Reflection-type holographic disk memory with random phase shift multiplexing

A reflection-type holographic disk memory system with random phase shift multiplexing is proposed. The experimental results show that a binary data page of 18x17 bits is recorded successfully at intervals of 4 mum in a Fe:LiNbO3 crystal with a thickness of 0.5 mm when six data pages are superimposed. Numerical results show that random phase modulation can improve the shift selectivity in shift multiplexing recording as well as in data security. Experimental and numerical results show that reflection-type holographic disk memory has a high potential for terabyte storage capacity as in transmission-type memory.     

Phase and light shift determination in an optically pumped cesium beam frequency standard

The determination of the phase shift and the light shift in our optically pumped Cs beam is reported here. We show that an error on the phase difference value determined by our fit method, without beam reversal, can be induced if the light shift is not taken into account. If the experimental data are corrected for the light shift, the results of the fit method are in very good agreement with the beam reversal results. This allows us to reduce the related uncertainty in the accuracy budget of our standard. The resulting overall accuracy of the standard is now estimated to be 6.3x10(-15).     

Computer simulation of reflective volume grating holographic data storage

The shift selectivity of a reflective-type spherical reference wave volume hologram is investigated using a nonparaxial numerical modeling based on a multiple-thin-layer implementation of a volume integral equation. The method can be easily parallelized on multiple computers. According to the results, the falloff of the diffraction efficiency due to the readout shift shows neither Bragg zeros nor oscillation with our parameter set. This agrees with our earlier study of smaller and transmissive holograms. Interhologram cross talk of shift-multiplexed holograms is also modeled using the same method, together with sparse modulation block coding and correlation decoding of data. Signal-to-noise ratio and raw bit error rate values are calculated.     

Optically stimulated luminescence from Al2O3:C irradiated with relativistic heavy ions.

Optically stimulated luminescence (OSL) from Al2O3:C (ALOC) irradiated with selected heavy ions (4He, 12C, 40Ar, and 56Fe) was examined for discussion on the effectiveness of ALOC for space radiation protection dosimetry. The OSL efficiency on the absorbed dose basis was almost unity for He (LETinfinity x H2O: 2.2 keV x microm(-1)) and decreased with increasing LET for C (14 keV x microm(-1)), Ar (91 keV x microm(-1)), and Fe (198 keV x microm(-1)); a notable reduction greater than 60%, was observed for Fe ions. The linearity in dose response and the angular independence for the heavy ions were fairly good (+/- <15%) Although further experimental studies are clearly necessary, these results suggest that small ALOC chips can be a part of an integrating dosimetry system in future space missions.     

Selectivity and tolerance calculations with half-cone-shaped reference beam in volume holographic storage

When developing a compact holographic storage system it is beneficial to use a reflection-type arrangement, where the entire optical system is on the same side of the storage material. For reflection type holographic discs, it is important to use half-cone-shaped spherical reference beams to avoid the ghost images caused by phase conjugate readout. The goal of this paper is to look for appropriate engineering tools to model diffraction efficiency of finite volume holograms created by half-cone-shaped reference beams. Two numerical methods – volume integral and beam propagation – were applied to calculate the shift selectivity curves. Simulation results show significant discrepancies between the shift selectivity curves corresponding to the approximated analytical equation and the numerically calculated shift selectivity curves; there are no Bragg zeros and there are no selective and nonselective directions. Beside the shift selectivity curves, track, focus, tilt and wavelength tolerance values are shown for finite volume holograms.     

Optimization of diffraction grating profiles in fabrication by electron-beam lithography

We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 microm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.     

Three-dimensional analysis of a hybrid photonic crystal-conventional waveguide 90 degree bend

We present a three-dimensional (3D) analysis of a hybrid photonic crystal-conventional waveguide 90 degree bend proposed previously [Opt. Express 10, 1334 (2002)] as an ultracompact component for large-scale planar lightwave circuit integration. Both rigorous 3D finite-difference time-domain modeling and a simple perfect mirror model analysis were carried out for different Si post heights in the photonic crystal region. Results show that the bend efficiency increases rapidly with Si post height. For a post height of 6.5 microm, this structure yields a bend efficiency of 97.3% at a wavelength of 1.55 microm for 90 degree bends in 2 microm x 2 microm square channel conventional waveguides with a refractive index contrast of 3.55%, which is very close to the bend efficiency of 98.2% for the corresponding two-dimensional problem. Our 3D analysis permits the examination of issues such as out-of-plane scattering loss and the effects of finite Si post height that are not considered in two dimensions.     

