Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-07-06
2003-01-14
Font, Frank G. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
Reexamination Certificate
active
06507009
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is concerned with an optical information reproducing apparatus (‘optical disk apparatus’) that is used to reproduce a data signal recorded in an optical information recording medium (‘optical disk’). Especially, the present invention is used to increase the performance in detecting various error signals which are used to control the position of an optical spot in an apparatus for reproducing information from an optical disk.
A knife edge detection method (Foucault detection method), beam size detection method, astigmatic detection method, etc. are conventionally used as methods to detect a focusing error signal in an optical disk apparatus. From various points of view, such as the simplicity of the optical system, the ease of adjustment thereof and the ease of combining such detection method with a detection method for a tracking error signal, etc., the astigmatic detection method has had the greatest general acceptance. However, the astigmatic detection method has an important problem in that, when an optical spot irradiated on the optical disk deviates from the record track of a disk, a disturbance may easily occur in the focusing error signal. The influence of this disturbance especially notably occurs when reproducing a disk having a land-groove structure such as a DVD-RAM disk, etc., of a type that is planned to be manufactured soon. The main reason why such a disturbance especially occurs on a disk having a land-groove structure is that the amplitude of the so-called push-pull signal produced by diffraction in a groove of the disk becomes enlarged because the width of the groove and the distance between grooves are almost equal on the optical disk having a land-groove structure, and the depth of the groove is set at ⅙~{fraction (1/7)} relative to the wavelength of the laser light used for reproduction (650 NM).
Conventionally, a method of shading the center part of a detection light beam (The Japanese patent Laid-open print No. 4-314500 official gazette) and a method of reducing a disturbance by a rotary adjustment of an objective lens (Japanese patent publication No. 5-68774 official gazette), etc. have been disclosed as a means to reduce the disturbance in the focusing error signal produced by an astigmatic detection method. However, in actual practice, the disturbance is sufficiently reduced by neither method. Therefore, in an optical disk apparatus for the DVD- RAM disk of the type mentioned above, it has been necessary to use a knife edge method and a beam size method conventionally, although the structure and coordinates of an optical system according to these methods are complex.
On the other hand, the three spot detection method and the push-pull method are representative methods which have been used for detecting a tracking error signal in an optical disk apparatus. The three spot detection method is widely used for conventional read-only disks, such as the CD and CD-ROM disks, from the point of view simplicity, ease of adjustment, and the resistance against production of a disturbance, etc., of an optical system. Optical disk apparatuses which are used to reproduce mass read-only disks, such as a DVD-ROM disk, widely use a differential phase detection method instead of the three spot detection method because (1) the sensitivity of a tracking error signal is not sufficiently obtained in DVD due to the influence of a narrow small record track pitch and (2) the precision of a relative position adjustment of the three optical spots that are irradiated on the disk in DVD must be remarkably finer than that of a CD. The differential phase detection method detects a tracking error signal in accordance with a time change of intensity distribution of the reflected light of one optical spot.
An optical disk apparatus cannot be used for reproducing recordable disks, such as DVD-RAM disks, because of the problem of offset resulting from the difference between the reflection light quantities of the forward subspot and that of a backward subspot. Thus, a push-pull method, that is another representative method for detecting a tracking error signal, is generally used in the optical disk apparatus for reproducing recordable disks.
Now, though the push-pull method has an excellent advantage in that a highly-sensitive tracking error signal is obtained by a relatively simple optical system, it also produces an important problem in that a large offset arises in the tracking error signal when an objective lens is displaced in the tracking direction. Thus, a so-called differential push-pull method has been proposed as a valid method for greatly reducing the offset of a tracking error signal resulting from objective lens displacement (Optical memory symposium '86 treatise collection (1986) PP. 127-132). This differential push-pull method irradiates an optical disk with three optical spots similar to the three spot detection method. However, this method cancels the offset caused by an objective lens displacement by performing specified subtraction processing on a tracking error signal detected by a push-pull method for each optical spot.
SUMMARY OF THE INVENTION
In actual practice, the most suitable methods for detecting a tracking error signal for each kind of optical disk are quite different. However, some of these optical disks have already experienced wide use and others are certain to become popular in the future. Therefore, for an optical disk apparatus, it is naturally desirable for all of these optical disks to be reproducible by a single apparatus. However, as mentioned above, there currently are unavoidable problems in that there is not a means of simple structure that can obtain a good focusing error signal that is not subject to the influence of a disturbance, and in that the structure of an optical head and the internal structure of an optical detector become very massively complex if designed to incorporate various tracking error signal detection means in one optical disk apparatus. That is, a quite new optical means is necessary to realize a useful optical disk apparatus that has a simple optical head or an optical detector with simple structure, and which can obtain a good focusing error signal that is not subject to the influence of a disturbance, and can realize all of the above-mentioned means for tracking error signal detection.
Considering the above situation, it is an object of the present invention to provide a new optical head that can greatly reduce the influence of a disturbance of the type that typically occurs when using an astigmatic detection method as a focusing error signal detection means with one simple detection optical system and a simple structured optical detector, and which can greatly reduce the influence of an offset that occurs because of displacement of an objective lens which results when using a push-pull method as a tracking error signal detection method.
Another object of this invention is to provide an optical detector and a signal processing circuit to realize the above-mentioned optical head.
A further object of present invention is to provide a useful optical disk apparatus that contains the above-mentioned optical head and which can reproduce information from various optical disks by selectively changing the focusing error signal detection method and tracking error signal detection method according to the kind of disk to be reproduced so as to use the most suitable method for each disk. To attain the foregoing objects, the present invention provides the following features.
An optical detector for detecting at least three optical beams reflected by an optical information recording medium and for outputting an electric signal, said optical detector comprising: three light receiving areas each of which receives one optical beam of said three optical beams and each of which has four light receiving surfaces;
twelve signal lines to transmit an electric signal obtained on each of said four light receiving surfaces in said three light receiving areas;
less than nine signal output
Fujita Shinji
Fukui Yukio
Inoue Masayuki
Nakamura Shigeru
Ohnishi Kunikazu
Antonelli Terry Stout & Kraus LLP
Font Frank G.
Hitachi , Ltd.
Nguyen Tu T.
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