Optimal precompensation in high density magnetic recording

Master'sMASTER OF ENGINEERIN

Enhanced coding, clock recovery and detection for a magnetic credit card

Merged with duplicate record 10026.1/2299 on 03.04.2017 by CS (TIS)This thesis describes the background, investigation and construction of a system for storing data on the magnetic stripe of a standard three-inch plastic credit in: inch card. Investigation shows that the information storage limit within a 3.375 in by 0.11 in rectangle of the stripe is bounded to about 20 kBytes. Practical issues limit the data storage to around 300 Bytes with a low raw error rate: a four-fold density increase over the standard. Removal of the timing jitter (that is prob-' ably caused by the magnetic medium particle size) would increase the limit to 1500 Bytes with no other system changes. This is enough capacity for either a small digital passport photograph or a digitized signature: making it possible to remove printed versions from the surface of the card. To achieve even these modest gains has required the development of a new variable rate code that is more resilient to timing errors than other codes in its efficiency class. The tabulation of the effects of timing errors required the construction of a new code metric and self-recovering decoders. In addition, a new method of timing recovery, based on the signal 'snatches' has been invented to increase the rapidity with which a Bayesian decoder can track the changing velocity of a hand-swiped card. The timing recovery and Bayesian detector have been integrated into one computation (software) unit that is self-contained and can decode a general class of (d, k) constrained codes. Additionally, the unit has a signal truncation mechanism to alleviate some of the effects of non-linear distortion that are present when a magnetic card is read with a magneto-resistive magnetic sensor that has been driven beyond its bias magnetization. While the storage density is low and the total storage capacity is meagre in comparison with contemporary storage devices, the high density card may still have a niche role to play in society. Nevertheless, in the face of the Smart card its long term outlook is uncertain. However, several areas of coding and detection under short-duration extreme conditions have brought new decoding methods to light. The scope of these methods is not limited just to the credit card

Investigation on target design for perpendicular magnetic recording channels

Master'sMASTER OF ENGINEERIN

Processing of Ex-Situ Acquired Signals from Magnetic Disks

The ubiquity and high performance of hard disk drives for nonvolatile digital data storage cannot be denied. As the magnetic recording industry continues to develop new techniques for increasing storage density and reducing cost per bit, diagnostic and forensic tools for characterizing and interpreting the magnetic patterns recorded onto disk drive media become increasingly important. Therefore, this dissertation presents developments to the uniquely suitable spin-stand-based method of imaging magnetization patterns on media extracted from commercial hard disk drives. The emphasis of the presented research is placed on the following three areas: microscopy enhancement techniques for longitudinal magnetic recording media, "drive-independent" characterization and reconstruction of disk data, and the exploration of spin-stand microscopy in the novel context of perpendicular magnetic recording. First, it is known that, while the spin-stand microscopy technique offers high-speed and massive scale imaging capabilities, the images obtained are corrupted by distortion due to the non-local sensing or finite spatial resolution of the imaging sensor. Two techniques for mitigating this distortion, one based on characterizing the head by means of its linear response function, and a new method based on spatial Hilbert transforms, are described and demonstrated. Furthermore, a two-dimensional extension of the Hilbert transform in the context of magnetic recording is derived based on physical arguments and its application to spin-stand imaging is demonstrated. Second, although magnetic media imaging is interesting in its own right, an extension of this capability is the identification of commercial hard disk drive write channels and the subsequent reconstruction of the data written to the associated disks in a "drive-independent" manner on the spin-stand. For fundamental and practical reasons, a multilayered encoding process is performed on digital data before it is written to the disk; the presented work details the theoretical and experimental results obtained in characterizing and reversing these codes. Finally, because perpendicular recording technology has recently come on the market in consumer disk drives, the spin-stand microscopy technique is extended to imaging the media employing this new mode of recording. In particular, the novel aspects of perpendicular recording are discussed and their impact on spin-stand microscopy is demonstrated

Multitrack Detection for Magnetic Recording

The thesis develops advanced signal processing algorithms for magnetic recording to increase areal density. The exploding demand for cloud storage is motivating a push for higher areal densities, with narrower track pitches and shorter bit lengths. The resulting increase in interference and media noise requires improvements in read channel signal processing to keep pace. This thesis proposes the multitrack pattern-dependent noise-prediction algorithm as a solution to the joint maximum-likelihood multitrack detection problem in the face of pattern-dependent autoregressive Gaussian noise. The magnetic recording read channel has numerous parameters that must be carefully tuned for best performance; these include not only the equalizer coefficients but also any parameters inside the detector. This thesis proposes two new tuning strategies: one is to minimize the bit-error rate after detection, and the other is to minimize the frame-error rate after error-control decoding. Furthermore, this thesis designs a neural network read channel architecture and compares the performance and complexity with these traditional signal processing techniques.Ph.D

