1. 南京航空航天大学机械结构力学及控制国家重点实验室，江苏 南京 210016
2. 上海卫星装备研究所，上海 200040
3. Université de Toulouse, Institute Clément Ader UMR CNRS 5312, INSA/UPS/ISAE/Mines Albi, France
Liquid Refractive Index Monitoring Method Based on SM-NCF Reflection Spectrum Characteristic Identification
YUAN Hui-ying1, ZENG Jie1*, WANG Ke2, GONG Xiao-jing3, LI Yu2, MA Chao1, LIANG Da-kai1
1. State Key Laboratory of Mechanics and Control of Mechanical Structures，Nanjing University of Aeronautics and Astronautics，Nanjing 210016，China
2. Shanghai Institute of Satellite Equipment, Shanghai 200040, China
3. Université de Toulouse, Institute Clément Ader UMR CNRS 5312, INSA/UPS/ISAE/Mines Albi, France
Abstract：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.
