JPH06111360A - Optical pickup device - Google Patents

Abstract

PURPOSE:To surely read a high density recorded information. CONSTITUTION:A second objective lens 20, which image forms high region signals, is placed adjacent to a first objective lens 13 which detects low region signals. The opening of the beam which passes the lens 20 is controlled by an aperture control diaphragm 21 at the image forming position of the lens 20. A signal demodulator 23 demodulates the information that indicates pit lengths contained in the formed image by the lens 20 and the information by the lens 13. Thus, the reproduced spatial frequency of an optical pickup is equivalently made wider, a short pit is identified without using a larger NA objective lens and light spot form variation caused by the aberration such as the tilt of an optical disk 14 is made small. Therefore, a disk information recorded at a high density is surely read.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device for reproducing a video signal, an audio signal, etc. recorded on an optical recording medium such as an optical disc, and particularly for reading an optical disc having an increased information recording density. The present invention relates to an optical pickup device for performing.

[0002]

2. Description of the Related Art As the amount of information to be recorded on an optical disk increases, it is required to increase the amount of information read. That is, for example, when recording or reading a high definition television signal, a wide band is required so that a signal up to a high frequency region can be recorded / reproduced.

In order to achieve this, it has been attempted to record signals more finely on an optical disc. That is, this is a method for recording / reproducing a large amount of information by increasing the number of pits read per unit time by increasing the number of pits recorded per unit length.

However, in the case of such a method, the pitch length on the optical disk is shortened, so that the reproduction spatial frequency of the optical pickup needs to be broadened in order to resolve it. For that purpose, it is necessary to shorten the wavelength of the light source or increase the numerical aperture of the objective lens.
However, in order to shorten the wavelength of the light source, it is necessary to use a gas laser, a laser using a non-linear optical element, or the like instead of the current semiconductor laser, so that the device becomes bulky and the cost of the device increases.

Further, if the numerical aperture of the objective lens is increased, the objective lens becomes large, making it difficult to make the objective lens, and the tilt of the optical disc and the unevenness of the optical disc surface affect the optical disc. The shape of the light spot changes greatly at the same time, and the signals of the surrounding pits are also read.

Even if an attempt is made to manufacture an optical pickup compatible with high-density information in consideration of the above, it is difficult to manufacture the optical pickup with the present technology, so that the optical disk can be highly densified. Not not.

Here, a basic form of an optical pickup currently in use is shown in FIG. 1, for example. In the figure, reference numeral 11 is a lateral single-mode semiconductor laser forming a point light source, reference numeral 12 is a half mirror for separating the light to be irradiated onto the optical disk 14 and the return light from the optical disk 14, and reference numeral 13 is recording on the optical disk 14. An objective lens for collecting light on the surface and a photodetector 15 for detecting the return light from the optical disk 14 are shown.

In the optical pickup having such a structure, the diameter of the light emitting point of the semiconductor laser 11 which is a point light source is 0.1 μm.
It is about m. The light emitted from the semiconductor laser 11 is
When it is folded back toward the objective lens 13 side by the half mirror 12, it is condensed on the recording surface of the optical disc 14 by the objective lens 13. At this time, the wavelength λ of the light emitted from the semiconductor laser 11 and the numerical aperture NA of the objective lens 13 are λ / N
A> 0.1 μm is selected. Therefore, the size of the spot on the optical disk 14 is limited by the so-called diffraction limit and is λ / NA.

Here, information as a modification of optical properties is written in advance on the optical disc 14. As an example thereof, a case where information is recorded as pits (pits) on the optical disc 14 will be described. Return light from the spot on the optical disc 14 causes diffraction according to the size of the pit, and the diffracted light After being made strong and weak by the incident portion of the objective lens 13, the half mirror 12
Be guided to the side.

A part of the light reaching the half mirror 12 is
The light is transmitted through the half mirror 12, is given astigmatism, and is imaged on the detection surface of the photodetector 15. At this time, the astigmatism causes a difference in the light-converging position between the vertical direction and the horizontal direction of the optical axis.
Is provided.

