Optical vortex application for a secure optical system

Abstract

In the paper a concept of an optical vortex application for the secure optical system was presented. The proposed system uses a spatial multiplexing of optical signals performed by creating two separate communication channels in one optical fiber in which the important data can be encrypted. The optical secure system consists of three parts, i.e.: light beam generator, optical fiber link and demodulation unit. Light from a single source is split into two types of light beams. One of them remains unchanged and preserves its Gaussian shape. The other one is transformed to an optical vortex by passing through the liquid crystal spiral phase plate. This liquid crystal cell requires spatially distributed electrodes which can be supplied independently to introduce spatially distributed phase shifts what changes this Gaussian beam into optical vortex. It was applied as a perdeuterated liquid crystal D5CB which has a small absorption in telecommunications’ spectral region. Both beams are coupled together to the specially designed photonic crystal fiber which supports propagation of fundamental and vortex modes launched to it. The cross section of the optical fiber has a honey comb lattice with two types of air-holes rings. External one with four rings confines all propagating modes and internal single ring which spatially separates the fundamental mode and the first group of higher order modes. Both types of spatially separated modes can be used for transmitting important data. The fiber link output requires a second spatial demodulator to decouple both modes and decrypt transmitted data. At this stage of the project a light beam generator is developed, fiber link and demodulation unit are under testing.