A new interferometric technique for the measurement of aspheric elements based on multiple test beams is
presented. By means of an array of sources (Point Source Array) an aspheric surface is illuminated under
different angles which allow the measurement of the zones where the local gradient of the test piece is
compensated. One of the main advantages of the system is that the measurement process is performed in
parallel (many sources are used at the same time) thus requiring extremely short measurement time in
comparison with other available subaperture testing techniques. Another important aspect is that the asphere
stays in the same position during the whole process; there are no mechanical movements of the test part
The technique allows the measurement of strong aspheric elements with departures from the best fit sphere up to
±10°. The method was developed to obtain accuracies of up to λ/30 and better. Simulations and first
experimental results are presented.
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.
Micro Mirror Arrays (MMAs) offer the potential of a high spatial and temporal resolution technology for wavefront control applications. In this paper a new Micro-Electro-Mechanical-System (MEMS) based MMA type is investigated. As opposed to most other MMA technologies which involve flip mirrors with only two possible orientations, this system can support two different mirror designs, piston-type mirrors for a continuous phase adjustment and tilt mirrors for light discarding purposes. The MMA's wavefront correction capabilities are being investigated in a breadboard which simulates continuous distortions and step errors, such as those that could be expected from lightweight primary mirrors of space telescopes or segmented mirrors, respectively. The wavefront is corrected by the MMA, then coupled into a monomode fiber. Four different correction methods have been tested, two stochastic approaches, a closed-loop Shack-Hartmann approach and an interferometric approach. Comparison of coupling efficiency is made between these approaches and against theoretical calculations.
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.
Interferometric testing of optical surfaces is problematic when strong asphericities are present. The spatial frequencies of the interference fringes exceed the detector resolution where the slope difference between test beam and reference beam is too large. CGHs are frequently used to avoid this effect but availability and flexibility is a problem. Alternatively we propose a new method to extend the dynamic range of interferometric measurements. For this purpose the reference beam in an interferometer is adapted. An active element containing a spatial light modulator (SLM) is used to vary the slope of the reference beam within a few degrees in both x- and y-directions. Hereby different areas of the interferogram become evaluable. Furthermore the active element can introduce phase shifts necessary for the phase shift interferometry algorithms. Several interferograms with different reference beam slopes are recorded and finally the phase functions are "stitched" together. By using an SLM for the reference beam tilt, no mechanical motion of any hardware which would limit the accuracy is necessary. A calibration of the tilts can be performed with interferometric accuracy.
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.
In the last decade optical tweezers became an important tool in microbiology. However, the setup becomes very complex if more than one trap needs to be moved. Holographic tweezers offer a very simple and cost efficient way of manipulating several traps independently in all three dimensions with an accuracy of less 100 nm. No mechanically moving parts are used therefore making them less vulnerable to vibration. They use computer-generated holograms (CGHs) written into a spatial light modulator (SLM) to control the position of each trap in space and to manipulate their shape. The ability to change the shape of the optical trap makes it possible to adapt the light field to a specific particle shape or in the case of force measurements to adjust the trapping potential. Furthermore the SLM can be used to correct for aberrations within the optical setup.
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.
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.