Self-referencing laser-based ultrasonic wave receiver

Abstract

An optical self-referencing ultrasonic receiver for detecting ultrasonic waves removes wavefront distortions imparted on the optical beams by diffusely reflecting readout surfaces or other sources, compensates for noise induced phase errors on the readout beam, compensates for amplitude noise present on the readout beam, substantially matches the wavefronts of the readout and reference beams, is capable of operating in a heterodyne mode and is self-aligning. In one embodiment, ultrasonic waves are measured by directing a signal beam and a reference beam to a surface of the workpiece so that the signal beam reflects off an area that is being vibrated by the ultrasonic waves, and the reference beam reflects off a different area of the surface. The signal beam gets phase modulated by the ultrasonic wave induced vibrations and also by other noise induced vibrations. The reference beam only gets phase modulated by the noise induced vibrations. The phase modulated signal and reference beams are directed to a wavefront compensator that overlaps the beams, substantially matches their wavefronts and removes wavefront distortions without altering their respective optical phases. The wavefront compensated beams are directed to a coherent detector whose output signal has a phase shift which corresponds to the difference between the optical phase shift on the signal beam (due to ultrasonic wave vibrations and noise vibrations) and the optical phase shift on the reference beam (due to noise vibrations only). Thus, noise induce vibrations common to both beams are canceled out.