JP2561253B2 - Track error detector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a track error detecting device for an optical head used for recording and reproducing so-called optical disc, digital audio disc, video disc, magneto-optical disc, and the like.

(Prior Art) Track error detection methods used in conventional optical heads of video discs, digital audio discs, and magneto-optical discs (hereinafter collectively referred to as optical discs) are mainly push-pull method, heterodyne method, three-track method. There is a beam method. The push-pull method and the heterodyne method are methods using a far-field pattern of reflected light of a main beam used for reading data from an optical disk, and the three-beam method is a method using an auxiliary beam for detecting a track error.

The error detection method using the far field pattern is
It utilizes that the light amount distribution of the reflected light diffracted by the pits or pregrooves formed on the surface of the optical disc changes depending on the positional relationship between the pits or pregrooves and the light spot.

FIG. 2 shows a disk on the four-division photodetector 1 shown in FIG. 3 placed in the far field under the condition that the pit depth is λ / 5, where λ is the wavelength of the light source used. 3 schematically shows how the light amount distribution of the reflected light 2 from the light changes depending on the relative position to each pit. Fig. 2 shows PP = (a + d)-(b + c) (1) HTD = (when the output of each segment A3, B4, C5, D6 of the 4-split photodetector is a, b, c, d. a + c)-(b + d) (2) RF = a + b + c + d (3) The three signal changes are shown. The PP signal is the difference signal of the sum signals of the segments arranged in the pit traveling direction, the HTD signal is the difference signal of the diagonal sum, and the RF signal is the total sum signal. The horizontal axis of FIG. 2 represents the relative position of the pit and the spot, and corresponds to the time axis.

The push-pull method is the method most closely related to the present invention, and uses the PP signal obtained by the equation (1) as a track error signal. The light amount distribution in the traveling direction of the track generated in each pit is asymmetric in the case of off-track. By performing the calculation of the formula (1) from the 4-division photodetector 1 by this asymmetric distribution, or by taking the output difference of the 2-division photodetector having a function equivalent to that of the 4-division photodetector in this case, the track is calculated. A track error signal corresponding to the direction of the light spot shift from is obtained. When the light spot passes through the center of the track as shown in FIG. 2 (b), the PP signal is always zero.

If the track moves from the state shown in FIG. 2A to the state shown in FIG. 2C as the disc rotates, the average value of the PP signal changes from plus to minus. Therefore, it suffices to apply the track servo according to the sign of the PP signal.

FIG. 4 shows a conventional track error detecting device by the push-pull method. The light emitted from the semiconductor laser 31 is condensed on the optical disc 40 by the converging lens 39 via the polarization beam splitter 42 and the λ / 4 plate 41. The reflected light from the optical disk 40 follows the reverse path, and the polarization beam splitter 42 bends the path thereof by 90 °, and the first photodetector 30 and the second photodetector 30 are separated by the optical axis as a boundary.
It is led to the photodetector 32. The track error signal is obtained from the difference signal between the outputs of the first photodetector 30 and the second photodetector 32.

Next, the heterodyne method will be described. Fig. 2 (b)
When the light spot passes through the center of the track like
The difference signal HTD of the diagonal sum of the 4-division photodetector 1 given by the equation (2) is always zero. (A) or (c)
As described above, the phase of the HTD signal is advanced or delayed by 90 ° with respect to the total signal RF depending on the direction of the deviation of the light spot. Therefore, if the phase of the HTD signal is detected with reference to the RF signal, a bipolar track error signal can be obtained. Heterodyne detection is often used as the phase detection means.

FIG. 5 shows a track error detecting device by the three-beam method. In addition to the main beam 53, the ± first-order diffracted lights 54 and 55 divided by the diffraction grating 51 are used as auxiliary beams for detecting a track error. FIG. 6 is a diagram for explaining the principle of track error detection by the three-beam method. The three light beams form three spots on the optical disc as shown in FIG. 6, and when the main spot 7 is on-track, 2
The two auxiliary spots 8 and 9 are imaged so as to be slightly off-track in the opposite directions. When the right off-track shown in FIGS. 6B to 6A is generated, the reflected light from the second auxiliary spot 9 is diffracted by the pits 10, and therefore, compared with the reflected light from the first auxiliary spot 8. Becomes weaker. Similarly, when the left off-track shown in FIG. 6C occurs, the reflected light from the first auxiliary spot 8 becomes weaker than the reflected light from the second auxiliary spot 9. Therefore 2
The track error signal can be obtained from the difference in the reflected light intensity from the two auxiliary spots.

