A new type of photodetector with cascaded waveguide grating filters as a bottom reflection mirror is proposed. A greatly improved spectral response is shown to follow by the integration of a waveguide grating into classical thin-film homogeneous layers. Calculation results for a single grating, a cascaded-double grating, and a cascaded-triple grating structure are demonstrated. An increasing rectangular spectral response is obtained by cascading two or three grating filters. Compared with a traditional photodetector with distributed Bragg reflectors, this new type of photodetector with the same materials requires significantly fewer layers while maintaining narrow flattop response, high peak efficiency, and low sideband reflectance.
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