JP4120519B2 - Spherical aberration corrector - Google Patents

Description

  In the present invention, when spherical aberration occurs between the optical recording medium and the optical system in the optical pickup, a beam of reflected light from the optical recording medium is detected by an eight-divided photodetector, and this eight-divided detection is performed. The present invention relates to a spherical aberration correction apparatus that can satisfactorily correct spherical aberration while feedback controlling a spherical aberration correction means based on a spherical aberration error signal obtained by calculating each detection signal from a type photodetector.

  In general, an optical recording medium such as a disk-shaped optical disk or a card-shaped optical card has a high density on a track in which information signals such as video information, audio information, and computer data are spirally or concentrically formed on a transparent substrate. When a recorded track is recorded and a recorded track is reproduced, a desired track can be accessed at high speed.

  For example, CDs (Compact Discs) and DVDs (Digital Versatile Discs) are already on the market as optical discs of this type of optical recording media. Development of an ultra-high density optical disc (Blu Ray Disc) capable of recording or reproducing information signals at an ultra-high density with a narrower track than the above-described CD and DVD has been actively conducted.

  The above ultra-high density optical disc is irradiated with a laser beam obtained by narrowing a laser beam having a wavelength of 450 nm or less with an objective lens having a numerical aperture (NA) of 0.75 or more, and is separated by approximately 0.1 mm from the beam incident surface. Development is progressing so that an information signal can be recorded or reproduced at an extremely high density on a signal surface at a certain position. At this time, the recording capacity of the ultra high density optical disk is around 25 GB (gigabyte) on one side when the diameter of the disk substrate is 12 cm.

By the way, although there are various structural forms of optical pickup devices for recording or reproducing high-density optical discs, as an example, a laser beam narrowed down by an objective lens is irradiated onto the signal surface of the optical disc and reflected from the optical disc. Spherical aberration can be corrected while controlling the liquid crystal based on a spherical aberration error signal obtained by detecting light by an eight-divided photodetector and calculating each detection signal from the eight-divided photodetector. There are an optical pickup, an information reproducing device, and an information recording device (see, for example, Patent Document 1).
Japanese Unexamined Patent Publication No. 2000-57616 (page 5-9, FIGS. 4, 5, and 8).

FIG. 12 is a block diagram showing a schematic configuration of a conventional information recording / reproducing apparatus,
FIG. 13 is a diagram for explaining a division form of an eight-divided photodetector provided in an optical pickup in a conventional information recording / reproducing apparatus;
FIG. 14 is a block diagram showing a detailed configuration of the signal processing unit shown in FIG.

  The conventional information recording / reproducing apparatus 100 shown in FIG. 12 is disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2000-57616) described above, and will be briefly described here with reference to Patent Document 1. .

  As shown in FIG. 12, an optical pickup 110 for recording or reproducing an optical disk 101 is provided inside a conventional information recording / reproducing apparatus 100.

  The optical pickup 110 described above emits a laser beam 111 for recording / reproduction, and reflects the laser beam L emitted from the laser diode 111 to turn about 90 ° and reflects the reflected light from the optical disc 101. A polarizing beam splitter 112 to be transmitted, a collimator lens 113 that converts the laser light L reflected from the polarizing beam splitter 112 into parallel light, and a liquid crystal that corrects spherical aberration based on a spherical aberration error signal described later. 114, a λ / 4 plate 115 that rotates the polarization plane of the laser light L that has been collimated by the collimator lens 113 and the polarization plane of the reflected light from the optical disc 101, and the laser light L that has passed through the λ / 4 plate 115. The laser beam LB obtained by narrowing down An outward optical system is configured by the objective lens 116 that condenses on the signal surface 101b through the protective layer 101a, and the objective lens 116 is attached to the optical disc 101 in a lens holder (not shown) incorporating the objective lens 116. An actuator 117 for controlling in the focus direction is attached.

  On the other hand, the reflected light reflected by the signal surface 101b of the optical disk 101 passes through the λ / 4 plate 115, the liquid crystal 114, the collimator lens 113, and the polarization beam splitter 112 in this order, and then condenses the lens 118 and cylindrical. An optical system of the return path is configured by being detected by an eight-divided photodetector (hereinafter, referred to as an eight-divided photodetector) 120 through a lens 119.

  Further, a signal processing unit 121 for calculating each detection signal obtained by detecting the reflected light from the optical disc 101 by the eight-divided photodetector 120 to generate a spherical aberration error signal Ske, a focus error signal Sfe, and an RF signal Srf. The spherical aberration error signal Ske obtained by calculation by the signal processing unit 121 is supplied to the liquid crystal 114 through the amplifier 122 and the driver 123 and connected to the eight-divided photodetector 120, and the signal processing unit 121. The focus error signal Sfe calculated in (1) is supplied to the actuator 117 via the amplifier 124 and the driver 125, and the RF signal Srf calculated by the signal processing unit 121 is supplied to the reproduction unit 126.

  Here, when the thickness of the protective layer 101a in the optical disc 101 is different, spherical aberration calculated by the signal processing unit 121 when spherical aberration occurs due to the laser beam LB from the objective lens 116 passing through the protective layer 101a. The error signal Ske is supplied to the liquid crystal 114, and the liquid crystal 114 gives a phase difference to the laser light L that has passed through the collimator lens 113 based on the spherical aberration error signal Ske so that the spherical aberration is canceled.

  Further, the focus error signal Sfe obtained by calculation in the signal processing unit 121 is supplied to the actuator 117, and the objective lens 116 is controlled in the focus direction with respect to the optical disc 101 based on the focus error signal Sfe.

  Further, the RF signal Srf obtained by calculation in the signal processing unit 121 is supplied to the reproduction unit 126, and the main data recorded on the signal surface 101b of the optical disc 101 is reproduced based on the RF signal Srf.

