JP4462298B2 - Optical head and optical disk apparatus - Google Patents

Description

  The present invention relates to an optical head and an optical disc apparatus, and more particularly to a technique for correcting a substrate thickness deviation and spherical aberration in a high NA dual objective lens.

  In recent years, the density of optical disks has been increasing, and 4.7 GB DVD-ROMs have been put on the market as compared to 0.65 GB CD-ROMs that are consumer-use reproduction-only optical disks. As a recordable large-capacity optical disk, a 2.6 GB DVD-RAM has already been put into practical use, and a large capacity version of 4.7 GB capacity is expected to be released in the first half of 2000. Such recordable DVDs are used not only as storage media for computers but also for video image recording that does not require rewinding or fast-forwarding. The video recorder used has been released. Video recorders using DVD-RAM are also expected to be supported from the 4.7 GB capacity version, and the market is highly expected in terms of compatibility with CDs and DVD-ROMs. However, the capacity is not limited to 4.7 GB, and with the future digitization of satellite broadcasting, it is desired to increase the capacity of 20 GB capable of recording high-definition moving images for 2 hours.

  The recording density of the optical disk is substantially limited by the size of the light spot λ / NA (λ: light wavelength, NA: objective lens numerical aperture) for recording and reproduction. Therefore, in order to increase the capacity, it is necessary to shorten the wavelength or increase the numerical aperture. In recent years, blue-violet semiconductor lasers with a wavelength of 410 nm have been developed. Since the current 4.7 GB DVD has a wavelength of 650 nm, by using only a blue-violet semiconductor laser, in principle, the capacity of about 12 GB can be realized by approximately 2.5 times the square of these wavelength ratios. . However, to further increase this to 20 GB, it is necessary to increase NA to 1.3 times, that is, 0.78 against 0.6 of the current DVD.

  As a conventional technique for increasing the NA in this way, for example, there is JP-A-11-195229 (first conventional example). Here, the NA is increased up to a maximum of 0.85 using two objective lenses in two groups. At this time, when the NA increases, there is a problem that aberrations caused by an optical system shift and a disc substrate thickness and tilt error increase. On the other hand, in the above conventional example, the thickness of the substrate is reduced to 0.1 mm in order to reduce the coma aberration caused by the disc tilt, and the spherical aberration caused by the substrate thickness error is reduced to the surface of the disc. The thickness of the substrate is detected from the difference in the defocus signal on the recording surface, and based on this, the distance between the two lenses is changed to compensate for spherical aberration.

  Still another conventional example is Japanese Patent Laid-Open No. 2000-057616 (second conventional example). Here, as described above, the control signal for compensating for the spherical aberration is detected by the difference signal of the defocus signal based on the astigmatism method which is detected by separating the inside and the outside of the light spot on the photodetector. . At this time, the sum signal is used as a defocus signal.

  However, in the first conventional example, the spherical aberration is detected by detecting the substrate thickness from the defocus signal from the surface and the recording film surface. In this case, the spherical aberration is directly detected. However, there is a problem that errors are easily generated due to the influence of the deviation of the refractive index of the substrate, the deviation of the photodetector, and the like, and the control is difficult.

  Further, in the second conventional example, as will be described in detail later, there is a problem that the defocus signal waveform is greatly deteriorated due to the spherical aberration itself, and the defocus range where the spherical aberration can be detected is narrow. Further, the offset of the defocus signal due to spherical aberration is large.

JP-A-11-195229 JP2000-057616

  In view of the above problems, the object of the present invention is to stably detect and correct spherical aberration due to deviations in the substrate thickness and optical system, and to correct this, and to detect a defocus signal with a small offset. It is another object of the present invention to provide an optical disc apparatus for recording and reproducing an optical disc.

  The optical head of the present invention for solving the above problems basically includes a semiconductor laser, an optical system for condensing the light on an optical disk, and a variable focus mechanism for changing the focal position of the collected light. A spherical aberration adding mechanism for adding variable spherical aberration to the collected light, a light branching element for branching the reflected light from the optical disk from the optical path from the semiconductor laser to the optical disk, and collecting the branched reflected light And a light receiving element that receives the light collected by the lens and converts it into an electrical signal.

  At this time, the reflected light branched by the light branching element is further split into a first light flux near the optical axis and a second light flux near the optical axis and branched so as to be condensed on the light receiving element. Add elements. This optical branching element is substantially a hologram. As the spherical aberration adding mechanism, an electrostatic actuator that can change the distance between two objective lenses in two groups or a liquid crystal filter that electrically controls the phase of transmitted light is used. A sum signal of these two defocus signals is used as a defocus signal. This is used to control the variable focus mechanism. The variable focus mechanism uses an electrostatic actuator that is substantially mounted with an objective lens and is movable.

  At this time, the optical system can be simplified by integrating the first and second optical branching elements.

  Moreover, the loss of light quantity can be suppressed by using the integrated light branching element as a polarization hologram.

  The first optical branching element is a polarizing element, the spherical aberration adding mechanism is a liquid crystal element, and the liquid crystal element is disposed between the semiconductor laser and the first optical branching element, thereby reducing the size of the optical head. By using the liquid crystal element only for the forward optical system, it is possible to suppress the light amount loss. Here, the polarizing element refers to an optical element that is dependent on incident polarization with respect to a branched light amount ratio, such as a polarizing beam splitter and a polarizing diffraction grating.

  In addition, the first optical branching element is a non-polarizing element, and the spherical aberration adding mechanism is a liquid crystal element. The first optical branching element is disposed between the first optical branching element and the objective lens. Since the spherical aberration acts in the reciprocating optical path, it is possible to avoid the influence of the spherical aberration offset described later. Here, the non-polarizing element refers to an optical element having no dependence on incident polarization with respect to the branched light quantity ratio, such as a non-polarizing beam splitter or a non-polarizing diffraction grating.

  In addition, in the optical element between the first optical branching element and the photodetector that does not affect the optical system from the semiconductor laser to the objective lens, detection can be performed by not arranging an optical element that generates spherical aberration such as a lens. It is possible to prevent an offset from occurring between the spherical aberration to be compensated and the spherical aberration on the optical disc surface to be compensated.

