GENERALIZED 2-DIMENSIONAL FOURIER-TRANSFORM INFRARED PHOTOACOUSTIC SPECTRAL DEPTH-PROFILING ANALYSIS

This paper reports the first application of a generalized two-dimensio nal (G2D) correlation method in photoacoustic spectral depth-profiling analysis of laminate/heterogeneous samples. In this method, photoacou stic magnitude spectra at different modulation frequencies are used to generate two-dimensional (2D) correlation spectra. The relative spati al origins of correlated signals are then determined from the signs of the corresponding contours. The unique features and advantages of thi s technique over previously reported depth-profiling methods are demon strated and discussed. These include simplification of step-scan photo acoustic phase-modulation experiments as well as 2D correlation-analys is rules, avoidance of phase-related negative-band ambiguities, and en hancement of both spectral and depth resolutions, etc. The recently de veloped software-based digital signal-processing (DSP) technique for s tep-scan photoacoustic measurements offers an efficient means (samplin g-depth multiplexing advantage) to collect within a single scan all ne cessary data for this type of 2D correlation analysis.