Scanning laser acoustic microscopy using derivative quadrature detection

Derivative quadrature detection is proposed for replacing the knife-edge de tector currently in use in scanning laser acoustic microscopy. This new sys tem employs an optical pi-step, a reference beam, an electronic quadrature detector and two RAMs for demodulating the laser beam which has been modula ted by the acoustically-obtained image information. The:pi-step senses the derivative of the object function, the quadrature detector determines its a mplitude and sign, and the RAMs combine the results thus obtained for recon structing the acoustic image. This detector is inherently capable of better resolution than the conventional knife-edge system. A second advantage ove r the knife-edge is that oblique insonification is not required and a Doppl er shift in the detected frequency of the transmitted zero-order acoustic b eam is thus avoided.