JP4368314B2 - Tracking control device - Google Patents

Description

  The present invention relates to a tracking error detection method in an optical head for recording and reproducing information on an optical information recording medium (hereinafter referred to as an optical disk), and tracking control for controlling tracking of a track on the optical disk based on the detected tracking error. Relates to the device.

  In an optical disc apparatus that reads and writes optical signals to and from an optical disc, in order to correctly irradiate a focused spot on a predetermined track in the optical disc, a tracking error indicating a spot deviation from a desired track is detected, Tracking is controlled so as to adjust the position of the objective lens based on the tracking error.

  There are DVD ± R, DVD ± RW, and DVD-RAM as optical discs, even with write-once and rewritable DVD optical discs alone. The track pitch of DVD-R and DVD-RW is 0.74 μm, the track pitch of DVD-RAM is 1.48 μm (RAM1), and the track pitch is 1.23 μm (RAM2). Therefore, it is desired that these optical discs having different track pitches can be reproduced / recorded by a single device, and it is required to detect tracking errors and control tracking by a method independent of track pitches.

  As a tracking error detection method that does not depend on the track pitch, a tracking error detection method based on an inline differential push-pull method has been proposed in Patent Document 1 for such a problem.

  In the inline type differential push-pull method, a laser beam 508 emitted from a light emitting element has a special structure in which two diffraction gratings 500a and 500b having the same period as shown in FIG. Diffracted by a diffraction grating (hereinafter referred to as a junction diffraction grating 500) to form 0th order diffracted light (hereinafter referred to as a main beam 510), + 1st order diffracted light (hereinafter referred to as a preceding subbeam 512), and −1st order diffracted light (hereinafter referred to as a delayed subbeam 514). To do.

  Further, Patent Document 2 proposes a differential push-pull method using a three-divided phase difference diffraction grating 550 as another tracking error detection method independent of the track pitch.

  In this method, the laser beam 508 emitted from the light emitting element is separated into zero-order diffracted light and ± first-order diffracted light using a three-part phase difference diffraction grating 550 as shown in FIG. Then, the main beam 510, the preceding sub beam 512, and the lagging sub beam 514 are formed.

  As shown in FIG. 16, the three-divided phase difference diffraction grating 550 includes diffraction gratings 550a, 550b, and 550c having the same period. Of these, the diffraction grating 550c has a narrower width w in the grating groove direction than other diffraction gratings. The three-divided phase difference diffraction grating 550 is configured such that these diffraction gratings are joined in the order of diffraction gratings 550a, 550c, and 550b with the phases of the adjacent diffraction gratings shifted by 90 degrees.

  The main beam 510, the preceding sub beam 512, and the lagging sub beam 514 are condensed on the recording surface of the optical disc. In the rewritable optical disc, guide grooves are concentrically spaced at equal intervals, and a track is formed on the recording surface of the optical disc. The cross section of the recording surface of the optical disc has a shape in which irregularities are periodically repeated in the radial direction. Of these, the concave portion is called a groove, and the convex portion is called a land. When there is no eccentricity of the optical disc, the main beam 510, the preceding sub beam 512, and the slowing sub beam 514 are condensed so that three beams are arranged in the groove or land direction as shown by 516, 517, and 518 in FIG. Ideally, these three beams are focused in the center of the same groove or land. These three beams are reflected from the recording surface of the optical disc, and are diffracted and separated into zero-order reflected light and ± first-order reflected light.

  As shown in FIG. 17, the photodetector 520 includes photodetecting elements 520A to 520F. The photodetectors 520A and 520B divide the region including the overlapping portion of the 0th-order reflected light and the ± 1st-order reflected light of the main beam 510 into two parts and receive light independently of each other. Similarly, the photodetectors 520C and 520D divide the region including the overlapping portion of the zero-order reflected light and the ± first-order reflected light of the preceding sub-beam 512 into two parts and receive light independently of each other. Further, the photodetector 520E and the photodetector 520F divide the region including the overlapped portion of the zero-order reflected light and the ± first-order reflected light of the slow sub-beam 514 into two parts and receive light independently of each other.

  The calculation unit 521 generates a difference (A−B) between detection values from the light detection element 520A and the light detection element 520B as the main push-pull signal MPP. Similarly, a difference (CD) between detection values from the light detection element 520C and the light detection element 520D is generated as a preceding sub push-pull signal FSPP, and a difference between detection values from the light detection element 520E and the light detection element 520F ( E−F) is generated as the delayed sub push-pull signal BSPP. Then, after the addition sub push-pull signal SPP, which is the sum of the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP (FSPP + BSPP), is amplified to the same extent as the main push-pull signal MPP as shown in Equation 1 (amplification factor) K), the difference between the main push-pull signal MPP and the added sub-push-pull signal SPP is calculated and generated as a tracking error signal DPP.

(Equation 1)
DPP = (A−B) −K ((C−D) + (E−F))
= MPP-K (FSPP + BSPP)
= MPP-K (SPP) (1)

  When the main beam 510, the preceding sub beam 512, and the delayed sub beam 514 are irradiated to the center of the groove or land, the signal intensity of the tracking error signal DPP is almost zero. On the other hand, the signal intensity of the tracking error signal DPP increases as the irradiation position of these beams moves away from the center of the groove or land. Therefore, the tracking control device performs tracking control by adjusting the position of the objective lens so that the tracking error signal DPP becomes 0 based on the tracking error signal DPP.

Japanese Patent Laid-Open No. 9-81942 JP 2000-145915 A

  In the tracking error detection method using the in-line differential push-pull method, if the objective lens is displaced in the radial direction of the disk (hereinafter referred to as objective lens shift) for tracking eccentricity, the tracking error detected according to the displacement amount The signal amplitude of the signal DPP is greatly reduced as compared with the case where there is no objective lens shift. Therefore, in order to obtain a practical tracking error signal DPP, it is necessary to limit the allowable displacement width of the objective lens to a narrow range, which is one of the factors that hinder the tracking performance.

  On the other hand, in the case of the differential push-pull method using the three-part phase difference diffraction grating 550, the preceding sub-push-pull signal FSPP and the delayed sub-push-pull signal BSPP with respect to the main push-pull signal MPP even when there is no objective lens shift. A reverse phase shift occurs between them.

  This phase shift will be described with reference to FIG. FIG. 18 shows the calculation results of the signal intensities of the main push-pull signal MPP, the preceding sub-push-pull signal FSPP, and the delayed sub-push-pull signal BSPP obtained by using the diffracted and separated beam using the three-part phase difference diffraction grating 550. Show. The calculation conditions at that time are shown below.

