ESPCI web site
Spectroscopic and holographic filtering applied to acousto-optic imaging of thick scattering media.
Imaging through a thick and scattering medium is difficult using conventional optical imaging techniques. Acousto-optic imaging is a multimodal imaging technique based on the interaction between a light wave and an acoustic wave.
This acoustic wave modulates the phase of the light field. It is then possible to access a local optical contrast at the millimetric resolution of ultrasound. Several methods are used to detect the ultrasound "tagged photons". They can be coherent or incoherent in nature and attempt to address the various issues imposed by imaging with multiple scattered light.
In this thesis we explored three kind of detections at 800 nm, in the optical therapeutic window (region of the spectrum where the absorption is minimal in biological tissues).
Spectral holeburning in a rare-earth ion doped crystal allowed us to achieve a hyperfine spectral filter centered on the tagged photons frequency at low temperatures.
A sensitive detection of tagged photons was then performed by heterodyne off-axis digital holography coupled with a long pulse laser. Acousto-optic profiles were obtained through several centimeters of thickness.
At last, wavefront adaption by photorefractive holography in a SPS:Te (tellurium-doped tin Hypothiodiphosphate) crystal allowed us to achieve a real-time imaging. Images in two and three dimensions were obtained through several hundreds of optical thicknesses. The process of phase conjugation by four-wave mixing has also been explored as a perspective.