Creating a Stable and Precisely Tunable Laser Light Source for Exciting Whispering Gallery Modes

The current investigation was inspired by an interest in whispering gallery mode resonators [WGMRs]. In these devices a wave travels around a circumference within a sphere or disk. The name whispering gallery is inherited from elliptical rooms in cathedrals, where a whisper that originates from one focal point can be heard with perfect clarity at the second focal point far away on the other side of the room. Whispering gallery mode resonators use the same principle except instead of sound, light undergoes total internal reflection within a circular structure as it travels around the interior.

WGMR's can achieve very high quality (Q) factors, which means that the incoming light must be precisely tuned to a specific cavity mode. A tunable diode laser would be ideal for searching for WGMR resonances but no such laser is available to us. Our alternative approach is to use an acousto-optic modulator (AOM) to shift the frequency of a stabilized single-longitudinal-mode HeNe laser. A laser of this type stable to around 15 MHz has been developed over the course of several past LTC projects. The AOM has a frequency shift range of 80 - 120 MHz and this amount can be doubled by passing the laser light through the AOM twice, using a mirror or retroreflector. Even 80 MHz is far smaller than the expected mode spacing in a small WGMR, making it quite possible that no resonance will be seen. Hopefully if several resonators are tested one will provide a detectable resonance.

We are currently creating a setup in which the double-passed frequency-shifted light from the AOM is compared with the unshifted light coming directly from the stabilized laser in a scanning Fabry-Perot resonator. The experimental challenge is to assure that the two beams enter the Fabry-Perot in precisely the same way as the frequency of the AOM is changed.