Abstract
Two‐wave mixing based interferometry has been demonstrated to be a powerful technique for non‐contact, broadband and speckle insensitive measurements of the small surface displacements produced by ultrasonic waves propagating in an object. When the object is in rapid motion along the line‐of‐sight of the probing laser or when the laser beam is rapidly scanned on a wavy surface, the two‐wave mixing photorefractive interferometer loses sensitivity to the point it could become useless. To circumvent the Doppler frequency‐shift produced by this relative motion, we propose a dynamic compensation scheme. We report a particularly simple scheme to implement this concept by monitoring the low‐frequency output signal of a balanced two‐wave mixing demodulator whose output is proportional to the frequency difference between the pump and signal beams, and feeding this signal back to the acousto‐optic shifter. With this new concept, the two‐wave mixing interferometer can operate on objects in rapid motion while maintaining its sensitivity to low frequency ultrasound.
Published Version
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