Mark O. Freeman

Scanned Laser Pico-Projectors: Seeing the Big Picture (with a Small Device)

Pico-projectors are the latest technology to prove that big things often do come in small packages. These tiny projectors are embedded in mobile devices to provide large-screen displays that can be viewed from anywhere.

Robust Focus and Tracking Detection for Holographic Digital Versatile Disc Optical Pickup-Head Modules

This paper introduces a robust focus and tracking detection method for use in holographic pickup heads. The method, which we call split-aperture servo detection, provides the focus error signal and both differential phase detection (DPD) and pushpull tracking signals but requires one less axis of alignment than the commonly used quad detector with astigmatic focus detection. A set of simulations designed according to Taguchis method is presented comparing the manufacturability of holographic modules using the split-aperture approach and the astigmatic approach. The split-aperture approach is seen to have strong advantages. Experimental verification of a prototype digital versatile disc (DVD) pickup is also presented.

Holographic Laser Module with Dual Wavelength for Digital Versatile Disc Optical Heads

Backward compatibility with CD-R necessitates having two laser sources in the digital versatile disc (DVD) pickup head. The dual-wavelength optical head complicates pickup design and enlarges the volume of the head. Moreover, it increases the number of components and assembly cost. In this paper, we present a new design for a dual-wavelength holographic pickup-head module. Packaged together in the module are two laser diodes, photodiodes and a holographic optical element (HOE) for diffracting reflected light from the disk onto the corresponding photodetectors. This module can be used as the core of an optical head capable of reading DVD, CD and CD-R.

High efficiency HOEs for holographic DVD pickup heads

It is well known that holographic hybrid pickup heads have a number of advantages in terms of size, modularity, and perhaps cost, but are less efficient than conventional discrete component type pickup heads. We introduce a new design for the HOE which places the 3-beam grating on the same surface with the holographic beamsplitter and utilizes light that would otherwise be wasted to create the satellite beams for 3-beam tracking. Efficiency is gained by avoiding the /spl sim/35% loss typically associated with the 3-beam grating and by blazing the holographic beamsplitter.

LCD diffractive element design to handle multiple disk thicknesses

We describe the design of a single diffractive LCD element placed adjacent to the objective lens that can be addressed to provide the required spherical aberration (SA) compensation for a plurality of disk substrate thicknesses. It is now commonplace that optical disk drives must be able to handle disks of more than one substrate thickness. The major problem is compensating for the SA introduced when the highly corrected objective lens is used with a disk substrate thickness other than that which it was specifically designed for. An abundance of methods for solving this problem in the specific case of CD/DVD backward compatibility exist in the literature; we use an active device to extend this to include HD-DVD as well.

A laser noise reduction method for holographic pickup heads

There are several methods to avoid optical feedback and reduce laser noise in optical recording systems. However, most of them utilize high cost optical components or electronics or increase the pickup head size. Furthermore, it is difficult to fit these components into in a small package like a holographic laser module. In this paper, a simple method for avoiding feedback noise in a holographic pickup head is proposed. A single wave plate is added to the optical system to rotate the polarization of the laser feedback to be orthogonal to the emitting polarization state. Experimental results show that it is feasible and practically reduces laser noise.

Polarized diffractive optical element design for a multibeam optical pickup head

This paper addresses the design and construction of an interesting polarization-switched diffractive optical element (DOE) that generates multiple beams incident on the disk and acts as a beamsplitter and servo-generating element for light returning from the disk. In this way, data speed is increased proportional to the number of beams on the disk, and, by combining three functions into a single optical element, allows a more compact and lightweight pickup to be realized. The polarization-switched DOE is constructed as a sandwich of two pieces of some birefringent material, with one rotated by 90 degrees relative to the other so that the ordinary and extraordinary axes are interchanged, and with a common index-match layer between them. A diffractive pattern is etched into each of the two birefringent pieces. Linearly polarized light traveling from the laser towards the disk is diffracted into multiple beams by one of the diffractive patterns while experiencing no diffraction from the other. Travelling the roundtrip from the DOE to the disk and back to the DOE, the light traverses a quarter-wave retarder two times thereby rotating its polarization direction by 90 degrees. It now experiences no diffraction from the multiple beam diffraction layer, but is diffracted by the second diffraction layer, which steers it onto the photodetectors and alters the beam to create useful focus and tracking error signals. This design is important in that it provides a way for two diffractive surfaces, each acting independently with high efficiency on orthogonal polarizations of light, to be combined into a single element. Implementation and application to a multiple-beam holographic pickup head module are presented.

Holographic Optical Element with High Assembly Tolerance for the Optical Module of Digital Versatile Disc Pickup-Heads

The astigmatic focus detection method and differential phase detection (DPD) tracking method are now widely used in a digital versatile disk (DVD) pickup head. The DPD tracking method requires the returning spot to remain centered in a quad detector and therefore makes the optical alignment very critical. In this paper, a novel holographic optical element (HOE) pattern design to relax the alignment tolerance and achieve a stable DPD tracking servo property is described. Simulations show that the servo signals are insensitive to photo-detector (PD) misalignment when compared to that of a conventional HOE design.