  By the way, the 8-divided photodetector 120 provided in the optical pickup 110 for obtaining the spherical aberration error signal Ske, the focus error signal Sfe, and the RF signal Srf is enlarged as shown in FIG. The square-shaped light-receiving surface is divided into eight, specifically, the outer periphery is divided into four squares to form four outer peripheral regions 120a to 120d, and the center in the outer peripheral regions 120a to 120d A region where the intensity of reflected light irradiation is strong is divided into four squares to form four inner peripheral regions 120e to 120h. At this time, the total area of the inner peripheral areas 120 e to 120 h in the eight-divided photodetector 120 is the area where the irradiation intensity at the central portion in the irradiation intensity distribution of the reflected light from the optical disc 101 is increased. It is set to be almost equal.

  In the above publication, instead of the eight-divided photodetector 120 in which the inner peripheral regions 120e to 120h are square as shown in FIG. 13A, FIG. 13B or FIG. As shown in FIG. 9, there is also disclosed a case of using an eight-divided photodetector 120 ′ or an eight-divided photodetector 120 ″ whose inner peripheral region is circular or octagonal.

  Each detection signal detected in each of the outer peripheral regions 120a to 120d and the inner peripheral regions 120e to 120h formed in the eight-divided photodetector 120 is sent to the signal processing unit 121 shown in FIG. The RF signal Srf, spherical aberration error signal Ske, and focus error signal Sfe are calculated using adders 130 to 138 and subtractors 139 and 140 provided in the unit 121.

  For example, the spherical aberration error signal Ske is based on the following equation using all the detection signals respectively detected in the outer peripheral areas 120a to 120d and the inner peripheral areas 120e to 120h formed in the eight-divided photodetector 120. It is obtained from the circuit diagram shown in FIG. Ske = (“120a” + “120c” + “120f” + “120h”) − (“120b” + “120d” + “120e” + “120g”).

  Further, the RF signal Srf and the focus error signal Sfe are also obtained from the circuit diagram shown in FIG.

  Therefore, in the conventional information recording / reproducing apparatus 100, the reflected light from the optical disc 101 is reflected by the eight-divided photodetector 120 particularly when spherical aberration occurs between the optical disc 101 and the optical system in the optical pickup 110. Since the spherical aberration is corrected while controlling the liquid crystal 114 based on the spherical aberration error signal Ske obtained by detecting and calculating each detection signal from the eight-divided photodetector 120, the optical disc 101 is protected. Spherical aberration that occurs when the thickness of the layer 101a is different can be corrected.

  The problem to be solved is that, as described above, when the spherical aberration error signal Ske for correcting the spherical aberration generated between the optical disc 101 and the optical system in the optical pickup 110 is obtained, the 8-split type light is used. Since all the detection signals detected in the outer peripheral areas 120a to 120d and the inner peripheral areas 120e to 120h formed in the detector 120 are all used, the signal processing unit 121 calculates the spherical aberration error signal Ske. As shown in FIG. 14, six adders 130 to 135 and one subtractor 139 are used.

  On the other hand, when developing information recording / reproducing devices and optical pickups in a compact and lightweight manner corresponding to the ultra-high density optical discs that are currently under development, it is necessary to reduce the number of components used together with the miniaturization and weight reduction of each component. As described above, when a total of seven arithmetic units are used to calculate the spherical aberration error signal Ske, a reduction in size and weight cannot be achieved, and a device for using a total of seven arithmetic units is used. Will be cost-effective.

  Further, in the conventional information recording / reproducing apparatus 100 described above, no consideration is given to the photodetector when the objective lens 116 is controlled in the tracking direction.

  Therefore, a spherical aberration error signal for correcting spherical aberration generated between the optical recording medium and the optical system in the optical pickup can be obtained more easily, and the spherical surface can be obtained even if the number of arithmetic units is reduced. There is a demand for a spherical aberration correction apparatus including a photodetector that can accurately obtain aberrations and can also cope with a focus error signal and various tracking error signals (PP signal, DPP signal, DPD signal). .

The present invention has been made in view of the above problems, and the first invention is a laser light source that emits laser light having a wavelength corresponding to an optical recording medium, and the laser light from the laser light source is incident, Obtained by narrowing down the laser light that has passed through the spherical aberration correcting means, and spherical aberration correcting means that controls to correct the spherical aberration generated between the laser light source and the optical recording medium with respect to the laser light. An objective lens that irradiates a signal surface of the optical recording medium with a laser beam, a photodetector provided with a plurality of light receiving regions therein to detect reflected light reflected by the signal surface of the optical recording medium, and the light a focus error circuit from the detection signals detected by the plurality of light receiving regions provided in the detector to generate by computation the focus error signal, the plurality of light receiving provided in the photodetector In the spherical aberration correcting device having at least a spherical aberration correcting circuit for feeding back the spherical aberration error signal is generated by calculating a spherical aberration error signal from the detection signal detected by frequency in the spherical aberration correction means,
The plurality of light receiving regions provided in the photodetector are divided into an inner peripheral light receiving region obtained by dividing the inner periphery of one light receiving surface that receives the beam of reflected light into four and an outer periphery of the inner peripheral light receiving region. It consists of a peripheral light receiving area,
The focus error circuit calculates the focus error signal by using the detection signals from the inner and outer light receiving areas.
The spherical aberration correction circuit is a spherical aberration correction device that calculates the spherical aberration error signal using only each detection signal from the outer periphery light receiving region.

The second invention is the spherical aberration correction apparatus according to the first invention described above.
In the spherical aberration correction device, the inner peripheral light receiving region in the photodetector is formed inside a spot diameter obtained when the beam having no spherical aberration is received.

