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 significantly reduced by more than 50 dB in the frequency range from 10 Hz to 10 kHz. Our experiment demonstrates the efficient laser linewidth reduction and phase noise suppression by the optical feedback technique.