Optimization And Design Of Chirped Fiber Gratings And Development Of Simulating Software For Fiber Gratings

Since 1990 the fiber grating written by ultraviolet light has developed into a criticalcomponent for many applications in fiber-optic communication and sensor systems. Thetechnologies for fabricating fiber grating have a fast development in recent years.Advantages of fiber gratings over competing technologies include small cubage, low cost,good compatibility to fiber system, low insertion loss, simple fabrication and so on. Fibergratings can be used in making fiber laser, semiconductor laser and ring laser etc, whichhave the advantages of wavelength accuracy and power accuracy. Fiber gratings can beused in fiber sensor systems as temperature sensors, strain sensors, pressure sensors,refractive index sensors and so on. Fiber gratings can also be used in fibercommunication system such as in filtering, in dispersion compensation, in gain flatteningof erbium-doped fiber amplifiers and so on.In recent years, plenty of communication business continuously emerges which rapidincrease of communication capacity. It is necessary to establish large-capacity andhigh-speed optical fiber communication system.In the future the object of fiber communication is to establish large capacity andhigh-speed system and the object of fiber sensor is to establish smart material, smartstructure and smart skin (3S) sensor system. For the above reasons, fiber gratings withbetter performance are demanded. The fiber grating's structure have a lot of kinds, eachkind has its own characteristics. So, for the fiber gratings designer, it is difficult toforecast the characteristics of a fiber grating with the arbitrary structure. On the basis oftheoretic model of fiber gratings, optimizing the parameters of fiber grating and analyzingthe performance of fiber grating is a significative work. This work can deduce a lot ofmanpower and resources.The mainly work of this thesis are focused on character analysis and optimizeddesign of fiber gratings and the development of simulation software for fiber gratings.There are mainly four factions in the thesis and the details are as followed:Firstly, we summarize the applications of fiber gratings in the fields of fibercommunication system and sensor systems. We introduce the development of UV writtenfiber gratings technology and the fabrication methods which are usually used now. Wededuced wave-guide modes of step-index fiber by use of optical fiber wave-guide theory.We deduced strictly couple mode theory of fiber gratings using weak wave-guideapproximation and introduce the transfer matrix method of fiber gratings. By using thecouple mode functions and transfer matrix method, we analyzed the characteristics offiber gratings when the grating parameters, such as index change, grating period, gratinglength, apodization function etc., have different values.Secondly, we studied the properties of the chirped fiber gratings. The linearlychirped fiber Bragg gratings are usually used as dispersion compensator in fibercommunication system. But the edges of reflection spectrum of linearly chirped fiberBragg grating are not steep. The reason is that the Bragg wave length of fiber gratings iflinearly with the grating period. For the linearly chirped fiber Bragg grating, themaximum and minimal values of grating period locate in the jumping-off point andend-point in all grating length. In these two points the coupled coefficient is lowest, so thecouple effect in edge wavelength is weakly compared with that in center wavelength. Thefinal result is that the edges of reflection spectrum of linearly chirped fiber Bragg gratingare not steep. We introduced a new grating structure to improve their performance. In thenew grating structure the fiber claddings are etched as hyperbolic function along thegrating length. The fiber grating exhibits a flattop spectrum with steep edges and highreflectivity and smoothly delay curve when tension is applied to it.Thirdly, we study the theory model of long period fiber gratings (LPGs). There is arefractive index modulation for LPGs. Based on the perturbation theory, the refractiveindex modulation can be considered as a perturbation of the fiber core refractive index. Inthe initially theoretical model of the grating, the perturbation can be regarded as asinusoidal function. For the LPGs fabricated by the amplitude mask method, theperturbation is a rectangular index modulation (RIM). Some errors would appear if wecompute the spectrum property of LPG with RIM by the sinusoidal model. Therefore, aLPGs model with RIM was proposed for avoiding the mistakes caused by the initiallymodel. In this paper we compared these two models of LPGs and indicated that the RIMmodel include the sinusoidal index modulation. Based on the RIM model, a novel modelof LPG is presented, which has a trapezoid index modulation (TIM). The advantage ofthe LPG with a TIM is that it can deduce the refractive index change. When the ofTIM is equal to zero, the LPG with TIM is transformed to the LPG with RIM. In this case,the LPG with TIM model can be used to simulate the LPG with RIM.dFinally, on the basis of the work above, we have developed a set of simulationsoftware, which can optimize parameters and analyze characteristics of fiber gratings.The simulation precision and function can compare beauty with the software developedby some other corporations. We give some recommendation of some modules of thesoftware by using a simple uniform Bragg grating. We also introduce the other functionsof this software, such as strain analysis, chirped fiber Bragg gratings analysis andapodization fiber gratings analysis etc. This software can be used as a tool of optimizedand pre-design for fiber gratings.The fiber gratings are mainly written in step-index fiber previously and so thetheoretic analysis of fiber grating is focused on this type fiber. In recent years, some newtype fibers have been fabricated, such as liquid crystal fiber, polymer fiber and photoniccrystal fibers. The fiber gratings written on these new type fibers are also reported. In thefuture, we will study the characteristic of the fiber gratings based on these new type fiberand improve the functions of our simulation software for fiber gratings.