JP2650646B2 - Tracking error detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking error detecting device for detecting a tracking error with respect to an optical disk.

[0002]

2. Description of the Related Art For example, in an optical disc player,
A tracking servo for making the optical system follow the recording track on the optical disk is necessary. To do so, first, a tracking error, that is, the relative displacement between the recording track and the optical system in a direction perpendicular to the recording track is calculated. It is necessary to detect the direction of this displacement.

As a method of detecting a tracking error, a push-pull method is widely used because it is the simplest and effective. In this push-pull method, as shown in FIG. 5, a photodetector 1 for detecting reflected light from an optical disk is composed of two photodetectors 1a and 1b.

If the spot of the irradiation light is shifted to the left with respect to the recording track composed of the pit row, the photodetector 1
As shown in FIG. 5, the upper reflected spot 2 is dark on the left and bright on the right. Therefore, a tracking error can be detected by calculating the difference between the outputs of the photodetectors 1a and 1b.

[0005]

However, in such a method, the position of the light detector 1 is shifted with respect to the spot 2 of the reflected light, and the portion of the spot 2 on the light detecting section 1a and the light detecting section 1b If the upper part is not the same area, a direct current component is generated in the outputs from the light detection units 1a and 1b, and the tracking error cannot be accurately detected.

[0006] For this purpose, it is necessary to position the two very accurately, but this is not easy. In addition, there is a possibility that the displacement between the two may also occur due to aging.
Accordingly, it is an object of the present invention to provide a tracking error detecting device capable of accurately detecting a tracking error even when the photodetector is not so accurately positioned, in view of such a problem.

[0007]

A tracking error detecting apparatus according to the present invention separates a light beam reflected on a recording surface of an optical disc into a light beam for tracking and a light beam for focusing in directions different from each other, and A hologram member is provided that separates the light beam into first and second light beam portions and separates the light beams into the first and second light detection portions of the photodetector.

In the tracking error detecting device according to the present invention, the light beam reflected on the recording surface of the optical disk is separated into a light beam for tracking and a light beam for focusing by a member made of a hologram, and this light beam for tracking is converted into the first light beam. And the first and second light detectors are separated and separated from each other and are incident on the first and second light detectors of the photodetector. Even if the two light flux units are displaced up to the separation distance, no DC component is generated in the outputs from the first and second light detection units.

In addition, since the light beam for tracking and the light beam for focus are separated in different directions,
It is possible to prevent the tracking light beam from being affected by the focusing light beam. Further, the light beam reflected on the recording surface of the optical disc is separated into a light beam for tracking and a light beam for focusing by only a single optical component, ie, a member made of a hologram, and the light beam for tracking is divided into first and second light beams. Since it is separated into light beams, the number of optical components is small and interference such as unnecessary reflection between the optical components is small as compared with a structure using a plurality of optical components for these.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention applied to an optical disk player will be described below with reference to FIGS. FIG. 2 shows the overall configuration of this example.
Coherent light 12 emitted from a semiconductor laser 11 such as a laser diode passes through a beam splitter 13 and is converged on a recording surface of an optical disk 15 by a lens 14.

The light 1 reflected on the recording surface of the optical disk 15
2 enters the lens 14 again. The light 12 converged by the lens 14 and reflected by the beam splitter 13 is incident on a member 16 such as a photographic plate made of a hologram, and is incident on the photodetector 1 via the member 16.

The hologram on the member 16 is composed of a first portion and a second portion recorded twice.
3 and 4 show a method for recording these first and second parts. The first portion is recorded by interference between the light beams 17a and 17b as shown in FIG. 3A, and the second portion is recorded by the light beam 17a as shown in FIG.
Recorded by the interference between the data and 17c.

The light 17a has an angle corresponding to the incident angle of the light 12 reflected on the recording surface of the optical disk 15 and incident on the member 16 as shown in FIG.
The light is incident on the surface on the opposite side of the light-incident surface 2 through the beam splitter 21.

The light 17 b is incident on the member 16 via the lens 22, the sheet-like diaphragm 23, and the beam splitter 21. The stop 23 is provided with a semicircular opening 23a as shown in FIG. 3B, and the lens 22 and the stop are arranged so that the light 17b passing through the opening 23a is incident only on the left half of the member 16, for example. 23 are arranged.

During recording using the light 17b, for example, the right half of the member 16 to which the light 17b does not enter is
The member 16 and the beam splitter 2 are arranged so that the incident light 7a does not enter.
1 and a shielding member 24 is disposed. Therefore, the first portion of the hologram, which forms a semicircle similar to the opening 23a, is recorded only on the left half of the member 16 due to the interference between the lights 17a and 17b.

The light 17c is light incident on the member 16 via the lens 25, the sheet-like aperture 26, and the beam splitter 21. The stop 26 is provided with a semicircular opening 26a opposite to the opening 23a as shown in FIG. 4B, and the light 17c passing through the opening 26a is incident only on the right half of the member 16, for example. Thus, the lens 25 and the aperture 26 are arranged.

During recording using the light 17c, for example, the light 1 is applied to the left half of the member 16 where the light 17c is not incident.
The member 16 and the beam splitter 2 are arranged so that the incident light 7a does not enter.
1 and a shielding member 27 is disposed. Accordingly, the second portion of the hologram, which is a semicircle similar to the opening 26a, is recorded only on the right half of the member 16 due to the interference between the lights 17a and 17c.

