Design, preparation, and characterization of high-performance narrow-bandpass filters for the infrared range

High performance narrow bandpass filters for the mid-infrared range of the spectrum have been designed, prepared and characterized. The filters must assure an optimal rejection with a mean transmission lower than 0.02%T and a maximum transmission lower than 0.1%T out of the pass band from 1 to 6 microns. The pass band must be centered in 4.52±0.04 microns, have a full width at half maximum of 0.09±0.01 microns and a maximum transmittance higher than 75% when measured at normal incidence. A 75-layer theoretical design meeting all the requirements is obtained by computer optimization using experimental values for the optical constants of the layers; SiO and Ge are the low and high index materials respectively. A tolerance analysis of the design indicates that the rejection band performance is very stable, but the pass band is very sensitive to thickness deviations in a group of seven layers, defined as the critical part of the coating. The filters are prepared by evaporation in high vacuum; the deposition rate and final thickness of each layer are controlled by a quartz-crystal monitor. Filters with the required performance are obtained applying a method based on the previous deposition and optical characterisation of the critical part of the coating. Finally, the parameters of the transmittance measurements have been optimized to assure the necessary accuracy of the spectra, including the blocking of the pass band to reliably define the rejection parameters.