REFERENCE-BEAM DETECTION FOR SCANNING LASER ACOUSTIC MICROSCOPY

Reference-beam detection is inherently superior to the knife-edge dete ction currently in use in scanning laser acoustic microscopy. This new detector makes use of a reference beam, retarded 90 degrees, which is mixed coherently in a photodiode with the acoustically obtained image -modulated beam. The new detector has an isotropic transfer function w hich is circularly symmetrical around its highest value, namely the ze ro-frequency point in the spatial spectrum. This property makes it pos sible to detect spatial frequencies in all directions simultaneously a nd with equal sensitivity and simplifies the associated electronics. I t also makes possible the employment of acoustic evanescent-wave detec tion so that ultrasound of low temporal frequency can be used and at t he same time high spatial frequencies can be detected for obtaining hi gh resolution. Oblique insonification, required for best operation in the knife-edge detector, is thus not preferred in the reference-beam d etector and the resultant Doppler shift in the detected frequency of t he transmitted zero-order acoustic waves is avoided. (C) 1998 Acoustic al Society of America.