Peter Varhegyi

Amplitude, phase, and hybrid ternary modulation modes of a twisted-nematic liquid-crystal display at ∼400 nm

Applicability of a commercial twisted-nematic liquid-crystal display is examined at ∼400 nm. Different modulation modes predicted by Jones-matrix calculus are experimentally tested. High contrast amplitude modulation with negligible loss, high contrast and low loss hybrid ternary modulation, and 1.5π continuous phase delay without intensity modulation and with low loss are presented. Simulation results of a 4f holographic system prove the usefulness of the high contrast for amplitude modulation, and the importance of π phase difference between high transmission white levels in a hybrid ternary modulation.

System modeling and optimization of Fourier holographic memory

A new fast-Fourier-transform-based model of a page-oriented holographic data-storage system is presented. The model accounts for essential system and storage material features (e.g. diffraction, noises, and saturation) and provides reliable results in the form of output images, histograms, or bit-error rates. The model is built on a modular basis and provides the possibility of working with different system versions, key components, and storage materials. Applications of the method are presented through examples of optimization of the data density, reference beam size at Gaussian beam illumination, and calculation of the storage mediums positioning tolerances in accordance with the results of test measurements.

A secure data storage system based on phase-encoded thin polarization holograms

A novel secure data storage architecture based on phase-encoded, thin film, Fourier-type polarization holograms is reported. Efficient data encryption and verification can be obtained by phase modulation of the reference wave introduced by a phase-type spatial light modulator that is imaged onto the hologram plane. A physical model is formulated including phase-encoding, non-linear storage medium behaviour and features of the proposed optical system. A method is proposed and implemented in the form of a custom modelling tool for generating code sets optimal in terms of code number and security level. The utility of the architecture and the achievable security level are experimentally demonstrated.

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.

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.

Holographic data storage in thin polymer films

We present our results on polarization holographic data storage in thin azobenzene side chain polymers. Two different systems have been demonstrated: a read only system with red diode laser and a read&write system with green frequency doubled solid state lasers. Error free operation have been proved at 2.77 bit/µm2 data density. We have also demonstrated enhanced security holographic storage by applying phase coded reference waves imaged onto the hololographic storage material. We also present the concept of extending the principle to multilayer holographic storage.

Read/write demonstrator of rewritable holographic memory card system

We present the improved demonstrator of our rewritable holographic memory card system. High density optical storage is realized in a non-commercial optical set-up. Fourier transformed recording is used in a polarization holographic arrangement realizing reading and writing from the same side of the data carrier which is a modified plastic card. Holograms containing binary information of 300 x 220 bits are as small as 0.0484 square mm. The storage layer is amorphous polyester providing repeated writing and erasure cycles and thousandfold readouts without loss of information. Alternate read only system providing non-volatile storage can be realized using 635 nm laser diode.

Data storage on holographic memory card

Our goal is to develop a re-writable holographic memory card system based on thin film polymer media on credit card size plastic carriers. Data is stored in our system in form of polarization holograms that present high efficiency and excellent suppression of higher orders even for thin material. Data is written on the card in a parallel way using spatial light modulators to encode the object beam that is Fourier transformed by a custom objective lens and interferes with the reference beam (of orthogonal polarization) on the card. We use reflective carrier in order to read out the data from the same side of the card. This allows us to have a compact system and standard ID 1 type carrier card. The optical system and the data organization are optimized to have a data density higher than 1bit/micrometers 2. We expect to pass the limit of 10 bit/micrometers 2 with the introduction of phase coded multiplexing that would provide more than 2Gbyte capacity if using half the card area as active surface.

Multilayer thin-film holographic storage-a new approach

Optilink has developed and presented a polarization holographic reading writing system using 1-2 /spl mu/m thick storage layer on a card form media. A storage density of 1.3 bit//spl mu/m/sup 2/ has been achieved without multiplexing with sparse code modulation and Fourier-filtering. Because of the use of a thin storage layer, the volume data density is extremely high in the system. Further enhancement of the areal density up to /spl sim/100 bit//spl mu/m/sup 2/ can be achieved in an improved new polarization holographic system using a multilayer media instead of the thin film. A 6f system for reading with a suitable second spatial filter allows separation of the signal from different layers. This technique combines the advantage of the thin film holography with the idea of the confocal filtering.