Research On Cascaded Optical Fiber Micro-cavity Sensor Based On Vernier Effect

In recent decades,optical fiber sensing technology has drawn more and more attention owing to its many advantages such as small size,high precision and easy integration.Compared with traditional sensor,the optical fiber F-P sensor is a new kind of sensor with the advantages of small size,simple structure,high sensitivity,strong ability of resisting electromagnetic interference and high resolution,and is widely used to measure the external environment parameters such as temperature,humidity,refractive index and strain.The Vernier effect is derived from Vernier calipers,which can improve the measurement sensitivity of length by using the small scale difference between the main ruler and the Vernier.In recent years,the Vernier effect has been used by researchers to increase the measurement sensitivity of optical fiber sensors.In this thesis,the two separated F-P cavities in single-mode fiber are designed and fabricated to realize the Vernier effect by cascading them.The two cascaded F-P sensor is applied to the measurement of temperature and refractive index.The main research contents are as follows:Research on the temperature sensor based on two cascaded fiber cavities and Vernier effect.One of the two cascaded F-P cavities is air cavity,and the other is filled with the water.To make ensure that Vernier effect can be realized,the air cavity,which is used as the reference F-P cavity,has the length of 270?m with free spectral ranges of 4.4nm,and the water cavity,which is for sensing cavity,has the length of 220?m with free spectral ranges of 4.1nm.This sensor possesses ultra-high sensitivity to temperature,since the water has much higher thermo-optic coefficient than the air.The experimental results show that the wavelength shift of the spectral envelope of the two cascaded cavities changes linearly with the temperature,and the sensitivity is-2.080nm/ when the temperature changes in the range of ? 40?~60?,which is about 10.9 times as high as that of the single water cavity.The experimental result is 0.08nm/ ? higher than the simulation result which is-2.00nm/?.The difference between the experiment and simulation is mainly caused by the fitting error of the peak position of the spectral envelope.Research on the refractive index sensor based on two cascaded fiber cavities and Vernier effect.One of the two cascaded F-P cavities is open cavity,and the other is air cavity.To make ensure that Vernier effect can be realized,the air cavity,which is used as the reference F-P cavity,has the length of 284?m with free spectral ranges of 4.23 nm,and the open cavity,which is for sensing cavity,has the length of 201?m with free spectral ranges of 4.48 nm.This sensor possesses ultra-high sensitivity to refractive index,since the thermal coefficient of the measured liquid is much larger than quartz.In experiment,we first prepared sucrose solution with different refractive index and filled it into the open fiber F-P cavity sequentially to obtain the double cavity cascade interference spectra of different refractive index sucrose solutions,The experimental results show that the translational amount of the envelope of the two-cavity interference spectrum changes linearly with the refractive index,and the sensitivity of the refractive index sensing is-23904.0nm/RIU when the concentrations of sucrose solution changes in the range of 10%~27%,which is about 22.7 times that of the single sensing F-P cavity.The difference between the experiment and simulation is mainly,and the simulation result is-27797.0nm/RIU.