Robust laser-based ultrasound vibrometer using a "cat" conjugator

Summary form only given. Laser-based ultrasound (LBU) has been shown to be a very effective diagnostic tool for remote inspection of opaque materials for internal defects (microcracks, delamination, hidden corrosion.) and for materials characterization (temperature, hardness, thickness, porosity). In LBU, a pair of lasers is used to replace contact transducers: one laser, typically a pulsed laser, is used to generate ultrasound in a sample via thermoelastic or ablative processes. The ultrasonic waves effectively probe the surface and interior of the material. The ultrasound signature is manifested by minute surface vibrations. A second laser, typically a low-power source, is used to as a vibrometer to sense the vibrations. A variety of nonlinear optical techniques have been investigated to provide for real-time wavefront compensation over wide bandwidths. Examples include optical phase conjugate mirrors (PCMs), such as externally pumped four-wave mixers, self-pumped conjugators, and double-pumped conjugators. In this presentation, we demonstrate a photorefractive barium titanate "cat" PCM (Feinberg, Opt. Lett. vol. 8, p. 480, 1983) to realize the required real-time wavefront compensation.