TuCoLens Tunable Compound Lens

Transcription

1 TuCoLens Tunable Compound Lens Within the priority program 1337, which is founded by the German Research Foundation (DFG), the opportunities of active micro-optics are investigated. In this national program new materials, components and actuation principles for active micro-optical systems are studied. In the project TuCoLens active, refractive components are used to overcome the limited resolution of compound eye systems with their small sub aperture sizes. To meet this fundamental issue, a scanning refractive element, a tunable micro-prism is introduced. This enables one to scan the object and take many pictures with a limited field of high resolution. These pictures are stitched together with image processing software resulting in a bigger picture with high resolution. Furthermore an active microlens is introduced in the system for a controlled refocusing on the object. The active micro-prism and lens are introduced to overcome the major disadvantage of compound eye systems, their limited resolution. In this approach we want to maintain the major advantage of these systems, their compact design. Therefore, all optical components are integrated on wafer-level. Furthermore, reactively sputtered aluminumnitride (AlN) is investigated for the active components. Due to its mechanical stability, thermal expansion and optical transparency, AlN is used for both, the definition of liquid air interfaces and the actuation. Thermal buckling due to local resistive heating of AlN thin films (less than 1 μm thick) is used as driving principle. Therefore, the properties of AlN, such as its coefficient of thermal expansion (CTE), its fracture strength and spectral transparency are investigated and optimized. Furthermore, thermal actuators are designed and fabricated, and their capability for the modification of liquid surfaces is studied. The optical performance of the single active elements and the integrated optical micro-system are simulated and characterized. Finally, the active compound eye system and the established stitching algorithm are tested for imaging purposes. Paetz, D.; Leopold, S.; Hoffmann, M.; Sinzinger, S., "Tunable anamorphotic imaging system based on fluidic cylindrical lenses," Optical MEMS and Nanophotonics (OMN), 2014 International Conference on, vol., no., pp.25,26, Aug doi: /OMN Steffen Leopold; Daniel Paetz; Fabian Knoebber; Oliver Ambacher; Stefan Sinzinger; Martin Hoffmann "Tunable cylindrical microlenses based on aluminum nitride membranes", Proc. SPIE 8616, MOEMS and Miniaturized Systems XII, (March 13, 2013); doi: / ; Leopold, Steffen; Geiling, Thomas; Fliegner, Caroline; Pätz, Daniel; Sinzinger, Stefan; Müller, Jens; Hoffmann, Martin: Multifunctional LTCC Substrates for Thermal Actuation of Tunable Micro-Lenses Made of Aluminum Nitride Membranes In: Proceedings 9th International Conference and Exhibition on Ceramic Interconnect and Ceramic Microsystems Technologies, Orlando, FL-USA, April , 7pp Leopold, Steffen; Pätz, Daniel ; Sinzinger, Stefan; Hoffmann, Martin: Adaptive Mikrolinsen basierend auf Aluminiumnitird Membranen In: Proceedings MikroSystemTechnik Kongress 2013, Oktober 2013, Aachen: VDE Verlag (2013) ISBN Steffen Leopold; Daniel Paetz; Fabian Knoebber; Oliver Ambacher; Stefan Sinzinger; Martin Hoffmann "Tunable cylindrical microlenses based on aluminum nitride membranes ", Proc. SPIE 8616, MOEMS and Miniaturized Systems XII, (March 13, 2013); doi: / ; S. Leopold; D. Paetz; F. Knoebber; T. Polster; O. Ambacher; S. Sinzinger; M. Hoffmann "Tunable refractive beam steering using aluminum nitride thermal actuators", Proc. SPIE 7931, MEMS Adaptive Optics V, 79310B (February 14, 2011); doi: / ; Daniel Pätz; Steffen Leopold; Fabian Knöbber; Stefan Sinzinger; Martin Hoffmann "Tunable compound eye cameras", Proc. SPIE 7716, Micro-Optics 2010, 77160K (May 13, 2010); doi: / ; /mms

2 aluminum nitride mechanical properties SEM picture of nanocrystalline, columnar structure of AlN layers on silicon Schematic sketch of cross-section trough a membrane. White light interferometric scan of a deflected membrane Load deflection characteristic for two cycles of one membrane before and after a long term load a) Membrane deflection for three different residual stresses at 20 kpa and b) their deviation from a spherical deflection /mms

3 aluminum nitride optical and thermal properties a) calculated reflection, absorption and transmission at an AlN membrane based on ellipsometry data and b) measured spectal transmission for a 500 nm thick membrane. a) measured temperature depended thin film stress of AlN and b) coefficient of thermal expansion (CTE). /mms

4 spherical AlN membrane micro-lenses Images of the test chart (USAF) (a) without and (b) through (d) with the tunable membrane lens at different pressures (b) 2 kpa, (c) 10 kpa, and (d) 20 kpa. The red rectangle indicates the increasing field of view. Refractive power versus pressure characteristic for three membranes with different residual stresses. Lines show the calculated characteristic based on a analytical model. a) Contour plot of refractive power versus residual stress and applied pressure according to the analytical model. The black curves represent lines of equal total membrane stress. b) potential refractive power and NA for different yield stresses. /mms

5 cylindrical AlN membrane micro-lenses Schematic Setup of two cylindrical lenses for individual focussion meridional and sagittal plane Shape of the b) optimized dogbone membrane compared to conventional long rectangular ones. FEM simulations of one quater of the dogbone membrane showing a optimum radius for a cylindrical deflection. a) fabricated AlN dogbone membranes on wafer level and b) white light interferometric scan of a deflected membrane, which shows the cylindrical profile /mms

6 cylindrical AlN membrane micro-lenses Schematic setup for shaping of a laser beam with two cylindrical lenses Images of the laser beam at different, individual pressures levels for the cylindrical lenses. /mms