Optical head device

Details Optical head device Optical head device

2001-07-02

2002-06-04

Edun, Muhammad (Department: 2653)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S109020, C369S044410

Reexamination Certificate

active

06400671

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head device.
2. Description of the Related Art
An optical memory technology using optical disks having pit patterns as high-density and large-capacity recording media has increasing applications such as digital audio disks, video disks, document file disks and data files. According to the optical memory technology, information is recorded onto and reproduced from optical disks with high precision and reliability through minutely converged light beams.
The precision and stability of the recording and reproduction depend entirely on the optical system.
Basic functions of the optical head device being the main part of the optical system are broadly divided into convergence to form diffraction limited minute spots, focus control and tracking control of the optical system, and detection of pit signals. These functions are realized by combinations of various types of optical systems and photoelectric conversion detection methods according to the purposes and uses thereof.
On the other hand, in recent years, high-density and large-capacity optical disks called DVDs have been put to practical use and spotlighted as information media capable of handling a large amount of information such as moving images. In DVD optical disks, in order to increase the recording density, the pit size on the information recording surface is small compared to compact disks (hereinafter, abbreviated as CDs) being conventional optical disks. Therefore, in the optical head devices for performing recording and reproduction of DVD optical disks, the wavelength of the light source for deciding the spot diameter and the numerical aperture (hereinafter, abbreviated as NA) of the converging lens are different from those in the case of CDs. In the case of CDs, the wavelength of the light source is substantially 0.78 &mgr;m and the NA is substantially 0.45, whereas in the case of DVD optical disks, the wavelength of the light source is substantially 0.63 to 0.65 &mgr;m and the NA is substantially 0.6. Therefore, to perform recording and reproduction of two kinds of optical disks of CDs and DVD optical disks by use of one optical disk drive, an optical head device having two optical systems is necessary.
On the other hand, in view of demands for smaller, thinner and lower-cost optical head devices, the trend is to use a common optical system for CDs and DVDs where possible. For example, a method is used in which a light source for DVDs is used as the light source and only as the converging lens, two kinds of converging lenses one of which is for DVD optical disks and the other of which is for CDs are used, or in which the converging lens is also shared and only the NA is mechanically or optically changed so as to be large for DVD optical disks and small for CDs.
Of the above-described optical head devices, the method to optically change the NA of the converging lens will hereinafter be described with reference to the drawings. In the x, y and z coordinates shown in the lower left part of the figures, the same coordinate axes represent the same directions on the figures.
FIG. 8
shows the structure of the optical system of the conventional optical head device. In
FIG. 8
, reference numeral
1
represents a semiconductor laser with a wavelength of substantially 0.65 &mgr;m. The semiconductor laser
1
is disposed so as to emit a light beam polarized in the direction of the x-axis of the x, y and z coordinates shown in the lower left part of FIG.
8
. Reference numeral
2
represents the light beam emitted from the semiconductor laser
1
. Reference numeral
3
represents a collimator lens that converts the light beam
2
into a parallel light beam. Reference numeral
4
represents a polarization anisotropic hologram disposed so as to transmit a polarized light beam having its plane of polarization within the x-z plane of FIG.
8
and diffract a polarized light beam having its plane of polarization within the y-z plane. The hologram pattern of the polarization anisotropic hologram
4
is formed so that the direction of diffraction is different between the central part and the peripheral part and that the diffracted light beam from the central part is converted into a plurality of light beams having different focus positions. Reference numeral
5
represents a quarter-wave plate that converts a linearly polarized light beam into a circularly polarized light beam. Reference numeral
6
represents an objective lens. The NA of the objective lens
6
is 0.6. Reference numeral
7
represents an optical disk. Reference numeral
9
represents a first diffracted light beam which is a light beam diffracted at the central part of the polarization anisotropic hologram
4
. Reference numeral
8
represents a second diffracted light beam which is the other light beam diffracted by the polarization anisotropic hologram
4
. The position of convergence of the first diffracted light beam
9
is closer to the collimator lens
3
than that of the second diffracted light beam
8
. Reference numeral
10
represents a photodetector comprising a plurality of photodetection areas.
The operation of the optical head device structured as described above will hereinafter be described.
In
FIG. 8
, first, the light beam
2
emitted from the semiconductor laser
1
is a linearly polarized light beam having its plane of polarization within the x-z plane of the x, y and z coordinates shown in the lower left part of the figure. After converted into a parallel light beam by the collimator lens
3
, the light beam
2
is incident on the polarization anisotropic hologram
4
. Since the polarization anisotropic hologram
4
transmits a polarized light beam having its plane of polarization within the x-z plane and diffracts a polarized light beam having its plane of polarization within the y-z plane, the light beam
2
is transmitted by the polarization anisotropic hologram
4
as it is and is then converted into a circularly polarized light beam by the quarter-wave plate
5
. The circularly polarized light beam is converged by the objective lens
6
to form a minute spot on the information recording surface of the optical disk
7
.
However, since the thickness from the substrate surface to the information recording surface is different between CDs and DVD optical disks, although a minute spot with hardly any aberration can be formed when the optical disk
7
is a DVD optical disk, when the optical disk
7
is a CD, a spot sufficient for reproduction of the CD cannot be obtained because of aberration generation.
It is known that for reproduction of CDs, by using only light, of within approximately 0.38 in terms of the NA, of the light passing through the objective lens
6
, the aberration generation is reduced and an excellent spot is obtained.
The light reflected at the information recording surface of the optical disk
7
passes through the objective lens
6
and the quarter-wave plate
5
to be converted into a linearly polarized light beam having its plane of polarization within the y-z plane, and is diffracted by the polarization anisotropic hologram
4
.
In the polarization anisotropic hologram, the direction of diffraction is different between the area of the central part through which light, of within approximately 0.38 in terms of the NA of the objective lens
6
, of the reflected light beam passes, and the area of the peripheral part. The light from the area of the central part becomes the diffracted light beams
9
and
8
, and is converged by the collimator lens
3
. At this time, the position of convergence is different between the diffracted light beam
9
and the diffracted light beam
8
. The diffracted light beam
9
is converged at a position closer to the collimator lens
3
than the diffracted light beam
8
.
The diffracted light beams
9
and
8
are incident on the photodetector
10
to be detected. By computing the output of the photodetector
10
, a focus error signal is obtained. The focus error signal is obtained by the above-describ

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