Path length distribution of multiple-scattered photons by low-coherence Doppler interferometry

Abstract

We report results of measurements by low coherence Doppler interferometry of the path length distribution of photons undergoing multiple scattering in a highly turbid medium. We use a Mach-Zehnder interferometer with multimode graded index fibers and a superluminescent diode as light source. The path length distribution is obtained by recording the heterodyne fluctuations arising due to the Brownian motion of particles in an Intralipid suspension as a function of the optical path length. The experimental path length distribution is in good agreement with predictions of Monte Carlo simulations. In the heterodyne spectrum an increase of the mean Doppler frequency with the path length is observed. The path length resolution of the setup was directly evaluated by replacing the turbid medium with randomly moving scatterers by a mirror attached to a harmonically oscillating piezo-element. The maximum (peak-to-peak) mirror displacement was 10% of the optical wavelength. We observed a narrow and strong (signal/noise ratio ~300) interference peak with the full width at the half maximum ~50 microns equal to the coherence length of the superluminescent diode. However, additional weaker satellite peaks are also observed, which may be caused by the intermodal dispersion in our multimode fibers. We demonstrate that our setup allows achieving high path length resolution for biological tissues where the width of the path length distribution is several millimeters.