Balazs Gombkoto

Computer simulation of reflective volume grating holographic data storage

The shift selectivity of a reflective-type spherical reference wave volume hologram is investigated using a nonparaxial numerical modeling based on a multiple-thin-layer implementation of a volume integral equation. The method can be easily parallelized on multiple computers. According to the results, the falloff of the diffraction efficiency due to the readout shift shows neither Bragg zeros nor oscillation with our parameter set. This agrees with our earlier study of smaller and transmissive holograms. Interhologram cross talk of shift-multiplexed holograms is also modeled using the same method, together with sparse modulation block coding and correlation decoding of data. Signal-to-noise ratio and raw bit error rate values are calculated.

Application of super image methods in digital holography

As in the classical holography, a major issue in digital holography is the enhancement of the resolution of the digital holograms. It means not only the enhancement of the image quality, but the extension of the upper measuring range too. One natural way can be the application of higher resolution recording devices. Unfortunately the price of a doubled resolution camera is approximately fourfold. In the presentation a different way of the resolution enhancement is shown. The resolution enhancement (building the super image) is based on the building of well sampled so called super images from a set of under sampled but dithered input images. Such methods are originated from the drizzle method, and from the Fourier spectrum combination method. Using these methods not only the resolution of the digital hologram can be increased, but the object distance also can be dramatically shortened.

Difference displacement measurement by digital holography by use of simulated wave fronts.

A concept called fringe compensation was first presented in phase-shifting electronic speckle-pattern interferometry. We apply a similar principle to digital holographic interferometry; here the phase of a wave front is known and can be manipulated. The basic mathematical formulation of fringe compensation and some experimental results are shown with relatively large, simple rigid-body rotation and circular membrane deformation.

Application of the fast-Fourier-transform-based volume integral equation method to model volume diffraction in shift-multiplexed holographic data storage

Numerical simulation of diffraction on thick holographic gratings in shift-multiplexed optical data storage application is presented. The grating is generated by the interference of a spherical reference wave and a plane signal wave corresponding to a single pixel of the input data page. To describe diffraction on this weak-index-modulated grating, we use the volume integral equation in the first Born approximation. This description yields a convolution integral that can be efficiently evaluated by a 3D fast Fourier transform (FFT) technique. For a 51.2 microm recording layer thickness, a serial-divided single personal computer code was built based on parallel FFT coding principles. Diffracted electric field and Poynting-vector distributions are calculated for probe beams spatially shifted with respect to the reference beams. The shift selectivity curves show significant differences from previous analytical calculations based on paraxial propagation and infinite gratings, as they have monotonic decrease in all three directions instead of sinclike functions with Bragg nulls. With the chosen numerical aperture of 0.6 and linear polarization, both the scalar and vector calculations provided similar results within 5%.

Difference displacement measurement using digital holograms as coherent masks

The rapid development of spatial light modulators in the past few years opened an exciting new area in coherent optical metrology. Commercially available liquid crystal spatial light modulators (LC SLMs) are capable to optically reconstruct digital holograms in quite good quality, so the reconstructed real image of an object can be used as a coherent illuminating mask in optical measurement methods like digital holography (DH) or Electronic Speckle Pattern Interferometry (ESPI). In our work we present experimental results of measuring the difference and sum of two displacements of an object pair (master and test object) using these two techniques. We describe the measurement setups in a DH and an ESPI arrangement, which are capable to project the real image of the master object -- using its previously recorded digital holograms in the SLM device -- onto the test object. If two digital holograms, recorded before and after the deformation of the master object, are used to illuminate the test object in its initial and deformed state, four images can be recorded either in the DH setup or in the ESPI setup. Using these four-four images, the contour fringes of the difference and sum of the master and test object displacements can be calculated. In the case of DH, these images are digital holograms, which are subject to numerical reconstruction, and in the case of ESPI the four images are plain speckled images, which can be used to obtain ESPI fringes (correlograms). As a result, we present several fringe images of our object pair made with these two methods.

Analogue and Digital Holography: Absolute and Comparative Measurements and Measuring Range Extension (Analoge und Digitale Holografie: Absolute und vergleichende Messungen sowie Messbereichserweiterungen)

Summary In the paper a brief survey of research at the Department is given that had a twofold aim: to support the project DISCO in indirect and direct ways as well. In this respect the physical state of the art of the analogue basis of DISCO is discussed: principle of the comparative technique named difference holographic interferometry (DHI) is analysed and applications for comparative deformation and shape control in analogue holography are presented. Peculiarities of holographic illumination as well as requirements for repositioning of the master and test objects at comparative measurements are treated. Investigations of the measuring range extension in holographic interferometry are described. Detailed analysis is devoted to the puzzle-read-out extension technique. Finally, application of the principles of DHI in comparative deformation control by digital holography and the developed portable laboratory device for comparative deformation control are presented.

Modeling multilayer microholographic storage with nonlocal and nonlinear storage material behavior

We present a model of microholograpic storage usable for system optimization, tolerancing and noise analysis. The storage material model includes monomer diffusion that returns the known saturation and the observed non-local behavior of microholograms.

Comparative displacement measurement by digital holographic interferometry

Digital holography is a powerful tool in NDT. Different measuring methods have been developed to perform more flexible measurements and to alleviate the drawbacks of this technique. The rapid development of spatial light modulators in the past few years opened an exciting new area in coherent optical metrology. Commercially available liquid crystal spatial light modulators (SLMs) are capable to optically reconstruct digital holograms in good quality, so the reconstructed real image of an object can be used as a coherent illuminating mask in optical measurement methods like digital holography. Combination of digital holography and TV holography (ESPI) is also possible. In the present work five methods of digital holography are investigated which are able to implement comparative measurement. Both the experimental arrangements and measuring results are presented.

Monte Carlo method in digital holography

The record of the interference pattern of the object wave and the reference wave is done by the application of high resolution photoplates in holography. After developing the photoplate the reconstruction of the object wave can be realized illuminating the hologram by the reference beam. In digital holography the photographic process is eliminated for the interference patterns are recorded by a CCD camera and the reconstruction can be done virtually using a computer. In general in-line reference beam is used coding the phases for the low resolution of the available CCD devices. The phase-shift technique (or application of two reference beams) can be applied to recover the phase and the amplitude of the image wve at the plane of the CCD matrix. The FFT algorithm is widely used for the reconstruction. Now it is proposed to apply the Monte-Carlo method simulating the diffracted wave to get the intensity distribution at the image plane. The application of Monte-Carlo simulation has a drawback, that is it can be slower than FFT, but its advantages can be significant, namely the intensity pattern of the diffracted wave can be determined along an inclined plane, just a part of the object can be examined if it is required and additionally there are no difficulties in extending it to phase holograms and 3-D objects.

New methods in recording and reconstruction of digital holograms

Digital holography gives new capabilities to the measurements of size, position, shape or velocity of different objects. In general in-line arrangement is used with a CCD device recording the interference pattern and the reconstruction is performed by numerical methods in computer. Some new ideas were used to improve the process of exposure and calculation techniques. The phase-shift technique is widely used in digital holography. Now it is proposed to use the two-reference-beams arrangement, which can replace the phase-shift methods at different cases. Three holograms -- the interference pattern of the object and the two reference waves and the three intensity distribution (of the beams, separately) are recorded and the phase evaluation can be done numerically. The advantage of FFT-algorithm is also applied reducing the computer running time in reconstruction of the image. We have developed another computational method for the reconstruction using Monte-Carlo simulation.