The third invention is the spherical aberration correction device according to the first or second invention described above,
The spherical surface other than the linear region in the S-shaped slope has a linear region where the value of the spherical aberration error signal changes substantially linearly with respect to the amount of the spherical aberration in the middle portion of the S-shaped slope. A spherical aberration corrector that corrects aberration toward the linear region.

  According to the spherical aberration correcting device according to the present invention, in particular, the plurality of light receiving regions provided in the photodetector have an inner circumference of one light receiving surface that receives a beam of reflected light. It consists of an inner periphery light receiving area divided into four and an outer periphery light receiving area obtained by dividing the outer periphery of this inner periphery light receiving area into four parts. Since the calculation is performed, the number of calculators in the spherical aberration correction circuit can be greatly reduced as compared with the conventional example, and the spherical aberration error signal can be easily obtained and can contribute to cost reduction. .

  According to the second aspect of the present invention, in particular, the inner peripheral light receiving region in the photodetector is formed inside the spot diameter obtained when a beam having no spherical aberration is received. When calculating the spherical aberration error signal using only the outer periphery light receiving area, the subtraction value in this area is subtracted with a slight amount of light in the area between the beam spot and the inner periphery light receiving area. Is zero, a noise-free spherical aberration error signal can be obtained, and a reduction in the S / N ratio of the spherical aberration error signal can be avoided.

  According to the third aspect of the present invention, in particular, when the linear region in which the value of the spherical aberration error signal changes substantially linearly with respect to the amount of spherical aberration is provided in the middle portion of the S-shaped slope, the S-shaped slope is provided. Since spherical aberration outside the linear region is corrected toward the linear region, the amount of spherical aberration for spherical aberration outside the linear region can be quickly transferred from outside the linear region into the linear region. Aberration correction can be reliably performed in the linear region based on normal servo theory.

  An embodiment of the spherical aberration correction apparatus according to the present invention will be described below in detail with reference to FIGS.

  The spherical aberration correction device according to the present invention is a disc-shaped ultra-high density optical disc (Blu Ray Disc) or card-like as an ultra-high density optical recording medium that is currently being developed with a narrower track than CD and DVD. When the spherical aberration occurs between the ultra high density optical recording medium and the optical system in the optical pickup, the beam by the reflected light from the ultra high density optical recording medium is configured. Is detected by an eight-divided photodetector, and spherical aberration is improved while controlling the spherical aberration correcting means based on the spherical aberration error signal obtained by calculating each detection signal from the eight-divided photodetector. In particular, the spherical aberration error signal can be detected more easily by using only four outer peripheral light receiving regions formed in the eight-divided photodetector.

FIG. 1 is a block diagram showing a spherical aberration correcting device according to the present invention.
FIG. 2 is an enlarged plan view showing the internal configuration of the photodetector shown in FIG.
FIG. 3 is a block diagram showing the internal configuration of the spherical aberration correction circuit shown in FIG.
FIG. 4 is a block diagram showing the internal configuration of the focus / tracking control circuit shown in FIG.

  As shown in FIG. 1, an optical pickup 20 for recording or reproducing an ultra high density optical disc (Blu Ray Disc) 11 is provided in the spherical aberration correction apparatus 10 according to the present invention in the radial direction of the ultra high density optical disc 11. It is provided to be freely movable.

  At this time, the ultra-high density optical disc 11 has a disc substrate thickness t between the beam incident surface 11a and the signal surface 11b set to be approximately 0.1 mm, and a reinforcing plate (FIG. (Not shown) to form a total thickness of about 1.2 mm.

  Further, in the optical pickup device 20 described above, a laser light L having a wavelength of 450 nm or less is emitted from a laser light source (hereinafter, referred to as a semiconductor laser) 21 using a semiconductor corresponding to the ultra high density optical disk 11. In the embodiment, the reference wavelength of the laser beam L is set to 405 nm, for example.

  The laser light L emitted from the semiconductor laser 21 is linearly polarized divergent light, and this divergent light is incident on the diffraction grating (grating) 22, and an uneven grating (not shown) formed in the diffraction grating 22. 3) are separated into three beams (hereinafter referred to as 3 beams) consisting of 0th order diffracted light and ± 1st order diffracted light according to the pitch and inclination angle, and then the 3 beams are incident on the polarization beam splitter 23. The

  In this embodiment, three beams are generated by the diffraction grating 22, but there is a configuration in which the diffraction grating 22 is not provided. In this case, the laser beam emitted from the semiconductor laser 21 is polarized with one beam remaining as it is. What is necessary is just to enter into the beam splitter 23 directly.

  The polarizing beam splitter 23 transmits the three beams from the diffraction grating 22 and reflects the reflected light from the ultra-high-density optical disk 11 so as to turn about 90 °. A film 23a is attached.

  Thereafter, the three beams transmitted through the semi-transmissive reflection film 23 a in the polarization beam splitter 23 are converted into parallel light by the collimator lens 24 and are incident on the spherical aberration correction unit 25.

  The spherical aberration correction means 25 described above is for correcting the spherical aberration generated by the optical system disposed between the semiconductor laser 21 and the signal surface 11b of the ultra high density optical disk 11, and is provided on the semiconductor laser 21 side. A concave lens (negative lens) 25A, a convex lens (positive lens) 25B provided on the later-described objective lens 26 side, and an actuator 25C that displaces the convex lens 25B along the optical axis direction. Then, as will be described later, the convex lens 25B is displaced in the optical axis direction with respect to the concave lens 25A by the actuator 25C based on the spherical aberration error signal SAE, and the distance between the concave lens 25A and the convex lens 25B is controlled to enter the objective lens 26. By adjusting the parallelism of the three beams and generating spherical aberration due to the magnification error of the objective lens 26 and canceling it with other spherical aberrations, the spherical aberration is corrected so that it becomes zero. is doing.

