Application of wave mixing in photorefractive materials to laser ultrasonics

Summary form only given. Laser ultrasonics is a novel technology that marries ultrasonics with lasers and optics. Ultrasonics allows us to probe inside materials to see hidden flaws, and it is widely used to measure thicknesses. By using lasers for the generation and detection of ultrasound, operations on complex-shaped parts or at elevated temperatures, which are difficult or not possible with conventional piezoelectric-based methods, are now readily feasible. Passive interferometers like the Fabry-Perot can be advantageously replaced by active or adaptive interferometers based on wave mixing in photorefractive materials. We have explored three approaches. The first one is two-wave mixing in which the signal beam, which acquires phase shift and speckle after reflection on the surface in motion, is mixed in the crystal with a pump wave to produce a speckle-adapted reference wave that propagates in the same direction as the transmitted signal wave and interferes with it. The second one uses the same mixing scheme without optical detectors, the signal being observed on electrodes attached to the crystal (photoelectromotive force). The third one strips the signal beam from its speckle by sending it onto a double-phase conjugate mirror. The speckle-free beam is then mixed with a reference beam in a conventional (preferably heterodyne) two-wave interferometer. Since optimum sensitivity and rapid response are required, the two-mixing approach is preferred.