Temperature-deep Device Study Of Composite Fiber Grating

Optical fiber sensors are a new type of sensor element with the advantages of high sensitivity,high accuracy,good stability and small size,and are widely used in the measurement of temperature,pressure,strain,chemistry and other fields.However,the sensitivity and response characteristics of a single type of optical fiber grating are limited at certain physical quantities,making it difficult to meet the needs of temperature and pressure measurements in complex underwater environments.Therefore,the main work of this paper is to explore the composite sensor based on fiber optic grating:（1）The process and method for the preparation of a new Asymmetric Misalign Tapered Multimode（ASTM）structure based on a conventional SMS interferometer structure using a common fiber fusion splicer are presented in detail,outlining the analysis of the ASTM optical field distribution and the coupling principle between core and cladding modes.（2）Based on the conventional single-mode convex cone structure,a process and method for the preparation of Bitaper Long Period Fiber Grating（BLPFG）structures were invented using a common fiber fusion splicer without fiber optical cutting process,directly using manual fusion discharge and stepper motor control parameters.The reasons for the selection of the main parameters are analysed.（3）RSoft is used to simulate and analyse the optical field distribution,and the coupling pattern between core and cladding modes for the ASTM and BLPFG structures,which is used as a guide for later sensor preparation.Two different pressure sensitization measurement devices are proposed for ASTM and BLPFG cascaded temperature-depth sensors for temperature rise and fall and pressure rise and fall measurements to analyse the sensitivity of their resonance peaks.The experimental results show that the maximum sensitivity for temperature is 26.98 pm/°C and for pressure is-28.76 pm/k Pa.The ASTM-BLPFG structure is a cascaded composite optical fiber grating sensor that can measure both temperature and pressure with high sensitivity to pressure.A matrix analysis method is also proposed to reduce the crosstalk sensitivity of the sensor temperature and pressure.（4）The main parameters for the preparation of a convex cone-seven-core fiber-convex cone type MZI interferometer sensor using a weakly coupled seven-core fiber are presented and numerically simulated,focusing on the effect of the convex cone structure parameters on the transmission spectrum.The main preparation process and parameter selection for LPFG inscription using a CO₂ marking machine are described in detail.An in-depth study of the sensor performance of the convex cone-seven-core optical fiber-convex cone tandem CO₂-type LPFG composite fiber grating sensor shows that the sensor has a maximum sensitivity of 73.42 pm/°C for temperature measurement and-1.21pm/k Pa for pressure measurement.The experiments use an ion sputtering instrument to the gold plating on the outer surface of the fiber optic sensor,and the results show that within a certain range,The results show that the temperature sensitivity of the optical fiber sensor can be increased to 84.52 pm/°C within a certain range of gold plating and that convex cone-seven fibers-convex cone tandem CO₂-LPFG composite optical fiber gratings can be used for both temperature and pressure measurements and are particularly sensitive to temperature variations.（5）To further reduce the influence of the complex underwater environment on the temperature and depth measurement,this paper proposes a temperature and pressure-encapsulated measurement device based on the antisymmetric design of the Boden tube.In addition,this paper summarises and analyses the work of a composite fiber optic grating sensor and proposes some improvements to improve the sensitivity of the sensor for temperature and pressure measurements.These works provide some reference value for the development of composite fiber optic sensors.In this thesis,ASTM,BLPFG,Convex Cone-Seven Core Fiber-Convex Cone Type MZI and CO₂ type LPFG structures are all made from common fiber optic materials and prepared by fiber optic fusion splicers and fiber optic marking machines for tandem compounding.These fiber optic sensor structures have a simple structure and controllable parameters,are cost-effective and are capable of measuring both temperature and pressure variations.In addition,these two composite optical fiber grating temperature-depth sensors,their temperature and pressure between the crosstalk sensitivity are low,so easy to promote the practical application.