Focal shift of focused truncated Lorentz-Gauss beam

Based on the Collins diffraction integral formula and the complex Gaussian expansion of the aperture function, an analytical expression for a Lorentz-Gauss beam focused by an optical system with a thin lens and a circular aperture has been derived. The focal shift of the focused truncated Lorentz-Gauss beam is investigated with numerical examples, and the dependence of the focal shift on the different parameters of the focused truncated Lorentz-Gauss beam is discussed in detail. This research is useful to the applications of highly divergent laser beams.     

Zone-boundary optimization for direct laser writing of continuous-relief diffractive optical elements

Enhancing the diffraction efficiency of continuous-relief diffractive optical elements fabricated by direct laser writing is discussed. A new method of zone-boundary optimization is proposed to correct exposure data only in narrow areas along the boundaries of diffractive zones. The optimization decreases the loss of diffraction efficiency related to convolution of a desired phase profile with a writing-beam intensity distribution. A simplified stepped transition function that describes optimized exposure data near zone boundaries can be made universal for a wide range of zone periods. The approach permits a similar increase in the diffraction efficiency as an individual-pixel optimization but with fewer computation efforts. Computer simulations demonstrated that the zone-boundary optimization for a 6 microm period grating increases the efficiency by 7% and 14.5% for 0.6 microm and 1.65 microm writing-spot diameters, respectively. The diffraction efficiency of as much as 65%-90% for 4-10 microm zone periods was obtained experimentally with this method.     

The contribution of attention in virtual moped riding training of teenagers

Riding a moped, like many other everyday activities, is a complex behavior in which attention plays a crucial role. This study aims to investigate the role of attention in enhancing the skills required to ride a moped simulator. Two experiments were conducted with 207 and 60 students (14–15 years old), respectively, using a moped simulator to ride on 12 different tracks. The assignment was to ride safely and avoid hazards. In experiment 1, we divided the hazard scenes of the tracks on the basis of the fact that a shift in attention was required to escape the danger. We showed that during the riding training, when no attentional shift was required, the ability to avoid hazards was constantly higher. In experiment 2, participants were asked to cope with the same basic experimental setting but with an additional attentive task. The results showed that they performed in such a way that not only did the attentive task not impair their performance, but it also produced an improvement in the ability to shift attentional focus, preserving performance efficiency. On the basis of these data, it can be claimed that, primarily, attentional shift plays a prominent role in accounting for accident circumstances. Secondarily, it can be claimed that attentional training contributes to improved processing efficiency so as to prevent mishaps.     

Fabrication of x-ray zone plates by surface-plasma chemical vapor deposition

A new cost-efficient sputter-slice technology for hard x-ray (10-30 keV) Fresnel zone plates fabrication, imposing no limitation to aspect ratio, is proposed. By means of a plasma chemical process, SiO(2)/Si(1-x)Ge(x)O(2) glassy film multilayer structures are deposited on a lateral surface of a silica rod, outermost layers being as thin as 100 nm. It has been shown by numerical simulation that for x=0.2 germanium fraction, 100-300 microm zone plate thickness and the number of zones of about 1000, first order diffraction efficiency as high as 20%-30% at the energy of approximately 20 keV can be achieved.     

Effective Fresnel number and the focal shifts of focused partially coherent beams

We define the effective Fresnel number of the cylindrical lens illuminated by a plane wave or Schell-model beams. On the basis of the concept of the effective Fresnel number, the focusing properties of the cylindrical lens illuminated by the Schell-model beam are investigated in a simple way. It is shown that the relative focal shift can be evaluated by an analytical formulation, which is expressed as a function of the effective Fresnel number. To evaluate our approach, we make the comparison between the results obtained by our method and the numerical calculation based on the diffraction integral. The results indicate that we can simply and exactly evaluate the focal shifts with our method.     

