Design And Research Of Nonreciprocal Filter Structure Of Non-magnetic Photonic Crystal

In an optical communication system,it is necessary to set the optical devices with non-reciprocal transmission in order to ensure the stability of the optical system and avoid the influence of reflected light on the work of the device in the system.Compared with traditional filters,phoonic crystal filter has the characteristics of small size,low energy consumption and easy integration,so the design and research of photonic crystal filter has great application value in optical communication.In this paper,based on the mode coupling theory of photonic crystals,the waveguide and resonator structures are designed by introducing different defect structures in a perfect two-dimensional photonic crystal,and the non-reciprocal filtering is realized by utilizing the matching between their respective local modes.The structure design and performance analysis of the non-reciprocal filter structure are carried out by using COMSOL and Rsoft software.This paper mainly focuses on the following aspects:(1)Based on the coupled mode theory of photonic crystal waveguide and resonator,a non-magnetic square lattice photonic crystal non-reciprocal single-channel filter structure is designed.The linear waveguide 1 is realized by removing a column of dielectric columns vertically and optimizing the position parameters of the dielectric columns on both sides.The waveguide is a dual-mode waveguide,which can carry the propagation of odd-mode and even-mode electromagnetic wave signals,and the odd-mode band range is 0.2462(a/λ)～0.4214(a/λ),and the range of the even mode band is 0.3540(a/λ)～ 0.4214(a/λ);The waveguide 2 is formed by removing a row of dielectric columns in the horizontal direction.The waveguide range is 0.3042(a/λ)～0.4225(a/λ),which can only carry the transmission of transverse even-symmetrical mode electromagnetic waves.A rectangular dielectric column resonator is designed at the intersection of waveguide 1 and waveguide 2,and the long side of the rectangular dielectric column is horizontal.The structural parameters of the rectangular dielectric column resonator are optimized so that the resonator has different resonance frequencies and symmetrical modes in the transverse and longitudinal directions.According to whether the resonant frequency of the resonant cavity is within the guided wave range of the waveguide,and whether the symmetrical mode of the resonant frequency is consistent with the guided wave symmetry of the waveguide,the non-reciprocal filtering transmission of light is realized,and the electromagnetic wave with the frequency of0.4106(a/λ)in odd mode is incident from the port of waveguide 1 and transmitted from the port of waveguide 2;the reverse is cut off.By optimizing the structure of the input waveguide 1 and optimizing the coupling part between the resonator and the waveguide,the optimal forward transmittance is 93.7%,reverse cut-off,and the transmittance is almost 0.(2)In terms of single-channel non-reciprocal filters,the structure in(1)is more complicated,so a simpler structure is designed with a higher forward transmittance.The adopted photonic crystal structure is the same as that in(1),and the incident waveguide is a graded straight waveguide 3similar to a water bottle shape,which can carry the transmission of multiple odd and even symmetrical mode electromagnetic waves;Two rows of dielectric columns are removed along the vertical direction of the photonic crystal to form a waveguide 4,which is a dual-mode waveguide and serves as an output waveguide.A square dielectric column resonator is designed at the intersection of waveguide 3 and waveguide 4.The resonator has a single resonance frequency electromagnetic wave locally,but has different symmetrical modes in the transverse and longitudinal directions.Non-reciprocal transmission is achieved using the matching between the waveguide and local electromagnetic wave symmetry modes of the resonator.After the optimization of the medium column in the resonator and the coupled medium column,the forward transmittance of the structure is increased to 95%,and the reverse transmittance is 0 due to the mode mismatch.(3)Based on the rectangular medium column resonator in(1)with multiple resonant frequencies and different transverse and longitudinal symmetries,a kind of T-type non-reciprocity dual-channel filter structure is designed according to the same coupling mode theory.Above the rectangular resonator,a row of dielectric columns is removed along the horizontal direction to form a waveguide 5,which has similar characteristics to the waveguide 2.The coupling output of the corresponding electromagnetic wave and the waveguide 5 is realized by utilizing the matching degree between the longitudinal symmetry of localized electromagnetic waves in the rectangular resonator and the wave guide characteristics of the waveguide 5.The results show that the odd-symmetric mode electromagnetic waves with frequencies of 0.4106(a/λ)and 0.4002(a/λ)are incident from waveguide 1,and output from waveguide 2 and waveguide 5,with reverse cutoff.The filter insertion loss was 0.4d B and 0.36 d B,and the inter-channel isolation was 33 d B and 22 d B.The designed dual-channel non-reciprocal filter structure has good filtering effect and has potential application value in the field of all-optical communication integration in the future.