Analysis of mode characteristics of fiber Bragg gratings in large-mode-area double-clad fibers

Fiber Bragg gratings (FBGs) in large-mode-area double-clad fibers (LMA-DCFBGs) are the key components for high-power all-fiber lasers. Mode characteristics analysis of LMA-DCFBGs is of great significance for mode selection and performance optimization for high-power fiber lasers. In this paper, the spectral characteristics of LMA-DCFBGs were studied theoretically and experimentally, and confirmed that there is a power conversion between the LP01 mode and the LP11 mode in LMA-DCFBGs. Then, the growth curves of coupling coefficients of LMA-DCFBG with an increasing irradiation dose of ultraviolet (UV) light were measured. The results demonstrated that coupling efficiency between the LP01 mode and the LP11 mode is independent of the polarization state of UV lasers. In high-power fiber lasers, according to the spectral characteristics of LMA-DCFBGs, it can be considered that mode selection of fiber lasers could be realized by a pair of FBGs inscribed in fibers with different structural parameters, and that the appropriate wavelength matching condition could effectively avoid the laser power conversion from the LP01 mode to the LP11 mode, which could improve the beam quality of the output laser.