JP2005276358A - Optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pickup device which can generate stably a tracking error signal in which an offset is canceled and, moreover, which has high utilization efficiency of light. <P>SOLUTION: A 1st grating 7 diffracts reflected light from an optical disk to separate it into a main beam (0th-order light) MB, a sub beam(+1st-order light) SB1 and a sub beam(-1st-order light) SB2. A phase difference is generated in a part of the sub beam SB1 and the sub beam SB2 by a phase difference arranged in a part of periodic structure of the 1st grating 7. A 2nd grating 9 arranged in the focal position of a 1st condensing lens 8 diffracts further respective condensed beams to separate them. The diffracted beams pass through a 2nd condensing lens 10 and a cylindrical lens 11 to be guided to a photodetecting part 12. Bisected photodetectors 12A to 12B respectively output a push-pull signal being the light intensity difference of a corresponding beam. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical pickup device, and more particularly to a tracking error signal detection method necessary for tracking servo of an optical pickup device.

  2. Description of the Related Art An optical recording / reproducing apparatus that records and reproduces an optical disk that is an optical recording medium includes an optical pickup device as an apparatus for stably recording information on the disk medium and faithfully reproducing the information.

  The optical pickup device collects laser light from a semiconductor laser and irradiates a target position on a disk medium with an objective lens that performs a focusing servo for tracking the surface vibration of the disk and a tracking servo for tracking the track vibration. A lens driving mechanism is mounted to automatically adjust the relative positional relationship between the target position on the disk medium and the laser light spot to be always constant.

  Here, with respect to the tracking servo in the optical pickup device, a tracking error signal which is error information for accurately scanning the laser beam is required. Conventionally, a three-beam method and a push-pull method are mainly used as a method for detecting this tracking error signal (hereinafter also referred to as TES).

  First, the three-beam method is a method of detecting a tracking error signal by irradiating the optical disk 108 with three light beams. Specifically, as shown in FIG. 8, on the signal recording surface of the optical disc 108, a main beam MB for reading a central signal and two sub beams SB1 and SB2 shifted before and after the main beam are arranged. In the most preferable state in which the main beam MB is directly above the track as shown in FIG. 8, the sub beams SB1 and SB2 have a portion applied to the land, which is a so-called mirror surface, in addition to the portion applied to the groove. This portion of light is reflected by the disk and reaches the photodetectors 210 and 211. The return light incident on each of the photodetectors 210 and 211 is amplified in the light amount difference in the differential arithmetic circuit 200 to generate a tracking error signal.

  As shown in FIG. 8, when the same amount of return light is detected from the sub beams SB1 and SB2, the tracking error signal is zero. On the other hand, when the main beam MB is shifted from directly above the track, a bipolar tracking error signal including information on the direction and amount of the shift is obtained.

  Next, as shown in FIG. 9, the push-pull method uses a two-divided photodetector 102P divided in half by a dividing line Px, and is diffracted and reflected by a pit (or track) formation portion again. This is a method of obtaining a bipolar tracking error signal by taking a difference in intensity of light incident on the objective lens 107 (hereinafter also referred to as a push-pull signal). This method is a simple method in which a tracking error signal can be obtained with one beam, whereas the previous three beam method required three spots to generate a tracking error signal.

  As described above, the 3-beam method and the push-pull method each have an advantage that the operation is reliable or the configuration is simple. On the other hand, these methods are also known to have the following problems.

  First, in the three beam method shown in FIG. 8, the interval between the sub beams SB1 and SB2 to be tracking error signals is wide. For this reason, when tracking is performed when information is recorded on the optical disc 108 to a portion where information is recorded, the difference in the amount of reflected light between the sub beams SB1 and SB2 increases. As a result, the differential signals of the photodetectors 210 and 211 are affected by the difference in the amount of reflected light other than the tracking error signal, and an offset occurs in the tracking error signal.

  Further, in the stage of assembling the optical pickup device, there is a problem that it takes time to assemble because it is necessary to accurately adjust the position so that the sub beams SB1 and SB2 irradiate a predetermined track.

  Further, in the three-beam method, since three beams are generated from one light source, it is necessary to increase the light source output in order to secure a sufficient amount of light for recording and reproduction in the main beam MB. There is a problem that power consumption increases as the burden increases.

  Next, for the push-pull method, the two-divided light detector 102P detects a light intensity difference at a symmetrical position about the dividing line Px with respect to the reflected light from the optical disc 108, and uses this as a tracking error signal. .

  Here, as shown in FIG. 10A, when the optical disc 108 is eccentric (dotted line), the optical axis of the reflected light from the optical disc 108 is deviated from that when there is no eccentricity (solid line). The beam center will deviate from the center of the two-divided detector 102P. At this time, the intensity distribution of the light applied to the two-divided photodetector 102P becomes asymmetrical with respect to the dividing line Px.

