MOEMS-based thin silicon membrane mirrors with a useable diameter of 5mm and fast (up to 1kHz) tunable focal length
(80 mm to 1m) have been realized. A ring shaped counter electrode is used to achieve a parabolic membrane deformation
by electrostatic forces. A circular kerf at the outer perimeter of the membrane provides a soft suspension to the rim and
thus reduces the needed driving voltage. FEM has been used for optimisation of the design, especially of the soft
suspension, which is realized by a controlled thinning of the outer rim of the Si-membrane.
A critical issue for demanding applications is the membrane distortion induced by material stress and the fabrication
process. Membrane residual stress reduction has been obtained by using SOI-technology (c-silicon) and by optimisation
of the Al deposition process (Al-coated Si-membrane).
For dynamic tests of the optical mirror properties a stroboscopic interferometer has been realized. A pulsed laser diode
with a pulse duration of 10μs is used as a light source which is synchronized with the modulated electrical field driving
the membrane mirror. The interference pattern is recorded with a CCD and evaluated with conventional phaseshift
techniques. The geometry is similar to a Mach-Zehnder interferometer. The reference path length can be varied with a
piezoceramic to induce the phase shift.
In the manufacturing process of aspheric glass lenses the grinding step plays a key role both in respect of the final quality of the polished lens as well as in respect of manufacturing costs. Therefore, the form of the grinded surface must be measured with high precision. The typically used tactile measuring machines provide sufficient precision regarding depth resolution but suffer from limited lateral resolution. In particular it is not possible to detect surface and sub-surface damages which essentially influence the duration of the subsequent polishing process. In order to detect these damages we set up and tested a scanning short-coherence interferometer very similar to optical coherence tomography. The aspheric lens under test is mounted on a rotation stage which can be translated in the lateral direction. The sensor beam of the interferometer is focused onto the sample and can be moved along the axial direction. The precision of the depth measurement is 0.25μm, the lateral positioning precision is 2μm. The system is used to optimize the grinding process for aspheric lenses to minimze sub-surface damages and therefore to maximize processing speed.
Depth of focus can be enhanced with cubic phaseplates located at the exit pupil of an optical system without significant loss of resolution. The enhancement factor is proportional to the strength of the phaseplate. The digital image is inversely filtered. The stronger the phaseplate is the stronger the inverse filter function must be. This causes increasing noise for high spatial frequencies in the restored image. Therefore, an optimum strength of the cubic phaseplate has to be chosen for the respective situation. A variable phaseplate system has been realized and tested. The performance of the setup has been experimentally studied. Applications and practical aspects are discussed in particular regarding barcode readers.
The Center of Optics Technology at the University of Applied Science, founded in 2003, is part of the School of Optics and Mechatronics. It completes the existing optical engineering department with a full optical fabrication and metrology chain and serves in parallel as a technology transfer center, to provide area industries with the most up-to-date technology in optical fabrication and engineering. Two examples of research work will be presented. The first example is the optimizing of the grinding process for high precision aspheres, the other is generating and polishing of a freeform optical element which is used as a phase plate.
Normally, depth of focus and resolution of an optical system are complementary parameters. According to an established technique known as wavefront coding the depth of focus can be enhanced by inserting a phaseplate with a cubic surface function into the exit pupil plane of an optical system. Although contrast is reduced the image quality can be restored very efficiently by inverse filtering because the modulation transfer function (MTF) almost does not change when the system is defocused. In addition the MTF has no zero crossings. Therefore inverse filtering has no singularities. Thus, three dimensional objects can be imaged with microscopes with a large depth of focus. The waveoptical performance of a commercial microscope equipped with a cubic phase plate has been simulated. The simulation results are discussed and compared with experimental data. The system has been analyzed both regarding effects specifically related to the phase plate design and regarding effects related to the optical properties which result from the combination of the phase plate with the microscope system. A variable phase plate design is also presented which can be adapted to specific objects and optical systems.
A stationary low coherence interferometer for optical coherence tomography (linear OCT, LOCT) based on Young's two-pinhole experiment is characterized theoretically. All OCT sensors either work in the time (TDOCT) or Fourier domain (FDOCT). In contrast to these setups, the interferometer described in this paper employs no moving parts in the reference arm and no spectrometers for depth profiling. Depth profiling is achieved by detecting the interference signal on a linear CCD-array. Different positions of the interference signal on
the CCD-array correspond to different depths inside the sample. The
interference signal of the setup and the sensitivity in the case of shot noise limited detection are derived theoretically and compared to sensors in the time domain. In-vitro images of porcine cornea demonstrate the clinical potential of the setup.
Optical coherence tomography (OCT) is a noninvasive imaging technology, which provides subsurface imaging of biological tissue with a resolution in the micrometer range. OCT sensors either work in the time or Fourier domain. We present a new interferometer setup based on a fiber double pinhole arrangement. Two fibers are placed in parallel similar to Young’s two-pinhole interference experiment with spatial coherent and temporal incoherent light. The interference pattern is observed on a linear CCD-array. A complete A-scan can be derived from a single readout of the CCD-array. The experimental setup is described in detail. The main parameters of the setup are derived theoretically and compared with experiments. First images of technical and biological samples are presented.
The optical imaging properties of the eye are both determined by optical aberrations and deviations of the eye length. We report on a device for simultaneous measurement of aberrations and eye length combining a Shack Hartmann sensor and a short coherence interferometer. The short measurement duration minimizes disturbing influences like tear film or eye length changes. The low irradiance of the atient's eye minimizes exposure time.
A Fourier transform spectrometer is used to simultaneously measure distance, dispersion and spectrum. It is shown that short coherence interferometry has the potential to measure the three-dimensional distribution of the spatial structure of a sample with a resolution determined by the coherence length of the light source, absorption spectrum with a resolution of 1 cm−1 and a dispersion with a resolution of up to 1025.
A Fourier transform spectrometer is used to simultaneously measure thickness, dispersion and absorption spectrum of a sample. It is shown that short coherence interferometry has the potential to measure the three dimensional distribution of the structure, the spectral absorption and dispersion of a sample.
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