Ferenc Ujhelyi

Physics and technology of optical storage in polymer thin films

Abstract We discuss different strategies for optical storage of information in polymeric films. An outline of the existing trends is given. The synthesis and characterization of side-chain azobenzene polyester films for holographic storage of information is described. A compact holographic memory card system based on polarization holography is described. A storage density of greater than 10MB/cm 2 has been achieved so far, with a potential increase to 100MB/cm 2 using multiplexing techniques and software correction. Finally the role of surface relief in azobenzene polymers on irradiation with polarized light is mentioned.

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter.

We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.

Modeling and Optimization of Scintillator Arrays for PET Detectors

A new Zemax model has been developed, which reliably predicts the light output of scintillator crystal pins for positron emission tomography. Different configurations, including pin shape, surface finish and reflector types are investigated. The simulations use the actual measured wavelength-dependent parameters of the scintillator crystal, of the reflector and of the PMTs components. Good agreement is demonstrated between simulated and measured light output figures for 33 different pin configurations. The highest light output was found by both simulation and experiment for the pins with polished superficies (either polished or diffuse ¿-side) with a non-coupled high reflectivity specular reflector around the superficies and an optically coupled high reflectivity diffuse reflector on the ¿ -side. We also present the measured light output homogeneity of a scintillator array fabricated according to one of the best pin configurations.

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.

Modeling material saturation effects in microholographic recording

Microholographic data storage system model is presented that includes non-linear and non-local behavior of the storage material for accurate simulation of the system and optimization of the writing process. For the description of the photopolymer material a diffusion based nonlocal material model is used. The diffusion equation is solved numerically and the modulation of the dielectric constant is calculated. Diffraction efficiency of simulated microholograms and measurements were compared, and they show good agreement.

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.

SPADnet: A fully digital, networked approach to MRI compatible PET systems based on deep-submicron CMOS technology

This paper is the first comprehensive presentation of the SPADnet concept. SPADnet is a fully digital, networked MRI compatible time-of-flight PET system, exploiting the speed and integration density of deep-submicron CMOS technologies. The core enabling technologies of SPADnet are a sensor device comprising an array of 8×16 pixels, each composed of 4 mini-SiPMs with in situ time-to-digital conversion, a multi-ring network to filter, carry, and process data produced by the sensor devices at 2Gbps, and a 130nm CMOS process enabling mass-production of photonic modules that are optically interfaced to scintillator crystals. The SPADnet photonic modules comprise a matrix of tightly packed sensor devices; each module is networked in multiple rings, where coincidence pairs are identified and readily used in reconstruction algorithms, enabling scalable, MRI compatible pre-clinical PET systems for multi-modal imaging.

First characterization of the SPADnet sensor: a digital silicon photomultiplier for PET applications

Silicon Photomultipliers have the ability to replace photomultiplier tubes when used as light sensors in scintillation gamma-ray detectors. Their timing properties, compactness, and magnetic field compatibility make them interesting for use in Time-of-Flight Magnetic Resonance Imaging compatible Positron Emission Tomography. In this paper, we present a new fully digital Single Photon Avalanche Diode (SPAD) based detector fabricated in CMOS image sensor technol- ogy. It contains 16x8 pixels with a pitch of 610x571.2 mm 2 .

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.

Evaluation of light extraction from PET detector modules using gamma equivalent UV excitation

Monolithic scintillator-based PET detector modules offer depth of interaction capability and lower price relative to conventional pixelated crystal detectors. However, making such constructions poses several problems that affect light extraction efficiency and accuracy of position determination. In this work we examine potential detector geometries proposed to solve these problems. The geometries were tested by our novel measurement method that utilizes point-like excitation of the scintillator. With the applied methods we successfully increased the light extraction with a factor of two relative to a conventional monolithic PET detector. The measurements were compared to results of complete optical CAD models of the investigated arrangements. We concluded that our optical CAD tool can be used to further optimize the current constructions.