KR20070016969A - Optical Pickup - Google Patents

Abstract

The optical pickup includes a semiconductor laser; A diffractive light element having a first diffraction region and a second diffraction region, the diffractive light element operable to diffract the beam emitted from the semiconductor laser; An objective lens operable to focus the diffracted beam on an optical disc; And a detector operable to receive the diffraction beam reflected by the optical disk to detect a tracking error signal. When a zero (zero) order diffraction beam is focused on the surface of the optical disk, the first diffraction beam is diffracted by the first diffraction region to be focused at a position between the objective lens and the optical disk, It is diffracted by the second diffraction region to be focused at the out of position. The primary diffraction beam has a beam width that is at least twice the track pitch of the optical disc in the radial direction in the optical disc.

Optical pickup, diffraction, tracking, error signal, optical disc, beam width, track pitch

Description

Optical Pickup

Fig. 1 shows the main structure of the optical pickup of the prior art example 1.

FIG. 2 shows a beam spot formed on the optical disk by the optical pickup of the conventional example 1. FIG.

3 shows the structure of the photodetector of the prior art example 1. FIG.

Fig. 4 shows the relationship between the position of the objective lens of the optical pickup of the conventional example 1 and the far field pattern received by the split photodetector.

5A to 5C are graphs showing the relationship between the displacement amount of the objective lens in the radial direction and the push pull signals MPP, SPP1, and SPP2.

6A and 6B show a diffractive light element according to the prior art example 2, where FIG. 6A is a plan view of the diffraction light element and FIG. 6B is a cross sectional view showing the optical path from the diffraction light element to the optical disk.

Fig. 7 shows beam spots formed on the optical disk by the main beam and the sub beams of the optical pickup of the conventional example 2;

Fig. 8 shows the structure of the diffractive light element according to the conventional example 3.

Fig. 9 shows the structure of the optical pickup of the first embodiment of the present invention.

Fig. 10 shows the main structure of the optical detector of the optical pickup of the first embodiment of the present invention.

Fig. 11 is a plan view showing the formation of diffractive light elements of the optical pickup of the first embodiment of the present invention.

Fig. 12 is a plan view showing a beam spot formed on an optical disk by the optical pickup of the first embodiment of the present invention.

Fig. 13 shows a beam spot formed when the objective lens of the optical pickup of the first embodiment of the present invention moves in the radial direction of the optical disk.

14A to 14C show the relationship between the push-pull signal and the displacement amount of the objective lens of the optical pickup according to the first embodiment of the present invention. FIG. 14A is a graph related to the main push-pull signal, and FIG. 14B is related to the sub-push pull signal. It is a graph, and FIG. 14C is the push pull signal ultimately acquired.

Fig. 15 shows the main structure of the optical pickup of the second embodiment of the present invention.

Fig. 16 is a plan view showing the structure of the diffractive light element of the optical pickup of the second embodiment of the present invention.

Fig. 17 is a plan view showing the structure of the diffractive light element of the modification (2) of the present invention.

TECHNICAL FIELD The present invention relates to optical pickups, and more particularly to a technique for performing tracking with high accuracy regardless of the standard followed by the optical disc.

This application is based on the application number 2005-228232 for which it applied to Japan, The content is taken in here as a reference.

When recording information on or reproducing information from the optical disc, the optical beam must accurately track the tracks of the optical disc. For this purpose, it is necessary to detect the tracking error signal which is an indicator of the light beam displacement from the track.

Conventional Example 1

Japanese Patent Laid-Open No. 4-34212 discloses a differential push-pull (DPP) method, which is a method of detecting a tracking error signal.

1 shows the main structure of the optical pickup 1 of the conventional example 1. FIG. As shown in FIG. 1, the output beam of the semiconductor laser 101 is divided into three beams by the diffractive light element 102, paralleled by the collimator lens 103 into parallel beams, and the objective lens 105. Is focused on the optical disc 111. The light reflected by the optical disk 111 is guided to the photodetector 107 by the beam splitter 104 and the collective lens 106.

