JPS6015840A - Optical pickup - Google Patents

Abstract

PURPOSE:To obtain an optical pickup with a low cost, high performance and a small size by increasing the number of apertures of a disc side more than the number of apertures at a semiconductor laser side so that the ratio of the number of apertures of the semiconductor laser side of an objective lens to the number of apertures of the disc side is a specified range. CONSTITUTION:Since a light irradiated from a semiconductor laser 3 is focused always onto a disc 5 by the focus control, even if the objective lens 1 moves to any positon, the luminous flux returns to the original optical path at all times. Thus, the image of the luminous flux separated by a half mirror 7 is formed to a position symmetrical to the semiconductor laser 3 at the opposite side of the half mirror 7 without fail and a normal signal is obtained. Since the ratio of the number of apertures of the object side to the image side, i.e., the magnifying power is fluctuated more or less through the objective lens 1 moved in the focus direction, the fluctuation width is increased if the ratio is too small and an aberration is produced. If too large, the optical path length from the semiconductor laser 3 to the disc 5 is longer, it results in increasing the size of the optical pickup. Let the movable range of the objective lens be + or -1mm., then the ratio of the number of apertures is 3-5 as a proper value taking the above into account.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a disk-shaped recording medium (hereinafter simply referred to as a disk).
The present invention relates to an optical pickup that optically reads information by projecting an optical sporatra onto an information track on which a high-density digital signal is recorded.

Conventional configurations and their problems In recent years, with the spread of digital audio discs, the development of optical pickups has been rapid. The optical pickup used for this uses a semiconductor laser as the light source to make it smaller and lighter. In this optical start-up, as a method for making the objective lens follow the disk warpage, surface runout, or eccentricity of the information I/rack relative to the rotation center of the disk, 2-dimensional movement, ■ As shown in Figure 2, only the objective lens 1 is moved in the focusing direction, and the entire optical pink-up 2 including the laser, light-receiving element, and objective lens 1 is moved in the tracking direction. As shown in FIG. 3, the entire optical pickup 2 may be moved in both the focusing direction and the tracking direction.

Among these methods, ■ and ■ are the most commonly used methods, but in the conventional methods including ■, all the light that enters the objective lens is a parallel beam VC0 This is because the diameter of the beam of light of the objective lens The performance of the objective lens does not change no matter what direction the objective lens is moved in a large parallel beam of light, and the conventional commercially available objective lenses are designed to be used in a parallel beam of light. This is because

However, as shown in FIG. 4, since the semiconductor laser 3 used as the light source is a diverging light beam, it is necessary to use a collimating lens 4 in addition to the objective lens in order to create a parallel light beam. Since the collimating lens 4 is expensive, it not only increases the cost, but also increases the aberration of the entire optical system due to the increase in the number of optical components. Also, the focal length of the collimating lens is usually 14
Since the length is as long as ~17 mm, the distance from the semiconductor laser 3 to the disk 5 becomes long as shown in FIG.
When the optical path is bent using the total reflection prism 8 as shown in the figure, there are problems such as increased cost, increased aberrations, and poor assembly precision due to the total reflection prism 8. In addition, the conventional type In order to move the objective 1/lens at In order to achieve this, the only option is to further lengthen the focal length of the collimating lens 1 or increase the numerical aperture, which has its own limitations.Purpose of the InventionThe present invention solves the problems of the prior art as described above. ,
This provides a low-cost, high-performance, compact optical pickup.

Structure of the Invention The present invention has a structure in which the objective lens is omnipotent in the diverging light beam of the semiconductor laser, and by not using a collimating lens, aberrations can be reduced and the movable range of the objective lens can be increased. At the same time, it realizes low cost and miniaturization.

DESCRIPTION OF EMBODIMENTS FIG. 8 shows the configuration of an optical pickup in an embodiment of the present invention. In FIG. 8, 1 is an objective lens movably arranged facing the disk, and 6 is an objective lens. A driving device 7 is a mirror 71 for separating all of the reflected light from the disk 6 toward a light receiving element 9, and 10 is a detection optical system for detecting all focus errors or tracking errors. Note that 3 is a semiconductor laser, which has the same configuration as the conventional example.

The operation of the optical pickup of this embodiment configured as described above will be explained below.

The light emitted from the semiconductor laser 3 passes through the No.-7 mirror 7 and enters the objective lens 1 as a divergent beam. This light beam is condensed by the objective lens 1 and completely falls on a minute spot on the disk 6. This is reflected by the signal surface of the disk 6, and an image is formed on the light receiving element by the No.-7 mirror 7. At this time, since the focus is always focused on the lens 6 by focus control, the objective lens 1 is Even if it moves to a different position, the light beam always returns to its original optical path, so l・-
The light beam separated by the mirror 7 is always imaged at a position symmetrical to the semiconductor laser 3 with respect to the l\-7 mirror 7, and a normal signal can be obtained.

At this time, as the objective lens 1 moves in the focus direction, the ratio of the numerical aperture on the object side (column side) and the image side (disk side), that is, the magnification, changes slightly, so this ratio becomes small. If it is too large, the fluctuation range becomes large and aberrations occur. If it is too thick, the optical path length from the semiconductor laser 3 to the disk 6 will become long, resulting in a large optical pickup. Taking these into account, and assuming that the movable range of the objective lens is ±1 mm (this is a practical value), the appropriate numerical aperture ratio is 3 to 6 (disk side > semiconductor laser side). 0 Furthermore, since the objective lens 1 moves with a diverging beam 4 as shown in Fig. 8, the light will not be eclipsed if it exceeds a certain range as in the conventional example, and the movable range of the objective lens 1 can be widened. O As described above, since a collimating lens is not used in this example, the cost is not reduced, and the performance is also improved because there is less aberration and the movable range of the objective lens is wide. , downsizing and thinning can be achieved at the same time.

In this embodiment, the half mirror 7 is arranged between the semiconductor laser 3 and the objective lens 1, but it may also be a combination of a polarizing beam splitter and a dead wave plate, and if a three-beam method is used for tracking control, a diffraction mirror 7 may be used. You can also insert grids.

In any of these cases, since the distance from the semiconductor laser to the objective lens is fixed, it is possible to arrange it without increasing the overall length.

Effects of the Invention As described above, the present invention does not use a collimating lens and moves the objective lens within the diverging light beam emitted from the semiconductor laser, thereby reducing costs.

■ Aberrations can be reduced.

■ The movable range of the objective lens can be increased.

■ Can be made smaller and thinner 0 This effect can be obtained.

[Brief explanation of the drawing]

Figures 1 to 3 are diagrams showing all methods of moving the light spot of a conventional optical pickup, Figure 4 is a diagram showing a method of creating a parallel light beam in a conventional example, and Figures 6 and 6 are diagrams showing a conventional example. Figure 7 is a side view of the conventional objective lens.
The figure is a side view of one embodiment of the present invention, and FIG. 9 is a diagram showing the movable range of the objective lens of the same embodiment. 1... Objective lens, 3... Semiconductor laser, 6... Disk, 6... Objective lens drive device, 7... Half mirror. 19...
. . . Light receiving element, 1o . . . Detection optical system. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3 Collection 4 Figure 45 Figure 6 Figure 7

Claims (1)

[Claims] An objective lens is movably disposed facing an encoded information track provided on a disc-shaped recording medium, and is configured to move in a diverging beam emitted from a semiconductor laser. An optical pickup characterized in that the numerical aperture on the disk side is larger than the numerical aperture on the semiconductor laser side so that the ratio of the numerical apertures on the semiconductor laser side and the disk side is 3 to 6.