Abstract: A multi-cavity optical device allowing selective control of a bandpass characteristic thereof . Fabry-Perot structures are coherently coupled together to form a narrow bandpass structure. At least one of the spacer regions in the device includes an active material that changes the optical thickness of the spacer to de-tune the device. In one embodiment, the active material is transparent and lies along the optical path through the device. In another embodiment, an active material that lies outside of the optical path tunes an air gap. Altering the first or last spacer in a multi-cavity structure provides variation in the transmission loss while generally retaining a flattop filter response with low passband ripple. Altering interior spacer regions provides high sensitivity to variations in the optical thickness of the spacer.