The angular spectrum method (ASM) is widely used in the design of computer-generated holograms (CGH) due to its strict adherence to the Helmholtz equations and the potential for accelerated computation using fast Fourier transform (FFT). However, when dealing with long distance propagation conditions, the reduction of effective spectral components and spectral aliasing will introduce significant computation errors, leading to a substantial degradation in holographic imaging quality. To achieve high-quality holographic imaging design in long distance, we introduce a novel angular spectrum computation method, the calculation process is divided into two steps: (1) First propagate a longer distance from the CGH plane to an intermediate plane. (2) Then continue to propagate to the observation plane after incorporating a spread spectrum phase factor to amplify the effective non-aliased bandwidth in the intermediate plane and eliminating the aliasing artifacts. Furthermore, the non-uniform fast Fourier transform (NUFFT) algorithm is adopted which allows for the flexible adjustments of the sampling interval and quantity in the spatial frequency domain, all while maintaining compliance with the Nyquist sampling theorem even in the context of bandwidth amplification. Both simulations and optical experiments are conducted to validate the feasibility and effectiveness of our proposed method.
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