Armin Kiessling

3D shape measurement of macroscopic objects in digital off-axis holography using structured illumination

We propose what we believe to be a novel approach to measure the 3D shape of arbitrary diffuse-reflecting macroscopic objects in holographic setups. Using a standard holographic setup, a second CCD and a liquid-crystal-on-silicon spatial light modulator to modulate the object wave, the method yields a dense 3D point cloud of an object or a scene. The calibration process is presented, and first quantitative results of a shape measurement are shown and discussed. Furthermore, a shape measurement of a complex object is displayed to demonstrate its universal use.

Self-focusing without external electric field in BaTiO3

We report on the first observation, to our knowledge, of self-focusing without an external electric field in barium titanate crystals under cw laser beam irradiance. This effect we observed at an intensity of 0.2W/cm(2) on the 633nm wavelength regime in the case of ordinarily as well as extraordinarily polarized light.

Enhanced resolution of microscopic objects by image inversion interferometry

We demonstrate in experiment that the resolution of a conventional light microscope can be enhanced by 26% with the help of an image inverting interferometer. In order to prove this statement, we measured the point spread function of the system as well as the resolution of two-point objects. Additionally, the contrast transmission function of the interferometric setup was measured and compared to the results gained with a conventional wide-field microscope. Using the interferometric system, the spatial frequencies near the cutoff-frequency were far better transmitted than by the conventional microscope. Finally, we demonstrate the improved resolution with the help of images of two-dimensional structures.

Beam self-trapping and self-bending dynamics in a strontium barium niobate crystal

We investigate experimentally and theoretically the self-trapping and the self-bending of a laser beam with input diameters between 18 and 40 μm in a photorefractive strontium barium niobate crystal dependent on the applied electric field, the input beam diameter, and the input beam intensity. We find out the parameters for the optimal focusing, i.e., the beam is self-trapped to a determined diameter independently on the input diameter.

Giant momentary readout produced by switching electric fields during two-wave mixing in sillenites

We show theoretically and experimentally that switching an applied square-wave field produces strong and short pulses of the outgoing signal during two-wave mixing in sillenite crystals. These pulses originate from the strong effect of the field on the optical eigenmodes and can be used in new optical schemes based on time-separated recording and readout processes.

Stereophotogrammetric 3D shape measurement by holographic methods using structured speckle illumination combined with interferometry.

We present a unique combination of the numerical three-dimensional (3D) reconstruction of the shape of an object with interferometric deformation measurements. Two cameras record several holograms of an object that is illuminated by structured illumination. This illumination is realized by speckle patterns. To improve the image quality, an inplace speckle reduction technique is combined with the structured illumination to reduce the effect of disturbing subjective speckles which appear in the reconstructed images. Stereophotogrammetric methods are applied to extract the 3D surface information of the object out of the reconstructed images. Since the recording is done by holography and because stereophotogrammetry enables a pointwise correlation between the two views, it is possible to combine other holographic techniques with the reconstructed 3D shape. This is demonstrated by an interferometric deformation measurement of an object cooling down. The resulting interferometric fringes are mapped onto the reconstructed 3D surface. Hence, the proposed method enables automatic and dense matching of interferometric fringe-maps recorded by spatially separated holograms onto the surface of the object, which has not yet been realized by existing techniques.

Dynamic holography with nonplane waves in sillenites

A three-dimensional model describing two-wave mixing in photorefractive crystals with finite beams of arbitrary shape is presented. The well-known longitudinal and transverse geometries are generalized by allowing an arbitrary orientation of the local grating vector. The coupled equations for the wave amplitudes are solved numerically in the small-angle approximation by introducing the light paths as characteristics. The influence of amplitude distributions and phase-front curvatures as well as the influence of geometrical arrangements and crystal properties (i.e. optical activity, externally applied voltage) on the energy exchange and the evolution of the polarization states is investigated. It is demonstrated that scanning the cross-section of the finite signal beam behind the crystal allows analysis of the coupling process inside the crystal which cannot be studied directly. The results of experiments with finite beams carried out on a BSO crystal are in good agreement with the numerical calculations.

A Simple Interferometric Method for Measuring the Fidelity of Phase Conjugated Beams

Abstract One of the most interesting properties of the process of nonlinear optical phase conjugation is the capability of compensating for aberrations. The quality of compensation can be determined by investigation of the wavefront. A simple interferometric method for measuring fidelity of phase conjugated beams which allows a quantitative evaluation with respect to the phase distribution is described. This method was used to investigate the quality of phase conjugation by four-wave mixing in saturable absorbers. Illustrative examples are presented for the case of a phase conjugating and an ordinary mirror.

Measurement of three-dimensional deformation vectors with digital holography and stereophotogrammetry.

We present a method to determine the three-dimensional (3D) deformation vectors of an arbitrary stressed object by combining stereophotogrammetry and digital holography in a setup with four cameras. The resulting data consists of a dense 3D point cloud, where every point is associated with a deformation vector. Our method is able to calculate the deformation without prior knowledge of the sensitivity vectors or the object surface. In the experimental setup only the base distance of the cameras needs to be known.

Optical information storage using refresh via phase conjugation

Abstract In this paper we discuss the possibility for realizing an optical memory using dynamic refreshment. Via phase-correct back-coupling by means of nonlinear optical phase-conjugation, the information stored in a photorefractive crystal is incessantly read out, transmitted into an auxiliary memory and from this back into the crystal again and in this way refreshed. Practical realizations and first results are presented.