针对分子生物学与环境监测领域高灵敏度特异性检测需求, 提出一种基于反射光谱特征辨识的单端反射式光纤折射率传感器模型, 并给出了这种基于多模干涉原理的单模光纤-无芯光纤(Single mode fiber-No core fiber, SM-NCF)串接结构传感机理及其理论模型。 无芯光纤实质上是一种结构特殊的多模光纤, 在实际应用中无芯光纤结构本身作为纤芯, 外界环境介质当作包层, 构成光波导结构。 这与普通多模光纤相比, 不需要采用氢氟酸对多模光纤的包层进行化学腐蚀, 不会降低光纤的机械性能, 也不会破坏芯模传输条件, 可以更好的实现对周围环境折射率的传感监测。 当无芯光纤所处外界环境折射率发生改变时, 其波导结构和包层有效折射率均会发生改变, 从而引起传输光信号的纵向传播常数和模场分布也会随之发生改变, 最终导致不同波长对应传输光功率的变化。 上述效应反映在反射光谱上, 即干涉波谷对应的谐振波长、 波谷峰值强度以及半波宽度发生相应变化, 通过辨识该反射光谱特征就可实现对外界环境折射率的测量。 借助光束传播法(BPM), 数值模拟得到无芯光纤长度分别为自映像距离和非自映像距离时的SM-NCF内部光场能量分布规律, 并制作了无芯光纤长度分别为自映像距离和非自映像距离的SM-NCF光纤折射率传感探头, 将作为传感区域的无芯光纤一端与标准单模光纤熔接, 采用磁控溅射技术在无芯光纤另一端面镀上金膜, 用以提升反射光谱强度。 在此基础上, 搭建了基于SM-NCF终端反射型的光纤折射率试验系统, 并开展了相关实验研究。 研究结果表明, 当无芯光纤长度是15 mm(自映像距离)时, 随着液体折射率从1.331 5依次增大至1.390 2, SM-NCF反射光谱逐渐向长波方向偏移, 其反射峰谐振波长对应的折射率灵敏度约为197.57 nm·RIU-1, 相关系数为0.93; 反射峰值强度也呈现逐渐降低趋势, 其折射率灵敏度约为-62.80 dB·RIU-1。 当无芯光纤长度是20 mm(非自映像距离)时, 随着液体折射率依次增大, SM-NCF反射光谱呈现明显双峰现象, 且均逐渐向长波方向偏移, dip2谐振峰波长折射率灵敏度约为133 nm·RIU-1, 相关系数为0.96; 反射峰值强度也呈现逐渐降低趋势, 其折射率灵敏度约为-31.66 dB·RIU-1。 对比分析可知, 不论是从反射峰谐振波长偏移的角度, 还是从反射峰值强度的角度, 自映像距离长度对应的 SM-NCF终端反射型光纤传感器均具有较高灵敏度。 对于相同折射率液体环境, 非自映像距离长度对应的SM-NCF反射光谱半波宽度与自映像距离长度相比, 呈现显著变窄趋势。 相对于SMS透射型传感结构, 当传感区域长度相同时, SM-NCF反射型结构能够实现对光波信号的往返两次调节。 这种终端反射型SM-NCF传感器改进了传统透射型折射率传感器不便与待测液体相接触的缺点, 具有结构简单、 易于制作、 抗电磁干扰能力强以及便于远程遥测等优点, 能够为后续生化与环保监测领域研究应用提供有益支持。

For the purpose of high sensitivity and specificity testing in the field of molecular biology and environmental monitoring, areflective single-ended optical fiber refractive index sensor model based on the characteristic identification of reflection spectrum is proposed, and the sensing mechanism and theoretical model of single-mode fiber-no core fiber (SM-NCF) cascaded structure based on multimode interference principle are given. No-core fiber is essentially a multimode fiber with special structure, where, the no-core fiber structure itself serves as the core, and the external environment medium serves as a cladding in practical application, which constitutes an optical waveguide structure compared with the ordinary multimode fiber, and it is unnecessary to use hydrofluoric acid to corrode the cladding of multimode optical fiber in the chemical way, so it will not reduce the mechanical properties of optical fiber, and it will not destroy the transmission condition of the core mode, therefore, it is better to realize the sensing monitoring of the refractive index of surrounding environment. When the refractive index of external environment where no-core fiber is surrounded changes, the effective refractive index of the waveguide structure and the cladding will change as well, causing the longitudinal propagation constant and the mode field distribution of the transmitted light to change accordingly, finally resulting in changes in different wavelength and optical power transmission. The above-mentioned effects are reflected in the reflection spectrum, which are the corresponding changes of the resonant wavelength, the trough valley intensity and the half-wave width of the interference valley, so the measurementof external environment refractive index can be achievedby identifying the characteristics of the reflection spectrum. Based on the beam propagation method (BPM), the numerical simulation of no-core fiber with a length of self-imaging distance and non-self-imaging distance is carried out in this paper, and the internal light energy distribution of the single-mode-no-coremode fiber sensing part is obtained. Then the real single-mode-no-core mode fiber sensing head is made with different lengths respectively, which is self-imaging distance and non-self-imaging mentioned as before. One end of the no-core fiber acts as the sensing area is welded with a standard single mode optical fiber, and a magnetron sputtering technique is used to plategold film on the other end of the no-core fiber in order to enhance the intensity of the reflection spectrum. On this basis, the optical fiber refractive index test system based on the SM-NCF terminal reflection type is built up and relevant experimental research is carried out. The results show that when the length of the no-core fiber is 15 millimeter (which corresponds to the self-image distance), the reflection spectrum of the single-mode-no-core-mode fiber sensing head gradually shifts to the longer wavelength as the refractive index of the liquid increases from 1.331 5 to 1.390 2, and the refractive index sensitivity corresponding to the resonant wavelength of the reflected peak is about 197.57 nm·RIU-1 and the correlation coefficient is 0.93. The peak reflection intensity also shows a decreasing trend, and the refractive index sensitivity is about -62.80 dB·RIU-1. When the length of the no-core fiber is 20 millimeter (which corresponds to the non-self-image distance), the reflectance spectrum of the single-mode-no-core-mode fiber sensing headexhibits a distinct bimodal phenomenon with the gradual increase of the liquid refractive index, and both of the two interference dips shift to the long-wave direction gradually. One of the obvious interference dip is dip 2. which has a refractive index sensitivity of 133 nm·RIU-1 from the aspect of resonant wavelength change, and the correlation coefficient is 0.96; On the other hand, the peak reflection intensity also shows a gradual downward trend, and the refractive index sensitivity is about -31.66 dB·RIU-1. The comparative analysis shows that the terminal reflection type optical fiber sensor of single-mode-no-core mode sensing head with the self-image distance length has higher sensitivity no matter from the prospect of the resonant wavelength deviation or the peak intensity of reflection. It can be seen clearly thatthe half-wave width from the reflection spectrum of single-mode-no-core mode fiber sensing head, when compared with the self-image distance length, there is a significant narrowing trendcorresponding to the non-self-image distance length for the same refractive index liquid environment. Compared with the single mode-no-core-single mode transmission fiber sensor structure, when the length of the sensing area is the same, the reflective structure of single mode-no-core fiber can realize the two adjustment of the round trip to the light wave signal. The terminal reflection type single mode-no-core fiber sensor improves the shortcomings of the traditional transmission type refractive index sensor, making contact with the liquid to be measured more convenient, and, havings the advantages of simple structure, being easy to fabricate, strong ability to resist electromagnetic interferenceand being convenient for remote telemetry, etc., hence it can provide useful support for subsequent research and application in the field of biochemical and environmental monitoring.