The development of single frequency 1.5 μm laser sources, e.g. distributed feedback (DFB) and coupled cavity (C3) lasers, has dramatically increased the capacity and performance of optical communication systems. These lasers, however, have several drawbacks. Firstly, DFB and lasers are more complex and hence are less reliable than regular Fabry-Perot lasers. Secondly, they suffer from chirp, i.e. a wavelength shift as the lasers are modulated. Thirdly, conventional single frequency lasers have rather broad linewidths, ~50–100 MHz which make them unsuitable as sources in coherent communication system. Further, DFB and C3 lasers are sensitive to optical reflections which may introduce mode partition noise, amplitude noise, and coherence collapse.

© 1987 Optical Society of America

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