Michael Warber

Dynamic holography using pixelated light modulators

Dynamic holography using spatial light modulators is a very flexible technique that offers various new applications compared to static holography. We give an overview on the technical background of dynamic holography focusing on pixelated spatial light modulators and their technical restrictions, and we present a selection of the numerous applications of dynamic holography.

3D Holographic Imaging and Trapping for Non-Invasive Cell Identification and Tracking

Real-time high-throughput identification, screening, characterization, and processing of biological specimen is of great interest to a host of areas spanning from cell biology and medicine to security and defense. Much like human biometrics, microorganisms exhibit natural signatures that can be used for identification. In this paper, we first overview two optical techniques, namely digital holographic microscopy and holographic optical tweezers which can non-invasively image, manipulate, and identify microorganisms in three dimensions. The two methods bear similarities in their optics and implementation. Thus, we have proposed a new approach to identification of micro/nano organisms and cells by combining the two methods of digital holographic microscopy and holographic optical tweezers which can be integrated into a single compact hardware. The proposed system can simultaneously sense, control, identify, and track cells and microorganisms in three dimensions. New possibilities that arise from the proposed method are discussed.

SLM-based phase-contrast filtering for single and multiple image acquisition

We report on the implementation of different phase contrast methods using an SLM in a microscope. The ease of generating complex phase filters with the SLM opens the possibility to realize standard filters adapted to the specimen and the possibility to develop new phase contrasting methods. Due to the real-time addressing we can obtain a number of different images from each specimen recorded nearly simultaneously with the same microscope objective. We demonstrate how to realize different versions of differential interference contrast imaging, Zernike-type imaging and dark-field imaging and the combination of the different images by simple post processing. Experimental results for biological as well as technical specimens are presented.

Holographic optical tweezers with real-time hologram calculation using a phase-only modulating LCOS-based SLM at 1064 nm

We present a method that enables the generation of arbitrary positioned dual-beam traps without additional hardware in a single-beam holographic optical tweezers setup. By this approach stable trapping at low numerical aperture and long working distance is realized with an inverse standard research microscope. Simulations and first experimental results are presented. Additionally we present first steps towards using the method to realize a holographic 4π-microscope. We will also give a detailed analysis of the phase-modulating properties and especially the spatial-frequency dependent diffraction efficiency of holograms reconstructed with the phase-only LCOS spatial light modulator used in our system. Finally, accelerated hologram optimization based on the iterative Fourier transform algorithm is done using the graphics processing unit of a consumer graphics board.

Scene-based wavefront correction with spatial light modulators

Spatial light modulators (SLM) are used in different microscopy setups. Examples are optical tweezers, programmable phase contrast imaging, confocal imaging, and aberration correction. We report on a method that measures and corrects specimen-induced aberrations in wide-field microscopy without additional optical components (e.g. Shack-Hartmann sensors) by taking advantage of the SLM that is already used in the setup. Different local gratings are written into the SLM which is positioned in a plane conjugate to the pupil of the imaging system. Multiple images are recorded and based on the shift of subimages we deduce the wavefront. We demonstrate first experimental results of this method for a system using a high resolution LCoS modulator.

Combination of scene-based and stochastic measurement for wide-field aberration correction in microscopic imaging

We report on a novel aberration correction technique that uses the sequential combination of two different aberration measurement methods to correct for setup-induced and specimen-induced aberrations. The advantages of both methods are combined and, thus, the measurement time is strongly reduced without loss of accuracy. The technique is implemented using a spatial-light-modulator-based wide-field microscope without the need for additional components (e.g., a Shack-Hartmann sensor). The aberrations are measured without a reference object by directly using the specimen to be imaged. We demonstrate experimental results for technical as well as biological specimens.

Vertical differential interference contrast using SLMs

In microscopy, several different phase contrast methods (e.g. Zernike contrast, differential interference contrast) have been developed to image phase objects. For these methods specialized microscopic equipment (modified microscope objectives, filters, etc.) is needed. The static elements within such microscopes are a trade-off, because phase contrast imaging depends strongly on the object and the information to be visualized. We show results of an implementation of many different phase contrast methods using a high resolution phase-only spatial light modulator (SLM) in the pupil plane. All implemented methods are realized by software. Therefore, it is not only feasible to change the different phase contrast methods in real time, but it is also possible to optimize the parameters. Images obtained from different settings are combined digitally to improve the final image quality. Furthermore, completely new phase contrast filters can be tested easily because the phase of each pixel can be changed arbitrarily. We use this method to implement a new phase contrast filter that is obtained by combining a focused and a defocused point spread function. We will present theoretical as well as experimental results of this vertical differential interference contrast filter.

Vertical differential interference contrast

We propose a new phase contrast filtering technique based on a combination of a focused and a defocused point-spread-function. This way, an axial shear is introduced in the imaging system. Compared to conventional differential interference contrast, an isotropic behavior is achieved. The lateral resolution is improved compared to conventional defocusing. Furthermore, the digital combination of multiple images leads to strongly enhanced visualization of small structures. We show simulated results as well as experimental results using a spatial-light modulator-based microscope.

SLM-based multipoint vibrometry

Heterodyne interferometry is a very accurate and robust technique for measuring vibrations under industrial conditions. Typically, instruments based on this principle measure on a single point and scanning is employed to obtain operational deflection shapes. For the simultaneous measurement of vibrations (e.g. to detect transients) it is possible to use a fixed arrangement of measurement beams. Dynamic steering of multiple beams, however, is not easy to achieve. In this contribution we present a solution for multipoint vibrometry using a high resolution (HDTV) programmable spatial light modulator as the core element. By using a liquid crystal display (LCD) for displaying a dynamic hologram it is possible to independently control multiple measurement spots in three dimensions with high repeatability and accuracy. Mechanical movements that might introduce noise do not oocur. The main challenge in designing such an LCD-based multipoint vibrometer is to avoid problems due to the unwanted spurious diffraction orders that will be present when using commercially available spatial light modulators for reconstructing holograms. We present a system in which the illuminating as well as the detection is programmable. One half of the high resolution LCD is used for illumination and the other half is responsible for the detection. Different possible methods to avoid spurious diffraction orders are shortly discussed. Emphasis will be laid on a method based on complex multiplexing (using hologram optimization) and a spatial multiplexing method based on Golay arrays. We show the optical design as well as first experimental results for a single detector. Hologram computation is based on a joint optimization of the detection and the illumination hologram using a modified Gerchberg-Saxton algorithm together with combinatorial optimization of the spot/detector mapping.

Advanced Scanning Laser‐Doppler Vibrometer with Computer Generated Holograms

In this paper we present a novel technique for steering the beam of a scanning laser‐Doppler vibrometer (LDV) using a Spatial Light Modulator (SLM). Computer Generated Holograms (CGH) are employed to obtain the phase maps displayed by the SLM. Due to this approach, spurious diffraction orders are generated. We present concepts to suppress these diffraction orders so as to realize a scanning vibrometer. We discuss the properties and limitations of this solution. Different SLMs have been evalutated and a compact scanning vibrometer based on a Holoeye Pluto SLM has been realized. First measurement results are presented. In addition, we demonstrate simulations on the reduction of speckle related signal dropouts. Drop‐Outs can be reduced by adapting the measurement‐beam wavefront with the CGH to maximize the light power collected with the vibrometer aperture. We have explored an approach to optimize the signal strength by adapting the coefficients of the Zernike polynomials of an additional wavefront shift.