Wavefront Sensor Design based on a Micro-Mirror Array for a High Dynamic Range Measurement at a High Lateral Resolution
Optical testing is confronted with the challenge of a flexible testing of precision aspheres. This challenge can be faced by test equipment with a high measurement range at a high resolution, i.e. a high dynamic range. Compared to other sensor types for optical testing the Shack-Hartmann sensor (SHS) features a high dynamic range. In SHS the measurement range is limited due to the fact that all measurement points are detected simultaneously by an imaging device and the signals must be separable thus the dynamic range is defined by the number of micro-lenses and the resolution of the imaging sensor. There are several approaches to enhance the dynamic range such as image processing approaches, e.g. spot tracking and unwrap algorithms  - or the use of additional optical elements, e.g. masks [2,3] or adaptive diffractive micro lenses as in the adaptive Shack-Hartman sensor . But despite these approaches the contradiction between dynamic range and lateral resolution couldn’t be resolved - a flexible wavefront testing of technically relevant aspheres is still restricted. In this paper an approach of wavefront sensing is proposed in order to increase the dynamic range and the lateral resolution at the same time. The basic idea is to select and thereby encode single sub-apertures of the wavefront under test and to measure their propagation direction consecutively in a scanning procedure. In difference to the LCD based approach described in , here the selection of the sub-apertures is performed by a digital micro-mirror array (DMD). The use of the DMD promises a high reflectivity and lateral resolution as well as a very fast scanning ability. But there are specific challenges as the diffraction effects caused by the small dimensions of the array and the angular stability of the signal which will be addressed in this paper.
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This work was part of the project “WaveSense” and was funded by the German Federal Ministry of Economics and Technology (BMWi). The project is supervised by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen "Otto von Guericke" e.V. (AiF) and the Forschungsvereinigung Feinmechanik, Optik und Medizintechnik e.V. (F.O.M.). Their support is gratefully acknowledged. Furthermore the project is accompanied by the companies of Rodenstock, Trioptics, Möller-Wedel, Ingeneric and Aixtooling. Their contributions are also gratefully acknowledged.
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