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Large aperture tunable ultra narrow band Fabry-Perot-Bragg filter

A novel type of filters based on a combination of a Fabry-Perot etalon and a volume Bragg grating is demonstrated. The proposed solid-spaced Fabry-Perot etalon consists of a high quality fused silica window with both faces having identical dielectric mirrors coatings. The transmission of this Fabry-Perot etalon is a discrete channel spectrum consisting of narrow lines (typically a few tens of picometers) which are separated by gaps with constant width defined as the free spectral range (typically between 0.1 and 10 nm). These discrete resonances define the addressable wavelengths. The second element of this complex filter is a volume Bragg grating. This component is obtained by recording of a sinusoidal refractive index modulation into the volume of a photo-thermo-refractive glass plate. Central wavelength of such element can be tuned over several tens of nanometers by rotating the Bragg grating and therefore changing the incidence angle. This element is thus used to select one of the Fabry-Perot resonances and switch between them. We present the features of this filter and two experimental demonstrations of different configurations of Fabry-Perot-Bragg filters combining different Fabry-Perot etalons and volume Bragg gratings. That way, the flexibility of such kind of filters is demonstrated.