  Further, as shown in FIG. 10B, even when the optical disk surface is tilted (tilt), the reflected light from the optical disk 108 is tilted and returned with respect to the objective lens 107. As a result, the center of the reflected light is shifted. Even in this case, the intensity distribution of the light applied to the photodetector 102P is asymmetrical with respect to the dividing line Px.

  FIG. 11 is a diagram for explaining the change of the push-pull signal when the optical disc 108 is eccentric or tilted.

  Referring to FIG. 11, the push-pull signal PP0 in the case where there is no eccentricity or tilt in the optical disk 108 is balanced in amplitude change between the plus direction and the minus direction.

  On the other hand, the push-pull signal PP1 in the case where the optical disk 108 is eccentric or tilted has an amplitude change shifted in the plus direction. As a result, a DC offset Δp occurs in the push-pull signal PP1.

  In the push-pull method, there is a problem that tracking cannot be performed satisfactorily even though the tracking is correct due to the offset Δp generated in the push-pull signal.

  Here, among the problems listed above, regarding the offset generated in the tracking error signal, as a means for canceling the offset, the tracking error signal using a differential push pull (DPP) method is used. A detection method has recently been proposed (see, for example, Patent Document 1).

  FIG. 12 is a diagram showing a schematic configuration of a conventional optical pickup device using a differential push-pull method.

  Referring to FIG. 12, the laser beam emitted from laser 101 is collimated by collimator lens 105, and then, by diffraction grating (hereinafter also referred to as grading) 113, main beam MB (solid line) and sub beam SB1, Divided into 3 beams consisting of SB2 (both are dotted lines). When these three beams pass through the beam splitter 115, they are condensed on the optical disk by the objective lens 107.

  Furthermore, when the light reflected from the optical disk 108 is reflected by the beam splitter 115 via the objective lens 107, the light is guided to the light detection unit 102D by the condenser lens 114.

  Specifically, the light detection unit 102D includes two-split photodetectors 230 to 232 each having a split line parallel to the track direction. The main beam MB is incident on the two-divided photodetector 230, the sub beam SB 1 is incident on the two-divided photodetector 231, and the sub beam SB 2 is incident on the two-divided photodetector 232.

  FIG. 13 is a diagram showing an example of the arrangement of the three beams in the guide groove 810 of the optical disc 108. As shown in FIG.

  Referring to FIG. 13, main beam MB is located in the groove of guide groove 810, and sub beams SB1 and SB2 are located in the land of guide groove 810, respectively. When the main beam MB is located in the groove of the guide groove 810, the sub beams SB1 and SB2 can be brought to the land position by adjusting the grading 113 in FIG.

  FIG. 14 is a circuit diagram for explaining an electrical configuration of the light detection unit 102D in FIG.

  Referring to FIG. 14, light detection unit 102 </ b> D includes two-divided light detectors 230 to 232, differential operation circuits 240 to 242, adder 250, gain adjustment circuit 251, and subtraction circuit 260.

  Corresponding beam ball patterns appear on the two-divided photodetectors 230 to 232. At this time, as shown in FIG. 14, the ball pattern (corresponding to the hatched portion) on the two-split photodetector 230 that receives the main beam MB is on the two-split photodetectors 231 and 232 that receive the sub-beams SB1 and SB2. Is symmetrical with respect to the ball pattern (corresponding to the shaded portion).

  The differential operation circuit 240 calculates the difference between the two signals output from the two-split photodetector 230 and outputs this as a push-pull signal PP30. The differential operation circuit 241 calculates the difference between the two signals output from the two-divided photodetector 231 and outputs this as a push-pull signal PP31. The differential operation circuit 242 calculates the difference between the two signals output from the two-split photodetector 232 and outputs this as a push-pull signal PP32.

  FIG. 15 is a waveform diagram showing waveforms of the push-pull signals PP30, PP31, PP32.

  As shown in FIG. 15, the push-pull signals PP31 and PP32 of the sub beams SB1 and SB2 are out of phase with respect to the push-pull signal PP30 of the main beam MB.

  Returning to FIG. 14 again, the adder 250 adds the push-pull signals PP31 and PP32 output from the two-split photodetectors 241 and 242. The gain adjustment circuit 251 adjusts the gain G1 of the push-pull signal (PP31 + PP32) added by the adder 250. The subtraction circuit 260 subtracts the output signal of the gain adjustment circuit 251 from the push-pull signal PP0 according to the equation (1), and outputs the subtraction result as the tracking error signal TES.

