Design And Analysis Of GaN-based Granting Polarizing Beam Splitter

The conventional polarization beam splitter is made of thin film or birefringent crystal.The thin film polarization beam splitter has many layers,which makes it difficult to manufacture.Birefringent crystals are usually sensitive to the angle of incidence,and they are large and expensive.Sub-wavelength grating polarizing beam splitter can separate the incident monochromatic plane wave into sub wavelengths with different polarization states: TE polarized light and TM polarized light.It has advantages of good polarization,compact structure,small volume,easy integration and so on.Two kinds of sub-wavelength grating polarizing beam splitter based on gallium nitride: transmission grating and reflected grating were designed here.Based on the equivalent medium theory and the finite element method,a reflective sub-wavelength grating polarization beam splitter for C band(1530nm to 1565 nm)is simulated by using COMSOL Multiphysics software.The grating is made of gallium nitride instead of the traditional material.The structure is a suspension single layer sub-wavelength grating.Its parameters were optimized to get the optimal structure during the process of simulation.Finally,the performance of the polarization beam splitter was analyzed by calculating the extinction ratio.The simulation results show that under normal incidence,when the sub-wavelength grating period is700 nm with the grating height of 265.3nm,and the grating width of 640.5nm,the transmittance of TE mode and TM mode is very high,and the extinction ratio is more than 20 dB.A transmission type grating polarization beam splitter based on the model method was designed in 1550 nm.The model method can transform the complicated diffraction problem into a pattern coherence mechanism.Four grating polarization beam splitters were calculated by the pattern method.Then the Matlab program based on strict coupled wave theory was used to analyze its performance,and then the optimal structure grating could be selected.When the diffraction efficiency is greater than 90%,the range of each parameter could be obtained with the incidence light wavelength of 1517nm-1582 nm,the incident angle of 68.9 degrees-85.3 degrees,the duty ratio of 0.477-0.521,the grating period of 803.7nm-828.9nm,the grating height of 449nm-1618 nm.The results show that the transmission diffraction efficiency of the optimal grating is high,and the calculated values are almost the same as those calculated by the model theory.