Rapid technological progress, especially in computers, CCD technology, color printers and scanners makes forgery and counterfeit of identification documents such as credit cards, or other important objects, increasingly simple. Current techniques such as the embossed hologram on credit cards are being defeated, and there is a strong need for a continuous development of new optical methods for security applications to stay ahead of counterfeiters. Optical security features can be inspected by either visual checking without special equipment or with the help of technical facilities for rapid screening. When such an optical system is required for security checking, its low manufacturing cost is a critical issue for its technological viability.
The proposed low-cost security verification system is based on the optical encoding of documents with pseudo-randomly generated phase masks and their inspection by performing all-optical spatial correlation of two phase-encoded images in a real-time optical recording medium and in a four-wave mixing configuration. One phase image is placed on the object to be verified, such as an ID card. The other is made available to the security systems for comparison with the input image. The practically invisible phase mask is permanently placed on the object to be verified, and can be manufactured using a number of techniques such as embossing on plastic films, encoding on photopolymer, etc. With the high resolution of commercially available optical materials, the phase mask can be of the order of a million pixels, and the mask size will be only a few millimeters square.
The recording medium is a key element in this type of all-optical architecture. The limited performance and/or the high cost of existing nonlinear optical materials has severely limited the technological potential of all-optical correlators: inorganic photorefractive crystals have been investigated but their processing and cost has limited their finding widespread applications. Due to limited optical material performance, other correlator designs have been proposed over the years: nonlinear joint-transform correlators, for instance, show good performance for pattern recognition and are capable of real-time operation. However, because these systems use either sophisticated liquid crystal light valves, CCD detectors, and/or a computer to perform Fourier transforms, they do not meet the low-cost requirement.
The proposed optical correlator uses highly efficient photorefractive polymers developed at the University of Arizona. These materials are at the cutting edge in plastics research and are promising for several applications, including holographic storage, optical processing, phase conjugation, and imaging. In the proposed optical correlation system the phase mask used is a 64-×-64-pixel binary random pattern. To authenticate the document it is compared with a master that is an exact copy. The hologram written by the interference of a reference beam and a laser beam going through the test mask forms a holographic filter for the master mask. If the two phase patterns match, light will be strongly diffracted by the photorefractive polymer. The figure shows the intensity distribution of the signal that is produced by two matching masks. This holographic filter is performed in real time with a low-power 675-nm laser diode. The active medium is a 105-micrometer-thick photorefractive polymer film sandwiched between two transparent indium-tin-oxide electrodes.
The security verification system proposed has the following features that make it practical for widespread applications. First of all, the use of a highly efficient photorefractive polymer as active material in an all-optical correlator configuration and its compatibility with semiconductor laser diodes keep the overall manufacturing cost to levels that are significantly lower than that of any previous proposed optical correlator. The system is fast because the processing is implemented optically in parallel. Furthermore, the high resolution of the photorefractive polymers allows the use of lenses with shorter focal lengths in the 4f correlator, thus making its design more compact compared with ones using liquid crystal light valves. In addition, all the components, including the laser source and the nonlinear material, can be manufactured in a very small size and the system can be easily further miniaturized. Finally, because the recording process is based on the photorefractive effect, the stored hologram can be erased and a new hologram written in real time. This reversible real-time recording and processing enables the testing of a variety of different documents encoded with different phase masks and their comparison with a corresponding master mask database.
This work was done by N. Peyghambarian, B. Kippelen, and colleagues at the Optical Sciences Center of the University of Arizona, Tucson, AZ, and by B. Javidi from the University of Connecticut. The project was funded by the Office of Naval Research through the MURI Center CAMP, by AFOSR, and NSF. For more information, call (520) 621-4649 or (520) 621-4341.