Photopyroelectric thin-film instrumentation and impulse-response detection. Part III: Performance and signal recovery techniques

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Abstract
Frequency-modulation time-delay spectrometry (FM-TDS) was compared with stochastic excitation in photopyroelectric measurements on thin solid samples. These methods involve broadband signal excitation with simultaneous detection of all frequency components within the response bandwidth of the photothermal system. Fast signal recovery was achieved by FFT methods. Both FM-TDS and wideband random-noise excitation were found to yield high-quality, band-limited impulse-response information. Random noise measurements were found to be much less susceptible to distortions and nonlinearities in the excitation wave train. FM-TDS showed superior coherence and signal-to-noise ratio (SNR) to random methods. The results of this work demonstrate a photopyroelectric effect spectrometer capable of yielding fast, time-resolved information equivalent to the response of a pulsed laser, with low peak power. This important feature shows excellent potential for nondestructive thickness, thermal diffusivity, and thermal conductivity measurements on materials which are susceptible to optical damage.

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