Laser-based surface acoustic wave spectrometer for industrial applications

A compact, robust, and fully automated optical system for laser generation and heterodyne detection of surface acoustic waves (SAWs) in the frequency range 0.1-1.3 GHz is presented. SAWs are generated by two short laser pulses forming an interference pattern at the sample surface, and detected via diffraction of a quasi-cw probe beam off the surface ripples produced by the SAWs. The detection is enhanced by optical heterodyning achieved through the introduction of a fourth (reference) beam. Both excitation and probe/reference beam pairs are produced by a phase grating optimized to provide maximum efficiency for the desirable diffraction orders at the two different optical wavelengths used for excitation and probing. The heterodyne arrangement has an excellent phase stability and provides a number of advantages compared to a conventional transient grating setup. These include enhanced signal level and suppression of "parasitic heterodyning" due to scattered light. The system provides reproducibility of SAW frequency measurements of similar to0.01% with measurement time of similar to1 s. Measurements can be performed on small test structures such as 50x50 mum boxes, with an integrated vision system helping position the measurement spot on the sample. (C) 2003 American Institute of Physics.