The Use of a Reference-Beam Detector Applied to the Scanning Laser Acoustic Microscope

Abstract

Scanning laser acoustic microscopy (SLAM) has become a reliable option for obtaining high-quality images on a microscopic scale with good resolution and contrast. Is the only acoustic microscope that produces it’s images in real time. The conventional technique for image data readout in SLAM is to deflection-modulate a laser beam by scanning it over a solid surface containing the acoustic field scattered from the object and then demodulate the beam by means of a knife-edge detector (KED). The probing beam is reflected form the surface of a mirrored coverslip onto which scattered acoustic waves, transmitted through the object, impinge. The image information is encoded as deflection modulation on the reflected beam and is detected by the KED. The system consists basically of a knife-edge placed in the focal field of a lens with a photodiode positioned behind the knife-edge. The KED is sensitive not only to the spatial frequencies of the sinusoidal components in the object but also to the direction in which the sinusoidal variations take place. Unfortunately, the KED transfer function is anti symmetric with zero response at zero spatial frequency and negative response at negative frequencies . Because of the nature of the transfer function, flexibility in choosing the optimum laser spot size is limited. In addition an obliquely incident insonification at an optimum angle of incidence is ordinarily employed in SLAM, with a resultant Doppler shift in the detected frequency of the transmitted zero-order acoustic beam, along with single side band detection. In spite of the use of these measures, the bandwidth of the spatial frequency spectrum used in