TES = PP0−G1 (PP1 + PP2) (1)
By generating the tracking error signal using the differential push-pull method in this way, the offset generated in the main beam MB can be canceled with the offset generated in the sub beams SB1 and SB2.
JP 2001-250250 A

  However, in the tracking error signal detection method using the differential push-pull method, the laser beam emitted from the laser 101 is divided into the main beam MB and the sub beams SB1 and SB2, so that the main beam used for recording is used. MB power tends to decrease, and high light utilization efficiency cannot be obtained. For this reason, it is widely used in optical systems for recordable CDs (Compact Disks) where high-power lasers can be used at a relatively low cost, but for DVDs (Digital Versatile Disks) that use red lasers where high-power output is relatively difficult. When this method is used in an optical system, the speed of an optical disk cannot be increased at present.

  If the positions of the sub-beams SB1 and SB2 with respect to the main beam MB are not accurately adjusted according to the guide groove 810, the reverse phase relationship between the push-pull signal PP30 and the push-pull signals PP31 and PP32 is lost, and the tracking error signal Since the AC amplitude is significantly reduced, there is a problem that optical disks having different guide groove structures cannot be interchanged.

  Further, there is a problem that an offset occurs in the tracking error signal or the servo becomes unstable due to a deviation in timing of irradiating the recorded portion and the unrecorded portion between the main beam MB and the sub beams SB1 and SB2. Has occurred.

  SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical pickup device that can stably generate a tracking error signal in which an offset is canceled and that has high light utilization efficiency.

  According to an aspect of the present invention, there is provided an optical pickup device for recording and reproducing information with respect to an optical disc, wherein the first diffraction that separates reflected light from the optical disc into a first light beam and a second light beam. Means, a first condensing lens for condensing the first and second light beams, a second diffracting means for diffracting the condensed first and second light beams, and a diffracted first A tracking error signal is detected based on a second condensing lens that condenses the first and second light beams and a light intensity distribution of the first and second light beams incident from the second condensing lens. And a light detection means. The light detection means includes a first and second split light detector that receives the first and second light beams, the light detection region being divided in the track direction, and the first and second light beams, respectively. Push-pull signal generating means for generating first and second push-pull signals based on the light intensity difference between the light detection regions of the first and second two-part photodetectors; and first and second pushes Error signal detecting means for detecting a tracking error signal from the pull signal. The first diffracting means includes phase difference adding means for giving a phase difference to a part of the second light beam so that the amplitude of the second push-pull signal becomes zero.

  Preferably, the error signal detecting means cancels an offset generated in the first push-pull signal using the second push-pull signal.

  Preferably, the phase difference adding means is formed when a part of the second light beam is set to have an origin at the center of the second light beam and a radial direction and a track direction of the optical disc are set to the X direction and the Y direction, respectively. One quadrant of the four quadrants is given, and a phase difference of 180 ° is given to one quadrant.

  Preferably, the first diffracting means further includes a diffraction grating having a first periodic structure and separating incident light into zero-order light and first-order light. In the first periodic structure of the diffraction grating, the phase difference adding means shifts one quadrant to ½ period with respect to the other three quadrants, thereby giving a 180 ° phase difference to one quadrant of the second light beam. give.

  Preferably, the second diffractive means is arranged at the focal position of the first condenser lens.

  Preferably, the second diffracting means includes a linear diffraction grating having a second periodic structure, and diffracts both of the zero-order diffracted light and the first-order diffracted light generated by diffracting each of the first and second light beams. The second periodic structure is set so that the efficiency is equal.

  Preferably, the second diffracting means interferes with an area where the 0th-order diffracted light and the 1st-order diffracted light overlap with a ball pattern area generated in the reflected light from the optical disc on the second condenser lens pupil. The second periodic structure is set.

  According to one aspect of the present invention, the offset of the tracking error signal, which has been a problem in the conventional optical pickup device, can be completely removed.

  Further, since the optical disk is configured to emit one beam, the light utilization efficiency of the pickup can be improved as compared with the conventional three-beam method and the differential push-pull method. As a result, the burden on the laser can be reduced and low power consumption can be realized.

  In addition, the push-pull signal generated from the sub-beam always has an amplitude of 0 regardless of the groove depth of the optical disk. Therefore, it is not necessary to adjust the position of the three beams in the conventional three-beam method. Can be greatly simplified.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts.

  FIG. 1 is an overall configuration diagram of an optical pickup device according to an embodiment of the present invention. In this figure, a direction corresponding to the radial direction of the optical disk 5 (hereinafter also referred to as a radial direction) is defined as an X direction, which is a tangential direction perpendicular to the X direction, and is a track length direction (hereinafter also referred to as a track direction). Is referred to as the Y direction.

  Referring to FIG. 1, the optical pickup device collects a laser 1, a collimator lens 2 that converts laser light emitted from the laser 1 into parallel light, a beam splitter 3, and parallel light transmitted through the beam splitter 3. And an objective lens 4 that emits light.

