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Pant, Kamal K.; Burada, Dali R.; Bichra, Mohamed; Gosh, Amitava; Khan, Gufran S.; Sinzinger, Stefan; Shakher, Chandra
Weighted spline based integration for reconstruction of freeform wavefront. - In: Applied optics, ISSN 2155-3165, Bd. 57 (2018), 5, S. 1100-1109

In the present work, a spline-based integration technique for the reconstruction of a freeform wavefront from the slope data has been implemented. The slope data of a freeform surface contain noise due to their machining process and that introduces reconstruction error. We have proposed a weighted cubic spline based least square integration method (WCSLI) for the faithful reconstruction of a wavefront from noisy slope data. In the proposed method, the measured slope data are fitted into a piecewise polynomial. The fitted coefficients are determined by using a smoothing cubic spline fitting method. The smoothing parameter locally assigns relative weight to the fitted slope data. The fitted slope data are then integrated using the standard least squares technique to reconstruct the freeform wavefront. Simulation studies show the improved result using the proposed technique as compared to the existing cubic spline-based integration (CSLI) and the Southwell methods. The proposed reconstruction method has been experimentally implemented to a subaperture stitching-based measurement of a freeform wavefront using a scanning Shack-Hartmann sensor. The boundary artifacts are minimal in WCSLI which improves the subaperture stitching accuracy and demonstrates an improved Shack-Hartmann sensor for freeform metrology application.



https://doi.org/10.1364/AO.57.001100
Cao, Xinrui; Sinzinger, Stefan
Effects of illumination on image reconstruction via Fourier ptychography. - In: Advanced Optical Technologies, ISSN 2192-8584, Bd. 6 (2017), 6, S. 467-474

https://doi.org/10.1515/aot-2017-0048
Bourgin, Yannick; Voigt, Daniel; Käsebier, Thomas; Siefke, Thomas; Kley, Ernst-Bernhard; Zeitner, Uwe D.
175 nm period grating fabricated by i-line proximity mask-aligner lithography. - In: Optics letters, ISSN 1539-4794, Bd. 42 (2017), 19, S. 3816-3819

https://doi.org/10.1364/OL.42.003816
Leopold, Steffen; Pätz, Daniel; Sinzinger, Stefan; Hoffmann, Martin
Stress-modulated tilt actuator for tunable optical prisms. - In: Sensors and actuators. Physical. - Amsterdam [u.a.] : Elsevier Science, 1990- , ISSN: 1873-3069 , ZDB-ID: 1500729-7, ISSN 1873-3069, Bd. 266 (2017), S. 328-337

A tunable optical prism MOEMS based on the deformation of a liquid droplet is presented. An aluminum-nitride membrane is tilted by a novel type of thermo-mechanical actuator. The actuation principle is based on a thermo-mechanical modulation of the intrinsic stress in aluminum-nitride beams. Based on an analytical model, the key parameters of the actuator are optimized. Furthermore, the influence of the intrinsic stress on the actuator properties is investigated. These dependencies and the model are verified by mechanical characterization of samples. Operation in air and with ambient fluid has been confirmed. An image shift of 30 mm is found in a microscopic setup which corresponds to 19 % of the field of view.



https://doi.org/10.1016/j.sna.2017.09.021
Burada, Dali Ramu; Pant, Kamal K.; Bichra, Mohamed; Khan, Gufran Sayeed; Sinzinger, Stefan; Shakher, Chandra
Experimental investigations on characterization of freeform wavefront using Shack-Hartmann sensor. - In: Optical engineering, ISSN 1560-2303, Bd. 56 (2017), 8, S. 084107, insges. 11 S.

