Characteristics of Helical Long-Period Gratings Inscribed in Seven-Core Fibers by CO² Laser

We demonstrated the fabrication of helical long-period gratings (HLPGs) in a seven-core fiber (SCF) using CO2-laser irradiation. The mode coupling from the fundamental core mode to both LP11 core mode and LP17 cladding mode can be realized using the HLPGs with different grating periods. The torsion, refractive index, temperature, axial strain, and polarization-dependent loss characteristics of the HLPGs in different cores were investigated experimentally. The HLPG was found to have a high torsion sensitivity of 0.61 nm/(rad/m), refractive index sensitivity of -4837.8 nm/RIU when the surroundings refractive index changes in the range of 1.445-1.454, the temperature sensitivity of 0.17 nm/°C and strain sensitivity of -9.6 pmμε. The proposed SCF-HLPGs could have applications as multi-channel high-sensitivity fiber sensors, and multicore few-mode mode converters for space division multiplexing.