Semiconductor lasers used in coherent lightwave communication systems require a linewidth of the order of 1 MHz or less. Previously, linewidths of a few kilohertz to a few hundred kilohertz have been achieved by the use of semiconductor lasers with a grating-loaded external cavity or by optical feedback. Alternatively, a linewidth of 1.5 MHz has been obtained in a distributed-feedback (DFB) laser hybridly coupled to a short-graded-index rod external cavity (GRECC).1,2 A similar value has been achieved with a BH laser integrated with a passive cavity.3 More recently, a new DFB laser has been monolithically integrated with a waveguide cavity which provides the optical feedback.4 The optical path length in the waveguide cavity can be tuned electrically to control the phase of the feedback to achieve linewidth narrowing. We report here the measured characteristics of linewidth narrowing and FM response of this new device.

© 1987 Optical Society of America

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