Microwave irradiation assisted rapid synthesis of Fe-Ru bimetallic nanoparticles and their catalytic properties in water-gas shift reaction

Fe-Ru bimetallic nanoparticles were prepared by a microwave irradiation assisted glycol reduction method using poly-N-vinyl-2-pyrrolidone (PVP) as protective agent. The structure and morphology of the nanoparticles were characterized by X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDXA) and high-resolution transmission electron microscopy (HRTEM). EDXA and XRD analysis confirmed the presence of Fe and Ru. The bimetallic nanoparticles were subsequently loaded onto an MgAl₂O₄ supporter with K₂O as promoters and used as catalyst for water-gas shift reaction. The results indicated that the FeRu bimetallic nanoparticles exhibit high catalytic activity for water-gas shift reaction due to the synergistic effect between iron and ruthenium. Potassium oxide can enhance the CO selectivity of the catalyst significantly besides increasing the catalyst activity.     

Three-dimensional shape measurement beyond the diffraction limit of lens using speckle interferometry

Speckle interferometry is generally known as a method for measuring the deformation of an object with rough surfaces. In this paper, a three-dimensional (3D) shape measurement method is proposed for superfine structures beyond the diffraction limit using the basic property of speckle interferometry. Since the differential coefficient distribution of the shape of such an object can be detected in speckle interferometry by imparting a known lateral shift to the measured object, the shape can be reconstructed by integrating the differential coefficient distribution. Based on experimental results obtained using diffraction gratings as measured objects, it is confirmed that the proposed method can measure 3D shapes that are beyond the diffraction limit of the lens.     

Characterization of gradient-index lens-fiber spacing toward applications in two-photon fluorescence endoscopy

We report on the experimental investigation into the characterization of two-photon fluorescence microscopy based on the separation distance of a single-mode optical fiber coupler and a gradient-index (GRIN) rod lens. The collected two-photon fluorescence signal exhibits a maximum intensity at a defined separation distance (gap length) where the increasing effective excitation numerical aperture is balanced by the decreasing confocal emission collection. A maximum signal is found at gap lengths of approximately 2, 1.25, and 1.75 mm for GRIN lenses with pitches of 0.23, 0.25, and 0.29 wavelength at 830 nm. The maximum two-photon fluorescence signal collected corresponds to a threefold reduction of axial resolution (38.5 microm at 1.25 mm), compared with the maximum resolution (11.6 microm at 5.5 mm), as shown by the three-dimensional imaging of 10 microm beads. These results demonstrate an intrinsic trade-off between signal collection and axial resolution.     

Tunable 6.8 W narrow bandwidth emission from a single-stripe continuous-wave broad-area laser diode in a simple external cavity

An antireflection-coated broad-area laser diode with an emitter size of 400 microm x 1 microm and a chip length of 1500 microm is operated in a simple external cavity. For wavelength stabilization and to narrow the bandwidth a diffraction grating in a Littrow configuration is used. At an injection current of 9 A up to 6.8 W of optical output power and a resulting slope efficiency of 0.8 W/A could be achieved. Further, the bandwidth could be narrowed to 100 pm (FWHM), and a tuning range of 40 nm around 976 nm was obtained.     

Relative phase shift in laguerre-gaussian beams propagating through an apertured paraxial ABCD system

Abstract

Based on the generalized Huygens-Fresnel diffraction integral, closed-form expressions for the relative phase shift of Laguerre-Gaussian beams propagating through an apertured paraxial optical ABCD system are derived. The dependence of the relative phase shift on the beam and system parameters, and the condition for eliminating phase shift are analysed and illustrated with numerical examples.     

Adaptive optics with four laser guide stars: correction of the cone effect in large telescopes

We study the performance of an adaptive optics (AO) system with four laser guide stars (LGSs) and a natural guide star (NGS). The residual cone effect with four LGSs is obtained by a numerical simulation. This method allows the adaptive optics system to be extended toward the visible part of the spectrum without tomographic reconstruction of three-dimensional atmospheric perturbations, resolving the cone effect in the visible. Diffraction-limited images are obtained with 17-arc ms precision in median atmospheric conditions at wavelengths longer than 600 nm. The gain achievable with such a system operated on an existing AO system is studied. For comparison, performance in terms of achievable Strehl ratio is also computed for a reasonable system composed of a 40 x 40 Shack-Hartmann wave-front sensor optimized for the I band. Typical errors of a NGS wave front are computed by use of analytical formulas. With the NGS errors and the cone effect, the Strehl ratio can reach 0.45 at 1.25 microm under good-seeing conditions with the Nasmyth Adaptive Optics System (NAOS; a 14 x 14 subpupil wave-front sensor) at the Very Large Telescope and 0.8 with a 40 x 40 Shack-Hartmann wave-front sensor.