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Dipartimento di Fisica - Politecnico di Milano

Diffuse Optical Spectroscopy (DOS)

The system provides the absorption and reduced scattering spectrum of highly diffusive media by time-resolved reflectance and transmittance measurements in the spectral range 560-1050 nm. The system is based on mode-locked tunable lasers (dyes, Ti:Sapphire laser) that are fully computer controlled. A set of fiber optics and fiber devices permits to select the laser source to be sent onto the sample, to split-off part of the signal to synchronize the acquisition, to adjust the injected laser power, and to monitor in real time the laser performances (wavelength, pulse width). The optical signal passed through the medium under study is collected by a fiber optic set on the surface of the medium and sent to a photomultiplier. An electronic chain for time-correlated single photon counting yields the time-distribution of photons re-emitted at the medium surface. The absorption and scattering spectra are then recovered in real time by interpreting the time-resolved curves at the different wavelengths using a model of photon migration (Diffusion theory). The system permits to perform a measurement over the whole spectral range in time interval compatible with an in vivo application (about 10 minutes).

A further development of the system made it possible to construct a parallel workstation operating with the supercontinuum generated by a photonic crystal fiber and with a multi-anode photomultiplier for the parallel acquisition of time-resolved curves at 32 wavelengths. The system is capable to achieve spectroscopy measurements in the 600-850 nm range with an acquisition time of 1 s, that is adequate for in vivo functional monitoring of optical parameters.