PHASE CONTRAST OPTICAL FILTERING FOR MAPPING SAW SECOND HARMONIC FIELDS.

An optical filtering technique has been devised which enables one to select in the Fourier plane of the diffracted light the contributions due to the first diffraction orders of the acoustic second harmonic from that due to second diffraction orders of fundamental wave. The filter that has been used in this technique is similar to that used in contrast phase microscopy, where an optical plate retards and attenuates the zeroth order central spot by proper amounts. The light signal is directly collected by a photomultiplier on a plane, where a suitably magnified image of the acoustic field is scanned. The electrical signal at 2 omega frequency is processed by etherodyning techniques. First results are presented relative to transversal scannings of SAW nonlinear fields in LiNbO//3 samples.