Disc: DVD-RAM 4.7 GB
Groove pitch: 1.23 μm
Wavelength: 658nm
Objective lens: NA = 0.65, f = 2.75 mm
Optical magnification: -1 / 6.5
Diffraction grating: Three-part phase difference diffraction grating 550

  The horizontal axis of FIG. 18 indicates the beam irradiation position in the radial direction of the optical disc 200, and the vertical axis indicates the signal strength of each push-pull signal MPP, FSPP, BSPP, SPP and tracking error signal DPP. As shown in FIG. 18, the preceding sub push-pull signal FSPP has a positive phase shift φx of the optical disc 200 with respect to the main pull signal MPP, and the lagging sub push-pull signal BSPP has a negative phase shift φy with respect to the main pull signal MPP. . The magnitudes of the phase shift φx and the phase shift φy are both equal. Thus, the reverse phase shift has arisen between the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP with respect to the main push-pull signal MPP.

  If there is such a phase shift between the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP, the tracking error signal is generated when the signal strengths of the preceding sub-push pull signal FSPP and the delayed sub-push pull signal BSPP are different. The DPP causes a phase shift with respect to the periodic structure of the recording surface of the optical disc. When tracking servo is applied based on such a tracking error signal DPP, detracking proportional to the phase shift occurs.

  As an example in which the signal strength of the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP is different, there is a case where recording is performed on an unrecorded disc of DVD-RAM. When recording on an unrecorded disc of the DVD-RAM, the lagging sub-beam is irradiated onto the unrecorded area, and the preceding sub-beam is irradiated onto the recorded area. The unrecorded area is occupied by a crystalline state, while the recorded area is a mixture of a crystalline state and an amorphous state. Since the reflectance of the amorphous portion is almost close to 0, the average reflectance of the recorded area is smaller than the average reflectance of the unrecorded area. Therefore, the signal strength of the preceding sub push-pull signal FSPP calculated using the reflected light from the recorded area having an average reflectance smaller than that of the unrecorded area is calculated using the reflected light from the unrecorded area. It becomes smaller than the signal strength of the preceding sub push-pull signal FSPP.

  FIG. 19 shows a simulation result assuming that recording is performed on an unrecorded disc of DVD-RAM by the differential push-pull method using the three-divided phase difference diffraction grating 550. In this simulation, considering that the preceding sub push-pull signal FSPP is a value detected using the reflected light of the recorded area having a low average reflectance, the signal strength of the preceding sub push-pull signal FSPP is the delayed sub push. The pull signal BSPP is 0.5 times. Other calculation conditions are the same as the calculation conditions described above.

  The horizontal and vertical axes in FIG. 19 indicate the beam irradiation position and the signal strength of each push-pull signal and tracking error signal DPP. As can be seen from FIG. 19, a phase shift occurs between the added sub push-pull signal SPP and the main push-pull signal MPP, and a phase shift φa also occurs in the tracking error signal DPP calculated based on the difference. When tracking servo is applied in a state where the tracking error signal DPP has a phase shift φa, detracking proportional to the phase shift φa occurs. In the case of FIG. 19, the detrack amount is about 20 nm, which exceeds the allowable range of the detrack amount in the DVD-RAM, and the recording / reproducing quality of information is remarkably deteriorated.

  In addition, when the phases of the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP are different, the signal strength of the tracking error signal DPP fluctuates when the track direction is inclined with respect to the direction in which three spots are arranged due to the eccentricity of the disk. However, there arises a problem that stable tracking servo becomes difficult.

  In the present invention, in view of the above-described problems of the prior art, the signal intensity of the tracking error signal DPP is small with respect to the objective lens shift, and a phase shift occurs between the preceding sub-push-pull signal FSPP and the delayed sub-push-pull signal BSPP. It is an object to provide a tracking error detection method that is difficult to perform.

  The present invention includes a junction type diffraction grating in which a first diffraction grating and a second diffraction grating having the same period as the first diffraction grating are joined with a phase shifted by 180 degrees on the same plane, and emitted from a light source. A tracking control device that detects tracking errors using the main beam and sub beams generated by diffracting the beam with a junction diffraction grating and controls tracking to the tracks on the disk based on the detected tracking errors. The junction type diffraction grating has a structure in which the first diffraction grating and the second diffraction grating are joined with a meshing region having a predetermined width in the grating groove direction.

  Here, in the junction type diffraction grating, the junction part of the first diffraction grating and the second diffraction grating has irregularities, respectively, and in the meshing region of the junction type diffraction grating, the convex part of one diffraction grating is the other. It is preferable to be formed in mesh with the concave portion of the diffraction grating.

  Furthermore, it is desirable that the area occupied by the first diffraction grating and the area occupied by the second diffraction grating in the meshing area of the junction type diffraction grating have the same area.

  Further, it is preferable that the first diffraction grating and the second diffraction grating in the meshing region of the junction type diffraction grating are meshed at a predetermined period.

  In addition, it is preferable that the first diffraction grating and the second diffraction grating have a comb shape, and one comb tooth is engaged with a portion without the other comb tooth.

  These tracking control devices irradiate an optical disk with a main beam, a preceding sub-beam, and a delayed sub-beam by means of a junction type diffraction grating, and a 0th-order diffracted reflected light and a ± 1st-order diffracted reflected light by the disk groove structure of the main beam. The first and second light receiving elements that receive the light by dividing the region including the overlapped portion into at least two or more and independently output detection values, and the zero-order diffracted reflected light by the disk groove structure of the preceding sub-beam and ± The region including the overlapping portion with the first-order diffracted reflected light is divided into at least two or more, and each receives light, and outputs a detection value independently, and 0 by the disk groove structure of the delayed sub beam. The region including the overlapping portion of the first-order diffracted reflected light and the ± 1st-order diffracted reflected light is divided into at least two parts, and each is received, and the detected value is independently obtained The main push-pull signal MPP generated from the detection values from the first and second light receiving elements and the detection values from the third and fourth light receiving elements are provided. The racking error signal DPP is calculated using the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP generated from the detection values from the fifth and sixth light receiving elements, and the calculated tracking error The tracking to the track on the disk is controlled based on the signal DPP.

  In this tracking control device, the tracking error signal DPP is calculated by DPP = MPP-K (FSPP + BSPP) (where K is a constant).

