In Optical transmission, coherent light sources have clear advantages over incoherent light sources. The word Coherent means logical or consistent or clear. Incoherent is opposite of Coherent. Thus incoherent is being not logical or inconsistent or not clear. In optical communication, an incoherent light is not consistent, while a coherent light is consistent in its output.
In optics, Coherent light is light in which the photons are all well streamed. In other words the change of phase within the beam occurs for all the photons at the same time. There are no abrupt phase changes within the beam. Light produced by lasers is both coherent and monochromatic . Monochromatic means being of one color.
Laser diodes gives coherent output of light in fiber optic communication. The light that comes from a laser, however, is coherent, parallel, monochromatic and in unbroken wave chains. Hence the light coming out from a Laser can travel longer distances before distortions happen.
Incoherent light sources on the other hand emit light with frequent and random changes of phase between the photons. Best examples that we see daily are Tungsten filament lamps and ordinary fluorescent tubes. They emit incoherent light. Incoherent light is broken and consist of slightly different wavelengths. Light from an incoherent source for example a light emitting diode has more spectral width compared to the one from Laser diodes.
Conventional light sources are incoherent sources. The transitions between energy levels in an atom is a completely random process and so there is no control over the process in which an atom lose the energy in the form of radiation. In contrast to the coherent light, light from an incoherent source travels less distance. Incoherent light gets affected by dispersion easily compared to the coherent light.
Coherent light sources are more expensive than the incoherent light sources. High bit rate optical transmission systems take advantage of the coherent technology. Low data rate transmission can be economical by employing incoherent light sources such as LEDs.