Abstract: A novel tunable optical filter has been proposed and investigated here, which is a thin film-slab substrate combination system, operating on principle of Dual Interference Effect introduced first time. According to theoretical analysis, the reflectance for a specific wavelength varies periodically with the changing of refractive index of the slab substrate for a given thickness of film. Experimentally, the tested samples are fabricated by depositing approximately 90 nm SnO$-2$/ semiconductor film on 3 mm Na-glass substrate, the measured ratio of reflectances at operating wavelength 632.8 nm/670 nm is as high as 15 dB.!4
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