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

摘要

Antireflection coatings have critical importance in thermal imaging system working in MWIR region (3-5 pm) since optics of high refractive index materials are used. Germanium (Ge) and Silicon (Si) optics areused extensively in the MWIR thermal systems. In this paper a study has been carried out on the designand fabrication of multi-substrate antireflection coating effective for Germanium and Silicon optics inMWIR (3.6-4.9 pm) region. The wave band 3.6-4.9 pm is chosen for the reported work because detectorsystem used in MWIR region has a band selection filter effective in the same wavelength region andatmospheric transmission window in MWIR region is effective in 3-5 pm spectral band. Comprehensivesearch method was used to design the multilayer stack on the substrate. The coating materials used in thedesign were Germanium (Ge), Hafnium oxide (Hf02) and Y-Ba-Fluoride (IR-F625). The fabrication of coat-ing was made in a coating plant fitted with Cryo pump system and residual gas analyzer (RGA). The evap-oration was carried out at high vacuum (2-6 x 10~(-6)mbar) with the help of electron beam gun systemand layer thicknesses were measured with crystal monitor. The result achieved for the antireflectioncoating was 98.5% average transmission in 3.6-4.9 pm band for Germanium and Silicon optics. This workwill be helpful in reducing the plant operation time, material and power consumption, as two differentkinds of optics are simultaneously coated in a single coating cycle.