Research On Microwave Photonic Filter Based On Few Mode Fiber

With the advent of the information age,the communication bandwidth is increasing,and the development of long-distance and large capacity communication becomes inevitable.With the rapid development of microwave technology,the frequency of microwave is increasing gradually,which makes the microwave technology of electronic devices have electronic bottleneck and limits the frequency and bandwidth of microwave communication link.Optical devices can realize optical communication with large bandwidth and high frequency.Microwave photonics technology combines microwave and photonics,gives full play to the characteristics of optical transmission link.Microwave photonic filter(MPF)is a key technology in microwave photonics.In the past,there are many researches on microwave photonic filter using wavelength division multiplexing and optical frequency comb technology.In addition,we can make use of the new dimension of mode,through the mode division multiplexing technology,using the characteristics of the few-mode fiber(FMF)can transmit different modes.Each mode constitutes a channel branch,and each channel has different weight and delay.By changing the weighting and delay of each channel,different transfer functions can be formed,and microwave photonic filters with different characteristics can be obtained.In the MPF system formed by FMF,the mode dimension is introduced,and the multi tap filter can be formed through a single fiber in a single wave.The formation of MPF structure is more simple,lower cost.Therefore,it is of great significance to study microwave photonic filter based on FMF.In this paper,we focus on the mode characteristics and mode division multiplexing technology in few-mode fiber,and analyze the low-mode fiber differential mode group delay,photonic lantern and low-mode fiber long period grating.Based on these,We study the theory and implementation of microwave photonic filter based on few-mode fiber.The main work of this paper is as follows:Firstly,the structure,classification and mode theory of few-mode fiber are introduced.The orthogonality of modes is derived.The principle and model of microwave photonic filter are described.Microwave photonic filters are classified and the main performance indexes are described.Secondly,the structure of the photon lantern is introduced,and the mode selective photon lantern is measured.The system theory of microwave photonic filter is studied,and the differential mode group delay of four-mode step fiber is calculated.Using the photon lanterns as the mode multiplexer and demultiplexer,the experimental system of microwave photonic filter based on few-mode fiber is built.The time delay of different modes of the system is measured,and the frequency spectrum of the microwave photonic filter is measured,and the filters with different number of samples with FSR of 0.7GHz are obtained.Then,the mode coupling theory of the few-mode optical fiber long period grating is introduced.The parameters of grating are analyzed,and the effects of period,modulation depth and length of refractive index on the transmission spectrum of grating are studied.The few-mode optical fiber long period gratings are used as the mode converter to form the optical true delay line.The few-mode fiber long period gratings which can excite different modes are fabricated,and the mode conversion of the gratings is measured.The experimental system of microwave photonic filter based on few-mode fiber long period grating is built.The spectrum of microwave photonic filter formed by different modes is measured,and the three-tap filter with FSR of8 GHz is realized.Finally,the main content and work of this paper are summarized,and the research of microwave photonic filter based on few-mode fiber is further prospected.