High efficiency antireflection coating in MWIR region (3.6–4.9 μm) simultaneously effective for Germanium and Silicon optics

Abstract

Antireflection coatings have critical importance in thermal imaging system working in MWIR region (3–5 μm) since optics of high refractive index materials are used. Germanium (Ge) and Silicon (Si) optics are used extensively in the MWIR thermal systems. In this paper a study has been carried out on the design and fabrication of multi-substrate antireflection coating effective for Germanium and Silicon optics in MWIR (3.6–4.9 μm) region. The wave band 3.6–4.9 μm is chosen for the reported work because detector system used in MWIR region has a band selection filter effective in the same wavelength region and atmospheric transmission window in MWIR region is effective in 3–5 μm spectral band. Comprehensive search method was used to design the multilayer stack on the substrate. The coating materials used in the design were Germanium (Ge), Hafnium oxide (HfO 2) and Y-Ba-Fluoride (IR-F625). The fabrication of coating was made in a coating plant fitted with Cryo pump system and residual gas analyzer (RGA). The evaporation was carried out at high vacuum (2–6 × 10 −6 mbar) with the help of electron beam gun system and layer thicknesses were measured with crystal monitor. The result achieved for the antireflection coating was 98.5% average transmission in 3.6–4.9 μm band for Germanium and Silicon optics. This work will be helpful in reducing the plant operation time, material and power consumption, as two different kinds of optics are simultaneously coated in a single coating cycle.