2 shows a beam spot that the optical pickup 1 forms on the optical disk 111. In FIG. 2, the X direction is a radial direction of the optical disc 111 (hereinafter, the radial direction of the optical disc 111 is simply referred to as a radial direction), and the Y direction is a direction parallel to the track of the optical disc 111. . As shown in Fig. 2, the light beam divided into three by the diffractive light element 102 is the beam spot 201 of the zero order diffraction beam (main beam), the + first order diffraction beam (sub beam). The beam spot 202 of the beam spot and the beam spot 203 of the -first-order diffraction beam (sub beam) are formed on the optical disk 111. The sub beams 202 and 203 are located at a distance from the main beam 201 that is half the distance of the track pitch T (distance T / 2) in the radial direction, respectively.

3 shows the structure of the photo detector 107. As shown in FIG. 3, the photodetector 107 has split photodetectors 301-303 that receive far field patterns of the light beams 201-203, respectively. The split photodetectors 301 to 303 each consist of a pair of photodetectors which receive one of two split beams which are divided by parallel beam spots on the optical disk in the track direction.

Push pull signals MPP, SPP1 and SPP2 are obtained as difference signals of the output signals of the split photodetectors 301 to 303. If the beam spot of the main beam 201 is at the track center on the optical disc, each photo detector receives approximately the same amount of light in pairs. However, if the beam spot of the main beam 201 deviates from the center of the track, each photo detector receives a significantly different amount of light for that pair.

4 shows the relationship between the position of the objective lens 105 and the far field pattern received by the split photodetector 301. As shown in FIG. 4, when the objective lens 105 moves in the radial direction, the far field pattern of the split photodetector 301 also moves in the radial direction. This causes an offset of the tracking error signal.

5A to 5C are graphs showing the relationship between the displacement amount of the objective lens 105 in the radial direction and the push pull signals MPP, SPP1, and SPP2. Referring to FIG. 5A, when the objective lens moves in the radial direction, an offset signal corresponding to the movement amount is generated in the push pull signal MPP. As shown in Fig. 5B, an offset signal corresponding to the movement amount is also generated in each of the push pull signals SPP1 and SPP2.

 Assuming that the main beam 201 and the sub beams 202 and 203 are irradiated at a distance of T / 2 from each other in the radial direction, the phase of the push pull signal MPP is opposite to the phase of the push pull signals SPP1 and SPP2. .

Using these push pull signals MPP, SPP1 and SPP2, the differential error amplifiers 311 to 314 and the amplifier 315 of the photodetector 107 generate a tracking error signal. Specifically,

DPP = MPP-k (SPP1 + SPP2)

This cancels the offset signal generated in the push-pull signal due to the movement of the objective lens 105 to obtain the tracking error signal.

k is a coefficient for correcting a difference in light intensity between the main beam 201 and the sub beams 202 and 203. Therefore, if the light intensity is 0th order diffraction beam: + 1st order diffraction beam: -1st order diffraction beam = a: b: c,

k = a / (2b).

In recent years, with the diversification of optical disc standards, it is necessary for signal optical pickups to be able to adapt to various optical disc track pitches. However, according to the conventional example 1 method, the main beam and the sub-beams should be irradiated to the optical disk at a distance T / 2 in the radial direction, and according to this method, a single optical pickup cannot adapt to multiple standards. .

Conventional Example 2

Japanese Patent Laid-Open No. Hei 10-162383 discloses a technique that allows more freedom by the irradiation positions of the main beam and the sub beam.

6A and 6B show diffractive light element 602 in accordance with the prior art example 2. FIG. FIG. 6A is a top view of the diffractive light element 602, and FIG. 6B is a cross sectional view showing the optical path from the diffractive light element 602 to the optical disk.

As shown in FIG. 6A, the diffractive light element 602 has a groove portion 602a at the center of the effective light flux of the light beam, and has a flat portion 602b surrounding the edge of the groove portion 602a. .

In the conventional example 1, all of the effective light flux of the light beam is diffracted by the diffractive light element, while in the conventional example 2, only a portion of the effective light flux is diffracted. For this reason, as shown in FIG. 6B, the beam diameter of the sub-beams 622, 623 irradiated from the diffractive light element 602 is smaller than the effective beam diameter (aperture diameter of the objective lens 605). . This is the same as the numerical aperture of the objective lens 605 is decreasing with respect to the sub beams 622 and 623.

