Research On Mode Tailoring Devices Based On Few-mode Fiber Grating

Single-mode optical fiber is widely used in communication,sensing and other fields.However,in recent years,single-mode optical fiber has some limitations in transmission capacity and sensing performance.Few-mode fiber can overcome the limitations of single-mode fiber in terms of transmission capacity and sensing performance,which has aroused widespread interest.Using different modes of the few-mode fiber as different information transmission channels,that is,mode division multiplexing technology,the communication capacity of the fiber can be multiplied.In the field of sensing,the higher-order mode of the few-mode fiber can participate in the sensing,which shows different characteristics from the single-mode fiber in the sensing mechanism and performance.This paper focuses on the grating devices based on few-mode optical fibers.In the mode multiplexer/demultiplexer and mode converter,the inter mode crosstalk is one of the important performance indexes.One of the ways to reduce the inter mode crosstalk is to use the filter.Just like the Wavelength-division multiplexing?WDM?systems,in the Mode-division multiplexing?MDM?systems,it also needs to be able to effectively filter out the mode independent devices.In this paper,a method of fabricating long-period fiber gratings in the local area of the core is proposed to realize selective filtering of specific modes in the few-mode fiber.To solve the problem of narrow bandwidth of single-stage grating,a cascaded grating structure composed of staggered gratings is proposed,which can effectively increase the bandwidth of the filter.The mode converter is one of the key devices in the MDM systems and the few-mode optical fiber sensing systems.How to realize its broadband and efficient work is one of the key researches on mode converters.In this paper,a mode converter based on long period fiber grating is proposed.By writing the grating in the local area of the few mode fiber core,the mode conversion between LP₀₁1 mode and LP₁₁1 mode is realized.Through the analysis of the position relationship between grating and mode field and the influence of grating cross-section diameter on mode conversion characteristics,the requirements of grating structure parameters for realizing broadband mode conversion are determined.The simulation results show that the bandwidth of the mode converter can reach 115.6 nm and the crosstalk is less than-20dB.In this paper,a kind of FBG sensor based on SMF is also studied.The temperature sensing and bending resistance of SMF are analyzed.The test results show that the existence of higher-order modes in SMF has little influence on the temperature sensing characteristics of FBG,and the temperature sensing sensitivity of the prepared SMF is equivalent to that of the ordinary single-mode FBG.This kind of fiber grating also has a strong bending resistance.When the fiber and grating are bent to a super small bending radius of 5 mm,the sensing sensitivity and detection limit of the few-mode fiber grating are still equivalent to that of the straight fiber.This kind of few-mode fiber grating can be used in the situation where the fiber and grating need to be bent with small radius.