(Problems to be Solved by the Invention) The conventional techniques described above have the following problems. In the push-pull method, when the focusing lens is moved in the direction perpendicular to the optical axis by the actuator based on the track error signal, as shown in FIG.
Then, the division line 13 of the two-division photodetector 12 is displaced. Since the reflected light from the optical disk surface is incident on the two-division photodetector 12 with the optical axis of the converging lens as the center, the amount of light is not correctly divided by the division line 13 of the two-division photodetector 12, and in FIG. More light will be incident on the photodetector 14 than on the B photodetector 15. As a result, DC offset occurs in the tracking error signal, and the control range of tracking error control is narrowed. Similar problems occur when the optical disc is tilted. The heterodyne method has a drawback in that a track error signal cannot be detected from a medium having only pregrooves without pits because it utilizes the change in the diffraction pattern generated at the edge of each pit. Further, since the heterodyne detection is used, there is a drawback that the electric circuit becomes complicated.

Further, in the three-beam method, if the carriage feed using the swing arm is adopted, there is a drawback that the signal strength changes depending on the rotation angle of the swing arm.

An object of the present invention is to provide a track error detecting device that solves the above drawbacks.

(Means for Solving the Problems) In order to solve the above problems, the means provided by the present invention is to collect reflected light that is focused on a pit or pre-groove of an optical recording medium through an objective lens and reflected. A hologram element for extracting the reflected light extracted outside the optical axis into the first and second photodetectors while extracting the reflected light outside the optical axis of the imaging optical system including at least a light source and an objective lens; The hologram optical element includes at least a first photodetector and the second photodetector, and the hologram optical element has a diameter F of the objective lens, a focal length f, and a track pitch of pits or pregrooves of the optical recording medium P. , Λ is the wavelength of the light source, and δ is an amount that satisfies the relationship of 0 ≦ δ ≦ (maximum movable distance of the objective lens in the track direction of the recording medium), and the recording medium has passed through the objective lens. The beam diameter of the reflected light is d, and in the vertical cross section of the light beam, the optical recording medium track and the parallel axis are the X-axis and X-axis.
When the axis orthogonal to the axis and the light beam is the Y axis, A light beam incident on a region satisfying the following condition to the first photodetector, And diffracting the light beams incident on the region satisfying the following condition so that they are incident on the second photodetector, and the light beams incident on the other regions are incident on the focus error detector. It is a characteristic track error detection device.

(Operation) Hereinafter, the operation of the device of the present invention will be comprehensively described.

The operation of the present invention is as follows. For simplicity, it is assumed that the reflected light from the optical disc is converted into collimated light by the objective lens and taken out of the optical axis of the imaging optical system. Figure 8 shows the reflected collimated light in the case of on-track.
The light intensity distribution in the cross section of 16 is schematically shown with X = Y = 0 as an optical axis in the case of a medium having a pre-groove. The X axis is parallel to the track alignment direction. The light intensity is divided into the first region 17, the second region 18, and the third region 19, and if the light intensity distribution of the light wave incident on the objective lens from the light source within the aperture of the objective lens is uniform, within each region Light intensity is substantially uniform, and the first region 17 and the third region 19 have equal intensity.

The second region 18 does not include the diffracted light other than the 0th order from the pre-groove, whereas the first region 17 and the third region 19 have the overlapping of the reflected light from the medium and the diffracted light from the brigrove. . Therefore, it is the first and third regions that show the strongest intensity change when off-track occurs.
That is, it is most sensitive to obtain the track error signal using the light in the first and third regions. Since the positions of the first and third regions are due to diffraction from the pre-grave, when reflected light is converted into collimated light, it is approximately given by the following equation.

