An optical instrument including: a thermo-optically tunable, thin film, free-space
interference filter having a tunable passband which functions as a wavelength selector,
the filter including a sequence of alternating layers of amorphous silicon and
a dielectric material deposited one on top of the other and forming a Fabry-Perot
cavity structure having: a first multi-layer thin film interference structure forming
a first mirror; a thin-film spacer layer deposited on top of the first multi-layer
interference structure, the thin-film spacer layer made of amorphous silicon; and
a second multi-layer thin film interference structure deposited on top of the thin-film
spacer layer and forming a second mirror; a lens for coupling an optical beam into
the filter; an optical detector for receiving the optical beam after the optical
beam has interacted with the interference filter; and circuitry for heating the
thermo-optically tunable interference filter to control a location of the passband.