  The laser beam condensed by the objective lens 4 enters the guide groove 6 of the optical disc 5. The objective lens 4 is driven in the focus direction and the tracking direction, and is controlled so that the size and position of the focused spot on the signal recording surface are optimized.

  The reflected light from the optical disk 5 passes through the objective lens 4 again to become parallel light and enters the beam splitter 3. The beam splitter 3 reflects the incident reflected light in a direction substantially orthogonal to the incident light, and separates it from the outgoing light (direction toward the optical disc 5).

  The reflected light separated by the beam splitter 3 is further separated or condensed by passing through a lens group shown below and guided to the light detection unit 12.

  Further, the optical pickup device selectively diffracts the incident reflected light, and travels straightly into the main beam (0th-order light) MB, the sub-beam (+ 1st-order light) SB1 and the sub-beam (−1st-order light) directed in a predetermined angular direction. Light) SB2, a first grading 7 for separating each beam, a first condenser lens 8 for condensing each beam, a second grading 9 for further diffracting and separating the collected beam, and a diffracted beam. A second condenser lens 10 for condensing the beam, a cylindrical lens 11, and a light detector 12.

  The second condenser lens 10 and the cylindrical lens 11 form a minimum circle of confusion on the two-divided photodetector 12A with respect to the reflected light from the optical disc 5, and generate a focus error signal by the astigmatism method.

  The light detection unit 12 detects a light intensity distribution of the sub-beam SB2 and a two-part light detector 12B that detects the light intensity distribution of the main beam MB, a two-part light detector 12B that detects the light intensity distribution of the sub-beam SB1. And a two-divided photodetector 12C.

  The two-divided photodetectors 12A to 12C have dividing lines corresponding to the track direction, detect light intensity differences at positions symmetrical with respect to the dividing lines, and generate push-pull signals PP10 to PP12. In the drawing, the track direction is rotated by 90 ° by the cylindrical lens 11 at the positions of the two-divided photodetectors 12A to 12C.

  In FIG. 1, only the 0th-order light of the diffracted light is shown for the optical path after the second grading 9 in the return light after being reflected by the optical disk 5 in order to avoid the complexity of the illustration. The light of the order of (± 1st order) is not shown. That is, the two-divided photodetectors 12A to 12C of the light detection unit 12 actually receive the main beam MB and the sub beams SB1 and SB2 separated into the 0th order light and the ± 1st order light, respectively.

  FIG. 2 is a circuit diagram showing an electrical configuration of the light detection unit 12 in FIG.

  Referring to FIG. 2, photodetection unit 12 includes two-divided photodetectors 12 </ b> A to 12 </ b> C, differential operation circuits 13 </ b> A to 13 </ b> C, an adder 14, gain adjustment circuit 15, and subtraction circuit 16.

  As shown in FIG. 2, the two-divided photodetectors 12A to 12C have dividing lines corresponding to the track direction, and the diffraction patterns (push-pull patterns) of the received main beam MB and sub beams SB1 and SB2 appear.

  The differential operation circuit 13A obtains a difference between the two signals output from the two-divided photodetector 12A and outputs this as a push-pull signal PP10. The differential arithmetic circuit 13B calculates a difference between the two signals output from the two-divided photodetector 12B and outputs this as a push-pull signal PP11. The differential operation circuit 13C obtains a difference between the two signals output from the two-divided photodetector 12C and outputs this as a push-pull signal PP12.

  The adder 14 adds push-pull signals PP11 and PP12 generated from the sub beams SB1 and SB2.

  The gain adjustment circuit 15 adjusts the gain k of the push-pull signal added by the adder 14. The subtraction circuit 16 subtracts the output signal of the gain adjustment circuit 15 from the push-pull signal PP10 of the main beam MB, and outputs the subtraction result as the push-pull signal PP1.

  FIG. 3 is a waveform diagram showing waveforms of the push-pull signals PP10, PP11, and PP12.

  Referring to FIG. 3, it can be seen that the push-pull signals PP11 and PP12 based on the sub-beams SB1 and SB2 have a reduced amplitude to a fraction of the push-pull signal PP10 based on the main beam MB. The reason why the amplitudes of the sub beams SB1 and SB2 decrease with respect to the main beam MB will be described in detail later.

  In addition, the push-pull signals PP10, PP11, and PP12 are offset on the same side (in phase) by ΔP and ΔP ′ according to the respective light amounts. These offsets are caused by the presence of eccentricity or tilt in the optical disc 5 as described in the push-pull method.

Therefore, in the light detection unit 12,
PP1 = PP10−k (PP11 + PP12) (2)
By performing this calculation, it is possible to obtain the push-pull signal PP1 in which the offsets ΔP and ΔP ′ are canceled.

