Two-dimensional finite-difference time-domain simulation for rewritable optical disk surface structure design

A novel two-dimensional finite-difference time-domain simulation for treating the interaction of a focused beam with a rewritable optical disk is detailed and experimentally validated. In this simulation, the real material properties of the rewritable multilayer stack and the aperiodic nature of the disk topography are considered. Excellent agreement is obtained between calculated and measured push-pull tracking servosignals for magneto-optical disks with pregrooves and infinite-length preformat pits. To demonstrate the utility of the simulation as a design tool, the design process for a 0.9-μm track pitch, continuous, composite servoformat magneto-optical disk is given.     

Influence of impurities on chemical etch pits in gypsum single crystals

Basal cleavages of gypsum are etched in nitric acid and potassium hydroxide solution. Terraced pits are commonly observed independent of the etchants used. Varying degrees of widths of terraces are observed on etching matched cleavage pairs as well as thin flakes. The terraced nature of etch pits has been attributed to the inhibitive action of the etchant due to the precipitation of impurities along dislocation lines. On prolonged etching of a thin flake holes are formed at some dislocation sites while at other dislocation sites, holes are not formed. Preferential etching of fission tracks gives rise to winged etch pits. The implications are discussed. Contribution No, 549.     

Optimization of groove depth for cross-talk cancellation in the scheme of land-groove recording in magneto-optic disk systems

In the scheme of land-groove recording in magneto-optic disk data-storage systems, it has been shown that an optimum groove depth exists at which the cross talk from adjacent tracks diminishes. Crosstalk cancellation, however, is very sensitive to various parameters of the system, and, in particular, the presence of substrate birefringence can have devastating effects on system performance. We analyze the origin of the observed effects by using scalar diffraction theory, and we show the reasons behind cross-talk cancellation. We also explain the relation between substrate birefringence and cross talk in simple analytical terms. Extensive computer simulations have been performed to verify and extend the theoretical results of this paper; the results of some of these simulations are also presented.     

提拉法Tm:YAG晶体的生长缺陷研究

采用提拉法(CZ)生长了质量优异的Tm:YAG晶体.部分晶片在1000℃的空气气氛中退火25h.借助光学显微镜、扫描电子显微镜(SEM),结合化学腐蚀法,对Tm:YAG晶体退火前后(111)面的缺陷特征进行了研究. Tm:YAG晶体(111)面的位错腐蚀坑呈三角形. 在偏光显微镜下观察了退火前后Tm:YAG晶体(111)面的应力双折射.同时应用高分辨X射线衍射法测定了晶体的完整性.实验结果表明,长时间空气气氛下高温退火有效降低了晶体中总的位错密度,提高了晶体质量.     

Soft capture of earth-orbiting hypervelocity particles with aerogel

The Orbital Debris Collection (ODC) experiment exposed 0.6 m² of SiO₂- based aerogel (0.02 g/cm³) for 18 months on the US Docking Module of the Mir space station. Post-flight inspection revealed hundreds of millimeter-sized hypervelocity features, ranging from deep penetration tracks to rather shallow pits. Intermediate morphologies suggest that deep tracks and shallow pits are the end members of a morphological continuum that seems controlled by impact velocity. Some 163 particles were recovered from the termini of deep tracks. Scanning (SEM) and Transmission Electron Microscopy (TEM) suggest that shock and thermal alterations are minimal and that individual phases and their textures are well preserved. SEM investigations combined with Energy Dispersive X-Ray Spectroscopy (EDS) reveal a wide variety of natural and man-made particle types, akin to those from other space-exposed surfaces. Very shallow aerogel depressions contain human-waste products. The transparent pits, 20% of all hypervelocity features, contain no residues detectable with SEM-EDS methods, suggesting that the utility of aerogel to soft capture hypervelocity impactors is velocity-limited. This velocity threshold is estimated at 18–20 km/s for the ODC aerogel.     

Measuring distribution of the ellipsoid of birefringence through the thickness of optical disk substrates

We have measured the birefringence of polycarbonate optical disk substrates, using ellipsometry. For a more comprehensive characterization, the probe beam was incident upon substrates in a wide range of polar angles and from different azimuths relative to track direction (?). Our measurements show that the ellipsoid of birefringence is tilted in the plane of radial (r) and normal (z) directions. The tilt angle varies through thickness, with a maximum value of approximately 10°. For beams passing through the substrate in the ?-z plane and at large incident angles, this tilt causes significant conversion (up to 100%) between p- and s-polarized components. Distributions of other parameters, such as the values of in-plane and vertical birefringence, are obtained simultaneously in the measurements.     

