Quantification of Doppler broadening in path length resolved diffusive light scattering using phase modulated low coherence interferometry

We describe path length resolved Doppler measurements of the multiply scattered light in turbid media using phase modulated low coherence Mach-Zehnder interferometer, with separate fibers for illumination and detection. A Doppler broadened phase modulation interference peak observed at the modulation frequency shows an increase in the average Doppler shift with optical path length. The path length dependent Doppler broadening of scattered light due to the detection of multiple scattered light is measured from the Lorentzian linewidth and the results are compared with the predictions of Diffusive Wave Spectroscopy. For particles with small scattering anisotropy, the diffusion approximation shows good agreement with our experimental results. For anisotropic scatterers. the experimental results show deviations from the Diffusion theory. The optical path lengths are determined experimentally from the Zero order moment of the phase modulation peak around the modulation frequency and the results are validated with the Monte Carlo technique.