Sub-kHz lasing of a CaF_2 Whispering Gallery Mode Resonator Stabilized   Fiber Ring Laser
  We utilize a high quality calcium fluoride whispering-gallery-mode resonator to stabilize a simple erbium doped fiber ring laser with an emission frequency of 196 THz (wavelenght 1530 nm) to a linewidth below 650 Hz. This corresponds to a relative stability of 3.3 x 10^(-12) over 16 \mus. In order to characterize the linewidth we use two identical self-built lasers and a commercial laser to determine the individual lasing linewidth via the three-cornered hat method. 
M. C. Collodo, F. Sedlmeir, B. Sprenger, S. Svitlov, L. J. Wang, and   H. G. L. Schwefel
physics.optics