Steffen Noehte

Computer-generated stratified diffractive optical elements

We present what is to our knowledge a new type of diffractive optical element (DOE), the computer-generated stratified diffractive optical element (SDOE), a hybridization of thin computer-generated DOEs and volume holograms. A model and several algorithms for calculating computer-generated SDOEs are given. Simulations and experimental results are presented that exhibit the properties of computer-generated SDOEs: the strong angular and wavelength selectivity of SDOEs makes it possible to store multiple pages in a computer-generated SDOE, which can be read out separately (multiplexing). The reconstruction of an optimized SDOE has a higher quality than the reconstruction of optimized one-layer DOEs. SDOEs can be calculated to have only one diffraction order.

Gradual and Random Binarization of Gray Scale Holograms

A new method called gradual and random binarization to binarize gray-scale holograms, based on an iterative algorithm, is proposed. The binarization process is performed gradually, and the pixels to be binarized are chosen randomly. Errors caused by this operation are spatially diffused. A comparison with other established methods based on error diffusion, direct binary search, and iterative stepwise quantization shows that the gradual and random binarization method achieves a very good compromise between computational complexity and reconstruction quality. Optical reconstructions are presented.

Vibration non-sensitive lithographic system for writing individualized holograms for data storage and security applications

A vibration non-sensitive lithographic system for writing individual computer-generated micro-holograms into a polymer material for data storage and security application is described. The robust lithograph is the central element of the data storage system consisting of the calculation of a computer-generated micro-hologram (CGH), the writing process and the retrieval of the data stored. The writing process is based on the pattern of a CGH, which projects a data pattern with an optimized signal-to-noise ratio. The hologram, which has the size of one square millimeter, consists of one million dots with a diameter of one micrometer each. In less than one second the laser scanning lithograph transfers the calculated hologram into a polyethelen perephtalate foil (PET) using a high-power single-mode laser diode. A thermal process locally converts the semi-crystalline structure of the foil into an amorphous structure. This leads to a change of the refractive index of more than 0.1 without the need of a post-processing5. Because of the diffractive nature of the introduced optical pattern a relative position accuracy of better than 100 nm has to be accomplished. A high-speed differential optical position detection system guarantees the required position accuracy even in a vibration-afflicted industrial environment. A position deviation of the writing spot caused by vibration is detected by a close correlation between the laser spot in the storage material and a second scanning laser spot on top of a grid mask. The position information is combined with the individual hologram pattern by a fast free-programmable gate-array (FPGA) processor, which again controls the laser diode. The reaction time of 26 ns ensures an interference compensation of up to one MHz. The quality of the reconstructed micro-hologram allows the retrieval of up to one kByte of machine-readable information.

Limit Cycles of a Planar Vector Field

This paper presents a solution to the problem to find isolated closed trajectories of two-dimensional dynamic systems. In contrast to the method of Bendixson’s ring regions, the new method is constructive. It allows the determination of the location of closed trajectories and therefore gives an upper bound for their number. The method is based on the idea to use inherent geometrical and physical extremal properties of these trajectories to transform the problem into an optimization task (isoperimetric problem of variational calculus) that can be solved by numerical algorithms, e.g., by hillclimbing.

tesa ROM and tesa WORM: multilayered optical data storage based on polymer films and using metal island films as tunable absorber system

A new optical data storage medium is introduced. Its cylindrical shape and multi-layer storage allows a storage capacity of ~ 3.5 GByte at a radius and a height of 15 mm. Metal island films are used as a tunable absorber system for multi-layer storage. The proposed absorber system provides high reflection and high absorption for each layer during the writing process. For the reading process the optical properties of each layer can be tuned to appropriate values. Results of a five-layered ROM as well as concept and first results of a five-layered WORM are given.

A new cylindrically shaped optical data storage medium based on adhesive tape-concept and first results

Many efforts have successfully been made in recent years to increase the storage capacities of optical discs from CD/DVD to HD-DVD using blue laser diodes. At the same time many consumer products became smaller and consequently small optical and magneto-optical discs were brought to market. In our search for new materials for optical data storage, we found tesa-film, a conventional adhesive tape, to be suitable for this purpose and in conjunction with a new geometry, our concept offers big advantages in the development of geometrically small write-once memories with a capacity of 3-6 Gbyte: tesa-film is used as a multilayer recording material forming a cheap cylindrically shaped optical data storage medium.

Local adjustment of the optical properties of metal island films by means of a laser

Metal island films are widely-used as reflection layers in optics and optical data storage. Their optical properties are dominated by the effect of plasmon resonance and, thus, by size and shape of the metal islands. By altering size and shape of these islands, the optical properties of the metal films can be adjusted. Depending on the wavelength range and type of metal, the film reflectance can be either increased or decreased. It is shown that changes in size and shape can be caused by means of a laser. This allows a well-defined adjustment of the optical properties of thin metal films. The optical properties can be adjusted locally. This technique can be used in different fields such as holography, adaptive optics or multi-layer optical data storage.

Weight Discretization due to Optical Constraints and Its Influence on the Generalization Abilities of a Simple Perceptron

The optical implementation of neural networks can be realized by storing the weights in holograms with a limited number of gray values. Motivated by this fact, we focused our investigation in this thesis on analyzing the dependence of the generalization and training errors of a simple perceptron with discrete weights, on the training set size, and on the number of allowed discrete values.