Here, F is the aperture of the objective lens, f is the focal length of the objective lens, λ is the wavelength, and P is the pitch of the pregroove.
If the track error signal is obtained from the total amount of light in the first area 17 and the total amount of light in the third area 19, the largest error signal can be taken out. When the optical axis of the convergent optical system and the optical axis of the error detection system are displaced by the operation of the actuator in the tracking direction, a DC offset is generated in the error signal. Therefore, let δ be an amount that satisfies 0 ≦ δ ≦ (maximum movable amount of the objective lens in the tracking direction), and from the first region, From the first part that satisfies the relation The track error signal is obtained from the amount of light incident on the second portion satisfying the relation. As shown in FIG. 9, the first portion 20 and the second portion 21 respectively have the first reflected light 22 from the optical disk 22 even when the optical axes are deviated from each other.
The amounts of light incident on the same area are extracted from the region 23 and the third region 24, respectively. Therefore, the occurrence of the track error signal offset can be prevented.
FIG. 10 shows the calculation of the track signal offset according to the present invention and the track signal offset by the conventional push-pull method with respect to the axis shift of the converging lens.

In the present invention, almost no track signal offset occurs.

Further, when the reflected light guided to the error detection optical system is not a collimated light but a diverging wave or a converging wave, the beam diameter at the desired position of the diverging wave or the converging wave is dn, and the diameter of the converging lens is If it is Fn, as the first part, As the second part, By extracting the light of the part satisfying the above condition, the same function as that of the collimated light can be obtained.

The optical element having the above effects can be realized by using any of the effects of reflection, refraction and diffraction, but as an optical axis separating means for extracting the reflected light from the optical disk out of the optical axis of the imaging optical system. An optical element having a combination of functions can be realized only by using a hologram optical element which is a diffractive optical element.

(Embodiment) FIG. 1 is a diagram showing a first embodiment of the present invention. In the present embodiment, the present invention is implemented by the effect of light diffraction by the hologram 26. A hologram 26 composed of four regions is formed on a transparent substrate 27. Hologram first
The region 28 and the second region 29 are formed at positions satisfying the expressions (8) and (9), respectively.
Interference fringes due to two diverging spherical waves whose light sources are the points on the photodetector 30 and the semiconductor laser 31, respectively, are recorded. Similarly, in the second area 29, interference fringes due to two diverging spherical waves having the points on the second photodetector 32 and the semiconductor laser 31 as light sources are recorded. The third area 33 and the fourth area 34 of the hologram are respectively point A 36 on the four-division photodetector 35,
Interference fringes are formed by two diverging spherical waves using the point B 37 and the semiconductor laser 31 as a light source. Hologram third area 33
Since the joining line 38 of the hologram fourth region 34 functions equivalently to the knife edge, the focus error signal can be obtained from the diagonal difference signal of the four-division photodetector 35. Of the light emitted from the semiconductor laser 31, the light that has passed through the hologram as the 0th-order diffracted light is condensed on the optical disc 40 by the converging lens 39. The reflected light from the optical disc is converged by the converging lens 39 into a converging spherical wave that converges on the light emitting point of the semiconductor laser and enters the hologram. Since the above-mentioned striped pattern is recorded on the hologram as surface irregularities, the light incident on the area satisfying the expression (8) is input to the first photodetector and the light incident on the area satisfying the expression (9) is It is diffracted so that it is incident on the second photodetector. Therefore, the track error signal can be obtained by taking the differential between the outputs of the first and second photodetectors. In this embodiment, the hologram also holds the function of separating the optical axes.

FIG. 11 is a diagram for explaining the second embodiment of the present invention. In this embodiment, the λ / 4 plate 41 and the polarization beam splitter 42 are used to separate the reflected light outside the optical axis of the imaging optical system. The reflected light extracted outside the optical axis is guided to the photodetectors 30, 32, 45 via the transparent substrate 44. The light of the area satisfying the expression (8) is transmitted to the first photodetector 30 on the transparent substrate 44,
A prism 46 that refracts light that satisfies the expression (9) so as to enter the second photodetector 32 is installed. Therefore, the track error signal can be obtained from the differential between the outputs of the first photodetector 30 and the second photodetector 32. Transparent substrate 44
Of course, it is also possible to obtain the focus error signal and the read signal by using the light in the region that does not satisfy the expressions (8) and (9). Further, instead of installing the prism on the transparent substrate, the same effect can be obtained by installing the same hologram as in the first embodiment.