That is, for example, as shown in FIG. 2, the detection surfaces 15a to 15d of the photodetector 15 have a divided shape, and when the focal position is far, the same figure (a) is obtained, and when the focal position is close. The position of the photodetector 15 is adjusted so as to be in the same figure (c) and in the same figure (b) at the in-focus position.

As a result, the detection surface 15a of the photodetector 15 is
In the detection of the received return light by ˜15d, the difference between focus and tracking is detected by calculating the intensity difference between the diagonal component and the radial component of the respective detection surfaces 15a to 15d, so that the intensity difference is zero. The servo control of the incident light on the recording surface of the optical disc 14 is performed by applying the focus and tracking servo so that

At this time, the detection surfaces 15a-
The image corresponding to the spot on the optical disk 14 is not imaged on 15d, but a so-called far-field image, which is deviated by a half of the difference of the condensing position, is imaged.

[0014]

As described above, in the conventional optical pickup described above, the numerical aperture N of the objective lens 13 is N.
By selecting A for λ / NA> 0.1 μm, the size of the spot on the optical disk 14 is set to λ / NA limited by the diffraction limit, and the intensity difference between the detection surfaces 15a to 15d of the photodetector 15 is set. Is calculated, and focus and tracking servo are applied so that the intensity difference becomes zero.

However, without changing the wavelength λ of the light emitted from the semiconductor laser 11 and the numerical aperture NA of the objective lens 13, the length of the information pit is shortened to improve the information recording density on the optical disc 14. In this case, since the diffracted light due to the pits is removed from the objective lens 13, the optical cutoff frequency exceeds 2NA / λ, and it becomes impossible to detect the information of the pits.

The present invention has been made in consideration of such a situation, and is for receiving the high-order diffracted light of the pit adjacent to the first objective lens for forming the light spot on the recording medium. By disposing the second objective lens,
An object of the present invention is to provide an optical pickup capable of reading high density information.

[0017]

The invention according to claim 1 is
An optical pickup device for reading information recorded on an optical recording medium, the first objective lens condensing light emitted from a light source onto the recording medium, and the first objective lens. A second objective lens which is disposed adjacent to the second objective lens and receives high-order diffracted light of the pit of the recording medium, and an image forming point of the second objective lens. An aperture limiting capturing means for capturing the higher-order diffracted light that has passed through the second objective lens by limiting the aperture of the transmitted light, and the return light obtained through the aperture limiting capturing means and the first The information recorded on the recording medium is read from the return light obtained through the first objective lens.

According to the second aspect of the invention, the size of the aperture at the image forming point is smaller than the size of the image of the light spot on the recording medium formed by the second objective lens. Characterize.

[0019]

In the optical pickup device of the present invention, of the diffracted light from the recording medium, the high-order diffracted light that is out of the first objective lens is the second diffracted light disposed adjacent to the first objective lens.
Collected by the objective lens of. However, the image by the return light collected by the second objective lens is larger than the size of the image that can be resolved by the second objective lens, and also includes adjacent pit information. Here, by limiting the beam width of the return light that has passed through the second objective lens or limiting the light receiving area of the light receiving surface of the photodetector that receives the return light, an extremely short pit on a predetermined track is formed. The information of can be read together.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings. In the drawings described below, parts common to those in FIGS. 1 and 2 are designated by the same reference numerals, and duplicated description will be omitted.

FIG. 3 shows an embodiment of the optical pickup of the present invention. Reference numeral 11 is a semiconductor laser, reference numeral 1
2 is a half mirror, 13 is a first objective lens, 14 is an optical disk, 15 is a first photodetector for detecting the reflected light collected by the first objective lens 13, and 20 is a first photodetector. The second objective lens for forming an image of the high-order diffracted light that has deviated from the first objective lens 13, reference numeral 21 is arranged at the image forming position of the second objective lens 20, and the second photodetector 22
Aperture limiting diaphragms for limiting the aperture of the light to be imaged are respectively shown. Reference numeral 23 denotes a signal demodulator for generating a signal corresponding to the pit length from the signals detected by the first photodetector 15 and the second photodetector 22.

