping. These optical waves are either in the visible light spectrum or in the IR spectrum which is able to provide very large (THz) bandwidth. Since these devices consume very little power, they are also suitable for mobile terminal systems. Besides many advantages, indoor optical wireless system is influenced by various impairments that impact the performance of the communication system. Some of the factors that lead to these impairments are (i) limiting speed of optoelectronic devices; (ii) large path loss; (iii) noisy indoor environment due to incandescent, fluorescent lighting or sunlight that contributes to noise in the detector; (iv) multipath dispersion; and (v) interference due to artificial noise sources. The range of the system is restricted as the average transmitted power is limited due to eye safety regulations [].