Abstract

Spectral linewidth of a semiconductor laser should be reduced for coherent optical communication, coherent optical measurement, and so on. One of the popular technique for linewidth reduction is to use an external mirror or an optical fiber, which has been called an optical feedback technique. The linewidth, as narrow as 1 kHz, has been realized by this technique[1]. However, it presents several problems, e.g., oscillation instabilities can be induced by the phase fluctuations of the reflected light. To overcome these difficulties, a novel and simple technique of electrical feedback is proposed here, in which the quantum FM noise can be directly controlled by the injection current. The linewidth can be stably reduced by this technique without changing the cavity structure. It has been pointed out that the electrical feedback can reduce the linewidth to a value limited by the noise of the feedback loop[2], which is far narrower than the one given by the modified Schawlow-Townes formula. This is a consequence of the analysis by using a quantum mechanical Langevin equation. From these reasons, the electrical feedback can be a more promising technique to realize an ultranarrow linewidth.

© 1985 Optical Society of America