Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-02-23
2004-10-05
Patel, Gautam R. (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230, C369S044420, C369S053230, C369S110030, C369S110040
Reexamination Certificate
active
06801491
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a focus error detecting apparatus of an optical pickup for detecting a focus error of light in the optical pickup for writing information to an optical disc by emission light from a light source or reading the information from the optical disc by return light from the optical disc and also relates to a focus error detecting method of the optical pickup.
2. Description of Related Art
An optical pickup having a light source, an optical system, and a photodetecting system is used widely. The operation of such a pickup is to irradiate emission light emitted from the light source onto an information recording surface of the surface of an optical disc such as CD (Compact Disc), CD-ROM, DVD (Digital Versatile Disc), or the like to write recording information such as music, data, or the like on the optical disk, or reading the recording information of the optical disk from the return light reflected and returned by the information recording surface of the optical disc.
With the optical pickup, in order to write the information surely onto the optical disc or to read the information surely from the optical disc, it is necessary to perform a control operation so as to always irradiate the emission light onto a recording position (for example, track) of the information recording surface of the optical disc (hereinafter, this control is referred to as a “tracking servo control”) and control so as to converge the emission light as a spot-like point to the recording position (hereinafter, this control is referred to as a “focusing servo control”).
As a method of the focusing servo control, for example, an “astigmatism method” and a “spot size method” are known.
The astigmatism method is a method wherein a cylindrical lens, a parallel flat plate, or the like is arranged in the optical system and the return light is received and detected by a 4-split detector.
With this construction, in the case where the emission light is focused onto the information recording surface of the optical disc (hereinafter, this state is referred to as “focused”), the return light is received like a circle to the center of the 4-split detector and a photosensing intensity of each photosensing surface of the 4-split detector is balanced. When the emission light is not focused on the optical disc, however, the return light becomes an elliptic shape which is inclined on the 4-split detector and the photosensing intensity of each photosensing surface of the 4-split detector is unbalanced. From this principle, in-focus or out-of-focus of the emission light on the optical disc can be detected by a signal obtained by performing a predetermined arithmetic operation to a photodetection electric signal obtained by photoelectrically converting the received light by each photosensing surface (hereinafter, this signal is referred to as a “focusing error signal”). A focusing servo control can be performed by controlling an objective lens or the like of a light transmission and reception optical system so as to feed back the focusing error signal.
The astigmatism method has a high detecting sensitivity of out-of-focus. Since the 4-split detector is used for photodetection, a tracking error signal for tracking servo control by a DPD (Differential Phase Detection) system can be easily obtained by arithmetic operations. The astigmatism method has an advantage that it can be easily applied also to an optical pickup of a 3-beam system using three light spots since the whole optical pickup can be miniaturized.
The spot size method is a method whereby the return light is divided into two optical paths by a photodetecting system and a focal point focused to a forward detector (hereinafter, this focal point is referred to as a “front focal point”) and a focal point focused to a rear detector (hereinafter, this focal point is referred to as a “rear focal point”) are caused.
With this construction, sizes of the return light spots of the front detector and the rear detector are equal in an in-focus state where the emission light is focused on the optical disc. If the emission light is in an out-of-focus state on the optical disc, however, the sizes of the return light spots of the front detector and the rear detector differ and the photosensing intensities of the detectors are unbalanced. The in-focus or out-of-focus of the emission light on the optical disc can be detected consequently by the focusing error signal obtained by performing a predetermined arithmetic operation to the photodetection electric signal obtained by photoelectrically converting the received light by each detector. A focusing servo control can be performed by controlling the objective lens or the like of the light transmission and reception optical system so as to feed back the focusing error signal.
In the spot size method, the focusing error signal is calculated by a difference between the photodetection electric signal from the front detector and the photodetection electric signal from the rear detector. When the emission light spot transverses the tracks on the optical disc, therefore, noises which are applied to the focusing error signal (hereinafter, referred to as “track transversal noises”) are set off by obtaining the difference between the pliotodetection electric signals of the two detectors, so that there is an advantage such that the signal is not influenced by the track transversal noises.
The conventional focus error detecting methods of the optical pickup, however, have the following problems.
1) According to the astigmatism method, when the optical pickup has an aberration (for example, astigmatism or the like), the signal is influenced by the track transversal noises. In the astigmatism method, when a thickness of optical disc is not constant but there is a thickness error depending on the position, a shape of return light spot on the detector is deformed, the light which cannot inherently be received leaks or enters the other photosensing surface, and an error occurs in the DPD tracking error signal.
2) According to the spot size method, in order to separate the return light to a plurality of optical paths, the optical pickup increases in size. It is difficult to use this method together with the 3-beam system because a structure becomes complicated.
OBJECTS AND SUMMARY OF THE INVENTION
The invention has been made to solve the problems and it is an object of the invention to provide a focus error detecting apparatus of an optical pickup and a focus error detecting method of an optical pickup, in which a signal is hard to be influenced by track transversal noises and an optical disc thickness error and the invention can be used together with the 3-beam system or DPD system.
To accomplish the object, according to the first aspect of the invention, there is provided a focus error detecting apparatus of an optical pickup, for detecting a focus error of an emission light in the optical pickup for writing optical disc recording information onto an information recording surface of an optical disc by the emission light emitted from a light source or reading out the optical disc recording information from return light which is emitted from the light source and reflected and returned by the information recording surface of the optical disc, comprising: a focus error detecting optical device having optical path separating means for separating the light existing in a first quadrant region and a third quadrant region on a plane that is perpendicular to an optical axis of the return light to a first optical path and separating the light existing in a second quadrant region and a fourth quadrant region on the plane that is perpendicular to the optical axis to a second optical path, first optical processing means for applying a first astigmatism to the light on the first optical path to thereby obtain first processing light, and second optical processing means for applying a second astigmatism in the direction that is inclined by 90° for the first astigmatism to the light on the second
Ogasawara Masakazu
Ohtaki Sakashi
Yanagisawa Takuma
Patel Gautam R.
Perman & Green LLP
Pioneer Corporation
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