Study On Optical In-fiber Interferometer Sensing Technology

Optical in-fiber interferometer has been widely applied in petrochemical,biomedicine,architecture,aerospace and many other sensing fields for the merits of compact structure,miniature size,electromagnetic immunity,high sensitivity and all optical fiber.In this thesis,four types of optical in-fiber interferometers based on the research of principle,fabrication,theoretical analysis and experimental study are proposed and fabricated.And the sensing characteristics of in-fiber interferometers with different structures response to liquid refractive index（RI）,liquid level,temperature and relative humidity（RH）are experimentally investigated.The main works and conclusions of this thesis are as follows:（1）An optical in-fiber Michelson interferometer based on single mode-multimode fiber（SM-MMF）structure is proposed and demonstrated,and the liquid RI sensing characteristics of the interferometer are experimentally investigated.The experimental results reveal that the liquid refractive index sensitivity of-92.43 d B/RIU of the 11-mm-length-MMF sensor is achieved with the RI range of 1.3333¹.3796 RIU.Temperature experiment exhibits that the measurement error of the liquid RI caused by temperature is-1.08×10^-4 RIU/℃.With good repeatability and stability,the sensor can realize the temperature-insensitive measurement of liquid RI at room temperature.（2）An optical in-line fiber Michelson interferometer based on single mode-thinned fiber（SM-TF）structure is designed and manufactured,which realizes high sensitivity measurement of liquid RI and liquid level.The experimental results show that the7-mm-length-TF sensor achieves a liquid RI sensitivity of-111.58 d B/RIU for RI range of 1.333¹.380 RIU,and the measurement error of the RI caused by temperature is -0.89×10^-6RIU/℃.The 12-mm-length-TF sensor gains a liquid level sensitivity of-0.129 nm/mm for liquid level range of 0⁹ mm and a temperature sensitivity of 0.038 nm/℃ for liquid temperature range of 20⁸0℃.（3）Combining optical fiber taper technology and process,an optical in-fiber Mach-Zehnder interferometer based on single mode-taper-thinned fiber-taper-single mode fiber structure is fabricated.The experimental results demonstrate that the 12-mm-length-TF sensor has a liquid level sensitivity of 0.63 nm/mm for liquid level range of 0²1 mm and atemperature sensitivity of 0.009 nm/℃ for water temperature range of 20⁵0℃,indicating that the liquid level measurement error of the liquid RI caused by temperature is 0.014 mm/℃.（4）Based on fiber chemical etching and fiber fusion splicing,a hybrid cavity optical in-fiber Fabry-Perot interferometer consists of air bubble with length of 76.70 μm,width of42.61 μm and a short section of MMF with length of 348 μm is proposed.The experimental results show that the sensor has a linear humidity sensitivity of-0.017 d B/%RH for the RH range of 40⁹0 %RH and a temperature sensitivity of 9.7 pm/℃ for the temperature range of25¹60℃.The sensor can realize simultaneous measurement of RH and temperature.The above sensors have advantages of compact structure,easy of fabrication,low cost and high sensitivity,which can realize temperature-insensitive liquid refractive index,liquid level and relative humidity measurement at room temperature,offering promising applications and practical value.