JPH1074326A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a DC offset in a tracking error signal obtained from a photodetector and to reduce the number of parts and also to simplify the assemblage. SOLUTION: A laser beam is converged on a disk 1 by an objective lens device 18 capable of moving in the direction of an optical axis and in the direction orthogonal to the optical axis. Reflected light reflected by the surface of the disk 19 is returned to the objective lens device 18. The surface of the objective lens device 18 on the side of a laser light source is formed with a light separating surface like a diffraction grating, and the return light is separated into at least two light beams to feed photodetectors 14 and 15 respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup, and more particularly to an optical pickup which mainly performs tracking servo by push-pull.

[0002]

2. Description of the Related Art An optical pickup plays an important part in detecting an optical signal and converting it into an electric signal. For example, a pickup used in a compact disk player performs focus servo and tracking servo. The tracking servo is corrected by the servo to track a track having a pitch of, for example, about 1.6 μm on the optical disk. The tracking servo is performed by moving all or a part of the optical pickup. The tracking servo detects the tracking error signal that appears due to the movement of the pickup and applies the servo. As a method of applying the tracking servo, a three-beam method, a push-pull method, and the like are known.

The tracking servo based on the push-pull method is a method of detecting an error signal with one beam, and is also referred to as a reflected light intensity distribution type or a diffracted light method.

FIG. 6 shows the structure of an optical pickup using the above push-pull method.

The light emitted from the semiconductor laser 40 is sent to an objective lens 42 via a half mirror 41, and the light forms an image of a focus on a pit on a disk surface 43 by the objective lens 42. The reflected light is reflected by the half mirror 41 and is detected by the photodetectors 44 and 45.
Is converted into an electric signal. The above half mirror 41
Is a polarizing beam splitter (PBS) and a quarter wave plate (Q
WP) has exactly the same function as the combination.

As described above, in the photodetectors 44 and 45,
The light is detected by the divided photodiode and converted into an electric signal, and the two signals are used as the tracking error detector 46.
, An error signal obtained by taking the sum and difference of the signals is output. The above-mentioned tracking error signal is a signal of both polarities, and tracking servo is applied based on the signal.

The tracking error signal changes due to the relative positional relationship between the pits and the beam spot as shown in FIG. 7 due to diffraction and reflection at the pits on the disk surface 43. This is because, when the track pitch on the disk surface becomes the size of the beam spot, the track looks like a diffraction grating. The intensity distribution changes.

FIG. 7 shows a positional relationship between a beam spot and a pit and a light intensity distribution of the pit appearing at that time.

When the position coincides with the pit 55 like the beam spot 54, the light intensity distribution 56 shows a symmetrical distribution, but the positional relationship between the beam spot and the pit is In the case where it is deviated to the left as shown in FIG. 50 or in the case where it is deviated to the right as shown in the beam spot 57 for the pit 58,
The light intensity distribution becomes asymmetric. The asymmetry of the light intensity distribution is opposite to the above-described relative positional relationship between the pit and the beam spot. This reflected light is detected by a photodetector and converted into an electric signal. This electric signal is used as a tracking signal and sent to a tracking error detector. The tracking servo is applied based on the tracking error signal output from the tracking error detector as described above.

[0010]

However, in order to perform tracking in this manner, an objective lens such as an actuator using a so-called two-axis device that moves the objective lens in the optical axis direction and in a direction perpendicular to the optical axis. If
Even if the control is performed so that the beam spot is accurately irradiated on the pits of the disc, a DC offset occurs in the tracking error signal by moving the objective lens in a direction orthogonal to the optical axis. The occurrence of a DC offset in the tracking error signal will be described with reference to FIG. FIG. 8 is a simplified representation of the optical path of the reflected light from the disk surface of FIG. 6 described above.

In FIG. 8, reflected light from a disk 43 is received by light receiving elements 44 and 45 via an objective lens 42. When the optical axis of the objective lens 42 and the optical center axis of the light receiving elements 44 and 45 match, a symmetric light intensity distribution is obtained in the light receiving elements 44 and 45 as shown by the solid line in FIG. On the other hand, when only the objective lens 41 is moved in a direction orthogonal to the optical axis, the reflected light
Shift as shown by the broken line. Therefore, each light receiving element 4
The balance of the light received by the light receiving elements 4 and 45 is lost, and in the illustrated example, the ratio of light received by the light receiving element 45 is higher than the rate of light received by the light receiving element 44. This appears as a DC offset in the tracking error signal.

As described above, when the objective lens 42 moves in a direction perpendicular to the optical axis as a part of the tracking operation,
The tracking error signal output from the tracking error detector 46 has a DC offset.

