Highly scattering optical system identification via frequency response analysis of NIR-TRS spectra

Frequency response analysis via pulse testing is often used for the characterization of engineering systems. Near infrared-time resolved spectroscopy (NIR-TRS) is a frequently used technique for the analysis of biological system properties. Since the TRS input is a very sharp photon pulse, a well designed TRS input pulse can produce a multi-frequency response over the useful frequency range for the system identification. This new approach for analyzing NIR-TRS provides new optical system parameters (e.g., magnitude ratio and phase shift at multi-frequencies, system time constant, system order, and steady state gain) that are not available by traditional TRS spectra analysis. In this paper, the basic theory of pulse reduction is introduced for the multi-modulation frequency response of TRS spectra. Homogeneous system response with various absorption and scattering properties were analyzed for the multi-system parameters. In heterogeneous systems, the position of the localized absorber is correlated with the multi-parameters, which can ultimately be used to enhance medical imaging.