Narrowband optical filter based on a Fabry–Perot interferometer with two waveguide—grating mirrors

The operation of a narrowband filter based on a Fabry—Perot interferometer formed by two waveguide—grating mirrors is studied upon normal incidence of light. In this case, two counterpropagating travelling waves and coupled modes are excited in a corrugated waveguide, while only one mode is excited in the case of oblique incidence of light on the filter. It is found that in the case of a small gap between the mirrors, the reflection spectrum of the interferometer depends on the phase shift Δφ of one corrugation relative to the other. If light is incident normally on the interferometer, two or three lines appear in the transmission spectrum if Δφ≠0 or Δφ≠π. The appearance of the additional resonances is attributed to symmetry breaking in the system. At large distances between the mirrors, the spectra at Θ=0 do not exhibit any peculiarities.