Design, Fabrication And Characteristics Study Of Multi-rectangular Cores Fiber

Integrated interferometers have been widely applied in sensing fields because of high sensitivity,application flexibility and wide measurement range.Multicore fibers play important rules in integrated interferometers,which have been widely researched in environmental sensing,curvature sensing.Multicore fiber integrated interferometers have many advantages such as small size and application flexibility.However,interferometric fiber-optic sensors commonly consist of conventional low-birefringence single mode optical fiber(SMF).Consequently they typically exhibit signal fading,the performance of them is influenced.In order to reduce signal fading,we proposed a multi rectangular cores fiber,due to the asymmetry of rectangular cores and cladding,high birefringence is expected.This paper mainly from the following aspects studied.(1)We proposed the multi rectangular cores fiber due to signal fading in integrated interferometers and made a brief introduction about the theory of step-index fibers.(2)Physical design of multi rectangular cores fiber.The solid multi rectangular cores fiber consists three rectangular cores which are doped by Ge,and pure silica cladding.Rectangular cores are on the circle.The hollow multi rectangular cores fiber consists of three parts,including central air hole,Ge-doped rectangular cores and pure silica cladding.(3)Full-vector finite element method based on COMSOL Multiphysics was used to invest the characteristics of multi rectangular cores fiber,including mode characteristic,mode birefringence,evanescent field characteristic and bending characteristic of hollow single-rectangular core fiber.Mode birefringence was studied in detail,including the effects of geometric parameters,material parameters and stress rod.(4)The fabrication method of multi rectangular cores fiber and its preparing steps,including fabricating technology of Ge-doped rods by modified-chemical-vapor deposition process and etching method. Stack-method was used to organize the perform and fiber drawing technology was also introduced.(5)Affecting factors and adjusting methods of hollow multi rectangular cores fiber.The size of rectangular cores,Ge doping concentration and the minimum distance between cores and air hole,all of these can be used to tune mode birefringence.When the central air hole is located with a stress rod,a larger inner stress will be produced,and mode birefringence will increase.More light power will escape from core into central hole region while different refractive index materials were injected into central air hole.Single mode can be obtained with single-rectangular core,if the fiber is bent with a proper radius.The investigation shows that the proposed fiber can achieve high birefringence,the fiber is expected to have some potential applications in in-fiber integrated devices to reduce polarization-induced fading.When the central air hole was filled with different refractive index material,anomalous dispersion value appears.The dispersion characteristics are sensitive to refractive index of material in air hole,so we expect that it can be further applied to chemical sensor to detect refractive index changes inherent to molecular absorption.Single rectangular core hollow fiber can achieve single polarization by choosing a appropriate bending radius.