Subtle effects of the substrate in optical disk data storage systems

Much attention has been focused on the effects of the disk substrate in optical data storage. In particular, substrate birefringence has been studied extensively because it causes significant problems in magneto-optic systems. We investigate certain subtle effects of the substrate, such as feedback into the laser diode, in compact disk and phase-change systems. Our analysis of the compact disk system led us to discover a new technique for the rapid measurement of the substrate birefringence. We also address the question of how focusing the laser beam through the substrate will affect the depth of focus.     

Effects of substrate birefringence on focusing and tracking servo signals in magneto-optical disk data storage

Using computer simulations based on the diffraction theory of high-N.A. systems, we examine the effects of substrate birefringence on servo signals in magneto-optical disk drives. Our attention is confined to systems that use the methods of push-pull tracking and astigmatic focus-error detection. We show that the amounts of birefringence typically observed in polycarbonate substrates do not in themselves cause problems for the servo channels. However, the presence of residual aberrations in the optical path (astigmatism in particular) can have devastating effects on the magnitude of the track-error signal and the stability of the focusing servo. We show that the combination of substrate birefringence and residual beam astigmatism can either improve or deteriorate the performance of the focusing servo, depending on the orientation of the aberration relative to the principal axes of the substrate.     

Effects of multilevel phase masks on interpixel cross talk in digital holographic storage

We study the interpixel cross talk introduced to digital holographic data storage by use of a multilevel phase mask at the data-input plane. We evaluate numerically the intensity distribution at the output detector for Fourier plane hologram storage in a limited-aperture storage medium. Only the effect at an output pixel of interpixel cross talk from the four horizontal and vertical neighboring pixels is considered, permitting systematic evaluation of all possibilities. For random two-level and pseudorandom six-level phase masks, the influence of the pixel fill factor, as well as the aperture size of the storage medium, is studied. Our simulations show that, for a given aperture size, a random two-level mask is more susceptible to interpixel cross talk than is a pseudorandom six-level mask. Decreasing the pixel fill factor below 94% with a pseudorandom six-level phase mask makes it theoretically possible to have a system with no errors from interpixel cross talk if one particular 5-pixel pattern is forbidden through modulation coding. Reducing the input fill factor below 85% means that no patterns need to be excluded.     

Free-space optical shuffle implementations by use of birefringence-customized modular optics

A stacked integration technique based on polarization optics is studied for implementing shuffle-based interconnection networks with three-dimensional solid-state modules. A basic building block of the proposed scheme consists of a hole-patterned half-wave retarder for birefringence customizations and a calcite slab for subsequent beam deflections. On the basis of a cascade of such building blocks the submodules of various shuffle-family permutations can be implemented. To minimize channel cross talk, we incorporated a collimating-relaying imaging system. To help design birefringence customization, we developed algebraic formulations of folded shuffle operations using separable shuffles. Proof-of-concept experimental results, as well as system design, fabrication, and integration issues, are discussed.     

Optical anisotropy induced by torsion stresses in LiNbO3 crystals: appearance of an optical vortex

We report the results of studies of the torsion effect on the optical birefringence in LiNbO(3) crystals. We found that the twisting of those crystals causes a birefringence distribution revealing nontrivial peculiarities. In particular, they have a special point at the center of the cross section perpendicular to the torsion axis where the zero birefringence value occurs. It has also been ascertained that the surface of the spatial birefringence distribution has a conical shape, with the cone axis coinciding with the torsion axis. We revealed that an optical vortex, with a topological charge equal to unity, appears under the torsion of LiNbO(3) crystals. It has been shown that, in contrast to the q plate, both the efficiency of spin-orbital coupling and the orbital momentum of the emergent light can be operated by the torque moment.     

Multilayered optical memory with bits stored as refractive index change. III. Numerical results of a conventional multilayered optical memory

In terms of the electromagnetic theories described in Part I of our current investigations [J. Opt. Soc. Am. A24, 1776 (2007)] and in [Opt. Express 16, 2797 (2008)], the characteristics of the cross talk and the modulation contrast and the variation of the power of the readout signals with the scanning position along the track are investigated in detail by computer simulations for a conventional multilayered optical memory (CMOM), where the two cases, i.e., the storage medium being homogenous and planar stratified homogenous, are considered. Results show that the feature sizes of bits, the distances between the two adjacent tracks, and the thickness of layers have significant effects on the cross talk and the modulation contrast. The polarization of the reading light also has significant effects on the cross talk, whereas it has only slight effects on the modulation contrast. Moreover, for a CMOM, the optimal polarization of the reading light is suggested.     

