JP2008052845A - Optical disk device and optical pickup - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical disk device and an optical pickup in which sure focusing control can be performed with simple constitution. <P>SOLUTION: A brightness and darkness pattern in which brightness of a bright part and a dark part are increased or decreased inversely each other in accordance with irradiation position movement of a sub-beam with respect to a track is caused in an overlap region of 0th order light and ±1st order light in a reflected sub-beam in which the sub-beam is diffracted and reflected by the track of an optical disk by changing the phase of a part of a region of luminous flux of the sub-beam by nearly 180° by a phase changing means 32A provided to a diffraction element 32, and the bright part and a dark part are offset against each other. Thereby, the brightness change of a whole overlap area and local brightness change in the vicinity of a division line of a photodetector accompanied by irradiation position movement are suppressed small, and a focus error signal which is hardly affected by the irradiation position change of the sub-beam with respect to the track can be obtained by generating the focus error signal by an astigmatism method using this reflected sub-beam. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical disc apparatus and an optical pickup, and more specifically, is suitable for application to generation of a focus error signal.

  2. Description of the Related Art Conventionally, an optical disc apparatus generates a tracking error signal indicating an irradiation position deviation of a light beam with respect to a desired track on a recording surface of an optical disc, and tracking that drives an optical pickup so that the irradiation position deviation is reduced based on the tracking error signal. In addition to performing the control, a focusing error signal indicating the focal position deviation of the light beam with respect to the recording surface is generated, and focusing control is performed to drive the objective lens so as to reduce the focal position deviation based on the focusing error signal.

  A push-pull method is widely used as a tracking servo method in an optical disc apparatus. As shown in FIG. 1A, convex grooves G on which data is recorded and lands L as guide grooves are alternately arranged on the recording surface of the optical disc 100. The pair of grooves G and A land L forms one track. Here, when the recording surface is irradiated with the light beam, the groove G and land L act as a diffraction grating on the light beam. Therefore, when the optical disc apparatus 1 irradiates the optical disc 100 with the light beam as the spot P, the secondary light consisting of the main light region AR0 where the reflected light beam is a zero order light beam and the ± first order light beam due to diffraction by the groove G and land L. The region AR ± 1 is divided into two overlapping regions W where the main light region AR0 and the sub-light region AR ± 1 overlap each other at both ends of the reflected light beam.

  In the two overlapping regions W, the main light region AR0 and the sub light region AR ± 1 interfere with each other, but the phase of the sub light region AR ± 1 changes according to the position of the spot P with respect to the groove G and the land L at this time. Therefore, the amount of light in the two overlapping regions W changes according to the phase difference of the sub-light region AR ± 1 with respect to the main light region AR0.

  Accordingly, as shown in FIG. 1B, when the reflected light spot Q is detected by the photodetector 1 having the light receiving areas 1A and 1B divided into two, the light receiving areas 1A and 1B corresponding to the reflected light spot Q are detected. A push-pull signal representing the light amount difference between the two overlapping regions W can be generated from the detected light amount difference. This push-pull signal represents the irradiation position deviation of the spot P with respect to the center of the groove G, and becomes zero when the spot P is irradiated on the center of the groove G, and indicates a sine wave having a wavelength corresponding to the track pitch. .

  By the way, in the optical disk apparatus, there are cases where only the objective lens is driven in the tracking direction while the optical pickup is fixed to finely adjust the irradiation position (so-called lens shift). At this time, the positional relationship between the objective lens and the photodetector 1 is Therefore, the center of the reflected light spot Q (hereinafter referred to as the spot center) is shifted from the dividing line of the light receiving regions 1A and 1B. In the push-pull method, it cannot be determined whether the detected light amount difference between the light receiving areas 1A and 1B is due to a lens shift or a light beam irradiation position shift.

  Therefore, in the optical disc apparatus, the optical beam is divided into a main beam composed of a zero order light beam and a sub beam composed of ± first order light beams and irradiated onto the optical disc 100, and a tracking error signal is generated based on the amount of reflected light. The so-called DPP (Differential Push Pull) method is used. In practice, in this DPP method, the light beam emitted from the laser diode is divided into a main beam and two sub beams by a diffraction grating, and the main beam is collected on the optical disc 100 as shown in FIG. The main spot PA formed by light is irradiated so as to be positioned at the center of the groove G in a desired track, and the sub-spots PB and PC obtained by condensing the sub-beams are ½ track in the opposite directions from the groove G. Irradiation is performed so as to be located on the land L shifted by minutes.

