Shape sensing of reflecting free form surfaces is achieved by deflection of an array of light pencils. The pencils showing very high depth of focus are produced by means of micro-optical components. Each pencil that interacts with the surface under test is redirected by reflection. Imaging intensity measurements, e.g. by a CCD-camera, at several propagation distances allow for determination of the propagation angle of each reflected pencil. By using the vector law of reflection the local slope of the surface at the location of reflection can be determined. Height data is obtained by depth from gradient methods known, e.g., from Hartman-Shack sensors. Both simulations and experimental results show a typical resolution of a view μm or 0.1 mrad for height or angle measurements, respectively.
The mask structured silver sodium ion exchange in glass (MSI) is a powerful tool for the realization of high precision refractive micro optical GRIN components. Commonly the distribution of the silver ions in GRIN elements and thus the index distribution is determined by the laws of thermal diffusion. By the use of a structured metal mask, which defines the areas of contact between the glass and the silver salt melt, an additional degree of freedom in optical design is introduced. A photolithographic pattern generator provides the accuracy of the mask structure to realize wave front optimized micro lens arrays with 100% filling factor in Cartesian, hexagonal and also in nearly any other arbitrary geometrical arrangements for several applications such as high precision Shack-Hartmann systems.
In this paper we want to discuss the potential and limits of this technique. We report on the family of optical functions, which can be realized with MSI. Furthermore we give an overview over the actual applications.
In this paper, we review the salient facts for a range of available atmosphere emulation technologies, and in the framework of the ESO Multi-Conjugate-AO demonstrator project, aptly called MAD, we present our phase screen test results for silver-sodium ion-exchange, transmissive phase screens. We find (a) that the measured power spectrum of phase fluctuations is consistent with the input Von Karman spectrum and (b) that by tracking the best focus of ten spots formed by a silver-sodium ion-exchange micro-lens array, it was found that the wavelength dependence of 1.266μm of phase-shift is 1.5±2.5% relative to air in the wavelength range 550nm to 800μm.
Additionally, we present our optical design and specifications for MAPS, the Multi-Atmospheric Phase screens and Stars instrument that will be used to test MAD before shipment to the VLT. It includes glass screens conjugate to the 0.25km, 3.0km, and 9.0km atmospheric layers above the telescope. We explain the reasoning behind the choice of pupil size and implications for phase screen proximity, footprint sizes, and wind speed gradients. Our design mimics the VLT Nasmyth F/15 focal plane in terms of plate scale, field of view, high Strehl, and field curvature.
The mask structured ion exchange in glass (MSI) is a powerful tool for realizing general planar phase distributions and in particular custom designed planar GRIN micro lenses with diffraction limited performance and high fill factor. For lens characterization the numerical aperture is a key parameter. However the classical geometrical definition of the N.A. disregards aberrations. Here we suggest an addition to this classical definition, which is based on diffraction limited performance. For a testing of micro lens arrays, global process parameters are assessed by interferometric measurements of a subset of the lenses. Local process variations typically result in small non-symmetric aberrations. These aberrations mainly lead to a lateral shift of the focus. Thus, for rapid quality control of micro lens arrays we analyze all focal positions in parallel. From the lateral deviations of the focal positions a quality criterion for each individual micro lens can be derived.
We suggest a new technique for fabricating a wide class of continuous refractive optical elements in glass by combining the technique of ion exchange with high precision structuring of metal masks. We call this technique mask structured ion exchange (MSI). We have demonstrated the potential of this method by fabrication of rectangular shaped microlenses with low numerical aperture for Hartmann-Shack wavefront sensing applications. The lenses, positioned on a 400 micrometers raster, had a fill factor of 100 %, a focal length of 33 mm and diffraction limited performance. Due to the special fabrication conditions, the lens shape, position and even the focal length can be varied spatially within one substrate. For realization of a high aperture microlens array by field assisted exchange process we could reduce proximity effects between adjacent mask apertures by MSI.
KEYWORDS: Lenses, Geometrical optics, Free space optics, Optics manufacturing, Integrated optics, Ion exchange, Ray tracing, Optical alignment, Glasses, 3D modeling
Inthe field of optical interconnection research, the topic perhaps closest to widespread applications is the realization of optical board to board and chip to chip interconnections [1]. The inherent limitations of electronic interconnects regarding bandwidth and electromagnetic interference are overcome by waveguide, fiber, or free space optics. With free space optics additionally the high interconnect density ofoptical imaging can be employed. One ofthe basic problems is the addressing of emitted signals to the intended detector. The concept of folding a free-space optical system into a thick transparent planeparallel substrate, was proposed by Jahns and Huang in 1989 [2]. Planar optics, also known as substrate-mode optics, fully employs the three dimensional nature of light propagation and also the fabrication methods known from integrated circuit manufacturing can be adopted. In addition to that the advantage ofthese technologies are compact packaging and simple alignment ofthe optical and electronical elements. The problem involved in folding optical light paths for imaging of extended data fields is the demand for good off-axis imaging properties ofthe optical elements. Besides diffractive solutions i.e. DOEs or CGHs [3] refractive optics offers the possibility for wavelength multiplexing since the wavelength dependence is only due to material dispersion. For off axis imaging in a folded zig-zag-path the optical system also has to be corrected by special astigmatic components.
Diffusion elements are fabricated by locally manipulating the ion composition of a glass substrate. For our experiments we use the silver sodium ion exchange technology. The effort of our work concentrates on the generation of arbitrary index distributions. Therefore we characterized two different types of special glasses--one of them showing linear, the other nonlinear diffusion response. To be able to optimize masks for the generation of arbitrary phase profiles we simulated the diffusion process numerically.
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