Hartmut Gruber

Liquid crystal display as spatial light modulator for diffractive optical elements and the reconstruction of digital holograms

Liquid crystal displays, originally designed and fabricated for projection systems, are often used as spatial light modulators in optical correlators or in fringe projection systems. Operating in a linearized phase mode, they can achieve a performance to be applicable as diffractive elements. An adapted driver electronics and adapting measurements of the phase modulation behavior can lead to a dynamic phase modulating system with an almost linear modulation and a maximum phase shift of 2(pi) . The electronic system can directly address the graphics card signal or can picture various video standards. So, computer generated holograms can be addressed at video frame rates. The limiting parameters of the performance are mostly due to physical boundary conditions, such as pixel number and size, response time, transmission etc.. We can assume that the fast growing micro-structuring technology will serve us soon with displays of higher resolutions and efficiency. Beam shaping elements, two-dimensional holograms and the reconstruction of digital holograms will be shown. The latter opens new possibilities for non-destructive testing devices especially in the field of holographic interferometry. We will show results of different systems and derive boundary conditions for applications in holographic reconstruction of coherent masks.

Fault detection and feature analysis in interferometric fringe patterns by the application of wavelet filters in convolution processors

The detection and classification of faults is a major task for optical nondestructive testing in industrial quality control. Interferometric fringes, obtained by real-time optical measurement methods, contain a large amount of image data with information about possible defect features. This mass of data must be reduced for further evaluation. One possible way is the filtering of these images applying the adaptive wavelet transform, which has been proved to be a capable tool in the detection of structures with definite spatial resolution. In this paper we show the extraction and classification of disturbances in interferometric fringe patterns, the application of several wavelet functions with different parameters for the detection of faults, and the combination of wavelet filters for fault classification. Examples for fringe patterns of known and varying fault parameters are processed showing the trend of the extracted features in order to draw conclusions concerning the relation between the feature, the filter parameter, and the fault attributes. Real-time processing was achieved by importing video sequences in a hybrid opto-electronic system with digital image processing and an optical correlation module. The optical correlator system is based on liquid-crystal spatial light modulators, which are addressed with image and filter data. Results of digital simulation and optical realization are compared.

Liquid crystal display as dynamic diffractive element

Liquid crystal panels, originally designed and fabricated for projection systems, are used as spatial light modulator in optical correlators or in fringe projection systems. An adapted driver electronics and measurements of the phase modulation behavior can lead to a dynamic phase modulating system with an almost linear modulation and a maximum phase shift of 2(pi) . The electronics of our Sony-LCD based system can directly address the graphics card signal or can picture various video standards. So, computer generated holograms can be addressed at video frame rates. We demonstrate examples of technical beam splitters, 2D holograms, reconstruction of digital holograms and animation of diffraction patterns. Here we will state to problems of light efficiency, intensity and wavelength dependent modulation. Furthermore, we will show first results of an FLC based system and derive boundary conditions for applications in holographic projection and micro- structuring.

Spatial light modulator system for application as a dynamic diffractive element and in optical image processing

The importance of spatial light modulators (SLMs) in optical systems is increasing. This paper presents our work on an SLM system based on Sony LCD. We show the optimization of the LCD modulation behavior concerning the application as dynamical diffractive element, and the implementation of images and filter functions in optical correlators and pattern projection systems. Measurements of the coupled amplitude and phase modulation and the planarity of the display at different wavelengths lead to correction functions and several optimized operation modes, e.g. gray scale or binary amplitude modulation. The optimization in the amplitude mode leads either to high contrast images of reduced gray levels or to a mostly linearized full gray scale mode. A similar way leads to optimized modes for the phase modulation and to a realization of a total phase shift of 2(pi) . The increase of the processing performance of phase-only filters in optical correlators is presented. Furthermore, diffraction efficiency (DE) measurements of Fourier holograms addressed to the SLM prove the optimization process, compared to the non-optimized and the theoretical DE-values. So, estimations can be made concerning the application of the SLM as dynamical diffractive element.

