Bound states in the continuum in fiber Bragg gratings

TL;DR

This paper demonstrates that bound states in the continuum (BICs) can be used to guide light in optical fibers with Bragg gratings, offering a new mechanism that does not require fine tuning and is robust against small index contrasts.

Contribution

It predicts the existence of bona fide and quasi-BICs in fiber Bragg gratings, expanding the understanding of light confinement mechanisms in optical fibers.

Findings

BICs exist in pure-polarization modes of fiber Bragg gratings.

Quasi-BICs are present in hybrid-polarization modes.

BICs are robust and do not require fine structural tuning.

Abstract

Optical fibers typically confine light through total internal reflection or through photonic bandgaps. 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 guided modes exist robustly without the need for fine structural tuning, and they persist even with the very small grating index contrasts that are available in conventional fiber Bragg gratings. The suppression of radiation loss arises from the coupling between a weakly-radiating mode and a strongly-radiating one. This finding opens the possibility of guiding light with BICs in optical fibers and their applications in distributed fiber sensors, in-line fiber filters, and high-power fiber lasers.