Technical University of Denmark

Programmable Phase Optics

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Research



The Generalised Phase Contrast method Advanced optical micro-manipulation Phase-only optical encryption and decryption Spatial phase-only modulation by the reverse phase contrast method GPC implemented in plana-integrated micro-optics Complex field coupling to advanced optical fibers 2D polarization encoding using Spatial light modulators

Optical Encryption/Decryption based on the GPC method

There is widespread interest in the development of encryption systems, which operate in the optical domain. The advantages inherent in an optical approach to encryption, such as a high space-bandwidth product, the difficulty of accessing, copying or falsification and the possibility of including biometrics are widely recognised (see further reading). In an encryption system, we wish to encode information in such a fashion that even if it is viewed or copied only the application of the correct key will reveal the original information. Our encryption approach is based on the direct mapping of an encrypted phase-mask and a decrypting phase key, resulting in the decryption of information completely within a phase-only domain. A schematic diagram of the generic phase-only decryption system is shown in Figure 1.


Figure 1. Schematic diagram showing the generic system architecture for the GPC-based decryption operation.

In this system, an encrypted phase mask is decrypted with a phase-only key and the decoded information is subsequently visualised using the generalised phase contrast (GPC) method. A plane polarised monochromatic wavefront illuminates the encrypted phase mask, which consists of a random array of phase-shifting pixels. This phase-mask is produced by electronically scrambling the original information we wish to encrypt with a random pattern and using this to generate an encrypted phase mask. The decrypting key effectively reverses the scrambling operation in the optical domain and results in the production of a wavefront in which the information of interest is encoded as a relative phase shift between different sections of the wavefront, in this case corresponding to the pixels.

The mask and key can be placed, either directly in contact with one another so that the decryption takes place in the same image plane, or alternatively they can be imaged onto one another with an optical system. By using a spatial light modulator the phase key can be scrolled electronically until it overlies the phase pattern of the encrypted mask removing the necessity for precise mechanical positioning in the optical system. An experiment utilising this electronic alignment feature is shown in Figure 2.


Figure 2. Results for a binary decryption of a 7x7-pixel test pattern, showing; the (a) encrypted phase-mask, the (b) decrypted image visualised by the PCF and for comparison, and (c) a graphical representation of the original information that was encoded.

It should be noted that although we use binary phase masks for the key and the encrypted information in figure 2, we are in fact not limited to this case. The encryption technique we present could equally well be applied to systems in which multiple phase levels are used for the masks and keys. However, the fabrication issues involved in the production of a multiple phase level fixed phase mask are more complicated than for the production of a binary mask, so for the purposes of the experimental demonstration of decryption binary 0/pi phase masks and phase keys have been used.

Further reading

Glückstad, J.; Daria, V.R.; Rodrigo, P.J.; Decrypting binary phase patterns by amplitude. Opt. Engineering. (in press) 

Daria, V.R.; Rodrigo, P.J.; Sinzinger, S.; Glückstad, J., Phase-only optical decryption in a planar-integrated micro-optics system, Opt. Engineering. (in press) 

Glückstad, J; Daria, V.R.; Rodrigo, P.J.; Comment on "Interferometric phase-only optical encryption system that uses a reference wave". Optics Letters (2003) 28, 1075-1076

Mogensen, P.C.; Glückstad, J., Phase-only optical decryption of a fixed mask. Appl. Opt. (2001) 40, 1226-1235

Mogensen, P.C.; Eriksen, R.L.; Glückstad, J., High capacity optical encryption system using ferro-electric spatial light modulators. J. Optics A. (2001) 3, 10-15

Mogensen, P.C.; Glückstad, J., Phase-only optical encryption. Opt. Lett. (2000) 25, 566-568

Mogensen, P.C.; Glückstad, J., A phase-based optical encryption system with polarisation encoding. Opt. Commun. (2000) 173, 177-183


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