  In this case, in the optical disc apparatus, as shown in FIG. 2B, the main reflected light spot QA, the sub reflected light spots QB, and QC corresponding to the main spot PA, the sub spot PB, and the PC are received by the photodetector 2.

  The photodetector 2 includes a main spot detector 3 having light receiving areas 3A, 3B, 3C, and 3D, a sub spot detector 4 having light receiving areas 4E, 4F, 4G, and 4H, and light receiving areas 5I, 5J, and 5K. And 5L, the main spot detector 3 is the main reflected light spot QA, the sub spot detector 4 is the sub reflected light spot QB, and the sub spot detector 5 is the sub reflected light spot. Each QC is received. The photodetector 2 generates detection signals a to d, e to h, and i to l corresponding to the received light amounts of the light receiving regions 3A to 3D, 4E to 4H, and 5I to 5L.

  The light quantity of the sub reflected light spots QB and QC by the sub beam is made smaller than the light quantity of the main reflected light spot QA by the main beam, thereby improving the light utilization efficiency of the main spot PA. .

  The signal processing unit of the optical disc apparatus uses detection signals a, b, c, and d corresponding to the amounts of light received in the light receiving areas 3A, 3B, 3C, and 3D, and both areas sandwiching a center line parallel to a desired track The difference in the amount of received light is calculated as the main push-pull signal MPP according to the following equation.

  MPP = a + d−b−c (1)

  Further, the signal processing unit detects detection signals e, f, g, and h corresponding to the received light amounts of the light receiving regions 4E, 4F, 4G, and 4H, and a detection signal i corresponding to the received light amounts of the light receiving regions 5I, 5J, 5K, and 5L. , J, k, and l, the sub push-pull signal SPP is calculated by the following equation. However, K is a coefficient for correcting the light amount difference between the main beam and the sub beam to increase the signal level of the sub push-pull signal SPP to the same level as the main push-pull signal MPP.

  SPP = K {(e + hf−g) + (i + 1−j−k)} (2)

  At this time, in the DPP method, the main spot PA by the main beam is irradiated so as to be positioned on the groove G, while the sub spots PB and PC by the sub beam are shifted by a half track so as to be positioned on the land L. is doing.

  For this reason, the light and darkness generated in the overlapping region W of the main reflected light spot QA and the sub reflected light spots QB and QC are reversed, and the phase of the sub push-pull signal SPP is 180 degrees out of phase with the main push-pull signal MPP. .

  Here, when the objective lens is at the reference position, the reflected light spots QA, QB, and QC are received at the centers of the main spot detector 3 and the sub spot detectors 4 and 5, respectively. 3 and the sub-spot detectors 4 and 5 are positioned on the light receiving area dividing line.

  On the other hand, if the spot centers of the reflected light spots QA, QB, and QC move from the centers of the main spot detector 3 and the sub spot detectors 4 and 5 due to the shift of the objective lens in the tracking direction, the main push-pull signal In the MPP and the sub push-pull signal SPP, a DC offset corresponding to the amount of spot movement is generated.

  Therefore, in an optical disc apparatus to which the DPP method is applied, a tracking error signal TE in which the DC offset is canceled is obtained by calculating a difference between the main push-pull signal MPP and the sub push-pull signal SPP according to the following equation.

TE = MPP-SPP
= (A + d-b-c) -K {(e + hf-g) + (i + l-j-k)} (3)

  On the other hand, the astigmatism method is widely used as a focusing servo method in an optical disc apparatus. This astigmatism method utilizes the fact that the spot shape of the reflected light beam reflected by the recording surface changes according to the positional relationship between the focal point of the light beam collected by the objective lens and the recording surface of the optical disk. The focus error signal is generated from the difference between the two diagonal sum signals in the light receiving element in which the light receiving area is divided into four parts, and has the advantage that it can be easily integrated with the tracking servo method by the push-pull method. is doing.

  That is, in the astigmatism method, in the main spot detector 3 (FIG. 2) that receives the main reflected light spot, a diagonal sum signal of the detection signals a and c output from the light receiving areas 3A and 3C located diagonally. By calculating a diagonal sum difference signal that is the difference between a + c and the diagonal sum signal b + d of the detection signals b and d output from the light receiving areas 3B and 3D, which are also located diagonally, according to the following equation: A focus error signal FE indicating the deviation of the focal position of the light beam with respect to the recording surface is obtained.

