Research On Optical Fiber Sensors Of Multi-Mode Interference Based On No-Core Fiber

Optical fiber sensing technology and fiber laser sensor technology have important application in sensing field,and become the focus of this filed.In this thesis,we focused on the single-mode-no-core-single-mode fiber(SNCS)structure as sensors and the expansion of this sensor coated or encapsulated functional materials.The ring fiber laser sensor based on SNCS structure is also studied.The work includes following parts.1.The mode propagate analysis method(MPA)for single-mode-multimode-singlemode fiber(SMS)structure is improved,so that it can be applied to the analysis of refractive index sensing properties of SNCS structure.From the view of optical waveguides,the far from cutoff model is adopted for no-core fiber.The transmission spectrum of SNCS structure is analyzed,and the sensing characteristics of temperature and refractive index are studied.The raliability of the improved method is verified by experimental comparison.It provides the theoretical basis and guidance for the design of optical fiber sensors using SNCS structure.2.A high precision and wide range liquid level sensor is realized based on the SNCS structure followed by FBG structure.Based on the temperature and refractive index sensing characteristics of SNCS structure,the liquid level sensor is designed,and two different spectra can be used in the liquid level sensor.On the basis of this,combining the SNCS structure and FBG structure,the reflection liquid level sensor is designed,and the structure of SNCS + FBG is parallel to extend the measurement range.The proposed liquid level sensing system has a sensitivity of about 0.2dB/mm in the liquid level range of 0-117 mm.3.The sensing application of the SNCS structure is extended using functional materials.The sensing properties of the SNCS structure are investigated by coating magnetic fluid and agarose gel.The simultaneous sensing of the magnetic field and temperature is realized by the multiple dip in the transmission spectrum,and the related sensing matrix is constructed.The humidity sensing characteristics of SNCS structure coated by agarose gel at different concentrations are studied experimentally,which is the basis of the further research of humidity sensors based on SNCS structure.4.The SNCS structure is embedded in the ring laser cavity,and the intracavity sensing technique based on fiber laser is realized.In the field of magnatic sensing,the experimental results show that the wavelength sensitivity of the magnetic sensor is about 50pm/mT,the intensity sensitivity is about-0.3dBm/mT.In the field of humidity sensing,the humidity sensitivity is about-68pm/%RH in the range of 35%RH to 95%RH.The rise time of relative humidity sensor is about 93 ms,and the recovery time is about 343 ms.It can be used as an optical fiber sensing platform due to easy fabrication and low cost.It will be applied more and more in the field of optical fiber sensors in the future.