Broadband optical detection of ultrasound by optical sideband stripping with a confocal Fabry–Perot

A method which allows broadband optical detection of ultrasound with large étendue from a scattering surface is presented. The wave scattered by the surface interferes with a reference wave which is derived from the scattered wave after stripping it from its optical sidebands by a confocal resonator. Simple implementation by a confocal Fabry–Perot used in reflection is explained and demonstrated experimentally.