100-Hz linewidth diode laser with external optical feedback

We present a narrow-linewidth diode laser with optical feedback from a high-finesse two-mirror nonconfocal cavity. The laser's full-width-at-half-maximum linewidth is reduced to 100 Hz, and its instantaneous linewidth to 30 Hz in the Lorentzian fitting. To the best of our knowledge, it is the narrowest linewidth ever achieved with the optical feedback method. The laser phase noise has been signifi...