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.     

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.     

A novel method for measuring the vertical birefringence of optical disk substrates

We describe a simple method of measuring vertical birefringence over the entire surface of an optical disk substrate.Our design consists of a linearly polarized He-Ne laser (1-2 mW) and a CCD camera interfaced to a computer. The measurement is non-intrusive, easy to set up, and needs only a few seconds to collect the data and plot a map of vertical birefringence over the surface area of the disk. The system described here is potentially useful as a qualitycontrol tool in substrate manufacturing environments.     

Temperature dependence of the birefringence of optical-disk substrates

Injection-molded polycarbonate substrates are used predominantly in read-only, write-once, phase-change and magneto-optic disks for data storage. The in-plane and vertical birefringences of these substrates adversely affect the performance of optical data-storage systems. The disks are typically expected to operate in the ambient temperature range of 5-50 °C. We have investigated the behavior of the in-plane and vertical birefringences of a polycarbonate disk substrate in this temperature range using a custom-built ellipsometer. This study reveals that the in-plane birefringence changes dramatically within the investigated range of temperatures, whereas the vertical birefringence remains essentially constant. We suspect that the change in birefringence is due primarily to thermally induced stress in the substrate.     

光盘基片平整度的相移干涉测量方法

使用工作波长为10.6mm的红外激光平面干涉仪对光盘基片的表面面形进行了移相式干涉法测试,对测试的波面数据进行多项式拟合,并用计算机处理测试结果,实现了光盘平整度的自动化测量。所测光盘基片平整度的峰谷值为7.88mm,均方根值为2.0mm。     

Structural and optical behavior due to thermal effects in end-pumped Yb:YAG disk lasers

We employ a Monte Carlo ray-tracing code along with the ANSYS package to predict the optical and structural behavior in end-pumped CW Yb:YAG disk lasers. The presence of inhomogeneous temperature, stress, and strain distributions is responsible for many deleterious effects for laser action through disk fracture, strain-induced birefringence, and thermal lensing. The thermal lensing, in turn, results in the optical phase distortion in solid-state lasers. Furthermore, the dependence of optical phase distortion on variables such as the heat transfer coefficient, the cooling fluid temperature, and crystal thickness is discussed.     

Simple birefringence measurement method for coated optical disks with a fixed incident angle ellipsometer

In a coated and laminated optical disk, only a reflectance method is available for oblique incidence retardation measurements to evaluate the birefringence in a polycarbonate substrate. I propose a simple measurement method that simultaneously measures both the lateral and vertical birefringence in a coated substrate. In this reflectance method, only two oblique incidence retardation values with a fixed incident angle are measured from the radial and the circumferential directions of the disk. Using the sum and the difference of these two retardation values, we can calculate both birefringences without any cumbersome curve-fitting procedure. This method can easily be introduced into the routine inspection for optical disk manufacturing.     

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.     

Analytical approach to thermal lensing in end-pumped Yb:YAG thin-disk laser

Thermal lensing in the thin-disk laser influences the output beam quality and optical efficiency significantly. In this paper, an analytical approach is taken to study the production mechanisms, features, and influences of thermal lensing in the end-pumped thin-disk laser. We calculate the distributions of temperature, stress, strain, and expansion in the disk and the curvature of the crystal using an analytic method. The expressions of the thermal lens focal length depending on the radius are presented. The optical path difference, a major cause of thermal lensing, is induced by the thermo-optical effect, the photoelastic effect, and inhomogeneous distribution of thermal expansion and the excited population. Thermal lensing is found to be aspheric with undesired aberrations and birefringence effects. Furthermore, a convex mirror due to the axial temperature gradient occurs in a free disk, and the convex mirror is found to be spherical in the center region of the disk. Based on the results of our analysis, the aspect ratio and size of the laser mode of the gain region may be adjusted to limit the damaging effects of thermal lensing.     

Triple-wavelength optical pickup head designs for compact disk,digital versatile disk and high-density digital versatile disk devices

We present a triple-wavelength design for a compact optical pickup head which uses two rhomboid prisms in parallel with three laser diodes. The design can be used for compact disk, digital versatile disk and high-density digital versatile disk devices. The specific prism lens used decreases the geometrical size and reduces the total number of optical components. The trade-off between tolerance and fabrication is also presented.     