On the other hand, the beam diameter of the main beam 621 is larger than the aperture diameter of the objective lens 605, so that a diffraction limiting beam spot corresponding to the aperture diameter of the objective lens 605 is formed in the optical disk 611.

7 shows beam spots formed by the main beam 621 and the sub beams 622 and 623 on the optical disk 611. As shown in Fig. 7, the beam spot diameter of the main beam 621 is approximately the same as the main beam spot diameter of the conventional example 1, but the beam spot diameters of the respective sub beams 622 and 623 are relatively large, and the sub beam Beam spots 622 and 623 span multiple tracks.

By increasing the beam spot diameter of the subbeams 622, 623, the high frequency signal components of the spatial frequency that occur when the subbeams 622, 623 are reflected at the boundary between the track groove and the land (hereinafter referred to as "tracks" Track component, so that the offset signal can be detected regardless of the distance between the main and sub beams. Thus, this structure can adapt to various optical disc track pitches.

Conventional Example 3

Japanese Laid-Open Patent Publication No. 2001-325738 discloses a technique for detecting an offset signal by reducing the size of a sub beam spot by a groove formed in a diffractive light element in a curved plan view.

8 shows the structure of the diffractive light element 802 according to the prior art example 3. As shown in FIG. As shown in FIG. 8, a curved groove is formed over the entire diffractive light element 802. This structure allows the sub beam spot to be large enough to span multiple tracks, thus achieving an effect similar to the conventional example 2.

However, in the conventional example 2, the intensity of the main beam 621 is weaker in the portion passing through the groove portion 602a than in the portion passing through the flat portion 602b of the diffractive light element 602. This weakening is proportional to the diffraction capability of the groove portion 602a. If this affects the profile of the main beam, read and write errors will occur.

In addition, the beam spot is enlarged in the injecting direction of the optical disk. For this reason, a difference occurs between the offset signals of the main beam 621 and the sub beams 622 and 623, and when the objective lens moves in the radial direction, the offset signal cannot be canceled properly.

In the prior art example 3, reducing the radius of curvature of the groove of the diffractive light element 802 will cause the light beam to have different intensity portions being used in the main beam and the sub beam. For this reason, the offset signal corresponding to the objective lens shift amount will be generated differently for the main beam and the sub beam, and the offset signal may not be completely canceled out.

Conversely, when the radius of curvature of the groove of the diffractive optical element 802 increases, the beam spot of the sub beam of the optical disk cannot be sufficiently large, and as a result, the track cross component cannot be completely suppressed. In addition, the phase difference between the push-pull signals of the main beam and the sub-beam varies with the amount of movement of the objective lens, resulting in tracking failure.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to provide an optical pickup that accurately performs tracking using a push-pull method regardless of track pitch.

These and other objects, advantages and features of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate specific embodiments of the present invention.

In order to solve the above problem, the present invention provides an optical pickup, comprising: a semiconductor laser; Diffractive light elements operable to diffract beams emitted from the semiconductor laser; An objective lens operable to focus the diffracted beam on an optical disc; And a detection unit operable to receive the diffraction beam reflected by the optical disk to detect a tracking error signal, wherein the diffractive light element has a plurality of first and second diffraction zones and is zero (zero). When the first diffraction beam is focused on the surface of the optical disc, the first diffraction region diffracts the first diffraction beam so that the first diffraction beam is focused at a position between the objective lens and the optical disc. The second diffraction area is configured to diffract the first diffraction beam so that the first and second diffraction areas are configured such that the first diffraction beam is focused at a position off the optical disk with respect to the objective lens. The first diffraction zone and the second diffraction zone are configured to have a beam width in the optical disc that is at least twice the track pitch of the optical disc in the radial direction.

By dividing the diffractive light element into a plurality of first and second diffractive areas, the phase deflection of the sub push pull signal caused by the movement of the objective lens with respect to the optical disk can be eliminated, and the tracking error signal can be detected accurately. have.

