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Étude et applications de l'imagerie sans lentille par diffraction cohérente

Abstract : This dissertation is dedicated to coherent diffractive imaging. Firstly, we present the conception and experimental implementation of a compact imaging system, working on this principle. It is made of an UV laser diode (λ = 400 nm), a CCD camera,and a platform to place the sample. The coherent beam coming from the diode illuminates the sample, and the diffraction pattern is recorded by the camera. Back-propagating the detected field should allow, in principle, to derive the sample’s profile. Nevertheless, the field’s phase, lost during the detection, forces us to use “phase retrieval” methods, this quantity being necessary to the inversion process. Several techniques have been used for that purpose. Fourier Transform Holography (FTH), for example, is a deterministic method thatconsists in using a circular reference, closely drilled nearby the sample. The phase is encoded in the diffraction pattern, in the form of interference fringes coming from the object and the reference. Then, a simple inverse Fourier Transform of the signal leads the profile of the sample. An iterative method has also been implemented, based on a set of constraints in the real and reciprocal spaces. In particular, the illuminated object must be “isolated”, i.e. smaller than the incident beam. Although this method is non-deterministic, we will see thatit is more robust and gives better resolutions than the holographic cases. This study is the starting point of three-dimensional imaging. We present a first deterministic method, based on FTH. For this purpose, a “holographic pupil” is used and serves as a support for a first 2D reconstruction of the field. The latter is then back-propagated towards the sample closely placed, in order to realize an entirely numerical focusing on it. The “isolation constraint” is then removed by the use of this pupil. However, with this method, the field of view is limitedby the pupil’s diameter. In order to observe larger samples, the “in-line holography” technique has been exploited as well. It consists in illuminating the object with a spherical wave and recording the interference fringes (or “hologram”). A back-propagation is made after the fact in order to do the focusing on the sample. The divergent nature of the beam allows for reaching several millimeters for the lateral field of view. The “twin image problem”, inherent to this configuration, is solved via an iterative algorithm coupled to the back-propagation process. Three-dimensional reconstructions have been made on varied samples, with these two methods — pupil reconstruction and in-line holography. In both cases, reconstruction interfaces have been implemented and work during the detection, in order to observe the object in real time. We then have a compact and complete lens-less imaging prototype. Finally, we present the application of a phase retrievaltechnique, named LIFT (LInearized Focal plane Technique), applied to a Shack-Hartmann wavefront sensor. Usually, such sensors have a spatial resolution that is limited by the micro-lenses size : only the local slopes, i.e. tip and tilt, are retrieved. The LIFT consists in determining the phase at the scale of each micro-lens, by exploiting the corresponding spot profile. Interaction matrices are calculated in order to linearize the relation between the real space (micro-lenses) and the reciprocal space (CCD chip), and an iterative loop allows for increasing this linearity domain. With this technique, a gain in spatial resolution by a factor 3 is expected.
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  • HAL Id : tel-01480706, version 1

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Julien Samaan. Étude et applications de l'imagerie sans lentille par diffraction cohérente. Optique [physics.optics]. Université Paris Saclay (COmUE), 2016. Français. ⟨NNT : 2016SACLS557⟩. ⟨tel-01480706⟩

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