High-quality helical long-period fiber gratings (HLPFGs) with a coupling strength greater than 30 dB and an insertion loss less than 0.5 dB are formed by applying CO2 laser to expose an optical fiber that is simultaneously translated and rotated to relieve the residual stress of the fiber along a helical pathway, rather than twisting the fiber under a high-temperature condition. The mode coupling characteristics of this type of HLPFG are investigated for the first time. Experimental results demonstrate that the helical refractive-index modulation of the HLPFGs can generate light with orbital angular momentum (OAM) modes, where the chirality of the OAM modes is insensitive to the polarization state of the input light. The HLPFGs provide excellent torsion characteristics, and the resonant wavelengths of OAM modes can be linearly tuned when applying torsions.

中文翻译：

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作为波长可调轨道角动量模式发生器的螺旋长周期光纤光栅

采用CO2激光曝光光纤，同时平移和旋转以释放残余应力，形成耦合强度大于30 dB、插入损耗小于0.5 dB的高质量螺旋长周期光纤光栅（HLPFG）光纤沿螺旋路径旋转，而不是在高温条件下扭曲光纤。首次研究了此类HLPFG的模式耦合特性。实验结果表明，HLPFG 的螺旋折射率调制可以产生具有轨道角动量 (OAM) 模式的光，其中 OAM 模式的手性对输入光的偏振态不敏感。HLPFG 具有出色的扭转特性，