  According to the present invention, in the recording / reproducing of a plurality of types of optical disks having different track pitches, the degree of decrease in the signal amplitude of the tracking error signal DPP with respect to the objective lens shift can be reduced, and the junction type diffraction grating 500 is used. Compared to tracking control, the permissible displacement width of the objective lens can be widened to achieve more stable tracking servo. Further, since tracking control can be performed with a smaller detrack when recording using a DVD-RAM unrecorded disk, the recording quality is higher than when tracking control is performed using the three-part phase difference diffraction grating 550. It is possible to record a good mark. Further, the fluctuation of the signal amplitude of the tracking error signal DPP due to the disk eccentricity is reduced, and stable tracking servo can be performed.

  As shown in FIG. 1, the tracking control circuit 100 according to the embodiment of the present invention includes a semiconductor laser 10, a diffraction grating 12, a beam splitter 14, a collimator lens 16, an objective lens 18, a sensor lens 20, a photodetector 22, a calculation unit. 24 and the objective lens driving unit 26.

  The semiconductor laser 10 emits light having a predetermined wavelength suitable for reading / writing on an optical disk. Light emitted from the semiconductor laser 10 is diffracted by the diffraction grating 12 and separated into a main beam and a sub beam. The diffracted light that has passed through the diffraction grating 12 passes through the beam splitter 14, is converted into parallel light by the collimator lens 16, and reaches the objective lens 18. The light reaching the objective lens 18 is condensed on the recording surface of the optical disc 200 by the objective lens 18.

  The light applied to the optical disc 200 is reflected by the recording surface of the optical disc 200. The reflected light reaches the beam splitter 14 through the objective lens 18 and the collimator lens 16. Then, it is reflected in the direction of the sensor lens 20 by the beam splitter 14, passes through the sensor lens 20, and enters the photodetector 22. The photodetector 22 outputs a signal intensity corresponding to the incident light to the calculation unit 24. The calculation unit 24 receives the detection signal and calculates a tracking error signal DPP based on the detection signal. Further, a control signal is output to the objective lens driving unit 26 based on the tracking error signal DPP, and the objective lens actuator 26 is driven by the objective lens driving unit 26 to adjust the position of the objective lens 18 in the tracking direction.

  FIG. 2 is a perspective view of the diffraction grating 12 used in this embodiment, and FIG. 3 is a plan view thereof. The diffraction grating 12 has a structure in which two diffraction gratings 12a and 12b having the same period are joined with the groove direction aligned on the same plane and shifted in phase by 180 degrees. At that time, the diffraction grating 12 is joined by a meshing region 38 having a predetermined width w as shown in FIG. 3 to form a joining boundary line 39 including a right angle shape.

  The definition of the meshing area in this specification will be described with reference to FIG. The diffraction grating 12a and the diffraction grating 12b shown in FIG. 4 have an arbitrarily-shaped joint surface. In the diffraction grating 12 of FIG. 4, the vertical direction corresponds to the groove direction. However, it is omitted in FIG. The diffraction grating 12 has a structure in which the diffraction grating 12a and the diffraction grating 12b are joined, and a joint boundary line 39 is formed at the joint. Here, both ends of the joint boundary line 39 are denoted by 39a and 39b. A straight line passing through an arbitrary point 39x on the joining boundary line 39 and perpendicular to the groove direction is defined as a boundary scanning line 40. Here, when the point 39x moves from the one end 39a to the other end 39b of the joint boundary line 39 through the joint boundary line 39, the region drawn by the boundary scanning line 40 is the meshing region 38. The length in the groove direction in the meshing region 38 is defined as the width w of the meshing region. Further, a straight line perpendicular to the groove direction and symmetrically dividing the meshing region 38 into two is defined as a pitch direction two-divided line 40c, and a straight line parallel to the groove direction and symmetrically divided into two. The straight line is defined as a groove-direction two-parting line 42. Then, the intersection of the groove direction two-parting line 42 and the pitch direction two-parting line 40 c is defined as the meshing region center 44.

  For example, in the case of the diffraction grating 12 of FIG. 4, a boundary scanning line 40a passing through a point 39d closest to the diffraction grating 12a in the junction boundary 39 and a point 39e closest to the diffraction grating 12b in the junction boundary 39 are represented. The boundary scanning straight line 40 b that passes through and the area surrounded by the outer periphery of the diffraction grating 12 are the meshing area 38.

  As described above, the meshing region 38 of the diffraction grating 12 includes the diffraction grating 12a included in the diffraction grating 12a and closer to the diffraction grating 12b than the pitch direction dividing line 40c, and the diffraction grating 12b included in the diffraction grating 12b and diffracting from the pitch direction dividing line 40c. There is a region near the lattice 12a, and a region is formed that enters into each other beyond the dividing line 40c in the pitch direction.

  FIG. 5 shows the phase of the wavefronts of the 0th-order diffracted light (hereinafter referred to as the main beam 110), the + 1st-order diffracted light (hereinafter referred to as the preceding sub-beam 112), and the −1st-order diffracted light (hereinafter referred to as the delayed sub-beam 114). It explains using.

  In general, the 0th-order diffracted light diffracted and separated by the diffraction grating has the same wavefront phase with respect to the incident beam. Therefore, even if the two diffraction gratings a and b are joined with their phases shifted, there is no wavefront phase difference between the 0th-order diffracted lights diffracted and separated from the two diffraction gratings a and b. On the other hand, the + 1st order diffracted light has a wavefront phase different from that of the incident beam. When the two diffraction gratings a and b are joined while shifting the phase, a wavefront phase difference is generated by the shifted phase. Similarly, for the −1st order diffracted light, a wavefront phase difference is generated by the amount of phase shifted. The wavefront phases of the + 1st order diffracted light and the −1st order diffracted light are in an opposite phase relationship, and the phase of the wavefront is 180 degrees different.

