Optical multiple-image encryption in diffractive-imaging-based scheme using spectral fusion and nonlinear operation. 
We report a new method for multiple-image encryption in diffractive-imaging-based encryption (DIBE) scheme.  The discrete cosine transformation (DCT) spectra of the primary images are extracted, compacted and then nonlinear-transformed before being sent to the DIBE, where they are encoded into a single intensity pattern.  With the help of a suggested phase retrieval algorithm, the original images can be recovered with high quality.  Furthermore, due to the introduction of the nonlinear operation, the proposal is demonstrated to be robust to the currently available cryptographic attacks.  The proposal probes a new way for multiple-image encryption in DIBE, and its effectiveness and feasibility have been supported by numerical simulations.