Broadband Optical Coupler Based on the Helical Long-Period Gratings Inscribed in Seven-Core Fibers

We propose a broadband optical coupler based on cladding mode coupling of identical helical long-period gratings (HLPGs) inscribed in different cores of seven-core fibers (SCFs). The influence of the surrounding refractive index and offset distance between the input grating and tapping grating on the coupling efficiency of the coupler was investigated numerically and experimentally. A total peak coupling efficiency of 75.7% was achieved experimentally under the conditions of a surrounding refractive index of 1.454 and an offset distance of 90 mm when the light is launched into the center core of the SCF. The proposed optical coupler could have promising applications in multi-core fiber amplifiers and space-division multiplexing optical communications.