Theoretical and experimental techniques are developed to study three common approaches to focus error signal generation for optical data storage applications. Specifically, the astigmatic, obstruction knife edge (a.k.a. shaded aperture), and critical angle prism systems are studied. These techniques are then applied to a new focus detection system, based on a lateral shearing interferometer, developed by this author. The sensitivity of a given optical system to the primary design point, alignment errors, aberrations and optical recording medium topology are presented. A new approach to constructing the focus error signal for the astigmatic and interferometric techniques is presented which greatly reduces the alignment sensitivity.

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The advantages of an optical spatial phase filter constructed by thickness variations are put forward and a method of fabricating such a device using vacuum deposition techniques is detailed. The design and construction of a vacuum system to produce such a device is outlined. The system comprises a vacuum chamber with a source holder for zinc sulfide, an electronically controlled shutter, an aperture, and a substrate and holder mounted on an x-y motion table driven by stepper motors. The system is controlled by a minicomputer and measurements of thickness are made by an ellipsometer controlled by the minicomputer. Experiments conducted with the system determine the spatial resolution and closed loop control capabilities to be adequate. An analysis of the results of the tests concludes that with further refinements it seems feasible to fabricate spatial phase filters by using vacuum deposition techniques. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A technique is presented that utilizes an on-axis intensity measurement to detect the reflected light from a multiple CD-ROM surface. The thesis includes a computer simulation of the detected signal for scans of various structures and discusses finite size detector effects. A suitably sized detector passes 77% of the total light flux at the detector plane without affecting the depth discrimination property of the technique. Experimentally, a breadboard Height Readout System (HRS) could detect height differences on test relief structures. A signal to noise ratio was obtained that corresponds to a noise equivalent height sensitivity of 30 Angstroms. Therefore, the HRS could discriminate between five levels on the test structure. An addition to the new technique uses a phase plate to determine if the focused beam is going into or out of a pit. Thus, the storage capability per unit area on the disk surface can be increased.

CD and DVD became the major portable and backup data storage devices because their reliability and economical cost when mass produced. As the computer technology grew, higher data storage density on CD/DVD disc was demanded. Using a shorter wavelength light source was the common technique to achieve this goal from both research and industry. However, the limit of wavelength had almost reached for applying it to optical storage. The nano-scale metallic wire polariser that was designed in this thesis provided a possible solution. This thesis introduces the method of using the nano-scale metallic wires to form a grating polariser as the data pit on CD/DVD disc. The polariser is a type of scattering polariser and could transmit one linear polarisation of the light and reject its orthogonal counter part. The designed pattern was tested by using XFDTD, an electromagnetic simulation program based on the finite difference time domain method. As the wave source was a red laser with a wavelength of 650nm, the simulation cell size was set to be 10nm. The dispersive materials were simulated by the Debye model. The electric field results were measured on X, Y, and Z components. The results were analysed by a pre-written Matlab program to find the transmission and crosstalk coefficients. The single polariser simulations showed that there are great potentials in this design. However, inter-cell crosstalk became the major problem in the polariser array simulations. The groove pattern and titanium material were used to optimise the polarisation effect. The simulation showed that a standard-sized disc with a titanium polariser array could have 5.5GB storage capacity and a 15 to 20dB inter-cell extinction ratio for an optical pickup with a red laser (650nm) and a numerical aperture (NA) of 0.6. Although the improvement is only marginal over existing optical data storage technology, there are many further researches possible to carry on such as the fabrication of the polariser arrays.

Three applications of Smith's model of magneto-optical effects in stratified magnetic media are presented. Each application involves the incident-angle dependence of magneto-optical effects which Smith's model treats explicitly. In addition. the three applications address current issues in magnetic and magneto-optical recording. The first problem considered is that of nonmagnetic surface layers which form on magnetic recording heads. The second is a theoretical treatment of effects which arise in magneto-optical recording when high numerical-aperture optical systems are combined with media exhibiting interference-enhanced optical properties. The final application is a novel technique for determining the entire dielectric tensor of materials used for magneto-optical recording. The incident-angle dependence of the longitudinal Kerr effect is employed to study nonmagnetic surface layers which form on ferrite recording heads. Calculations based on the Smith model show a correlation between the inert-layer thickness and the incident-angle dependence of the longitudinal Kerr effect. Experimental results confirm the shift in the angular position of the signal peak. in agreement with the theory for finite inert-layer thickness. An unambiguous determination of the inert-layer thickness for ferrite materials is prevented because of a periodic dependence of the peak angle on the inert-layer thickness. Jones-matrix algebra is used with the Smith model to quantitatively determine the effects of obliquely-incident rays on the reflectance and magneto-optical readout signals for various magneto-optical media structures. The most antireflective structure is found to be very sensitive to incident-angle effects. Structures with moderate reflectance (>10%). however. are not significantly affected by incident-angle effects. Finally. a technique for the determination of the optical and magneto-optical constants from multiple incident-angle measurements of reflectance and Kerr rotation is presented. The experimental system employs a Helium-Neon laser and a rotating differential detection system. The technique is used to investigate the optical properties of a series of Cu/Co multilayers and a comparison with a model based on the bulk optical. constants of Cu and Co is made. Anomalous behavior is observed for multilayers with very thin periods.

Light can be coupled out of a waveguide to a focused point by a focusing grating coupler and has possible applications in optical data storage. The grating can be fabricated with either e-beam techniques or holographic techniques. Two design methods are demonstrated that model the focusing grating coupler with holographic optical elements. Both methods take a geometrical optics approach to designing the holographic optical elements and both methods make use of commercially available ray trace programs. The first method uses complicated non-rotationally symmetric construction optics and requires either a modified ray trace program or special user defined surfaces. The second method involves a much simpler approach which did not require any changes to an existing ray trace program and requires only rotationally symmetric elements to correct the aberrations.

This study develops the foundation of a digitally implemented control system for the radial positioning of the read/write heads of an optical disk drive system. Topics addressed are: sample rate selection, state reconstruction, closed-loop system response, disk track-following filters, and measurement noise filters. Consideration is given to the unmodeled dynamic's influence on system performance, system sensitivity to parameter variations, and a one sample computational delay. What has been concluded from this work is that the system must be further desensitized to parameter variations, and, at this stage of the development of the control scheme, neither a computational delay nor the unmodeled dynamics degrade system performance significantly.

Thesis (Ph. D.)--University of Washington, 1989. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references (p. 120-125). Also available in electronic version.