Micromirrors with electrically tunable curvature

Micromirrors with current-controlled curvature have been fabricated and tested. The principle behind the devices is that resistive heating changes the temperature of the micromirrors, causing a change in optical coating stress. For highly symmetric mirrors, the variable stress induces a uniform and reversible change in curvature. The mirrors used in this study were surface-micromachined from polysilicon and had gold coatings. The radius of curvature of a typical mirror could be tuned from 2.5 mm to 8.2 mm over a current-induced temperature range from 22/spl deg/C to 145/spl deg/C (estimated). The corresponding change in focus over this range, as measured by the Seidel parameters of the wavefront, was 0.38/spl lambda/ while the largest aberration (astigmatism) varied from its zero current value by less than 0.02/spl lambda/. These results should be readily extendable to tuning or correcting flat mirrors by changing the design stress of the reflective coating.