1：Experimental Design and Method Selection:

The study involved fabricating a helical long-period grating (HLPG) in graded-index few-mode fiber (GI-FMF) using a CO2 laser technique. The sensing characteristics on torque, curvature, temperature, and refractive index were experimentally investigated.

2：Sample Selection and Data Sources:

A segment of GI-FMF was spliced with single-mode fiber at both ends. Light from a broadband light source was launched into the GI-FMF, and the output light was monitored by an optical spectrum analyzer.

3：List of Experimental Equipment and Materials:

CO2 laser (H-10, Han’s Laser), optical spectrum analyzer (OSA, AQ6375, YOKOGAWA), rotator, transition platform, broadband light source, index matching liquid.

4：Experimental Procedures and Operational Workflow:

The GI-FMF was rotated with a certain angular speed by a rotator and moved along the fiber axial direction by a transition platform while the CO2 laser scanned on the GI-FMF along a fixed path. The processes of axial movement, rotation, and CO2 laser exposure were managed by a computer.

5：Data Analysis Methods:

The transmission spectrum of the fabricated HLPG was recorded, and the wavelength shifts of resonance dips were analyzed to determine sensitivities to torque, curvature, temperature, and refractive index.