Applications of the high-resolution optical reconstruction of digital holograms

Adressable spatial light modulators with as much as possible ideal phase modulation are the precondition for their application in digital holography. An adapted driver electronics for the modulator and a correct knowledge of the modulation behavior can lead to a dynamic phase modulating device with nearly linear characteristic curve and a maximum phase range of 2π. We show a system for recording and reconstruction of digital holograms applying a spatial light modulator for the optical reconstruction and the digital processing of the holograms. The data of a CCD-camera are taken to a PC and sent to a spatial light modulator. In that sense we realised an analog-digital converter for recording and a digital-analog converter for the optical hologram reconstruction. We discuss the resolution of the reconstruction and their applications, especially possibilities for the manipulation with the reconstructed wave field.

Nematic LCoS spatial light modulators: performance in diffractive optics

Approaching the ideal high resolution phase addressable modulator is a main task for current SLM (Spatial Light Modulator) development. Different technologies, such as optically addressed, modal and electrically addressed spatial light modulators compete in performance and applicability. The requested high Space-Bandwidth-Product (SBWP) can be served by the actual micro-structuring technology used to fabricate LCoS (Liquid Crystal on Silicon) micro-displays. Liquid crystal displays in different modes are suitable due to their birefringence properties and wide transmission range.

Optical processing for the detection of faults in interferometric patterns

The detection and classification of faults is a major task for optical nondestructive testing in industrial quality control. Interferometric fringes, obtained by real-time optical measurement methods, contain a large amount of image data with information about possible defect features. This mass of data must be reduced for further evaluation. One possible way is the filtering of these images applying the adaptive wavelet transform. The wavelet transform has been proved to be a capable tool in the detection of structures with definite spatial resolution. In this paper it is shown the extraction and classification of disturbances in interferometric fringe patterns, the application of several wavelet functions with different parameters for the detection of faults, and the combination of wavelet filters for fault classification. Furthermore the implementation of complex valued wavelet filters and correlation filters is shown. We will present an algorithm to classify interferometric fringe patterns. In order to achieve real-time processing a hybrid opto-electronic system with a digital image processing and an optical correlation module is favored. The calculated wavelet filters are implemented into the optical correlator system that is based on liquid-crystal spatial light modulators. So, all discussed items were verified experimentally in the optical setup.

Analysis of interferometric fringe patterns by optical wavelet transform

For the application of interferometric methods in industrial quality control and particularly in non-destructive material testing an extensive analysis of the obtained image data is needed. In the field of optical image processing wavelet transformation has been proved to be a capable tool in the detection of structures with definite spatial resolution. The selection of the analyzing wavelet function and the variation of parameters lead to a wide range of selective wavelet filters, which are able to perform detection and classification of structures. The presented paper demonstrates numerical simulations and their optical realization in an SLM- based correlator. To locate defect patterns of different classes in interferometric fringe patterns, several wavelet functions, which are optically realizable by a transmission distribution, were utilized. The optical experiments indicate a good accordance with the computer simulations.

High resolution coherent optical reconstruction of digital holograms and their applications

Liquid crystal panels, originally designed and fabricated for projection systems, are used as spatial light modulator in optical correlators or in fringe projection systems. An adapted driver electronics and measurements of the phase modulation behavior can lead to a dynamical phase modulating system with an almost linear modulation and a maximum phase shift of 2π. We built up a system with a LCD based spatial light modulator in order to realize the optical the reconstruction of digital holograms. Hologram reconstructions and the use of holographic interferograms for deformation detection are presented and future developments and applications are discussed.

Optical characterization of liquid-crystal-on-silicon displays

Electronically addressed spatial light modulators (SLMs) are key elements for the reconstruction of digital holograms. Reflective liquid-crystal-on-silicon displays (LCOS) have great potential to fulfill this task due to their high fill factors of over 90% and their small pixel sizes of less than 15 μm. In order to obtain maximum diffraction efficiency of the holographic reconstruction, analog phase holograms have to be implemented making a maximum phase shift of 2π in each LCOS pixel necessary. Therefore, each LCOS display has to be thoroughly characterized prior to its use as a holographic element. In this publication, we report on a specially designed LCOS test bench. Here, displays can be characterized with respect to their phase and amplitude modulation (i.e. the complex transmittance) under a varying angle of the incident linearly polarized light. Additionally, the Jones matrix of the displays can be measured, which allows computation of the response of the displays to light of arbitrary polarization. The measurement of panel flatness is also possible which is necessary to compensate wave front aberrations. Results of measurements of two LCOS dis-plays are presented and a comparison to other measurement methods is given.