Retroreflecting ellipsometer for measuring the birefringence of optical disk substrates

A retroreflecting ellipsometer has been constructed for measuring the birefringence of optical disk substrates. In contrast to conventional ellipsometers with two mechanical arms, this system has only one arm along which both the incident and reflected beams travel. This construction eliminates the mechanical limitations of conventional ellipsometers, thereby permitting normal incidence on the sample. In addition, the single arm is adjustable in two dimensions, with the polar incident angle, θ(inc), varying from 0° to 70°, and the azimuthal incident angle, Φ(inc), varying from 0° to 360°. The condition of normal incidence permits accurate measurement of in-plane birefringence. The adjustability of both θ(inc) and Φ(inc) is necessary for the measurement of possible tilts of the index ellipsoid, and also for the variation of birefringence through the substrate thickness. Measurement results showing the useful features of the equipment are presented. The optics of the hemispherical assembly used for retroreflection as well as for the elimination of undesirable refractions are also studied by use of the ZEMAX lens design program.     

Birefringence effects in metal overlayers on Langmuir-Blodgett films

The structure observed between crossed polarizers from silver layers that have been vacuum deposited slowly on top of fatty acid Langmuir-Blodgett (LB) films are shown to be related to the “crystallites” revealed by the intrinsic birefringence of the film. The anisotropy giving rise to the birefringence of the silver layer can be explained in terms of the positions of nucleation of metal grains. These are such that the nearest neighbours of a given grain always tend to lie in a specific direction relative to the crystalline packing of the underlying LB film.     

Dislocation etch pits in zinc selenide

It is shown that triangular etch pits are produced at the points of emergence of dislocation lines on the (111) zinc faces of single crystals of zinc selenide. Conical etch pits can be produced on the (¯1¯1¯1) selenium faces with a solution of one part HCl to three parts HNO₃. Etch patterns produced following indentation confirm the identity of the etch pits. Triangular etch figures which are associated with stacking fault tetrahedra have also been observed.     

琼斯矩阵在分布式光纤传感器偏振态分析中的应用

针对基于Sagnac原理的分布式光纤传感器中光波偏振态在双折射影响下所带来的干涉信号"偏振诱导衰落"问题,运用琼斯矩阵分析法,建立了传输光偏振态影响系统功率传输系数的数学模型;根据仿真分析的结果,发现使用反射镜作为反射元件,只能消除光纤圆双折射的影响,而不能消除线性双折射的影响.因此,提出了使用法拉第旋转镜提高系统抗偏振衰落能力的改进方法,仿真结果表明可以很好地消除传感光纤的线性双折射和圆双折射的影响.     

Investigation of a half-wave method for birefringence or thickness measurements of a thick, semitransparent, uniaxial, anisotropic substrate by use of spectroscopic ellipsometry

A half-wave method of measurement of wafer birefringence that is based on interference fringes recorded from a uniaxial wafer by use of a standard phase-modulated spectroscopic ellipsometer is investigated. The birefringence of uniaxial wafers is calculated from the extremal points in the recorded oscillating intensities. A formalism is developed to incorporate the change in birefringence with wavelength as a correction factor. The correction explains the overestimation of the birefringence from previous similar research on thick uniaxial sapphire substrates. The enhanced derivative of the birefringence that is due to polarization-dependent intraconduction band transitions is detected. Furthermore, for well-characterized wafers it is shown that this method can be used in wafer-thickness mapping of 4H-SiC and similar uniaxial high-bandgap semiconductors.     

Birefringence in gradient-index rod lenses: a direct measurement method and interferometric polarization effects

Gradient-index (GRIN) media can contain stress birefringence resulting from the variation in material composition. Anisotropy in a GRIN rod lens affects ray propagation and can degrade image quality. A technique, believed to be new, for measuring birefringence in GRIN rod lenses has been developed. The change in optical path difference (OPD) for orthogonal polarizations is measured directly. With this method, effects on OPD from standard imaging aberrations are excluded. Birefringence measurements for various GRIN rod samples are presented. The data are compared with results obtained previously by use of a Twyman-Green measurement method. Also, the polarization effects on tilt fringes observed with the direct measurement method and the Twyman-Green method are presented and modeled theoretically. Tilt fringes for large birefringence test cases are also modeled.     

Correlation between corrosion pits and stresses in Al alloys

Pitting corrosion is commonly observed in a wide range of aluminum alloys that are being used for aerospace applications. There is a need to study the stress environment around pits in order to predict the nucleation of cracks. The objective of this study is to investigate the correlation between pits and stresses in aluminum alloys. Corrosion experiments were carried out on aluminum 2024-T3 alloy samples and imaged through Atomic Force Microscopy (AFM) to obtain the pit profiles. An analysis procedure was developed using CAD and finite element analysis to predict stresses resulting from corrosion pits. Based on the analysis, it was observed that the stress distribution and levels on the corroded surface varied due to irregularity in the corrosion process. The results obtained indicate that the stress initially increases and reaches a plateau with increasing corrosion time. From these stresses it is possible to estimate the initiation of cracks, from which the life can be estimated for failure in the material.     