  A method of displacing the concave lens (negative lens) 25A with respect to the convex lens 25B in the optical axis direction may be used.

  Further, as the spherical aberration correcting means, the combination of the concave lens 25A, the convex lens 25B and the actuator 25C is used in the embodiment, but instead, a wavefront modulation element using liquid crystal or the like as described in the conventional example is applied. Is also possible.

  Thereafter, the three beams that have passed through the spherical aberration correcting means 25 are incident on an objective lens 26 designed for an ultra-high density optical disk. The objective lens 26 has a numerical aperture (NA) of 0.75 or more corresponding to the ultra-high density optical disc 11, and at least one of the first and second surfaces facing each other is aspheric. It is formed. The objective lens 26 in this embodiment is a single lens having a numerical aperture (NA) of 0.85. At this time, the objective lens 26 is attached to an upper portion in a lens holder (not shown), and the objective lens 26 is controlled on the outer periphery of the lens holder in the focus direction and the tracking direction of the ultra high density optical disc 11. An actuator 27 is attached.

  Then, the three beams incident thereon are narrowed down by the objective lens 26 to obtain a main beam of O-order light and a pair of sub-beams of ± primary light, and the main beam and the pair of sub-beams are beams of the ultra high density optical disc 11. The light is incident from the incident surface 11 a and irradiated onto the signal surface 11 b of the ultra high density optical disk 11.

  Accordingly, the semiconductor laser 21, the diffraction grating 22, the polarization beam splitter 23, the collimator lens 24, the spherical aberration correcting means 25, and the objective lens 26 constitute an outward optical system.

  Thereafter, the reflected light reflected by the signal surface 11b of the ultra-high density optical disk 11 passes through the objective lens 26, the spherical aberration correcting means 25, and the collimator lens 24, and then in the polarization beam splitter 23, contrary to the above. After being reflected by the semi-transmissive reflective film 23a having the above-mentioned polarization property and turned in the direction of approximately 90 °, the light passes through the cylindrical lens 28 and reaches the photodetector 29, thereby constituting a return optical system.

  Then, based on the reflected light from the signal surface 11b of the ultra-high density optical disk 11 by the photodetector 29, the spherical aberration error signal SAE is obtained by the spherical aberration correction circuit 30, and this spherical aberration error signal SAE is converted into spherical aberration correction means. The focus aberration control circuit 40 obtains a focus error signal FE and various tracking error signals (PP, DPP, DPD) and feeds back each signal to the actuator 27 for the objective. The lens 26 is controlled in the focus direction and the tracking direction, and the main data signal RF is appropriately processed by the RF signal processing circuit 60 according to the signal format of the ultra-high density optical disk 11.

  Here, the photodetector 29 constituting the main part of the present invention is an 8-split type light for detecting the main beam MB by the 0th-order diffracted light obtained by the diffraction grating 22, as shown in an enlarged view in FIG. It comprises a detector 29A and a pair of two-divided photodetectors 29B and 29C for detecting the pair of sub-beams SB1 and SB2 by the ± first-order diffracted light obtained by the diffraction grating 22, and is divided into eight. A pair of two-divided photodetectors 29B and 29C are separated from the eight-divided photodetector 29A on the left and right sides of the mold-type photodetector 29A, and are integrally disposed on a semiconductor substrate (not shown).

  The eight-divided photodetector 29A described above forms a single square light-receiving surface in the XY plane formed by the X axis and Y axis orthogonal to each other, and receives the main beam MB having no spherical aberration. When a circular spot is obtained when light is received at the center of the surface, a square that circumscribes the spot of the main beam MB and has the X and Y axes as vertices is formed in the square light receiving surface. The inside of the square is divided into four with the X axis and the Y axis as ridges to form inner circumferential light receiving areas A to D, and further, the X axis and the Y axis are folded outside the inner circumferential light receiving areas A to D. The outer periphery light receiving regions E to H are formed by dividing the region into four.

  The pair of two-divided photodetectors 29B and 29C described above, when the circular sub-beams SB1 and SB2 are respectively irradiated to the center of the light-receiving surface formed in a square shape, The regions I and G are formed in the two-divided photodetector 29B so as to be divided into two along the track direction, and the regions K and L are formed in the two-divided photodetector 29C.

  Next, the spherical aberration error signal SAE, the focus error signal FE, and various tracking error signals (PP, DPP,...) Using the above-described eight-divided photodetector 29A and a pair of two-divided photodetectors 29B and 29C. DPD) and the case of obtaining the main data signal RF will be described below.

  First, a spherical aberration error signal SAE (Spherical Aberration Error) for correcting spherical aberration generated between the ultra high density optical disk 11 and the optical system in the optical pickup 20 via the spherical aberration correcting means 25 (FIG. 1). Is obtained by using the four outer peripheral light receiving areas E to H formed in the eight-divided photodetector 29A, and calculating in the spherical aberration correction circuit 30 shown in FIG. is doing. SAE = (E + G) − (F + H).

  That is, when spherical aberration occurs, as is apparent from FIGS. 6 and 8 described later, the spot shape by the main beam MB in the eight-divided photodetector 29A is in the right diagonal direction (E, A, C, G) side or left diagonal direction (F, B, D, H) side and the spot shape in the inner peripheral light receiving areas A to D with respect to the left and right diagonal directions (± 45 ° direction). Therefore, when the spherical aberration error signal SAE is obtained, only the outer peripheral light receiving areas E to H are used to determine the right diagonal direction (E, G) side and the left diagonal direction (F, H) side. The detected value difference is detected.

