Rugate filters fabricated from dense and porous magnetron sputtered WO3 films.

Single-material interference filters are obtained from magnetron sputtered alternating layers of WO3 deposited at different pressures. The stop band is investigated with respect to the refractive index contrast and the total number of layers. Some features of rugate filters are also investigated. We show experimentally that a sine-wave index profile modulation can be used to suppress the harmonic stop bands located at odd fractional wavelengths. We also show experimental evidence that sidelobes near the stop band can be attenuated by using a sine-wave index profile that has a variable amplitude.