The present invention has been made in view of such circumstances, and has as its object to provide an optical pickup in which a DC offset does not occur when the objective lens is moved in a direction perpendicular to the optical axis. It is.

[0014]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a light source for generating a laser beam, a laser beam from the light source condensed on a disk, and an optical axis direction and an optical axis. An objective lens movable in a direction perpendicular to the laser light source, wherein the objective lens has a light separating surface formed on the side of the laser light source for converging the laser light into at least two light fluxes.

Here, examples of the objective lens include a lens in which a hologram is integrally formed on the surface on the laser light source side and a lens in which a diffraction grating is integrally formed on the surface on the laser light source side.

The laser beam from the light separation surface formed on the objective lens is received by the light receiving element to obtain a tracking error signal, so that each laser beam is moved in a direction orthogonal to the optical axis of the objective lens. The balance of light to the light receiving element is prevented from being lost, and the occurrence of DC offset can be prevented.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to description of a preferred embodiment of an optical pickup according to the present invention, referring to FIG. 1, referring to FIG. 1, an optical element which is arranged so as to be movable integrally with an objective lens and separates a laser beam. An example of an optical pickup provided with is described.

In the configuration of the main part of the optical pickup shown in FIG. 1, the light emitted from the laser light source 1 is integrated with a hologram plate 3 and a holding member 8 for holding an objective lens 4 so as to have an integrated structure. The image is formed on the pits of the disk 5 via 2. Next, the light reflected by the pits of the disk 5 is split into two or more optical paths by a hologram plate 3 having a beam splitting function via an objective lens 4 of an objective lens device 2 and received by photodiodes 6 and 7. . Here, as shown in FIG.
With the optical axis connecting the laser, hologram, objective lens, and pit of the disc as the center, the first hologram plate 3a in the upper semicircle portion and the second hologram plate 3b in the lower semicircle portion in the figure are divided. The first hologram and the second hologram are configured such that light refraction directions are different from each other. The reflected light via the objective lens 4 of the objective lens device 2 is diffracted so as to reach the light receiving portion of the photodiode 6 in the first hologram 3a and to the light receiving portion of the photodiode 7 in the second hologram 3b. The beam is split (split) so as to form an optical path. FIG. 2 is an enlarged view of the light receiving element portion (light receiving elements 6 and 7) of the example shown in FIG. 1, and further shows a differential amplifier 16 for tracking error detection.

In FIG. 1, in order to make clear the separated light beams in the optical path from the hologram plate 3 to the light receiving section, they are shown with different diagonal lines.

Here, even if the objective lens 4 is moved in a direction perpendicular to the optical axis, the return light of the laser light condensed on the disk 5 is split into two optical paths by holograms 3a and 3b which are divided into two. Equalizing the balance of light to the two light receiving elements 6 and 7 will be described with reference to FIG.

As described above, when the objective lens device 2 integrally mounted with the hologram plate 3 and the objective lens 4 shown in FIG.
a and the second light beam 1 by the second hologram plate 3b.
The light is separated into light beams 2 and 13 and received by light receiving elements 6 and 7, respectively. At this time, the irradiation area of each of the beams 12 and 13 is equal to each other in the light receiving elements 6 and 7, and is converted into an electric signal, sent to the tracking error detector 16, and subjected to tracking servo. On the other hand, the light beams 10, 11
Shows a case where the objective lens 4 is moved by the biaxial actuator in a direction orthogonal to the optical axis. At this time, since the hologram plate 3 moves integrally with the objective lens 4,
With respect to the light beams 10 and 11 separated by the first hologram plate 3a in the upper part and the second hologram plate 3b in the lower part in the figure, the irradiation area to each light receiving element 6 and 7 does not change. Or, even if it changes, they change by the same amount, and no DC offset occurs in the difference signal between the output signals from these light receiving elements 6 and 7. Therefore, even if the tracking operation of the objective lens device 2 is performed by the two-axis actuator, no DC offset occurs in the output of the tracking error signal from the tracking error detector 16.

By the way, in the example shown in FIG. 1, the objective lens 4 and the hologram plate 3 which is an optical element for laser beam separation are integrally connected by the holding member 8, but the number of parts is further reduced. It is conceivable to form a light separating surface on the laser light source side of the objective lens in order to reduce the number of assembly steps.

In view of the above, in the present invention, a light separating surface for converging laser light into at least two light beams is formed on the laser light source side of the objective lens.

