The technique of detecting the laser-ultrasonic vector displacement with a confocal Fabry-Perot interferometer

Generally, a confocal Fabry-Perot interferometer is only able to detect the out-of-plane component of a displacement field; while the in-plane component often has the information about the material which cannot be found in this out-of-plane component. Based on a confocal F-P interferometer set-up for detecting the out-of-plane component of a laser generated acoustic field, a novel two-channel confocal F-P interferometer is developed. The scattering lights at scattering angle +θ and -θ from sample surface are received by this F-P interferometer. The displacement vector can be obtained with simple signal processing of two channel output signals. The out-of-plane and in plane displacement component of Lamb wave generated by a laser pulse in a Ti film are measured with this F-P interferometer. The experimental results are in good agreement with the theoretical predictions.