  Here, the coefficient k is for correcting the difference in light intensity between the 0th order light (corresponding to the main beam MB) diffracting the first grading 7 and the ± 1st order light (corresponding to the sub beams SB1 and SB2). It is. For example, if the intensity ratio is 0th order light: + 1st order light: -1st order light = a: b: b, the coefficient k is k = a / 2b.

In this embodiment, since the amplitudes of the sub-beams SB1 and SB2 are small, it is not necessary to perform calculations using two signals PP11 and PP12, and one of PP11 and PP12 is used.
PP1 = PP10-2k (PP11) (3)
Or
PP1 = PP10-2k (PP12) (4)
The push-pull signal PP1 with the offset canceled can be obtained by the calculation of

  The push-pull signals PP11 and PP12 of the sub beams SB1 and SB2 can also be used as radial position detection signals of the objective lens 4.

  Referring to FIG. 1 again, the optical pickup device according to the present embodiment is different in the grading arrangement position from the conventional optical pickup device using the differential push-pull method shown in FIG.

  Specifically, in the conventional optical pickup device, the grading 113 is disposed on the forward light side and is disposed between the collimator lens 105 and the beam splitter 115. That is, the laser light that has become parallel light is diffracted and divided into three, and then incident on the optical disk 108.

  On the other hand, the optical pickup device according to the present embodiment includes two gradings (first grading 7 and second grading 9), and both grading and beam splitter 3 on the return light side and the light The difference is that it is arranged between the detector 12 and the light reflected from the optical disk 5 is diffracted and separated.

  First, as a major reason that the first grading 7 is arranged with respect to the reflected light, the high light that is the merit of this method is adopted by adopting the one beam method in which the beam irradiated to the optical disc 5 is one beam. One example is to obtain utilization efficiency. According to this, since the light output of the laser 1 can be suppressed, the life of the laser 1 is extended, and the burden of the laser 1 is also imposed on an optical pickup device that requires a high laser output such as a double-layer disc or double speed recording / reproduction. Can be reduced.

  Next, details of the first grading 7 and the second grading 9 unique to the present invention will be described.

  FIG. 4 is a diagram showing the structure of the groove portion of the first grading 7 in FIG.

  Referring to FIG. 4, the first grading 7 has the feature in the shape of the groove.

  More specifically, assuming that the radial direction is the X direction and the track direction is the Y direction, the groove portion of the first grading 7 has a lattice groove in the first quadrant and the other three quadrants on the XY plane. The shape is shifted by ½ pitch.

  As described above, the reflected light from the optical disk 5 is incident on the first grading 7, and the 0th-order light (corresponding to the main beam MB) traveling straight and the ± first-order light (sub-beams) traveling in a predetermined angular direction. SB1 and SB2).

  At this time, in the sub beams SB1 and SB2 diffracted by the groove, a phase difference of 180 ° corresponding to a 1/2 pitch shift of the grating groove is generated between the first quadrant and the second to fourth quadrants. As will be described later, this phase difference decreases the amplitude of the push-pull signals PP11 and PP12 using the sub beams SB1 and SB2 with respect to the push-pull signal PP0 of the main beam MB having no phase difference.

  Referring to FIG. 1 again, the main beam MB and the sub beams SB1 and SB2 generated in the first grading 7 are both condensed by the first condenser lens 8, and then the first condenser lens. The light is incident on the second grading 9 arranged at the focal point 8.

  Here, the configuration in which the three beams are condensed by the first condenser lens 8 and is incident on the second grading 9 eliminates the influence of the tilt that occurs when the second grading 9 is disposed. Because.

  The second grading 9 is a linear lattice in which linear grooves are formed according to a predetermined lattice pitch p (p is a positive number).

  The main beam MB and the sub beams SB1 and SB2 incident on the second grading 9 are diffracted in the groove portions and separated into 0th order diffracted light, + 1st order diffracted light, and −1st order diffracted light. At this time, since diffracted light overlaps and interferes with each other for each beam, a diffraction pattern (push-pull pattern) is formed on the lens pupil surface of the second condenser lens 10 on which the diffracted light enters.

  FIG. 5 is a diffraction pattern generated on the second condenser lens pupil by the second grading 9. In the drawing, the diffraction pattern of the sub beam SB1 among the three beams incident on the second grading 9 is illustrated.

  Referring to FIG. 5, sub beam SB <b> 1 collected in the groove of second grading 9 is separated into 0th order diffracted light 110, + 1st order diffracted light 111, and −1st order diffracted light 112.

  At this time, the + 1st order diffracted light 111 and the −1st order diffracted light 112 travel in the direction of the diffraction angle corresponding to the pitch p of the groove. If this diffraction angle is within a predetermined range, regions N1, N2 where the 0th-order diffracted light 110 and the ± 1st-order diffracted light 111, 112 as shown in FIG. 5 overlap on the lens pupil of the second condenser lens 10. Occurs. The overlapping regions N1 and N2 contribute to the generation of the push-pull signal PP11 in the two-divided photodetector 12B of the photodetector 12.

