Brillouin Lasing with a ${\mathrm{CaF}}_2$ Whispering Gallery Mode Resonator

Stimulated Brillouin scattering with both pump and Stokes beams in resonance with whispering gallery modes of an ultrahigh $Q$ calcium fluoride resonator is demonstrated for the first time. The resonator is pumped with 1064 nm light and has $3\mu \mathrm{W}$ Brillouin lasing threshold. The scattering is observed due to the unique morphology of the resonator reducing the phase mismatch between the optical modes and the hypersound wave.

Figure 1

Characteristic frequency of the Brillouin scattering as a function of the azimuth angle in a resonator. Frequencies of two tangential and one longitudinal (blue line on top) acoustic waves involved in Brillouin scattering are plotted for the fluorite resonator. Frequencies are changing for different points along the resonator’s circumference (angle $\varphi$) due to varying orientation of the acoustic wave vector relative to the crystalline axes.

Figure 2

Fluorite resonators. WGM localization areas are highlighted with the white brackets on the left. Shadow photographs of the highlighted regions are on the right. Diameters are 5.52 mm for resonator 1 and 5.21 mm for resonator 2.

Figure 3

Experimental setup diagram. Laser is locked to a WGM via simplified Pound-Drever-Hall technique. Optical power of clockwise and counterclockwise WG modes is monitored.

Figure 4

(a) Output of the resonator 1, coupler $B$. Pump laser wavelength is 1064.35 nm. Low power at the spectrum analyzer is due to undercoupling. Frequency shift is 17.5 GHz for the weak Stokes and 35 GHz for the strong Stokes line. (b) Backward signal from output $C$ for second resonator. Pump laser wavelength is 1064.34 nm. Stokes offset is 18 GHz for the first and 35.5 GHz for the second line. The width of each line is limited by the optical spectrum analyzer resolution of 0.012 nm.

Figure 5

Cascaded Raman lasing observed at pump power above $10\mu \mathrm{W}$ in the second cavity. Intensity modulation resembles Brillouin spectrum near the pump wavelength.

Figure 6

Typical optical spectra from couplers $B$ and $C$, representing forward and backward signals. Stokes offset is 17.7 GHz. Pump laser wavelength is 1064.34 nm. Spectrum analyzer resolution is 0.012 nm.