Design And Application Of Single-Hole Fiber Sensor Devices

Fiber optic sensors have been widely used in many fields such as aerospace,bridge construction,industrial production,biomedical,etc.with the advantages of small size,high sensitivity,anti-electromagnetic interference,and easy integration.However,as more and more research has being conducted in various fields,the fiber optic sensors based on single-mode fibers（SMFs）are not able to comply with the increasing performance requirements.The development and application of special optical fiber,especially micro-hole fiber,provides great vitality to the design and application of novel optical fiber sensor devices.Due to the presence of air hole,micro-hole fiber sensor devices provide superior performance in fluid sensing and combination with functional materials compared to traditional SMF sensors.The single-hole fiber,as a simpler structure of micro-hole fiber,is more suitable for large-scale industrial applications because of its less difficult preparation process and lower production cost.In this thesis,two types of single-hole fibers,hollow suspended-core fiber（HSCF）and single-hole dual-core fiber（SHDCF）,have been utilized for the design and fabrication of fiber grating,fiber interferometer and directional coupler,which are applied to the sensing of various physical parameters.This study is significant to promote the development of fiber sensing technology.The main works of this thesis are listed as follows:1.A refractive index（RI）sensor based on fiber Bragg grating（FBG）in HSCF is proposed.The FBGs were written on the cores of hollow suspended-core fibers by the phase mask method and the femtosecond point-by-point method,respectively,and applied to refractive index sensing.The samples prepared by the femtosecond point-by-point method have slightly higher sensing characteristics.The sensor obtained a refractive index sensitivity of 14.24 nm/RIU in the RI range of 1.41 to 1.44.In addition,In addition,due to the off-center geometry of the core of the HSCF.This sensor can be used for one-dimensional bend sensing.The maximum bending sensitivity is-146.6 pm/m^-1 in the curvature range of 1 to 4 m^-1.2.The Fabry-Perot interferometer（FPI）was prepared based on the hollow core fiber（replaced with HSCF）by using the precision cutting technique and fiber fusion splicer.In addition,a high sensitive gas pressure sensor with vernier effect is obtained by serially connecting open-cavity FPI and closed-cavity FPI with approximate cavity length.The proposed sensor has a high barometric sensitivity of-44.31 nm/MPa over the barometric range of 0～2.2 MPa.and a low temperature cross-sensitivity of-0.6 KPa/°C.The designed gas pressure sensor has a large measurement range,high sensitivity,and low temperature cross-crosstalk.3.A dual-mode Mach-Zehnder interferometer（MZI）magnetic field sensor based on HSCF is proposed.Mode interference is formed by the simultaneous excitation of LP₀₁ mode and LP₁₁in the core,by offset fusion of the two ends of the HSCF with the SMFs.The microfluidic channel is then filled with magnetic fluid,which is constructed using a high frequency CO₂laser.The magnetic fluid（MF）is then filled into the microfluidic channel,which is constructed by using a high frequency CO₂ laser.The principle of the sensor is the modulation of the RI of the MF by the magnetic field,which changes the effective RI difference between the two modes in the fiber core,thus achieving the sensing measurement of the magnetic field.Dual-mode interference based on LP₀₁ and LP₁₁ modes allows the sensor to have a more concise interference spectrum and spatial spectrum,reducing the difficulty of subsequent modulation and demodulation,and enhancing the practicality of the sensor.4.A dual parametric sensor with integrated dual-mode MZI and directional coupler was designed and prepared based on SHDCF for the simultaneous measurement of liquid’s salinity and temperature.The center core and suspension core of a SHDCF are resonantly coupled near phase-matched wavelengths.In addition,the slightly larger core diameter of the suspended core allows supporting both LP₀₁ and LP₁₁ modes until 1410 nm.Mode interference can be formed by offset fusion with SMF.The dual parametric sensor is constructed by offset fusing the suspended core ends of a single coupling length SHDCF to SMF.Micro-holes are fabricated on SHDCF by using a femtosecond laser.The salinity sensitivity of the interfering peak and coupling peak were 1.357nm/%and 1.907nm/%,and the temperature sensitivity were-158.4pm/°C and-387.4 pm/°C,respectively.Dual parametric sensing of salinity and temperature of Na Cl solution can be achieved by constructing a sensitivity matrix equation.