  Accordingly, when calculating the spherical aberration error signal SAE, two adders 31 and 32 and one subtractor 33 may be used in the spherical aberration correction circuit 30, so that FIG. As compared with the conventional example, the number of arithmetic units in the spherical aberration correction circuit 30 can be greatly reduced as compared with the conventional example, and the spherical aberration error signal SAE can be easily obtained. At the same time, it can contribute to cost reduction.

  Next, when a focus error signal FE (Focus Error) for controlling the objective lens 26 (FIG. 1) in the focus direction is obtained, the inner circumference of the eight-divided photodetector 29A is obtained by a known astigmatism method. Using the light receiving areas A to D and the outer periphery light receiving areas E to H, calculation is performed by the focus circuit unit 40A in the focus / tracking control circuit 40 shown in FIG. FE = (A + C + E + G) − (B + D + F + H).

  That is, when the focus error signal FE is obtained, the right diagonal direction (E, A, C, G) side and the left diagonal direction (F, B, D, H) side of the spot shape by the main beam MB are obtained. The detected value difference is calculated by six adders 41 to 46 and one subtractor 47 provided in the focus circuit unit 40A.

  Next, in order to control the objective lens 26 (FIG. 1) in the tracking direction, any one of the following various tracking error signals can be selectively applied here. As the various tracking error signals described above, there are a push-pull signal PP (Push Pull), a differential push-pull signal DPP (Differential Push Pull), and a phase difference signal DPD ((Differential Phase Detection). In order to calculate a signal, an eight-divided photodetector 29A and a pair of two-divided photodetectors 29B and 29B are used.

When obtaining the tracking error signal by the push-pull signal PP described above, the inner peripheral light receiving areas A to D and the outer peripheral light receiving areas E to H in the eight-divided photodetector 29A are used by a known astigmatism method. Based on the following equation, the calculation is performed by the tracking circuit unit 40B in the focus / tracking control circuit 40 shown in FIG.
FE = (A + D + E + H) − (B + C + F + G ).

  That is, when the tracking error signal is determined by the push-pull signal PP, the detected values on the upper (E, A, D, H) side and the lower (F, B, C, G) side of the spot shape by the main beam MB. The difference is calculated by six adders 48 to 53 and one subtractor 54 provided in the tracking circuit unit 40B. At this time, since the tracking error signal based on the push-pull signal PP uses only the eight-divided photodetector 29A, the tracking error signal can also be applied to a one-beam method in which the diffraction grating 22 (FIG. 1) is not provided.

On the other hand, when obtaining the tracking error signal by the differential push-pull signal DPP, the result of the push-pull signal PP and the pair of two-divided photodetectors 29B and 29C are used. 22 (FIG. 1) is suitable for the three-beam system according to the configuration of the embodiment, and the tracking circuit unit 40B (FIG. 3) in the focus / tracking control circuit 40 performs calculation based on the following formula, Again, the circuit configuration is not shown.
DPP = PP−α × {(I + K) − (J + L)} where α is a coefficient.

  Further, when obtaining the tracking error signal based on the phase difference signal DPD described above, the tracking circuit unit 40B (see FIG. 5) in the focus / tracking control circuit 40 is used based on the following formula using only the 8-divided photodetector 29A. Although the calculation is performed in 3), the circuit configuration is not shown here. DPD = the phase difference between (A + E) + (C + G) and (B + F) + (D + H).

  Since the tracking error signal based on the phase difference signal DPD also uses only the eight-divided photodetector 29A, the tracking error signal can be applied to a one-beam method in which the diffraction grating 22 (FIG. 1) is not provided.

  Next, when the main data signal RF is obtained, it is shown in FIG. 1 based on the following equation using the inner periphery light receiving areas A to D and the outer periphery light receiving areas E to H in the eight-divided photodetector 29A. Although the calculation is performed in the RF signal processing circuit 60, the circuit configuration is not shown here. RF = A + B + C + D + E + F + G + H.

  Next, the spherical aberration will be described more specifically with reference to FIGS. 1 and 2 used earlier and new FIGS.

FIG. 5 is a diagram showing the focus error dependency on the change in the spot shape of the main beam received by the eight-divided photodetector when there is no spherical aberration.
FIG. 6 is a graph showing the focus error dependency on the change in the spot shape of the main beam received by the 8-split type photodetector when there is spherical aberration.
FIG. 7 is a diagram showing the dependence of the focus position on the focus error signal and the spherical aberration error signal when there is no offset in the focus error signal.
FIG. 8 shows that when the ultra-high density optical disc is at the best focus point, the spherical aberration is overcorrected with respect to the main beam received by the eight-divided photodetector, the case where there is no spherical aberration, and the case where the spherical aberration is corrected. Figure showing under case and
FIG. 9 shows the dependence of the focus position on the focus error signal, when the spherical aberration error signal is overcorrected, and when the spherical aberration error signal is undercorrected when the spherical aberration amount is 0.05λ · rms. Figure showing
FIG. 10 is a diagram showing a spherical aberration error signal with respect to the spherical aberration amount.

  First, in FIGS. 5A to 5C, an 8-division type is shown in the case where the objective lens 26 provided in the optical pickup 20 is designed for the ultra high density optical disk 11 and there is no spherical aberration. Changes in the spot shape of the main beam MB received by the photodetector 29A were calculated by the ray tracing method, and focus error dependency was examined.

  Here, as shown in FIG. 5B, when the objective lens 26 is at the best focus point with respect to the ultra-high density optical disc 11, the spot by the main beam MB on the center of the eight-divided photodetector 29A. Becomes substantially circular. On the other hand, as shown in FIG. 5A, when the ultra high density optical disk 11 is close to the 1 μm objective lens 26 with respect to the focal point of the objective lens 26, or as shown in FIG. When the ultra-high density optical disk 11 is far from the 1 μm objective lens 26 with respect to the focal point of the lens 26, the direction of the focal line of astigmatism due to the main beam MB on the eight-divided photodetector 29A is the left diagonal direction ( The focus error signal FE is detected by changing in the −45 ° direction) or the right diagonal direction (+ 45 ° direction). Astigmatism is given in the direction of 45 °.