The metrology of freeform wavefront can be performed by the use of a noninterferometric method, such as a Shack-Hartmann sensor (SHS). Detailed experimental investigations employing an SHS as metrology head are presented. The scheme is of nonnull nature where small subapertures are measured using an SHS and stitched to give the full wavefront. For the assessment of complex misalignment errors during the spiral scanning, a library of residual slope errors has been created, which makes the alignment process fast converging for minimizing the scanning errors. A detailed analysis of the effects of slope and positioning error on reproducibility is presented. It is validated by null test where a null diffractive optical element has been used in a Mach-Zehnder configuration for compensating the freeform shape. A freeform optics is measured by both measurement schemes, and the results are in good agreement. Further, the nonnull-based scanning subaperture stitching scheme is also validated by performing measurements on an aspheric surface and compared with the measurements from the interferometric method (Zygo Verifire).



https://doi.org/10.1117/1.OE.56.8.084107
Karthaus, Daniela; Giehl, Markus; Sandfuchs, Oliver; Sinzinger, Stefan
Modeling of light-emitting diode wavefronts for the optimization of transmission holograms. - In: Applied optics, ISSN 2155-3165, Bd. 56 (2017), 18, S. 5234-5241

The objective of applying transmission holograms in automotive headlamp systems requires the adaptation of holograms to divergent and polychromatic light sources like light-emitting diodes (LEDs). In this paper, four different options to describe the scalar light waves emitted by a typical automotive LED are regarded. This includes a new approach to determine the LEDs wavefront from interferometric measurements. Computer-generated holograms are designed considering the different LED approximations and recorded into a photopolymer. The holograms are reconstructed with the LED and the resulting images are analyzed to evaluate the quality of the wave descriptions. In this paper, we show that our presented new approach leads to better results in comparison to other wave descriptions. The enhancement is evaluated by the correlation between reconstructed and ideal images. In contrast to the next best approximation, a spherical wave, the correlation coefficient increased by 0.18% at 532 nm, 1.69% at 590 nm, and 0.75% at 620 nm.



https://doi.org/10.1364/AO.56.005234
Kampmann, Ronald; Sinzinger, Stefan
Optical tweezers affected by monochromatic aberrations. - In: Applied optics, ISSN 2155-3165, Bd. 56 (2017), 5, S. 1317-1326

The performance of optical tweezers in the presence of monochromatic aberrations is investigated theoretically and experimentally. For the calculation of optical forces we use a ray-optics-based force simulation, which is embedded in a ray tracing routine. By considering optical path differences caused by monochromatic aberrations, we are able to simulate their effect on the trapping forces. Optical trapping experiments considering the influence of aberrations were performed with an optical tweezers system, specially adapted to trap particles in gaseous surrounding. The conformity of these measurements with the theoretical predictions verifies the correct performance of the optical force simulation routine. Based on the reliable simulation tool, further theoretical studies are performed on how optical aberrations affect the function and performance of optical tweezers.



https://doi.org/10.1364/AO.56.001317
Kreismann, Jakob; Sinzinger, Stefan; Hentschel, Martina
Three-dimensional lima¸con: properties and applications. - In: Physical review, ISSN 2469-9934, Bd. 95 (2017), 1, 011801, insges. 6 S.

https://doi.org/10.1103/PhysRevA.95.011801
Grewe, Adrian; Feßer, Patrick; Sinzinger, Stefan
Diffractive array optics tuned by rotation. - In: Applied optics, ISSN 2155-3165, Bd. 56 (2017), 1, Seite A89-A96

In this work, we apply the Alvarez-Lohmann principle for varifocal lenses to diffractive off-axis elements tuned by rotation. Two methods to combine multiple elements into arrays are presented. Further, we show that inverse phase sections result from a 2π ambiguity of the rotation. Quantization techniques are applied to eliminate these unwanted sections. As proof of concept, a retrofocus lens using a tunable diffractive lens array is presented.



https://doi.org/10.1364/AO.56.000A89
Bichra, Mohamed; Sabitov, Nail; Meinecke, Thomas; Sinzinger, Stefan
Wavefront sensing by numerical evaluation of diffracted wavefields. - In: Applied optics, ISSN 2155-3165, Bd. 56 (2017), 1, Seite A13-A22

A novel wavefront sensor principle based on diffraction theory and Fourier analysis with a modified angular spectrum propagator has been developed. We observe the propagation of a wavefront behind a two-dimensional cross grating and derive a universal method to extract the phase gradient directly from a captured intensity image. To this end the intensity distribution is analyzed in the spectral domain, and the processing is simplified by an appropriate decomposition of the propagator kernel. This method works for arbitrary distances behind the grating. Our new formulation is verified through simulations. The wavefront generated by a freeform surface is measured by the new method and compared with measurements from a commercial Shack-Hartmann wavefront sensor.



https://doi.org/10.1364/AO.56.000A13