High Sensitivity And Wide Range Optical Fiber Sensing Based On Vernier Effect

Optical fiber Fabry Perot(F-P)sensors are widely used in aerospace,mechanical manufacturing,medical imaging,biological sensing,structural health monitoring,petrochemical industry,environmental detection and other fields because of their unique characteristics of easy reuse,high temperature resistance,high sensitivity,fast response,low manufacturing cost,small size,high precision,simultaneous measurement and anti-electromagnetic interference.Based on Vernier effect theory,aiming at the bottleneck of sensitivity amplification and large-scale measurement,in this dissertation,we innovate the matching condition theory,and design and verify the strain and liquid refractive index optical fiber sensor.In this dissertation,the sensing mechanism of single F-P cavity type optical fiber sensor is re summarized and deduced based on the basic flat plate interference principle.According to the development of Vernier effect,the principle of sensitivity amplification is deduced.The effect of flexible transformation of matching conditions on Vernier is studied,and a theoretical model of sensitive amplification and large range measurement is established,which promotes the development and application of interferometric optical fiber sensors based on this theory.Aiming at solving the bottleneck of sensitivity amplification of Vernier effect strain sensor,and the problems of mixed cavity interference and low envelope contrast existing in cascaded F-P cavity.In this dissertation,an optical fiber strain sensor with tunable sensitivity amplification is proposed.The two F-P cavities of the sensor are all optical fiber open cavities based on the air cavity formed by dislocation fusion.The single-mode fiber core at both ends is completely exposed to the air by large amount of dislocation fusion.Two flat optical fiber end faces form two reflective surfaces of F-P.In this dissertation,the Vernier effect can be controlled by adjusting the matching F-P cavity with glycerin solution.The matching error can be eliminated,and the ultra-high sensitivity amplification can be realized.The strain sensitivity is 2.55 nm/??.Compared with the single dislocation fusion F-P cavity strain sensor,the sensitivity is increased by 57.95times.At present,the sensitivity of strain sensing using Vernier effect is the highest.Moreover,the air cavity structure formed by fusion splitter is beneficial to reduce the influence of temperature and air pressure,which further widens the application scenarios of the sensor.In order to solve the problem of failure of traditional Vernier effect measurement in a wide range of physical quantities.An all fiber structure liquid refractive index sensor is proposed in this paper.The sensor is composed of two F-P cavities,the sensing cavity is the air F-P cavity formed by fusion splitter,and the matching cavity is the F-P cavity adjusted by the fusion splitter.In this dissertation,the traditional Vernier effect sensor with fixed matching condition is experimentally demonstrated,which has the defect of extremely small sensitivity when measuring physical quantities with large range of variation.The detection sensitivity of alcohol and two refractive index matching solutions decreased from 2.53×10~4 nm/RIU to 5.45×10~3 nm/RIU.Then the sensor proposed in this dissertation is tested experimentally.Through the adjustment of the fusion splicer,the liquid refractive index sensitivity of sample 2(refractive index value 1.55?1.559)and sample 3(refractive index value 1.65?1.659)are corrected to 2.34×10~4 nm/RIU and2.43×10~4 nm/RIU respectively.The splicer continues to adjust,and the sensors achieve high sensitivity of 3.27×10~4 nm/RIU and 3.33×10~4 nm/RIU respectively.