Resonator modes beyond the paraxial approximation: Spin-orbit coupling of light

Laser light at 780 nm is coupled into a resonator with a finesse of 4.5 x 105, and the output light is detected by a photomultiplier and a camera. A part of the measured frequency spectrum of the resonator is shown in this paper. Two sets of modes are observed at distinct frequencies with the set of modes depending on the polarization of the uncoupled light, confirming the predicted spin-orbit coupling. The measured frequency differences between consecutive modes with equal polarization closely match the predicted values.The work has implications for the construction of resonators with many degenerate modes, which could significantly improve cavity cooling of atoms and molecules.