FIG. 12 is a diagram for explaining the third embodiment of the present invention, which is an example in which the transparent substrate 44 of the second embodiment is replaced by a reflecting mirror 47. In addition, the reflected light from the disk is extracted off-axis as collimated light. The sub-mirror so that the reflection angle of the corresponding portion is different from that of the other portions so that the light in the regions satisfying the expressions (8) and (9) is guided to the first and second photodetectors 30 and 32, respectively. 48 are installed.
Of course, it is also possible to obtain the focus error signal and the read signal by using the light in the region that does not satisfy the expressions (8) and (9). Further, instead of changing the reflection angle of the reflected light from the area satisfying the expressions (8) and (9), a reflection hologram is installed in the corresponding area, and the reflected light from the optical disc is detected by diffraction. Of course, it is possible to lead to a vessel. At that time, the stripe pattern on the reflecting surface to be recorded as a hologram becomes an interference fringe formed by two diverging spherical waves having a semiconductor laser and a point on the photodetector as a light source on the reflecting surface.

(Effect of the Invention) According to the present invention, it is possible to realize a track error detection device in which an offset is hardly generated in a track error signal. Further, the optical element used in the present invention can be easily manufactured by the conventional molding technique, and an inexpensive track error detecting device with high mass productivity can be obtained.

[Brief description of drawings]

1 is a perspective view for explaining a first embodiment of the present invention, FIG. 2, FIG. 3, FIG. 4, FIG. 5 and FIG. FIG. 7 is a diagram for explaining the problems of the prior art, FIGS. 8, 9, and 10 are diagrams for explaining the operation of the present invention, and FIG. 11 is a second embodiment of the present invention. And FIG. 12 is a sectional view for explaining the third embodiment of the present invention. In the figure, 1,35 ... 4-division photodetector, 2 ... Reflected light, 3 ... Segment A, 4 ... Segment B, 5 ... Segment C, 6
...... Segment D, 7 ...... Main spot, 8 ...... First auxiliary spot, 9 ...... Second auxiliary spot, 10 ...... Pit, 11
…… Converging lens optical axis, 12 …… 2-division photodetector, 13 …… Dividing line, 14 …… Photodetector A, 15 …… Photodetector B, 16 …… Reflected collimated light, 17,23 …… 1st area of reflected light, 18 ... 2nd area of reflected light, 19, 24 ... 3rd area of reflected light, 20 ... 1st
21 ... Second part, 22 ... Reflected light, 25 ... Error detection system, 26 ... Hologram, 27,44 ... Transparent substrate, 28 ...
… Hologram first area, 29 …… Hologram second area, 30
...... First photodetector, 31 ...... Semiconductor laser, 32 ...... Second photodetector, 33 ...... Hologram third area, 34 ...... Hologram fourth area, 36 ...... A point, 37 ...... B point , 38 …… Joining line, 39
...... Convergent lens, 40 …… Optical disc, 41 …… λ / 4 plate, 42
...... Polarizing beam splitter, 43 …… Reflected light, 45 …… Photodetector, 46 …… Prism, 47 …… Reflecting mirror, 48 …… Sub mirror, 49 …… Collimating lens, 50 …… Focus error, Read signal detection System, 51 ... Diffraction grating, 52 ... Concave lens, 53 ... Main beam, 54 ... + 1st order diffracted light, 55 ...−
First-order diffracted light.

Claims (1)

(57) [Claims] 1. A pit or a pre-groove of an optical recording medium, which is condensed through an objective lens and is reflected and reflected out of an optical axis of an image forming optical system including at least a light source and an objective lens, The hologram optical device includes at least a hologram element that causes the reflected light extracted outside the optical axis to be incident on first and second photodetectors, and the first photodetector and the second photodetector. The element is the aperture of the objective lens is F, the focal length is f, the track pitch of the pits or pregrooves of the optical recording medium is P, the wavelength of the light source is λ, and 0 ≦ δ ≦ (track of the recording medium of the objective lens. The maximum movable distance in the direction), δ, the beam diameter of the reflected light from the recording medium that has passed through the objective lens, and d is the light beam in the vertical cross section. When recording medium track and that the X-axis parallel axes, an axis perpendicular to the X axis and the light beam is Y axis, of the light beam, A light beam incident on a region satisfying the following condition to the first photodetector, Diffracting the light beams incident on the area satisfying the following condition so as to be incident on the second photodetector, and causing the light beams incident on the other areas to be incident on the focus error detector. Track error detection device characterized by.