Experimental demonstration of linewidth reduction of laser diode by compact coherent optical negative feedback system

We demonstrate the linewidth reduction of a laser diode from 6.4 MHz to 6.5 kHz using a compact coherent optical negative feedback system, which consists of only a lens and an optical filter. The FM noise power spectral density (PSD) and the relative intensity noise (RIN) of the laser output are measured to discuss the performance of the system. The FM noise PSD is reduced by ∼30 dB at <100 MHz. The RIN of ∼130 dB/Hz is not found to be increased by the feedback at <7 GHz, where the system is driven under negative feedback conditions.