提出基于衍射光学器件的光学图像级联加密系统。一般来说，一个n级系统可由n个透镜串联组成。经过优化设计的衍射光学元件如相位板等相应的放置在系统的空间域和频谱域。当系统在平面波照明下，便能在输出平面获得解密图像。这些衍射元件可通过级联迭代傅里叶变化算法设计。计算机模拟结果显示，算法具有很快的收敛速度，而且所应用于的系统的级次越多，相应的收敛速度越快．级联系统能够高质量的恢复原始图像，使之与原始图像的均方差小于5×10^－30。采用密钥共享方案，级联系统可以用作多用户系统，这样能使安全性明显提高。

Cascaded systems for optical image encryption based on diffractive optical elements are presented. Generally,a cascaded system with n-stages is a 2n-f system constituted with n lens aligning in serial. N diffractive optical elements(DOES)such as phase masks are located in the spatial and the frequency domains of the system so as to decrypt the primary image at the output when the system is illuminated with a corresponding plane wave. These DOEs are designed with the cascaded iterative Fourier transform(CIFT)algorithm. Computer simulations indicate that such algorithms converge very fast,and that the larger the stages number,the faster the convergent speed for the corresponding algorithm. The cascaded systems can perform high quality decryption. Numerical results show that the mean square error between the decrypted image and the primary image can be less than 5×10^－30. Furthermore,the proposed systems can be used as key-sharing implementation in a multiple users system,and higher security is thus achieved.