| Information on refractive index is an important reference for sugar industry,drug screening,medical research and other fields.In the traditional chemical reagents and biological methods,the sample processing is a complicated problem.There are too many unpredictable or controlled mutagenesis factors,and the prism sensor is large;The fiber sensor is favored because of its advantages of small diameter and light mass,easy coupling and reuse of spectrum,and suitable for all kinds of hard and bad environment.Grating sensor has attracted the attention of many scholars for its simultaneous measurement of multi parameters,by means of cascading,array layout and distributed design.Based on the development of refractive index sensors,a reflective fiber optic refractive index sensing system is builded in this thesis.The tilted fiber Bragg grating(TFBG)produces a sensitive cladding mode to the refractive index of the environment because of out of the vertical between its grating plane and the optical fiber axis.The double conical structure can transfer the part of the back to the core to propagate in the core,and the paper combines the advantages of double conical and TFBG to expand the work of this subject.With the variety of working environment in the field,the sensing system not only has to realize the basic performance of anti electromagnetic interference/corrosion,micromation,and no damage to the structure and activity of the object to be measured,and it is also a problem that the research institute should pay attention to in real time on-line monitoring,on-site application,no marking,heavy coupling reuse,quick detection and so on.This paper is devoted to the study of the structure of reflective double conical-TFBG fiber sensor.Through the process of numerical simulation,fabrication,experimental analysis and feedback modification,the feasibility of the design is verified gradually.The main contents of this paper are as follows:1.By summarizing the domestic and foreign development status of the measurement of the refractive index of solution and the monitoring of the growth density of microorganism,the research background of this subject is expounded.A reflective double conical-TFBG optical fiber sensing system is proposed,and the coupled mode equation and transmission theory involved in the reflective double conical-TFBG structure are analyzed.The performance of the moving splice TFBG and the double cone reflector structure is compared,and the refractive index sensing processes and characteristics of the two systems are described.2.Using COMSOL to achieve simulation of double conical-TFBG sensing structure,the light leakage degree of four conical structures of step type,continuous cone type,cone column type and cone cone type are analyzed.The cone column type is selected as part of the structure of the sensitive area.Comparing the cladding modes around TFBG at different gate length,the conclusion is drawn that the mode field distribution around the sensitive area varies with the external medium.And the writing process of TFBG grid phase mask method and the processing method of cone region structure are introduced.3.The reflection spectra of the conical-TFBG sensitive region of different sizes were compared,the parameter of the conical waist diameter of 45um and gate length of 20mm was selected for the follow-up experiment,and the test experiment system and the strain monitoring platform were built.The test of the refractive index of Sodium Chloride Solution was carried out,and the reflection spectrum was shifted to the large wavelength direction with the increase of the refractive index.The result shows that the sensitivity is 7.4188nm/RIU and the linear fitting coefficient reaches 0.9906.The performance and feasibility of the sensor structure are verified by the measurement of the refractive index of solution.The test experiment of ethanol water solution has been carried out.With the volatilization of ethanol,the refractive index of the solution decreases and the wavelength of the reflectance spectrum is blue shift.The monitoring experiment of the growth state of E.coliTG1 strain was carried out to observe the change of the reflectance spectrum in the whole process,and the experimental results were good,thus the measurement of the bacterial concentration change was realized by the reflective double conical-TFBG sensing system.At the end of this paper,the completion of the current topic is summarized,and the deficiencies in the progress of the research are put forward,and the suggestions for the direction of further optimizing the sensor structure are provided. |
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