Laser phase noise reduction for industrial interferometric applications

Laser ultrasound is a technique used for the ultrasonic inspection of composites during manufacturing of advanced jet fighters. With this technique laser interferometry is used to detect ultrasonic displacements generated by a laser. In theory, the signal-to-noise ratio is proportional to the square root of the collected detection light. In practice, laser phase noise limits the signal-to-noise ratio above a certain collected light level. Two techniques are presented to decrease effects due to laser noise. In one technique the dual-cavity Fabry-Perot currently used is replaced by an interferometer based on a photorefractive crystal. The other technique has a high-finesse Sagnac cavity that filters the phase noise from the detection laser. Experimental results demonstrate that these two techniques significantly reduce limitations due to laser noise. (C) 2004 Optical Society of America.