  In the embodiment of the present invention, since the main beam 110 is 0th order diffracted light, the phase of the wave front is uniform in the entire region of the main beam 110. On the other hand, in the case of the preceding sub-beam 112, one half surface region 112a is a region of primary diffraction light generated by being diffracted by the diffraction grating 12a, and the other half surface region 112b is a region of primary diffraction light generated by being diffracted by the diffraction grating 12b. is there. Therefore, the phase of the wave front is 180 degrees different between the half surface region 112a and the half surface region 112b of the preceding sub beam 112. The shape of the boundary line 115 between the half surface region 112a and the half surface region 112b corresponds to the junction boundary line 39 (see FIG. 3) of the diffraction grating 12 and includes a right angle. As a result, the shape of the half surface region 112a and the half surface region 112b in the preceding sub-beam 112 is such that part of the half surface region 112a enters the half surface region 112b side, and conversely, part of the half surface region 112b enters the half surface region 112b side. ing. In such a region diffracted from the meshing region 38 of the diffraction grating 12, the half surface region 112a and the half surface region 112b enter each other, and the average phase is zero. As with the preceding sub-beam 112, the retarding sub-beam 114 has a wavefront phase difference of 180 degrees in one half-surface region 114a and the other half-surface region 114b. The shape of the boundary line 115 between the half surface region 114a and the half surface region 114b is a shape including a right angle, and the half surface regions 114a and 114b are inserted into each other. In the region diffracted from the meshing region 38 of the diffraction grating 12, the half surface region 112a and the half surface region 112b enter each other, and the average phase is zero. The preceding sub-beam 112 and the delayed sub-beam 114 have a relationship of + 1st order diffracted light and −1st order diffracted light, and the phase of the wavefront in the corresponding half-surface region is 180 degrees different.

  The main beam 110, the preceding sub beam 112, and the lagging sub beam 114 are collected on the optical disc as shown in the left diagram of FIG. Guide grooves are formed in the recording surface of the optical disc 200 in a concentric manner at equal intervals, and the recording surface of the optical disc has a periodic uneven shape in the radial direction. Of the irregularities, the convex part is called a land, and the concave part is called a groove. The main beam 110, the preceding sub beam 112, and the lagging sub beam 114 are condensed so that three beams are arranged in the groove or land direction when the optical disc 200 is not decentered. Further, when ideal tracking is performed, these three beams are collected at the center of the same groove or land.

  Each beam collected on the recording surface of the optical disc 200 is reflected, diffracted by the uneven shape of the recording surface, and further diffracted by 0th order diffracted reflected light (hereinafter referred to as 0th order reflected light) and ± 1st order diffracted reflected light ( Thereafter, it is diffracted and separated into ± first order reflected light). At this time, the 0th-order reflected light is given the phase by the diffraction grating 12 as it is. Therefore, with respect to the beam incident on the recording surface of the optical disc 200, the phase difference of the wave front of the zero-order reflected light is constant regardless of the irradiation position of the beam. On the other hand, in the case of ± first order reflected light, the phase due to the diffraction grating 12 and the phase due to diffraction of the guide grooves formed periodically of the optical disc 200 are added. The phase due to diffraction of the guide groove depends on the periodic uneven shape in the radial direction on the recording surface of the optical disc 200. Therefore, the phase difference of the wavefront of the ± first order reflected light with respect to the beam incident on the recording surface of the optical disc 200 periodically changes as the beam irradiation position moves in the radial direction of the optical disc 200.

  The 0th-order reflected light and the ± 1st-order reflected light of the main beam 110 diffracted and separated on the recording surface of the optical disc 200 reach the photodetector 22, respectively. At this time, as shown in FIG. 7, the 0th order reflected light 160 and the −1st order reflected light 162 form a superimposed region A, and the 0th order reflected light 160 and the + 1st order reflected light 164 form a superimposed region B. The distance between the center of the zero-order reflected light 160 and the centers of the ± first-order reflected lights 162 and 164 on the photodetector 22 is uniquely determined by the focal length and numerical aperture of the objective lens 18 and the track pitch of the disk recording surface. . Similarly, the 0th-order reflected light and the ± 1st-order reflected light of the preceding sub-beam 112 and the delayed sub-beam 114 form overlapping regions A and B on the photodetector 22.

  As shown in FIG. 6, the photodetector 22 includes photodetectors 22 </ b> A and 22 </ b> B that detect reflected light of the main beam 110, photodetectors 22 </ b> C and 22 </ b> D that detect reflected light of the preceding sub-beam 112, and reflection of the delayed sub-beam 114. Photodetectors 22E and 22 for detecting light are provided.

  The photodetectors 22A and 22B are arranged adjacent to each other. The positions of the photodetectors 22A and 22B are adjusted so that the center of the 0th-order reflected light of the main beam 110 hits the boundary between the photodetectors 22A and 22B. The region including B is divided into two to receive light. The photodetector 22A receives a region including the superimposed region A in the reflected light of the main beam 110, and the photodetector 22B receives a region including the superimposed region B, and outputs a detection value independently of each other.

  The photodetectors 22C and 22D are arranged adjacent to each other. The positions of the photodetectors 22C and D are adjusted so that the center of the zero-order reflected light of the preceding sub-beam 112 hits the boundary between the photodetectors 22C and D, and the overlapping portions A and 0 of the zero-order reflected light and the ± first-order reflected light are adjusted. The region including B is divided into two to receive light. The photodetector 22C receives a region including the superimposed region A in the reflected light of the preceding sub-beam 112, and the photodetector 22D receives a region including the superimposed region B and outputs a detection value independently of each other.

  The photodetectors 22E and 22F are arranged adjacent to each other. The positions of the photodetectors 22E and 22F are adjusted so that the center of the zero-order reflected light of the slow sub-beam 114 hits the boundary between the photodetectors 22E and 22F, and the overlapping portions A and 0 of the zero-order reflected light and the ± first-order reflected light are adjusted. The region including B is divided into two to receive light. The photodetector 22E receives the region including the superimposed region A in the reflected light of the delayed sub-beam 114, and the photodetector 22F receives the region including the superimposed region B, and outputs the detection value independently of each other.

  In the photodetector 22 in the present embodiment, the two photodetectors are handled as a set of photodetectors, and the region including the overlapping portions A and B of the zero-order reflected light and the ± first-order reflected light is divided into two parts, respectively. It is designed to receive light. However, the four photodetectors are used as a set of photodetectors so that the region including the overlapping portions A and B of the zeroth-order reflected light and the ± first-order reflected light is divided into four parts and received respectively. It is also possible to do.

  As shown in FIG. 6, the arithmetic unit 24 includes four differential amplifiers 27 </ b> A to 27 </ b> D, an adder 28, and an amplification amplifier 30. The differential amplifier 27A calculates a difference (A−B) between output signals from the photodetectors 22A and 22B and generates a main push-pull signal MPP. The differential amplifier 27B calculates a difference (C−D) between output signals from the photodetectors 22C and 22D, and generates a preceding sub push-pull signal FSPP. The differential amplifier 27C calculates a difference (E−F) between output signals from the photodetectors 22E and 22F and generates a delayed sub push-pull signal BSPP.

