Optical sources for fiber transmission systems

Two types of semiconductor devices are available for use as light sources in fiber transmission systems. The simpler device, the light-emitting diode (LED), emits light in many directions and is useful with large core diameter, multimode fibers. The more complicated semiconductor laser emits more collimated light and can couple 10-100 times more power into a multimode fiber and can also be efficiently coupled to small-core single-mode fibers. LED´s will be adequate for use in the more numerous data bus and low-to-medium capacity transmission systems, and will be used whenever possible because of their simpler driving circuit requirements, lower temperature sensitivity. and lower cost. Semiconductor laser with their higher power and narrower emission spectrum will be required for high-capacity and long-span transmission systems.