1. CHRONOLOGICAL DEVELOPMENT OF OPTICAL FIBER COMMUNICATION

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. With the LASER coming into the picture the research interest of optical communication got a stimulation. A powerful coherent light beam together with the possibility of modulation at high frequencies was the key feature of LASER. Kao and Hockham proposed the transmission of information via dielectric waveguides or optical fiber cables fabricated from glass almost simultaneously in 1966. In the earlier stage optical fibers exhibited very high attenuation (almost 1000 dB/km)which was incomparable with coaxial cables having attenuation of around 5 to 10dB/km. Nevertheless, within ten years optical fiber losses were reduced to below 5dB/km and suitable low loss jointing techniques were perfected as well. Parallely with the development of the optical fibers other essential optical components like semiconductor optical sources (i.e. injection LASERs and LEDs) and detectors (i.e. photodiodes and phototransistors) were also going through rigorous research process. Primarily the semiconductor LASERs exhibited very short lifetime of at most a few hours but by 1973 and 1977 lifetimes greater than 1000 hr and 7000 hr respectively were obtained through advanced device structure.
The first generation optical fiber links operated at around 850 nm range. Existing GaAs based optical sources, silicon photo detectors, and multimode fibers were used in these links and quiet understandably they suffered from intermodal dispersion and fiber losses. With the advent of optical sources and photo detectors capable of operating at 1300 nm, a shift in transmission wavelength from 850nm to 1300nm was possible which inturn resulted in a substantial increase in the repeaterless transmission distance for long haul telephone trunks. Systems operating at 1550nm provided lowest attenuation and these links routinely carry traffic at around 2.5Gb/s over 90 km repeaterless distance. The introduction of optical amplifiers like Erbium-doped fiber amplifiers (EDFA) and Praseodymium-doped fiber amplifiers (PDFA) had a major thrust to fiber transmission capacity. The use of Wavelength Division Multiplexing along with EDFA proved to be a real boost in fiber capacity. Hence developments in fiber technology have been carried out rapidly over recent years. Glass material for even longer wavelength operation in the mid-infrared (2000 to 5000nm) and far-infrared (8000 to 12000nm) regions have been developed. Furthermore, the implementation of active optoelectronic devices and associated fiber components (i.e. splices, connectors, couplers etc.) has also accelerated ahead with such speed that optical fiber communication technology would seem to have reached a stage of maturity within its developmental path.