Study On Few-Mode Fiber Bragg Grating Based High Order Mode Obtaining Technology

In the past few years,the demand for communication bandwidth has increased exponentially due to the development of the Internet,the Internet of things and IPTV technology.In order to avoid the situation of"capacity crunch",a variety of technologies such as Wavelength Division Multiplexing(WDM),Polarization Division Multiplexing(PDM)and Orthogonal Frequency Division Multiplexing(OFDM)have been proposed.However,most of these technologies are based on Single Mode Fiber Communication System.The capacity of SMF is restricted by the nonlinear Shannon limit.Therefore,mode division multiplexing(MDM)based on few-mode fiber(FMF)has been proposed to overcome the capacity limits of SMF as a promising approach.In a MDM system,signal is modulated on a specific mode channel by high order mode obtaining device.Then a mode multiplexer is used to multiplex all the channels onto a FMF.MDM will increase the system capacity by a factor of the number of modes.In addition,the impairment of nonlinear effects can be minimized due to the inherent large effective area of FMF.In this paper,we have focus on the high order mode obtaining technology in MDM system.In a MDM system,how to obtaining high order mode effectively is vitally important.The existing high order mode obtaining methods can't meet the commercial system's requirement entirely.We propose a novel high order mode obtaining method based on few-mode fiber Bragg gratings(FM-FBG)in this paper.The FM-FBG can reflect a high order mode at the specific Bragg wavelength.If the center wavelength of light source matches the Bragg wavelength,we can obtain the high order mode at the reflection end of the FBG.The main work of this paper is organized as follow:At first,the mode characters is analyzed by deducing the Maxwell's equations which lays the foundation for the subsequent discussion.Then we introduce the existing high order mode obtaining technology,such as free space optics based mode conversion technology and the mechanical long period fiber grating based mode conversion technology.In addition,the asymmetric waveguide based mode converter is simulated.Then we study the transmission characters of FM-FBG by coupled mode theory and discuss the effects of the asymmetric transversal distribution of refractive index of the fiber which is caused by the UV-side illumination.After that,we experimentally analyze the mode transmission character of the three reflection peak of a two-mode FBG A CCD imaging system is used to observe the optical field distribution of both the FBG's transmission end and reflection end.By using the characters of the two-mode FBG,LP11 mode is obtained.In addition,the insertion loss of LP11 mode is optimized by adjusting the offset splicing distance between the single mode fiber and the two mode fiber.At last we generalize the results above and design a high order mode obtaining method based on few-mode FBG which can obtain any high order mode.Moreover,the performance of this technology is also evaluated in this paper.