Research On Long Period Grating In Eccentric Core Fiber And Its Characteristics

Long period fiber grating has been widely used in the field of optical fiber sensing and optical fiber communication because of its unique grating forming principle,simple fabrication process,no backscatter,low insertion loss and so on.Compared with the traditional single-mode fiber,the eccentric core fiber has stronger evanescent wave and more sensitive to the changes of the external environment.The long period grating written in eccentric core fiber will have the characteristics of both the long period grating and the eccentric core fiber.In this paper,long period fiber gratings are fabricated on single core-eccentric core fiber and twin core-eccentric core fiber by CO₂laser writing technology.The coupling and sensing characteristics of the long period grating in eccentric core fibers are studied theoretically and experimentally.The main research work of this paper is as follows:（1）Based on the theoretical analysis of long period fiber grating in single mode fiber,according to the coupling mode theory,the modeling and simulation of long period fiber in single core-eccentric core fiber are carried out combined with the finite element analysis and numerical method.The modes and optical field distribution are obtained,and the coupling characteristics between modes are analyzed.（2）A high frequency CO₂ laser is used to fabricated the long period grating on a single core eccentric core fiber.The influence of the writing parameters（refractive index modulation depth）and the exposure angle of the laser beam is analyzed.The experimental results show that different exposure angles（the position of the fiber core relative to the laser pulse）have a great influence on the energy and the spectrum of the grating.（3）The bending,temperature and strain sensing characteristics of the long period fiber gratings in single core-eccentric core fiber with different exposure angles were measured,and the influence of different refractive index modulation directions on the sensing characteristics of fiber gratings was studied.（4）Based on the coupling mode theory,COMSOL Multiphysics is used to model and simulate the Twin core eccentric core fiber,to get the modes and mode field distribution,and study the coupling between the fundamental core mode and cladding modes.The feasibility of realizing wavelength selective couplers by using twin core-eccentric core fiber is analyzed.（5）The long period fiber grating is written on the twin core-eccentric core fiber by CO₂ laser.The bending,temperature and stress sensing characteristics of the long-period fiber grating of the core positioned in the middle axis of the fiber are studied.（6）Compared with the theoretical simulation,the feasibility of wavelength selective couplers based on long period fiber gratings in twin core-eccentric core fiber is verified by experiment,and the influence of grating parameters（period and refractive index modulation depth）on the coupling characteristics is analyzed.At the same time,the effect of temperature and stress on the wavelength tuning of coupler is analyzed.