Exploration on in-situ diagnosis methods for the evaluation of mechanical and magnetic performance of hard disk drives

Master'sMASTER OF ENGINEERIN

Optical Magnetometry Using Multiphoton Transitions

Optical magnetometry plays a critical role in low-energy precision measurements and numerous other applications. In particular, permanent electric dipole moment (EDM) searches impose strict requirements on magnetic field sensitivity of the underlying atomic or molecular species. Other magnetometer properties -- such as chemical reactivity, dielectric strength, and interaction cross-sections with other species -- also impose limitations on experimental conditions. Here, we explore a novel approach to optical magnetometry, using multiphoton transitions of diamagnetic atoms to detect Larmor precession of polarized nuclei. Resonant probes are possible at moderate ultraviolet wavelengths, and hyperfine structure couples spin precession to fluorescence transitions with negligible backgrounds; paramagnetic rotation due to intensity-dependent dispersion may also be detectable. Nuclear spins and nonlinear optical excitation introduce new degrees of freedom, and evade limitations arising from rapid electronic decoherence. This dissertation reports progress towards two-photon optical magnetometry using ytterbium, rubidium, and xenon. We characterize the influence of probe polarization and magnetic fields on fluorescence spectra, for one- and two-photon continuous-wave (cw) excitation of ytterbium. Resolved hyperfine and isotope structure allow us to use spin-zero isotopes for diagnostics and normalization, and we develop analysis for overlapping two-photon resonances. We also report measurements of two-photon excitation in ytterbium and rubidium using picosecond laser pulses, and in xenon using a cw laser. Although hyperfine structure is unresolved, the rubidium measurements are sensitive to probe field polarization. Fluorescence spectra from two-photon excitation of ytterbium with femtosecond pulses show modulation when the repetition rate changes. Although techniques for polarizing noble gas nuclei are mature, existing cell designs are incompatible with two-photon magnetometry. We describe development of silicate-assisted hydroxide-catalysis bonding for both aluminosilicate EDM cells with silicon electrodes, and sapphire-windowed cells that transmit ultraviolet excitation light. Progress in measuring the 129Xe nuclear EDM is discussed. Absolute referencing of the picosecond laser to potassium transitions is proposed for two-photon spectroscopy of ytterbium and xenon, and a compatible frequency-tripling method is outlined to produce excitation light for xenon. Novel possibilities including spatial resolution and multiphoton optical pumping of nuclear spins are considered.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135807/1/sdegen_1.pd

