We developed a helical long-period fiber grating (HLPG), which was formed by twisting step-index double-clad fiber (SDF) under the irradiation of a CO2 laser, and can generate stable orbital angular momentum (OAM) mode. The mode coupling and transmission characteristics of the SDF-HLPG were investigated both theoretically and experimentally. The results prove that this grating can couple the light wave into the inner cladding mode of the SDF, and directly generate OAM mode. Subsequently, the tolerance of the SDF-HLPG to environmental distur-bance was also evaluated. The OAM mode has low sensitivities to mechanical torsion and bending, and has almost no responses to strain and surrounding refractive index. This indicates that the SDF provides a channel for the OAM mode to avoid high external interference. Therefore, this kind of SDF-HLPG has the potential for communication applications.
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