  Further, by fixing the objective lens and the spherical aberration adding mechanism as one body, it is possible to eliminate the influence of the axial deviation of the spherical aberration correction that occurs due to the lens eccentricity accompanying the tracking control.

  In addition, the effective light beam system of the objective lens is 1 mm or less, and the semiconductor laser, the spherical aberration adding mechanism, the first and second light branching elements, the objective lens, and the photodetector are fixed together and mounted on the variable focus mechanism. The optical head can be miniaturized and the influence of the axial deviation caused by the spherical aberration correction caused by the lens decentering accompanying the tracking control can be eliminated. In the case of an optical disk having a substrate thickness of 0.1 mm, a working distance of 0.1 mm or more can be secured with one lens even if the effective diameter of the objective lens is 1 mm. The reason why the effective light beam system of the objective lens is 1 mm or less will be described with reference to FIG. FIG. 33 shows a condition where NA is 0.85, the disk substrate thickness is 0.1 mm, the substrate refractive index is 1.62, the objective lens refractive index is 1.8, and the radius of curvature of the first surface of the single lens is 1/2 of the beam diameter. It is the calculation result of the working distance with respect to the effective diameter. Here, based on Yoshiya Matsui's “Lens Design Method” (Kyoritsu Shuppan, 1989, first edition, 7th edition), the working distance is analyzed analytically at the lens thickness where the value of spherical aberration derived analytically from aberration theory is minimized. Asked. The condition that the radius of curvature of the first surface is ½ of the light beam diameter is the last condition for geometrically forming a lens, but since it is actually an aspherical shape, if it is in this vicinity, the lens Holds. As a result, it can be seen that even with an NA of 0.85 and a substrate thickness of 0.1 mm, an effective diameter of 1 mm and a working distance of about 0.1 mm can be secured. This is almost equivalent to the working distance of 0.13 mm of the double-lens lens having an effective diameter of 3 mm shown in FIG.

  When the spherical aberration adding mechanism and the objective lens are not integrated, when the objective lens moves in the radial direction for tracking control, the axis of the spherical aberration generated by the spherical aberration adding mechanism is the optical axis of the objective lens. This effectively shifts coma aberration. This can be compensated for by adding a coma aberration adding mechanism. Further, when the spherical aberration adding mechanism and the objective lens are integrated, it is effective for coma aberration that occurs when the optical disk substrate is tilted.

  Further, when the optical head is configured in a small size, assembly adjustment can be facilitated by forming the semiconductor laser chip integrally on the substrate of the photodetector.

  Further, when the track pitch of the optical disk is fine, the offset due to the movement of the objective lens accompanying the tracking control can be canceled by combining the spherical aberration detection compensation with the tracking by the differential push-pull method. In this case, a diffraction grating that diffracts the inner and outer sides of the light beam in different directions is provided in the light beam directed toward the objective lens, and the outer light beam is diffracted in a substantially tangential direction of the disk and the inner light beam is diffracted in a substantially radial direction. . In particular, the outer light beam is arranged so as to be substantially shifted by 1/2 of the period of the guide groove or pit row of the optical disk on both sides of the zero-order light that is not diffracted. Here, “substantially” means that the deviation of the ± first-order light that is diffracted from the 0th-order light is substantially the same whether it is a ½ period or a (n + ½) period where n is an integer. Even if the orientation of the diffraction grating is adjusted with the intention of 1/2 period, the effect is intended to be substantially the same in an error range (within ± 1/8 period) that does not affect the signal. Yes. Similarly, the meaning of “substantially tangential direction” means the tangential direction including the above “substantial” deviation.

  By doing so, a tracking signal whose polarity is reversed from that of the 0th order light can be obtained from the outer light flux while the offset due to the lens movement remains the same as that of the 0th order light. Accordingly, by taking these differential outputs, it is possible to obtain a tracking signal in which the offset is canceled. If the track pitch is fine, the inner beam will not be affected by the interference of diffracted light. Therefore, the diffraction angle of the diffracted light can be easily suppressed below the allowable range of the angle of view of the objective lens by disposing it in a substantially radial direction. be able to. The meaning of “substantially in the radial direction” here is that the influence of interference by the diffracted light does not appear in the inner light beam as described above, and the relative position to the guide groove is almost irrelevant, so that it is separated from the outer light beam at the time of detection. Is within the possible range.

  When the inner and outer light beams are separated in the reflected light path from the disk, the number of signal output lines from the photodetector can be reduced by using the astigmatism method as the focus detection method. In this case, a pattern that adds astigmatism to the diffraction grating for separating the inside and outside of the light beam may be used.

  An optical disc apparatus for solving the above-described problems is composed of at least an optical circuit and an arithmetic circuit for obtaining a reproduction signal and a defocus signal from an electric signal of the light receiving element. Then, the first and second defocus signals are detected independently for each of the first light flux and the second light flux, and a signal substantially proportional to the spherical aberration is obtained from the difference signal. Using this, the spherical aberration addition mechanism is controlled to reduce the spherical aberration of the focused spot. Here, the term “substantially” means the calculation of the spherical aberration detection signal because the order of the circuit calculation is that the first beam defocus signal and the second beam defocus signal are: Not only the order in which each difference is calculated independently but then the difference signal is calculated, but also other orders in which the results of the calculations are substantially equivalent (for example, contribute to the + polarity of the result) And the like, after adding all of the components and the components that contribute to the negative polarity, the difference between them is included.

