Helical long-period grating manufactured with a CO₂ laser on multicore fiber.

We have demonstrated a helical long-period grating (HLPG) manufactured from multicore fiber (MCF) combined with twist and CO₂-laser heating technology. The MCF we used behaves as a kind of single mode fiber. The nineteen cores of MCF are distributed hexagonally which make the helical structure only exist in core area after heating and twisting. There are several strong couplings between the core and cladding modes at different wavelengths when the grating pitch was 900 µm. The extinction ratio of the main coupling reached 20 dB at 1550 nm. We have investigated the sensor characteristics of the HLPG. It is found that the grating presents clear sensitivity to refractive-index while exhibiting smaller cross-sensitivity to environmental temperature than conventional long-period gratings. The HLPG also shows great potential as a twist sensor, which has a sensitivity up to 0.198 nm/(rad/m) and can apply for measuring twist directions.