  FE = (a + c) − (b + d) (4)

  However, when the astigmatism method is used as a focusing servo system, a phenomenon called a focus leak occurs when reading / writing from / to an optical disk having a large push-pull signal amplitude. In the case of using the above-described push-pull method as the tracking servo method, this focus leak is caused by the change in the light / dark pattern in the spot that occurs when the irradiation position of the spot with respect to the track shifts. When the two sets of diagonal sum signals are unbalanced, the focus servo malfunctions.

  In order to cope with such a problem, a focusing servo method called a differential astigmatism method has been proposed (see, for example, Patent Document 1). In this differential astigmatism method, the diagonal sum difference for each of the sub-spot detectors 4 and 5 that receive the sub-reflected light spot with respect to the diagonal sum difference signal in the main spot detector 3 described above according to the following equation: By adding the signals (e + g) − (f + h) and (i + k) − (j + l) over a predetermined coefficient α, a focus error signal FE with little focus leak can be obtained.

FE = (a + c) − (b + d) + α {(e + g) − (f + h) + (i + k) − (j + l)}

...... (5)
JP 2004-63073 A

  In order to perform focus servo using the differential astigmatism method described above, the phases of the main push-pull signal MPP and the sub push-pull signal SPP need to be shifted by 180 degrees.

  Here, in an optical pickup of an optical disc apparatus corresponding to a plurality of types of optical discs, it is conceivable to arrange a plurality of objective lenses according to the wavelength of the laser beam in parallel in the track direction of the optical disc. One objective lens is arranged at a position away from the radius of the optical disk (this is called an offset arrangement).

  The objective lens arranged offset moves on a line separated by the lens interval with respect to the disk radius. The positional relationship between the two sub beams irradiated from the objective lens and the track depends on the disk center of the objective lens. The phase difference between the main push-pull signal MPP and the sub push-pull signal SPP changes from 180 degrees due to this influence, and the differential astigmatism method described above cannot be applied. At the same time, a normal DPP signal cannot be obtained.

  In order to obtain a normal DPP signal from the optical pickup having such a configuration, it is conceivable to suppress the amplitude of the sub push-pull signal. For example, in a method called the phase shift DPP method, as shown in FIG. 3, a band-like delay unit 6A that delays the phase of the passing light by 180 ° with respect to the diffraction grating 6 for generating the sub-beam, and a delay are generated. The non-delayed portion 6B that is not to be disturbed is periodically provided in a state inclined at a predetermined angle (for example, 45 °) with respect to the track direction of the optical disc, whereby the optical beam irradiated to the optical disc through the diffraction grating 6 is fringed. As shown in FIG. 4, due to this interference, the overlapping region W in the sub reflected light spots QB and QC on the sub spot detectors 4 and 5 causes a checkered pattern inclined with respect to the track direction. This produces a light and dark pattern.

  The overlapping region W in the sub reflected light spots QB and QC does not change much in lightness even if the irradiation position of the sub beam is moved due to the above-described influence of the light / dark pattern, thereby suppressing the amplitude of the sub push-pull signal. it can. For this reason, even in an optical system using an objective lens arranged offset, it is possible to obtain the tracking error signal TE in which the offset of the DC component is canceled without being affected by the displacement of the irradiation position of the sub beam.

  However, when such a diffraction grating 6 is used, since the sub push-pull signal amplitude is suppressed, the focus leak reduction effect due to differential astigmatism is greatly reduced. In addition, it is inevitable that the phase difference between the main push-pull signal MPP and the sub push-pull signal SPP changes from 180 degrees, and the differential astigmatism method is still used in an optical system using an objective lens that is offset. There was a problem that it was impossible to perform the focusing control.

  The present invention has been made in consideration of the above points, and intends to propose an optical disc apparatus and an optical pickup capable of performing reliable focusing control with a simple configuration.

  In order to solve this problem, in the optical disk apparatus of the present invention, the diffraction element that diffracts the light beam emitted from the light source to generate the main beam and the sub beam, and the phase of the partial region of the sub beam change by approximately 180 °. And a phase change means for causing the light receiving surface to be divided into four light receiving regions by a dividing line parallel to the track of the optical disk and a dividing line perpendicular to the track, and a reflected sub beam reflected by the optical disk is photoelectrically generated in each light receiving region. A focus error signal based on a difference between a photodetector that converts and outputs a detection signal, and a sum of detection signals of one pair of light receiving areas located diagonally and a sum of detection signals of the other pair And a signal processing means for generating.

