Natl. Res. Lab. of Metrol., 1-1-4, Umezono, Tsukuba-shi, Ibaraki 305, Japan
An optical system has been developed for detection of laser-generated ultrasound on rough surfaces at a distance over 1 m. A long pulse Nd:YAG laser is used to prove samples. The laser provides a light pulse of 60 (mu)s in duration and 60 mJ in energy with a single frequency. The scattered light from the sample is focused into a multimode fiber and introduced to the modified confocal Fabry--Perot interferometer which has a wave plate in the cavity. The wave plate gives the different optical lengths of the cavity depending on the beam polarization. By adjusting the difference of the optical lengths, the ultrasound signals detected from the orthogonally polarized beams can be out of phase. Taking the difference of the signals produces the valance output which is less sensitive to intensity modulations of the pulsed laser. Longitudinal and shear waves have been observed in several metallic samples without signal averaging.