Opt. Express 23, 655–666 (2015)

An optical encryption scheme for images that makes use of random polarization vector keys has been developed by researchers in Spain and the USA. The system uses a Mach–Zehnder interferometer. Both of the arms of the interferometer feature translucent liquid crystal displays that act as spatial light modulators (SLMs), making it possible to encode the polarization state at each point of the light's wavefront. In this way, a random polarization distribution created by appropriate programming of the SLM can be thought of as an encryption key. In addition, one of the arms features the plain text image. Light linearly polarized at 45° enters the interferometer through a polarizing beam splitter and light existing the system, following the recombination of the arms, is imaged by a CCD camera. An authorized user knowing the polarization key can retrieve the plain text image.