  By changing the phase of a partial region of the sub-beam light beam by the phase changing means by approximately 180 °, the sub-beam is diffracted and reflected by the track of the optical disc in the overlapping region of the zero-order light and the ± first-order light. A bright / dark pattern in which the brightness of the bright part and the dark part increase or decrease in reverse with each other according to the movement of the irradiation position of the sub beam with respect to the track is generated, and the bright part and the dark part cancel each other. Focus error that is less susceptible to changes in the irradiation position of the sub-beam on the track by suppressing the local brightness change near the dividing line of the photodetector and generating a focus error signal by the astigmatism method using this reflected sub-beam A signal can be obtained.

  In the optical pickup of the present invention, a diffraction element that diffracts a light beam emitted from a light source to generate a main beam and a sub beam, and a phase change unit that changes a phase of a partial region of the sub beam by approximately 180 °. The light-receiving surface is divided into four light-receiving areas by a dividing line parallel to the optical disc track and a perpendicular dividing line, and the sub-beams reflected by the optical disk are photoelectrically converted in each light-receiving area and detected respectively. And a photodetector for outputting a signal.

  By changing the phase of a partial region of the sub-beam light beam by the phase changing means by approximately 180 °, the sub-beam is diffracted and reflected by the track of the optical disc in the overlapping region of the zero-order light and the ± first-order light. A bright / dark pattern in which the brightness of the bright part and the dark part increase or decrease in reverse with each other according to the movement of the irradiation position of the sub beam with respect to the track is generated, and the bright part and the dark part cancel each other. A focus error signal that is less susceptible to changes in the irradiation position of the sub beam by generating a focus error signal by the astigmatism method using this reflected sub beam by suppressing the local brightness change near the dividing line of the photodetector. A signal can be obtained.

  According to the present invention, the phase change means changes the phase of a partial region of the sub-beam light by approximately 180 °, so that the sub-beam is diffracted and reflected by the track of the optical disc and the zero-order light and the ± first-order light in the reflected sub-beam. In the overlapping area of light, a bright and dark pattern in which the brightness of the bright part and the dark part increase or decrease in reverse according to the movement of the irradiation position of the sub beam to the track is generated, and the bright part and the dark part cancel each other, thereby moving the irradiation position. By reducing the local brightness change in the entire overlapping area and in the vicinity of the dividing line of the above-mentioned photodetector, the focus error signal is generated by the astigmatism method using this reflected sub-beam, thereby changing the irradiation position of the sub-beam. A focus error signal that is not easily affected by noise can be obtained. Thus, reliable focusing control can be achieved with a simple configuration. An optical disk device and an optical pickup that can be performed can be realized.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

(1) Overall Configuration of Optical Disc Device In FIG. 5, reference numeral 10 denotes an optical disc device as a whole, and the control unit 12 performs overall control. The optical disc apparatus 10 corresponds to a CD (Compact Disc), a DVD (Digital Versatile Disc), and a Blu-ray Disc (registered trademark, hereinafter referred to as BD), and an optical disc 100 of any of these three types is mounted. In response to a reproduction instruction from an external device (not shown), the controller 12 controls the drive unit 13 and the signal processing unit 14 to read information recorded on the optical disc 100.

  In practice, the drive unit 13 rotates the optical disc 100 at a desired rotational speed by the spindle motor 15 in accordance with the control of the control unit 12, and the sled motor 16 moves the optical pickup 17 in the tracking direction which is the radial direction of the optical disc 100. Move it a lot.

  In parallel with this, the signal processing unit 14 irradiates a desired track of the optical disc 100 from the optical pickup 17 with a light beam having a wavelength corresponding to the type of the mounted optical disc 100, and displays the detection result of the reflected light. Based on this, a reproduction signal is generated, and this reproduction signal is sent to an external device (not shown) via the control unit 12.

  In other words, the optical pickup 17 condenses the emitted beam having a wavelength corresponding to the type of the mounted optical disc 100 with the objective lens, irradiates the recording layer of the optical disc 100 with the focal point, and at the focused recording layer. After the reflected beam reflected is collected by the objective lens, it is photoelectrically converted by a photodetector, and various detection signals are generated and supplied to the signal processing unit 14.

  At this time, the drive unit 13 finely drives the objective lens based on the focus error signal and the tracking error signal supplied from the signal processing unit 14 to match the focal point of the light beam with a desired track in the recording layer of the optical disc 100. . The signal processing unit 14 performs predetermined signal processing on the reproduction signal supplied from the optical pickup 17 and then outputs the reproduction signal to the outside via the control unit 12.

