为了克服折射率测量过程中温度交叉敏感的影响, 提出并制备了一种少模光纤长周期光栅传感器.该传感器利用CO2激光器在少模光纤上先写入周期为654 μm、长度为30 mm的长周期光栅, 然后用旋转平台将光纤旋转180°, 再写入相同长度周期为819 μm的长周期光栅制作而成, 其传输光谱在1 487.2 nm和1 533.0 nm处出现两个由不同模式耦合形成的谐振峰, 通过监测两个谐振峰差值的变化减少温度串扰, 实现折射率的测量.实验结果表明：两个谐振峰差值在折射率1.333 3~1.376 6范围内的灵敏度为143 nm/RIU, 在温度20~70 ℃范围内的灵敏度为-0.002 5 nm/℃, 温度灵敏度远低于折射率灵敏度, 具有对温度不敏感的特性.与传统光纤传感器相比, 该传感器具有温度干扰小, 折射率灵敏度高等优势, 并且尺寸较小、结构紧凑, 可在工业、水利、医学等领域广泛应用.

In order to avoid the temperature cross-sensitivity during refractive index measurement, a sensor based on a superimposed long period gratings fabricated in few mode fiber was proposed. The superimposed long period grating was inscribed under the CO2 laser irradiation as following: first, a long period fiber grating with a period of 654 μm and length of 30 mm was inscribed, then the few mode fiber was rotated 180° to inscribe second long period fiber grating with a period of 819 μm and the same length. Two resonance dips at 1 487.2 nm and 1 533.0 nm appeared in the transmission spectrum, which were formed by coupling between different guided modes. The difference of two resonance dips increases with the refractive index while is constant with temperature increase, which can eliminate temperature cross-sensitivity during refractive index measurement. The experimental results show the temperature sensitivity of the sensor is -0.002 5 nm/℃ in 20~70 ℃ temperature range and the refractive index sensitivity of 143 nm/RIU in 1.333 3~1.376 6 refractive index range. The temperature sensitivity of the sensor is much lower than the refractive index sensitivity, which means that it is insensitive to temperature. Compared with traditional optical fiber sensors, the sensor has the advantages of small temperature cross-sensitivity, high refractive index sensitivity, small size and compact structure, and it can be widely used in industry, water conservancy, medicine and other fields.