Hereinafter, a first embodiment of the present invention will be described with reference to FIG. In FIG.
Light from the laser light source is reflected on the surface of the disk 19 via the objective lens device 18 formed integrally with the hologram and the like. Here, in the objective lens device 18, a diffraction grating (grating) for diffracting a beam in directions different from each other is formed in the upper semicircular portion and the lower semicircular portion in the drawing by directly forming a groove in the surface or the like. ing. Therefore,
The reflected light from the disk 19 is transmitted to the objective lens device 18
The beams are separated into upper and lower sides of the optical axis in the figure by the above-mentioned gratings. On the other hand, photodetectors 14 and 15 which are sufficiently large are provided on the upper and lower sides of the optical axis in the figure, respectively, to receive the separated beams.

The operation principle and operation of the optical pickup according to the first embodiment of the present invention shown in FIG. 3 are the same as those shown in FIGS. 1 and 2 except that a diffraction grating is used for light separation. Is the same as

Therefore, according to the first embodiment,
When the moving operation is performed in the same optical axis direction and the direction orthogonal to the optical axis as the actuator as in the examples of FIGS. 1 and 2 described above, the return position of the separated light beam changes, but is received by the light receiving element. Light balance is maintained equally,
DC offset can be prevented from occurring. Further, since a diffraction grating is formed directly on the surface of the objective lens to form a light separating surface, an individual light separating optical element such as the hologram plate 3 of FIG. It becomes unnecessary, the number of parts can be reduced, and the number of assembly steps can be reduced.

FIG. 4 shows an objective lens device used in an optical pickup according to a second embodiment of the present invention.

In FIG. 4, the objective lens device 20
In the figure, a hologram plate 22 in the upper half of the pupil of the objective lens in the drawing and a hologram plate 23 in the lower half of the pupil in the drawing are integrated on the surface of the objective lens 21. In the optical pickup using the objective lens device 20, the same effects as in the first embodiment can be obtained.

FIG. 5 shows an objective lens device used in an optical pickup according to a third embodiment of the present invention.

In the objective lens device 30 shown in FIG. 5, diffraction halves (gratings) 32a and 32b are provided on each half (semicircle) of the pupil on the surface of the objective lens 31, respectively.
Are integrally formed. Specifically, there is a method in which a groove is formed directly on the surface of a plastic lens. Also in the optical pickup using the objective lens device 30, the effect of preventing the occurrence of DC offset can be obtained as in the above-described embodiments.

As described above, by using plastic as the material of the objective lens and manufacturing it by a so-called molding method or the like, it is possible to manufacture a large number of objective lens devices having the same performance. By using such a plastic objective lens device, an optical pickup can be supplied at low cost.

It should be noted that the present invention is not limited to only the above-described embodiment, and for example, various light separating surfaces for separating beams can be used.

[0033]

As is apparent from the above description, according to the optical pickup of the present invention, the light separating surface having the function of separating the beam is formed integrally with the objective lens, so that the objective lens Since the above-mentioned optical element moves simultaneously with the movement, the DC offset in the tracking error signal obtained from the light receiving element can be reduced, and the number of parts can be reduced as compared with the case where individual optical separation optical elements are used. Reduction and simplification of assembly man-hours can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a main part of an example of an optical pickup for explaining an optical pickup according to the present invention.

FIG. 2 is an explanatory diagram showing an enlarged light receiving element section in FIG. 1;

FIG. 3 is a diagram showing a main part of a first embodiment of the optical pickup according to the present invention.

FIG. 4 is a sectional view showing an objective lens and an optical element used in a second embodiment of the present invention.

FIG. 5 is a sectional view showing an objective lens and an optical element used in a third embodiment of the present invention.

FIG. 6 is an explanatory diagram for explaining an operation principle of an optical pickup by a push-pull method.

FIG. 7 is a diagram showing a relationship between a beam spot position with respect to a pit and a light intensity distribution.

FIG. 8 is a diagram for explaining a DC offset.

[Explanation of symbols]

1 laser light source, 2, 18, 20, 30 objective lens device, 3, 3a, 3b, 22, 23 hologram plate,
4, 21, 31 objective lens, 5, 19 disk, 6, 7, 14, 15 light receiving element, 16 tracking error detector, 32 diffraction grating

Claims (3)

[Claims] 1. A light source for generating a laser beam, and an objective lens which focuses the laser beam from the light source on a disk and is movable in an optical axis direction and in a direction orthogonal to the optical axis. An optical pickup characterized in that a light separation surface for converging the laser light into at least two light beams is formed on the laser light source side. 2. The optical pickup according to claim 1, wherein the objective lens has a hologram integrally formed on a surface on the side of the laser light source. 3. The optical pickup according to claim 1, wherein the objective lens has a diffraction grating integrally formed on a surface on the side of the laser light source.