Research Foe FP Cavity Based On Super Imposed Same Chirped Bragg Gratings

In recent years, taking the advantages of the high accuracy, the long distance of measuring, multiple-point measurement, the long lifetime, adapted to all kinds of severe environment and so on, the applications of quasi-distributional optical fiber grating sensing technique, are getting more and more widespread in the long-time and real-time health monitoring of modern projects, then as the key device of the whole sensing system, Fabri-Perot cavity filter(FP cavity filter)is one of hot spots of research at present.This article makes use of the technique of superimposing chirped Bragg gratings to form FP cavity filter with different methods, analysis its theory with the coupled mode theory, makes comparison to each method combining with the results of experiments and simulations. The major and innovation work is as follows:1. Summarized the development process and the current application situation of optical fiber as well as the optical fiber sensing technique, introduced several main kinds of FP cavity filter's structures and the characteristics.2. Elaborates the FP cavity filter's principle in detail, analyzed the mechanism structure of FP cavity filter, harmonious process as well as free spectral range; Introduced the theoretical model of fiber Bragg gratings, and using pair matched molds theory, analysis the spectral characteristic and the modulation of refractive index of superimposed chirped Bragg gratings. Proposed the FP cavity filter formed by superimposed chirped Bragg gratings based on the research of FP cavity filter and the chirped Bragg gratings.3. Compared three methods of forming FP cavity filters, method of chirped Bragg gratings series, superimposing chirped Bragg gratings with method of forcing stress, superimposing chirped Bragg gratings with method of phase mask shifting.The structure is simple in using method of series chirped Bragg gratings, but there is no obvious comb-like transmission spectrum because the chirped Bragg gratings are too long. There is obvious comb-like transmission spectrum after superimposing chirped Bragg gratings with method of forcing stress. This method had proven the feasibility of forming the FP cavity filter with superimposing chirped Bragg gratings.. With the method of method of phase mask shifting to superimpose chirped Bragg gratings, there is also comb-like transmission spectrum, and the gap between two chirped Bragg gratings' central wavelength can be controlled accurately, in addition, the free spectral range in transmission spectrum is wider obviously.4. Analyzed the characteristics of FP cavity filters by superimposing chirped Bragg gratings with method of phase mask shifting in theory, then obtained the parameters of resonance peak with nice fineness, and analyzed the influence of chirp quantity to the performance of FP cavity filters. The simulation result indicated that after superimposing chirped Bragg gratings with chirp quantity of 3nm, there is resonance peak with nice fineness, and the free spectral range could reach 1.3nm; after superimposing chirped Bragg gratings with chirp quantity of 4nm, the free spectral range could reach 1.7nm; after superimposing chirped Bragg gratings with chirp quantity of 5nm, the fineness of resonance peak also remained nice,the free spectral range could reach 2nm.It is enough for single channel demodulation of wavelength.5. Made experiments with methods of stress forcing and phase mask shifting separately, superimposed chirped Bragg gratings in the same fiber to form FP cavity filter. There are both comb-like transmission spectrums with both two methods. And the free spectral range could reach 0.6nm with the method of phase mask shifting. It indicate that it can formed FP cavity filter with the mature and cheap technique of chirped Bragg gratings superimposing by the result of the experiments.