Svetomir Stankovic

Generation of doughnut laser beams by use of a liquid-crystal cell with a conversion efficiency near 100%

We present a novel technique for producing a doughnut laser beam by use of a liquid-crystal cell. It is demonstrated that the liquid-crystal cell exhibits an efficiency in energy conversion near 100%. One of the main advantages of this method is its capability of dynamic switching between a Gaussian mode and a doughnut mode of different topological charges. The liquid-crystal cell is also dynamically tunable over the visible and near-infrared wavelength range. These advantages make the device appealing for laser trapping methods used in single-molecule biomechanics and for optical guiding of cold atoms.

Fast switching liquid crystal lenses for a dual focus digital versatile disc pickup

We investigated two types of liquid crystal (LC) lenses, the convex LC lens and the adaptive LC lens. Both lens types were designed and fabricated for integration in a novel pickup head for dual layer digital versatile discs (DVDs). The polarization dependent focal length of LC lenses was used to read both DVD information layers simultaneously. For fast and near diffraction limited performance, a new circular electrode structure with 64 ring electrodes was studied. Short response times of 50 ms for blazed Fresnel lens profiles and less than 10 ms for binary lens profiles could be realized using the transient nematic effect. Both lens types were characterized in terms of diffraction limited operation, focus efficiency and switching time.

Analytic phase-factor equations for Talbot array illuminations.

Under specific circumstances the fractional Talbot effect can be described by simplified equations. We have obtained simplified analytic phase-factor equations to describe the relation between the pure-phase factors and their fractional Talbot distances behind a binary amplitude grating with an opening ratio of (1/M). We explain how these simple equations are obtained from the regularly rearranged neighboring phase differences. We point out that any intensity distribution with an irreducible opening ratio (M(N)/M) (M(N) < M, where M(N) and M are positive integers) generated by such an amplitude grating can be described by similar phase-factor equations. It is interesting to note that an amplitude grating with additional arbitrary phase modulation can also generate pure-phase distributions at the fractional Talbot distance. We have applied these analytic phase-factor equations to neighboring (0, pi) phase-modulated amplitude gratings and have analytically derived a new set of simple phase-factor equations for Talbot array illumination in this case. Experimental verification of our theoretical results is given.

Phase codes of Talbot array illumination for encoding holographic multiplexing storage

Abstract Phase codes of Talbot array illumination, originally derived from Talbot effect for array illumination at fractional Talbot distance, are now suggested as a set of orthogonal phase codes for phase-encoded holographic multiplexing storage in volume-holographic media. The most attractive feature of the Talbot encoding method is that an arbitrary number of images can be phase-encoded while the previous Hadamard encoding methods can only encode the number of stored images, M=2n, and/or M=4n (with n a positive integer). Thus the Talbot encoding method is highly interesting to take full use of the available pixel numbers of spatial light modulators and to achieve the maximum possible storage capacity of a phase-code multiplexed holographic memory.

Effective spherical aberration compensation by use of a nematic liquid-crystal device

The next generation of optical data storage system beyond DVDs will use blue laser light and an objective lens with a high numerical aperture of 0.85 to increase storage capacity. Such high numerical aperture systems have an inherent higher sensitivity to aberrations. In particular, the spherical aberration caused by cover layer thickness tolerances and--more obvious--by dual-layer disks with a typical separation of approximately 20 microm between the two layers must be compensated. We propose a novel transmissive nematic liquid-crystal device, which is capable of compensating spherical aberration that occurs during the operation of optical pickup systems.

High-frequency gratings for applications to DVD pickup systems

Binary gratings with feature sizes smaller than the illumination wavelength were designed, fabricated and experimentally tested for operation in transmission at 650 nm. The elements were designed for integration in DVD pickup systems. We used rigorous coupled wave analysis for the design of diffractive optical elements such as polarizing beam splitters (PBSs), phase retardation plates, deflection gratings and beam splitter gratings. Electron-beam-lithography and reactive ion etching techniques were employed to produce the PBSs and the phase retardation plates in quartz glass. For the PBS elements diffraction efficiencies of higher than 75% were measured for TE and TM polarization. Furthermore a phase retardation plate, which showed a phase difference of 44.8° (theoretical, 45°) between TE and TM polarized light, was realized.

High-frequency gratings as polarization elements

Binary gratings with feature sizes smaller than the illumination wavelength were fabricated in quartz glass by means of microstructuring techniques. Using rigorous coupled wave analysis polarization elements like polarizing beam splitters and phase retardation plates were designed for operation in transmission at the wavelength of 650 nm. High frequency polarization gratings with feature sizes down to 140 nm and aspect ratios up to 7 were realized. For the polarization selective beam splitting elements we measured diffraction efficiencies of about 80% in the -1st order for TE polarization, and 90% in the 0th order for TM polarization. The values are in good agreement with the theoretical values. Furthermore we realized phase retarding elements e.g. (lambda) /8-plates which showed a phase difference of (Phi) equals 44.8 degree(s) ((Phi) theor. equals 45 degree(s)) between TE and TM polarized light. The design and the fabrication process as well as the optical properties of our high frequency binary phase gratings will be presented. Experimental results will be compared with theoretical values.

Integrated optical pickup for optical disks

We report on a new integrated optical pickup for double layer DVDs. The optics is almost integrated by means of diffractive optical elements. Dual focus as well as focal control is done by a liquid crystal cell.

Fast electro-optical lenses using liquid crystals

Summary form only. In many optical designs, movable optics are used to modulate, focus or redirect light fields. Solutions that allow more compact and shock resistant designs are needed. We present two electro-optical liquid crystal lenses which satisfy these requirements when focus tracking without any mechanical components.

Dual layer digital versatile disk pickup integrating opto-electronic and diffractive elements

Summary form only. A pickup using an optoelectronic, nematic LC lens and two diffractive optical elements (DOEs) was designed and a demonstrator model built. It was successfully demonstrated that the LC lens could focus onto both layers of a dual layer DVD. Focus switching of below 10ms were achieved. The working principle and the fabrication methods of the DOEs and the design and working of the LC lens are described. Measurements of the integrated pickup using the astigmatism method are presented.