Wherein the first and second diffractive regions are divided from each other by a boundary line that is substantially orthogonal to the track of the optical disc, each of the first and second diffractive regions forming a groove set therein. Each set of grooves has a substantially arc shape. In addition, the first diffraction zone and the second diffraction zone may be arranged alternately, such that the concentric center is outside the diffraction zone where the groove set is formed, and the groove of the first diffraction zone with respect to the boundary line. Each groove set may be formed such that the groove set is substantially a mirror image of the groove set of the second diffraction region.

In addition, the diffractive light element may be configured such that none of the first and second diffractive regions exist at a position through which the portion of the emission beam that contributes to the zeroth order diffraction beam passes.

According to the above structure, in the diffractive light element, at the position where the emission beam portion contributing to the zero-order diffraction beam passes, none of the first and second diffraction regions exist, so that the intensity of the main beam is Do not attenuate This prevents read and write errors caused by attenuation of the main beam.

In addition, the semiconductor laser emits a plurality of beams each having a different wavelength, and with respect to the radial direction of the optical disk, concentric arc-shaped grooves provided in the first and second diffraction regions of the major surface of the diffractive optical element. The concentric center of the web can be placed between the plurality of beams.

According to this structure, the tracking error signal can be accurately detected for various optical disks using lasers having different wavelengths.

As described above, according to the present invention, since the tracking error signal can be detected accurately, the accuracy of tracking can be improved. In addition, since the utilization efficiency of the light beam can be increased, recording and reproduction can be performed stably. In addition, in the prior art, it was necessary to adjust the diffractive light elements in order with great precision when assembling the optical pickup, but the present invention eliminates the need for such sequential adjustment. Therefore, assembly time and manufacturing cost can be reduced.

The following describes an embodiment of the optical pickup of the present invention with reference to the drawings.

1. First embodiment

(1) optical pickup structure

The following describes the structure of the optical pickup of the present invention.

9 shows the structure of the optical pickup 9 of one embodiment of the present invention. As shown in FIG. 9, the optical pickup 9 includes a semiconductor laser 901, a diffractive light element 902, a collimator lens 903, a beam splitter 904, an objective lens 905, and an aggregate lens 906. , And a photo detector 907. The optical pickup has a function of reading information from the optical disk 911.

The semiconductor laser 901 emits a laser beam.

The diffractive light element 902 is a transmission grating, which divides the laser beam irradiated by the semiconductor laser 901 into a zeroth order diffraction beam (main beam) and a + 1st and -1st order diffraction beam (subbeam).

The collimator lens 903 makes the main beam and the sub beams parallel with the parallel beams. The parallel main beam and the sub beam pass through the beam splitter 904 and are irradiated to the objective lens 905.

The objective lens 905 focuses the main beam and the sub beam on the recording surface of the optical disk 911, and also makes the main beam and the sub beam reflected by the recording surface of the optical disk 911 substantially parallel.

The beam splitter 904 guides the main beam and the sub beams reflected from the recording surface of the optical disk 911 to the collective lens 906.

The collective lens 907 focuses the main beam and the sub beam on the photo detector 907.

The photo detector 907 receives the main beam and the sub beam and generates a tracking error signal.

(2) Structure of the photo detector 907

Next, the structure of the photo detector 907 will be described.

10 shows the main structure of the photo detector 907. As shown in FIG. 10, the photodetector 907 is composed of split photodetectors 1001 to 1003, differential amplifiers 1011 to 1014, and amplifier 1015. The far field patterns 1021 of the main beam are received by the split photodetector 1001, and the far field patterns 1022 and 1023 of the sub beam are received by the split photodetectors 1002 and 1003, respectively.

Each of the split optical detectors 1001 to 1003 can receive each far field pattern (field pattern on the optical disk 911) of beam spots divided in two by dividing lines parallel to the track direction of the optical disk 911. So that it is divided into two parts. The photodetectors 1001 to 1003 output push-pull signals MPP, SPP1 and SPP2 as differential signals of the received far field patterns, respectively.

Hereinafter, with respect to the beam spot, the direction parallel to the track direction of the optical disk 911 is simply referred to as the "track direction", and the direction parallel to the radial direction of the optical disk 911 is simply referred to as the "radial direction".

