Gabor Szarvas

Ternary phase-amplitude modulation with twisted nematic liquid crystal displays for Fourier-plane light homogenization in holographic data storage

Summary Holographic data storage applications often use liquid crystal displays as spatial light-amplitude modulators for writing data images. The hologram created in the Fourier plane is usually applied to store the information, since this plane supplies optimal data density. A well known technique for homogenizing the light distribution in the Fourier plane is the application of external random phase modulating masks. The requirement for pixel by pixel matched positioning of the phase modulating mask and the pixels of the spatial light-amplitude modulator is hard to solve in the optical systems and any positioning error leads to significant signal degradations. The article analyses the possibilities of realizing the required simultaneous amplitude and phase modulation of light with the application of a single LCD. Twisted nematic LCDs with different maximal birefringence are numerically investigated using the Jones matrix method. Elliptical incident and exit polarizations are proposed, by which ternary phase-amplitude modulation (+1,–1,0) can be realized. Test measurements are also presented that demonstrate the validity of the calculated results.

Polarization holographic data storage using azobenzene polyster as storage material

Polarization holographic read/write and read only demonstrator systems have been developed using ~2 µm thick azobenzene polyester on a card form media. The thin-film holographic system has practical advantages, e.g. high diffraction efficiency, no cross talk between the holograms, reading in reflection mode, no hardware servo, different wavelengths for writing and reading (non-volatile storage), data encryption possibility, no problem with material shrinkage, etc. The candidate azobenzene polyester has good thermal, room temperature and ambient light stability and good optical properties for the purpose of thin film application. Using thin-film holography the possibilities of multiplexing are limited, however, raw data density as high as 2.77 bit/µm2 has been achieved in an optimized Fourier holographic system using high numerical aperture (NA³ 0.74) objective in a 8f arrangement with sparse code modulation and Fourier-filtering at 532 nm. High density polarization holographic demonstrator systems have been developed using ~2μm thick azobenzene polyesters on reflective card form media. FFT computer simulation of the system including saturation model of the material allows optimization of system components including data density and capacity. A raw density as high as 2.77 bit/μm2 has been achieved without multiplexing in a compact, portable read/write sytem at 532 nm allowing more than 1000 readout without data loss. A separate read only system working at 635 nm realizes non-volatile readout and allows card exchange at a data density of 1.3 bit/μm2. Security level of the presents holographic optical card systems can be further increased by using phase encoded reference beam. Advantageous applications of the proposed encrypted holographic card system are also outlined.

Fourier description of the propagation and focusing of an extraordinary beam in a planar uniaxial medium.

The scalar-angular-spectrum approach is used to examine a focused extraordinary wave propagating along an arbitrary direction in a homogeneous uniaxial planar medium, and the Fraunhofer diffraction formula is generalized for this case. The size of the focal spot is found to be inversely proportional to an effective index, depending on the refractive indices and the propagation direction. The validity of the paraxial model is checked by nonparaxial (but scalar) numerical calculations. They show that the paraxial formulas predict the spot size correctly, but if the beam propagates neither parallel with nor perpendicular to the optic axis, they do not reproduce the symmetries of the amplitude distribution in the focal line.

Tolerancing surface accuracy of aspheric lenses used for imaging purposes

Tolerancing aspheres and preparing the corresponding drawing indications significantly differ from techniques used at spherical lenses due mainly to surface waviness, an error caused by most asphere fabrication technologies. Standard (ISO) regulations proved to be adequate for several kinds of aspheric lenses (e.g. laser focusing/collimation) made by the traditional diamond turning method, but sometimes are not general enough for recent fabrication techniques (such as CNC polishing of glass aspheres), and today’s more demanding lenses (eyepieces, Fourier objectives, relays etc.). A new, generalized tolerancing technique has been developed to accurately constrain surface waviness, quite independently of fabrication technology, and to provide easy verification of the results. Operation of the method is demonstrated on a Fourier-type objective comprising a glass aspheric lens, by computer simulation and testing of the fabricated prototypes.

Rewritable holographic memory card system

The new rewritable holographic memory card (HMC) of Optilink provides a novel solution for high density optical storage of personal data. In contrast to most holographic storage systems using highly multiplexed transmissive volume holograms in bulk materials, our approach is adapted to the card format using a polymer thin film holographic storage material, operating in reflection mode, allowing writing and reading to be accomplished from the same side of the card with a small optical head. This allows card drives with dimensions equal or smaller to common PC peripherals (e.g. CD drive). Such card can be potentially used wherever custom parameters of conventional cards (credit cards with magnetic strip, smart cards, security cards or LaserCards); i.e. storage capacity, reading and writing speed or data security are not adequate to the applications.

