Studies On The Sensor Of Refractivity Of Fiber Grating

Optical fiber grating senor has many advantages, such as delicacy structure, fine measurement, strong anti-interference, wavelength multiplexing and distributive sense. Especially, fiber grating senor use wavelength to coding and sensing information, it overcomes shortcoming of strength tuning senores which must compensate loss of fiber connections and coupler, and the power fluctuate of light source. Thus, fiber sense technology represents development trend of new senor generation.Potential application of fiber grating senor has industrialized. Although its function need improved in some degree, the technique has approached to mature. In the field of aviation, fiber grating has successfully used for non-damage detection. At the same time, fiber grating senores are also used in Chemistry, industry, electrical power fields and so on.However, the research on refraction index changing of fiber grating has not fully development. In biology and chemistry application, the refraction senor has more value, since many structure and density of materials could be detected through refraction index.In the thesis, we design a new kind fiber grating senor—fiber grating F-P cavity refraction index senor. This new senor was constituted by two fiber gratings. Compared with one fiber grating, the new one has narrower resonant peak and could be used for more exact measurement. This refraction index senor is used for measure refraction index of different liquid, expressing capacity for detecting small change of refraction index.The first part Research on Fiber gratingMake an introduction about senor technology. Give a general expression about the development of fiber grating senor, the character of such senor and its application.On the base of analyzing fiber and fiber grating, do research work on different kind of fiber grating according to distribution of refraction index along the axis-direction. Then, do research work on several methods of making fiber grating in order to write clear strip on fiber core and get high quality fiber grating. At last, making a deduction of fiber Bragg grating by using coupled mode theory and analyze every parameters which could make effect on fiber Bragg gratings.The second part Analysis on fundamental theory and rectification technique of Fiber grating senorThe sensing process of fiber grating senor is that through physical parameter from outside to modulate fiber grating wavelength, it is a kind of wavelength modulation fiber grating sensor.Combined with coupled mode theory, we could get the reasons of fiber Bragg grating sensor. The Bragg wavelength fluctuate induced by temperature and stress could be written asIn the above equation, k_T and k_εare fiber Bragg grating responsive index of temperature and stress. The displacement of fiber grating wavelengthΔλis very sensitive corresponding to stress changeΔεand temperature changeΔT. Using this character, we could set fiber grating into intellect materials and structure. Changing acting physical parameter to the variable quantity of stress and temperature to sense and detect relative physics parameters.Fiber grating sensor signal modulation technique is the key point of sensor system. Fiber grating senor is absolute wavelength coding. It is the key point to make it more quickly and high finely distinguish the location of distribution fiber grating senor and displacement of measurement of Bragg wavelength. And in this thesis, we make a detail analysis on signal modulation about several fiber grating senores: spectrum measurement, tunable F-P cavity, edge filtering modulation, non-balance M-Z interference modulation and tunable laser match method.The third part Design and research on new fiber grating F-P cavityThe structure of this new fiber grating F-P cavity is shown in Fig 1. It is formed by one fiber which writes two Bragg grating with nearly same parameters. We covered fiber. Under the premise of no effect on fiber grating, selectively erode the area between two fiber grating and remove the fiber of this area, make core part of this fiber to contact with measuring materials fully.We use transfer matrix to analyze characters of this new type of fiber grating F-P cavity. And this cavity is divided into five parts, we could deduce transfer matrixes of every part and then calculate the transfer matrix of the whole F-P cavity.Make application of edge conditions, the reflective index of this cavity could be expressed asAnd, is the reflective index of single fiber Bragg grating; is the propagate constant of the area which the measure material should be put into, and is the propagate constant of non-exposure area. is the phase factor of fiber Bragg grating at wavelength ofλ.Realize the numeric analysis through transfer matrix by using Matlab program. According to given parameters, use Matlab program to realize reflection spectrums with different refractive index of alcohol by using this new type fiber grating F-P cavity sensor. Refection situation is shown in Fig 2.Compared with common F-P cavity, the resonant requirement is not only the phase difference of two close light beams, but also the phase difference which is the influence of fiber grating. So the requirement of resonance in the fiber grating F-P cavity senor is thatIn the equation(4.21),φ_qrepresents the phase difference of two close light beams in F-P cavity, and its detail expressions are that and is the center wavelength of resonance in F-P cavity.Insertφ₁ andφ_B into equation (3) and m = 0,±1,±2,...From equation(4), we could get relationship between resonant wavelengthλm of F-P cavity and refractive index n_a of measuring material.According to the relationship between resonant wavelengthλ_m of F-P cavity and refractive index n_a of measuring material of new fiber grating F-P cavity, when refractive index n_a of measuring material changes, the change of the resonant peakλ_m of center wavelength meets andPut alcohol liquids in the measurement areas for detection. Use Matlab to simulate reflection spectrum of this new F-P cavity senor. Base on the given parameter of this new senor shown in Fig 2, combined with equation (5), we could get variation curve of the changeΔλ_m of the resonant peak of center wavelength of ethanol along with the changeΔn_a of refractive index of measuring material, shown in Figure 3.Making an analysis on distinguish ability of refraction senor. Compared with fiber grating senor, this new type senor has a relative high distinguish ability for it has sensitive resonant character. Because the spectrum width is one order small than FBG, the whole quality of the F-P cavity senor is finer than traditional fiber grating senor. It realizes more accurate measurement with high distinguish capacity.Make research on different parameters, which could affect on the new fiber grating F-P cavity senor. The biggest advantage of this senor is that it has high sensitivity of refractive index. And it provides foundation for refine designing of this kind of senor.However, this new has its shortcomings. The range of measurement is very limited and the sensitivity is confined by the steady of the whole measure system.This new type F-P cavity fiber grating senor could especially be used in biology and chemistry field. It has advantages of high fine measurement of single under the premise of without damaging of cell. In chemistry field, it could make a high fine measurement about refractive index of liquid, which could provide more exact parameters for judging the element and density of the liquid.