Jakub Svoboda

Optical security elements based on waveguide effects

Optical document security represents an important field of application of analogue and synthetic diffractive structures. Most of the security elements are based on visual effects formed by diffraction on a structure with the details in the order of hundreds of nanometers. However, to improve the anti-counterfeiting properties of these structures, various types of hidden features are included within the area of the security elements. They are not visible under normal lighting but it is possible to easily reveal the hidden information under specially-defined geometry and/or type of illumination. In this paper, theory and application of a novel type of hidden diffractive security element are presented. It combines standard visual properties of synthetic holograms with waveguide effects. The hidden information is recorded using a special grating, which is not visible under normal observation geometry. The encoded image can be reconstructed only when the proper guided mode appears in a substrate. During the reconstruction, light is coupled into a waveguide (holographic foil) using a grating coupler and after traveling through the substrate in a chosen direction it is selectively out-coupled within the areas containing the hidden information. Several elements with different properties have been designed, fabricated and compared with theory. Principles of diffraction and waveguide effects, realization technology and properties of the realized test samples are presented. The advantage of the combination of diffractive and waveguide effects is that the resulting hidden effect is sophisticated but easily readable with no additional tools.

Advanced optical document security elements

ABSTRACT Synthetic diffractive structures represent an important tool in the optical document security. Their macroscopic visual behavior is based on properties of a very fine micro-structure which cannot be copied using common copying techniques. The visual effects can be easily observed by a common observer without any special inspection tools. However, when a high level of security is needed, additional features are often included based on an optical encryption of information. In this paper, a novel encryption technique is presented, which is based on utilizing the plastic holographic foil as a waveguide and special diffractive structures as coupling elements. When an in-coupling area is illuminated with a defined light beam, the light is coupled into the waveguide and travels to an out-coupling part. The encrypted information is encoded either in the shape of the out-coupling area or it can be formed from an out-coupling hologram in free space above the element. Both laser and normal white light sources can be used for reading the information. The coupling areas can be mixed with diffractive micro-structures forming visual effects and can be invisible during a normal observation of the hologram. The couplers can be realized using the technology fully compatible with the standard process for mastering and replication of the security elements. Several extensions of the described idea of waveguide cryptograms are also included. Finally, a set of real samples of the security elements is presented, which were realized using an advanced matrix laser lithography technique.

Synthetic Image Holograms

This chapter is dedicated to synthetic image holograms the elements which can create a reconstruction of a 3D object for observation with the human eye. Holography as a technique of image recording and reconstruction has been extensively developed from sixties of the twentieth century. During this time there have been various attempts to synthesize holograms artificially without the presence of the real object in the classical recording setup. Different approaches have been used, several trying to synthesize the three-dimensional object from two-dimensional views using the classical recording setup, the others trying to calculate the microstructure of the hologram completely in a computer. Today, we can divide synthetic holography into twomajor streams, the first containing the methods for creating the image for observation by human eye and the second consisting of approaches for designing the synthetic diffractive structures for general wavefront generation. The former techniques can exploit various imperfections of human vision and omit several parameters of the optical wave. The latter techniques are usually based on the direct calculation of the microstructure and they try to create the reconstruction in its full complexity. Only the first group of synthetic image holograms will be analyzed in this chapter. The synthetic approach to hologram creation can have several advantages, but also noticeable disadvantages. The most important advantages are connected with flexibility in modifying the recorded object. First, the object need not to exist in reality in a form of a physical model. For most synthetic approaches, it is fully sufficient to have a 3D computer model for preparing the recording data. Also for real physically existing objects it could be tricky to perform the recording process in a classical setup. For example, various outdoor scenes such as buildings and others could not be included in the laboratory setups. Generally, the scaling possibility is very limited in classical holography, so the recorded object (or its model) must be of final size. On the other hand, it is easy to scale the computer model of an object. The next problem is in various corrections of color properties, surface textures, and general fine tuning of the recorded object. While such operations are very simple in the case of computer models, they could bring insoluble problems for real physical models. The stability of the object is also very important. It is crucial to highly stabilize the object for recording in classical holographic recording setup (when exposing with a continuous-wave laser), whereas in a computer stability is not a problem. This can apply also for holograms of living objects or dynamic scenes, where it is easy to take snaphots using photographic techniques, but holographic exposure is almost impossible. Finally, according to the recording technology chosen, other parameters of the synthetic hologram can be highly superior to those of classical holograms (e.g. fidelity of color mixing, contrast of the image, etc.). 10

Realization of spatial image using diffractive structures

Spatial imaging is nowadays realized using a number of principles and technologies. Application of stereography in combination with the diffractive structures is thus only one of many possible solutions. We introduce two methods of synthetic stereography using diffractive structures. The two-step method of holographic stereography records a 2D matrix of primary holograms in its first step, using an original recording device. This multiplexed master is then reconstructed in a typical holographic scheme and a full-color rainbow hologram can be recorded in the second step. The second presented method, the method of direct writing, records the diffractive structure of a full-color synthetic stereogram in a single step, using an optical lithograph. The recorded structure is composed of elementary diffraction gratings and is completely calculated using a computer. Benefits and specific problems of both methods are discussed.

Device for synthetizing computer-generated holograms

According to the needs of creating holograms of 3D computer models, an original device for synthetizing non focused holograms has been designed and manufactured. The device composes holographic masters by projecting series of 2D views of a subject. These views are displayed by a SLM and projected through a special designed objective. The hologram is then subsequently recorded near the plane of the output pupil of the writing objective. The 3D holographic stereogram is then made by the H1-H2 copying. The main goal of the device is to create rainbow single parallax hologram masters. However, the device is able to write the holo-pixels in x-y directions, which allows us to use both parallaxes. This is used for the research in the field of RGB rainbow masters, reflection hologram masters and implementation of kinetics to the final stereogram. In this paper, the method and the design of the recording device is presented.

Design of a Device for Synthetizing RGB Color Rainbow Holograms

According to the needs of creating RGB color rainbow holograms of 3-D computer models, an original device for synthetizing non focused holograms has been designed. The device and the principle of the method is presented.