On-chip band-stop filter using multimode waveguide Bragg grating in alumina visible-light waveguides

Efficient elimination of unwanted light while transmitting lights with desired wavelengths is crucial in integrated photonics. We demonstrate a band-stop filter in alumina low-loss visible-light waveguides that eliminates green light while transmitting red light, applicable to diamond spin-based quantum computing. The filter employing a multimode waveguide Bragg grating (WBG) where forward TE₀ and backward TE₂ modes couple strongly, exhibits single-mode red light transmission and superior green light elimination compared to conventional TE₀–TE₀ coupling. The elimination wavelength changes linearly with the WBG period, offering tunability to target unwanted green wavelengths. A minimum transmittance of 10−3 is achieved for N = 10 000 periods. This band-stop filter is a key component for integrated photonics, especially for on-chip photonic quantum processors.