Semiconductor lasers: A choice structure for lightwave communications

Since its first demonstration in the early 1960s, the semiconductor laser has become the center of a tremendously important communications technology — fiber-optic communications. In terms of structure, design, and operation, the semiconductor laser has much in common with all other forms of lasers. It has an active medium that provides optical amplification (gain) under the appropriate circumstances. This active medium is typically a compound semiconductor from the III-V family (combinations of elements from columns III and V of the periodic chart), such as gallium arsenide (GaAs). The III-Vs are direct energy gap semiconductors that, unlike the silicon used in integrated circuitry, are very efficient light emitters. The feedback cavity for semiconductor lasers is easily obtained by using the natural reflectivity of the parallel cleavage planes of these single-crystal materials. Finally, the semiconductor laser is very efficient and requires only a few volts at modest current levels (200 milliamperes).