Passive intrinsic-linewidth narrowing of ultraviolet extended-cavity   diode laser by weak optical feedback
  We present a simple method for narrowing the intrinsic Lorentzian linewidth of a commercial ultraviolet grating extended-cavity diode laser (TOPTICA DL Pro) using weak optical feedback from a long external cavity. We achieve a suppression in frequency noise spectral density of 20 dB measured at frequencies around 1 MHz, corresponding to the narrowing of the intrinsic Lorentzian linewidth from 200 kHz to 2 kHz. The system is suitable for experiments requiring a tunable ultraviolet laser with narrow linewidth and low high-frequency noise, such as precision spectroscopy, optical clocks, and quantum information science experiments. 
Polnop Samutpraphoot, Sophie Weber, Qian Lin, Dorian Gangloff, Alexei   Bylinskii, Boris Braverman, Akio Kawasaki, Christoph...
physics.optics