Cross talk and ghost talk in a microbeam free-space optical interconnect system with vertical-cavity surface-emitting lasers, microlenses, and metal-semiconductor-metal detectors

A diffraction-based beam-propagation model is used to study optical cross talk in microbeam free-space optical interconnection (FSOI) systems. The system consists of VCSEL's, microlenses, and metal-semiconductor-metal (MSM) detectors, with the detectors modeled as amplitude gratings with low contrast ratio (based on experimental results). Different possible cross-talk sources are studied. Results show that, in an optimized system, the cross talk caused by diffractive scattering is not an issue. However, in such systems the principal reflection from a MSM detector surface creates two problems: VCSEL coupling and ghost talk. The coupling of the reflected beam into the VCSEL's may cause power oscillation (and increase the bit error rate), whereas ghost talk will limit the distance-bandwidth product of the interconnect system. This optical system is also abstracted in hspice together with the laser driver and receiver circuits to analyze ghost talk in this system. Results show that at high speed (1 Gbit/s or more) these effects negatively affect system performance.     

Polarization microscopy of magnetic domains for magneto-optical disks.

Polarization microscopes are widely used to image the magnetic domains of a magneto-optical disk and to characterize the birefringence of the disk substrate. For high-resolution imaging, unfortunately, the coupling of the polarization rotation from the Kerr signal, the effect of Fresnel's reflection coefficients, and the substrate birefringence severely deteriorate the image contrast obtained from conventional observations. Here we present the technique of differential polarization microscopy, which replaces the analyzer with a Wollaston prism, for providing better image contrast. Images of a magnetic pattern obtained with both conventional and differential methods are observed for objective lenses that have different numerical apertures and magneto-optical disks with and without a birefringent substrate. The computer simulations and experimental results show that the use of this differential method improves the image contrast and provides excellent tolerance for defects of the optical system.     

Effects of substrate birefringence and tilt on the irradiance and phase patterns of the return beam in magneto-optical disk data storage

Substrate birefringence in a magneto-optical disk system is shown to have a predictable effect on the return beam. The irradiance and phase patterns of the return beam at the exit pupil of the objective lens are calculated and experimentally verified for the cases of no substrate birefringence, birefringence aligned with the incident polarization, and birefringence aligned at 45° to the incident polarization. The irradiance at the exit pupil is also calculated (and experimentally verified) for a grooved substrate for various amounts of substrate tilt.     

Effect of tilted ellipsoid of birefringence on readout signal in magneto-optical disk data storage

In the 2× format for second-generation magneto-optical (MO) disks, there are recording tracks next to preformat pits. It has been observed that the MO signals from the tracks adjacent to the preformat pits can be affected. We present numerical modeling results on the effect of a tilted ellipsoid of substrate birefringence on the MO readout signal. Theoretical calculations show that the observed effects can be explained if a certain tilt of the ellipsoid of birefringence exists around the preformat pits.     

Evaluation of seeing-induced cross talk in tip-tilt-corrected solar polarimetry

We reanalyze the effects of atmosphere-induced image motions on the measurement of solar polarized light using a formalism developed by Lites. Our reanalysis is prompted by the advent of adaptive optics (AO) systems that reduce image motion and higher-order aberrations, by the availability of liquid crystals as modulation devices, and by the need to understand how best to design polarimeters for future telescopes such as the Advanced Technology Solar Telescope. In this first attempt to understand the major issues, we analyze the influence of residual image motion (tip-tilt) corrections of operational AO systems on the cross talk between Stokes parameters and present results for several polarization analysis schemes. Higher-order wave-front corrections are left for future research. We also restrict our discussion to the solar photosphere, which limits several important parameters of interest, using some recent magnetoconvection simulations.     

Multiple scattering in optical coherence tomography. II. Experimental and theoretical investigation of cross talk in wide-field optical coherence tomography

We present a comprehensive study of multiple-scattering effects in wide-field optical coherence tomography (OCT) realized with spatially coherent illumination. Imaging a sample made of a cleaved mirror embedded in an aqueous suspension of microspheres revealed that, despite temporal coherence gating, multiple scattering can induce significant coherent optical cross talk. The latter is a serious limitation to the method, since it prevents shot-noise-limited detection and diffraction-limited imaging in scattering samples. We investigate the dependence of cross talk on important system design parameters, as well as on some relevant sample properties. The agreement between theoretical and experimental results for the wide range of parameters investigated was very good, in both the lateral and the axial dimensions. This further confirms the validity of the model developed in our companion paper [J. Opt. Soc. Am. A 22, 1369-1379 (2005)].     

Analysis of optical channel cross talk for free-space optical interconnects in the presence of higher-order transverse modes

We analyze the effect of cross-talk noise on the performance of free-space optical interconnects (FSOIs). In addition to diffraction-caused cross talk, we consider the effect of stray-light cross-talk noise, an issue that, to the best of our knowledge, was not addressed previously. Simulations were performed on a microlens-based FSOI system using the modal composition and beam profiles experimentally extracted from a commercial vertical-cavity surface-emitting laser. We demonstrate that this cross-talk noise introduces significant degradation to interconnect performance, particularly for multitransverse-mode laser sources. A simple behavioral model is also developed that accurately approximates the cross talk noise for a range of optical sources and interconnect configurations.     

