A hybrid optical feedback method for narrowing and frequency-stabilizing diode lasers

Lasers with narrow linewidths and long-term frequency stability are required in various applications such as precision measurement and optical frequency reference. Here, we propose a hybrid method that combines techniques of optical feedback and optical heterodyne modulation locking to an external Fabry-Perot cavity to reduce the linewidth and frequency drift of the laser. The method is demonstrated on a distributed feedback laser with a free-running linewidth of 2 MHz. The frequency noise power density spectrum shows a reduction of 50 dB in the low-frequency range and 30 dB for white noise, and the linewidth has been reduced to 20 kHz. The lock can be maintained for days. This method can be applied to various lasers of different wavelengths.