Optical encryption and verification technique for information coding in multiple-wavelengths in Fresnel domain

We propose a new optical encryption and verification technique in which multiple frames of images recorded by different wavelengths can be synchronously encrypted into double pure phase masks (PMs). The height distributions of the two surface-relief PMs are designed by using an iterative phase-retrieval algorithm in combination with the conjugate-gradient method. Two PMs are located at the predesigned positions in the verification system. When a plane wave with a correct wavelength illuminates the optical verification system, the corresponding image is extracted to display on the output plane. Another incident plane wave with other different wavelength which was used in recording can produce another image. Only when all the frames of images recorded by different wavelengths are obtained, the user can then pass through the verification procedure. Compared with the previous encryption and verification technique, these predesigned wavelengths can be used as additional verification keys, which should significantly enhance the security of system. Numerical simulation shows that the proposed technique can successfully reconstruct the verification images for the authorized user having all the correct keys, but never for any unauthorized intruder.