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.
In biological micromanipulation image aberrations are introduced not
only by the optical system, but also by the immersion liquid. Whereas
optical system aberrations are constant and it is relatively easy to
measure and correct for them, the immersion caused aberrations are
variable in time and space. In this paper a method using a spherical
microparticle as an artificial point source for aberration control is
presented. The particle is positioned by optical tweezers at the
location of the biological sample. In the experiment holographic
tweezers are used. They are based on computer generated holograms,
written into spatial light modulators, which create light traps for
the microparticle in the object plane. The light traps can be moved
without any mechanically moving parts, just by changing the
hologram. The particle strongly focuses the light, therefore an
artificial point source in the object space is created. The
illumination light is filtered, so that only the signal corresponding
to a spherical wave is analyzed by the wavefront detection system. The
information about the wavefront distortion is used to dynamically
correct for it. This can be done by using spatial light modulators.
The method is suitable for biophotonic imaging systems, where
refractive index variations in the sample plane are significant. The
integration with holographic tweezers is advantageous since it offers
flexibility in positioning and imaging the particles.
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.
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.
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