Computer simulations of effects of disk tilt and lens tilt on push-pull tracking error signal in an optical disk drive

We quantify the effects of disk tilt and objective lens tilt on the push-pull tracking error signal of an optical disk data storage system. For a grooved disk, such as a recordable compact disk that operates at a laser wavelength of lambda, it is found that disk tilt produces a tracking offset of 0.05lambda per degree of tilt, whereas objective lens tilt produces an offset of 0.012lambda per degree of tilt. The amplitude of the tracking error signal decreases by 2.5% at the disk tilt angle of 0.3 degrees and by 5% at the objective lens tilt of 0.3 degrees . We achieved these simulations with the computer program Diffract, which performs a combination of diffraction and ray-tracing calculations through the entire optical path, from the light source to the detectors.     

Optimum depth of the information pit on the data surface of a compact disk

Abstract

The substrate of a compact disk with data storage structure (pits and lands) on one side is part of the read-out system. One would not expect the optimum pit depth to be a fixed value (a quarter wavelength, as is customarily assumed) because this value is independent of the parameters of the optical system. The quarter wavelength value is based on an inappropriate plane wave model. A model has been developed, based on the interference of converging spherical waves, for the optimum depth of the information pit on a compact disk. A simple expression was found that gives the dependence of optimum pit depth on parameters of the read-out system. In different systems the optimum pit depth will, in general, differ, ranging from a half wavelength to much smaller values. The results provide a theoretical basis for the optimum design of future systems with higher storage densities. In the present paper, the analysis is extended and several diagrams are presented, which show the dependence of detector signal contrast on system parameters.     

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.     

Three-dimensional computer model for simulating realistic solid-state lasers

We developed an accurate and fast three-dimensional computer model for simulating realistic solid-state laser systems. An iteration of the beam propagation calculation was developed to account for the counterpropagation of the laser beams in the saturated gain medium and eventually obtain the converged solution for the output beam. An analytic method was devised to account for the curved cavity mirror and the surface deformation of the gain medium induced by the temperature gradient due to pump absorption. The temperature gradient induced thermal lensing and stress birefringence is also properly included in the model calculation. This model has been validated and shown to be very accurate and efficient for modeling three-dimensional laser systems in a personal computer.     

High density magneto-optical recording using 0.67 μm band highpower laser diode

High-density optical recording on a magnetooptical disk has been achieved by a compact optical head that uses a 0.67-μm-band high-power visible-light laser diode. The recording density has been improved to 1.5 times that for conventional optical recording, using a 0.83-μm laser diode, as a result of the increase in linear density and track density. The C/N ratio for the readout signal is greater than 50 dB     

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.     

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.     

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.     

Multiwatt High-Repetition-Rate 2-microm Output from an Intracavity KTiOPO(4) Optical Parametric Oscillator

We have obtained 6.5 W of 2-mum output from a near-degenerate, type II KTiOPO(4) (KTP) optical parametric oscillator (OPO) pumped within the cavity of a Q-switched diode-pumped Nd:YAG laser that operates at 3 kHz. We adopted the simplest configuration with a compact diode-pumped Nd:YAG module pumping the doubly resonant KTP OPO in its randomly polarized resonator with an acousto-optic Q switch. Attempts to increase the 2-mum output power by pumping this intracavity KTP OPO with a polarized laser beam by use of thermal birefringence compensation configurations are discussed.     

Polarization properties of fiber lasers with twist-induced circular birefringence

We have experimentally observed and theoretically analyzed the polarization properties of fiber lasers with twist-induced birefringence. Twisting a fiber induces the circular birefringence of a fiber laser cavity, and this birefringence reduces the effects of intrinsic linear birefringence on the polarization properties of fiber lasers. The frequencies of their polarization eigenmodes coincide with each other gradually as the twist rate increases, and the directions of polarization eigenmodes deviate from the birefringence axis at a much larger twist rate than the magnitude of intrinsic linear birefringence. We describe the successful experimental results for Nd and Er fiber lasers.