Study On Mode Filtering And Transmission Characteristics Of Band GAP Optical Waveguide Devices

In recent years the application of few mode fibers has aroused great interest among researchers.Due to the nonlinearity of the optical fiber and the bandwidth of the amplifier,the communication capacity of the single mode fiber gradually approaches to its limit.In mode division multiplexing?MDM?system,each mode in few mode fibers acts as a single information transmission channel,as a result,the communication capacity of few mode fiber can be increased effectively.The use of selective mode incentives allows few mode optical fiber to operate in a single mode.Compared with single mode fiber,the mode field area of a few mode fiber is larger,which can effectively reduce the nonlinear effect and achieve long-distance transmission of communication.The way to manipulate the mode in a few mode fiber becomes one of the key technologies for the application of few mode fibers.This paper focuses on several new types of few mode fiber devices and analyzes their transmission performance.Mode converters and multiplexer/demultiplexers are key components of the MDM system,one of its key technical indicators is crosstalk.Reducing device mode crosstalk is one of the important measures to improve its performance.Optical filters can purify the mode output from the mode converter or the mode-division multiplexer,so that the crosstalk of other modes can be reduced and the signal-to-noise ratio of the system can be improved.Similar to filters in wavelength division multiplexing,filters that enable selective filtering modes in few mode fibers can also have important applications in mode multiplexing systems.This paper proposes a solution to filter fiber.The surface plasmon polariton?SPP?mode in the fiber has a strong loss,according to this feature,this paper introduces the use of gold filaments in the fiber to utilize its high-loss surface plasmon excitation mode to enhance the loss of the specific mode in the fiber,and then achieves the role of the filter mode The loss characteristics of the modes in the four-mode fiber and the dual-mode fiber were analyzed by using the finite element method.By optimizing the fiber parameters and structure,high-loss filtering of specific modes in the fiber and low-loss transmission of other modes were achieved.and the length of fiber was only centimeters in size.In order to achieve a specific mode of filtering,this paper also proposeed to introduce the bandgap structure theory to the mode waveguide.By arranging a microstructure defect in the waveguide cladding region,the fundamental mode of the mode waveguide is coupled to the defect mode.After this,introducing a high refractive index dielectric region to destroy the total internal reflection transmission mechanism of the defect mode,which had a strong leakage characteristic,so that the fundamental mode of the waveguide generated a strong leakage loss.In addition,the high-order mode and the defect mode did not couple to each other,and low loss transmission was maintained.The large mode-area optical fiber has a large mode field area,which can reduce the nonlinear effect of the optical fiber.Using such an optical fiber can effectively reduce the thermal damage of the high power fiber laser.This kind of optical fiber should usually work in the single-mode state,and should also allow a certain degree of bending.For this reason,a large mode field bandgap fiber is designed in this paper.This fiber can transmit two modes without bending.When the fiber is bent,its cladding mode generates strong leakage,the core LP₁₁ mode and the cladding mode are filtered out because of the coupling of the refractive index matching conditions.Simultaneously,based on the band gap transmission principle,the fundamental mode of the optical fiber is confined to the core region,thereby the equivalent single mode and large mode field transmission is achieved.