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Reference : UMR7587-ALEAUB-010
Workplace : PARIS 05
Date of publication : Tuesday, July 13, 2021
Scientific Responsible name : Alexandre Aubry
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 4 October 2021
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
In wave imaging, we aim at characterizing an unknown environment by actively probing it and then recording the waves reflected by the medium. It is, for example, the principle of ultrasound imaging, or optical coherence tomography for light. However, wave propagation from the sensors to the focal plane is often degraded by the heterogeneities of the medium itself. They can induce wave-front distortions (aberrations) and multiple scattering events that can strongly degrade the resolution and the contrast of the image. Aberration and multiple scattering thus constitute the most fundamental limits for imaging in all domains of wave physics. In this thesis, we propose to solve these two fundamental problems (aberrations and multiple scattering) in optical imaging by extending the reflection matrix approach  recently developed at the Langevin institute to dynamic scattering media.
In optical imaging, we often have to deal with dynamic scattering media. This is the case for optical microcopy in which living tissues exhibit a decorrelation characteristic time ranging from 50 ms to 2.5 s depending on the level of immobilization . This is also the case for sub-aquatic imaging in turbid water. This decorrelation time is thus a fundamental issue in optical imaging that imposes a time constraint on experimental measurements.
The first step of the PhD thesis will be to set up an optical device that allows to meet this requirement. To that aim, we will inspire ourselves from the recent work of the Wonshik Choi's team  that built an experimental set up dedicated to the ultra-fast measurement of high-dimension reflection matrices. This set up will be coupled to an adaptive optics scheme in order to perform a real-time correction of low-order aberrations.
The second part of the PhD thesis will consist in developing a dynamic matrix approach of optical imaging. It will consist in taking advantage of the Wigner Smith time-delay operator in order to discriminate scattering paths as a function of their decorrelation time . On the one hand, we will be able to extract ballistic and forward multiple scattering contributions since they are less sensitive to slight movements than fully random trajectories. On the other hand, moving speckle will be taken advantage of to optimally extract the transmission matrix between sensors and medium voxels from reflection measurements. Moving speckle actually gives access to large number of speckle realizations at each voxel which can be used in return to extract a local aberration phase law for each voxel of the medium . At last, we will take advantage of dynamic scattering to reveal novel contrast in tissues by taking advantage of the time dependence of the reflection matrix .
With regards to sub-aquatic imaging applications, a modeling of aberrations and scattering in underwater environments will be first performed [7,8]. Short-scale experiments will be then conducted in turbid water using the matrix microscope built during the first part of the PhD. One key point will be to identify the coherence time of the propagation channels according to the water turbidity. Post-processing tools developed in the context of optical microscopy will be then applied to sub-aquatic imaging for the detection and imaging of targets immersed in turbid water.
 A. Badon, V. Barolle, K. Irsch, A. Boccara, M. Fink, and A. Aubry, “Distortion matrix concept for deep imaging in optical coherence microscopy,” Sci. Adv. 6, eaay7170 (2020).
 M. Jang, H. Ruan, I. M. Vellekoop, B. Judkewitz, E. Chung, and C. Yang, “Relation between speckle decorrelation and optical phase conjugation (OPC)-based turbidity suppression through dynamic scattering media: a study on in vivo mouse skin”, Biomed. Opt. Exp. 6, 72-85 (2015).
 S. Yoon, et al. “Laser scanning reflection-matrix microscopy for aberration-free imaging through intact mouse skull”. Nat Commun 11, 5721 (2020).
 P. Ambich et al., “Focusing inside Disordered Media with the Generalized Wigner-Smith Operator”, Phys. Rev. Lett. 119, 033903 (2017).
 B. F. Osmanski, G. Montaldo, M. Tanter and M. Fink, “Aberration correction by time reversal of moving speckle noise”, IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 59, 7, 1575-1583 (2012).
 C. Apelian, F. Harms, O. Thouvenin, and A. C. Boccara, “Dynamic full field optical coherence tomography: subcellular metabolic contrast revealed in tissues by interferometric signals temporal analysis”, Biomed. Opt. Exp. 7, 1511-1524 (2016).
 M.V. Jamali et al., "Statistical Studies of Fading in Underwater Wireless Optical Channels in the Presence of Air Bubble, Temperature, and Salinity Random Variations," in IEEE Transactions on Communications 66, 4706-4723 (2018).
 S. Matt, W. Hou, W. Goode, and S. Hellman. “Introducing SiTTE: A controlled laboratory setting to study the impact of turbulent fluctuations on light propagation in the underwater environment”, Opt. Exp. 25, 5662-5683 (2017).
This thesis will take place at the Langevin Institute (1 rue Jussieu - 75005 PARIS) in the Matrix Imaging team. This thesis is funded by the ERC project REMINISCENCE and ONERA.
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