Abstract

A coherent optical system offers two advantages over the conventional system with Intensity modulation and direct detection. First, the receiver sensitivity may approach the shot-noise limit; and, second, the much greater frequency selectivity may allow more channels to be transmitted at close wavelength separation than by conventional wavelength multiplexing in direct detection systems. The frequency selectivity is specially suitable for applications in the local access and exchange networks. Optical sources used in such coherent systems, however, require narrower linewidth and greater frequency stability than that normally exhibited in a solitary semiconductor laser. Various schemes to reduce the linewidth of a semiconductor laser have been demonstrated.1–3 Recently, we reported4 a new coupled-cavity laser consisting of a distributed feedback laser (DFB) coupled to a graded-index rod (GRIN rod). Use of the DFB laser has improved the frequency stability over that of the cleaved-coupled cavity (C3 ) laser previously investigated.3 The linewidth of the new coupled-cavity laser is comparable to or narrower than that obtained with a C3 laser. We report, for the first time, the dynamic linewidth properties of the DFB-GRIN-rod external coupled-cavity (DFB-GRECC) laser under direct high-frequency modulation by the injection current. The implication of the results for its application in a FSK system is discussed.

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