Research On Microwave Photonic Filters Based On Multicore Fiber Space-division Multiplexing Technology

Microwave photonics is an interdisciplinary subject that combines microwave and optical domain technologies.By realizing functions such as signal generation,transmission and processing in the optical domain,the performance of microwave systems can be improved in terms of bandwidth,loss,size,weight,tunability and reconfigurability.Therefore,microwave photonics is a powerful means to break through the“electronic bottleneck”dilemma.As an important branch of microwave photonics,microwave photonic filters have a wide range of applications in the fields of communication,sensing,military and security.With the development of the 5^th generation mobile communication(5G)technology and the advent of the Internet of Everything,in emerging applications scenarios such as fiber wireless access networks,one of the main challenges in microwave photonics is to reduce the system's size,weight and power consumption while ensuring large operating bandwidth,low transmission loss,good tunability and reconfigurability.Space-division multiplexing(SDM)technology,as a novel technique that uses the spatial dimension to realize channel multiplexing,has been intensively explored and studied in the fields of optical fiber communication and sensing.However,for SDM technology,there is still a huge scope for research in the field of microwave photonics.This dissertation focuses on the research of microwave photonic filters based on multicore fiber space-division multiplexing technology.By the combination of optical fiber microstructure processing technology,optical fiber grating inscription technology,nonlinear optical technology,novel all-fiber structure fabrication technology and other optical technologies and principles,a variety of new microwave photonic filters have been proposed.The main research results and innovations of this dissertation are as follows:(1)Two types of infinite impulse response microwave photonic filters(IIR-MPFs)with cascaded and parallel configurations have been proposed,which contain tapered homogeneous multicore fibers and a pair of self-developed multicore fiber multiplexer/demultiplexer.The Q-factors of the cascaded and parallel IIR-MPFs have been improved from 43,68 to 143,and from 32,37 to 136,respectively,which are higher than that of an individual IIR-MPF.The geometric parameters of the taper structure and the number of spatial cores provide new dimensions for manipulating the performance of the microwave photonic filter.By using more cores of the multicore fiber to realize multi-stage cascaded or parallel IIR-MPFs,the Q-factors can be further improved.(2)Two kinds of reconfigurable finite impulse response microwave photonic filters(FIR-MPF)have been proposed based on multicore fiber long-period gratings in dimensions of wavelength and space,respectively.For the dimension of wavelength,the operation wavelength is tuned to change the inter-core power coupling according to the wavelength-dependent characteristic of long-period gratings.On the other hand,for the dimension of space,directional bending of long-period gratings is generated to achieve switchable spatial channel allocation.(3)A new scheme of fast reconfigurable microwave photonic filter based on long-period gratings inscribed in multicore fibers incorporating a semiconductor optical amplifier-nonlinear optical loop mirror(SOA-NOLM)switch has been proposed.Controlled by the pump light,the SOA-NOLM subsystem can realize fast signal switching between two spatial channels with an extinction ratio over 16 d B and a switching rise and fall time of 3.4 ns and 3 ns respectively.With the help of multicore fiber long-period gratings,inter-core power coupling differs in different input cores as well as operation wavelengths.Therefore,appropriate operation wavelength,input and output cores are selected to implement a reconfigurable two-tap MPF with a rejection ratio exceeding 38d B.(4)An all-fiber reconfigurable seven-tap microwave photonic filter has been realized using a multicore-multimode-multicore fiber structure.The multimode interference generated from the all-fiber structure can be manipulated in dimensions of space and wavelength simultaneously,thus configuring the power distribution in all the taps of the microwave photonic filter.A fully-connected neural network is also proposed to predict the operation wavelength and spatial input core when a target amplitude-frequency curve is input.The testing results show that the fully-connected neural network has high prediction accuracy and good generalization ability.