Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-01-12
2001-07-31
Huber, Paul W. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112170, C369S118000
Reexamination Certificate
active
06269057
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the generation of focus error signals for use in the auto-focussing of optical data storage and retrieval systems. More specifically, the present invention relates to a method and apparatus for generating focus error signals based upon creating two focal planes in a return read beam using a birefringent lens, filtering the read beam using a confocally configured pinhole and deriving a focus error signal related to the difference in distance between the resulting points of focus within the focal planes and the pinhole.
2. Background
As a prerequisite to successful optical data storage, the optical head containing the focussing optics must be positioned properly within the storage layer of the optical storage medium. When recording or retrieving optical data it is essential that the optical head be positioned precisely at the desired storage point. Proper positioning of the optical head is typically carried out through auto-focusing techniques implemented by a servo system within the optical head. The signals that drive the auto-focussing process are generated by Focus Error Signal (FES) generator devices that are incorporated into the overall scheme of the optical delivery and detection system.
FES generator devices within optical delivery and detection systems have typically only been required to provide signals in instances where data storage is limited to a single or to a few layers within the optical storage medium. Current technology is generally limited to performing optical data storage on a minimal number of layers within the optical storage medium. In most instances, these layers have, generally, about 60 micron separation between adjacent layers. When such a pronounced distance separates the layers, the current FES generator devices are sufficient because layer separation does not present an issue.
However, as technological advances in data storage are made, the capability presents itself to store data on numerous layers within the storage medium. See for example U.S. patent application Ser. No. 09/016,382 filed on Jan. 30, 1998, in the name of inventor Hesselink et. al. (assigned to the assignors of the present invention) entitled “Optical Data Storage by Selective Localized Alteration of a Format Hologram” for a detailed discussion of layer definition by format hologram grating structures. That disclosure is hereby expressly incorporated herein by reference as if set forth fully herein.
When data storage is performed on multiple layers the distance separating such layers is minimized. As the separation between the layers shrinks to the about 3 to about 10 micron ranges, the ability to separate out these layers during focus error signal detection becomes more of a concern. The prior art methods are not capable of delineating between layers that are packed so closely together. The present invention serves to provide an FES generator device and a method for FES generation that is capable of differentiating the layers in optical storage medium that have numerous tightly packed layers separated at distances comparable to the Rayleigh range of the illuminating beam. Additionally, the FES generator device and the method of FES generation described herein can be used with a data storage device having multiple storage layers residing at discrete media layers spaced at distances that can be comparable to or substantially longer than the Rayleigh range.
BRIEF DESCRIPTION OF THE INVENTION
Briefly, and in general terms, one embodiment of the present invention comprises an improved focus error signal generator device including two optical lenses in series; a birefringent optical lens followed by a standard optical lens, the lenses being disposed in the path of a return read beam wherein the birefringent optical lens has a first and second focal plane and the standard optical lens serves to minimize the focal lengths. A pinhole is disposed in the path of the return read beam in close proximity to the first and second focal planes. A polarizing beam splitter is positioned after the second focal plane and serves to split the return read beam into two light beams of polarization associated with the first and second focal planes. First and second detectors are positioned so as to read the two light beams signals output from the polarizing beam splitter and the detectors are connected to an electrical differencing circuit having an output to an optical head servo system.
Another aspect of the present invention comprises a method for focus error signal generation including the steps of focussing a return read beam at first and second points of focus; providing a pinhole in the path of a return read beam located in close proximity to the first and second points of focus; providing for a polarizing beam splitter disposed in the path of the return read beam after the second point of focus that splits the return read beam into first and second polarization beams; providing for first and second detectors, respectively, in the paths of the corresponding first and second polarization beams; and generating a focus error signal related to the difference between the output of the first detector and an output of the second detector.
Additionally, another embodiment of the present invention comprises an optical data delivery and detection system comprising a laser source emitting a light beam, an optical head that receives the light beam, optical lenses within the optical head that focus the light beam on an optical storage media, a data detector that receives the light beam on the beam's return path and provides data signals and a focus error generator device as described herein.
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McDonald Mark E.
McLeod Robert R.
Huber Paul W.
Sierra Patent Group Ltd.
Siros Technologies, Inc.
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