Two-state Optical Filter Based on Micromechanical Diffractive Elements

We have designed a robust two-state filter for infrared gas measurement, where the filter transmittance alternates between a single bandpass function, and a double-band offset reference. The device consists of fixed and movable diffractive sub-elements, micromachined in the device layer of a bonded silicon on insulator (BSOI) wafer. Switching between the two states of the filter is obtained by actuation of the movable sub- elements between idle and pull-in positions, which affects the interference of reflected light The characteristics of the filter are defined by a diffractive microrelief pattern etched on top of the sub-elements and by the position of the movable sub-elements at pull-in, the latter mechanically defined by the buried oxide layer. Thus, no accurate electrical control is needed to operate the filter. The first test components operate at 2 mum wavelength using a displacement of 500 nm and an actuation voltage of 5 V. No sticking or change in filter characteristics have been observed after repeated pull-in operations. The simplicity of fabrication and operation is likely to make the two-state filter an attractive component for sensors such as non-dispersive infrared gas detector.