  In such an optical disc apparatus, when a liquid crystal element or the like is used in an optical system combining a polarization beam splitter and a λ / 4 plate, a phase difference is added by the liquid crystal element only to a linearly polarized light component in one direction. The spherical aberration acts only on the light beam traveling in the outward direction of the disk. Here, the λ / 4 plate has two optical axes orthogonal to each other so that incident linearly polarized light can be transmitted as linearly polarized light in a plane perpendicular to the optical axis. An optical element that gives a phase difference of a quarter wavelength to incident light of two orthogonally polarized light polarized in directions. Such an optical element transmits linearly polarized light parallel to the two optical axes when it is incident with the same amplitude and in the same phase, that is, when linearly polarized light inclined by 45 ° with respect to the optical axis is incident. There is an effect of converting light into circularly polarized light. This is because when light directed to the disk passes through the λ / 4 plate, it becomes circularly polarized light, and when the light beam reflected back from the disk passes through the λ / 4 plate again, it becomes linearly polarized light with a polarization direction orthogonal to the forward path. . However, since the spherical aberration detected by the light detector and calculated by the calculation reflects the spherical aberration added by both the forward path and the return path, when this signal is fed back to the spherical aberration compensation mechanism as it is, a round-trip spherical surface is obtained. Since the aberration is compensated only in the forward optical path and is controlled so that there is no spherical aberration on the photodetector, the spherical aberration for reciprocation is the same as the spot on the optical disk where only one-way spherical aberration was originally added. Since the compensation is made only in the forward path, the spherical aberration remains with the opposite sign by the same amount as the original spherical aberration. Therefore, in order to prevent this, the feedback system is configured so that the spherical aberration on the disk surface becomes zero. The word “substantially” here means that, as already mentioned, the order of circuit operations also includes other sequences of operations in which the results of the operations are substantially equivalent. .

  For example, for this purpose, a loop for feeding back a drive signal for electrically driving the spherical aberration adding mechanism to a system for amplifying the spherical aberration error is provided.

  Furthermore, in these optical disk apparatuses, only the spherical aberration on the disk surface is compensated. Therefore, the spherical aberration on the return path is not compensated, and in this case, an offset occurs in the defocus signal. In order to compensate for this, in order to compensate for the offset of the defocus signal in accordance with the detected spherical aberration signal, the spherical aberration signal is multiplied by an appropriate coefficient and added to the drive signal of the variable focus mechanism. Drives the variable focus mechanism.

  If the spherical aberration adding mechanism and the objective lens are not integrated, as described above, the axis of the spherical aberration generated by the spherical aberration adding mechanism when the objective lens moves in the radial direction for tracking control. Deviates from the optical axis of the objective lens and effectively produces coma. In an optical disk apparatus using an optical head having a coma aberration adding mechanism, the amount of movement of the objective lens is detected to compensate for this, and the coma aberration adding mechanism is driven using this.

  Further, for example, in order to detect the amount of movement of the objective lens, an imbalance in the disk radial direction of the light beam near the branched optical axis is detected. When the track pitch of the disk is sufficiently narrow, the diffracted light by the guide groove of the disk is biased to the peripheral part of the light beam, so that the light beam near the optical axis is not affected by the interference of the diffracted light. Therefore, the movement of the objective lens in the radial direction accompanying tracking can be detected by independently detecting the light in this region in the two regions divided by the diameter in the disk tangential direction and obtaining the difference between them.

  In addition, when the track pitch of the optical disk is fine (that is, when the numerical aperture NA of the objective lens is less than the track pitch λ / NA μm with respect to the wavelength λ), spherical aberration detection compensation is performed by a differential push-pull tracking. By combining with, offset due to the movement of the objective lens accompanying tracking control can be canceled. In this case, as described above, a diffraction grating that diffracts the inner side and the outer side of the beam in different directions is provided in the beam toward the objective lens, the outer beam is in the tangential direction of the disk, and the inner beam is a radius. An optical head that diffracts in the direction is used. In particular, the outer luminous flux is arranged so as to be shifted by a half of the period of the guide groove or pit row of the optical disc on both sides of the zero-order light that is not diffracted. By doing so, a tracking signal whose polarity is reversed from that of the 0th order light can be obtained from the outer light flux while the offset due to the lens movement remains the same as that of the 0th order light. Accordingly, by taking these differential outputs, it is possible to obtain a tracking signal in which the offset is canceled. If the track pitch is fine, the inner beam will not be affected by the interference of the diffracted light, so the diffraction angle of the diffracted light can be easily suppressed below the allowable range of the field angle of the objective lens by arranging it in the radial direction. Can do. The tracking control mechanism is controlled by the tracking signal thus obtained. At the same time, defocus detection is performed for each of the light beams separated inside and outside the light beam, and spherical aberration is detected from the difference between them, and defocus is detected from the sum, and the spherical aberration addition mechanism and the defocus control mechanism are respectively controlled.

  Alternatively, as another method, in an optical disc apparatus having a spherical aberration control mechanism, after focus control is started on the recording layer on the optical disc, the focus position is moved back and forth from the focus position, and the spherical signal is detected from the change in the amplitude of the tracking signal. The spherical aberration control mechanism is driven by detecting the aberration.

  As described above, according to the present invention, the spherical aberration in the optical disc apparatus can be detected accurately and easily and inexpensively, and this is fed back to the spherical aberration compensation mechanism, so that the quality of the focused spot can be maintained high and stably high. Recording and reproduction of a high density optical disc can be performed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a basic embodiment of an optical disc apparatus according to the present invention.

Light from the semiconductor laser 101 is converted into parallel light by the collimator lens 102, passes through the beam splitter 103, and is focused on the recording film surface of the optical disk 108 through the substrate by the two groups of two objective lenses 106 and 107. The beam splitter corresponds to the first optical branching element recited in the claims. The first lens 106 is mounted on the two-dimensional actuator 104 and is driven in the optical axis direction and the radial direction of the optical disk. The second lens 107 is mounted on a spherical aberration correcting actuator 105 that is driven integrally with the first lens, and the interval between the two lenses is varied to generate a spherical aberration corresponding to the interval. The light reflected from the optical disk 108 is reflected by the beam splitter 103 and is incident on the light separation hologram 109. The light near the optical axis (not shown) and the light in the peripheral part are separated in different directions, both by the condenser lens 110, The light enters the photodetector 112 through the cylindrical lens 111. The photodetector 112 has a plurality of light receiving areas, and detects and divides these lights in the plurality of light receiving areas and converts them into photocurrents. The defocus signal detection circuit 113, tracking error signal detection circuit 114, spherical aberration signal detection circuit 115, and reproduction signal detection circuit 116 output these signals as voltage signals. Focus error signal is fed back as a drive signal for the focus direction of the two-dimensional actuator 104 is always best image point on the optical disk is controlled to be imaged. The tracking error signal is fed back as a drive signal of the two-dimensional actuator 104 in the disk radial direction. The spherical aberration signal is fed back to the spherical aberration correcting actuator 105 and controlled so as to compensate for the spherical aberration due to the variation in the substrate thickness of the optical disk 108 and the lens interval deviation. The reproduction signal detection circuit 116 reproduces a signal recorded on the optical disk, including current-voltage conversion, waveform equalization processing, binarization processing, and the like. In FIG. 1, the collimating lens 102 can be shared between the condenser lens 110 and disposed between the beam splitter 103 and the first lens 106. In order to improve the light utilization efficiency, a quarter-wave plate may be provided between the beam splitter 103 and the first lens 106, and the beam splitter 103 may be a polarization beam splitter. In this embodiment, the cylindrical lens 111 is arranged to show the case of using the astigmatism method as the defocus detection method. However, this is not necessary when using, for example, the knife edge method or the beam size method. Become. Also in the case of the astigmatism method, any element that generates astigmatism may be used, and for example, an inclined parallel flat plate can be substituted. In this embodiment, the spherical aberration compensation mechanism feeds back to the distance between the two groups of two objective lenses. For example, the collimator lens 102 may be mounted on an actuator and moved. Further, the wavefront may be directly modulated using a voltage-driven liquid crystal variable phase modulation element.

