Surrogate modeling using coupled mode theory for accurate prediction of the optical response of long Bragg gratings

Abstract In this study, we propose a surrogate modeling approach that uses coupled mode theory (CMT) for accurate and efficient prediction of the reflectance and transmittance spectra of Bragg gratings. It is particularly useful for long structures (e.g. 2 mm), for which finite-difference time-domain (FDTD) simulations are too complex and time-consuming. We simultaneously fit the reflectance and transmittance spectra of shorter structures obtained from 2D and three-dimensional (3D) FDTD simulations using CMT and evaluate the performance of models with three and five parameters. The three-parameter model demonstrates superior accuracy with lower error and more physically meaningful predictions, while the five-parameter model better captures asymmetries caused by radiation losses. By overcoming the challenges of using FDTD directly for modeling long structures, our surrogate model provides a scalable and efficient framework for accurate estimation of the optical response. Validated against 3D FDTD simulations, it not only offers accuracy but also reduces computational costs.