A novel method is proposed for simulating free-space diffraction propagation. This method is an improvement of the angular spectrum (AS) method and the band-limited angular spectrum (BLAS) method. Due to the sampling problem about the transfer function, AS is not suited for the long-distance propagation. For BLAS, the calculation accuracy would decrease when the propagation distance is relatively large. Using a wide calculation window can largely reduce the numerical error of AS and BLAS. However, a wide calculation window generally causes a huge calculation burden in the simulation. For the proposed method, the calculation window size is chosen to make sure all the sampling points in the frequency domain are effective in the sense of Nyquist theorem. The calculation complexity is independent of the calculation window size because of the use of the linear convolution. The linear convolution can be evaluated effectively by fast Fourier transform. This method can produce simulation results with high accuracy for far and near field propagation.

Band-limited angular spectrum (BLAS) methods can be used for simulating the diffractional propagation in the near field, the far field, the tilted system, and the nonparaxial system. However, it does not allow free sample interval on the output calculation window. In this paper, an improved BLAS method is proposed. This new algorithm permits a selective scaling of observation window size and sample number on the observation plane. The method is based on the linear convolution, which can be calculated by fast Fourier transform effectively.

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