Electrical feedback and its network analysis for linewidth reduction of a semiconductor laser

The center frequency of the field spectrum of a 1.5-μm InGaAsP distributed-feedback laser was stabilized by negative electrical feedback. The resultant residual frequency fluctuation was σ=3.6×10^-11 for an integration time of 10 s. The linewidth of the field spectrum was simultaneously reduced by using another electrical feedback loop. Its minimum value was 360 kHz, which was 1/27 that of the free-running condition. This linewidth was narrower than the one determined by the magnitude of the spontaneous emission of the free-running laser. The field spectrum was very stable, and the value of the linewidth was constant for more than 40 h. Network analysis was carried out to realize further reductions of the linewidth