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 CO
₂
-laser irradiation. The mode coupling from the fundamental core mode to both LP
₁₁
core mode and LP
₁₇
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, temperature sensitivity of 0.17 nm/°C and strain sensitivity of −9.6 pm/μϵ. The proposed SCF-HLPGs could have applications as multi-channel high-sensitivity fiber sensors, and multicore few-mode mode converters for space division multiplexing.