CaF2 Whispering-Gallery-Mode Resonator Stabilized Narrow Linewidth Laser
  A fiber laser is stabilized using a Calcium Fluoride (CaF2) whispering-gallery-mode resonator. It is set up using a semiconductor optical amplifier as a gain medium. The resonator is critically coupled through prisms, and used as a filtering element to suppress the laser linewidth. Using the self-heterodyne beat technique the linewidth is determined to be 13 kHz. This implies an enhancement factor of 10^3 with respect to the passive cavity linewidth. The three-cornered hat method shows a stability of 10^(-11) after 10 \mu s. 
B. Sprenger, H. G. L. Schwefel, Z. H. Lu, S. Svitlov, and L. J. Wang
physics.optics