  FIG. 2 shows a schematic diagram of the pattern of the light separation hologram 109 in the embodiment of FIG. The boundary 202 is set with a radius that is almost equally divided by the amount of light with respect to the diameter of the incident light beam 201, and the direction of the diffraction grating is made different between the inner region 203 and the outer region 204. As a result, the inner side and the outer side of the light beam are separated and condensed on the detector 112. Here, an example using the astigmatism method is shown. However, in the knife edge method, for example, the direction of the diffraction grating may be made different for a region that is further divided with at least one diameter that divides the light beam into two. In the case of the beam size detection method, the grating may be a curved grating so that the lens action is applied to the diffracted light while the grating directions are different between the inside and the outside in FIG.

  FIG. 3 is a schematic diagram showing a light receiving surface pattern of the photodetector 112 in the embodiment of FIG. 1 and a circuit calculation method for obtaining a defocus signal, a spherical aberration signal, a tracking error signal, and a reproduction signal from the output signal. The light separated by the light separation hologram 109 into the inside and outside of the light beam is received by the four light receiving areas 301, 302, 303, and 304. Out of these, the outer light beam first-order diffracted light 305 and the inner light beam first-order diffracted light 306 are received by the four-divided light-receiving regions 301 and 302, and the outer light beam-first-order diffracted light 307 and the inner light beam-first-order diffracted light 308 are received by the non-divided light-receiving regions 303 and 304. Is received. The outputs of these light receiving areas are converted into voltages by the buffer amplifier 309 and the differential amplifiers 311, 312, 313, 314, 315, and 316 are added or subtracted with an appropriate gain determined by the resistor 310. At this time, after adding two sets of diagonal areas for the inner luminous flux and the outer luminous flux, respectively, the difference is calculated to obtain independent defocus signals, and then the defocus signals are added to the inner and outer defocus signals. A spherical aberration signal is obtained by obtaining and subtracting the signal. At this time, if the light amount ratio between the inner and outer divisions of the light beam is not uniform, the resistor 310 may be adjusted as a variable resistor. Here, the tracking error signal uses a push-pull method, and is calculated so as to obtain a difference between the detected light amounts of two regions divided by the diameter in the radial direction of the disk with respect to the light beam. In the normal push-pull method, the difference between the received light amounts of the two regions divided by the tangential diameter is calculated, but the astigmatism defocus detection method is used in the embodiment of FIG. In the minimum circle of confusion due to, the direction of the light beam is rotated by 90 °, and the diffraction pattern by the guide groove of the disk appears in the tangential direction. Therefore, the division is performed with a radial diameter.

  Next, the principle of detecting spherical aberration will be described with reference to FIG. If there is spherical aberration, as shown in the figure, among the light rays collected by the lens 401, the focal point position differs between a light ray close to the optical axis and a light ray far from the optical axis. Therefore, when the light beam is separated into the inner side and the outer side, the respective defocus signals are shifted with the shift of the focal position. Therefore, the difference between the defocus signals on the inside and outside of the light beam represents spherical aberration. Also in the second conventional example described above, since the light beam is separated on the inner side and the outer side on the detector in the detection of astigmatism defocusing, the spherical aberration can be detected on the basis of substantially the same principle as in the present invention. However, when separated on the detector, when the aberration is large, the inner and outer rays of the light beam overlap and cannot be completely separated, and the signal deteriorates due to interference caused by the overlapped light. Therefore, in the present invention, the inner luminous flux and the outer luminous flux are separated before entering the detector.

  As will be described later with reference to the drawings, the separation position (position of the second optical branching element) can be selected according to the difference in effect. First, the space between the first optical branching element and the photodetector can be considered. In this case, it is preferable to separate the inner luminous flux and the outer luminous flux into different photodetector positions using a non-polarizing diffraction grating (non-polarized diffraction grating). At this time, by appropriately selecting the phase of the diffraction grating so as not to diffract 100% but to leave 0th-order light, it is possible to simultaneously detect light that does not separate the inner side and the outer side. For example, if this is used as an RF signal, light in the entire region of the light beam is detected without dividing the photodetector, so that it is possible to prevent a plurality of noises from current-voltage conversion amplifiers from being mixed. it can. Second, a space between the first light branching element and the objective lens can be considered. In this case, a polarizing diffraction grating is preferably used. Then, for example, by preventing the light from being diffracted by the light flux (outward light flux) directed to the disk and diffracting by the light flux reflected back by the disk (return light flux), it is possible to suppress the loss of the light amount. Compared with a non-polarized diffraction grating, a polarizing diffraction grating may have both a condition in which it is not diffracted by an outward light beam and a condition in which a zero-order light is left in a return light beam, and the accuracy of film thickness may be difficult in manufacturing. However, it is not impossible in principle. Further, in this case, a configuration in which the light is diffracted by the outward light beam and is not diffracted by the backward light beam is possible. At this time, light spots of a plurality of separated light beams are generated on the disk surface. If these are used, for example, a tracking control signal by a differential push-pull method described later can be obtained. This tracking method is effective when the track pitch is smaller than the light spot diameter. Still further, the third position may be between the semiconductor laser and the first optical branching element. In this case, since the light beam reflected from the disk does not pass through the separation element again, it is preferable to separate the inner light beam and the outer light beam using a non-polarized diffraction grating. Non-polarized diffraction gratings can be freely selected for diffraction efficiency, and at the same time have the advantage of being cheaper than polarizing diffraction gratings. In this case, a plurality of light spots are formed on the optical disc by the diffracted light beam. As described above, also in this case, it is effective to use the differential push-pull method when the track pitch is narrower than the light spot diameter. It is conceivable that the fourth position is shared with the first optical branching element. This is effective in the case of a laser module in which the semiconductor laser and the photodetector are mounted in the same package. At this time, a polarizing diffraction grating is preferably used as the light branching element. By sharing with the first optical branching element, the 0th-order light that is not diffracted returns to the semiconductor laser, so that it is not diffracted by the forward light beam but is diffracted by the return path, and all the 0th-order light is not diffracted. It is better to do so.