  Here, the regions N1 and N2 where the diffracted light of the second grading 9 overlaps are desirably larger than the ball pattern region generated by the diffracted light in the guide groove 6 of the optical disc 5.

  The reason for this and the measures for making the regions N1 and N2 where the diffracted beams overlap larger than the ball pattern region will be described below.

  Referring to FIG. 1 again, when the light spot is focused on the guide groove 6 of the optical disc 5, the grooves and lands formed on the signal recording surface of the optical disc 5 function as a reflective diffraction grating, and the incident light Is diffracted into 0th order diffracted light and ± 1st order diffracted light. The diffracted light travels in the direction of the objective lens 4 as reflected light from the optical disk 5. As a result, a ball pattern, which is a region where the 0th-order diffracted light and the ± 1st-order diffracted light overlap, appears in the diffraction pattern on the objective lens pupil. This ball pattern greatly affects the light intensity distribution (bright / dark pattern) on the objective lens pupil. Note that the way in which the diffracted light overlaps in the ball pattern is uniquely determined by the diffraction angle of the ± first-order diffracted light. This diffraction angle is obtained from the relationship between the pitch of the guide groove 6 and the wavelength of the incident laser beam.

  The reflected light from the optical disk 5 in which the ball pattern is generated is separated into three beams by the first grading 7, so that the light intensity distribution by the ball pattern is generated in the main beam MB and the sub beams SB 1 and SB 2, respectively. become.

  Therefore, when the push-pull signals PP1 and PP2 are generated using the sub beams SB1 and SB2, an offset appears in the push-pull signals PP1 and PP2 due to the influence of the light intensity distribution due to the ball pattern. Therefore, in order to remove the offset component from the push-pull signals PP1 and PP2, it is desirable to eliminate the light intensity distribution of the sub beams SB1 and SB2.

  Therefore, in the present embodiment, the sub-beams SB1 and SB2 are further diffracted by the second grading 9, and the regions N1 and N2 where the diffracted light overlap and the ball pattern region interfere with each other to thereby have the sub-beams SB1 and SB2. The light intensity distribution will be eliminated.

  As a first means for eliminating the light intensity distribution, the focal length of the first condenser lens 8 and the grating pitch of the second grading 9 are set so that the regions N1 and N2 where the diffracted light overlaps are larger than the ball pattern region. It is mentioned to design so that it may become.

  Specifically, it is known that the regions N1 and N2 increase as the focal length of the first condenser lens 8 is shorter or as the grating pitch p of the second grading 9 is larger. Therefore, first, the size of the ball pattern area is calculated by diffraction calculation using the numerical aperture NA of the objective lens 4 and the pitch of the guide groove 6 of the optical disk 5, and the obtained ball pattern areas are represented by areas N1 and N2. The focal length and the grating pitch are set so as to cover.

  As a second means for eliminating the light intensity distribution, in the second grading 9, setting is made so that the diffraction efficiencies of the 0th-order diffracted light and the ± 1st-order diffracted light are equal.

  This is because if the light intensity of the 0th-order diffracted light is different from that of the ± 1st-order diffracted light, the component having the larger light intensity remains when they interfere with each other, and the ball pattern cannot be erased sufficiently. It is.

Therefore, in order to equalize both diffraction efficiencies, the groove depth of the second grading 9 and the groove: duty duty must be set appropriately. In general, a light beam incident on a linear diffraction grating is separated into 0th order diffracted light and ± 1st order diffracted light. However, in the case of a diffraction grating having a rectangular grating groove cross section, the light intensity I _{0 of the 0th order} diffracted light and the + 1st order diffracted light of the light intensity I _1, upon the light intensity of the incident light is 1, it can be expressed by the following equation.

I ₀ = 1 + 2β (β-1) (1-cosα) (5)
I ₁ = 2 / π ² · sin ² (πβ) (1-cos α) (6)
α = 2π / λ · (n−1) h (7)
β = w / p (8)
Here, w is the grating groove width of the diffraction grating, p is the grating period, and h is the grating groove depth. N is the refractive index of the transparent member on which the linear diffraction grating is engraved, and λ is the wavelength of the laser beam.

Now, considering that the range of β in equation (8) is 0 <β <1, from equations (5) to (8), the closer β is to 0.5, that is, the lattice groove width is lattice. It can be seen that the light intensity I ₁ of the ± 1st order diffracted light becomes stronger and the light intensity I ₀ of the 0th order diffracted light becomes weaker as it approaches half of the period.

  As described above, the light intensity distribution of the sub-beams SB1 and SB2 can be eliminated by separating the sub-beams SB1 and SB2 into three diffracted lights and interfering with each other by the second grading 9, and the push-pull signal An offset occurring in PP1 and PP2 can be canceled.

