Christian Lingel

Optimizing the diffraction efficiency of SLM-based holography with respect to the fringing field effect

The fringing field effect of liquid-crystal displays causes a crosstalk between neighboring pixels, so that a desired sharp phase edge gets blurred. This blurring effect influences the diffraction efficiency of holograms, which are displayed on the spatial light modulator (SLM). In this paper, we show two different simulation models for the SLM, one based on the measured subpixel Jones matrices of the SLM and the other based on a direction-dependent convolution model. Using these models we optimize different blazed gratings written in the SLM according to their diffraction efficiency followed by an experimental verification.

Multipoint vibrometry with dynamic and static holograms

We report on two multipoint vibrometers with user-adjustable position of the measurement spots. Both systems are using holograms for beam deflection. The measurement is based on heterodyne interferometry with a frequency difference of 5 MHz between reference and object beam. One of the systems uses programmable positioning of the spots in the object volume but is limited concerning the light efficiency. The other system is based on static holograms in combination with mechanical adjustment of the measurement spots and does not have such a general efficiency restriction. Design considerations are given and we show measurement results for both systems. In addition, we analyze the sensitivity of the systems which is a major limitation compared to single point scanning systems.

Advanced multipoint vibrometry using spatial light modulators

We present a new type of system for measuring the vibration simultaneously at multiple positions (multipoint vibrometry) using multiple (at the moment 14) individual channels, each consisting of a complete heterodyne interferometer. Control of the three-dimensional position of the measurement positions is achieved using dynamic holography implemented by a liquid crystal on silicon (LCoS) spatial light modulator. Compared to full field vibration measurement techniques like ESPI, a high temporal bandwidth in combination with a high dynamic range is achieved for the interference signal. Transient phenomena at different points can be acquired. We analyse the system concerning the light efficiency and show first experimental results.

A benchmark system for the evaluation of selected phase retrieval methods

In comparison to classical phase measurement methods like interferometry and holography, there are many phase retrieval methods which are able to recover the phase of a complex valued object without the necessity of a reference wave. Due to the large number of different methods, iterative as well as non-iterative ones, it is hard to find the method which is appropriate for a given application or object. We propose a system which is based on different criteria, some of which can be calculated by analyzing the phase retrieval result compared to the original object. Other criteria like the complexity of the optical system are also taken into account. For testing the benchmark system we use a software which is suitable, first, to simulate the acquisition process of the intensity measurements, second, to run the phase retrieval algorithm itself and, third, to calculate the values of the benchmark criteria. Having determined the values of the different criteria we assign points for every criterion which can be weighted by importance and are summed up for getting an overall benchmark score. This final score can be used to compare different phase retrieval methods and by having a closer look at the single criteria it is possible to analyze the strengths and weaknesses of the method. We will show the detailed proceeding of calculating the benchmark value by means of a selected phase retrieval method and a phase only object (USAF target). We have to emphasize that the results strongly depend on the object.

Parallelized genetic optimization of spatial light modulator addressing for diffractive applications

We describe a new technique for optimizing the addressing of spatial light modulators in dynamic holographic applications. The method utilizes 200 times parallelization using imaging of subholograms in combination with genetic optimization. Compared to a fixed linear addressing curve for all different gratings, the diffraction efficiency can be improved by up to 25% for a Holoeye Pluto LCoS modulator.

Characterization and demonstration of a 12-channel laser-Doppler vibrometer

Scanning laser-Doppler vibrometry is the standard optical, non-contact technology for vibration measurement applications in all areas of mechanical engineering. The vibration signals are measured from the different measurement points at different time points. This requires synchronization and the technology is limited to repeatable or periodic events. We have explored a new solution for the optical setup of the sensing system of a multi-channel vibrometer that we present in this paper. Our optical system is a 12-channel vibrometer and consists of a 12-channel interferometer unit which is connected with 12 optical fibers to a sensor head with 12 fiber-coupled objective lenses. Every objective lens can be focused manually and is placed in a sphere which can be tilted and fixed by a blocking screw. Thus it is possible to adjust a user defined measurement grid by hand. The user can define the geometry of the measurement grid in a camera image displayed in the software by just clicking on the laser foci. We use synchronous analog-digital conversion for the 12 heterodyne detector signals and a digital 12-channel-demodulator which is connected via USB to a computer. We can realize high deflection angles, good sensitivity, proper resolution, sufficient vibration bandwidth, and high maximum vibration amplitudes. In this paper, we demonstrate the optical and electrical setup of the manually adjustable 12-channel vibrometer, we present the experimentally evaluated performance of our device, and we present first measurements from real automotive applications.

SLM-based Multipoint Vibrometry

We present a spatial light modulator-based multipoint vibrometer realizing 14 independent fast heterodyne interferometry channels for measuring vibrations. The positions of the measurement spots on the object are controlled using dynamic holography.

Examination and optimizing of a liquid crystal display used as spatial light modulator concerning the fringing field effect

Due to the small pixel sizes of modern LC-based microdisplays the crosstalk between adjacent pixels has to be taken into account when considering the performance of LCDs used as spatial light modulators in optical setups. This crosstalk effect is called fringing field effect. Measurements show that this effect is anisotropic and therefore has different influences on the diffraction efficiencies of vertical and horizontal gratings. Because such microdisplays are employed in many different holographic applications it is desirable to improve the diffraction efficiency as much as possible. Recently a simulation model of an LCD which takes the fringing field effect into account has been presented. Using this model optimization of blazed gratings is realized to improve the diffraction efficiency.

Reduction of chromatic dispersion using multiple carrier frequency patterns in SLM-based microscopy

Typically, spatial light modulator (SLM)-based microscopy is implemented using a carrier frequency in order to avoid disturbances due to the non-ideal modulation behavior of most SLMs. However, in combination with polychromatic light, this leads to strong chromatic aberrations due to the dispersion at the grating formed by the carrier frequency. In this contribution, we introduce a method based on the evaluation of multiple images obtained with different carrier frequency orientations. This way, chromatic aberrations and the limitation concerning the object field can be strongly reduced.

A 14-channel multipoint vibrometry system using dynamic holography

We report on a new system for measuring the vibration simultaneously at multiple positions (multipoint vibrometry) using multiple individual channels, each consisting of a complete heterodyne interferometer. The core element for controlling the three-dimensional position of the measurement spots is a liquid crystal on silicon (LCoS) HDTV spatial light modulator. Unwanted modulation effects, especially unwanted diffraction orders and crosstalk effects are taken into account for the design of the optical system and the computation of the holograms. We present a general discussion with an emphasis on the LCoS related considerations and first measurement results.