Optical system of holographic memory card writing/reading equipment

We developed a standard credit card-shaped general-purpose data carrier, a reflective Holographic Memory Card (HMC), and the appropriate equipment for its handling. Data recording and retrieval are accomplished by polarisation Fourier holography using a thin layer of photo-anisotropic polymer as the storage material. The data density is about 1 bit/micrometers 2, the maximum storage capacity of the card is around 10 Mbytes assuming a 10 x 10 mm storage area. Data is stored in the form of microholograms, from which 40x40 pieces are recorded on the HMC. The optical system involved performs data writing/reading/erasing and also locates the position of the microholograms. Main components of the optical system are an SLM and CCD for opto-electronic conversion, a frequency-doubled solid-state laser source, a beam shaping system that provides homogeneous illumination of the SLM, an interferometer for hologram construction, special Fourier transforming objectives and a random-phase mask for optimised hologram recording. Our results include conceptual planning, design, fabrication and assembling of the optical system. In the present paper we describe principle of operation including layout of the elements, and explain the operation of the equipment in detail.

Optimization of the storage density in thin polarization holograms

We are developing a holographic memory card drive using thin polymer storage layer on credit card sized plastic carriers. The main features of the card are high storage density, re- writability and resistance to harsh environment (e.g. electromagnetic noise, mechanical intolerance, high temperature and humidity). The optical system of this device uses Fourier holograms to record information. The present communication deals with the optimization of the storage density by computer modeling of the system. The model is based on fast Fourier transformations, taking the parameters of the optical processes into account. The model was used to assess the performance of various parameter sets. The results are tested experimentally. The work suggests that storage density higher than 1 bit/micrometers 2 may be achieved in the thin holograms of this memory card at raw bit error rate values below 103.

Design of high-numerical-aperture Fourier objectives for holographic memory card writing/reading equipment

A pair of special Fourier transforming objectives intended for use in a Holographic Memory Card (HMC) writing/reading equipment have been designed and fabricated. At writing in, the objective Fourier transform a binary pattern, representing the data displayed by an SLM, into the storage medium of the HMC, where the Fourier transform is recorded as a polarization hologram. At reading out, the objectives inverse Fourier transform the reconstructed hologram onto the surface of a CCD array. The Fourier space NA of the objectives is high enough to achieve a theoretical data density of 1 bit/μm2. For comparison reasons we designed two optically identical objectives of basically different structures: one is an aspheric glass doublet, the other is an all-spherical five-element system (arranged in two lens groups). Computer analysis of the objectives shows that both systems are diffraction limited in object and Fourier space and have a distortion of less than 1%. In this paper we overview the theory of Fourier objectives, present our design method, describe the optical behavior of the designed systems, show our test results performed on the fabricated aspheric objective and present our experiences at manufacturing aspheric glass lens prototypes.

Single-element refractive optical device for laser beam profiling

A simple, single-element, afocal, refractive optical device with two aspheric surfaces has been designed and fabricated for transformation the Gaussian intensity profile of a He-Ne laser into a collimated beam of uniform profile. The working principle, the method of design, the method of fabrication are presented. Optical and geometrical properties of the fabricated sample have been tested. Device parameters and simulated behavior are compared with test results in detail.

Selectivity and tolerance calculations with half-cone-shaped reference beam in volume holographic storage

When developing a compact holographic storage system it is beneficial to use a reflection-type arrangement, where the entire optical system is on the same side of the storage material. For reflection type holographic discs, it is important to use half-cone-shaped spherical reference beams to avoid the ghost images caused by phase conjugate readout. The goal of this paper is to look for appropriate engineering tools to model diffraction efficiency of finite volume holograms created by half-cone-shaped reference beams. Two numerical methods – volume integral and beam propagation – were applied to calculate the shift selectivity curves. Simulation results show significant discrepancies between the shift selectivity curves corresponding to the approximated analytical equation and the numerically calculated shift selectivity curves; there are no Bragg zeros and there are no selective and nonselective directions. Beside the shift selectivity curves, track, focus, tilt and wavelength tolerance values are shown for finite volume holograms.