(3) formation of the diffractive light element 902

The following describes the shape of the diffractive light element.

11 is a plan view showing the shape of the diffractive light element 902. As shown in FIG. 11, the diffractive light element 902 is divided into a plurality of regions whose boundaries extend in the radial direction (X direction) of the optical disk 911. Specifically, the diffractive light element 902 is provided in the order described in the region 11a, the region 11b, the region 11c, and the region 11d, and the region 11a is nearest to the main beam.

Each of the regions described above is one of two kinds of regions, specifically, the region 1101 and the region 1102, depending on the shape of the groove provided on the surface. In both regions 1101 and 1102 the grooves are arc-shaped when viewed in plan view, with substantially equal spacing between them. In other words, the relationship between the grooves of two kinds of regions is such that each curvature sign is reversed.

According to the groove thus provided, regions 1101 and 1102 produce a sub-beam. When the main beam is parallel to the optical disk with a diffraction limit determined according to the wavelength of the main beam and the aperture diameter of the objective lens 905, the sub-beam generated by the area 1101 causes the objective lens 905 and the optical beam to be parallel. The sub beam generated by the region 1102 is focused at the position between the objective lens 905 and the optical disc 911. Thus, both sub-beams having radially expanded diameters are formed in the optical disk 911.

The surface areas of the regions 1101 and 1102 are substantially equal to each other at each side of the position through which the main beam passes in the diffractive light element 902. Further, the total surface area of the regions 1101 and 1102 at one side of the position where the main beam passes in the diffractive light element 902 is equal to the total surface area at the other side.

(4) the shape of the beam spot

Next, the shape of the beam spot which the optical pickup 9 forms in the optical disk 911 is demonstrated.

12 is a plan view showing a beam spot that the optical pickup 9 forms on the optical disk 911. As shown in FIG. 12, with respect to the beam spot of the main beam 1021, each beam spot of the sub beams 1022 and 1023 is shifted by half of the track pitch in the radial direction. Each of the sub beams is composed of four portions corresponding to four regions 11a to 11d of the diffractive light element 902, respectively.

Beam spots of the sub beams 1022 and 1023 extend in the radial direction of the optical disk 911. Specifically, the beam diameter of the radial sub beam spot of the optical disc 911 is approximately three times the track pitch. This suppresses the generation of the track cross component. Note that the sub beams 1022 and 1023 have substantially the same light intensity.

(5) Characteristics of the optical pickup 9

Next, the characteristics of the optical pickup 9 will be described.

In the diffractive light element 902, the grooves of the region 1101 are bent substantially in the opposite direction to the grooves of the region 1102. Further, the regions 1101 and 1102 receive substantially the same amount of light and are alternately arranged in the track direction (Y direction) of the optical disk 911. In other words, the regions 1101 and 1102 are divided by boundaries that are parallel lines extending in the radial direction (X direction) of the optical disk 911.

For this reason, when the objective lens 905 moves in the radial direction of the optical disk 911, each of the beam spots formed in the optical disk 911 is one of the region 1101 and the region 1102 of which the sub beam 1022, Depending on whether the diffraction 1023 is diffracted, it moves toward the inner edge or the outer edge of the optical disk 911.

13 shows the beam spot formed when the objective lens 905 moves in the radial direction of the optical disk 911. Compared to the beam spot shown in FIG. 12, a part of each beam spot of the sub beams 1022 and 1023 shown in FIG. 13 is moved toward the inner edge or the outer edge of the optical disk 911.

As a result, the phase difference between the main push pull signal MPP and the sub push pull signal SPP1 and the phase difference between the main push pull signal MPP and the sub push pull signal SPP2 are the same. If this phase difference is canceled between the sub push pull signals SPP1 and SPP2, the phase of the offset signal of the main push pull signal and the offset signal of the sub push pull signal will be the same.

In other words, the objective lens 905 moving in the radial direction of the optical disk 911 is equivalent to the diffractive optical element 902 moving in the radial direction of the optical disk 911, and the beams of the sub-beams 1022 and 1023. The positional relationship of the beam spot of the main beam 1021 with respect to the spot changes.

