High-dimensional coding/decoding information with braiding period in free-space optical communication link

We present in principle a free-space optical coding/decoding method that based on the longitudinal braiding period of the structured laser beam. Different from the conventional techniques that mainly focuses on the field of time, frequency, amplitude, polarization and spatial structure, the longitudinal structure is added as a new freedom in our optical link. By combining the topological charge and propagation angle in the transmitter, the longitudinal intervals between the detectors are utilized as the key for decoding. The system using the braiding period as the information carrier has an advantage in security for it is difficult to be eavesdropped. The realization of the hexadecimal coding/decoding suggests that employing the braiding period as a new dimension may have a potential in increasing the capacity or security of the OAM-based free-space data transmission.