  In addition, since the second grading 9 is a linear diffraction grating, the same diffracted light is generated no matter where the grading is irradiated with the spot, thereby eliminating the influence of the placement position tolerance of the second grading 9. Can do.

  FIG. 6 is a diagram showing a beam pattern of the sub beam SB1 received by the light detection unit 12. As shown in FIG. 6 is obtained by projecting the diffracted lights 110, 111, and 112 of the sub beam SB1 shown in FIG. 5 onto the two-divided photodetector 12B through the cylindrical lens 11.

  Referring to FIG. 6, in sub-beam SB1, zero-order diffracted light 110 has a phase advance of 180 ° in the third quadrant (corresponding to a mesh region). Further, in the fourth quadrant, the + 1st order diffracted light 111 has a phase advance of 180 ° (corresponding to the hatched region).

  Here, a radial dividing line n is virtually considered in the two-divided photodetector 12B divided by the dividing line m in the track direction, and the detection areas in the first quadrant to the fourth quadrant are designated as 12B-a and 12B-, respectively. b, 12B-c, 12B-d, and the output of each region is Ia, Ib, Ic, Id.

  Normally, the push-pull signal PP11 is the sum (Ia + Ib) of the detection areas 12B-a and 12B-b of the two-divided photodetector 12B in FIG. It is obtained by calculating the difference from (Ic + Id), which is the sum of the outputs of c and 12B-d.

PP11 = (Ia + Ib)-(Ic + Id) (9)
In addition, the push-pull signal PPbc obtained from the difference between the outputs Ib and Ic of the detection regions 12B-b and 12B-c is PPbc = (Ib−Ic). The push-pull signal PPbc obtained from the difference between the outputs Ia and Id of the detection regions 12B-a and 12B-d is PPad = (Ia−Id).

  FIG. 7 is a waveform diagram showing waveforms of push-pull signals PPad, PPbc, PP11 in the two-divided photodetector 12B.

  Here, the region where the 0th-order diffracted light 110 and the + 1st-order diffracted light 111 overlap in the detection regions 12B-b and 12B-c is the same as the 0th-order diffracted light 110 in the detection regions 12B-a and 12B-d, as shown in FIG. Each of the regions where the −1st order diffracted light 112 overlaps has a phase difference of 180 °.

  As a result, the push-pull signal PPbc = (Ib−Ic) obtained from the difference between the outputs of the detection regions 12B-b and 12B-c and the push-pull signal PPad = (Ia) obtained from the detection regions 12B-a and 12B-d. -Id) is a signal having a reverse phase with a phase difference of 180 °.

Here, since Equation (9) is rewritten as the following equation,
PP11 = (Ia−Id) + (Ib−Ic) = PPad + PPbc (10)
As a result, the push-pull signal P11 obtained from the two-divided photodetector 12B is PP11 = PPad + PPbc = 0, and the amplitude is always 0. Similarly, in the push-pull signal PP12 obtained from the sub beam SB2, the amplitude is always 0.

  This is because the push-pull signal is not detected regardless of the position of the track, so that the sub-beams SB1 and SB2 are almost the same regardless of whether they are arranged on the same track as the main beam MB or on different tracks. Means.

  Also, the push-pull signal of the sub beam always indicates that the amplitude is zero regardless of the groove depth of the target optical disc, regardless of the groove depth.

  In the present embodiment, the case where a phase difference occurs only in the first quadrant in the first grading 7 has been described. However, the present invention is not limited to this, and there is a phase difference in any of the second to fourth quadrants. The same effect can be obtained in.

  Furthermore, in the present embodiment, the case where there is a phase difference in one quadrant in the first grading 7 is shown. However, the region related to the push-pull signal is from the second grading 9 shown in FIG. Since the regions N1 and N2 overlap the diffracted light, it is not necessary to add a phase difference to the entire quadrant, and a sufficient effect can be obtained by adding a phase difference to the corresponding portion of the first quadrant.

  As described above, according to the embodiment of the present invention, the offset of the tracking error signal, which has been a problem in the conventional optical pickup device, can be completely removed.

  Further, since the optical disk is configured to emit one beam, the light utilization efficiency of the pickup can be improved as compared with the conventional three-beam method and the differential push-pull method. As a result, the burden on the laser can be reduced and low power consumption can be realized.

  In addition, the push-pull signal generated from the sub-beam always has an amplitude of 0 regardless of the groove depth of the optical disk. Therefore, it is not necessary to adjust the position of the three beams in the conventional three-beam method. Can be greatly simplified.

