| In the past decades of the development of fiber grating which has attracted attention of many researchers,because of the advantages of small size,light weight,and anti-electromagnetic.In recent years,fiber grating has developed rapidly in many sensing fields including physics,biology,and chemistry.The refractive index?RI?sensor is one of the most important sensor and one of the most widely used sensor.In order to improve the sensing performance of RI sensor,some new RI sensing characteristics and sensor structures are proposed in this thesis.These new characteristics and the proposed sensor structure make the sensing performance of the RI sensor optimized,especially in terms of sensitivity,and at the same time provide some new directions for the fiber grating RI sensor design.With these new characteristics and sensor structures,RI sensors with excellent sensing properties can be designed.These RI sensors can be used for high-sensitivity detection in many fields.The first chapter of the thesis introduces the development history of fiber grating,the theoretical basis of fiber grating,the development status of fiber grating RI sensor and the background.The coupled mode theory of LPFG and FBG is introduced.We analyzes the difference of the two kinds of fiber grating and give the coupled mode equations.The analysis of the current research status of RI sensor shows that the new RI sensing characteristics and sensor structures proposed in this thesis have better sensing performance than the traditional RI sensor.On the one hand,the sensitivity of the RI sensor is improved,and on the other hand,the sensor structure is simplified to some extent.At the end of the chapter,we summarizes the content and structure of the full thesis.The second chapter of the thesis proposes a method of combining cascaded LPFG and PMTP to reduce the bandwidth of the loss peak and improve the sensitivity of LPFG sensor.This sensor can be designed for intensity modulated and wavelength modulated,which has a strong application flexibility.In general,an LPFG operating at PMTP has the highest RI sensitivity to surrounding,but the bandwidth of the loss peak is very large at the same time,which is not conducive to sensing,and it is one of the difficulties that need to be overcome in the practical application of PMTP.Therefore,we cascade two LPFGs operating at PMTP,which not only reduces the bandwidth of the loss peak,but also improves the resolution of LPFG to 1.97×10-9,which is increased by nearly three orders of magnitude.In the third chapter,a novel wavelength-modulated LPFG liquid level sensor based on superimposed dual-peak?SDP?is proposed.The liquid level sensor overcomes the shortcoming that the traditional LPFG liquid level sensor cannot be designed into wavelength-modulated sensor.The essential principle of this liquid level sensor is the dual-peak resonance.By designing the period and length of the LPFG,the two groups of dual-peak are superimposed to form a SDP.The experimental results show that the sensing characteristics of the liquid level sensor are in good agreement with the theoretical results.The experimental data show that the resolution of the liquid level sensor is available to 10-3 mm,which is two orders of magnitude higher than the resolution of some reported liquid level sensors.The fourth chapter of the thesis re-investigates the phenomenon of“mode transition”by analyzing the definition of the effective refractive index?ERI?of cladding modes,and further studies the sensing characteristics of“mode transition”.We modify some of the briefs that lack theoretical support,and rename the“mode transition region”to“mode barrier region”.Through in-depth analysis,two new characteristics of the mode barrier region are proposed,one is the dual-peak resonance and the other one is high sensitivity of coupling coefficient of cladding mode.These two characteristics are meaningful for the sensitivity improvement of the LPFG sensor.In addition,the mode barrier region is also one of the method to design high-sensitivity fiber grating sensor,and providing theoretical guidance for the design of coated fiber grating RI sensors.In the fifth chapter,a transmissive FBG RI sensor is designed by combining the mode barrier region and the thin cladding FBG.The sensor has the advantages of mode barrier and thin cladding FBG,so the sensitivity is further improved.By studying the relationship between the optimal overlay thickness?OOT?of the overlay and the cladding radius,the relationship between the two parameters is obtained,which plays an important guiding role in the practice.The sixth chapter summarizes the work in the thesis,summarizes some key conclusions,points out shortcomings of the thesis and looks forward to the future development direction of fiber grating RI sensor. |
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