  In addition, aberrations can be substantially reduced by separating and detecting the light flux. As for aberrations, the RMS value of the normal wavefront aberration is used as a direct evaluation index. However, when the luminous flux is divided and limited, the RMS wavefront aberration is reduced for each luminous flux. For this reason, it is expected that the defocus signal is less deteriorated and offset is reduced. Further, the sign of spherical aberration in the following description was defined as shown in the figure.

  FIG. 5 shows the result of confirming the detection principle of spherical aberration in the present invention by computer simulation. The simulation was based on scalar diffraction theory, and the light intensity distribution on the detector was obtained by Fourier integration. The defocus detection method is an astigmatism method. Calculation conditions are wavelength 655 nm, rim intensity 0.57, objective lens NA 0.6, detection system condensing lens NA 0.088, detection system astigmatism difference 0.92 mm, quadrant photodetector size 100 μm □, detector split line width It is 10 μm and the luminous flux dividing boundary diameter effective aperture ratio is 70.7%. The horizontal axis of the graph is the amount of defocus of the spot on the optical disk, and the vertical axis is the defocus signal normalized by the amplitude. (A) is a spherical aberration with Seidel's wavefront aberration coefficient of -0.6λ, (b) no aberration, and (c) + 0.6λ, respectively. This is a signal when the whole is detected simultaneously. It can be seen that the defocus signal inside and outside the light beam is shifted due to spherical aberration.

  FIG. 6 shows the result of calculating the spherical aberration signal in the present invention using this result. In (a), the horizontal axis indicates the defocus amount on the optical disk, and the spherical aberration is changed while the spherical aberration signal is indicated on the vertical axis. It can be seen that a signal proportional to the spherical aberration is obtained in a range of about ± 3 μm centering on the in-focus position. (B) shows spherical aberration signals with spherical aberration on the horizontal axis and defocusing changed. It can be seen that although there is an offset to the spherical aberration signal when there is a day focus, a signal proportional to the spherical aberration can be detected satisfactorily.

  For comparison, FIG. 7 shows the calculation of the respective defocus signals by dividing the inside and outside of the light beam on the detector, not in the light beam, according to the second conventional example. Compared to FIG. 5, it can be seen that the waveform of the signal is considerably deteriorated. In particular, when there is spherical aberration, a DC offset occurs inside and outside.

  FIG. 8 shows a calculation similar to that shown in FIG. 6, based on the conventional example, calculated by dividing the light beam on the detector. It can be seen that the spherical aberration signal changes abruptly with respect to defocusing as compared with FIG. For this reason, as shown in (b), it can be seen that the sensitivity of the signal with respect to the spherical aberration is abruptly decreased due to defocusing, and the DC offset is increased.

  FIG. 9 shows the calculation result of the offset of the defocus signal with respect to the spherical aberration. It can be seen that the offset of the defocus signal becomes large due to spherical aberration in all light beams, but the offset becomes very small when the inner and outer light beams are separated and detected according to the present invention.

  FIG. 10 is a calculation model for confirming the compensation effect of a two-lens lens that compensates the detected spherical aberration by the lens interval. This is the lens shape shown in the first conventional example, which is a two-group objective lens with a wavelength of 410 nm and NA of 0.85. The disk substrate 108 has a thickness of 0.1 mm.

  FIG. 11 shows the surface shape of this lens. Surface numbers are numbered sequentially from the left in FIG.

FIG. 12 shows the calculation result of spherical aberration that occurs when the lens interval is changed. The vertical axis represents Seidel's spherical aberration coefficient and is shown in wavelength units. It can be seen that the spherical aberration changes depending on the surface spacing.

FIG. 13 shows an embodiment of the optical disc apparatus of the present invention in which the first and second optical branching elements are integrated. Here, the semiconductor laser 1303 and the photodetectors 1302 and 1304 are integrated into one package 1301, and the two optical branching elements are a composite optical branching element 1305 in which a quarter-wave plate and a polarizing diffraction grating are integrated. It has become. In the composite light splitting element 1305, the polarization diffraction grating on the incident side does not act on the polarized light from the incident semiconductor laser, and the light reflected by the disk 108, which is circularly polarized by the ¼ wavelength plate on the output side, is again ¼. The light is incident on the wave plate and becomes linearly polarized light whose polarization direction is rotated by 90 ° from that emitted from the semiconductor laser, and is incident on the polarizing diffraction grating. At this time, the phase shift of the diffraction grating acts to diffract, and the light is collected by the collimator lens 102 onto the photodetectors 1302 and 1304. When the astigmatism method is used as the defocus detection method as described above, the polarizing diffraction grating pattern may be a curved diffraction grating that diffracts in the detector direction and simultaneously generates astigmatism. As the light receiving surface pattern of the photodetector, the semiconductor laser 1303 may be arranged so that the laser emission point is arranged at the center of FIG. For example, if silicon is used for the photodetector substrate, a mirror tilted at 45 ° can be easily formed by anisotropic etching. If this mirror is used to raise the emitted light of the semiconductor laser, the photodetector is A semiconductor laser and a photodetector can be integrated in a compact manner simply by arranging them around.

