Building-up and suppression of cladding mode coupling in fiber Bragg gratings

Abstract

The building-up and suppression of cladding-mode coupling in UV exposure fiber Bragg gratings (FBGs) is investigated experimentally and theoretically. Experimental results reveal that fundamental mode can simultaneously couple to cladding modes from different groups in the un-tilted FBGs as the transmission spectrum takes on multilevel. Theoretical results indicate that due to the non-uniform refractive index profile on the transverse plane of the FBGs, core-guided fundamental mode not only couples to axis-symmetrical cladding modes LP0n, but also transfers to circular-symmetrical cladding modes like LP1n if they distribute in the core region. Based on the symmetry property of the coupled cladding modes, the ‘ring-by-ring’ inscription is theoretically proposed to suppress the cladding mode coupling for the first time, which plays a significant role in high-power fiber lasers research and application.