The Basic Research On Fiber Grating Sensing Technology

Fiber Grating is a new kind of all-fiber passive device that results from the periodicity micro perturbation of the refractive index in the optical fiber core. Because of the excellent properties such as compatible with optical fiber, easy fabrication, little insert loss, small volume and low price, fiber gratings have wide application in optical fiber communication and sensor field. The application of fiber grating is at the highly prosperity. It is a central issue of interest all along.Bragg fiber grating (FBG) can couple the forward propagating core mode to one the backward propagating core modes. It is a type of reflection gratings whose reflective wavelength depends on the product of grating's effective refractive index and grating's period, and grating's reflectivity and bandwidth depend on the change in refractive index along grating and grating's length. Then, designing grating's period and modulating the change in index can fabricate Bragg fiber grating with different reflective wavelength and reflectivity; in addition, Bragg fiber grating has especially sensitivity to the temperature and strain. It is these unique properties that make Bragg fiber gratings have sensing applications for some physical parameters.A long period fiber grating (LPFG) can couple the forward propagating core mode to one or a few of the forward propagating cladding modes, and has no backward reflection. It is a typical transmission device. Because LFPG resonance peaks are changed with different environment index, LPFG can be used as the chemical sensor to measure solution concentration or index.The work of this thesis are deeply studying the principle, fabrication, mechanism, properties and applications of sensing of both Bragg fiber gratings and long period fiber gratings. The main achievements of this thesis are shown as following:1. Research backgroundThe interrelated knowledge of the thesis are introduced and analyzed. It include the develop, research actuality , manufacture and sensing application of fiber grating, also the study of electrostatic self assembled technology.2. The fabrication of fiber gratingsThe making methods are studied, combined with the technology and equipment conditions of our laboratory, we used uv-exposure to make a series of FBG whose center wavelength range were 1546.908nm-1558.24nm and a series of LPFG whose were periods were 350, 460, 550 and 640μm by the methods of phase-mask method and amplitude-mask method respectively.3. The study on the sensing theory and sensing characteristic of FBGWe deduced the waveguide mode of single mode fiber based on the Maxwell equation, We also deduced the mode field distribution of reflective-type FBG by the couple mode theory, obtained the corresponding relation between the period and wavelength of the fiber gratings based on the weakly guiding optical fiber approximate theory. It's the theoretical basis of the analysis, design and making of the FBG. Secondly, we built the sensing model of strain and temperature respectively based on the study of the theory, the temperature, strain sensing characteristic of the FBG, which were designed by ourselves, were studied.4. Special cantilever stress sensorFirstly put forward a new method to solve the problems of cross sensitivity, studied a stress sensor based on a special cantilever structure, a simple and convenient sensitivity enhancement method was realized by using the wavelength difference of the reflection center of FBG in the different region of cantilever. By using this structure of the special cantilever, we solved the cross sensitivity of temperature and strain successfully, and the measure sensitivity was increased by a factor, the strain measurement error caused by temperature is eliminated automatically, and the single parameter measurement and dynamical adjusting measurement range of strain has been realized by using FBG.5. The study on the sensing theory and sensing characteristic of long-period fiber gratingBased on Maxwell equation, we strictly deduced mode field distribution of transmission-mode long period fiber gratings by using of couple mode theory and the theory of weak waveguide approximation, analyzed the relation between wavelength of resonance peaks and ambient refractive index changes. The experiment studied the principle of long-period fiber grating chemical sensing and refractive index sensitivity.6. Basic research on long period fiber grating chemical sensingFirstly put forward a new nitrite sensor based on long-period fiber grating, we studied three methods to enhance the sensitivity of the sensor.1) Etching the cladding of LPFGCladding diameter was decreased using etching method by Hydrofluoric acid; this method can adjust the resonant wavelength and amplitude, extend the spectral range of LPFG to some extent, and enhance the sensitivity of it. The experiments indicated that the measurement sensitivity was enhanced by 14 times when the cladding of LPFG was etched compared with the naked LPFG. The lowest measurable nitrite concentration was 3×10-4mol/L, this value was better than the National standard of nitrite, which was 4.2×10-3mol/L.2) Self-assembled multilayersBuilt the sensing theory mode of LPFG which was coated by weak absorption thin film based on the sensing theory of LPFG, and analyzed the influence of refractive index on the transmission spectrum of LPFG. Polyelectrolyte (PSS,PDDA) was deposited on the surface of LPFG used self-assembled multilayers technology, We used this method (construct bimolecular multilayer) to change the morphology of the LPFG's surface, adjust the effective refractive index of the cladding mode and improve the sensitivity of the LPFG, which was used to chemical and biological measurement. The experiment indicated that when the overlay was 70, the highest sensitivity of LPFG was obtained, the lowest measurable nitrite concentration was 3×10-4mol/L, this value was better than the National standard of nitrite, which was 4.2×10-3mol/L, and meet demand of National standard.3) Refractive index matching methodForm the sensing theory of LPFG, we know that when the ambient refractive index match to the cladding refractive index, its sensitivity of refractive index sensing is highest. Hereby, we used the sucrose solution, which concentration was 65%, as the matching liquid of coated LPFG to measure nitrite; the sensitivity was enhanced by 30% in this condition.The experiment showed that any kinds of above methods or combined method can improve the sensitivity of LPFG. Compared with the chemical methods, this method has the characteristics of high precision, fast, non-toxic, no need of chemical reagent and low costs, and provides a new method on food safety inspection. The study above laid a theoretical and experimental foundation on the study of fast and undivided measurement of biochemical substances, such as protein, enzymes and DNA and so on, the study will provide theoretical and experimental basis to the fabrication and development of high sensitivity and multi-parametric biochemical sensor.