JP2001229565A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the reduction of utilization efficiency of light in a pickup capable of reproducing different kinds of disks such as a DVD and a CD. SOLUTION: The light emitting wavelength of a light receiving and emitting module M1 for the DVD is different from that of a light receiving and emitting module M2 for the CD. The numerical aperture of an objective lens 1 is determined matching the wavelength for the DVD, and a diaphragm means is provided in the optical path of the light receiving and emitting module M2 for the CD. Then, the diameter of a spot focused on the recording surface of the DVD is different from that of the CD. Also, the optical path length from the light receiving and emitting module M2 for the CD is shorter than the optical path length of the light receiving and emitting module M1 for the DVD, and whereby the influence of aberration due the difference between the substrate thickness T1 of the DVD and the substrate thickness T2 of the CD is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk applicable to different types of disks having different track pitches and substrate thicknesses, such as a DVD (digital versatile disk) and a CD (compact disk). Type pickup.

[0002]

2. Description of the Related Art Optical recording or magneto-optical recording disks are used for various purposes. Some of these disks have the same diameter and the like. In some cases, a compatible configuration that enables playback of a disc may be required.

For example, a DVD (Digital Versatile Disc) and a CD (Compact Disc) both have a diameter of 120 mm and a central hole diameter for clamping of 15 mm. The recording method of the disk is both a light modulation method using pits, and the rotation control method is also a CLV (constant linear velocity). Therefore, it is possible to develop an upward compatible playback apparatus that can play both discs.

However, the major difference between a DVD and a CD is the data transfer speed and the recording capacity.
6 to 8 times the storage capacity of The track pitch of the CD is 1.6 μm, while the DV standard of the SD standard is used.
D is 0.725 μm.

[0005] Due to such a difference in recording density, the diameter of an irradiation spot of a laser beam applied to the recording surface of a disk is
It is necessary to greatly reduce the diameter of a DVD as compared with a CD. The diameter of the irradiation spot formed on the recording surface of the disc by being converged by the objective lens is proportional to the wavelength of light and inversely proportional to the numerical aperture (NA) of the objective lens. Therefore, in an optical pickup corresponding to a DVD of high density recording, it is necessary to shorten the emission wavelength of the light source and / or increase the numerical aperture of the objective lens.

Therefore, in the optical pickup for DVD, the use of a red semiconductor laser (wavelength: 635 nm to 650 nm) shorter in wavelength than the light source (wavelength: 780 nm) for CD has been studied. Is 0.45, whereas 0.52-
It is being studied to make it about 0.6.

In the DVD of the SD standard, the thickness of the substrate is T2 = 0.6 mm, and the thickness of the CD substrate is T1 = 1.2 mm.
mm. Therefore, SD standard DVD
In order to ensure compatibility between a CD and a CD, the difference in substrate thickness shown in FIG.
2 = 0.6 mm), it is necessary to change the focal position on the disk side.

As a compatible reproducing apparatus which has been conventionally studied, one optical pickup is provided with both an objective lens for DVD and an objective lens for CD, and both objective lenses are mechanically positioned at a light beam passing position. Some are switched and positioned. However, in this method, since two objective lenses must be mounted on the optical pickup and a mechanism for switching the objective lens must be mounted, the pickup becomes large and the cost increases. Further, there is a disadvantage that power consumption increases because a mechanism for switching the objective lens is operated.

Also, as shown in FIG. 3, the use of an optical pickup provided with a single objective lens having a hologram has been studied. One or both surfaces of the objective lens L are aspherical lenses, and a hologram F is integrally formed on one surface (the incident surface in FIG. 3).
The objective lens L with a hologram is a bifocal lens, and has two focal points for a single light beam.
Thus, the focal position and the substantial numerical aperture of the objective lens are changed between the Ba component and the Bb component of the light beam, and recording and reproduction on both SD standard DVDs and CDs are enabled.

[0010]

However, in the optical pickup using the objective lens L with hologram shown in FIG. 3, when one light beam passes through the objective lens L,
Ba and Bb components having different focal points are separated, and only one component is used for reproduction. For this reason, the light use efficiency is deteriorated.

An object of the present invention is to solve the above-mentioned conventional problems, and to provide an optical pickup capable of performing a reproducing operation on discs of different types such as DVD and CD using a single objective lens. It is an object.