  The adder 28 receives a preceding sub push-pull signal FSPP that is an output signal of the differential amplifiers 27B and 27C and a delayed sub push-pull signal BSPP that is an output signal of the differential amplifier 27C. The adder 28 calculates the addition (FSPP + BSPP) of these signals to obtain an added sub push-pull signal SPP. The amplifying amplifier 30 receives an addition sub push-pull signal SPP that is an output signal of the adder 28. The amplification amplifier 30 amplifies the added sub push-pull signal SPP with an amplification factor K to a signal level equivalent to that of the main push-pull signal MPP. The output signal of the differential amplifier 27A and the output signal of the amplification amplifier 30 are input to the differential amplifier 27D. The differential amplifier 27D calculates the difference between the main push-pull signal MPP and the signal obtained by amplifying the added sub-push-pull signal SPP and outputs it as a tracking error signal DPP.

  In such a circuit configuration, the tracking error signal DPP is calculated as shown in Equation (2).

(Equation 2)
DPP = (A−B) −K ((C−D) + (E−F))
= MPP-K (FSPP + BSPP)
= MPP-K (SPP) (2)

  The signal strengths of the main push-pull signal MPP, the preceding sub push-pull signal FSPP, and the delayed sub push-pull signal BSPP calculated according to the present embodiment will be described with reference to FIG. The upper diagram in FIG. 8 shows a cross-sectional view of the optical disc 200 and beam irradiation positions 201a to 201e. The horizontal axis of the lower graph in FIG. 8 indicates the beam irradiation position on the optical disk, and corresponds to the beam irradiation position shown in the upper diagram. The vertical axis indicates the signal strength of the main push-pull signal MPP, the preceding sub push-pull signal FSPP, the delayed sub push-pull signal BSPP, and the tracking error signal DPP.

  As shown in FIG. 8, since the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP are both in reverse phase with respect to the main push-pull signal MPP, the tracking error signal DPP calculated by Equation (2) is The signal strength is larger than that of the main push-pull signal MPP. Therefore, in order to obtain a tracking error signal DPP having a greater strength, it is desirable that not only the main push-pull signal MPP but also the signal strengths of the preceding sub-push pull signal FSPP and the delayed sub-push pull signal BSPP are large.

  In the main push-pull signal MPP, the preceding sub push-pull signal FSPP, the delayed sub push-pull signal BSPP, and the tracking error signal DPP, as shown in FIG. 8, when the beam is irradiated to the centers 201a, 201c, and 201e of the grooves and lands. The signal intensity becomes zero. The signal intensity of the tracking error signal DPP increases as the beam irradiation position moves away from the center of the groove or land, and the signal intensity becomes maximum when the beam is irradiated at exactly the middle between the groove center and the land center. Therefore, tracking control is performed based on the tracking error signal DPP so that the signal intensity becomes zero.

  In the present embodiment, the light intensity of the reflected light of the preceding sub-beam 112 detected by the photodetectors 22C and 22D with and without the objective lens shift will be described with reference to FIG. The upper diagram of FIG. 9 shows the irradiation position of the preceding sub-beam 112 on the optical disk, and shows the case where the center of the preceding sub-beam 112 is irradiated to the boundary of the groove or land. 9 is a diagram in the case where there is no objective lens shift, and shows the 0th-order reflected light 121, the −1st-order reflected light 122, and the + 1st-order reflected light 123 of the preceding sub beam 112. The diagram on the right side of the middle stage of FIG. 9 is a diagram in the case where there is an objective lens shift of about one-eighth of the diameter of the objective lens, and the 0th-order reflected light 124, the −1st-order reflected light 122, and +1 of the preceding sub-beam 112 Next reflected light 123 is shown. Lower 127 and 128 are diagrams showing the interference between the zero-order reflected light and the ± first-order reflected light, and 129 and 130 are diagrams showing the light intensity on the photodetector. The light intensity on the photodetector indicates that the light intensity is smaller as the hatched area is denser and the light intensity is larger as the hatched area is sparse.

  The wavefronts of the 0th-order reflected lights 121 and 124 of the preceding sub-beam 112 are 180 degrees out of phase with respect to the boundary line. The shape of the boundary line includes a right angle corresponding to the junction boundary line 39 of the diffraction grating 12. In the region corresponding to the meshing region of the diffraction grating 12 among the regions of the zero-order reflected light 121 and 124 of the preceding sub-beam 112, a part of the left-side a-plane (hereinafter referred to as a-plane convex portion) is the right-side b-plane. On the contrary, a part of the left b-plane (hereinafter referred to as a b-plane convex portion) enters a part of the right a-plane. The same applies to the ± first-order reflected lights 122, 123, 125, and 126 of the preceding sub-beam 112.

  As shown in FIG. 9, when the center of the preceding sub-beam 112 is irradiated on the track boundary, the wavefront (left) of the minus first-order reflected light of the preceding sub-beam does not generate a phase difference with respect to the zero-order reflected light. The wavefront distribution is as shown at 122. On the other hand, the wavefront (right) of the + 1st order reflected light has a wavefront distribution as indicated by 123 because a phase difference of 180 degrees is generated compared to the 0th order reflected light.

  In the overlapping region of the 0th order reflected light and the ± 1st order reflected light, the phase difference between the 0th order reflected light and the ± 1st order reflected light becomes brighter due to interference, and when the phase difference is ± 180, it becomes darker due to interference. . When the phase difference between the 0th-order reflected light and the + 1st-order reflected light (or -1st order reflected light) is Δφ, the light intensity after interference is proportional to 1 + cos (Δφ).

  When there is no objective lens shift, as shown in the left figure, a bright region 131 appears in a part of the superimposed region detected by the photodetector 22C, and a dark region 132 appears in a part of the superimposed region detected by the photodetector 22D. The bright region 131 appears in a superimposed region including the convex portions of the a- and b-planes of 0th-order and −1st-order reflected light. The dark region 132 appears in the overlapping region including the convex portions of the a and b surfaces of the 0th and + 1st order reflected light. As described above, both the bright region and the dark region appear in the overlapping region of the zero-order reflected light and the ± first-order reflected light. Therefore, compared to the preceding sub push-pull signal FSPP obtained by using the junction type diffraction grating 500 to completely separate the bright and dark regions in the photodetectors 22C and 22D, the preceding sub push-pull obtained in the present embodiment. The signal strength of the signal FSPP is slightly reduced.

