Acoustically tagged photons for ultimate sensitivity imaging

Summary form only given. Getting resolved images through scattering media is a crucial concern as it is related to important issues. For instance one can cite high resolution biological imaging. Such an imaging system could provide us information about deep tissues without requiring invasive biopsy followed by histological sections.When light propagates through turbid media, the major problem is to find the trade-off between the sensitivity and the resolution. The first possibility is to measure the multi-scattered photons [1,2]; the advantage is the good sensitivity at large depths but at the price of a highly degraded resolution (multi-scattered photons have lost their initial direction). On the contrary one can choose to measure only ballistic photons and thus to get an optically resolved image. The main drawback is that their number decreases exponentially during the propagation, impacting signal to noise ratio and limiting the accessible depth. Laser Optical Feedback Imaging (LOFI) microscope [3] is a setup belonging to the second family, a laser acts both as an emitter and a receiver of photons, and an image is obtained point by point by scanning the target with galvanometric mirrors. Photons are emitted, frequency shifted near the relaxation frequency of the laser, retroreflected by the target and finally reinjected into the laser cavity. This leads to a beating at the frequency shift of the laser mode, which is detected by a photodiode and demodulated in amplitude and in phase. Finally, one has an equivalent detector which benefits from the dynamics of the laser with an ultimate shot noise sensitivity. The counterpart of these advantages is the time of an image acquisition (at least 30 seconds for 512*512 pixels). Another interesting property of this system is the ability to keep the resolution of microscope objective beyond the working distance of the microscope objective. Indeed this setup can be seen as an autodyne interferometer with both amplitude and phase information on the reinjected electric field. As a result it is possible to use Synthetic Aperture (SA) techniques to treat numerically raw defocus images (from fig. 1a to 1b), with a final resolution corresponding to the microscope objective resolution.The major problem with synthetic aperture LOFI is the photometric balance which degrades with the defocus during raw recording (the coupling of the reinjected electric field with the laser cavity acts as a confocal pinhole) limiting the accessible lattitude of numerical refocusing. This problem coupled with parasitic reinjected photons from reflection on all interfaces (lenses, mirrors) before the target prevents the system from reaching the shot noise limit. To handle this problem, a system coupling the LOFI and an the acoustic tagging [4] is proposed. In this setup only photons tagged (by an acoustical transducer) just before the target are taken into account, leading to an elimination of this parasitic echoes (fig. 1c) restoring the LOFI full sensitivity.