Method and apparatus to measure and compute the amplitude point spread function and associated parameters of a coherent optical imaging system

The invention concerns a method of and an apparatus for the measurement of the Amplitude Point Spread Function (APSF), of an Optical Imaging System (OIS), wherein the APSF is defined as the complex field distribution of the beam delivered by the OIS and originating from a point source. The apparatus comprising a) an illumination source b) a point object, PO1 irradiated by said illumination source c) an object beam coming from said point object, collected by the Optical imaging System and transmitted! to a Wavefront Sensing Device, WSD and, d) a computer with processing means adapted to acquire the data from said WSD and adapted to compute the Amplitude Point Spread Function and derived parameters associated with the OIS. In a generic embodiment, the WSD is realized by means of a Mach-Zehnder interferometer providing a digital hologram to the processing means. The invention may be advantageously used in the field of Digital Holographic Microscopy (DHM).

Low coherence digital holographic microscopy

Pia Massatsch

A new holographic technique combining digital holographic microscopy with the use of low coherence light has been developed and the potential of the resulting method for vision in turbid media and biomedical imaging has been evaluated. In digital holography the hologram is recorded on a CCD camera and reconstructed numerically on a PC, giving direct access to the phase and the amplitude of the reconstructed wave front. This technique is combined with low coherence light, creating the possibility to perform optical sectioning. Series of holograms corresponding to slices at different depths in the specimen are recorded and each of these holograms contain phase and amplitude information of the light originating from the corresponding tomographic slice in the object. Sub micrometer lateral resolution is obtained experimentally and the phase distribution of the reconstructed object corresponds to a depth resolution of approximately 10 nm for a depth range of a couple of micrometers. A second level depth resolution is achieved using low coherence illumination. The thickness of a tomographic layer in water is 11 µm. Vision through 18 mean free paths (mfp) of turbid media (micro sphere suspension) is demonstrated with depth resolved reconstruction of the phase. Both amplitude and phase is reconstructed through 17 mfp's of turbid media. Tomographic imaging of a USAF test target hidden behind a 100 µm thick onion cell membrane is performed with phase and amplitude reconstructions of the test target as well as the surface of the onion cell membrane. Tomography of a biological specimen is demonstrated by the images of an onion shell, with structures visible down to a depth of 1 mm and by the images of a St-Paulia leaf presenting two cellular layers separated by 200 µm. Two structures of the anterior eye, the iris and the cornea, are studied on enucleated porcine eyes. A tomography of the iris border close to the pupil was performed and the penetration into the iris tissue is good. The cornea is first imaged without contact medium giving images strongly dominated by the air-cornea reflection. These images contain valuable information on the superficial epithelial cells in spite the presence of the tear film on the cornea. A water immersion configuration is also applied to enable a tomography of the cornea. A z-scan profile of the cornea is obtained and phase and amplitude images are reconstructed for different locations in the cornea (epithelium, stroma and endothelium).

EPFL2003

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

Polarization imaging by use of digital holography

Tristan Colomb, Etienne Cuche, Christian Depeursinge, Pierre Marquet

We present what we believe to be a new digital holographic imaging method that is able to determine simultaneously the distributions of intensity, phase, and polarization state at the surface of a specimen on the basis of a single image acquisition. Two reference waves with orthogonal polarization states interfere with the object wave to create a hologram that is recorded on a CCD camera. Two wave fronts, one for each perpendicular polarization state, are numerically reconstructed in intensity and phase. Combining the intensity and the phase distributions of these two wave fronts permits the determination of all the components of the Jones vector of the object-wave front. We show that this method can be used to image and measure the distribution of the polarization state at the surface of a specimen, and the obtained results indicate that precise quantitative measurements of the polarization state can be achieved. An application of the method to image the birefringence of a stressed polymethyl methacrylate sample is presented.