Characterization and validation of the frequency-modulated continuous-wave technique for assessment of photon migration in solid scattering media

The frequency-modulated continuous-wave (FM-CW) technique, based on the beat signal of a Mach–Zehnder interferometer employing a frequency-ramped light source, is studied for solid scattering media applications. The method is used to evaluate the mean time-of-flight (MTOF) of light traveling in scattering media, specifically polystyrene foams. We assume that each the time-of-flight (TOF) time corresponds to different light scattering paths resulting in a different phase shift. The phase shift variations produce a speckle pattern, which together with the frequency leakage induced by the discrete Fourier transform (DFT) cause “spikes” in the power spectrum of the beat signal, thus decreasing the accuracy of the measured MTOF values in solid scattering media. For comparison, time-of-flight spectroscopy (TOFS) is also employed to evaluate the MTOF for the same samples, while the geometrical difference between these two techniques is compensated for by using diffusion theory. The MTOFs measured by the FMCW and TOFS techniques agree well, which demonstrates a great potential to develop a robust FMCW setup for simplified MTOF assessment.