Line narrowing in a single-mode injection laser due to external optical feedback

The effect of external optical feedback on the linewidth of a single-mode injection laser is considered theoretically. A set of three rate equations with Langevin noise sources is used to obtain the power spectrum. If the high-frequency structure in the power spectrum is ignored, the line shape is Lorentzian and exhibits broadening or narrowing depending on the external-cavity phase shift. Particular attention is paid to the line narrowing after including the effect of carrrier-induced index changes.