从理论和实验上研究了角向二阶少模长周期光纤光栅的扭转响应特性。理论分析发现角向高阶长周期光纤光栅的耦合谐振波长的扭转响应度依赖于光栅角向模式的阶数,角向二阶模式的扭转波长响应度约是角向一阶的2倍;同时,光栅扭转引起的相位失配使得光栅谐振峰强度在小扭转率区间内随光栅扭转率增加呈准线性减小,减小的快慢与相位失配量成反比。实验表明:角向二阶少模长周期光纤光栅的谐振波长的扭转响应度约是一阶的1.5倍,在顺逆时针扭转的情况下分别达到0.72 nm·(rad·m -1) -1 和0.82 nm·(rad·m -1) -1;而谐振峰强度在小扭转率区间随扭转率的增加呈线性变化,对应顺逆时针的线性灵敏度分别为 0.81 dB·(rad·m -1) -1和0.72 dB·(rad·m -1) -1,在大扭转率状态下,对应谐振强度变化幅度小且具有波动的特点,实验结果与理论分析基本一致。角向二阶少模长周期光纤光栅的谐振峰波长和强度随扭转的响应特性在高精度的扭转力学量传感(例如扭转量、扭转速度和加速度),以及多参量同时测量等方面有潜在的应用前景。

This study experimentally and theoretically examines the response characteristic of twisting the second-azimuthal-order few-mode long-period fiber grating (FM-LPFG). The theoretical analysis shows that the twisting responsivity of the coupled resonant wavelength of the high-order FM-LPFG is closely related to the azimuthal order of the grating, i.e., the twisting responsivity of the resonant wavelength of the second-azimuthal-order FM-LPFG is almost two times larger than that of the first-azimuthal-order FM-LPFG; the twisting-induced phase mismatching leads to an almost linear decay of the intensity of the resonant peak in the transmission spectrum of the grating with the increase of grating twisting rate in a relatively small twisting-rate domain. This decay rate is inversely related to the value of the phase mismatch. Further, the experimental results show that the twisting responsivity of the resonant wavelength of the second-azimuthal-order FM-LPFG is approximately 1.5 times larger than that of the first-azimuthal-order FM-LPFG, which reaches 0.72 nm·(rad·m ^-1) ^-1 and 0.82 nm·(rad·m ^-1) ^-1 in the cases of clockwise and counter-clockwise twisting, respectively. The intensity of the resonant peak varies linearly with the increase of the twisting-rate in the small twisting-rate domain, and the linear sensitivities are 0.81 dB·(rad·m ^-1) ^-1 and 0.72 dB·(rad·m ^-1) ^-1 in the cases of clockwise and counter-clockwise twisting, respectively; however, the corresponding intensity of resonant peak has the characteristics of small variation amplitude and fluctuation in the large twisting-rate domain, indicating that experimental results are generally in agreement with the theoretical analysis. These twisting response characteristics of the wavelength and intensity of the resonant peak of the second-azimuthal-order FM-LPFG have potential applications in high-precision sensing of twisting mechanical parameters (such as twisting capacity, twisting speed, and acceleration) and simultaneous measurement of multiple parameters in the same monitoring twisting-rate domain.