Dynamically deformable micromirror array for defined laser beam shaping and homogenizing

We present a dynamic laser beam shaper than can generate smooth flat top and gaussian intensity profiles. It consists of a 100% fill factor membrane, supported by beams or posts, that deforms in a shape similar to a concave linear or 2D microlens array. The dynamic and tunable focal lengths allow to change the diffusion angle or to switch between flat top or gaussian output optical profiles. The whole array is fabricated over a rotating stage that enables the averaging of the interference effects caused by the array. Experiments showed that an angle of only 0.18° smooth the interferences to a contrast of 0.04. Optical simulations were performed to design the device. A first prototype using a magnetic scanner was tested to validate the simulations. Fabrication of the rotating stage and the membrane was accomplished using surface and bulk micromachining using parylene trench refilling.

I. Introduction

Microlenses array are widespread in optics and they are used for collimation, imaging, laser beam shaping and homogenizing. They introduce interferences in the output intensity profile when they are used with coherence light. To correct these effects diffusers are often used in combination with beam shaping optics. In this paper we propose a single dynamic optical device that is able to shape a laser beams and smooth the interferences. It consists of a randomized concave micromirror array with tunable focal lengths. The whole array is directly fabricated over a rotating stage that provides beam smoothing by averaging of the interferences effects (fig. 1). In the next sections we present the concept and the design of the device. The fabrication process is detailed followed by the presentation of optical simulations and experimental results. Fig. 1. A dynamically deformable membrane is supported by posts or beams that are fabricated on a 5 mm wide rotating stage. The membrane is electrostatically actuated by a parallel plate actuator and the stage is scanned at resonance by in plane comb drives. The stage is supported by 2 large springs for high heat conductivity.