DIGITAL SIGNAL-PROCESSING FOR STEP-SCAN FOURIER-TRANSFORM INFRARED PHOTOACOUSTIC-SPECTROSCOPY

A software-based digital signal processing (DSP) method using the data system processor has been developed to demodulate the photoacoustic r esponses of a sample to the fundamental phase modulation frequency and its harmonic frequencies (up to the ninth harmonic) in step-scan FT-I R photoacoustic measurements, without the use of any additional hardwa re, The DSP algorithm and its sampling depth multiplexing advantages a re compared to conventional hardware demodulation, Comparison of resul ts from the DSP method to those from hardware demodulators are shown a t both the phase modulation frequency and the harmonics, and applicati on of the DSP method to step-scan photoacoustic measurements with phas e modulation is discussed as it applies to obtaining depth profile inf ormation in heterogeneous materials.