[0012]

An optical pickup according to the present invention is provided with an objective lens facing a recording medium, two light sources having different emission wavelengths from each other, and a position where the optical paths of the respective light sources intersect. Spectral means for providing light from each light source to the recording medium via the objective lens and returning light from the recording medium to the light receiver, and narrowing the luminous flux diameter of light emitted from the light source having a long emission wavelength. Diaphragm means for reducing a substantial numerical aperture of the objective lens with respect to the light flux is provided, and an optical path length from the light source having a long emission wavelength to the objective lens has an optical path length from the light source having a short emission wavelength to the objective lens. It is characterized in that it is set shorter than the optical path length.

In the optical pickup of the present invention, of the light beams emitted from the plurality of light sources, those that have passed through the aperture means can be used for reproduction on a small-capacity recording medium having a large track pitch, and the other light beam is transmitted on the recording medium. Since the spot diameter is small, it can be used for reproduction from a large-capacity recording medium having a small track pitch.

In addition, the thickness of the substrate is different by emitting light having a long emission wavelength and making the optical path length from the light source having the aperture means to the objective lens shorter than the other optical path length. When a recording medium is used, it is possible to eliminate the influence of aberration when reproducing a recording medium having a small capacity and a large thickness of the substrate.

[0015]

FIG. 1 is a side view showing the structure of an optical pickup according to the present invention. The disk D serving as a recording medium is a DVD or a CD, and the objective lens 1 faces the recording surface of the disk D. The objective lens 1 has a spherical surface or an aspherical surface on both sides or one side, and is a plastic lens or a glass lens. The numerical aperture (NA) of the objective lens 1 is determined so that a small-diameter irradiation spot corresponding to the recording capacity of the DVD can be formed.

FIG. 1 shows an optical pickup capable of reproducing both DVD and CD of the SD standard. In the DVD of the SD standard, the thickness of the substrate (polycarbonate) through which the light flux passes is T2 = 0.6 mm. The wavelength of the laser light emitted from the light source of the light emitting and receiving module M1 for DVD is 635-650 nm, and the numerical aperture of the objective lens 1 is 0.6.

In the optical pickup shown in FIG.
A light receiving / emitting module M2 is provided, and the wavelength of laser light emitted from the light source of the light receiving / emitting module M2 is 780 nm. The thickness of the CD substrate is T1 =
1.2 mm.

As shown in FIG. 2, the structure of the light emitting / receiving module M1 for DVD and the structure of the light emitting / receiving module M2 for CD are the same, and both have the light source 2 and the light receiver 6, and The light emission to the disk and the return light from the disk can be detected in the inside. However, DV
The light emitting / emitting module M1 for D and the light receiving / emitting module M2 for CD have different emission wavelengths from the light source.

In each of the light emitting / receiving modules M1 and M2,
Dispersing means (polarizing beam splitter or half mirror) 4 for transmitting light from the light source 2 toward the disk or returning light from the disk to the light receiver 6 is provided. A diffraction grating 3 as a light beam separating means for separating a light beam into three beams is provided in a light emission path from the light source 2. A cylindrical lens 5 for detecting a focus error is provided on a path to the light receiver 6. The light receiver 6 is
It is configured to be able to detect three beams of return light, and it is possible to detect a tracking error by the three beams of return light. The main beam detector of the light receiver 6 is divided into four parts, and a focus error can be detected by light transmitted through the cylindrical lens 5.

The objective lens 1 is supported so as to be finely movable and can be driven by a focus and tracking servo mechanism S. The objective lens 1 is driven by the servo mechanism S based on the focus error detected by the light receiver 6 of one of the light emitting / receiving modules M1 or M2 and the tracking error of the three-beam detection.

At the intersection of the optical path of the light emitting and receiving module M1 for DVD and the optical path of the light emitting and emitting module M2 for CD, a light splitting means 10 is provided.
Thus, the light from each of the light receiving and emitting modules M1 and M2 can be applied to the disk D, and the reflected return light from the disk D can be applied to each of the light receiving and emitting modules.
The spectral unit 10 is, for example, a polarization beam splitter. In this case, by changing the direction of the polarization plane of the light beam from the light receiving / emitting module M1 and the direction of the polarization plane of the light beam from the light receiving / emitting module M2 by 90 degrees, the polarization beam splitter allows the light beam from each light receiving / emitting module to be changed. Almost 100 of luminous flux
% Can be given to disk D. Alternatively, the spectral unit 10 may be a half mirror.

