In coherent lightwave communication systems, a narrow line width single frequency laser is required for phase shift keying and homodyne schemes. To obtain sub-MHz linewidth, distributed-feedback (DFB) lasers have been made with a cavity length longer than one millimeter.1,2 Even with multiple-quantum well (MQW) active layer, the threshold current of these lasers is 20 mA or higher. By incorporating compressive strain into the quantum well, both the threshold current3 and the line width enhancement factor4 are expected to be reduced theoretically. Recently, we have reported sub-milliampere- threshold compressive-strained MQW Fabry- Perot lasers at 1.5 μm wavelength.5 In this work, we are reporting the sub-MHz (600 kHz) DFB lasers with a low threshold current of 5.5 mA by the use of the compressive-strained MQW active layer and the strong grating.

© 1991 Optical Society of America

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