Judit Reményi

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.

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.

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.

Phase modulation configuration of a liquid crystal display

Phase-mostly modulation mode of a Sony LCX016AL-6 liquid crystal display has been found for appropriate elliptical polarization states of the input and output light with calculation using Jones matrices. Phase delay measurement with an accuracy of 2° were performed using a simple diffractive technique at different gray levels. Our measured results agree well with the theoretical calculation with Jones matrices. Both gave about 10% intensity variation and more than 200° phase modulation in the whole range.

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.

True time delay line for short pulses based on optical path-length dispersion: experimental proof of functioning

A true time optical delay line for short pulses by means of acousto-optic signal processing is presented. A theoretical description of the heterodyning delay line based on spatial Fourier decomposition and differential optical phase shift of the pulse frequency components is given. 2 /spl mu/s time delay of 0.5 /spl mu/s long pulses with maximum optical phase shift of 1.2 /spl pi/ was experimentally realised. Implementation of the delay line for an array with more than 10000 antenna elements, for more than 50% fractional bandwidth is theoretically examined, and a compact optical system with two-dimensional acousto-optic deflector and LCD SLM is proposed.

Holographic data storage using phase-only data pages

The application of phase-only input data pages has several advantages with respect to conventional amplitude modulated holographic storage: It avoids the saturation of the storage material by providing a smooth Fourier plane, improves the response in associative read-out, increases the light efficiency of the recording object wave and provides the opportunity of data encryption. However, if the information is carried by the phase of object wave front recovery of the data from the reconstructed beam is problematic with simple intensity sensitive devices as a CCD camera. To solve this problem we propose a compact phase to amplitude data page conversion method and apply it to the output of a Fourier holographic data storage system. The phase to amplitude conversion uses a birefringent crystal to generate two equal intensity copies of the reconstructed data page that are geometrically shifted by an integer number of pixels with respect to each other each other. The interference of these two phase modulated images is projected on the detector field of the camera. The interference pattern contains low and high intensity pixels if the phases of the interfering pixels are opposite and identical respectively. Using proper data coding, the original data matrix recovered from the intensity pattern of the CCD. Fourier plane homogeneity, bit error rate and positioning tolerances of the proposed holographic storage method are investigated by computer modeling and a comparison is provided with amplitude modulated data pages.

Application of a phase-SLM and low-pass Fourier filtering to generate spatial patterns simultaneously modulated in phase and amplitude

Holographic data storage systems convert several bits of information into data pages, and record them in the form of holograms. The data to be stored is usually displayed as a binary pattern by an amplitude modulation spatial light modulator (SLM). For practical reasons, the Fourier transform of the SLM pattern is recorded in the hologram in most cases. In order to avoid the unfavourable effects of high intensity peaks, characteristic to such Fourier transforms, the SLM pattern should also be modulated in phase. We studied the behaviour of the method by simulations, and verified its operation experimentally.

Optical encryption and encrypted holographic storage using phase- only data pages

We propose a simple and robust method for the recovery of phase data pages. We provide experimental proof of the concept and investigate its applicability to optical encryption and encrypted holographic storage. Finally we discuss a possible compact optical implementation of the method.

Variable true time delay realized with acousto-optics and liquid crystal spatial light modulator

Optical heterodyne acousto-optic delay line applying frequency dependent phase shift provided by liquid crystal spatial light modulator is presented. We experimentally demonstrate continuously variable true time delay of 1 /spl mu/s long microwave pulses between /spl plusmn/300 ns.