Abstract

We present and demonstrate a method that uses intracavity saturable absorptive wave mixing of the oscillating beams in lasers to reduce the oscillation linewidth and to obtain a single longitudinal mode operation. The demonstration is done with an erbium-doped fiber laser. The narrowing effect seems to be surprising since we know that coupling and interference give rise to spatial hole burning that cause a degradation of the oscillation quality of lasers, a reduction of the coherence, and an increase of the linewidth. However, these problems result from saturation of an amplifying medium and for saturation of an absorber the outcome is quite different. This can be understood by considering the reflections from the induced saturable gain grating due to the interference pattern. These reflections interfere destructively (180° out of phase) with the corresponding copropagating waves¹ and therefore they provide a negative feedback, which in turn tends to eliminate the grating (from which they originate) by a reduction of the coherence. However, in a saturable absorber the nonlinear wave mixing has an opposite effect. The reflections from the induced saturable absorbing grating are in phase and provide positive feedback. The same conclusion can be obtained¹ by realizing that the absorption is lower for light intensity that has a nonuniform periodic distribution along the saturable absorber, than it is in the uniform case. Therefore, in the presence of a saturable absorber, coherence and single longitudinal mode operation induce the standing wave and are thus favored. Moreover, the effect on a third wave with a different frequency is the opposite: the absorption is higher for nonuniform distribution than for the uniform case. Thus the saturable absorber favors elimination of all other longitudinal modes. We note that passive mode-locking in such a system that contains erbium-doped fiber as a saturable absorber cannot be simply used for passive mode-locking because of its long time response (≈10 msec).

© 1994 IEEE