Optical Cryptosystems Based on Spiral Phase Modulation

Optical information security techniques have several advantages over digital counterparts such as ability to process information parallelly, use of physical parameters as security keys, efficient storage capability etc. In last few years, several optical cryptosystems have been designed based on different optical aspects. In this chapter, we discuss optical cryptosystems based on spiral/vortex phase modulation in details. The orbital angular momentum (OAM) associated with a spatially helical phase or vortex beam can be utilized to design enhanced security protocols. Moreover, since the OAM has theoretically unlimited values of topological charges (TCs) and have the orthogonality of OAM modes with different integer TCs, it is an excellent candidate for designing high-capacity secure optical cryptosystems. Here, the spiral phase functions have been first introduced with different TCs and then the 2D spiral phase transform (SPT) and several optical cryptosystems based on it are discussed in detail with possible optical configurations for practice applications. Numerical simulation results for three cryptosystems are discussed showing their feasibility. The security analysis in terms of keys sensitivity and robustness against existing attacks is also performed and discussed for these cryptosystems.