An aperture means 11 is provided between the light receiving / emitting module M2 for CDs having a long emission wavelength and the spectral means 10. The aperture of the light beam given from the light receiving / emitting module M2 to the objective lens 1
Is limited by Therefore, the substantial numerical aperture (apparent numerical aperture) of the objective lens 1 with respect to the light flux from the light emitting / receiving module M2 is small.

In this optical pickup, the light emitting / receiving module M1 operates when a DVD is loaded, and the light emitting / emitting module M2 operates when a CD is loaded.
When a DVD is loaded, a short wavelength (635 nm or 650 nm) emitted from the light source 2 of the light emitting / receiving module M1 is used.
The laser light of m) is transmitted to the disk D through the spectral means 10 and the objective lens 1. The diameter of the irradiation spot formed on the disk D becomes smaller due to the wavelength of the laser light and the numerical aperture of the objective lens 1, so that a large-capacity DVD can be reproduced.

When a CD is loaded, the light beam from the light emitting / receiving module M2 is reflected by the spectroscopic means 10,
It is given to the disk D via the objective lens 1. The light from the light receiving / emitting module M2 has a long wavelength (780 nm),
Further, the aperture of the light beam is restricted by the aperture means 11, and the substantial numerical aperture of the objective lens 1 is reduced. Therefore, an irradiation spot having a large diameter can be formed on the disk.

FIG. 1 shows a light emitting / receiving module M2 for CD.
The optical path length L2 from the light source to the spectroscopic means 10 (or the optical path length from the light source to the objective lens 1) is changed by the optical path length L1 from the light receiving / emitting module M1 for DVD to the spectroscopic means 10 (or the optical path length from the light source to the objective lens 1). ).

Thus, the light emitting / receiving modules M1 and M2
Can be made different from each other in the imaging position of the light beam on the disk side, and the influence of aberration due to the difference in substrate thickness can be eliminated.

Each of the light emitting / receiving modules M1 and M2 has a light source and a light receiver individually. Accordingly, the tracking error can be detected by the three-beam method, and a highly accurate tracking servo can be realized.

In the configuration example shown in FIG. 1, a diaphragm plate having an opening at the center is used as the diaphragm means 11.
As the stop means 11, a hologram element or the like for limiting the aperture of the light beam may be used.

Further, in the configuration example shown in FIG. 1, a finite optical system in which the divergent light from the light source 2 is focused by the objective lens 1 is used, but the divergent light from the light source 2 of the light receiving / emitting modules M1 and M2 is used. May be provided with an infinite optical system in which a parallel light beam is converted into a parallel light beam by a collimating lens, and the parallel light beam is focused by an objective lens.

Different types of discs are DVD and C
The disk is not limited to D and may be another disk.

[0031]

As described above, according to the present invention, of the light beams emitted from a plurality of light sources, those which have passed through the aperture means can be used for reproduction on a small-capacity recording medium having a large track pitch, and the other light beam is used for recording. Since the spot diameter on the medium is small, it can be used for reproduction on a large-capacity recording medium with a small track pitch.

In addition, the thickness dimension of the substrate is different by emitting light having a long emission wavelength and making the optical path length from the light source having the aperture means to the objective lens shorter than the other optical path length. When a recording medium is used, it is possible to eliminate the influence of aberration when reproducing a recording medium having a small capacity and a large thickness of the substrate.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical pickup of the present invention,

FIG. 2 is a configuration diagram illustrating an example of a configuration of a light receiving and emitting module.

FIG. 3 is a side view showing a configuration of a conventional pickup;

[Explanation of symbols]

 D Disk M1 Light emitting / receiving module for DVD M2 Light emitting / receiving module for CD S Servo mechanism 1 Objective lens 2 Light source 3 Diffraction grating as light beam separation means 4 Spectral means 6 Light receiver 10 Spectral means 11 Stop means

Claims (1)

[Claims] 1. An objective lens facing a recording medium, two light sources having different emission wavelengths from each other, and a light source provided at a position where an optical path of each light source intersects to allow light from each light source to pass through the objective lens. Means for supplying the light to the recording medium and returning the light from the recording medium to the light receiving device, and a substantial numerical aperture of the objective lens for the light flux by narrowing a light flux diameter of the light emitted from the light source having a long emission wavelength. Aperture means for lowering the optical path length, wherein an optical path length from the light source having a long emission wavelength to the objective lens is set shorter than an optical path length from the light source having a short emission wavelength to the objective lens. Optical pickup.