Gabor Erdei

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.

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.

Telecentric/inverse-telecentric objective for optical data storage purposes

In most optical storage methods data bits are stored in the form of microscopic pixels on the surface of an appropriate storage material. Some currently developed techniques apply parallel data processing by multiple data bit access simultaneously. Such methods require special imaging systems for data recording and retrieval. In our laboratory a page- organized optical memory card reading/writing equipment is under development. According to the basic principle 256 by 256 data bits are processed at the same time, the corresponding pixels are arranged in a 2D array format. The same objective is used to image the selected data page both at writing in and reading out. This objective performs diffraction limited imaging in an extended field, it has low distortion, and it images each pixel of the same value with the same intensity. To achieve all these specifications a telecentric/inverse telecentric imaging system (a special type of afocal systems) offered a suitable solution. This paper describes the advantages of telecentric/inverse telecentric systems in optical imaging by detailed presentation of our objective. The discussion includes specification and design process of the objective together with our test results performed on the fabricated prototype.