SOURCES AND DETECTORS
The optical source is often considered to be the active component in an optical fiber communication system. Its fundamental function is to convert electrical energy in the form of a current into optical energy (light) in an efficient manner which allows the light output to be effectively launched or coupled into the optical fiber. Three main types of optical light source are available. These are:
ü wideband ‘continuous spectra’ sources (incandescent lamps);
ü monochromatic incoherent sources (light-emitting diodes, LEDs);
ü monochromatic coherent sources (lasers).
To aid consideration of the sources currently in major use, the historical aspect must be mentioned. In the early stages of optical fiber communications the most powerful narrowband coherent light sources were necessary due to severe attenuation and dispersion in the fibers. Therefore, gas lasers (helium-neon) were utilized initially. However, the development of the semiconductor injection laser and the LED, together with the substantial improvement in the properties of optical fibers, has given prominence to these two specific sources