Micromachined tunable Fabry-Perot filters for infrared astronomy

Micromachined Fabry-Perot tunable filters with a large clear aperture (12.5 to 40 mm) are being developed as an optical component for wide-field imaging spectroscopy. This program applies silicon micromachining fabrication techniques to miniaturize Fabry-Perot filters for astronomical science instruments. The filter assembly consists of two reflector plates that form a tunable Fabry-Perot etalon. One plate is fixed and the second plate is free to move along the optical axis on silicon springs. The moving plate is actuated electrostatically by capacitance pads on the stationary and moving plates. To reduce mass, both reflectors are fabricated by applying optical coatings to a thin freestanding silicon nitride film held flat in drumhead tension. In this paper, we discuss the etalon design, electromechanical modeling, fabrication, and initial results. In the current design, the transmission aperture is 11.0 mm in diameter, the moving plate is 26.3 mm in diameter, and the stationary plate is 32.6 mm in diameter. The plates and springs are nominally 350 μm thick, the electrical and mechanical spacing between plates is 18 μm, and the uncoated optical spacing is 15 μm.