Narrowband reflector based on a side-coupled transverse-mode-conversion Bragg grating resonator

We propose a narrowband reflector (NBR) on the Si₃N₄ platform based on a side-coupled transverse-mode-conversion Bragg grating resonator. The NBR is constructed of the bus waveguide consisting of two S-bends, and the resonator waveguide consisting of two anti-symmetric Bragg gratings (ASBGs) and a uniform Bragg grating (UBG). Due to the support of UBG, two ASBGs can form two resonant modes, in which the fundamental transverse electric (TE₀) and the first order transverse electric (TE₁) modes are interconnected in different propagation directions. The backward TE₁ mode can be coupled to the bus waveguide and converted to TE₀ mode, resulting in reflection at the input port. The proposed NBR does not require tuning with the Vernier effect. The reflection spectrum is not Lorentzian in shape. It can achieve a box-like reflection spectrum when the resonant splitting is introduced. The NBR remains effective even when the laser wavelength undergoes slight chirping. The proposed NBR can serve as an external cavity for emitting narrow linewidth lasers at specific wavelengths. The simulation results demonstrate that the reflection of 0.829 (-0.814dB), full width at half maximum (FWHM) of 0.34 nm, and side lobe suppression ratio (SLSR) of 44 dB are achieved for the proposed reflector when the grating length is 476.8 µm.