Lars Seifert

The adaptive Shack–Hartmann sensor

Abstract Replacing the static microlens array in a Shack–Hartmann sensor by a liquid-crystal display offers a very flexible measurement approach. The liquid-crystal display is used for the generation of an array of Fresnel microlenses. The design parameters of the microlenses like focal length, number of lenses, position, and aperture size can be quickly adapted to solve difficult measurement tasks. This adaptive Shack–Hartmann sensor allows a pre-correction of the wavefront to take care of larger aberrations and to achieve a higher measurement accuracy. This makes new strategies possible to determine the wavefront.

Using graphics boards to compute holograms

Todays,consumer graphics boards incorporate highly integrated, parallel-working graphics processing units (GPUs) with transistor counts and performance that exceed those of CPUs. In addition to playing the latest 3D video game, you can use the graphics boards power to solve computational problems in science or engineering work. Current GPUs are programmable and flexible enough to transfer computational problems from the CPU to the GPU.Why shift computational work to the GPU? The short answer is processing power. As we demonstrate, using a standard graphics board can considerably speed up the overall performance of tasks such as computing Fourier holograms in real time.

Application of SLMs for optical metrology

For wavefront sensing, wavefront shaping, and optical filtering, spatial light modulators can be very useful. With the availability of high resolution liquid crystals (LC) spatial phase modulators and micromechanical systems (MEMS) containing large arrays of micromirrors, new applications in optical metrology become possible. For wavefront analysis and correction, dynamic CGHs are used. A correction hologram for the aberrated system is computed from which the lens shape can be derived. For Hartmann sensors, usually static microlenses are used. It was found advantageous to generate dynamic microlenses in order to correct for local wavefront aberrations. Optically addressed spatial light modulators can be applied very effectively for the characterisation and defect analysis of primarily periodic structures such as microchips or microlens arrays. For triangulation based methods, better results can be obtained by adapting the projected fringes to the object in terms of shape and brightness. Examples and experimental results are discussed.

Adaptive Shack-Hartmann sensor

We present a new type of Shack-Hartmann sensor. Replacing the static microlens array in a Shack-Hartmann sensor by a liquid crystal display results in a greater measurement flexibility. The liquid-crystal display in the adaptive Shack-Hartmann Sensor is used for the generation of an array of Fresnel microlenses. The design parameters of the microlenses like focal length, number of lenses, position, and aperture size can be quickly adapted to solve difficult measurement tasks. Furthermore the adaptive Shack-Hartmann sensor allows a pre-correction of the hologram to the wave-front to take care of larger aberrations and to achieve a higher measurement accuracy. We compare the adaptive sensor with a conventional Shack-Hartmann sensor and discuss new strategies to determine the wave-front that are possible with this active system.

Fast hologram computation and aberration control for holographic tweezers

Holographic tweezers offer a very versatile tool in many trapping applications. Compared to tweezers working with acousto optical modulators or using the generalized phase contrast, holographic tweezers so far were relatively slow. The computation time for a hologram was much longer than the modulation frequency of the modulator. To overcome this drawback we present a method using modified algorithms which run on state of the art graphics boards and not on the CPU. This gives the potential for a fast manipulation of many traps, for cell sorting for example, as well as for a real-time aberration control. The control of aberrations which can vary spatially or temporally is relevant to many real world applications. This can be accomplished by applying an iterative approach based on image processing.

Active wavefront sensing and wavefront control with SLMs

Active systems are becoming more and more prevalent in the the field of optical technology. These systems require not only means of controlling wavefronts but mostly also of sensing wavefronts. This paper shows in four examples how spatial light modulators can be utilized to perform active wavefront sensing and wavefront controlling tasks. The examples include an interferometer with a flexible reference, a method to determine and compensate wave aberrations in focusing optical systems, an adaptive Shack-Hartmann sensor with a microlens array generated by a liquid crystal display and wavefront error compensation strategies with a piston micro mirror array.

Active wavefront shaping and analysis

Active and adaptive optics are becoming more important in the field of beam shaping and wave front analysis. One of the main reasons for the progress and recent activities in active optics is the availability of new active elements like deformable mirrors, micro mirrors and liquid crystals. The application of spatial light modulators for wave front adaption, shaping and sensing will be discussed. An important application is a flexible testing procedure for testing aspheric surfaces using adaptive optics such as deformable mirrors liquid crystals and micro mirrors.

Wavefront sensing by an aperiodic diffractive microlens array

Shack-Hartmann sensors are commonly used wavefront sensors in a large field of applications, like adaptive optics, beam characterization and non-contact measurements. They are popular because of the ease of use and the robustness of the sensor. We introduce a new way to improve the performance of miniaturized and mass-producible optical wavefront sensors for industrial inspection: A sensor design due to an aperiodic diffractive element working as microlens array allows the use of small and cost-efficient detector chips. The diffractive element was optimized using raytracing and thin element approximation (done in Zemax). As an example, we present the design and realization of a sensor for laboratory use with a measurement diameter of 20mm. We show an example measurement and results concerning dynamic range. The measurement accuracy was determined by measuring spherical waves.

Fast hologram computation for holographic tweezers

We have shown that it is possible to accelerate considerably the computation of phase-only Fourier holograms by using a consumer graphics board (MSI 6800GT) instead of the ordinary CPU. Our fastest CPU solution (Pentium 4 @ 3.0 GHz) — using handcoded assembly code together with the multimedia extensions SSE — was outperformed by a factor of more than thirty resulting in an average performance of 14.1 GFlops for 100 doughnuts. The presented algorithm can be used for the computation of holograms for an arbitrary (up to 250) number of traps located at different positions in three dimensions and having independent trapping potentials.

Dynamic holography and its application in measurement systems

Spatial light modulators are of growing interest not only for optical correlators but also for new optical measurement and processing methods. We present different applications of dynamic phase holograms based on liquid crystal elements in the field of optical measurement and manipulation. Within digital holography, modern modulators can be used in order to test the geometry as well as the behavior of objects under external load. A direct comparison between the test objects and a master object at different locations around the world is possible. Holographic tweezers are used in order to position small particles in three dimensions and to measure very small forces. We also present results of novel methods for testing aspheric surfaces and the application of dynamic hologram reconstructions for the ablation of complex patterns on the microscopic scale.