JPH04271027A - Multibeam optical head - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for an optical disk using two beams.

0002

2. Description of the Related Art A conventional two-beam optical disk device uses a common objective lens for beams of two different wavelengths. In such devices, one of the two beams is used exclusively for signal recording, and the other is used only for reproduction. The outline of its configuration is as follows. A light beam of a certain wavelength is emitted from a first semiconductor laser, passes through a collimator lens, a dichroic mirror, and an objective lens to form a light spot on the recording surface of an optical disk. On the other hand, a light beam having a wavelength different from that of the first semiconductor laser is emitted from the second semiconductor laser, passes through a collimator lens, and is reflected by a dichroic mirror.
A light spot is formed on the recording surface of an optical disk through an objective lens.

[0003] During recording, the recording state is checked immediately after writing by leading the recording beam and immediately following it with the reproducing beam to read the recording. Therefore, it is necessary to slightly shift the spot positions of the two beams on the disk, and the two beams are usually configured to be slightly inclined relative to each other. As shown in FIG. 4, the currently adopted example is to adjust the two beams so that they have the same angle of inclination with respect to the optical axis of the objective lens 14. Here, 15 is an optical disk, the dashed line is the optical axis of the objective lens, and the broken line and solid line indicate the irradiation directions of the two beams.

0004

In the multi-beam optical head as described above, since beams of different wavelengths are used, chromatic aberration, that is, a difference in the focal position of the two beams, occurs. Therefore, an achromatic lens that corrects chromatic aberration is used as the objective lens. At this time, the difference in the focal positions of the two beams in the direction perpendicular to the recording surface is 0.3
- Must be 0.2 μm or less. This is because, taking into account focus servo residuals and the like, if a difference larger than this occurs, there is a risk that signal recording and reproduction performance will deteriorate, such as omissions in recording and reading of recorded information.

However, for example, if the wavelength is 830 nm
If an attempt is made to sufficiently achromatize the two beams of 780 nm and 780 nm using a lens, the number of objective lenses will increase, resulting in an increase in the weight of the lens. This increases the burden on the focus actuator that drives the objective lens optical system. On the other hand, when designing a lens with two lenses, for example, in order to reduce the weight of the lens, in order to suppress chromatic aberration and other aberrations, the glass materials that can be used are limited, and complicated aspherical surfaces are required. must be adopted. These pose problems in terms of manufacturing and cost.

The present invention has been made in view of the above-mentioned conventional problems, and even if a lens with a simple structure and relatively mild achromatic performance is used, the two-dimensional image in the direction perpendicular to the recording surface An object of the present invention is to provide a multi-beam optical head in which there is no difference in focal position of two beams.

[0007]

[Means for Solving the Problems] To achieve the above object, the present invention provides a multi-beam optical head that uses a common objective lens for two light beams having different wavelengths. The structure is such that the difference in the focal positions of the two beams is canceled out using the field curvature of the objective lens. That is, the multi-beam optical head is characterized in that the two beams have different irradiation angles with respect to the objective lens optical axis so that the difference in focal position is canceled out by field curvature. provide.

[0008]

[Operation] The operation of the present invention will be explained below. FIG. 3 shows an example of the shape of the image plane at two beam wavelengths. Here, the horizontal axis represents the distance (image height) measured perpendicularly from the optical axis of the objective lens, and the vertical axis represents the height measured from the disk surface toward the objective lens in a direction perpendicular to the recording surface of the disk. In FIG. 3, the solid line is the image plane of one beam (tentatively referred to as beam A), and the broken line is the image plane of the other beam (beam B).
). δ is the difference in focal position due to chromatic aberration on the optical axis of the objective lens. Now, if the irradiation direction of beam A is set parallel to the objective lens optical axis,
The light spot of beam A is focused on point A in FIG. In order to image the light spot of beam B at the same height as point A, the light spot of beam B should be positioned at the same height as point A on the image plane of beam B, that is, point B in the figure. Just do it. For this purpose, beam B is tilted with respect to the optical axis of the objective lens so that it is focused on point B. By doing this, it is possible to correct the difference in the focal positions of the two beams in the direction perpendicular to the disk recording surface.

