Heterodyne Detection of Ultrasound from Rough Surfaces Using a Double Phase Conjugate Mirror

Abstract

Ultrasonic excitation of a solid sample (optically opaque) can be detected by directing a laser beam at one of its surfaces. Surface motion causes a transient phase shift upon the scattered light, which has to be demodulated into an intensity variation prior to its detection by a photodetector. Classical reference beam interferometry (homodyne or heterodyne) is a well-known technique for performing this demodulation. It is characterized by a broad detection bandwidth, but is, following the antenna theorem [1], essentially limited to the detection of one speckle, when used on rough surfaces. In order to circumvent this limitation (i.e., in order to increase the étendue of the interferometer), two different approaches for adapting the signal and reference wavefronts have been considered. The first approach proceeds by creating a reference beam that matched the wavefront of the signal beam. This can be done by using a Fabry-Pérot (FP) [2] which is a self-reference interferometer and means that the reference beam is generated by the signal beam. It can also be done by using two-wave mixing (TWM) in a photorefractive crystal [3,4]. In this case, the reference beam is created by the diffraction of a plane wave pump beam by the hologram written by both pump and signal beams. Alternatively the signal beam wavefront can be adapted to the reference wavefront, which requires, since the reference beam can usually be approximated by a plane wave, the transformation of the speckled beam to a beam with a plane wavefront. Devices using externally pumped [5] or self-pumped phase conjugate mirrors (SPCM) [6] have been reported.KeywordsPump BeamBarium TitanateReference BeamSignal BeamPhase ConjugateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.