Scientists of the National Research Nuclear University MEPhI (Russia) have proposed a scheme for optical encoding of information based on the formation of wave fronts, and which works with spatially incoherent illumination. This scheme is effective in creating highly secure, high-speed coding systems. Security is provided by the two-dimensionality of dynamically replaced coded keys. The study is published in Laser Physics Letters.
Scientists from around the world are actively conducting research on the creation of optical encoding systems. The main direction of such studies is the encoding of advanced optical information systems with fully coherent laser illumination. Such studies are incompatible with standard cameras, because they require the use of complex holographic methods and are focused on a next-generation elemental base.
The development of optical coding using spatially incoherent quasi-monochromatic (single color) illumination is regarded as a more effective approach. This provides the possibility of hardware implementation on the basis of commercially available photo and video cameras.
In this case, information for encoding is displayed as a QR code on the LCD amplitude-spatial light modulator that is illuminated by monochromatic laser radiation. Radiation is passed through a rotating frosted diffuser, which destroys the spatial coherence. The researchers used a liquid-crystal phase light modulator as the encoding element, where pre-synthesized diffraction optical elements are displayed. The camera's image sensor detects the optical convolution of the image that is produced by the amplitude modulator with the pulse response of the diffractive element, derived on the phase modulator.
The use of such modulators allows changing encoding keys in real-time. Decoding is performed by software-based method of digital deconvolution (inverse convolution in signal processing) with stabilization solution.
The scientists successfully encoded and decoded the images of QR codes with a size up to 129×129 elements. The percentage of erroneously decoded pixels didn't exceed 0.05 percent. This is a high ratio of signal to noise.
"The novelty of our work is, first, in the application of monochromatic spatially incoherent illumination of the encoding scene – it allows you to avoid the speckle noise and does not require holographic methods of registration. Second, the use of computer-synthesized phase diffractive elements allows you to generate required coding wave front and minimizes the loss of radiation in the system," said Associate Professor Vitaly Krasnov.
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P A Cheremkhin et al. QR code optical encryption using spatially incoherent illumination, Laser Physics Letters (2017). DOI: 10.1088/1612-202X/aa5242