  Next, FIGS. 6A to 6C show the case where the objective lens 26 provided in the optical pickup 20 is designed for the ultra high density optical disc 11 and has spherical aberration. As in the case of FIGS. 5A to 5C, the change in the spot shape of the main beam MB received by the eight-divided photodetector 29A was calculated by the ray tracing method, and the focus error dependency was examined.

  In this case, when the wavelength λ of the laser beam L is 405 nm, the spherical aberration amount is 0.05λ · rms. This amount of spherical aberration corresponds to the case where the disk substrate thickness error Δt is about 5 μm when the disk substrate thickness t of the ultra-high density optical disk 11 is set to be approximately 0.1 mm. Here, as will be described later, the objective lens 26 is swung in the focus direction in a state where the correction amount of the spherical aberration is under (undercorrected), and as shown in FIG. When the focus is at the best focus point with respect to the optical disk 11, the spot by the main beam MB extends in the right diagonal direction on the center of the eight-divided photodetector 29A. become. On the other hand, as shown in FIG. 6A, when the ultra high density optical disk 11 is close to the 1 μm objective lens 26 with respect to the focal point of the objective lens 26, or as shown in FIG. When the ultra high density optical disk 11 is far from the 1 μm objective lens 26 with respect to the focal point of the lens 26, if there is spherical aberration, the spot due to the main beam MB is left as shown in FIG. 5 (a) or FIG. 5 (c). Although it is the same direction as the diagonal direction (−45 ° direction) or the right diagonal direction (+ 45 ° direction), it has a bulging shape with respect to FIG. 5A or FIG.

In FIG. 7, when the focus error signal FE has no offset, the dependency of the focus position on the focus error signal FE and the spherical aberration error signal SAE is calculated and evaluated. Here is counted by calculating the number of rays entering each region A~H of 8 division type optical detector in 29A, the abscissa indicates the off O carcass error ([mu] m), the vertical axis represents a focus error signal FE And the spherical aberration error signal SAE, the difference in the number of rays (error output) corresponding to the subtraction results of the above-described equations when calculating the spherical aberration error signal SAE. As can be seen from FIG. 7, the occurrence of the offset to the focus error signal FE is very slight despite the change in the spot shape of the main beam MB irradiated onto the 8-split photodetector 29A. , it was found that sometimes the spherical aberration error signal SAE also become zero of the focus error signal FE is zero, a problem in the detection of the focus error signal FE does not occur.

  Next, FIGS. 8A to 8C show a case where the spherical aberration is overcorrected (overcorrected), a case where there is no spherical aberration, and a spherical surface when the ultrahigh density optical disk 11 is at the best focus point. A change in the spot shape of the main beam MB received on the eight-divided photodetector 29A when the aberration is under-corrected (undercorrected) is shown. Also in this case, when the wavelength of the laser beam is λ = 405 nm, the spherical aberration amount is 0.05λ · rms. This amount of spherical aberration corresponds to the case where the disk substrate thickness error Δt is about 5 μm when the disk substrate thickness t of the ultra-high density optical disk 11 is set to be approximately 0.1 mm.

  As is apparent from FIGS. 8A to 8C, it can be seen that due to the presence of spherical aberration, the spot shape changes in the direction of the astigmatism focal line due to the main beam MB. Furthermore, the direction of change of the spot shape of the main beam MB may change in a diagonal direction, which is the direction of astigmatism, depending on the polarity of whether the spherical aberration correction is over or under spherical aberration correction. Recognize.

  As described above, the spot shape of the main beam MB on the eight-divided photodetector 29A is the above-described spherical aberration correction overrun by the calculation of the spherical aberration error signal SAE in the spherical aberration correction circuit 30 shown in FIG. It is qualitatively shown to detect a change in polarity as to whether or not the spherical aberration correction is under.

  Next, FIG. 9 shows a case where the focus error signal FE and the spherical aberration error signal SAE are overcorrected and a case where the spherical aberration error signal SAE is undercorrected when the spherical aberration amount is 0.05λ · rms. The dependence of the focus position is shown. The calculation here differs only in the amount of spherical aberration under the same conditions as in FIG. According to FIG. 9, the focus error signal FE is zero when there is no defocus, and when the spherical aberration error signal SAE is overcorrected at the point where the focus error signal FE is zero, a positive output is obtained. On the other hand, when the spherical aberration error signal SAE is under-corrected, a negative output is generated.

  Next, FIG. 10 shows a calculation result of the dependence of the spherical aberration error signal SAE on the spherical aberration amount. The horizontal axis is the spherical aberration amount λ · rms value. For convenience, when the spherical aberration is undercorrected, it is displayed as a negative value, while when the spherical aberration is overcorrected, it is displayed as a positive value. ing. The vertical axis indicates the spherical aberration error signal SAE (relative value).

  As is apparent from FIG. 10, the value of the spherical aberration error signal SAE has a linear region that changes substantially linearly with respect to the amount of spherical aberration (spherical aberration amount) in the middle portion of the S-shaped slope. At the same time, the value of the spherical aberration error signal SAE is saturated with respect to the amount of spherical aberration near both ends of the S-shaped slope, and the value of the spherical aberration error signal SAE decreases when the amount of spherical aberration subsequently increases to the + side. On the other hand, when the amount of spherical aberration increases toward the negative side, the value of the spherical aberration error signal SAE increases. That is, in the S-shaped slope of the spherical aberration error signal SAE, the spherical aberration error signal SAE has a positive (plus) slope when the spherical aberration amount is in the range of approximately −0.1λ · rms to + approximately 1.1λ · rms. When the spherical aberration amount increases on the ± side beyond the linear region, the value of the spherical aberration error signal is saturated, and the larger spherical aberration amount is in the linear outer region. The spherical aberration error signal is inverted to have a negative (minus) slope. At this time, it can be seen that the offset of the focus error signal FE does not occur within the linear region in the S-shaped slope of the spherical aberration error signal SAE.

