Dichroic coatings at 45° on germanium substrate for dual channel thermal imager

Abstract

Dichroic coatings have crucial importance in separating two closely spaced wavelength regions for use in dual channel thermal imaging system working in 3–5 μm and 7.5–10.5 μm wavelength region. In this paper a study has been carried out on the design and fabrication of high- and low-wave pass dichroic coatings on germanium substrate at 45°. Comprehensive search method of optimisation is used to design the multilayer stack on the substrate where the upper/lower limit of thicknesses are 0.25 λ and 0.025 λ respectively and the layer numbers are restricted to seven layers. The result achieved for the high-wave pass dichroic coating average reflection in 3–5 μm band is 91% and the average transmission in 7.5–10.5 μm is 88% and for the low-wave pass dichroic coating average transmission from 3 to 4.5 μm is 85% and average reflection from 7.5 to 10.5 μm region is 99%.