Reference-beam detection for scanning laser acoustic microscopy

Abstract

Reference-beam detection is inherently superior to the knife-edge detection 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 which is circularly symmetrical around its highest value, namely the zero-frequency point in the spatial spectrum. This property makes it possible to detect spatial frequencies in all directions simultaneously and with equal sensitivity and simplifies the associated electronics. It also makes possible the employment of acoustic evanescent-wave detection so that ultrasound of low temporal frequency can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. Oblique insonification, required for best operation in the knife-edge detector, is thus not preferred in the reference-beam detector and the resultant Doppler shift in the detected frequency of the transmitted zero-order acoustic waves is avoided.