  FIG. 14 shows still another embodiment. Light from the semiconductor laser 1401 is converted into parallel light by the collimator lens 1402, a beam having an elliptical intensity distribution is converted into a circular beam by the beam shaping prisms 1403 and 1404, and spherical aberration is added by the liquid crystal phase compensation element 1405. The liquid crystal phase compensation element is a liquid crystal sandwiched between two glass substrates patterned with transparent electrodes, and the phase of transmitted light can be changed by applying an AC voltage to the electrodes. The electrodes are separated into a plurality of regions in accordance with the wavefront shape of the spherical aberration, and a voltage is applied to reduce the phase difference in each region. The light transmitted through the liquid crystal is condensed on the optical disk 1412 through the polarization beam splitter 1406, the λ / 4 plate 1407, the rising mirror 1408, and the objective lens 1411. The objective lens is mounted on the actuator 1410 and performs focus control and tracking control. The reflected light from the optical disk 1412 returns to the polarization beam splitter through the same path, is reflected here, and enters the λ / 2 plate 1413. The λ / 2 plate 1413 is used to adjust the light beam separation ratio in the next polarizing beam splitter 1414 by rotating around the optical axis. The light that has passed through the polarization beam splitter 1414 is divided into four light beams by the radial and tangential diameters in order to detect the tracking signal by the diffraction grating 1415 and is incident on the photodetector 1417, and the output is output to the tracking servo circuit. 1422 and the reproduction signal circuit 1423 are operated to detect a tracking signal and a reproduction RF signal, respectively. The tracking signal is fed back to the actuator 1410 together with a defocus signal described later. On the other hand, the light reflected by the polarization beam splitter 1414 is reflected by the reflection mirror 1418, the light beam is divided by the diffraction grating 1419, and collected by the condenser lens 1420 on the photodetector 1421. An AF servo circuit and a spherical aberration servo circuit detect a defocus signal and a spherical aberration signal from the output from the light detector 1421, respectively. The defocus signal is fed back to the actuator 1410, and the spherical aberration signal is fed back to the liquid crystal phase compensation element 1405. Is done. Here, unlike the previous embodiment, the configuration of the diffraction grating 1419 and the photodetector 1421 will be described below in the case of using the knife edge method for focus detection.

  FIG. 15 shows a form of the diffraction grating 1419. This diffraction grating separates the light beam into and out of the disk, and at the same time, also divides the diameter of the disk in the radial direction of the reflected light beam from the disk, and detects it separately, thereby detecting the focus error signal from the inner light beam and the focus error from the outer light beam. Detect signals independently.

  FIG. 16 shows light diffracted by the photodetector 1421 and the diffraction grating 1419 incident thereon. For the sake of convenience, the diffracted light shows a case where there is a defocus, and the zero-order light is not shown. It is easy to adjust the grating depth of the diffraction grating 1419 so that almost no zero-order light is generated. Alternatively, zero-order light may be generated and a light receiving unit may be provided at the center to detect the total amount of light. Here, the diffracted light is received by the four-split light detection areas 1601 and 1602 and the two-split light detection areas 1603 and 1604. The spherical aberration signal SAS can be detected by detecting the outputs from the quadrant light detection areas 1601 and 1602 as shown in the figure and performing calculations as shown in the lower part of the figure. Here, in order to compensate for the influence of the intensity distribution variation of the semiconductor laser, the gain G is multiplied by the inner defocus signal in the calculation of the difference between the outer defocus signal and the inner defocus signal. The defocus signal FES is obtained from the outputs of the two-divided light detection areas 1603 and 1604 by similarly performing calculations as shown in the figure. Although different diffracted lights are used here, it is of course possible to obtain a defocus signal by adding the inner defocus signal and the outer defocus signal. In such detection using the knife edge method, when the light spot on the disk surface is in focus, the light spot on the light detector surface also focuses. Therefore, for example, when applied to a two-layer disc, the crosstalk from other layers can be reduced by designing the size of the light receiving surface to be small to some extent.

が成り立ち、これより

（式２）

が導ける。したがって、

（式３）

となるようにゲインγを調節することにより、

（式４）

となり、ゲインｋが十分大きければ、ディスク面上の球面収差δを０に漸近させることができる。
In the embodiment shown in FIG. 14, since the liquid crystal element acts only on the optical path of the light beam directed to the disk, the compensating spherical aberration acts only one way. However, since the spherical aberration caused by the substrate thickness error of the disk is caused not only in the incident light but also in the reflected light, the spherical aberration detected by the photodetector is a reciprocal amount. Therefore, if the liquid crystal phase compensation element is directly driven by the spherical aberration signal and the spherical aberration of the detected light beam is controlled to be zero, the spherical aberration of the focal point on the disk surface is overcorrected. This is the same even when a liquid crystal element is inserted between the polarizing beam splitter 1406 and the objective lens 1411. This is because a liquid crystal element usually adds a phase difference only with linearly polarized light in a specific direction. However, when the optical isolation by the λ / 4 plate 1407 and the polarization beam splitter 1406 is not used, a phase can be added by a reciprocating light beam by a liquid crystal element, but the light use efficiency is lowered. FIG. 17 shows a schematic diagram of a control system for compensating for the spherical aberration of the light spot on the focal plane in such a case. Here, the optical system is shown in a simplified manner. When the spherical aberration W ′ acts due to the liquid crystal phase compensation element 1405 and the one-way W spherical aberration acts due to the substrate thickness deviation of the disk, the spherical aberration of the focal point on the disk surface becomes W−W ′ = δ. Further, since spherical aberration also acts on the light beam in the return path, the spherical aberration of the light beam at the position of the photodetector 1421 is 2W−W ′ = δ + W. It is assumed that this is detected as SAS = α (δ + W) as a spherical aberration signal by the detection system. α is the gain of the detection system, and is usually negative for feedback control. This signal is input to the differential amplifier 1701, and a difference signal from the signal fed back from the previous signal is further amplified k times by the amplifier 1702. This signal is multiplied by .gamma. By the amplifier 1703 and used as the previous feedback signal and simultaneously input to the liquid crystal driving circuit 1704. The spherical aberration -W 'added by the liquid crystal phase compensating element 1405 shown above is supplied to the liquid crystal driving circuit. Β times the input signal. At this time,

(Formula 1)

Is true, more than this

(Formula 2)

Can lead. Therefore,

(Formula 3)

By adjusting the gain γ so that

(Formula 4)

If the gain k is sufficiently large, the spherical aberration δ on the disk surface can be made asymptotic to zero.

