Improved micro topography measurement by LCoS-based fringe projection and z-stitching

Author(s): X. Schwab, C. Kohler, K. Körner, N. Eichhorn, W. Osten

Abstract:

“Fringe projection is a commonly used method for 3D surface metrology. Numerous applications have demonstrated a measurement field from a few millimeters to several meters. To enable the measurement of micro systems with this method, a zoom stereo microscope from Leica was used as the basis for the implementation of a fringe projection microscope. A state of the art twisted nematic WUXGA LCD was used for flexible fringe generation. The high fill factor of this reflective LCoS in combination with a 500 Lumen LED and a 12 bit CCD camera delivers fringe patterns with high contrast. This allows us to measure objects with both a strong reflectivity variation and a low reflectivity. The second main objective was to increase the measurement field and the depth of field. Using the zoom system and exchangeable microscope objectives, the measurement fields could be changed quickly from 4 cm² to less than 1 mm². Depending on the measurement field, the depth of field was between 5.22 mm and 0.018 mm. However, this was often not sufficient to measure the complete depth of a 3D-object. The microscope system also features an integrated high precision motor stage, which is already used for system calibration. Based on this, we implemented a new z-stitching method where n measurements at different well determined z-positions of the motor stage were performed. The n resulting topography maps can be stitched together to get the complete depth map of the entire object. Thus the depth measurement range is only limited by the mechanics of the z-stage. ”

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Publication: SPIE Proceedings
Issue/Year: Proc. SPIE, Vol. 6995, 69950Q (2008);
DOI: 10.1117/12.781822

Spatial amplitude and phase modulation using commercial twisted nematic LCDs

Author(s): E. G. van Putten, I. M. Vellekoop, and A. P. Mosk

Abstract:

“We present a method for full spatial phase and amplitude control of a laser beam using a twisted nematic LCD combined with a spatial filter. By spatial filtering we combine four neighboring pixels into one superpixel. At each superpixel we are able to independently modulate the phase and the amplitude of light. We experimentally demonstrate the independent phase and amplitude modulation using this novel technique. Our technique does not impose special requirements on the spatial light modulator and allows precise control of fields even with imperfect modulators.”

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Publication: Applied Optics
Issue/Year: Applied Optics, Vol. 47, Issue 12, pp. 2076-2081, 2008
DOI: 10.1364/AO.47.002076

Spectral and temporal response of liquid-crystal-on-silicon spatial light modulators

Author(s): Martin Bock, Susanta Kumar Das, Ruediger Grunwald, Stefan Osten, Peter Staudt and Gero Stibenz

Abstract:

“Spectral and temporal phase response of selected types of liquid-crystal-on-silicon spatial light modulators were studied using femtosecond pulses, determining specific transfer functions of the devices. The phase response resulting from programed gray level distributions was detected by analyzing the diffraction characteristics and by spectral phase interferometry for direct electric-field reconstruction. The results indicate the appearance of distinct parameter ranges that enable minimum spatiotemporal distortion. Weak oscillations in the spectral phase are explained by Gires–Tournois resonances [F. Gires and P. Tournois, Acad. Sci. Paris, C. R. 258, 6112 (1964)].”

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Publication: Applied Physics Letters
Issue/Year: Appl. Phys. Lett. 92, 151105 (2008)
DOI: 10.1063/1.2909115

Optical tweezers of programmable shape with transverse scattering forces

Author(s): Alexander Jesacher, Christian Maurer, Severin Fuerhapter, Andreas Schwaighofer, Stefan Bernet, Monika Ritsch-Marte

Abstract:

“We propose a non-holographic method to create line traps of arbitrary shape in the sample plane. Setting the phase gradient along theses lines gives control over the transverse forces acting on the confined particles. Phase structures, displayed on a spatial light modulator, are optically processed by a spiral phase filter and imaged onto the object plane of a microscope objective. The resulting bright line structures can be used to trap microparticles. Additionally, they exert transverse scattering forces, which can be exploited for inducing orbital motions or for creating “attracting” or “repelling” points, respectively. We give theoretical and experimental evidence that these scattering forces are proportional to the curvature of the line tweezers.”

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Publication:Optics Communications
Issue/Year: Optics Communications, Volume 281, Issue 8, 15 April 2008, Pages 2207-2212
DOI: 10.1016/j.optcom.2007.12.042