Optical reconstruction of transparent objects with phase-only SLMs

Three approaches for visualization of transparent micro-objects from holographic data using phase-only SLMs are described. The objects are silicon micro-lenses captured in the near infrared by means of digital holographic microscopy and a simulated weakly refracting 3D object with size in the micrometer range. In the first method, profilometric/tomographic data are retrieved from captured holograms and converted into a 3D point cloud which allows for computer generation of multi-view phase holograms using Rayleigh-Sommerfeld formulation. In the second method, the microlens is computationally placed in front of a textured object to simulate the image of the textured data as seen through the lens. In the third method, direct optical reconstruction of the micrometer object through a digital lens by modifying the phase with the Gerchberg-Saxton algorithm is achieved. (C) 2013 Optical Society of America

Numerical aberrations compensation and polarization imaging in digital holographic microscopy

Tristan Colomb

In this thesis, we describe a method for the numerical reconstruction of the complete wavefront properties from a single digital hologram: the amplitude, the phase and the polarization state. For this purpose, we present the principle of digital holographic microscopy (DHM) and the numerical reconstruction process which consists of propagating numerically a wavefront from the hologram plane to the reconstruction plane. We then define the different parameters of a Numerical Parametric Lens (NPL) introduced in the reconstruction plane that should be precisely adjusted to achieve a correct reconstruction. We demonstrate that automatic procedures not only allow to adjust these parameters, but in addition, to completely compensate for the phase aberrations. The method consists in computing directly from the hologram a NPL defined by standard or Zernike polynomials without prior knowledge of physical setup values (microscope objective focal length, distance between the object and the objective...). This method enables to reconstruct correct and accurate phase distributions, even in the presence of strong and high order aberrations. Furthermore, we show that this method allows to compensate for the curvature of specimen. The NPL parameters obtained by Zernike polynomial fit give quantitative measurements of micro-optics aberrations and the reconstructed images reveal their surface defects and roughness. Examples with micro-lenses and a metallic sphere are presented. Then, this NPL is introduced in the hologram plane and allows, as a system of optical lenses, numerical magnification, complete aberration compensation in DHM (correction of image distortions and phase aberrations) and shifting. This NPL can be automatically computed by polynomial fit, but it can also be defined by a calibration method called Reference Conjugated Hologram (RCH). We demonstrate the power of the method by the reconstruction of non-aberrated wavefronts from holograms recorded specifically with high orders aberrations introduced by a tilted thick plate, or by a cylindrical lens or by a lens ball used instead of the microscope objective. Finally, we present a modified digital holographic microscope permitting the reconstruction of the polarization state of a wavefront. The principle consists in using two reference waves polarized orthogonally that interfere with an object wave. Then, the two wavefronts are reconstructed separately from the same hologram and are processed to image the polarization state in terms of Jones vector components. Simulated and experimental data are compared to a theoretical model in order to evaluate the precision limit of the method for different polarization states of the object wave. We apply this technique to image the birefringence and the dichroism induced in a stressed polymethylmethacrylate sample (PMMA), in a bent optical fiber and in a thin concrete specimen. To evaluate the precision of the phase difference measurement in DHM design, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers. A 6 degrees phase difference resolution is obtained, comparable with standard imaging polariscope, but with the advantage of a single acquisition allowing real-time reconstruction.

EPFL2006

Automatic procedure for aberrations compensation in digital holographic microscopy

Nicolas Aspert, Florian Charrière, Tristan Colomb, Etienne Cuche, Christian Depeursinge, Jonas Kühn, Mihaela Marian, Pierre Marquet, Frédéric Montfort

Digital Holographic Microscopy (DHM) is a powerful imaging technique allowing, from a single amplitude image (amplitude and phase), reflected acquisition (hologram), the reconstruction of the entire complex wave front by or transmitted through an object. Because holography is an interferometric technique, the reconstructed phase leads to a sub-wavelength axial accuracy (below lambda/100). Nevertheless, this accuracy is difficult to obtain from a single hologram. Indeed, the reconstruction process consisting to process the hologram with a digital reference wave (similar to classical holographic reconstruction) seems to need a-priori knowledge about the physical values of the setup. Furthermore, the introduction of a microscope objective (MO), used to improve the lateral resolution, introduces a wave front curvature in the object wave front. Finally, the optics of the set-up can introduce different aberrations that decrease the quality and the accuracy of the phase images. We propose here an automatic procedure allowing the adjustment of the physical values and the compensation for the phase aberrations. The method is based on the extraction of reconstructed phase values, along line profiles, located on or around the sample, in assumed to be flat area, and which serve as reference surfaces. The phase reconstruction parameters are then automatically adjusted by applying curve-fitting procedures on the extracted phase profiles. An example of a mirror and a USAF test target recorded with high order aberrations (introduced by a thick tilted plate placed in the set- up) shows that our procedure reduces the phase standard deviation from 45 degrees to 5 degrees.

SPIE2006

Numerical aberrations compensation and polarization imaging in digital holographic microscopy

Tristan Colomb

EPFL2006