Analysis Of Chirped Fiber Bragg Grating Reflection Characteristics And Experimental Study Of Grating Tuning Technology Based On Compression Bar

As a new kind of optical device, the fiber grating is a fiber structure that the refractive index of the core is modulated periodically. Based on the photosensitivity of Ge-doped fiber, fiber gratings can be fabricated by successive laser exposure of single mode fiber to UV light beam, excimer laser, femtosecond laser, CO2 laser. Also, they can be inscribed by ion implantation technique, arc discharge method, HF etching technique. Because of the excellent characteristics, such as good compatibility to fiber, fine reflectivity performance, low insertion loss, small volume, low cost and high sensitivity to the changes of environmental temperature, stress and strain, fiber gratings have been and will be used in fiber communication and fiber sensing domain extensively. In actual applications, we always need to design proper fiber grating devices to meet the special demands of filters, which is related to fiber grating synthesis algorithm.The thesis pays more attention to the characteristics of chirped fiber Bragg gratings. Apodization technique, parameters reconstruction algorithm and grating tuning technique of chirped fiber grating are theoretically and experimentally studied.The main contents are as follows:1. The development history of fiber gratings, photosensitivity of fiber materials and the sorts of grating are briefly introduced. And write techniques of fiber grating are summarized in chapter 1.2. Common analysis theories of fiber gratings is introduced. Based on index modulation model, the coupled mode equations are derived by eigenmode expansion of light field. By the given boundary conditions, the reflectivity formula of uniform fiber Bragg grating is obtained. The influences of grating length and index modulation depth on uniform Bragg grating are analyzed. Reflection characteristics of chirped fiber Bragg grating are described by transfer matrix method which divides the grating into many uniform gratings. The influences of grating length, chirp parameter and index modulation depth on chirped fiber grating are demonstrated in detail in chapter 2. 3. Symmetric apodization of fiber grating can seriously reduce the bandwidth of reflection spectrum. In chapter 3, we show the advantages of asymmetric apodization of fiber grating in broadening reflection bandwidth and reducing time delay ripples. We take chirped fiber Bragg grating as an example to discuss the changes of reflection bandwidth and time delay ripples when the two ends of grating have different apodization ratios. A reasonable apodization ratio is presented after sufficient simulations. To sum up, we should take a compromise consideration between reflection bandwidth and time delay ripples in apodization design process. 30% apodization on long wavelength end and 20% apodization on short wavelength end is a proper scheme when design a CFBG for dispersion compensation.4. Theories of fiber grating parameters reconstruction are introduced. Especially, basic principles and computation steps of discrete layer peeling algorithm are demonstrated. We get the relationship of coupling coefficients and fiber index distribution by taking an effective approximation of coupled mode equation and transfer matrix. Finally, we apply this algorithm to synthesizing three kinds of special fiber grating filters, dispersionless FBG filter, CFBG filter for dispersion compensation and triangular spectrum FBG. Results show its effectiveness in synthesizing process.5. A new grating tuning mechanism is presented. Based on elastic compression bar, the mechanism can realize both wavelength tuning and bandwidth tuning. By attaching the fiber grating to the middle of the compression bar, the center wavelength tunes 35.43nm bidirectionally. When attached it to the quarter part of bar, 12.6nm bandwidth tuning is realized.In the end, we summarize the main work of the thesis and predict the direction for further study.