  Furthermore, the optical pickup device according to the present embodiment uses light such as a reproduction-only pit disk, a phase change disk capable of recording / reproducing / erasing, a magneto-optical disk, or a recordable / recordable write-once disk. Although it is intended for all optical discs for playback / recording, even in mass-produced pit discs where the eccentricity of the optical disc during servo is large, no offset occurs in the tracking error signal, and the signal can be reproduced stably. Can do.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

FIG. 1 is an overall configuration diagram of an optical pickup device according to an embodiment of the present invention. It is a circuit diagram which shows the electrical structure of the photon detection part 12 in FIG. It is a wave form diagram which shows each waveform of push pull signal PP10, PP11, PP12. It is a figure which shows the structure of the groove part of the 1st grading 7 in FIG. This is a diffraction pattern generated on the second condenser lens pupil by the second grading 9. It is a figure which shows the beam pattern of the diffracted light received in the photon detection part. It is a wave form diagram which showed each waveform of push pull signal PPad, PPbc, PP11 in 2 division | segmentation photodetector 12B. It is a figure explaining the detection method of the tracking error signal using 3 beam method. It is a figure explaining the detection method of the tracking error signal using the push pull method. It is a figure which shows typically the light intensity distribution on the 2-part dividing photodetector 102P in case eccentricity or tilt exists in the optical disk 108. FIG. FIG. 10 is a diagram for explaining a change of a push-pull signal when there is eccentricity or tilt in the optical disc 108. It is a figure which shows schematic structure of the conventional optical pick-up apparatus using the differential push pull method. 6 is a diagram illustrating an example of the arrangement of three beams in a guide groove 810 of an optical disc 108. FIG. FIG. 13 is a circuit diagram for explaining an electrical configuration of a light detection unit 102D in FIG. It is a wave form diagram which shows each waveform of push pull signal PP30, PP31, PP32.

Explanation of symbols

  1,101 laser, 2,105 collimator lens, 3,115 beam splitter, 4,107 objective lens, 5,108 optical disk, 6,810 guide groove, 7 first grading, 8 first condenser lens, 9 first 2 grading, 10 second condensing lens, 11 cylindrical lens, 12 light detection unit, 12A to 12C, 102P, 230 to 232, two-part photodetector, 13A to 13C, 200, 240 differential operation circuit, 14, 250 adder, 15,251 gain adjustment circuit, 16,260 subtraction circuit, 110 0th order diffracted light, 111 + 1st order diffracted light, 112-1st order diffracted light, 113 grading, 114 condenser lens, 210, 211 photodetector,

Claims (7)

An optical pickup device for recording and reproducing information with respect to an optical disc,
First diffraction means for separating reflected light from the optical disc into a first light beam and a second light beam;
A first condenser lens for condensing the first and second light beams;
Second diffracting means for diffracting each of the collected first and second light beams;
A second condenser lens for condensing the diffracted first and second light beams;
Light detection means for detecting a tracking error signal based on the light intensity distribution of the first and second light beams incident from the second condenser lens;
The light detection means includes
A first and a second two-divided photodetector for dividing a light detection region in the track direction and receiving the first and second light beams, respectively;
Pushes that generate first and second push-pull signals for the first and second light beams, respectively, based on a difference in light intensity between the light detection regions of the first and second split photodetectors. A pull signal generating means;
Error signal detection means for detecting the tracking error signal from the first and second push-pull signals,
The first diffraction means includes
An optical pickup device comprising phase difference adding means for giving a phase difference to a part of the second light beam so that the amplitude of the second push-pull signal becomes zero.   2. The optical pickup device according to claim 1, wherein the error signal detection unit cancels an offset generated in the first push-pull signal using the second push-pull signal.   The phase difference adding means is formed when a part of the second light beam is set to have an origin at the center of the second light beam and a radial direction and a track direction of the optical disc as an X direction and a Y direction, respectively. The optical pickup device according to claim 1, wherein any one of the four quadrants is set, and a phase difference of 180 ° is given to the one quadrant. The first diffracting means further includes a diffraction grating having a first periodic structure and separating incident light into zero-order light and first-order light,
In the first periodic structure of the diffraction grating, the phase difference adding unit shifts the first quadrant to the first quadrant of the second light beam by shifting the first quadrant by a half period with respect to the other three quadrants. The optical pickup device according to claim 3, which gives a phase difference of 180 °.   The optical pickup device according to claim 1, wherein the second diffracting unit is disposed at a focal position of the first condenser lens.   The second diffracting means includes a linear diffraction grating having a second periodic structure, and diffraction efficiency of both the 0th order diffracted light and the 1st order diffracted light generated by diffracting each of the first and second light beams. The optical pickup device according to claim 5, wherein the second periodic structure is set so as to be equal to each other.   In the second diffracting means, on the second condenser lens pupil, an area where the 0th-order diffracted light and the 1st-order diffracted light overlap and a ball pattern area generated in reflected light from the optical disc interfere with each other. The optical pickup device according to claim 6, wherein the second periodic structure is set as described above.

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