Research On Photonic Crystal Low-pass Spatial Filter

In the high-power laser systems,spatial filter is a very important optical device. 4F spatial filter is used in the traditional filter system,which is huge in size and can not work in near field and need vacuum environment.The photonic crystal (PhC) spatial filter is on focused,non-vacuum work and plug and play which can effectively overcome the shortage of the 4F spatial filter. The research on PhC is described and the numerical calculation methods for theoretical study are introduced and a new method to calculate the angle transmission coefficient is proposed.Then based on the abnormal refraction in PhC,we research the design of PhC low-pass spatial filter. Main contents of the thesis are summarized as follows:In the first, a optically thinner medium is constructed with PhC to realize low-pass spatial filter. Take a two-dimensional (2D)triangle-lattice PhC for example, with rational setting of its structure and materials,we present a medium whose absolute effective index is less than 1 and simulate its low-pass filtering performance by numerical calculation method.The result shows that a PhC whose positive effective index less than 1 is rare and usually occurs in high-level photonics band and has a strong anisotropy. Otherwise a PhC whose negative index greater than-1 is very easy to design,which can achieve good low-pass spatial filtering.Secondly, the self-collimation in PhC is another way to design low-pass spatial filter. Taking a 2D square-lattice PhC as study object, we summarize the principle in design spatial filter based on self-collimation and demonstrate its filter effect by numerical simulations.Theoretical analysis and simulation results show that the low-spatial-frequency components can coupled to the self-collimating modes effectively. Otherwise,the high-spatial-frequency components are reflected totally. So the high-spatial-frequency components are filtered effectively.Thirdly, the design parameter of PhC spatial filter is investigated in analytic theory and numerical simulation.We take a 2D square-lattice which is composed by rectangular dielectric rods for example and study the impact on its filter performance by changing the permittivity, dielectric fill factor and rotation angle of the rods respectively. The research shows that the critical angle is well controlled by the design parameter. The simulation result is well tallied with the theory analysis and this research contributes important significance to the design of PhC spatial filter.