When the distance between the center of each beam spot in the radial direction of the optical disk 911 deviates from that of one half of the track, the phase difference of the main push pull signal MPP for the sub push pull signals SPP1 and SPP2 deviates from 180 degrees. . Since the amplitudes of the push-pull signals SPP1 and SPP2 are not small enough to be negligible, there is a risk that the accuracy of the tracking error signal is reduced due to the deflection of the phase difference.

However, in this embodiment, since the phase difference between the main push pull signal MPP and the sub push pull signals SPP1 and SPP2 cancel each other as described above, the accuracy of the tracking error signal is maintained.

14A to 14C show the relationship between the push-pull signal and the displacement amount of the objective lens of the optical pickup of the first embodiment of the present invention. Specifically, FIG. 14A is a graph relating to a main push pull signal, FIG. 14B is a graph relating to a sub push pull signal, and FIG. 14C is a graph relating to an ultimately acquired push pull signal.

As shown in the figure, in the sub push pull signal obtained by adding the sub push pull signals SPP1 and SPP2, the phase difference between the sub push pull signals SPP1 and SPP2 is canceled so that the sub push pull has the same phase as the main push pull signal MPP. Get the signal (FIGS. 14A and 14B). Therefore, the offset signal is accurately removed from the obtained push pull signal using Equation 1.

On the other hand, since the regions 1101 and 1102 are alternately arranged in the radial direction of the optical disk 911 and not divided in the track direction, it would have been so by the diffraction effect if the diffractive light element 902 was divided into regions. The beam spot does not extend in the radial direction of the optical disc.

Therefore, the offset signal caused by the movement of the objective lens 905 in the radial direction of the optical disk 911 or by the tilting of the optical disk 911 is linear with the same phase as the main beam and the sub beam and is linear. . This allows the offset signal to be more accurately canceled than in the prior art.

This characteristic is the same regardless of the distance between the sub beams 1022 and 1023 and the main beam 1021 in the radial direction of the optical disk 911. Therefore, according to this embodiment, the tracking error can be accurately canceled with respect to the optical disc, regardless of the tracking pitch of the standard followed by the optical disc.

2. Second Embodiment

The following describes a second embodiment of the present invention. The optical pickup of this embodiment is basically the same as the optical pickup of the first embodiment in structure, but differs in that it includes a plurality of laser light sources. The following description focuses on aspects different from the first embodiment.

Fig. 15 shows the main structure of the optical pickup 15 of this embodiment. As shown in FIG. 15, the optical pickup 15 includes the semiconductor lasers 1501 and 1521, the diffractive light element 1502, the collimator lens 1503, the beam splitter 1504, the objective lens 1505, and the collective lens 1506. ), And a photo detector 1507. The optical pickup 15 has a function of reading information from the optical disk 1511.

16 is a plan view showing the structure of the diffractive light element 1502. As shown in FIG. 16, the regions 1601 and 1602 are alternately disposed in the track direction of the optical disc at the major surface of the diffractive light element 1502. Arc-shaped concentric grooves are formed in each region, and grooves in region 1601 bend in the opposite direction to grooves in region 1602.

Light irradiated from the semiconductor lasers 1501 and 1521 passes through the regions 1621 and 1622. The solid lines 1611 and 1612 represent center positions of the circular regions 1621 and 1622 in the radial direction of the optical disk 1511, respectively. These center positions are positions through which the main light rays emitted from the semiconductor lasers 1501 and 1521 pass.

The dashed line 1613 represents the center position of the arc-shaped groove formed in the area 1601 in the radial direction of the optical disk 1511. Also, dashed line 1614 represents the center position of arc-shaped groove formed in region 1602 in the radial direction of optical disk 1511.

In this embodiment, grooves are formed such that dashed lines 1613 and 1614 lie between solid lines 1611 and 1612. This may not only allow the amplitude of the sub push pull signal to be suppressed, but also allow the variation of the phase difference between the main push pull signal and the sub push pull signal to be suppressed. Therefore, the tracking error can be correctly solved for both the semiconductor lasers 1501 and 1502.

3. Variation Example

Although the present invention has been described based on the preferred embodiments, the present invention is not limited to the above embodiments. The following are examples of possible variations.

