Time-Delay-Domain and Pseudorandom-Noise Photoacoustic and Photothermal Wave Processes: A Review of the State of the Art

Abstract

The application and development of correlation and spec- tral analysis methods to photoacoustic and photothermal wave detec- tion are examined. The purposes of this review are to 1) describe the two most prominent techniques available for photothermal correlation and spectral processing, namely the pseudorandom noise and the fre- quency-modulation (FM) time-delay (or frequency sweep) optical ex- citation; 2) compare the technical features of these techniques, such as measurement dynamic range properties, amount of measurement time required and instrumentation requirements; and 3) present advantages of the techniques over the widely used conventional frequency domain and pulsed laser excitation, as well as a detailed comparison between themselves. Attention is focused on the conceptual and mathematical details of signal generation, processing, and interpretation. Subtle but significant differences in the signal dynamic range, impulse response, and transfer function are shown to determine the limits of each of the two major techniques and are corroborated by the available experi- mental evidence to date. A review of the steadily increasing literature on spectroscopic and thermal imaging applications of these techniques is a powerful indicator of the promise they hold for conducting fun- damental and applied studies, primarily due to 1) the superior wealth of information obtained through pseudorandom photothermal exci- tation, as compared to the conventional frequency dispersive tech- niques and 2) the less destructive nature of the pseudorandom im- pulse response, compared to pulsed laser excitation.