| Refractive index is an important parameter that reflects the characteristics of material optical properties,purity and concentration.It also is an indispensable index in industrial production process control,and has a very wide range of applications in the chemical,pharmaceutical,food and other fields.For example,by measuring sucrose solution's refractive index,the size of its concentration and ensure the safety and quality of food can be determined,so the refractive index detection is very significant.With the continuous development and perfection of the optical fiber technology,Fiber grating has become the most valuable passive optical components in the field of optical fiber sensing at present.Due to its sensitive to environments,simple-manufacture and low-cost,Long-period fiber grating has received many researchers' attention of the domestic and foreign countries.In order to improve the sensitivity to the surrounding refractive index,most researchers chose to remove the optical fiber cladding or coating,the fiber become fragile and deterioration on mechanical properties,in addition,to measure refractive index by measuring the wavelength shift or the variation of transmission energy makes the cost of demodulation higher and easily influenced by the light source fluctuation.To reduce the impact of factors above,combining Long-period fiber grating with Fiber Loop Ring-down,which has many excellent properties in the sensing field,such as strong anti-interference capacity,immunity from light source disturbance and fitness of long-distance transmission.In this dissertation,refractive index sensing based on the method of combining Long-period fiber grating with Fiber Loop Ring-down is proposed.The main research contents are as follows:(1)The basic principle of Fiber Loop Ring-down spectroscopy technology was mainly researched and the main factors which influence the accuracy of the system on fiber loop ring were analyzed,the parameters of the device components were determined by the theoretical basis in the Fiber Loop Ring-down spectroscopy system.(2)On the basis of the coupled mode theory,the transmission spectrum of Long-period fiber grating has been simulated by MATLAB,through the simulation analysis.The influence of grating period,grating length,refractive index modulation depth on the transmission spectrum of long period fiber grating has been obtained.According to the results of theory simulation,the production parameters of the sensing element LPFG can be selected.(3)A refractive index sensing system of the Long-period fiber grating based on Fiber Loop Ring-down has been designed.The parameters of the Long-period fiber grating has been obtained by simulation,the resonant wavelength is 1581nm,the transmission depth is-30dB,The real parameter is that the resonant wavelength is 1581.5nm,the 3dB bandwidth is 1.33nm,the transmission depth is-27dB,and the refractive index of a Long-period fiber grating was tested.The parameter of the fiber loop ring-down system has been selected by the theoretical analysis and the experimental,the splitting ratio of coupler is 10:90;fiber loop length is 200m;pulse frequency is 50 kHz and duty ratio of 1%.(4)The refractive index detection system has been set up,and the experimental data of sodium chloride solution with different refractive index has been realized.The experimental data is processed by Bessel filter,and the sensitivity of the system is 2404ns/RIU,and the refractive index resolution is 3 × 10-3RIU.In this thesis,the refractive index of sodium chloride solution has been measured by theoretical and experimental study of the Long-period fiber grating based on cavity ring down.However,Long-period fiber grating parameters is dependent on the spectra of the light source,which limits the sensitivity of the system.In addition,there is no effective solution to reduce the internal loss of the system,and the measurement result will deviate from the true value,so reducing the internal loss are also the problems which need to be further studied. |
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