  FIG. 18 shows another embodiment of the optical disk apparatus according to the present invention.

The light from the semiconductor laser 101 is converted into parallel light by the collimator lens 102, passes through the beam splitter 103 via the spherical aberration correction actuator 1801, and is applied to the recording film surface of the optical disk 108 by the two groups of objective lenses 106 and 107. The light is collected through the substrate. The beam splitter corresponds to the first optical branching element recited in the claims. The first lens 106 is mounted on the two-dimensional actuator 104 and is driven in the optical axis direction and the radial direction of the optical disk. The second lens 107 is driven integrally with the first lens. The light reflected from the optical disk 108 is reflected by the beam splitter 103 and is incident on the light separation hologram 109. The light near the optical axis (not shown) and the light in the peripheral part are separated in different directions, both by the condenser lens 110, The light enters the photodetector 112 through the cylindrical lens 111. The photodetector 112 has a plurality of light receiving areas, and detects and divides these lights in the plurality of light receiving areas and converts them into photocurrents. The defocus signal detection circuit 113, tracking error signal detection circuit 114, spherical aberration signal detection circuit 1802, and reproduction signal detection circuit 116 output these signals as voltage signals. Focus error signal is fed back as a drive signal for the focus direction of the two-dimensional actuator 104 is always best image point on the optical disk is controlled to be imaged. The tracking error signal is fed back as a drive signal of the two-dimensional actuator 104 in the disk radial direction. The spherical aberration signal is fed back to the spherical aberration correcting actuator 1801 and controlled so as to compensate for the spherical aberration due to the variation in the substrate thickness of the optical disk 108 and the lens interval deviation. The reproduction signal detection circuit 116 reproduces a signal recorded on the optical disk, including current-voltage conversion, waveform equalization processing, binarization processing, and the like. In FIG. 18, the collimating lens 102 can be shared with the condenser lens 110 and disposed between the beam splitter 103 and the first lens 106. In order to improve the light utilization efficiency, a quarter-wave plate may be provided between the beam splitter 103 and the first lens 106, and the beam splitter 103 may be a polarization beam splitter. In this embodiment, the cylindrical lens 111 is arranged to show the case of using the astigmatism method as the defocus detection method. However, this is not necessary when using, for example, the knife edge method or the beam size method. Become. Also in the case of the astigmatism method, any element that generates astigmatism may be used, and for example, an inclined parallel flat plate can be substituted. In this embodiment, as the spherical aberration compensation mechanism, for example, the collimator lens 102 may be mounted on an actuator and moved. Further, the wavefront may be directly modulated using a voltage-driven liquid crystal variable phase modulation element. In this embodiment, since the spherical aberration correction is applied only to the incident light, the control system described so far, which detects the spherical aberration with the reflected light and applies the feedback as it is, has the following problems.

  The spherical aberration error detected with this configuration is assumed to be ε. The incident light beam is given a wavefront aberration of y by the spherical aberration compensation mechanism and enters the disk surface. When the spherical aberration generated on the disk surface is x, the aberration generated on the disk surface is 2x when viewed from the reflected light, but the aberration given by the spherical aberration compensation mechanism remains as it is. Then, if control is performed so that the detection error ε of the spherical aberration detection system is zero, y = 2x, and overcompensation is performed by x on the disk surface. Unless control is performed so that x = y, the spherical aberration on the disk surface cannot be made zero.

  Therefore, consider the block configuration of the control system as shown in FIG.

となる。また、閉ループ伝達関数Ｈは、式６のように表せる。

（式６）

さらに、ｘから見たｙへの伝達関数は２・Ｈとなり、以下の式７のように表せる。

（式７）

（式８）

ｘ−ｙをゼロにするためには、少なくともｘの取りうる帯域で、以下の式が成立する必要がある。

（式９）

すなわち

（式１０）

従って、式９の条件を満たすようにＧ２を選択すればよい。

Ｇ２として以下の実施例が考えられる。
式ｘにしたがって、Ｇ１とＧ３の逆関数の積を作成する。例えば、球面収差補償アクチュエータとして、液晶板を使用すると、駆動入力に対する波面位相量の周波数特性は図４６のような低域フィルタの特性をしめす。したがって、Ｇ３を以下の伝達関数で表す。

（式１１）

Ｇ１は対象とする帯域ではほぼ定数、Ｋ１と見なせる。したがって、Ｇ２は以下の伝達関数となればよい。

（式１２）

Block 1901 represents the doubling of aberrations that occur in the reflection process. Block 1902 represents a spherical aberration detector and outputs an aberration error ε. The system basically includes an aberration detector 1902, a control compensation system 1904, and a spherical aberration compensation actuator 1905. The control amount is the spherical aberration y of the incident light and is driven by the spherical aberration compensation actuator. Although it is impossible to directly measure the target value x, since it is possible to measure a deviation amount of 2xy, a feedback control system in which xy becomes zero is configured using this system. The difference from the normal control system is that a control compensation system 1904 is inserted. Then, the open loop circuit G from the phase detector to the spherical aberration compensation actuator is

(Formula 5)

It becomes. Further, the closed loop transfer function H can be expressed as shown in Equation 6.

(Formula 6)

Further, the transfer function from x to y is 2 · H, which can be expressed as the following Expression 7.

(Formula 7)

(Formula 8)

In order to make xy zero, it is necessary to hold the following expression at least in the band that x can take.

(Formula 9)

Ie

(Formula 10)

Therefore, G2 may be selected so as to satisfy the condition of Expression 9.

The following examples can be considered as G2.
A product of inverse functions of G1 and G3 is created according to the equation x. For example, when a liquid crystal plate is used as the spherical aberration compensation actuator, the frequency characteristic of the wavefront phase amount with respect to the drive input shows the characteristic of a low-pass filter as shown in FIG. Therefore, G3 is expressed by the following transfer function.

(Formula 11)

G1 can be regarded as a substantially constant K1 in the target band. Therefore, G2 may be the following transfer function.