Measurement of the modal birefringence of single-mode K(+) ion-exchanged planar waveguides with polarimetric interferometry

The polarimetric interference pattern on the surface of single-mode planar waveguides made by potassium ion exchange in soda-lime glass substrates can be observed through a 45 degrees analyzer, which allows for the high-precision measurement of the modal birefringence of samples in a wide range of 0 to 5 x 10(-4). Using this method, believed to be new, we investigated the effects of exchange temperature and time on the modal birefringence of single-mode potassium ion-exchanged waveguides. The modal birefringence profile was achieved by measurement of the variation of the phase difference between the TE(0) and the TM(0) modes with hydrofluoric-acid-etching depth of the sample.     

钢铁点蚀坑内腐蚀产物对点蚀扩展的影响

为了探讨钢铁点蚀坑内部生成的腐蚀产物对点蚀生长的影响,使用自制的2种铁微电极,通过电化学三电极体系,使之在阳极极化条件下发生溶解,一定程度上模拟点蚀生长的特征。结果表明:采用微型注射器可以将2种铁微电极表面黑色腐蚀产物沉积层冲洗到模拟点蚀坑外部,消除其对点蚀扩展的影响;通过对比冲洗前后的阳极溶解电流随时间的变化趋势,可以计算该腐蚀产物对模拟点蚀生长速率的影响;借助2种铁微电极的特殊性,提出了对其点蚀内部腐蚀产物影响进行量化的方法,将其用于修正点蚀内部金属离子的扩散系数,在一定程度上可完善点蚀的生长和预测模型。     

Effect of Cl<Superscript>−</Superscript>on the corrosive wear of AISI 321 stainless steel in H<Subscript>2</Subscript>SO<Subscript>4</Subscript> solution

The effect of Cl⁻ on the corrosive wear behaviour of AISI 321 stainless steel in H₂SO₄ solution was studied via the corrosive wear rate, the load bearing capacity of passive film and the relationship between pitting and corrosive wear. There is a critical load at natural potential, below which the corrosive wear rate is slightly lowered by Cl⁻, while above which is increased. At natural potential there are more pits at low load than that at a higher one in the wear tracks and the pits are also deeper. The load bearing capacity is lowered by Cl⁻ at passive region and then the corrosive wear rate increased.     

A DNA-based molecular motor that can navigate a network of tracks

Synthetic molecular motors can be fuelled by the hydrolysis or hybridization of DNA. Such motors can move autonomously and programmably, and long-range transport has been observed on linear tracks. It has also been shown that DNA systems can compute. Here, we report a synthetic DNA-based system that integrates long-range transport and information processing. We show that the path of a motor through a network of tracks containing four possible routes can be programmed using instructions that are added externally or carried by the motor itself. When external control is used we find that 87% of the motors follow the correct path, and when internal control is used 71% of the motors follow the correct path. Programmable motion will allow the development of computing networks, molecular systems that can sort and process cargoes according to instructions that they carry, and assembly lines that can be reconfigured dynamically in response to changing demands.     

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.     

Magnetic birefringence in a suspension containing carbon nanotubes

We have studied the magnetic birefringence in aqueous suspensions prepared from cathode deposits containing up to 5% of multi-walled carbon nanotubes and in suspensions of some other carbon materials. The magnetooptical effect observed in the suspensions of materials with nanotubes can be considered as a superposition of two contributions, one being related to the presence of graphite and graphitelike structures in suspension and the other, to the multi-walled nanotubes. The magnetic birefringence technique can be used to determine the content of multi-walled nanotubes and to obtain carbon materials enriched with nanotubes.     

Ion-exchanged glass waveguides with low birefringence for a broad range of waveguide widths

Optical communications networks require integrated photonic components with negligible polarization dependence, which typically means that the waveguides must feature very low birefringence. Recent studies have shown that waveguides with low birefringence can be obtained, e.g., by use of silica-on-silicon waveguides or buried ion-exchanged glass waveguides. However, many integrated photonic circuits consist of waveguides with varying widths. Therefore low birefringence is consequently required for waveguides having different widths. This is a difficult task for most waveguide fabrication technologies. We present experimental results on waveguide birefringence for buried silver-sodium ion-exchanged glass waveguides. We show that the waveguide birefringence of the order of 10(-6) for waveguide mask opening widths ranging from 2 to 10 microm can be obtained by postprocessing the sample through annealing at an elevated temperature. The measured values are in agreement with the values calculated with our modeling software for ion-exchanged glass waveguides. This unique feature of ion-exchanged waveguides may be of significant importance in a wide variety of integrated photonic circuits requiring polarization-independent operation.