Research On Fiber Refractive Index Sensor Based On Mode Interference

Refractive index as an important physical parameter,its detection technology plays an important role in food processing,biochemistry,medical and other fields.Electronic sensors（such as resistive and capacitive）on the market are widely used because of their low cost,easy to use and other advantages.However,in some flammable,explosive,strong acid and alkali,strong electromagnetic interference and other harsh environment,the traditional electrochemical sensors can not work properly.Fiber optic sensor has attracted wide attention because of its high sensitivity,corrosion resistance and electromagnetic interference resistance.Mach-Zehnder interferometer（MZI）has been widely used in the measurement of refractive index because of its compact structure and easy signal detection.The current online Mach-Zehnder interferometer sensor is mainly composed of fiber splicing with different core diameter.The external refractive index has little influence on the fiber mode.When light is transmitted in the fiber core and cladding,the interference of cladding mode and core mode only depends on the evanescent wave,which leads to the low sensitivity of this sensor to the refractive index change.The refractive index sensitivity of the sensor can be improved effectively by fabricating micro-structures on the sensor,including the misplacement of fiber core welding structure and fiber cone structure.In order to obtain higher sensitivity,based on the principle of Mach-Zehnder interferometer,two kinds of interferometric fiber sensors are proposed based on the single mode-multimode-single mode（SMS）fiber structure,and their refractive index sensing characteristics are studied.The main research contents are as follows:1.An ultra-high sensitivity refractive index sensor based on double bias fiber Mach-Zehnder interferometer is proposed.The interference mechanism of the biased Mach-Zehnder interferometer is analyzed theoretically.Through beam propagation method（BPM）,the refractive index sensing characteristics of multi-mode bias optical fibers and the changes of refractive index sensitivity in different bias lengths and refractive index ranges are simulated and analyzed,and the sensing characteristics of single bias and double bias optical fibers are compared.The simulation results show that the sensitivity of single offset Mach-Zehnder interferometer is-5557 nm/RIU and that of double offset Mach-Zehnder interferometer is-14071 nm/RIU in the range of 1.3330～1.3340.The results provide an important theoretical basis for the realization of high sensitivity liquid refractive index measurement.The sensor has a compact structure and the whole length is only 1200μm,which has important application value in the field of liquid refractive index measurement.2.Based on the Mach-Zender interferometer principle,a high-sensitivity fiber refractive index sensor with the structure of multimode fiber-taper single mode fiber-Multimode fiber（MTSM）is designed.Multimode interference（MMI）generated by the coupling of higher-order mode excited by cladding and fundamental mode in single-mode optical fiber core due to the mismatch of fiber core caused by the fusion of single-mode fiber and multi-mode fiber is used to measure the external refractive index.The proposed MTSM sensor is numerically simulated by Rsoft software,and the optical field distribution inside the sensor is analyzed.The influence of the tapered fiber’s waist diameter on the sensing performance is discussed.The MTSM structure sensor was optimized and fabricated,and the transmission spectra of several groups of H₂SO₄solutions with different concentrations were tested.The experimental results show that the smaller the waist diameter,the higher the sensitivity of the proposed sensor.In the range of refractive index 1.3330～1.3460,the sensitivity of the proposed fiber sensor can reach up to 541.5 nm/RIU,and the sensitivity of the sensor to refractive index can be further improved by reducing the waist diameter of the conical fiber.