Our approach first consists in measuring a time-gated reflection matrix associated to a scattering medium using a spatial light modulator at the input and a CCD camera at the output. An interferometric arm allows to discriminate the scattered photons as a function of their time of flight. Inspired by previous works in acoustics, a random matrix approach then allows to get rid of multiple scattering. This improves by far the detection and imaging of targets embedded in or hidden behind a highly scattering medium. As proof of concept, we tackle with the issue of imaging ZnO micrometric beads across a highly scattering paper sheet whose optical thickness is of 12.5 ls, with ls the scattering mean free path. This experimental situation is particularly extreme, even almost desperate for imaging. The ballistic wave has to go through 25 ls back and forth, thus undergoing an attenuation of 10^-11 in intensity. For an incident plane wave, 1 scattered photon over 1000 billions is associated to the target beads. In optical coherence tomography, the single-to-multiple scattering ratio is of 5×10^-4 which prevents from any target detection and imaging. On the contrary, our approach allows to get rid of most of the multiple scattering contribution in this extreme situation. By means of the time-reversal operator, the ballistic echoes associated to each bead are extracted and allow to reconstruct a satisfying image of the targets. The perspective of this work is to apply this promising approach to in-depth imaging of biological tissues.
Amaury Badon, Dayan Li, Geoffroy Lerosey, Claude Boccara, Mathias Fink, and Alexandre Aubry, "A matrix approach for optical detection and imaging through highly scattering media
(Conference Presentation)," Proc. SPIE 9717, Adaptive Optics and Wavefront Control for Biological Systems II, 97170W (Presented at SPIE BiOS: February 14, 2016; Published: 27 April 2016); https://doi.org/10.1117/12.2211556.4848767678001.
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