  When there is an objective lens shift, the position of the boundary line of the wavefront of the zero-order reflected light 124 of the preceding sub-beam 112 is shifted from the center according to the objective lens shift. Similarly, for the ± first-order reflected lights 125 and 126 of the preceding sub-beam 112, the position of the boundary line of the wavefront is shifted from the center in accordance with the objective lens shift. Further, according to the objective lens shift, the center of the zero-order reflected light 124 deviates from the boundary line between the photodetectors 22C and 22D.

  As described above, since the position of the boundary line of the wavefront is shifted according to the objective lens shift, the bright region 131 is enlarged in the superimposing region detected by the photodetector 22C, while the dark region is dark in the superimposing region detected by the photodetector 22D. Region 132 is enlarged. In this way, the areas of the bright region 131 and the dark region 132 change in the superposed region, and the signal strength of the preceding sub push-pull signal FSPP is reduced as compared with the case where there is no objective lens shift. However, since there are convex portions on the a and b surfaces of the 0th and ± 1st order reflected light, the amount of change in the area of the bright region 131 and the dark region 132 in the overlapping region caused by the influence of the objective lens shift is the junction type diffraction grating. Compared to the case of using 500, it is halved. Therefore, as compared with the case where the junction type diffraction grating 500 is used, the degree of decrease in the signal strength of the preceding sub push-pull signal FSPP with respect to the objective lens shift amount is small.

  The preceding sub-beam 112 and the delayed sub-beam 114 are beams having a phase difference of 180 degrees at the boundary 115 and the delayed sub-push-pull signal BSPP is the same as the preceding sub-push-pull signal FSPP if the conditions are the same. Is obtained.

  As described above, according to the present embodiment, in the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP, it is possible to reduce the extent to which the signal intensity decreases with respect to the objective lens shift. As a result, also for the tracking error signal DPP, the degree of decrease in the signal intensity with respect to the objective lens shift can be reduced.

  FIG. 10 shows the result of calculating the signal amplitude of the tracking error signal DPP with respect to the objective lens shift amount in the radial direction. FIG. 10 shows the calculation results when the junction type diffraction grating 500 is used, when the three-divided phase difference diffraction grating 550 is used, and when the diffraction grating 12 is used. The horizontal axis in FIG. 10 represents the objective lens shift amount, and the vertical axis in FIG. 10 represents the signal amplitude of the tracking error signal DPP. FIG. 10 shows the relative value of the tracking error signal DPP when each diffraction grating is used with the signal amplitude of the tracking error signal DPP set to 1 when the objective lens shift amount is 0 using the junction type diffraction grating 500. .

  When the three-divided phase difference diffraction grating 550 is used, the length w of the central diffraction grating 550c in the groove direction is 10%, 20%, The calculation result in the case of 30% is shown. Further, for the diffraction grating 12, calculation results when the width of the meshing region 38 is 10%, 20%, and 30% of the length w of the diffraction grating 12 in the groove direction are shown.

  When the junction type diffraction grating 500 is used, the signal amplitude of the tracking error signal DPP is greatly reduced as the distance of the objective lens shift increases. On the other hand, when the diffraction grating 12 is used, the degree of decrease in the signal amplitude of the tracking error signal DPP is smaller than when the junction diffraction grating 500 is used.

  Thus, according to the present embodiment, the degree of decrease in the signal amplitude of the tracking error signal DPP when there is an objective lens shift is small, and it is less affected by the objective lens shift than in the case where the junction type diffraction grating 500 is used. Tracking control is possible.

  Next, the tracking error detection method according to the present embodiment will be described with reference to FIG. 11 to indicate that no phase shift occurs between the preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP. The upper diagram in FIG. 11 shows the irradiation position of the preceding sub beam 112 on the optical disk, and shows that the center of the preceding sub beam 112 and the delayed sub beam 114 is irradiated to the center of the groove. The left diagram in the middle of FIG. 11 shows the zeroth-order reflected light 141 of the preceding subbeam 112, the −1st order reflected light 142 of the preceding subbeam 112, and the + 1st order reflected light 143 of the preceding subbeam 112. The diagram on the right side of the middle stage of FIG. 11 shows the zero-order reflected light 144 of the slowing sub-beam 114, the −1st-order reflected light 145 of the slowing sub-beam 114, and the + first-order reflected light 146 of the slowing sub-beam 114. The lower 147 and 148 are diagrams showing the interference between the zero-order reflected light and the ± first-order reflected light, and 149 and 150 are diagrams showing the light intensity on the photodetector. As in FIG. 9, the light intensity on the photodetector indicates that the light intensity is lower in the area where the hatched eyes are denser and the light intensity is higher in the area where the hatched eyes are sparse.

  As shown in FIG. 11, when the center of the preceding sub-beam 112 is irradiated to the center of the groove, the phase difference Δφ between the zero-order reflected light and the ± first-order reflected light in the overlapping region is ± 90 degrees as shown by 147. Become. Therefore, the light intensities in the photodetectors 22C and 22D that detect the reflected light of the preceding sub beam 112 are equal to each other, and the signal intensity of the preceding sub push-pull signal FSPP is zero. Similarly, the light intensities in the photodetectors 22E and 22E that detect the reflected light of the delayed sub beam 114 are equal to each other, and the signal intensity of the delayed sub push-pull signal BSPP becomes zero. Similarly, the signal intensity of the preceding sub-push pull signal FSPP and the delayed sub-push pull signal BSPP is 0 in the case where irradiation is performed so that the centers of the preceding sub-beam 112 and the retarding sub-beam 114 are in the center of the land.

  In this way, if the diffraction gratings are joined by shifting the phases of the two diffraction gratings by 180 degrees as in the present embodiment, the phase shift between the preceding sub-push-pull signal FSPP and the delayed sub-push-pull signal BSPP occurs almost. do not do. Therefore, even when recording on an unrecorded disk of DVD-RAM, the detrack is small and tracking control is possible.

  FIG. 12 shows a simulation result assuming that the diffraction grating 12 is used to record on an unrecorded disc of DVD-RAM by the differential push-pull method. At that time, the simulation was performed under the following conditions.

Disc: DVD-RAM 4.7 GB
Groove pitch: 1.23 μm
Wavelength: 658nm
Objective lens: NA = 0.65, f = 2.75 mm
Optical magnification: -1 / 6.5
Other: The signal strength of FSPP is 0.5 times that of BSPP.