(1) In the above embodiment, grooves of the regions 1101 and 1102 are formed at substantially equal intervals in the track direction of the optical disk 911, but the present invention is not limited to this structure. As an alternative, a Fresnel pattern can be used. Specifically, the spacing between the grooves of each of the regions 1101 can be made larger as the groove is further away from the center of the main beam in the track direction of the optical disc 911, and the grooves of each of the regions 1102 are formed. The spacing between the grooves may be smaller as the groove is further away from the center of the main beam in the track direction of the optical disk 911.

(2) In the above embodiment, grooves are formed in the regions 1101 and 1102 provided in a part of the diffractive light element 902, but the present invention is not limited to this structure. The following alternative structure may be used.

17 is a plan view showing the structure of the diffractive light element 17 of this modification. In FIG. 17, circular region 1711 is a region through which the main beam passes. As shown in FIG. 17, the diffractive light element 17 is divided so that the regions 1701 and 1702 are alternately arranged in the track direction of the optical disc.

Regions 1701 and 1702 also exist in circular region 1711. In addition, the grooves are formed to have different curves, so that the curve of the region 1701 has a curvature sign different from that of the region 1702.

According to this structure, the amount of light lost from the main beam can be suppressed. Improve light utilization efficiency

(3) Although not described in the above embodiments, the size of the sub beam spot should be at least twice the track pitch in the radial direction of the optical disc.

In particular, in order to make the amplitude of the sub-push pull signal sufficiently small, the size of the sub beam spot is preferably at least three times the track pitch. However, since the relatively large sub beam spot will be even larger at the photo detector surface and will require a larger photo detector, the size of the sub beam spot is preferably less than six times the track pitch.

Although the present invention has been fully described through examples with reference to the accompanying drawings, it should be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as belonging to the present invention.

According to the present invention, since the tracking error signal can be detected accurately, the accuracy of tracking can be improved. In addition, since the utilization efficiency of the light beam can be increased, recording and reproduction can be performed stably. In addition, in the prior art, it was necessary to adjust the diffractive light elements in order with great precision when assembling the optical pickup, but the present invention eliminates the need for such sequential adjustment. Therefore, assembly time and manufacturing cost can be reduced.

Claims (5)

As an optical pickup, Semiconductor lasers; Diffractive light elements operable to diffract beams emitted from the semiconductor laser; An objective lens operable to focus the diffracted beam on an optical disc; And A detection unit operable to receive the diffracted beam reflected by the optical disk to detect a tracking error signal, The diffractive light element has a plurality of first and second diffractive regions, When a zero (zero) order diffraction beam is focused on the surface of the optical disk, the first diffraction region diffracts the first diffraction beam so that the first diffraction beam is focused at a position between the objective lens and the optical disk. Wherein the second diffractive region is configured to diffract the first diffraction beam such that the first diffraction beam is focused at a position off the optical disk relative to the objective lens, And the first diffraction zone and the second diffraction zone are configured such that the first diffraction beam has a beam width that is at least twice the track pitch of the optical disc in the radial direction in the optical disc. The method according to claim 1, The first diffraction zone and the second diffraction zone are divided from each other by a boundary line substantially perpendicular to the track of the optical disc, And wherein each of said first and second diffractive regions forms a set of grooves therein, each set of grooves having a substantially arc shape. The method according to claim 2, The first diffraction zone and the second diffraction zone are alternately arranged, A mirror image of the groove set of the second diffractive region substantially such that the concentric center is outside of the diffractive region in which the groove set is formed, and the groove set of the first diffractive region with respect to the boundary line is substantially Wherein each set of grooves is formed. The method according to claim 1, Wherein in the diffractive light element, none of the first and second diffractive regions are present at a position through which the portion of the emission beam that contributes to the zeroth order diffraction beam passes. The method according to claim 1, The semiconductor laser emits a plurality of beams having different wavelengths, With respect to the radial direction of the optical disc, the concentric center of the concentric arc-shaped grooves provided in the first and second diffraction regions of the major surface of the diffractive light element lies between the plurality of beams. Optical pickup.

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