The requirement of the linewidth of semiconductor lasers used in coherent optical communication systems is very stringent. DFB semiconductor lasers operating in the free-running state in the longer-wavelength region typically have lens of megahertz linewidths, which are still too wide to be useful. The electrical negative feedback control of the laser frequency has been proposed as one method for narrowing the linewidth. A 200-kHz linewidth of 1.5-μm DFB lasers has been achieved by using this scheme.1

© 1987 Optical Society of America

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