MEMS Piston Mirror Arrays for Computer Generated Holography

Computer generated holography (CGH) offers the best possible solution for very interesting applications like virtual, augmented and mixed reality. To get the images from the computer into the real world, spatial light modulators (SLMs) are required that fulfil very demanding specifications. Unfortunately, none of the currently available kinds of SLMs can meet this challenge fully. Within the European Union funded Project REALHOLO we are therefore developing a novel kind of MEMS (micro electro mechanical system) SLM especially for CGH applications. The challenge is to modulate the phase of incoming coherent light with millions of individually controllable pixels. The pixels have to be only a few micrometers in size for acceptable diffraction angles and still have a stroke range of half the wavelength of visible light, about 350nm. Within this range, each pixel needs to be set very precisely to one of many deflection levels at frame rates of more than one kHz. This paper discusses the challenge and our solution: an innovative MEMS comb drive actuator array, monolithically integrated on top of a CMOS backplane. The advantages of this design are compared to other types of SLMs and its superior performance is shown by FEM simulations. We also discuss the impact of effects like charging and fabrication imperfections on the deflection precision. Our newly developed MEMS technology and SLM will also enable many other applications that may benefit from the fast and precise phase modulation by a large number of pixels, like wave front shaping or quickly re-programmable diffractive optical elements (DOEs).