Raman lasing and other nonlinear effects based on ultrahigh-Q CaF2 optical resonator

The calcium fluoride (CaF₂) whispering gallery mode crystalline resonator is an excellent platform for nonlinear optical applications because of the decreasing in threshold caused by ultrahigh quality (Q) factor. In this paper, we achieved the observation of Raman lasing, first-order Raman comb, and second-order Raman lasing in a CaF₂ disk resonator with a diameter of 4.96 mm and an ultrahigh-Q of 8.43× 10⁸ at 1550-nm wavelength. We also observed thermal effects in CaF₂ disk resonator, and the threshold of thermo-optical oscillation is approximately coincident with Raman lasing, since the intracavity power increases rapidly when the power reaches the threshold, and higher input pump power results in longer thermal drift and higher Raman emission power. With a further increase in pump power, the optical frequency combs range is from 1520 nm to 1650 nm, with a wavelength interval of 4× m FSR. It is a promising candidate for optical communication, biological environment monitoring, spectral analysis, and microwave signal sources.