3-kHz Spectral Linewidth Laser Assembly with Coherent Optical Negative Feedback

We demonstrate the stable operation of a narrow-linewidth semiconductor laser source based on a coherent optical negative feedback system. The linewidth was reduced to 3 kHz from 13.5 MHz. The power spectral density of frequency modulation (FM) noise was reduced by 35 dB, while the relative intensity noise was low enough for a light source for optical communications (less than -140 dB/Hz). Additionally, the system showed stable operation more than an hour. Our results indicate that the instability of a laser system is caused by fluctuations in the feedback loop length due to the mechanical vibrations, which can be suppressed by the robust integration of the system.