Interlayer cross talk in dual-layer read-only optical disks

Volumetric optical disks comprising multiple data layers have been proposed to multiply recording density. Owing to the presence of out-of-focus data layers, interlayer cross talk is induced in readout. An optical model was developed to study the readout process and the effect of interlayer cross talk on the readout of dual-layer read-only optical disks. Schemes to improve the readout characteristics by suppression of the interlayer cross talk were proposed. Experiments that agreed well with the simulation resulted.     

Investigation of certain diffraction effects in an optical disk

We report certain diffraction effects that are pertinent to the operation of double-layer optical recording media. For simulating cross-talk effects for double layers, the diffraction of light from the out-of-focus layer and the resulting distribution on the in-focus layer are studied by use of computer simulations. The findings are then verified qualitatively by direct measurements. We also describe a technique for analyzing (by computer simulation) the focus-error signal (FES), taking into account the cross talk between two layers, in systems that use the astigmatic method in conjunction with the double-layer disk. The results of our computer simulations of the FES give us a 10% cross-talk contribution to the original signal. The results of the FES evaluation are compared with those measured in an actual disk drive; good agreement between computation and measurement is obtained.     

Birefringence measurement in double-clad fiber lasers with large cross section

We report on birefringence measurements in double-clad fibers with large cross section, doped with neodymium and ytterbium. The experimental results for rectangular double-clad fibers are compared with those for single-clad circular neodymium-doped fibers, taking into account existing models of stress- and geometry-induced birefringence. We demonstrated that the ellipticity of an outer silica cladding has no effect on birefringence in large-area double-clad fibers. The stress-induced birefringence is shown to depend on the ratio between the diameter of an internal silica support and the linear dimensions of the rectangular outer cladding. The stress-distribution pattern is derived to prove the experimental results.     

Cross-talk reduction by correcting the subpixel position in a multiview autostereoscopic three-dimensional display based on a lenticular sheet

A method is proposed to alleviate the cross talk in multiview autostereoscopic three-dimensional displays based on a lenticular sheet. We analyze the positional relationship between subpixels on the image panel and the lenticular sheet. According to this relationship, optimal synthetic images are synthesized to minimize cross talk by correcting the positions of subpixels on the image panel. Experimental results show that the proposed method significantly reduces the cross talk of view images and improves the quality of stereoscopic images.     

Coupling and cross-talk effects in 12-15 microm diameter single-mode fiber arrays for simultaneous transmission and photon collection from scattering media

We examine signal degradation effects in fiber arrays from fiber-to-fiber coupling and from cross talk attributable to backscatter from the sample medium originating from adjacent fibers in the array. An analysis of coupling and cross talk for single-mode fibers (SMFs) operating at 1310 nm with different core diameters, interaction lengths, core center spacing, and numerical apertures (NAs) is evaluated. The coupling was evaluated using beam propagation algorithms and cross talk was analyzed by using Monte Carlo methods. Several multimode fiber types that are currently used in fiber image guides were also evaluated for comparative purposes. The analysis shows that an optimum NA and core diameter can be found for a specific fiber center separation that maximizes the directly backscattered signal relative to the cross talk. The coupling between fibers can be kept less than -35 dB for interaction lengths less than 5 mm. The calculations were compared to an experimentally fabricated SMF array with 15 microm center spacing and showed good agreement. The experimental fiber array without a lens was also used in a coherent detection configuration to measure the position of a mirror. Accurate depth ranging up to a distance of 250 microm from the tip of the fiber was achieved, which was five times the Rayleigh range of the beam emitted from the fiber.     

一种用于宽带自噪声有源抵消的无延时子带多通道系统

降低测量平台处本艇自噪声能够有效地提高被动声纳设备的作用距离.提出了一种用于宽带自噪声有源抵消的无延时子带多通道系统.这种系统与传统的子带结构和算法相比,不但保持了乘加运算量小、收敛速度快的优点,而且在抵消通道没有引入任何时延.计算机仿真和真实海试数据都验证了系统的有效性.     

Optomechanical design and characterization of a printed-circuit-board-based free-space optical interconnect package

We present a proof of concept and a feasibility demonstration of a practical packaging approach in which free-space optical interconnects (FSOI's) can be integrated simply on electronic multichip modules (MCM's) for intra-MCM-board interconnects. Our system-level packaging architecture is based on a modified folded 4f imaging system that has been implemented with only off-the-shelf optics, conventional electronic packaging, and passive-assembly techniques to yield a potentially low-cost and manufacturable packaging solution. The prototypical system as built supports 48 independent FSOI channels with 8 separate laser and detector chips, for which each chip consists of a one-dimensional array of 12 devices. All the chips are assembled on a single substrate that consists of a printed circuit board or a ceramic MCM. Optical link channel efficiencies of greater than 90% and interchannel cross talk of less than -20 dB at low frequency have been measured. The system is compact at only 10 in.(3) (25.4 cm(3)) and is scalable, as it can easily accommodate additional chips as well as two-dimensional optoelectronic device arrays for increased interconnection density.