Brilliant radiation sources by laser-plasma accelerators and optical undulators

Die vorliegende Arbeit beschäftigt sich in Experiment und Theorie mit Laser-Plasma beschleunigten Elektronen und optischen Undulatoren zur Erzeugung von brillianter Synchrotronstrahlung. Zum ersten Mal wird experimentell nachgewießen, dass laserbeschleunigte Elektronenpulse kürzer als 30 fs sind. Ferner werden solche Elektronenpulse erstmalig in einem Demonstrationsexperiment durch einen magnetischen Undulator als Synchrotronstrahlenquelle genutzt. Aufbauend auf diesen experimentellen Erkenntnissen, sowie umfangreichen numerischen Simulationen zur Thomsonstreuung, werden die theoretischen Grundlagen einer neuartigen Interaktionsgeometrie für Laser-Materie Wechselwirkungen entwickelt. Diese neue, in der Anwendbarkeit sehr allgemeine Methode basiert auf raum-zeitlicher Laserpulsformung durch nichtlineare Winkeldispersion wie diese durch VLS- (varied-line spacing) Gitter erzeugt werden kann und hat den Vorteil nicht durch die Fokussierbarkeit des Lasers (Rayleighlänge) begrenzt zu sein. Zusammen mit laserbeschleunigten Elektronen ermöglicht dieser traveling-wave Thomson scattering (TWTS) benannte Ansatz neuartige, nur auf optischer Technologie basierende Synchrotronstrahlenquellen mit Zentimeter bis Meter langen optische Undulatoren. Die hierbei mit existierenden Lasern erzielbaren Brillianzen übersteigen diese bestehender Thomsonquellen-Designs um 2-3 Größenordnungen. Die hier vorgestellten Ergebnisse weisen weit über die Grenzen der vorliegenden Arbeit hinaus. Die Möglichkeit Laser als Teilchenbeschleuniger und auch optischen Undulator zu verwenden führt zu bauartbedingt sehr kompakten und energieeffizienten Synchrotronstrahlungsquellen. Die hieraus resultierende monochromatische Strahlung hoher Brillianz in einem Wellenlängenbereich von extremen ultraviolett (EUV) zu harten Röntgenstrahlen ist für die Grundlagenforschung, medizinische Anwendungen, Material- und Lebenswissenschaften von fundamentaler Bedeutung und wird maßgeblich zu einer neuen Generation ultrakurzer Strahlungsquellen und freien Elektronenlasern (FELs) beitragen.This thesis investigates the use of high-power lasers for synchrotron radiation sources with high brilliance, from the EUV to the hard X-ray spectral range. Hereby lasers accelerate electrons by laser-wakefield acceleration (LWFA), act as optical undulators, or both. Experimental evidence shows for the first time that LWFA electron bunches are shorter than the driving laser and have a length scale comparable to the plasma wavelength. Furthermore, a first proof of principle experiment demonstrates that LWFA electrons can be exploited to generate undulator radiation. Building upon these experimental findings, as well as extensive numerical simulations of Thomson scattering, the theoretical foundations of a novel interaction geometry for laser-matter interaction are developed. This new method is very general and when tailored towards relativistically moving targets not being limited by the focusability (Rayleigh length) of the laser, while it does not require a waveguide. In a theoretical investigation of Thomson scattering, the optical analogue of undulator radiation, the limits of Thomson sources in scaling towards higher peak brilliances are highlighted. This leads to a novel method for generating brilliant, highly tunable X-ray sources, which is highly energy efficient by circumventing the laser Rayleigh limit through a novel traveling-wave Thomson scattering (TWTS) geometry. This new method suggests increases in X-ray photon yields of 2-3 orders of magnitudes using existing lasers and a way towards efficient, optical undulators to drive a free-electron laser. The results presented here extend far beyond the scope of this work. The possibility to use lasers as particle accelerators, as well as optical undulators, leads to very compact and energy efficient synchrotron sources. The resulting monoenergetic radiation of high brilliance in a range from extreme ultraviolet (EUV) to hard X-ray radiation is of fundamental importance for basic research, medical applications, material and life sciences and is going to significantly contribute to a new generation of radiation sources and free-electron lasers (FELs)

Brilliant radiation sources by laser-plasma accelerators and optical undulators

This thesis investigates the use of high-power lasers for synchrotron radiation sources with high brilliance, from the EUV to the hard X-ray spectral range. Hereby lasers accelerate electrons by laser-wakefield acceleration (LWFA), act as optical undulators, or both. Experimental evidence shows for the first time that LWFA electron bunches are shorter than the driving laser and have a length scale comparable to the plasma wavelength. Furthermore, a first proof of principle experiment demonstrates that LWFA electrons can be exploited to generate undulator radiation. Building upon these experimental findings, as well as extensive numerical simulations of Thomson scattering, the theoretical foundations of a novel interaction geometry for laser-matter interaction are developed. This new method is very general and when tailored towards relativistically moving targets not being limited by the focusability (Rayleigh length) of the laser, while it does not require a waveguide. In a theoretical investigation of Thomson scattering, the optical analogue of undulator radiation, the limits of Thomson sources in scaling towards higher peak brilliances are highlighted. This leads to a novel method for generating brilliant, highly tunable X-ray sources, which is highly energy efficient by circumventing the laser Rayleigh limit through a novel traveling-wave Thomson scattering (TWTS) geometry. This new method suggests increases in X-ray photon yields of 2-3 orders of magnitudes using existing lasers and a way towards efficient, optical undulators to drive a free-electron laser. The results presented here extend far beyond the scope of this work. The possibility to use lasers as particle accelerators, as well as optical undulators, leads to very compact and energy efficient synchrotron sources. The resulting monoenergetic radiation of high brilliance in a range from extreme ultraviolet (EUV) to hard X-ray radiation is of fundamental importance for basic research, medical applications, material and life sciences and is going to significantly contribute to a new generation of radiation sources and free-electron lasers (FELs)