Harnessing forward multiple scattering for optical imaging deep inside an opaque medium
Ulysse Najar, Victor Barolle, Paul Balondrade, Mathias Fink, Claude Boccara, Alexandre Aubry
Abstract
As light travels through a disordered medium such as biological tissues, it undergoes multiple scattering events. This phenomenon is detrimental to in-depth optical microscopy, as it causes a drastic degradation of contrast, resolution and brightness of the resulting image beyond a few scattering mean free paths. However, the information about the inner reflectivity of the sample is not lost; only scrambled. To recover this information, a matrix approach of optical imaging can be fruitful. Here, we report on a de-scanned measurement of a high-dimension reflection matrix R via low coherence interferometry. Then, we show how a set of independent focusing laws can be extracted for each medium voxel through an iterative multi-scale analysis of wave distortions contained in R. It enables an optimal and local compensation of forward multiple scattering paths and provides a three-dimensional confocal image of the sample as the latter one had become digitally transparent. The proof-of-concept experiment is performed on a human opaque cornea and an extension of the penetration depth by a factor five is demonstrated compared to the state-of-the-art. Microscopy exploits light scattering to image biological tissues, but heterogeneities distort wave trajectories, creating a foggy image. A digital approach is proposed to harness these complex paths and reveal the inner structure of opaque media.