Cascaded diffractive optical element for high-fidelity optical information encryption

Cascaded diffractive optical element (DOE), consisting of multiple DOE layers, is a type of multi-layer architecture that introduces additional design freedom, e.g. rotation angle, wavelength or polarization state, enabling more flexible and precise modulation of light field compared to a single-layer DOE. This enhanced modulation capability endows it with significant potential for applications in the field of information encryption. For this application, the fidelity of image reconstruction is critically important to the performance of the cascaded DOE. In this work, we propose a new cascaded DOE design framework with the integration of an optimized Harvey-s model, enabling larger modulation bandwidth compared to conventional angular spectrum method (ASM), thereby increasing the information capacity of cascaded DOE, as well as the accuracy of reconstructed images. To validate the proposed method, we design a cascaded DOE for four distinct images encryption. The correlation coefficient of decrypted images is improved by 37% compared to the result that used ASM-based design method. Future work includes fabricating the designed DOE using a two-photon polymerization (2PP) technique and verifying its performance experimentally.