Dual-mode optical-frequency discriminator

A dual-mode optical-frequency discriminator is described. Its operation is based on the fact that beam modes in a wave-beam resonator (focused Fabry-Perot) are degenerative modes. This degeneracy is removed by placing a birefringent crystal in the resonator to shift the resonant frequencies of two perpendicularly polarised modes. The desired discriminator response is obtained by detecting each mode separately and then taking the difference of each detector response. Measurements on a dual-mode discriminator are also described.