Enhanced measurement of ultrasonic surface acoustic waves with TV holography by correction of phase mismatch between laser cavities

Abstract

The use of a pulsed twin-cavity laser allows to study transient events and fast phenomena, like the ultrasonic surface acoustic waves that are the object of our interest. However, slight differences between the output beams can lead to a systematic phase mismatch that yields a broad fringe pattern (even in the absence of any phase change due to the measurand) which can hinder the detection and measurement of waves of very low amplitude. We profit from the fact that this mismatch changes slowly to propose an enhancement of the original TV holography technique to remove most of this undesired effect. For each measurement we record in fast sequence four primary correlograms. The first two (one with each cavity) are taken with the object at rest; the other couple records two mechanical states of the excited surface. Each primary correlogram is recorded with an off-axis configuration of the reference beam, which allows to recover its phase by applying the spatial Fourier transform method. The optical phase-difference calculated from the first pair of primary correlograms represents the phase mismatch between laser beams, and can be subtracted from the optical phase-difference map that carries the information of interest. The whole process is implemented in a single step by using complex arithmetic.