Next, the operation of the optical pickup having such a configuration will be described. First, the emitted light from the semiconductor laser 11 is made into a light spot having a size of λ / NA on the optical disk 14 by the first objective lens 13 as in the conventional configuration, and the reflected light is the first objective lens 13. And the first photodetector 15 via the half mirror 12.
The first photodetector 15 detects a signal in a portion where the pit density is low, and also performs focus and tracking servo.

At this time, the reflected light on the surface of the first objective lens 13 is in a state as shown in FIG. That is, assuming that the pit period is d, this first-order diffracted light is sin
It emerges away from the 0th-order light by an angle θ such that θ = λ / d. Here, θ becomes larger as the pit period d becomes shorter, and the amount of the first-order diffracted light incident on the first objective lens 13 gradually decreases, so that the modulation degree becomes smaller. Finally, the diffracted light is removed from the first objective lens 13.

Since the second objective lens 20 is disposed at a position outside the first objective lens 13 of the diffracted light, the second objective lens 20 allows the first objective lens 13 to solve the problem. Diffracted light from a short pit that cannot be imaged is captured.

However, since the reflected light collected by the second objective lens 20 is larger than the size of the pits in which the light spot diameter on the optical disk 14 can be resolved, the reflected light in the far field image is different from the conventional one. If the total intensity is detected, the information of several pits will be mixed, and it will be difficult to obtain the desired pit information.

For this reason, the aperture of the light passing through the second objective lens 20 is limited by the aperture limiting diaphragm 21 having a predetermined aperture provided at the image forming point of the second objective lens 20. Only desired pit information can be obtained. In addition,
The same effect can be obtained by limiting the detection area of the detection surface of the second photodetector 22 instead of limiting the aperture of the light passing through the second objective lens 20 by the aperture limiting diaphragm 21.

By forming a pit image by the second objective lens 20, an image in which only the high frequency component of the original pit shape is extracted is formed. Here, FIG.
4A is a graph showing the pit shape on the optical disc 14, and FIG. 4B is a graph showing the intensity change of the image formed by the second objective lens 20.

As described above, since the image formation by the second objective lens 20 includes the information indicating the pit length, the signal demodulator 23 combines this information and the signal from the first objective lens 13. By demodulating the signal by, the reproduction spatial frequency of the optical pickup is broadened equivalently.

Here, by directing the main axis of the second objective lens 20 in the direction of the light spot formed by the first objective lens 13, the influence of image deterioration due to the image height of the second objective lens 20 is avoided. be able to.

As described above, in this embodiment, the second objective lens 20 for focusing a high-frequency signal is arranged adjacent to the first objective lens 13 for focusing a low-frequency signal, and At the image forming position of the second objective lens 20, the aperture limiting diaphragm 21 limits the aperture of the light passing through the second objective lens 20.

Therefore, the signal demodulator 23 demodulates the information indicating the pit length included in the image formed by the second objective lens 20 and the information obtained by the first objective lens 13 together to obtain an equivalent signal. Since the reproduction spatial frequency of the optical pickup can be widened, it is possible to identify a short pit without using a large NA objective lens, and to change the shape of the light spot due to the aberration such as the inclination of the optical disk 14. Since it can be made small, the disc information recorded at high density can be read reliably.

In this embodiment, the second objective lens 2
The case where the signal detection is performed at the image forming position of 0 has been described. However, in order to perform this detection, it is necessary to form an image only in the direction in which the objective lenses are lined up. The detection may be performed at the focal position on one side of the obtained light flux.

Further, in this embodiment, the first objective lens 13
Based on the return light from the optical disk 14 collected by
Although the case where focus and servo control are performed has been described, the present invention is not limited to this example, and the servo control may be performed based on the detection result of the signal level by the second photodetector 22. The servo control may be performed based on the detection results of the photodetector 15 and the second photodetector 22.

Further, in this embodiment, the second objective lens 20 is used.
Although the case where the arrangement direction of the second objective lens 2 is the circumferential direction of the optical disc 14 has been described, the present invention is not limited to this example.
By arranging 0 in the radial direction of the optical disk 14, it is possible to detect a signal even if the rack pitch is short.

