Design And Analysis Of ZnO-based Sub-wavelength Granting Polarizing Beam Splitter

Polarization beam splitter is an important device in the field of optical communication.It is mainly used in the coherent optical system of polarization multiplexing.It is also widely used in optical switch,optical routing,polarization imaging,optical isolator and many other fields.It has developed from traditional natural crystal and multilayer structure to photonic crystal and waveguide structure,greatly reducing the size of devices and reducing the size to micron level.However,due to the material and process constraints,the size is mostly in the micron level,which can not be further refined.Subwavelength grating is precise in structure and simple in design.Because of its periodic structure,the boundary conditions of electromagnetic field are different.Therefore,subwavelength grating has good polarization characteristics.In addition,the sub-wavelength grating has adjustable equivalent refractive index,which is more flexible and easy to design than crystal materials.In this paper,TE wave and TM wave are transmitted separately to achieve polarization beam splitting according to the polarization characteristics of sub-wavelength gratings.Using zinc oxide material instead of traditional silicon and gallium nitride material,a new type of polarization splitter with different side splitting is designed based on rigorous coupled wave theory(RCWA),equivalent medium theory and leakage mode resonance effect.The performance of the polarization splitter is simulated and analyzed by using finite element tool COMSOL Multiphysics.Finally,the optimum structure of the polarization splitter is obtained as the grating period is 1150 nm,the grating duty cycle is 0.35 and the grating thickness is 263 nm.The simulation results show that the polarization beam splitter maintains a very high reflectivity for TE mode in the range of 1450-1600 nm at normal incidence,and the reflectivity of TM mode is less than 5%.The extinction ratio of reflected light can reach more than 15 dB in the range of 1450-1600 nm,16 dB at 1550 nm.The extinction ratio of transmitted light can reach over 20 dB in the range of 1500-1580 nm,and 33 dB at 1550 nm.In addition,the diffraction efficiency of TE mode and TM mode are more than 95%,so that the effect of power splitting is good.The inverse problem of grating design is solved by mode method.The structure parameters of grating are deduced inversely.Several kinds of zinc oxide sub-wavelength grating polarizers with same side beam splitting in a certain scale are designed.The performance of sub-wavelength grating polarizer is analyzed by using MATLAB program based on rigorous coupled-wave method.In addition,the polarizer with the smallest size is analyzed by using MATLAB to evaluate the effect of grating structure parameters on the polarization performance of the polarizer.The extinction ratio of the two diffraction orders(-1 and 0)in the transmission direction of the polarization beam splitter can reach more than 25 dB at the incident wavelength of 1550 nm.The design results are mostly consistent with the theoretical analysis results.To guarantee the extinction ratios of the two directions above 20 dB,the parameters of the new type of zinc oxide sub-wavelength grating polarization beam splitter should be from 1530 nm to 1565 nm for wavelength.The incident angle should be from 61.9 to 67 degrees.The duty cycle should be from 0.49 to 0.515 for and the grating thickness should be from 860 nm to 880 nm.