| Precise control of micromirror shape is critical in many optical microsystems. The optical performance of micromirrors is seriously degraded by undesired mirror deformation and thermal expansion-induced deformation becomes a major effect in micromirrors as the mirror diameter exceeds 100mum.; In this project, we demonstrate that one can use the mechanical properties of multilayer structures to create mirrors with stable curvature across temperature. We demonstrate the fabrication of such thermally invariant mirrors using dielectric coatings. Micromirrors are demonstrated that maintain their design curvature to within lambda/60 for lambda = 633nm across an operating range from 21°C to 58°C.; We also demonstrate micromirrors with current-controlled curvature. The working principle is that resistive heating changes the temperature of the micromirrors and thermal expansion induces a controlled curvature whose magnitude is determined by coating design. For example, for wide focal-length tuning, the radius of curvature of a gold-coated mirror was tuned from 2.5 to 8.2mm. For fine focal-length tuning, the radius of curvature of a dielectric-coated (SiO2/Y2O3 lambda/4 pairs) mirror was tuned from -0.68 to -0.64mm. These results should be readily extendable to mirror flattening or real-time adaptive shape control.; The performance of optical micro-cavities is limited by spectral degradation resulting from thermal deformation and fabrication imperfections. In this project, we study the spatial mode properties of micromirror optical cavities with respect to commonly seen aberrations. A simple current-based method is used to control the configurations of micro-cavities that are compatible with electrostatic spectral tuning and small array architectures. The shapes of the micromirrors are changed using Joule heating with thermal expansion deformation. Significant differences in mirror tilt, curvature, and astigmatism are measured, but the tilt has by far the biggest impact on cavity finesse and resolution. We demonstrate that unwanted higher order spatial modes can be suppressed electrically and an amplitude reduction for the higher order modes of over 60% has been obtained with a tuning current of 5.5mA. A fundamental mode finesse of approximately 60 is maintained throughout tuning. These tunable cavities have great potential in applications using cavity arrays or requiring dynamic mode control. |
