Tunable Fabry–Perot filter and grating hybrid modulator to improve dispersive spectrometer resolution

We describe a tunable Fabry–Perot filter and grating hybrid modulator to achieve a higher spectral resolution compared with that produced by a single grating with the same period. In the hybrid modulator, a tunable Fabry–Perot filter is designed with a long cavity to accommodate a multi-order narrowband pre-filter. A grating is then utilized to separate these multi-orders spatially. Scanning the air gap of the tunable Fabry–Perot filter within 1/2 wavelength, the entire spectrogram can be achieved by compositing each group of transmitted multi-orders. Light passes first through the Fabry–Perot cavity and then into the grating. Thus, all of the light is incident on the Fabry–Perot cavity at a given angle, which can reduce the requirement for incident beam alignment and simplify the operation of the hybrid modulator. The structural matching conditions of the tunable Fabry–Perot filter and grating were presented based on the operating law of the hybrid modulator. In terms of the Rayleigh criterion, the practical spectral resolution of the hybrid modulator can be increased by at least twice that of the single grating. Experiments with a neon lamp revealed that the spectral resolution of the hybrid modulator was nearly double that of a single grating.