Holograms are classified into several types according to their recording methods. In this embodiment, phase holograms are used. The phase hologram diffracts the reproduction light by phase modulation, and in this specific example, the light is converged by this diffraction. In the phase hologram, light 17a, 17b,
By selecting the intensity 17c and the exposure time, it is possible to make the utilization ratio of the reproduction light 100%.

As described above, a photographic dry plate or the like is usually used as the member 16, but in the case of a phase hologram, the material of the member is appropriately selected according to the above-mentioned utilization rate. Gelatin is used as a member in order to obtain a higher utilization factor than silver salts used in ordinary photographic plates.

When gelatin is used as a member, the recording light beams 17a and 17a depend on the material of the photosensitive material.
Usually, b and 17c and the light 12 as the reproducing light have different wavelengths. For example, the wavelength of light 12 is 0.8 μ
m, the light 17a, 17b, 1
The wavelength 7c is a shorter wavelength band than this band.

When light having different wavelengths is made incident on the hologram, the emission angles are different even if the incident angle is the same. Therefore, in order to obtain the same convergence point, the incident angle is different for light having different wavelengths. It needs to be different.

By the way, orthogonal coordinates are set with the center of the member 16 as the origin and the direction perpendicular to the member 16 as the z-axis direction, and the light 17b or 17c travels in reverse from the member 16 only in the air and converges. Let the coordinates of the point be (x ₁ , y ₁ , z
₁ ), the coordinates of the light source of the light 17 a are (p ₁ , q ₁ , r ₁ ), and the coordinates of the point where the light 12 in FIG. 2 is converged by the hologram on the member 16 are (x ₂ , y ₂ , z ₂₎ ), The coordinates of a point simply transmitted through the hologram and converged by the action of only the lens 14 are (p ₂ , q ₂ , r ₂ ), and the wavelengths of the recording light beams 17a, 17b, 17c are λ ₁ , Assuming that the wavelength of the light 12 as the reproduction light is λ ₂ and the magnification of the hologram after recording is m, the imaging formula of the hologram is represented by the following equation.

[0023]

(Equation 1)

Assuming that the hologram on the member 16 is an on-axis hologram as shown in FIGS. 3A and 4A, the light 17b or 17c travels in reverse from the member 16 only in the air. The position of the converging point, that is, the positional relationship of the photodetector 1 of FIG.

The light 16a and the light 17a are applied to the member 16 as described above.
Holograms, which are diffraction gratings, are recorded by 7b and 17c, but the intensity of the diffracted light changes according to the incident angle of the light to the diffraction grating, and the highest diffraction efficiency is obtained when the Bragg angle condition is met. At the time of hologram recording,
A mirror or the like may be used instead of the beam splitter 21.

When the hologram on the member 16 is recorded as described above, the first hologram of the hologram on the member 16 in the light flux of the light 12 reflected on the recording surface of the optical disk 15 as shown in FIG. The light beam portion incident on the portion is turned into a light beam portion having a semicircular cross section which is converged by the first portion and travels in the opposite direction to the light 17b. In the light flux of the light 12, the light flux part incident on the second portion of the hologram becomes a semicircular light flux part having a cross section which is converged by the second part and travels in the opposite direction to the light 17 c.

As a result, when the light 12 irradiates a position other than the pits on the optical disk 15, a single circular spot 2 is formed on the photodetector 1 as shown in FIG. Instead, as shown in FIG. 1, a semicircular first spot 31 and a second spot 32 are formed separately from each other.

Also, as in the case of FIG. 5, if the spot of the light 12 is shifted to the left with respect to the recording track, of the light flux of the light 12 reflected by the optical disk 15, the light is directed to the second portion of the hologram. Since the incident light beam portion is strong and the light beam portion incident on the first portion is weak, the spot 32 is bright and the spot 31 is dark. Therefore, as described with reference to FIG.
The tracking error can be detected by calculating the difference between the outputs b and b.

Although the hologram recorded on the member 16 is an on-axis hologram in the above description, it is an off-axis hologram in this specific example. Therefore, the tracking light beam can be separated in a direction different from that of the focusing light beam so that the two do not affect each other.

[0030]

In the tracking error detecting device according to the present invention, even if the first and second light detecting portions are displaced to the distance between the first and second light flux portions, the first and second light detecting portions are displaced. Since a DC component does not occur in the output from the detection unit and the tracking light beam is not affected by the focusing light beam, a tracking error can be accurately detected. Also, since the number of optical components is small, a tracking error can be detected with a simple configuration, and interference such as unnecessary reflection between the optical components is small, so that the tracking error can be detected with higher accuracy.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing a spot formed on a photodetector in a hologram used in one embodiment of the present invention.

FIG. 2 is a schematic side view showing the overall configuration of one specific example.

FIG. 3 is a schematic side view showing a hologram recording method used in one specific example.

FIG. 4 is a schematic side view showing a hologram recording method used in one specific example.

FIG. 5 is a schematic plan view showing a spot formed on a photodetector in a conventional example of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Photodetector 1a, 1b Photodetector 12
Light 15 optical disk 16 member 31,
32 spots

Claims (1)

(57) [Claims] 1. A light beam reflected on a recording surface of an optical disk is separated into a light beam for tracking and a light beam for focusing in directions different from each other, and the light beam for tracking is separated into first light beams.
And a tracking error detection device having a hologram member which is separated into a second light beam portion and separated from each other and is incident on the first and second light detection portions of the photodetector.