Spectral linewidth reduction in semiconductor lasers by an external cavity with weak optical feedback

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Abstract
The linewidth narrowing of a semiconductor laser due to weak optical feedback is analysed, taking into account both phase condition and threshold change for the feedback-induced modes. The achievable linewidth reduction lies in between two limiting cases, 1/(1 + X √(1 + α2))2 and 1/(1 + X)2, where α and X are the linewidth enhancement factor and the feedback parameter, respectively.