(2) Configuration of Optical Pickup FIG. 6 shows the configuration of the optical pickup 17. In FIG. 6, the wave plate and the aberration correction element are omitted. As described above, the optical disk device 10 is compatible with three types of optical disks, CD, DVD and BD. For this reason, the optical pickup 17 has a BD optical system 20 for accessing a Blu-ray disk, and an access to the CD and DVD. The CD / DVD shared optical system 30 that performs the two independent optical systems is used, and the control unit 12 of the optical disk apparatus controls the BD optical system 20 or the CD / DVD shared optical depending on the type of the loaded optical disk 100. One of the systems 30 is selected and used.

  The BD optical system 20 of the optical pickup 17 emits a light beam having a wavelength corresponding to BD of about 405 nm from the BD laser diode 21, and diffracts the light beam by the BD diffraction element 22 to be zero-order light. Two side beams composed of a main beam and ± primary light are generated, and a total of three light beams are incident on the beam splitter 23.

  The beam splitter 23 reflects the light beam (the main beam and the two side beams) from the diffraction grating 22 based on the polarization direction and makes the light beam incident on the collimator lens 24. The collimator lens 24 converts a light beam made of divergent light into parallel light and makes it incident on the BD objective lens 26 via the rising mirror 25.

  The BD objective lens 26 condenses the main beam and the two side beams of the light beam and irradiates the recording surface of the optical disc 100 at a predetermined interval.

  Further, the BD optical system 20 collects the reflected light beam reflected by the recording layer of the optical disc 100 by the objective lens 26 and re-enters the polarization beam splitter 23 through the rising mirror 25 and the collimator lens 24 sequentially. The polarization beam splitter 23 transmits the reflected light beam based on the polarization direction, and forms an image on the BD photodetector 28 via the cylindrical lens 27. The BD photodetector 28 generates various detection signals according to the amount of the received reflected light beam and supplies the detection signals to the signal processing unit 14 (FIG. 1).

  The signal processing unit 14 generates a tracking error signal and a focus error signal using the detection signal supplied from the BD photodetector 28, and performs tracking and focusing control using these signals.

  On the other hand, the CD / DVD optical system 30 of the optical pickup 17 emits a light beam having a wavelength corresponding to CD of about 780 nm or a wavelength corresponding to DVD of about 650 nm from the CD / DVD laser diode 31, and outputs the light beam to the CD / DVD. Diffraction is performed by the DVD diffraction element 32 to generate a main beam composed of zero-order light and two side beams composed of ± first-order light, and these three light beams are incident on the beam splitter 33.

  The beam splitter 33 reflects the light beam from the diffraction grating 32 for CD / DVD based on its polarization direction and makes it incident on the collimator lens 34. The collimator lens 34 converts a divergent light beam into parallel light and makes it incident on the CD / DVD objective lens 36 via the rising mirror 35.

  The CD / DVD objective lens 36 condenses the main beam and the two side beams of the light beam and irradiates the recording surface of the optical disc 100 at a predetermined interval.

  Further, the optical system 30 for CD / DVD collects the reflected light beam reflected by the recording layer of the optical disc 100 by the objective lens 36 and re-enters the polarization beam splitter 33 through the rising mirror 35 and the collimator lens 34 in order. . The polarization beam splitter 33 transmits the reflected light beam based on the polarization direction, aligns the optical axes of the reflected light beam from the CD and the reflected light beam from the DVD by the optical axis combining element 37, and then moves the cylindrical lens 38. Then, an image is formed on the photodetector 39 for CD / DVD. Then, the CD / DVD photodetector 39 generates various detection signals according to the amount of the received reflected light beam and supplies the detection signals to the signal processing unit 14 (FIG. 5).

  The signal processor 14 generates a tracking error signal and a focus error signal using the detection signal supplied from the CD / DVD photodetector 39, and performs tracking and focusing control using these signals.

  Incidentally, as described above, the optical pickup 17 has the BD objective lens 26 and the CD / DVD objective lens 36 in the BD optical system 20 and the CD / DVD shared optical system 30, respectively. The objective lenses are mounted on a common lens holder 29 so as to be arranged in parallel in the track direction of the optical disc 100, and are driven in a focus direction and a tracking direction by a biaxial actuator (not shown).

  Here, as the configuration of the objective lens arrangement of the optical pickup 17, the CD / DVD objective lens 36 is arranged on the radius of the optical disc and the BD objective lens 26 is offset, and the BD objective lens 26 is arranged on the optical disc. It is conceivable that the CD / DVD objective lens 36 is arranged on the radius of the CD. Here, the case where the BD objective lens 26 is arranged on the radius of the optical disk will be described as an example. However, the present invention can also be applied to the case where the CD / DVD objective lens 36 is arranged on the radius of the optical disk.

