Fabry-Perot interferometer in the PolyMUMPS process

A Fabry-Perot interferometer is formed by an adjustable optical cavity between two partially reflecting mirrors, as shown schematically in 

The distance between the mirrors can be adjusted with a MEMS actuator to enable a tunable interferometer. When the distance between the mirrors is adjusted to 2/2, the light passes through the interferometer while light with other wavelengths is blocked. The Fabry-Perot interferometer acts as a narrow bandpass filter for light, with the bandpass is dependent on the finesse of the optical cavity. 

For light that is incident normally on a Fabry-Perot filter with a mirror separation d and index n in the cavity between the mirrors, the transmission function is given by 

While not the optimal layer thickness for the Bragg stack, which would have dielectric layer thicknesses that are one-quarter of the wavelength of light that is to be filtered, it makes good use of the available layers in the 

The top mirror support is four springs acting in series. Each individual spring is comprised of a set of three fixed-guides beams connected end-to-end and acting in parallel. 

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