Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aberration detection device for an optical system used for an optical information recording and reproducing apparatus for recording information on an optical information recording medium (also referred to as “information carrier” in the following), such as an optical disk, and/or reproducing recorded information.
The present invention also relates to an optical information recording and reproducing apparatus for recording large amounts of information on an optical information recording medium (information carrier) with laser light, and for reproducing the recorded information. This aspect relates in particular to an optical information recording and reproducing apparatus for an information carrier having a plurality of information recording layers, such as an optical disk.
2. Description of the Prior Art
A conventional aberration correction system for optical disks is published in Publication of Unexamined Japanese Patent Application (Tokkai) No. Hei 8-212611.
is a diagram of such a conventional wavefront aberration correction method. In
denotes an optomagnetic disk, numeral
denotes a semiconductor laser, numeral
denotes a collimator lens for collimating the divergent light bundle emitted by the semiconductor laser
into a parallel light bundle, numeral
denotes an anamorphic prism for correcting the light bundle into a light bundle with circular cross section, numerals
denote reflecting mirrors, numeral
denotes an object lens, and numeral
denotes a liquid crystal element. Moreover, numeral
denotes a complex prism, numeral
denotes an APC sensor for detecting and controlling the power of the laser light, numeral
denotes a &lgr;/2 plate, numeral
denotes a polarization beam splitter, numerals
denote light receiving elements, numeral
denotes a liquid crystal control circuit, and numeral
denotes a microcomputer.
In the device in
, the liquid crystal control circuit
is driven based on data from a memory to control the liquid crystal element
so as to perform aberration correction. In particular, when aberrations occur, the phase of the liquid crystal aberration correction element
is controlled by an open loop, so that the wavefront aberration becomes minimal. Also, to correct wavefront aberration changes due to temperature influences, the temperature is detected, and the wavefront aberration is corrected on the basis of the detected temperature and previously stored control data relating to the temperature.
In the example in
, the signals from the light receiving elements
for signal detection and the light receiving element
for error signal detection are entered into the microcomputer
, which determines the voltages that the liquid crystal control circuit
applies to the elements of the liquid crystal element
, so that the detection signal of the light receiving elements is improved.
A method for detecting aberration disclosed in the same publication measures the wavefront aberration with an interference system. Moreover, after determining the disk type and the necessary data for controlling the liquid crystal so as to correct the wavefront aberration occurring when that disk type is used, the correction of the wavefront aberration is performed based on a pre-arranged table. To do so, a measurement device comprising an interference system is arranged on the outside to measure the wavefront aberration, but the publication does not disclose a specific configuration of the interference system.
To optimize the S/N ratio with these conventional aberration correction methods, the wavefront aberration is changed by trial and error, and a closed loop is formed that minimizes the wavefront aberration as a result.
However, judging with these methods whether the signal improves or deteriorates, the determination of the optimal point becomes tedious (i.e. trial and error), so that the detection takes time and it is not possible to perform control with a closed loop with fast response.
Types of so-called read-only optical information recording media that reproduce signals using laser light include compact disks (CDs), laser disks (LDs), and digital video disks (DVDs).
Presently, the read-only optical information recording medium with the highest signal recording density on the market is the DVD-ROM with 4.7 GB.
Standardized formats for read-only DVDs with a diameter of 120 mm include the single-side single-layer type with 4.7 GB maximum user capacity, the double-side single-layer type with 9.4 GB maximum user capacity, and the single-side double-layer type with 8.5 GB maximum user capacity.
shows an example of the structure of a single-side double layer optical disk. In this optical disk, by irradiating a laser beam from the side of a substrate
, signals recorded in either a first information recording layer
or a second information recording layer
can be reproduced through the substrate
. Between the first information recording layer
and the second information recording layer
, an optical separation layer
is provided, which optically separates the laser light entering through the substrate
to the first information recording layer
and the second information recording layer
. Below the second information recording layer
, a protective substrate
for protecting the second information recording layer
is provided. A method for manufacturing such a multi-layered read-only optical disk is disclosed, for example, in U.S. Pat. No. 5,126,996.
Moreover, types of optical information recording media on which a signal can be recorded and reproduced using laser light include phase-changing optical disks, optomagnetic disks, and dye disks.
In recordable phase-changing optical disks, a chalcogen compound is normally used as a material for the recording thin film. Usually, the crystalline state of this recording thin film material is regarded as the unrecorded state, and signals are recorded by irradiating laser light and changing the recording thin film material into the amorphous state by melting and cooling it quickly. Conversely, to erase signals, laser light is irradiated at lower power than for the recording, and the recording thing film is crystallized.
As an attempt to increase the recording density of recordable or recordable/erasable optical disks, the so-called “land & groove recording” has been proposed (see for example Tokkai Hei 5-282705), wherein signals are recorded in both the guide grooves and the guide lands provided in a substrate surface.
Moreover, as an attempt to increase the recording capacity of recordable or recordable/erasable phase-changing optical disks, double-layer disks have been suggested (see for example Tokkai No. Hei 9-212917).
To raise the recording/reproducing density of these disks, it is desirable to perform recording and reproducing with an object lens that has a high numerical aperture (NA). Among conventional optical disk devices, there is no example of a device using an object lens with a NA that is high enough so that errors in the thickness of the substrate may have become a problem, and irregularities in the substrate thickness have not been a particular problem.
An idea of how to correct spherical aberrations of a double-layer disk with the reproducing apparatus is mentioned in Tokko Hei 7-77031. In this publication, a predicted aberration amount of spherical aberration that occurs when using an object lens and a double-layer disk is corrected. As an element for generating an optical phase difference to correct the aberration, a liquid crystal layer is mentioned in an example embodiment. For low NAs, this method provides sufficient correction.
This means, even when the disk substrate is produced with high precision, there are still thickness irregularities of
Matsushita Electric Industrial Co. Ltd
Merchant & Gould P,C,
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