Low-pass rugate spatial filters for beam smoothing

A new application of rugate structures is proposed as low-pass spatial filters for beam smoothing. By using the transfer matrix method to analyze the spatial properties of the bandgap of rugate structures, the low-pass rugate spatial filters with both an almost ideal flat bandpass and a rather steep switching between pass- and stop-bands are designed. The angle-domain bandwidth of the spatial filters can be adjusted by changing the parameters of rugate structures for a given light frequency. The near-field simulations carried out by using the finite-difference time-domain technique confirm the possibility of an efficient light smoothing.