The Whispering Gallery Mode And Sensing Characteristics Of Bending Fiber

The bending loss of optical fiber under the condition of small radius seriously affects the sensing application of optical fiber devices.The bending loss decreases exponentially with the decrease of bending radius,which makes it difficult to further miniaturization of the fiber hydrophone and its array.Due to the sharp increase of bending loss,it is always hard to improve the performance indicators of some optical fiber devices in the manufacturing process.The test results under the condition of small radius show that the bending loss of the fiber does not change monotonically with the bending radius,but in the form of increasing oscillation.It is of great significance to further reduce the diameter of fiber hydrophone and its array.And it is necessary to improve the performance of related fiber optic optical components to study the minimum loss under the small radius of bending fiber.This paper focuses on the loss of optical fiber under small radius bending.Under this condition,the cladding and coating layer of the bending fiber will have a strong convergence effect on the light which leak out of the core.Some of the leaking light returns to the core to generate a strong whispering gallery mode(WGM),which will affect the loss characteristics of the bending fiber.The existing research cannot quantitatively describe the bending loss under the small radius.So,the existing theory should to be modified.In this paper,the optical transmission characteristics of bending fiber are modeled.For the case of small radius,considering the bending deformation of the core which caused by bending,the change of refractive index profile of optical fiber and the change in the optical field distribution which caused by the symmetry broken in the bending waveguide.The traditional theory of five layers slab waveguide model is modified by model transformation.The WGM distribution and characteristics are simulated and analysis.The bending loss of the fiber is obtained under the small radius bending model.A fiber bending loss measurement system with a minimum radius is established.The results show that the theoretical model established in this paper can better approximate the test results.The theoretical analysis shows that the WGM is sensitive to the outside refractive index in the bending fiber without coating layer,which can be used as the refractive index sensing mechanism for salinity sensing.In this paper,the loss of optical fiber with different bending radius is measured,and the bending radius of single-mode half-stripping optical fiber can be used for temperature sensing and salinity sensing.Generally,the fiber ring with radius of 4.934 mm and 4.536 mm is selected for sensing the refractive index of the solution.