Multi-layer dielectric thin-film optical filters for beam folding applications

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ABSTRACT Multi-layer reflective thin film filters optimized for oblique incidence angles were deposited on glass substrates using Electron Beam evaporation technique with in situ thickness monitoring. The present study involves deposition and optical characterization of 5 layered multi-layer structures of TiO 2 -Al 2 O 3 and TiO 2 -SiO 2 having different thicknesses for varied wavelength ranges in the visible region. Three TiO 2 -SiO 2 multi-layer thin film filters were deposited having peak reflectance at 480 nm, 540 nm and 675 nm respectively corresponding to light sources in the blue, green and red wavelength regions. Similarly, a TiO 2 -Al 2 O 3 multi-layer was fabricated having peak reflectance of around 64% at 610nm. These filters were deposited at an elevated temperature of 250° C in an oxygenated reactive environment for better adhesion, mechanical strength and proper stoichiometr y. Reflectance measurements of these multi-layer filters at oblique incidence angles reveal high reflectance of around 70 ~ 75% with a reasonably broad reflection band which can have wide applications in beam steering, shaping and folding applications in various complex optical systems facing constrained space and we ight requirements. Keywords: Thin films, optical coatings, interference filters, fo lding mirror, multi-layers, electron beam deposition.