FM NOISE AND SPECTRAL LINEWIDTH REDUCTION BY INCOHERENT OPTICAL NEGATIVE FEEDBACK

A new, 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 up to several hundreds of megahertz. The spectral linewidth and the optical-frequency drift are simultaneously experimentally reduced to about 140 KHz and less than 11 MHz, respectively. The maximum reduction ratio of the power-spectral density of FM noise is about 22 dB.