Abstract

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 ${\mathrm{LP}}_{01}$ mode and the ${\mathrm{LP}}_{11}$ 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 ${\mathrm{LP}}_{01}$ mode and the ${\mathrm{LP}}_{11}$ 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 ${\mathrm{LP}}_{01}$ mode to the ${\mathrm{LP}}_{11}$ mode, which could improve the beam quality of the output laser.

© 2019 Optical Society of America

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