  Since the BD objective lens 26 is arranged on the radius of the optical disk, the CD / DVD objective lens 36 is thread-moved on a line separated by the lens interval with respect to the disk radius. The positional relationship between the two sub-beams irradiated from the objective lens 36 and the track changes according to the distance from the center of the disc of the CD / DVD objective lens 36. Due to this influence, the main push-pull signal MPP and the sub push-pull signal SPP are changed. As a result of the focus servo method, the diagonal sum difference of the sub reflected light spot is added to the diagonal sum difference signal of the main reflected light spot. Applying the differential astigmatism method (equation (5)) that generates the focus error signal FE by adding the signals Can not.

  For this reason, in the optical disk apparatus 10 of the present invention, the sub-beam having the characteristic that the brightness change of the sub-reflection light spot is small even when the irradiation position on the track is changed is generated by the CD / DVD diffraction element 32 and only the sub-reflection light spot is generated. Is used to generate a focus error signal FE so that reliable focusing control using the astigmatism method can be performed even in the CD / DVD shared optical system 30 in which the objective lens is offset.

  As shown in FIG. 7, the CD / DVD diffraction element 32 is divided by a dividing line 32X passing through the center of the light beam passing through the CD / DVD diffraction element 32 and parallel to the track direction of the optical disk. In this area, a delay part 32A as a phase changing means for delaying the phase of the passing light by 180 ° is formed in a checkered pattern parallel to the track direction, and the part other than the delaying part 32A does not delay the phase of the passing light. A non-delay portion 32B is formed.

  The delay part 32A and the non-delay part 32B have a phase difference of 180 degrees in the periodic structure, and a diffractive element having such a region is used in the invention of Japanese Patent Application Laid-Open No. 2004-145915, for example. With this structure, the phase of the wave front of the + 1st order diffracted light that has passed through the region of the delay unit 32A is shifted by 180 degrees compared to the + 1st order diffracted light that has passed through the region of the non-delay unit 32B. If the polarity of this shift is positive, the phase of the wavefront of the −1st order diffracted light that has passed through the area of the delay section 32A is shifted by −180 degrees compared to the −1st order diffracted light that has passed through the area of the non-delay section 32B. . Hereinafter, the phase change of the ± first-order diffracted light is expressed as “the phase is delayed by 180 degrees”.

  As a result, the two sub-beams generated when the light beam passes through the CD / DVD diffraction element 32 has a phase of 180 in one of the light beam regions divided by the dividing line parallel to the track direction. The delayed delay region is formed in a checkered pattern parallel to the track direction, and the other light flux region is in a state without phase delay in all parts. Note that the phase of the wavefront of the main beam, which is zero-order light, is not affected.

  The main beam and the two sub beams are condensed by the CD / DVD objective lens 36 and irradiated onto the recording surface of the optical disc 100 as a main spot and two sub spots. Since the main beam and the two sub beams are diffracted when reflected by the recording surface, the reflected main beam and the two reflected sub beams include 0th order light and ± 1st order light, respectively. The reflected main beam and the two reflected sub-beams are received by the CD / DVD photodetector 39 in a state where the respective 0th-order light and ± 1st-order light are overlapped.

  That is, FIG. 8 shows a CD / DVD photodetector 39, which includes a main spot detector 41 having light receiving areas 41A, 41B, 41C and 41D, and a sub spot detector 42 having light receiving areas 42E, 42F, 42G and 42H. , The sub spot detector 43 having the light receiving areas 43I, 43J, 43K and 43L, the main spot detector 41 is the main reflected light spot QA, the sub spot detector 42 is the sub reflected light spot QB, The detector 43 receives each of the sub reflected light spots QC. Then, the CD / DVD photodetector 39 generates detection signals a to d, e to h, and i to l corresponding to the received light amounts of the light receiving regions 41A to 41D, 42E to 42H, and 43I to 43L.

  Since the overlapping region W in the sub-reflected light spots QB and QC is formed by overlapping the 0th order light and the ± 1st order light diffracted on the recording surface, the light flux region having the checkered pattern-like delay region described above. And a light flux region having no phase delay interfere to form a checkered pattern of light and dark parallel to the track direction.

  In the checkered pattern of light and darkness formed in the overlapping region W, the lightness of the bright part and the dark part respectively increase or decrease in accordance with the movement of the irradiation position of the sub beam with respect to the track (when one lightness increases, the other decreases). However, when the bright part and the dark part cancel each other, the change in the average brightness in the entire overlapping region W becomes small.

