optical fiber communication

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General overview of optical fiber

Like all other communication system, the primary objective of optical fiber communication system also is to transfer the signal containing information (voice, data, video) from the source to the destination. The general block diagram of optical fiber communication system is shown in the figure9.
The source provides information in the form of electrical signal to the transmitter. The electrical stage of the transmitter drives an optical source to produce modulated light wave carrier. Semiconductor LASERs or LEDs are usually used as optical source here. The information carrying light wave then passes through the transmission medium i.e. optical fiber cables in this system. Now it reaches to the receiver stage where the optical detector demodulates the optical carrier and gives an electrical output signal to the electrical stage. The common types of optical detectors used are photodiodes (p-i-n, avalanche), phototransistors, photoconductors etc. Finally the electrical stage gets the real information back and gives it to the concerned destination.



Definition of optical fiber
Optical fiber is a dielectric waveguide or medium in which information (voice, data or video) is transmitted through a glass or plastic fiber, in the form of light. The basic structure of an optical fiber is shown in figure 1. It consists of a transparent core with a refractive index n1 surrounded by a transparent cladding of a slightly less refractive index n2. The refractive index of cladding is less than 1%, lower than that of core. Typical values for example are a core refractive index of 1.47 and a cladding index of 1.46. The cladding supports the waveguide structure, protects the core from absorbing surface contaminants and when adequately thick, substantially reduces the radiation loss to the surrounding air. Glass core fibers tend to have low loss in comparison with plastic core fibers. Additionally, most of the fibers are encapsulated in an elastic, abrasion-resistant plastic material which mechanically isolates the fibers from small geometrical irregularities and distortions. A set of guided electromagnetic waves, also called the modes of the waveguide, can describe the propagation of light along the waveguide. Only a certain number of modes are capable of propagating through the waveguide.




History of optical fiber
The visible optical carrier waves or light has been commonly used for communication purpose for many years. Alexander Graham Bell transmitted a speech information using a light beam for the first time in 1880. Just after four years of the invention of the telephone Bell proposed his photophone which was capable of providing a speech transmission over a distance of 200m. In the year 1910 Hondros and Debye carried out a theoretical study and in 1920 Schriever reported an experimental work. Although in the early part of twentieth century optical communication was going through some research work but it was being used only in the low capacity communication links due to severe affect of disturbances in the atmosphere and lack of suitable optical sources. However, low frequency (longer wavelength) electromagnetic waves like radio and microwaves proved to be much more useful for information transfer in atmosphere, being far less affected by the atmospheric disturbances. The relative frequencies and their corresponding wavelengths can be known from the electromagnetic spectrum and it is understandable that optical frequencies offer an increase in the potential usable bandwidth by a factor of around 10000 over high frequency microwave transmission.