Effect of laser linewidth reduction systems on coherence length and interferometer noise

We analyze the effect of linewidth reduction systems on the degree of coherence of a laser and the noise levels measured at the output of an interferometer. We show results computed from the power spectral density of frequency noise measured on a DFB fiber laser and a DFB semiconductor laser narrowed with electrical feedback. Results show that the increase of coherence and interferometer noise is significant only over specific range of interferometer length, and that the improvement of interferometer noise might be proportionally much less important than the improvement in linewidth and coherence length.