  The horizontal axis of FIG. 12 indicates the beam irradiation position in the radial direction of the optical disc, and the vertical axis indicates the signal strength of each push-pull signal MPP, FSPP, BSPP, SPP and tracking error signal DPP.

  The preceding sub push-pull signal FSPP and the delayed sub push-pull signal BSPP have no phase difference with respect to the main push-pull signal MPP as shown in FIG. Therefore, the addition sub push-pull signal SPP has no phase shift with respect to the main push-pull signal MPP, and as a result, there is no phase shift of the tracking error signal DPP. As described above, according to the method for detecting the tracking error signal DPP in the present embodiment, there is no phase shift with respect to the periodic structure of the guide groove formed on the optical disk even when recording on a DVD-RAM unrecorded disk. The tracking error signal DPP can be detected. Then, by performing tracking control based on the tracking error signal DPP, tracking control with a smaller detrack amount is possible compared to tracking control using the three-divided phase difference diffraction grating 550.

  Subsequently, FIG. 13 shows the result of calculating the amplitude characteristic of the tracking error signal DPP with respect to the disk eccentricity. The calculation conditions are as follows.

Disc: DVD-RAM 4.7 GB
Groove pitch: 1.23 μm
Wavelength: 658nm
Objective lens: NA = 0.65, f = 2.75 mm
Optical magnification: -1 / 6.5
Distance between main spot and sub spot: 25 μm
Main spot location: Track radius 25mm

  In FIG. 13, the horizontal axis indicates the disk eccentricity, and the vertical axis indicates the amplitude of the tracking error signal DPP. In FIG. 13, the amplitude value of each tracking error signal DPP is a value normalized with the amplitude of the tracking error signal DPP being 1 when the disk eccentricity = 0.

  As shown in FIG. 13, when the three-part phase difference diffraction grating 550 is used, the amplitude at an eccentricity of 100 μm varies by 5% to 20% compared to when the eccentricity = 0. On the other hand, when the diffraction grating 12 according to the present embodiment is used, the fluctuation range of the amplitude at the eccentricity of 100 μm is less than 5% compared to when the eccentricity = 0.

  As described above, with the tracking error detection method according to the present embodiment, since the fluctuation range of the signal amplitude with respect to the eccentricity is small, it is possible to obtain a good tracking error signal even for a bad disk with a large eccentricity. When the center of the objective lens is deviated in the tangential direction with respect to the spindle motor axis, the same effect as decentration is produced, but even in such a case, it is acceptable compared with the case where the three-part phase difference diffraction grating 550 is used. The range can be large.

<Modification>
In the above embodiment, as shown in FIGS. 2 and 3, the diffraction grating 12 has a shape in which the junction boundary line 39 includes a right angle. By such a joint boundary line 39, the meshing region 38 is composed of two quadrangles 38a and 38b.

  However, the meshing region 38 of the diffraction grating 12 is not limited to such a shape, and may be configured such that the diffraction grating 12a and the diffraction grating 12b are joined at a joining boundary line 39 that is non-perpendicular to the groove direction as shown in FIG. 14a. Even in such a configuration, there is a region in the diffraction grating 12a and the diffraction grating 12b that enter each other beyond the dividing line 40c in the pitch direction, and the presence of this region allows the preceding sub push-pull signal FSPP and the objective lens shift amount to be This is because the degree of decrease in the signal strength of the delayed sub push-pull signal BSPP can be reduced.

  In addition, as shown in FIG. 4, the diffraction grating 12a and the diffraction grating 12b each have a concavo-convex shape in the groove direction at the joint portion, and the diffractive portion formed by engaging the convex portion of one diffraction grating with the concave portion of the other diffraction grating. Even in the case of the grating 12, there are regions in the diffraction grating 12a and the diffraction grating 12b that enter each other beyond the dividing line 40c in the pitch direction, and signals of the preceding sub-push-pull signal FSPP and the delayed sub-push-pull signal BSPP with respect to the objective lens shift amount. This is because the degree of strength reduction can be reduced.

  Further, in the meshing region 38 in the diffraction grating 12, the region occupied by the diffraction grating 12a and the region occupied by the diffraction grating 12b are in a direction perpendicular to the groove direction of the diffraction grating 12, as shown in FIGS. 14b and 14d. It is desirable that they are engaged at a predetermined cycle. This is because, in the diffraction grating 12 having such a meshing region 38, the average phase of the regions diffracted and separated by the meshing region 38 in the preceding sub-beam 112 and the slowing sub-beam 114 is 0, and the amount of shift relative to the objective lens shift amount A signal that can reduce the degree of signal amplitude reduction of the preceding sub-push-pull signal FSPP and the delayed sub-push-pull signal BSPP, and is stable with little phase difference between the preceding sub-push-pull signal FSPP and the delayed sub-push-pull signal BSPP. Is obtained.

  Further, among the meshing regions 38 in the diffraction grating 12, the diffraction grating 12 has the same area as the region occupied by the diffraction grating 12 a and the region occupied by the diffraction grating 12 b. In the preceding sub-beam 112 and the slowing sub-beam 114, the meshing region 38. The average phase of the region diffracted and separated by 0 becomes 0, and the same effect as when the diffraction grating 12 shown in FIGS. 14b and 14d is used can be obtained. In particular, the region occupied by the diffraction grating 12 a and the region occupied by the diffraction grating 12 b are point-symmetric with respect to the meshing region center 44, or the region occupied by the diffraction grating 12 a and the region occupied by the diffraction grating 12 b is a straight line passing through the meshing region center 44. A shape that is line-symmetric with respect to the surface is preferable.

  In the above embodiment, as shown in FIGS. 2 and 3, in the diffraction grating 12, the junction boundary line 39 has a square shape, but this is replaced with a junction boundary line 39 like a comb shape as shown in FIG. 14c. The diffraction gratings 12a and 12b may be joined as described above. However, in this case, when increasing the number of comb-shaped protrusions, it is desirable that the width of the comb-shaped protrusions is larger than the width of the lattice grooves.

  In the diffraction grating 12, the shape of the meshing region 38 of the diffraction grating 12 is such that the structure having the joint boundary line 39 in which irregularities enter the groove direction in the vicinity of the center 44 of the meshing region has a preceding sub push-pull with respect to the objective lens shift amount. This is preferable because the degree to which the signal amplitude of the signal FSPP and the delayed sub push-pull signal BSPP decreases can be made smaller. For example, the shape of the meshing region 38 having a joint boundary line 39 having irregularities in the groove direction in the vicinity of the joint region center 44 as shown in FIG. 14c from a smooth joint boundary line 39 as shown in FIG. 14a. Is more preferable.

