Femtosecond laser direct inscription of long period gratings in multicore fiber.

We demonstrate femtosecond laser direct writing long period gratings (LPGs) on multicore fibers (MCFs). By adopting the line-by-line inscription technique, LPGs can be independently fabricated at arbitrary cores of the MCF without removing the coating or altering fiber positions. Theoretical and experimental analyses were conducted to assess the laser energy distribution in the MCF and the impact of LPG parameters on filtering characteristics. LPGs inscribed in different cores under the same condition exhibited good spectral consistency, with a coupling efficiency of 90% (10 dB). Fabricated LPGs with various periods achieved multi-wavelength filtering effects in a single piece of the MCF with a widely adjustable wavelength range. The femtosecond laser direct inscription technique for fabricating spatial structures in the MCF demonstrates significant potential for space-division multiplexing (SDM) and wavelength-division multiplexing filtering applications.