Wavelength stabilization of electrostatically actuated micromechanical infrared Fabry-Pérot filters

In this work, an application of electrostatically tunable optical infrared filters in a closed-loop control system is presented. The filters are based on a Fabry-Perot architecture, fabricated in a bulk micromachining process. Compared to surface micromachined devices, this design opens a path to higher optical performance due to the high planarity and low roughness of substrates but also introduces the drawback of acceleration sensitivity because of a moving mass. To overcome this problem, the filter is driven by a closed-loop control system, with feedback from a charge amplifier that measures the capacitance of the control electrodes. A PI-based controller that adapts to nonlinear stiffness and damping by real-time calculation of the deflection dependent controller parameters is developed. Basic system parameters are derived from optical calibration measurements and step-response results. At 5 Hz sinusoidal accelerations of 1 g, the filter wavelength accuracy can be improved from ±35 nm in open-loop case down to ±2 nm in closed-loop operation.