Distributed-Feedback Semiconductor Lasers

Abstract

We have seen in Chapter 6 that a conventional semiconductor laser does not emit light in a single longitudinal mode. In general, the mode closest to the gain peak is most intense, and a few percent of the output power is carried by other longitudinal modes lying close to the gain peak. Furthermore, even when these side modes are reasonably suppressed under CW operation, their power content increases significantly when the laser is pulsed rapidly. Direct modulation of a semiconductor laser at frequencies in the gigahertz range is commonly employed in optical communication systems. In the presence of chromatic dispersion, the unwanted side modes limit the information transmission rate by reducing the fiber bandwidth. It is therefore desirable to devise means for a semiconductor laser to emit light predominantly in a single longitudinal mode even under high-speed modulation. Such Lasers are referred to as single-frequency or single-longitudinal-mode Lasers and in view of their potential application in optical communication systems were studied extensively during the 1980s.1 They were used in commercial lightwave systems by 1990.