Optical negative feedback for linewidth reduction of semiconductor lasers

We propose a coherent optical negative feedback method for reducing linewidth of single-mode semiconductor lasers, which requires only an optical filter. Results of numerical analysis indicate that the power spectral density of FM noise can be reduced by more than 40 dB and the subkilohertz spectral linewidth can be obtained with our method when the spectral linewidth is 10 MHz under a free running condition. From a proof-of-concept experiment, we confirmed that the spectral linewidth of a single-mode semiconductor laser is reduced when using our method from 6.4 MHz to 6.5 kHz. This result confirms that the spectral linewidth can be reduced by at least 30 dB, which proves the validity of our method.