It is the block diagram which showed the tracking control apparatus in embodiment of this invention. It is a perspective view of the diffraction grating in the embodiment of the present invention. It is a top view of the diffraction grating in the embodiment of the present invention. It is a conceptual diagram for description of the engagement area | region in the diffraction grating in embodiment of this invention. It is sectional drawing of the beam diffracted and separated by the diffraction grating in embodiment of this invention. It is a block diagram of the photodetector and the calculating part in embodiment of this invention. It is a pattern on the photodetector of zero-order reflected light and ± first-order reflected light reflected by the optical disc. It is a figure which shows the relationship of the signal strength with respect to the irradiation position of the beam in embodiment of this invention. It is a figure which shows the light intensity on the phase and phase of 0th-order reflected light and ± 1st-order reflected light when the center of a preceding sub beam is irradiated to the boundary of a track. It is a graph which shows the relationship of the signal amplitude with respect to the objective lens shift of the radial direction of an optical disk. It is a figure which shows the light intensity on the phase and phase of 0th order reflected light and the ± 1st order reflected light when the center of a preceding sub beam is irradiated to the center of a land. It is a figure which shows the signal strength with respect to the irradiation position of the radial direction of each beam when recording on the DVD unrecorded disc in embodiment of this invention is assumed. It is a figure which shows the relationship of the signal amplitude of the tracking error signal DPP with respect to disk eccentricity. One of the diffraction gratings suitable for the embodiment of the present invention, in which the diffraction grating 12a and the diffraction grating 12b form junction boundary lines that are not perpendicular to the groove direction, respectively. One of the diffraction gratings suitable for the embodiment of the present invention, in which the meshing regions of the diffraction gratings 12a and 12b are meshed at a predetermined period. One of the diffraction gratings suitable for the embodiment of the present invention, in which the diffraction grating 12a and the diffraction grating 12b form a junction boundary line like a comb shape. One of the diffraction gratings suitable for the embodiment of the present invention, in which the meshing regions of the diffraction gratings 12a and 12b are meshed at a predetermined period. It is a perspective view of the junction type diffraction grating used by the differential push pull method by an in-line system. It is a perspective view of a three-part phase difference diffraction grating. This is a circuit configuration of a light spot focused on an optical disk by a differential push-pull method using an in-line method, a photodetector, and an arithmetic unit. It is a figure which shows the signal strength with respect to the irradiation position of the radial direction of each beam at the time of using a 3 division phase difference diffraction grating. It is a figure which shows the simulation result supposing the case where it records on the unrecorded disk of DVD-RAM by the differential push pull method using the three-part phase difference diffraction grating 130.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Semiconductor laser, 12 Diffraction grating, 14 Beam splitter, 16 Collimator lens, 18 Objective lens, 20 Sensor lens, 22 Photo detector, 24 Calculation part, 26 Objective lens drive part, 27 Differential amplifier, 28 Adder, 30 Amplification Amplifier, 38 Interlocking area, 39 Joint boundary line, 40 Boundary scanning line, 40c Groove direction dividing line, 42 Groove direction dividing line, 44 Intermeshing area center, 100 Tracking control circuit, 110 Main beam, 112 Leading sub beam, 114 Delay Sub beam, 121, 124, 141, 144 0th order reflected light, 122, 125, 142, 145-1st order reflected light, 123, 126, 143, 146 + 1st order reflected light, 127, 128, 147, 148 0th order reflected light And ± 1st order reflected light interference, 129, 130, 14 , 150 Light intensity on photodetector, 131 bright region, 132 dark region, 1600th order reflected light, 162-1st order reflected light, 164 + 1st order reflected light, 200 optical disc, 201 optical disc recording surface, 500 junction type diffraction grating 550 Three-part phase difference diffraction grating.

Claims (5)

The first diffraction grating and the second diffraction grating having the same period as the first diffraction grating are provided with a junction type diffraction grating that is joined on the same plane by shifting the phase by 180 degrees, and the beam emitted from the light source is joined. A tracking control device that detects a tracking error using a main beam and a sub beam generated by diffracting by a diffraction grating, and controls tracking to a track on a disk based on the detected tracking error,
In the junction type diffraction grating, the first diffraction grating and the second diffraction grating are joined with a meshing region having a predetermined width in the grating groove direction, and the first diffraction grating occupies the meshing region of the junction type diffraction grating. A tracking control device , wherein the area and the area occupied by the second diffraction grating have the same area . The tracking control device according to claim 1,
The junction between the first diffraction grating and the second diffraction grating has irregularities, and the meshing region of the junction diffraction grating is formed by the convex part of one diffraction grating meshing with the concave part of the other diffraction grating. A tracking control device. The tracking control device according to claim 1 ,
A tracking control device characterized in that the first diffraction grating and the second diffraction grating in the meshing region of the junction diffraction grating are meshed at a predetermined period . The tracking control device according to claim 2,
A tracking control device characterized in that the first diffraction grating and the second diffraction grating have a comb shape, and one comb tooth is engaged with a portion without the other comb tooth . In the tracking control device according to any one of claims 1 to 4 ,
The main beam, the preceding sub-beam and the lagging sub-beam are irradiated onto the optical disc by the junction type diffraction grating,
The first and second outputs are divided into at least two regions including the overlapping portion of the 0th-order diffracted reflected light and the ± 1st-order diffracted reflected light by the main beam disc groove structure, and the detection values are output independently. Two light receiving elements;
The third and the second outputs the detection value independently by dividing the region including the overlapping portion of the 0th-order diffracted reflected light and the ± 1st-order diffracted reflected light by the disk groove structure of the preceding sub beam into at least two parts. Four light receiving elements;
Fifth and fifth outputs each of the regions including the overlapping portion of the 0th-order diffracted reflected light and the ± 1st-order diffracted reflected light by the delayed sub-beam disk groove structure are divided into at least two and output detection values independently. With six light receiving elements,
A main push-pull signal MPP generated from detection values from the first and second light receiving elements, a preceding sub push-pull signal FSPP generated from detection values from the third and fourth light receiving elements, The tracking error signal DPP is calculated using the delayed sub push-pull signal BSPP generated from the detection value from the sixth light receiving element, and tracking to the track on the disk is performed based on the calculated tracking error signal DPP. tracking control apparatus and controls.

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