Differential confocal fabry-perot for the optical detection of ultrasound

The best detection limit of an optical system for the detection of ultrasound is obtained for a system limited by the shot noise. However, typical laser oscillators are not free of intensity and phase fluctuations, and such laser noise exceeds the shot-noise level above a given laser light power. Moreover, since typical industrial surfaces are optically rough and absorbing, laser amplifiers are frequently used to increase the light power collected back from the surface. Such amplifiers add both intensity and phase fluctuations. While intensity fluctuations can be eliminated by a simple differential scheme when using a confocal Fabry-Perot, no simple solution has been given to reduce the phase noise. In this paper, differential schemes that reduce both laser intensity and phase noises are proposed for the transmission and reflection configurations and experimental results on the noise reduction reached are presented. The advantages of this differential approach to image variable reflectivity surface parts and to relax detection laser requirements are then discussed.