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 stoichiometry. 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 weight requirements.
Rugate filters are a special kind of graded-index films that may provide advantages in both, optical performance and mechanical properties of the optical coatings. In this work, design and optimization of a high reflection rugate filter having reflection peak at 540nm has been presented which has been further optimized for side-lobe suppression. A suitable number of apodization and matching layers, generated through Quintic function, were added to the basic sinusoidal refractive index profile to achieve high reflectance of around 80% in the rejection window for normal incidence. Smaller index contrast between successive layers in the present design leads to less residual stress in the thinfilm stack which enhances the adhesion and mechanical strength of the filter. The optimized results show excellent side lobe suppression achieved around the stopband.
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