  When the signal surface 11b is a single-layer ultrahigh density optical disk 11, the amount of spherical aberration is sufficiently small in the linear region in the S-shaped slope of the spherical aberration error signal SAE, but the illustration is omitted here. However, in the case of a two-layer type ultra-high density optical disk or a multilayer type ultra-high density optical disk, spherical aberration outside the linear region occurs. When the spherical aberration error signal SAE is outside the linear region, the spherical aberration is corrected via the spherical correction circuit 30 and the spherical aberration correction means 25 (FIG. 1) simply according to the S-shaped slope of the spherical aberration error signal SAE. When trying to control, there is a concern that control may not be successful due to oscillation. This can be solved, for example, as described in the following (a) and (b).

  (A). As described above, when the linear region in which the value of the spherical aberration error signal SAE changes substantially linearly with respect to the amount of spherical aberration (spherical aberration amount) is provided in the middle portion of the S-shaped slope, A spherical aberration other than the linear region is corrected toward the linear region. Specifically, when the value of the spherical aberration error signal SAE is positive (plus) with respect to the amount of spherical aberration and is in a negative slope area in the S-shaped slope, it moves toward a linear area having a positive slope. A linear region that corrects spherical aberration and has a positive slope when the value of the spherical aberration error signal SAE is negative (minus) with respect to the amount of spherical aberration and is in a negative slope region in the S-shaped slope. The spherical aberration is corrected toward. Thereby, the amount of spherical aberration can be quickly transferred from outside the linear region to inside the linear region. Then, after entering the linear region in the S-shaped slope, the spherical aberration can be reliably corrected in the linear region based on normal servo theory. In this case, it is necessary to determine that the linear region has been entered, but whether to use the zero cross of the spherical aberration error signal SAE, or to set and determine a threshold value for the spherical aberration error signal SAE, or As long as the amplitude of the main data signal RF increases, there is a determination method such as ignoring the S-shaped slope of the spherical aberration error signal SAE.

  (B). As another countermeasure, there is a method of performing fine adjustment after detecting the linear region by largely changing the correction amount of the spherical aberration as a rough adjustment first when correcting the spherical aberration. In the case of this method, since the fluctuation of the spherical aberration in the ultra high density optical disk 11 can be expected to be smaller than that in the linear region, one coarse adjustment is sufficient when the ultra high density disk 11 is mounted once.

  Next, in the spherical aberration correcting device 10 according to the present invention, FIG. 11 shows a modified example in which only the area division form is modified with respect to the eight-divided photodetector 29A in the photodetector 29 provided in the optical pickup 20. This will be briefly described.

  FIG. 11 is an enlarged plan view showing a modified example in which only the area division mode is modified with respect to the eight-divided photodetector in the photodetector.

  11 (a) and 11 (b) show a modified example in which only the area division mode is modified with respect to the 8-split type photodetector 29A provided in the photodetector 29 described with reference to FIG. Eight-divided photodetectors 29A ′ and 29A ″ are shown. 2 is the same as that of the previous embodiment, the illustration of the pair of two-divided photodetectors 29B and 29C is omitted here.

  First, the eight-divided photodetector 29A ′ of the modification shown in FIG. 11A forms one square light receiving surface in the XY plane formed by the X axis and Y axis orthogonal to each other. In addition, when a circular spot is obtained when the main beam MB having no spherical aberration is received at the center of the light receiving surface, an area slightly smaller than the spot diameter (spot diameter) inside the spot of the main beam MB. The inner circumference light receiving areas A ′ to D ′ are formed by dividing the inside of this circle into four with the X axis and the Y axis as the ridges. The outer peripheral light receiving areas E ′ to H ′ are formed by dividing the X axis and the Y axis into four parts on the outer side of D ′. At this time, a pair of signal line extraction regions 29a and 29b for extracting detection signals to the left and right on the X axis are formed narrow.

  Next, the eight-divided photodetector 29A ″ of the modification shown in FIG. 11B has a single square light receiving surface in the XY plane formed by the X axis and the Y axis orthogonal to each other. In the case where a circular spot is obtained when the main beam MB formed and having no spherical aberration is received at the center of the light receiving surface, the area inside the spot of the main beam MB is slightly smaller than the spot diameter. In addition, a square having the X axis and the Y axis as vertices is formed, and the inside of the square is divided into four with the X axis and the Y axis as ridges to form inner peripheral light receiving areas A ″ to D ″. Further, the outer periphery light receiving areas E ″ to H ″ are formed by dividing the X axis and the Y axis into four parts on the outside of the inner periphery light receiving areas A ″ to D ″. Here too, a pair of signal line lead-out regions 29a and 29b for pulling out detection signals to the left and right on the X axis are formed narrow.

