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Diffuse Optical Spectroscopy (DOS)

A light beam injected into a diffusive medium does not propagate as a straight line but, due to the scattering, photons spread out in a glow-ball and a simple attenuation measurement 
does not provide the correct absorption. To retrieve the correct absorption it is needed a deep understanding of the light propagation in a diffusive media like a biological tissue. In this laboratory the light propagation in diffusive media is investigated using time-resolved techniques in the spectral range 600-1350 nm. 

The Time-Resolved technique

A pulse on the picoseconds scale is injected into the diffusive medium and collected on a different place of its external surface. The temporal distribution of the photons time-of-flight provides information about the absorption and scattering probabilities. In particular the broadening and the delay of the collected pulse is related to the scattering and the asymptotic slope  to the absorption. Absorption and scattering spectra over a broad range provide information about the composition and the structure of the sample. As an example if the sample is a biological tissue it is possible to estimate the concentrations of water, lipid, collagen and hemoglobin.

The setup

The system is based on a mode-locked fiber laser generating supercontinuum radiation in the range 450 - 1750 nm. The supercontinuum light is spectrally dispersed by a prism to allow the wavelength selection with a FWHM ranging from about 4 nm to 15 nm. The light is typically injected into the sample and collected  from it using optical fibers mounted on custom probes. The detection is typically performed using Photomultiplier Tubes(PMT) and Single-Photon Avalanche Diode (SPAD) operating in different spectral ranges and providing different temporal resolutions (from 50 to 300 ps). The collection of the photons time-of-flight histogram is operated by a Time-Correlated Single Photon Counting(TCSPC) board mounted on a computer. The acquisition is fully automated and controlled by a home-made software. The acquisition time of a typical spectrum takes about 10 minutes.