Research On Thermal Diffusion Methodology For Fiber Integrated Optical Devices

Optical fiber devices have always been a research hot topic in fiber-optic communication technology and fiber-optic sensing technology.A fiber-integrated optical device that integrates multiple optical paths or multiple functional units into one optical fiber focuses on fiber integration,has the advantages of compact structure,versatility,and ease of connection to existing fiber optic networks.And it plays an irreplaceable role in the construction of all-optical networks.Optical fiber thermal difffusion technology is one of the important methods to realize fiber-integrated optical devices.Its wide applicability,easy operation,maintain fiber size and good mechanical strength are beneficial to the development of novel fiber-integrated optical devices with complex functions.The research on thermal diffusion of optical fiber is usually limited to a lower degree,and lack of more in-depth research.The refractive index changes induced by thermal diffusion in the axial and radial degrees of freedom of the optical fiber can build a complex and smooth transition three-dimensional refractive index structure in the optical fiber,thus realizing the functions of optical field transformation,mode field coupling,optical path interaction and mode field adaptation.In this thesis,the application potential of fiber thermal diffusion technology in the field of fiber-integrated optical devices is discussed.The mode field adapter,fiber microlens and multi-core fiber beam splitter based on fiber thermal diffusion technology are studied by means of simulation analysis.This thesis mainly includes the following research contents:1.The thermally diffused process of dopants in optical fibers under high temperature conditions is theoretically derived and verified by simulation results.The high temperature irreversible thermal diffusion gradually evolves the internal refractive index distribution of the fiber from the initial form to the Gaussian form while maintaining the outer diameter of the fiber.The thermal diffusion effect of a single fiber appears an extension of the transmission mode field.The thermal diffusion effect of a multi-segment fiber is to construct a complex,gently transitioning three-dimensional refractive index structure inside the fiber.In a gradient temperature field that is sufficiently slow,the reshaped three-dimensional refractive index transition region by thermal diffusion can achieve adiabatic transformation of the transmission mode field in the fiber.The thermally diffused evolution of the refractive index distribution of the fiber is demonstrated by taking a variety of optical fibers as an example.2.Using the fiber thermally diffused technology,a mode field adaptation scheme of the sandwich structure is proposed,which realizes the ultra-high efficiency fundamental mode field adaptation between various specialty fibers and conventional single mode fibers.The double-clad fiber possess specific parameters is used as the bridge fiber between the special mode field fiber and the single mode fiber.On the one hand,the fundamental mode field of the double-clad fiber and the single-mode fiber is almost identical.On the other hand,after a proper thermal diffusion,the double-clad bridge fiber is matched to the refractive index profile of the special mode field fiber to be adapted.The thermal diffusion of dopant builds a three-dimensional refractive index transition region in the fiber.The thermal diffusion region is centered on the fusion point of special mode field fiber and double clad fiber.The fundamental mode field of the special mode field fiber is adiabatically transformed into that of the double clad fiber(or the single mode fiber)through the transition region.3.A scheme for the fabrication of microlens,microlens group,and microlens array using simple step index fiber is proposed.Step index multimode fiber can be equivalent to gradient index fiber after moderate thermal diffusion.On this basis,positive radial gradient thermal diffusion fiber microlens is prepared,which is equivalent to self focusing fiber microlens.Another kind of thermal diffusive fiber microlens with anti radial gradient index distribution can be used as fiber microconcave lens.Thermally diffused fiber microlens provides more solutions for the fabrication of fiber integrated optical devices,especially all fiber devices4.A graded-index dual-lens system is constructed,which coaxially separates the coaxially positioned and closely spaced core channels of multi-core fibers,and couples them into individual multiple single-mode fibers respectively,achieving low loss and low crosstalk multi-core fiber optical splitter.The efficient beam splitting interconnection between the dual-core fiber and the single-mode fiber was verified by the beam propagation method and the ray matrix model,respectively.Fine-tuning the lens gap can compensate for the mismatch of optical component parameters,and coaxial assembly contributes to the preparation of a compact and easy-to-operate multi-core fiber beam splitter.