According to the modified 8-divided photodetector 29 A ′ or 8-divided photodetector 29 A ″ formed as described above, a circular area slightly smaller than the spot diameter inside the spot of the main beam MB. or by forming the four divided inner light receiving regions a'-d 'or the inner circumferential receiving regions A''~D''square, 8 split photodetector 29 a' or in 8 divided photodetector When the spherical aberration error signal SAE is calculated using only the outer peripheral light receiving areas E ′ to H ′ or the outer peripheral light receiving areas E ″ to H ″ formed in 29A ″, the spot of the main beam MB and the inner peripheral light receiving are calculated. Noise is subtracted in a state where there is a slight amount of light in the area between the areas A ′ to D ′ or the inner peripheral light receiving areas A ″ to D ″, and the subtraction value in this area becomes zero. To obtain a spherical aberration error signal SAE with no distortion and avoid a decrease in the S / N ratio of the spherical aberration error signal SAE. be able to. In addition, by dividing the area within the 8-split photodetector 29 A ′ or the 8-split photodetector 29 A ″ as described above, the spot shape of the main beam MB becomes an error of an actual optical system (for example, When it changes due to the focal length of the detection system lens, etc., it is possible to prevent the spot diameter of the main beam MB from being smaller than that of the inner peripheral light receiving region, thereby preventing a dead zone from being generated when spherical aberration is detected.

It is the block diagram which showed the spherical aberration correction apparatus which concerns on this invention. It is the top view which expanded and showed the internal structure of the photodetector shown in FIG. FIG. 2 is a block diagram showing an internal configuration of the spherical aberration correction circuit shown in FIG. 1. FIG. 2 is a block diagram showing an internal configuration of a focus / tracking control circuit shown in FIG. 1. FIG. 6 is a diagram showing a focus error dependency on a change in the spot shape of a main beam received by an 8-divided photodetector when there is no spherical aberration. It is the figure which showed the focus error dependence with respect to the change of the spot shape of the main beam received with the 8-part dividing type photodetector, when there exists spherical aberration. FIG. 10 is a diagram showing the dependence of the focus position on the focus error signal and the spherical aberration error signal when there is no offset in the focus error signal. When an ultra-high-density optical disk is at the best focus point, when the spherical aberration is overcorrected, when there is no spherical aberration, or when the spherical aberration is undercorrected with respect to the main beam received by the 8-segment photodetector FIG. When the spherical aberration amount is 0.05λ · rms, the dependence of the focus position on the focus error signal, the case where the spherical aberration error signal is overcorrected, and the case where the spherical aberration error signal is undercorrected are shown. FIG. It is the figure which showed the spherical aberration error signal with respect to the amount of spherical aberration. It is the top view which expanded and showed the modification which deform | transformed only the area | region division form with respect to the 8-part dividing type photodetector in a photodetector. It is the block diagram which showed schematic structure of the conventional information recording / reproducing apparatus. It is a figure for demonstrating the division | segmentation form of the 8-part type | mold photodetector provided in the optical pick-up in the conventional information recording / reproducing apparatus. It is the block diagram which showed the detailed structure of the signal processing part shown in FIG.

Explanation of symbols

10 ... Spherical aberration correction device,
11 ... Ultra high density optical disk, 11a ... Beam incident surface, 11b ... Signal surface,
20 ... optical pickup,
21 ... Laser light source (semiconductor laser), 22 ... Diffraction grating (grating),
23 ... Polarizing beam splitter, 24 ... Collimator lens,
25 ... spherical aberration correction means, 25A ... concave lens (negative lens),
25B ... convex lens (positive lens), 25C ... actuator,
26 ... Objective lens, 27 ... Actuator, 28 ... Cylindrical lens,
29 ... photodetector
29A, 29A ′ , 29A ″... 8 split type photodetector,
A to D, A ′ to D ′, A ″ to D ″...
E to H, E ′ to H ′, E ″ to H ″, inner peripheral region,
29B, 29C ... A pair of two-divided photodetectors,
30: Spherical aberration correction circuit,
40. Focus / tracking control circuit,
40A: Focus circuit unit, 40B: Tracking circuit unit,
60 ... RF signal processing circuit,
FE: Focus error signal, SAE: Spherical aberration error signal,
PP ... push-pull signal, DPP ... differential push-pull signal, DPD ... phase difference signal,
RF: Main data signal,
L: Laser light, MB: Main beam, SB1, SB2: A pair of sub beams.

Claims (3)

A laser light source that emits a laser beam having a wavelength corresponding to an optical recording medium, and a spherical surface that is incident between the laser light source and the optical recording medium with respect to the laser light that is incident on the laser light from the laser light source A spherical aberration correcting unit that controls to correct aberrations; an objective lens that irradiates a signal surface of the optical recording medium with a laser beam obtained by narrowing down the laser beam that has passed through the spherical aberration correcting unit; and the optical recording A photodetector that has a plurality of light receiving areas therein to detect reflected light reflected from the signal surface of the medium, and a focus error from each detection signal detected in the plurality of light receiving areas provided in the light detector. a focus error circuit for generating the operation signal, the operation of the spherical aberration error signal from the detection signals detected by the plurality of light receiving regions provided in the photodetector In the spherical aberration correcting device having at least a spherical aberration correcting circuit for feeding back the spherical aberration error signal in the spherical aberration correction means form,
The plurality of light receiving regions provided in the photodetector are divided into an inner peripheral light receiving region obtained by dividing the inner periphery of one light receiving surface that receives the beam of reflected light into four and an outer periphery of the inner peripheral light receiving region. It consists of a peripheral light receiving area,
The focus error circuit calculates the focus error signal by using the detection signals from the inner and outer light receiving areas.
The spherical aberration correction circuit, wherein the spherical aberration correction circuit calculates the spherical aberration error signal using only each detection signal from the outer periphery light receiving region. The spherical aberration correction apparatus according to claim 1,
A spherical aberration correction device, wherein the inner peripheral light receiving region in the photodetector is formed inside a spot diameter obtained when the beam having no spherical aberration is received. In the spherical aberration correcting device according to claim 1 or 2,
The spherical surface other than the linear region in the S-shaped slope has a linear region where the value of the spherical aberration error signal changes substantially linearly with respect to the amount of the spherical aberration in the middle portion of the S-shaped slope. A spherical aberration corrector for correcting aberration toward the linear region.

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