  Most of the changes in the detection signals e to h and the detection signals i to l obtained when the sub spot detectors 42 and 43 receive the sub reflected light spots QB and QC are mostly shifted in the focal position of the light beam. This is caused by the change in the spot shape caused by the movement of the sub reflected light spots QB and QC due to the shift of the objective lens 36 in the tracking direction.

  For this reason, the focus error signal FE with less focus leak can be obtained using only the detection signals e to h and the detection signals i to l based on the received light amounts of the sub reflected light spots QB and QC.

  That is, the signal processing unit 14 adds the diagonal sum difference signals (e + g) − (f + h) and (i + k) − (j + l) for each of the sub-spot detectors 42 and 43 by using the following equation, thereby obtaining a focus error. A signal FE is generated.

  FE = (e + g) − (f + h) + (i + k) − (j + l) (6)

  FIG. 9 shows the change of the focus error signal FE with respect to the spot irradiation position shift with respect to the track. The focus leak in the focus error signal FE (shown by a solid line) generated by using the diffraction element 32 of the present invention and Equation (6) is It can be seen that this is significantly suppressed as compared with the focus error signal FE (shown by a broken line) generated using the conventional diffraction element and the equation (5).

  Furthermore, in the optical pickup 17 of the present invention, the diffraction element 32 is configured as shown in FIG. 7, so that the checkered pattern of the overlapping region W is parallel to the track direction, unlike the invention of Japanese Patent Application Laid-Open No. 2004-145915. Therefore, the sub-spot detectors 42 and 43 are irradiated with the sub reflected light spots QB and QC as compared with the tilted checkered light and dark pattern (FIG. 4) of the conventional diffraction grating 6 (FIG. 3). Fluctuations in the signals eh, fg, il, and jk when the angle shifts in the track direction can be reduced, thereby further suppressing focus leak.

  Note that the signal processing unit 14 generates the tracking error signal TE in which the DC offset is canceled using the above-described equation (3) as in the conventional phase shift DPP method, and this also applies to the sub reflected light spots QB and QC. Therefore, it is possible to obtain the tracking error signal TE in which the offset of the DC component is canceled without being affected by the deviation of the irradiation position of the sub beam.

(3) Operation and Effect In the above configuration, in the optical disk apparatus 10, the objective lens 36 is parallel to the track direction in the CD / DVD diffraction element 32 of the CD / DVD shared optical system 30 in which the objective lens 36 is offset from the disk radius. A delay portion 32A for delaying the phase of the passing light by 180 ° is formed in a checkered pattern parallel to the track direction in one light flux region divided by a simple dividing line.

  As a result, checkered light and darkness parallel to the track direction is formed in the overlapping region W of the zero-order light and the ± first-order light in the sub reflected light spots QB and QC due to interference of the checkered delay region. The

  The brightness of the checkered pattern and the dark part increase or decrease according to the movement of the irradiation position of the sub beam with respect to the track. Therefore, when the bright part and the dark part cancel each other, the overlapping area W generated by the movement of the irradiation position is reduced. The brightness change can be greatly suppressed as compared with the conventional case.

  In this optical disk apparatus 10, the focus error signal FE is generated by the astigmatism method using only the sub reflected light spots QB and QC with little change in brightness in the overlapping area W, thereby changing the irradiation position of the sub beam on the track. It is possible to obtain a focus error signal FE that is largely unaffected and greatly suppresses the focus leak.

  For this reason, even in the CD / DVD shared optical system 30 in which the irradiation position of the sub beam with respect to the track changes between the inner periphery and the outer periphery due to the offset arrangement of the objective lens 36, a good focus error signal FE is always generated. Obtainable.

  Further, since the focus error signal FE is not affected by the change of the irradiation position of the sub beam on the track, a good focus error signal FE can be obtained from various optical discs 100 having different track pitches.

  According to the above configuration, the sub-beam having a small change in brightness in the overlapping region W of the sub-reflected light spots QB and QC is generated by the CD / DVD diffraction element 32, and only the sub-reflected light spots QB and QC are used astigmatism. By generating the focus error signal FE by the aberration method, it is possible to generate a high-quality focus error signal FE that is not affected by the change in the irradiation position of the sub beam with respect to the track, and to perform reliable focusing control.

