Research On Fiber Optic Fluid Pressure Sensor With Extended Air Cavity Based On FP Interferometer

Optical fiber sensors have the characteristics of small volume,immune to electromagnetic interference and enabling remote detection,these are suitable for biomedical,industrial,and environmental safety monitoring.The Fabry-Perot(FP)interferometric structure has the advantages of high sensitivity,compact structure,remote detection and easy to single-ended operation among of all fiber optic sensors.So it is specially suitable for applications in confined spaces and extreme environments.In this paper,it is applied to the detection of fluid pressure according to the structural characteristics of the Fabry-Perot interferometer.For its sensing mechanism and performances,experimental measurements and theoretical analysis were carried out.The main work includes:1.The background and significance of this research are expounded.The basic principles of several interferometers are briefly introduced and the fiber optic Fabry-Perot interferometers are classified.Then,the research status of fiber optic pressure sensors in domestic and foreign is reviewed.The Fabry-Perot interferometer is theoretically analyzed,and the relationship between the relative light intensity of the reflected light and transmitted light in the FP cavity and the optical phase,the interference free spectral range(FSR)and the FP cavity length is analyzed by simulation.2.A fiber-optic Fabry-Perot interferometric(FPI)microfluidic static pressure sensor with an extended air cavity is presented.The structure is made of single-mode fiber and silica capillary with hydrophobic membrane,and the FP cavity is composed of a fixed fiber end face and a floating liquid surface,including the Sensing Air Cavity(SAC)and Extended Air Cavity(EAC).The variety of pressure will cause the cavity length of the FP to change because the liquid surface to drift.The free spectral width of the interference spectrum will decrease with the increase of the external pressure,and the interference fringes will show the red-shift phenomenon.Therefore,the pressure can be measured by detecting the free spectral width of the interference spectrum and the shift of the FPI spectrum.The dynamic curvature of the liquid surface in the cavity is analyzed,and the influence of gravity on the shape of the liquid surface is discussed.The sensor with EAC showed higher cavity length sensitivity.By analyzing the dynamic curvature of the liquid surface in the cavity get the influence of gravity to determine the specific usage attitude of the sensor.3.Based on theoretical analysis and simulation processing,the relationship between instantaneous sensing air cavity length L_xand pressure P under constant temperature,and the cavity length sensitivity expression of FPI is deduced.At the same time,the expressions of free spectral width in FPI and wavelength drift sensitivity with extended air cavity of the sensor are obtained.The condition of constant external temperature and different external pressure,the experiment analyzes the influence of the free spectrum width and the interference fringe drift in the interference spectrum by changing the length of the air cavity.The experimental results show that the extended air cavity will show higher cavity length sensitivity under the same external pressure change.When the lengths of the sensing air cavity and extended air cavity are about 500μm,3003μm,the cavity length sensitivity for pressure can be as high as 32.4μm/k Pa,and the wavelength drift sensitivity is 138.9 nm/k Pa.4.The experiment explores the influence of the external ambient temperature for the sensitivity of the sensor,and obtains the temperature cross-sensitivity of the sensor.In the temperature range from 293.15 K to 303.15 K,the temperature-induced cavity length sensitivity is 10.7μm/K and the temperature cross-sensitivity is 0.33 k Pa/K at a constant pressure of 10 k Pa.Finally,the measurement error and repeatability performance of the sensor samples are tested,the maximum fluctuation is 2 nm in the quantile range of25%-75%,and the maximum measurement error of the sensor samples is 18 Pa.5.Based Graded-Index Fiber(GIF)of a quarter-cycle length was designed as a fiber collimator to optimize the fiber optic fluid pressure sensor with an extended air cavity.By welding SMF to GIF,the air cavity between the GIF end face and the gas-liquid surface is used as a new FP sensing cavity.In the optimization process,the introduction of GIF increases the fringe contrast by 6.5 d B.Using GIF as a collimator can enhance fringe visibility without sacrificing the structural stability of the sensor and without increasing the sensor size,thereby extending the dynamic range of FPI.In this paper studied the FPI fluid pressure sensor is characterized by simple structure,convenient preparation and high sensitivity for static pressure measurement.It can be reasonably prepared according to the measurement accuracy and range,and has certain application prospects.