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基于调频光谱原理,提出一种可用于在线测量的表面等离子体共振光纤传感器结构。理论分析、数值仿真和实测结果表明,基于表面等离子体共振效应的光纤传感头,相当于具有中心对称吸收谱的吸收介质,且吸收峰中心波长随待测折射率的增加而红移;基于此吸收谱函数,以微分光谱替代直接光谱测量,可有效克服吸收峰处一阶微分谱为零对测量灵敏度的消极影响;参考光路及闭环负反馈结构的引入,可有效抑制源于光源和光路中光强波动等的影响。以镀有50nm厚金薄膜的多模石英光纤组成传感头,搭建相应的闭环系统,对折射率为1.33000～1.43000范围内不同液体进行折射率测量,并与阿贝折射率仪、光纤光谱仪及理论仿真结果进行比较。结果表明,在1.33200～1.37580RIU(RIU表示单位折射率)范围内,测量精度可达0.00016RIU,与波长分辨率为0.4nm的光谱仪测量结果精度相当;在1.37500～1.42640RIU范围内,测量精度可达0.00071RIU,优于0.00090RIU的光谱仪测量结果,较好地验证了该方案用于在线测量的可行性和检测结果的可靠性。
Based on the principle of FM spectroscopy, a surface plasmon resonance fiber sensor structure which can be used for on-line measurement is proposed. Theoretical analysis, numerical simulation and measured results show that the optical fiber sensing head based on surface plasmon resonance is equivalent to an absorption medium with central symmetry absorption spectrum, and the central wavelength of absorption peak shifts red with the increase of the refractive index to be measured. The absorption spectrum function, which uses the differential spectrum instead of the direct spectrum measurement, can effectively overcome the negative influence of the first-order differential spectrum at the absorption peak on the measurement sensitivity. The introduction of the reference light path and the closed loop negative feedback structure can effectively restrain the light source from the light source and the light path In the light intensity fluctuations and so on. A multi-mode quartz optical fiber coated with a 50 nm thick gold film was used to form a sensing head. A corresponding closed-loop system was set up to measure the refractive index of different liquids with a refractive index ranging from 1.33000 to 1.43000. The refractive index was measured with Abbe refractometer, fiber spectrometer and Theoretical simulation results are compared. The results show that the measurement accuracy can reach 0.00016 RIU in the range of 1.33200 ~ 1.37580 RIU (RIU means unit refractive index), which is comparable with the spectrometer with wavelength resolution of 0.4 nm. The accuracy of measurement can be within the range of 1.37500 ~ 1.42640 RIU The results of spectrometer measurement up to 0.00071 RIU and better than 0.00090 RIU validated the feasibility of this scheme for on-line measurement and the reliability of test results.