Bound States in the Continuum in Fiber Bragg Gratings

Copyright © 2019 American Chemical Society. Optical fibers typically confine light through total internal reflection or through photonic band gaps. Here, we show that light can be perfectly guided in optical fibers through a different mechanism based on bound states in the continuum (BICs). In fibers with periodic Bragg gratings, we predict bona fide BICs in pure-polarization modes as well as quasi-BICs in hybrid-polarization modes. These BICs and quasi-BICs are topologically protected and exist robustly without the need for fine structural tuning. With a coupled-wave framework that is especially accurate for gratings with small index contrasts, we analytically prove that these BICs persist even with the very small index contrasts that are common in realistic fiber Bragg gratings. The suppression of radiation loss arises from the destructive interference between a weakly radiating local mode and a strongly radiating one. This finding opens the possibility of guiding light with BICs in optical fibers and their applications in fiber sensors, filters, and high-power fiber lasers.