Furthermore, in the present embodiment, the optical disc 14 has been described in which information is formed by forming pits, but the present invention is not limited to this example, and other optical discs in which information is recorded by changes in reflectance and polarization state. It is also possible to apply to.

In the present embodiment, the second objective lens 20
The case where the aperture limiting diaphragm 21 is provided at the image forming position and the desired pit information is obtained by using the aperture limiting diaphragm 21 has been described. However, the present invention is not limited to this. The second photodetector 22 may be arranged at the image forming position in a state where the aperture is reduced. In this case, the same effect as when the aperture limiting diaphragm 21 is provided can be expected, and The aperture limiting diaphragm 21 can be omitted.

Further, in the present embodiment, the case where the first photodetector 15 and the second photodetector 22 are separately provided has been described, but the present invention is not limited to this example, and the second objective lens 20 is used. By disposing the mirror so as to guide the collected light to the first photodetector 15, the second photodetector 22 can be omitted.

Furthermore, in this embodiment, the case where the two signals of the first photodetector 15 and the second photodetector 22 are individually demodulated by the signal demodulator 23 has been described. The present invention is not limited to this, and various methods may be adopted, such as performing demodulation after adding both signals in advance at a stage before being taken into the signal demodulator 23, or changing the demodulation method according to the type of pit detected. Good.

In this embodiment, the second objective lens 20
Although the NA and the shape thereof are not particularly described, the NA can be selected according to the signal frequency recorded on the optical disc 14, and the shape is circular as shown in FIGS. In addition, various shapes such as a rectangle, a semicircle, and a crescent shape can be used.

Furthermore, in the present embodiment, the case where the first and second objective lenses 13 and 20 are separately provided has been described, but the present invention is not limited to this, and the two may be integrally formed by resin molding, for example. Furthermore, the first and second objective lenses 13 and 2
The focal length of 0 may be the same, and in this case, the two photodetectors 15 when the optical disk 14 moves up and down,
It is possible to suppress the defocus amount of 22 to be small.

[0041]

As described above, according to the optical pickup device of the present invention, of the diffracted light from the recording medium, the high-order diffracted light that is out of the first objective lens is located adjacent to the first objective lens. The return light collected by the second objective lens limits the beam width of the return light passing through the second objective lens or receives the return light. Since the light receiving area of the light receiving surface of the photodetector is limited, the information of the extremely short pits on a predetermined track can be read together.

Therefore, by arranging the second objective lens for receiving the high-order diffracted light of the pit adjacent to the first objective lens for forming the light spot on the recording medium, it is equivalent. Since the reproduction spatial frequency of the optical pickup can be broadened, short pits can be identified without using an objective lens having a large NA, and the change in shape of the light spot due to aberration such as tilt of the recording medium can be reduced. Therefore, high-density information can be read.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a conventional optical pickup.

FIG. 2 is a diagram showing a detection surface of the photodetector of FIG.

FIG. 3 is a diagram showing an embodiment of an optical pickup of the present invention.

FIG. 4 is a diagram showing a pit shape on the optical disc of FIG. 3 and a change in intensity of an image formed by a second objective lens.

5A and 5B are diagrams showing another embodiment in which the shape of the second objective lens in FIG. 3 is changed.

[Explanation of symbols]

 11 semiconductor laser 12 half mirror 13 first objective lens 14 optical disk 15 first photodetector 20 second objective lens 21 aperture limiting diaphragm 22 second photodetector 23 signal demodulator

Claims (2)

[Claims] 1. An optical pickup device for reading information recorded on an optical recording medium, comprising: a first objective lens for condensing light emitted from a light source onto the recording medium; A second objective lens disposed adjacent to the first objective lens for receiving higher-order diffracted light of the pit of the recording medium, and an image forming point of the second objective lens. By limiting the aperture of light that has passed through the second objective lens,
An aperture limiting capturing means for capturing the higher-order diffracted light that has passed through the second objective lens, and return light obtained through the aperture limiting capturing means and return light obtained through the first objective lens. An optical pickup device characterized in that information recorded on the recording medium is read out from light. 2. The size of the aperture at the image formation point is smaller than the size of the image of the light spot on the recording medium formed by the second objective lens. The optical pickup device described.