(4) Other Embodiments In the above-described embodiment, the case where the present invention is applied to the CD / DVD shared optical system 30 in which the objective lens 36 is offset from the radius of the optical disk has been described. The present invention is not limited to this, and the present invention may also be applied to the BD optical system 20 in which the objective lens 26 is disposed on the radius of the optical disc to generate a sub-beam with little change in brightness in the overlapping region. . Even in this case, it is possible to generate a high-quality focus error signal FE that is not affected by a change in the irradiation position of the sub beam on the track, and to perform reliable focusing control.

  Furthermore, the present invention can be applied to various optical discs other than CD, DVD and BD, and various optical disc apparatuses using a plurality of them.

  In the above-described embodiment, the sub-reflected light is formed by forming the checkered pattern-like delay portion 32A parallel to the track direction in one region divided by the dividing line 32X in the CD / DVD diffraction element 32. A checkered pattern of light and dark parallel to the track direction is formed in the overlapping region W of the spots QB and QC. However, the present invention is not limited to this, and as in the diffraction element 42 shown in FIG. A band-shaped delay section 42A parallel to the track direction is periodically provided in one area divided by the above-mentioned area, and a band-shaped delay section 42A perpendicular to the track direction is periodically provided in the other area. As shown in FIG. 7, the sub reflected light spots QB and QC with little change in brightness of the overlapping region W can be formed.

  Further, in the above-described embodiment, the case where the present invention is applied to the optical pickup 17 of the optical disc apparatus 10 has been described. However, the present invention is not limited to this, and the present invention may be applied to other various types of optical pickups. . That is, the optical pickup 17 may be other than that incorporated in the optical disc apparatus 10.

  The present invention can be widely applied to various optical disk devices.

It is a basic diagram which shows the relationship between a reflective spot and an overlapping area | region. It is a basic diagram which shows the spot and reflection spot by DPP method. It is a basic diagram which shows the diffraction element used by the phase shift DPP method. It is a basic diagram which shows the sub reflected light spot by a phase shift DPP method. 1 is a schematic diagram illustrating an overall configuration of an optical disc device. It is a basic diagram which shows the structure of an optical pick-up. It is a basic diagram which shows the diffraction element by this invention. It is a basic diagram which shows a photodetector and a reflective spot. It is a characteristic curve figure which shows the focus error signal by this invention. It is a basic diagram which shows the diffraction element by other embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Optical disk apparatus, 12 ... Control part, 13 ... Drive part, 14 ... Signal processing part, 15 ... Spindle motor, 16 ... Thread motor, 17 ... Optical pick-up, 20 ... Optical system for BD 21 ... BD laser diode, 22 ... BD diffraction element, 23 ... beam splitter, 24 ... collimator lens, 25 ... rise mirror, 26 ... BD objective lens, 27 ... cylindrical lens, 28 .. Photodetector for BD, 29 .. Lens holder, 30 .. Optical system for CD / DVD, 31 .. Laser diode for CD / DVD, 32 .. Diffraction element for CD / DVD, 33 .. Beam splitter , 34... Collimator lens, 35... Vertical mirror, 36... Objective lens for CD / DVD, 37... Optical axis synthesizing element, 38. ... CD / DVD optical detector.

Claims (4)

A diffraction element that diffracts a light beam emitted from a light source to generate a main beam and a sub beam;
Phase changing means for changing the phase of a partial region of the sub-beam luminous flux by approximately 180 °;
The light receiving surface is divided into four light receiving regions by a dividing line parallel to the track of the optical disk and a dividing line perpendicular to the track, and a reflected sub beam obtained by reflecting the sub beam on the optical disk is photoelectrically converted in each light receiving region. A photodetector for outputting detection signals,
Signal processing means for generating a focus error signal based on a difference between a sum of the detection signals of one pair of the light receiving regions located on the diagonal and a sum of the detection signals of the other pair. An optical disk device. The phase changing means is formed in a checkered pattern parallel to the track in one region divided by a dividing line parallel to the track in the diffraction element. 1. An optical disc device according to 1. The optical disc apparatus according to claim 1, wherein the objective lens that collects the main beam and the sub beam and irradiates the track of the optical disc is disposed at a position away from the radius of the optical disc. A diffraction element that diffracts a light beam emitted from a light source to generate a main beam and a sub beam;
Phase changing means for changing the phase of a partial region of the sub-beam luminous flux by approximately 180 °;
The light receiving surface is divided into four light receiving regions by a dividing line parallel to the track of the optical disk and a dividing line perpendicular to the track, and a reflected sub beam obtained by reflecting the sub beam on the optical disk is photoelectrically converted in each light receiving region. An optical pickup comprising: a photodetector that outputs a detection signal.

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