[0009] Due to the characteristics of general lenses, it is advantageous to use the central part of the lens, so here we have described the case where one beam is parallel to the optical axis of the lens. However, the essence of the multi-beam optical head of the present invention is that, as shown in FIG. Since the irradiation angles of both beams are set to different angles with respect to the optical axis of the objective lens so that they are substantially located on a plane, both beams may be tilted with respect to the optical axis of the objective lens. In that case, for example, in FIG. 3, the beams may be tilted so that beam A is imaged at point C and beam B is imaged at point D, and the disk recording surface is placed at a position that includes the line segment CD. Further, although an example has been described in which the recording surface is perpendicular to the optical axis of the objective lens, it is also possible to tilt the disk surface.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1 is an explanatory diagram schematically showing the basic configuration of a multi-beam optical head according to the present invention. In the figure, the light emitted from the first semiconductor laser 1 is transmitted through a collimator lens 2, a dichroic mirror 3, and an objective lens 4.
, to form a light spot on the recording surface of the optical disc 5. On the other hand, the light emitted from the second semiconductor laser 6 having a wavelength different from that of the first semiconductor laser passes through the collimator lens 7, is reflected by the dichroic mirror 3, passes through the objective lens 4, and is used for recording on the optical disc 5. Form a light spot on the surface. FIG. 2 shows details around the objective lens 4 and optical disk 5 in FIG. 1. In this figure, the broken line represents a beam with a wavelength of 780 nm, and the solid line represents a beam with a wavelength of 830 nm. The conditions in this example are as follows. Objective lens focal length: 4mm Field curvature: 240h2 (μm
) Beam wavelength: 780nm,
830nm residual chromatic aberration: wavelength 78
In the beam light of 0 nm, the focal position of the beam light of wavelength 830 nm is 0.0 nm on the side moving away from the objective lens.
5 μm where h is the image height (mm). In this example, the beam with a wavelength of 830 nm is set parallel to the optical axis of the objective lens. As mentioned above, the residual chromatic aberration of this system, in other words, the difference between the focal positions of the two beams in the optical axis direction of the objective lens is 0.5 μm, so 240h2
It is sufficient to tilt the beam having a wavelength of 780 nm so that the image is formed at the position of the image height h given by =0.5. In this system, the angle of such tilt would be approximately 0°39'.

In the system of this example, the focal length of the shorter wavelength of the two beams is longer, but depending on the configuration of the optical system, the relationship between wavelength and focal length may be reversed. It may happen. Further, here, since it is preferable to use the central portion of the objective lens in terms of lens performance, an example has been described in which one beam is guided parallel to the optical axis of the objective lens, but both beams may be tilted together.

0012

[Effects of the Invention] As described above, according to the present invention, by utilizing field curvature, the difference in focal position between two beams can be canceled without using a lens with high performance chromatic aberration correction. I can do it. Therefore, the objective lens can be made smaller and lighter.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the basic configuration of a multi-beam optical head according to an embodiment of the present invention.

FIG. 2 is a detailed view of a portion of FIG. 1;

FIG. 3 is an explanatory diagram of the operation of the present invention.

FIG. 4 is a layout diagram of a conventional multi-beam optical head in a beam irradiation direction.

[Explanation of symbols]

1,6 Semiconductor laser 2,7 Collimator lens 3 Dichroic mirror 4 Objective lens 5 Optical disc

Claims (2)

[Claims] 1. In a multi-beam optical head that uses a common objective lens for two light beams with different wavelengths, the difference in focal position of the two beams that occurs in the direction perpendicular to the recording surface is calculated from the image plane of the objective lens. A multi-beam optical head characterized in that the two beams have different irradiation angles with respect to the optical axis of an objective lens so as to cancel each other due to curvature. 2. The multi-beam optical head according to claim 1, wherein of the two beams, the beam having a shorter focal length at the objective lens is guided parallel to the optical axis of the objective lens. beam light head.