Theoretical And Experimental Research On Few Mode Fiber Bragg Grating Based On Ultraviolet Single-side Exposure

With the progress of society and the development of science,the demand for communication capacity is increasing,and the traditional single-mode optical fiber(SMF)will not be able to meet the demand for the rapid growth of communication capacity.Therefore,exploring the new multiplexing dimension of optical fiber has become the current research hotspot.Mode division multiplexing uses spatial orthogonal modes as independent channels,which can greatly increase the maximum transmission capacity of the communication system.The conversion and generation of higher-order modes in the mode-division multiplexing system are the basis for its further application.Aiming at the problem of mode conversion and generation in few-mode fiber,this paper studies the mode coupling conversion problem based on few-mode fiber Bragg grating(FBG).The specific contents are as follows:(1)Based on the coupled mode theory of FBG,the coupling characteristics of the fundamental mode to the higher-order mode in the few-mode step index FBG and fewmode ring-core FBG under UV single-side exposure are studied,and the influence of the grating parameters on the mode coupling characteristics is analyzed.(2)For the FBG based on step index fiber and ring-core fiber,the effect of attenuation factor on mode coupling is studied.Under UV single-side exposure,with the increase of the attenuation factor,the reflection peak value of the fundamental mode gradually decreases,and the spectrum width also gradually changes narrow.For higherorder modes,with the increase of attenuation factor,the corresponding reflection peak value tends to increase firstly and then decrease.The optimal attenuation factor for step index fiber is about 0.2,while the optimal attenuation factor for ring-core fiber is about 0.3.(3)Based on the modified chemical vapor deposition method,a few-mode ring-core fiber was fabricated,and the FBG was inscribed in the homemade fiber through the phase mask method under UV single-side exposure.In addition,the reflection spectrum and transmission spectrum of the FBG are analyzed,and theoretical analysis and verification are carried out.The fiber Bragg grating proposed in this paper can realize the coupling conversion from the fundamental mode to the higher-order mode,which provides a method for the generation and conversion of the higher-order mode in the few-mode fiber,and it is of great significance to the application of the mode division multiplexing technology.