SILICON-GAP FABRY-PEROT FILTER FOR FAR-INFRARED WAVELENGTHS

We developed a far-infrared Fabry-Perot filter constructed from a sing le silicon substrate. The limiting resolving power caused by beam dive rgence of a silicon-gap Fabry-Perot filter is approximately 10 times h igher than that of a vacuum-gap Fabry-Perot filter because of the larg e index of refraction of silicon. The filter thus permits compact, hig h-throughput optical systems. Metal mesh patterns microlithographed on each face provide enhanced, wavelength-dependent face reflectivity. W e tested the performance of filters with metal mesh patterns consistin g of inductive crosses and capacitive squares. A Fabry-Perot filter de veloped for a rocketborne astrophysics experiment with a capacitive sq uare metal mesh pattern achieves a resolving power of lambda/Delta lam bda(FWHM) = 160 at lambda = 158 mu m, with a peak transmittance of 37% over an active aperture of 6.9 mm for an f/3.8 optical beam at 15 deg rees incidence. The absorptivity of a 240-mu m thick silicon substrate patterned with capacitive metal mesh is A less than or similar to 1% per pass, including loss in both the silicon and the metal mesh.