Microwave Photonic Filter Based On Few-mode Fiber With Uniform Group-delay Difference

The rapid development of space division multiplexing brings a new opportunity for microwave photonic technology.Combined with the orthogonality of modes,the design freedom of microwave photonic filter can be effectively improved by using mode dispersion for signal delay operation.Combined with the chromatic dispersion of each mode channel,the number of taps of the microwave photonic filter will be further increased,or the number of independent light sources will be reduced.Therefore,microwave photonic filter based on few-mode fibers can simplify the complexity of system structure,reduce the system energy consumption,and has a good application prospect in the field of Radio over Fiber and radar system.In this paper,according to the characteristics of step and gradient refractive index distribution with trench few-mode fiber,the design method of uniform group-delay difference few-mode fiber is proposed,and the optimization design of three modegroups and four mode-groups few-mode fiber with uniform group-delay difference is carried out.Through parameter scanning,the design of three mode-groups fiber with uniform group-delay difference is realized by using step and gradient refractive index distribution with trench fiber structure for the first time.For the step and gradient refractive index distribution fiber,the maximum deviation of refractive index difference between adjacent modes in the C band is 2.15% and 1.48%,respectively.For the four mode-groups uniform group-delay difference fiber,within the range of structural parameters,the deviation of group refractive index difference between adjacent modes of the step fiber is greater than 15%,while the maximum deviation of group refractive index difference between adjacent modes of the grade fiber is 2.12%,which can be used for the design of four mode-groups fiber with uniform group-delay difference.The few-mode fiber with uniform group-delay difference can be used to realize the microwave photonic filter system with finite impulse response.The microwave photonic filter based on 1 km four mode-groups fiber with uniform group-delay difference is constructed.Its free spectrum range is 5.7 GHz,3-d B bandwidth is2.64 GHz,and the main side lobe rejection ratio is 11.42 d B.By studying the effect of mode coupling on the filter,it is found that the inter-mode coupling has more effect on the spectral response than the inter-mode coupling.In the C-band,the normalized transfer function of the filter is analyzed,and it is proved that the filter has a wide spectrum.Considering the limitation of fiber fabrication process,when the mode group refractive index difference deviates from the ideal value to the maximum,its free spectrum range decreases by 0.52%,which verifies the feasibility of the filter.