Ultrasensitive plenoptic microscope for imaging through turbid media

Summary form only given. Plenoptic photography was first introduced by the French Nobel Prize Gabriel Lippmann in 1908 [1] under the name "integral photography". Unlike conventional imaging systems where only the impact position of photons is recorded, a device based on plenoptic technology also records their initial direction of propagation, all the information about the incident light is therefore accessible. This property allows to obtain images with unusual properties such as the possibility to refocus a blurry photo after the picture has been taken or to have an extended depth of field without reducing the aperture of the lens and thus degrading the photometric balance [2]. The opportunity to correct the objective\´s aberrations by digital post-processing has also been demonstrated [2].An experimental configuration was developed by Ren Ng at Stanford University [2] in which a microlens array is placed before the CCD (Fig. 1a) each microlens covers N pixels. The impacting photons position are sampled by each microlens while one have N informations on the angle of arrival of each pixel. This device therefore suffers from a trade-off spatial / angular resolution which depends on the choice of N. In view of making images through turbid media (low number of ballistic photons), there is also a problem caused by low sensitivity of CCD. An holographic microscope (Fig. 1b) based on Laser Optical Feedback Imaging (LOFI) [3,4] is proposed. In this setup, a laser acts as an emitter and also a receiver of photons; 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 an optical beating at the frequency shift which is detected by a photodiode and demodulated in amplitude and/or in phase. This device is an autodyne (the reference is the laser cavity itself) interferometer o- fering direct access to wavefront from the sample and therefore “full” position / direction information of photons are accessible. The advantage is that the resolution is given by the microscope objective and therefore there is no longer a spatial / angular resolution trade-off. Another advantage is the sensitivity: the dispositive is now shot noise limited thanks to the use of the gain from the laser dynamics. This sensitivity can be reached with little signal and through highly scattering media, which is well suited for imaging biological media. These benefits are counterbalanced by the slow acquisition, due to point by point scanning (around 30 s for a 512*512 pixels image).