FM noise and spectral linewidth reduction by incoherent optical negative feedback

An all-optical, stable, external feedback method, the incoherent optical negative feedback method, is proposed and demonstrated for spectral linewidth reduction and optical-frequency stabilization of distributed-feedback (DFB) lasers. The power-spectral density of frequency modulation (FM) noise is shown to be reduced in a wide range of up to several hundreds of megahertz. The linewidth and the optical-frequency drift are simultaneously experimentally reduced to about 140 kHz and less than 17 MHz, respectively. The frequency range of the effective FM noise reduction and the reduction ratio of the FM noise are up to 1 GHz and 22 dB, respectively