Fig. 2: Principle of double-encryption optical communication technique.
From: Partial coherence and multi-channel vortices enhance high-security free-space optical communication
a shows a double-encryption optical communication technique, where the encryption operation is represented by the yellow key symbol. The first key is a varying average-mode number, and the process is exemplified in the dashed black box. The second encryption is a spatial nine-grid decoding trajectory (key 2), where the data is encoded in a grid-like spatial representation with a specific direction, start and end point. b shows a flowchart of average-mode number encryption. The solid lines with different colors represent different channels. The blue dashed boxes correspond to the phase modulation operations of data encoding and average-mode number encryption. The blue solid boxes correspond to the specific operations of data encoding, transmission, and data decryption and decoding.
