Low-coherence Doppler interferometry to determine pathlength distribution of multiple-scattered photons

We report here first results of measurements of the pathlength distribution of scattered photons by low- coherence Doppler interferometry. Laser Doppler Flowmetry (LDF) is used to non-invasively monitor the blood micro circulation in biological tissue. On the other hand, the LDF response is also affected by the optical properties of the tissue itself, which complicates the problem of the exact evaluation of the blood perfusion. In a scattering medium like skin photons travel along different paths of variable length. The longer the pathlength, the higher the change for the photon to be scattered by a blood cell. An aqueous suspension of Intralipid is used to mimic the most important properties of the skin. Using a free beam Michelson interferometer we measure the AC component of the intensity of the pattern formed by interfering the scattered light with a coherent component from the reference channel. In each measurement the mirror in the reference channel is kept fixed while the AC component arises due to the Doppler effect in light scattered by the micro-particles experiencing Brownian motion. The pathlength distribution is extracted from the dependence of the detected signal on the reference mirror position.