(Formula 12)

That is, G2 is a transfer function having a transfer function having a gain that is the reciprocal of the product of K1 and K3, a transfer function of a derivative of the time constant T, and a constant 1. G2 is expressed as shown in FIG. In order to construct an actual control system, the differential is included in this, so that the noise is not increased in the high frequency range and from a frequency higher than the control band so as not to affect the control system. A low-pass filter for decreasing the gain is inserted between G1 and G3. The low-pass filter described above is preferably inserted after the differentiation circuit as shown in FIG.
(2) In this embodiment, a system in which the aberration detection error is zero is considered as a basic system. It comprises an aberration detector 1902, phase compensation element 4902, amplifier 4901, and spherical aberration compensation actuator 1905. The control amount is the spherical aberration y of the incident light and is proportional to the input of the spherical aberration compensation actuator. Although it is impossible to directly measure the target value x, it is possible to measure a deviation amount of 2x−y. Therefore, when feedback is applied as usual, 2x = y.

Therefore, the movement of the spherical aberration compensating actuator is electrically simulated in block 4903, and the spherical aberration simulated in block 4904 is converted into an electrical signal and subtracted from the aberration detection signal. Then, the signal after subtracting from the aberration signal becomes a signal proportional to (2x−y) −y = 2 (xy). When feedback is applied with this signal, x = y is controlled. A block diagram for realizing this is shown in FIG. The transfer function of block 4903 may be G3, and the transfer function of block 4904 may be G1. In this configuration, when the transfer function of the phase compensation element 4902 is g1, and the transfer function of the amplifier 4901 is g2, G2 is expressed as follows.

(Formula 13)

となり、Ｇ２がＧ１とＧ３の積の逆関数となる。
When g1 · g2 is sufficiently larger than 1,
(Formula 14)

G2 is an inverse function of the product of G1 and G3.

  In order to configure the above control system, the block of G2 has a configuration as shown in FIG. The spherical aberration signal is input to the plus terminal of the differential amplifier 2005. The drive voltage of the drive circuit 2002 for driving the spherical aberration compensation actuator is input to the minus terminal and the transfer function of the spherical aberration compensation actuator (including the characteristics of the actuator drive circuit). A signal that passes through the simulation circuit 2001 having a transfer function in which the transfer functions of the aberration detectors are connected in series is input. The output of the differential amplifier 2005 is input to the amplifier 2004, and the output is input to the phase advance compensation circuit 2003 via the secondary integration circuit 2006, and the output is the drive voltage of the drive circuit 2002 that drives the spherical aberration compensation actuator. It becomes. The output of the drive circuit 2002 is input to the spherical aberration compensation actuator 1905 to give the incident light spherical aberration.

The control system design method will be described below by taking the configuration of the control system (2) as an example. The phase compensation circuit 4902 includes a secondary integration circuit 2006 and a phase advance circuit 2003. The secondary integration circuit has a frequency characteristic in which the gain decreases with a minus 40 dB / dec with respect to the frequency. The characteristic of the secondary integration circuit is represented as K / (s ² ). Here, s = jω. K is adjusted so that this becomes a frequency characteristic as shown in FIG.

Further, in order to improve the response characteristic of the control system, a phase advance / delay circuit having a frequency characteristic shown in FIG. 22 is inserted. Frequency compensation by the phase advance circuit stabilizes the servo system. In order for the servo system to be stable, according to Nyquist's simple stability determination method, the phase is -180 degrees at a frequency ωc (crossing frequency) at which the gain of the loop transfer function (in this case, G1G2G3) is 0 dB. It must be more than that. If the phase is 180 degrees behind, it will oscillate.

  Therefore, how much the phase at the crossover frequency ωc is more than −180 degrees is an evaluation value of stability. Therefore, the control system stabilizes the servo system by raising the phase at ωc from −180 degrees and providing a phase margin of 40 to 50 degrees with a phase advance circuit having frequency characteristics as shown in FIG. When the gain characteristic of the phase soot compensation circuit is set to 0.1, the gain is increased by 20 dB at high frequencies.

FIG. 23 shows the open loop characteristics after incorporating the phase lead compensation circuit. In this embodiment, the crossing frequency is set to 1.7 kHz.

  FIG. 24 shows an embodiment in which a non-polarizing beam splitter 2401 is used. Since a non-polarizing beam splitter is used, even when the liquid crystal phase compensation element 1405 is used, if this is inserted between the beam splitter 2401 and the objective lens 1411, spherical aberration acts on both the forward and backward light fluxes. To do. For this reason, since the detected spherical aberration signal has a value proportional to the spherical aberration on the disk surface, the feedback control circuit includes the amplifier 1702 and the liquid crystal drive circuit 1704 as in the conventional spherical aberration control circuit 2402. Can be used.

  FIG. 25 shows an embodiment in which a lens that may cause spherical aberration is not disposed in an optical path through which light passes only one way, such as between a semiconductor laser and an optical branching element, and between an optical branching element and a photodetector. It is. The light from the semiconductor laser 101 is converted into parallel light by the collimator lens 102 through the polarization beam splitter 1406, and spherical aberration is added by the liquid crystal phase compensation element 1405. Further, the light passes through the polarizing diffraction grating 2501 and the λ / 4 plate 1407 mounted on the objective lens actuator 104 and is condensed on the optical disk 108 by the objective lens 1411. The reflected light is converted into linearly polarized light orthogonal to the incident time by the λ / 4 plate 1407, diffracted by the polarizing diffraction grating 2501, transmitted through the liquid crystal phase compensation element 1405, and condensed by the collimator lens 102. The light is reflected by the beam splitter 1406 and received by the photodetector 2502. In this way, optical components that may cause spherical aberration, such as lenses, always pass through both directions, so that a method that takes into account that the aberration applied by the liquid crystal phase compensation element during spherical aberration compensation is only one way. Thus, the occurrence of offset can be prevented.

  FIG. 26 is a diagram showing a pattern of the polarizing diffraction grating 2501 used here. The light beam near the optical axis and the light beam in the peripheral part both generate astigmatism in the 45 ° direction of the same magnitude in the diffracted light, and at the same time, the diffracted light is separated vertically in the vicinity of the optical axis and vertically in the peripheral part. It is a simple lattice pattern.

FIG. 27 shows the photodetector 2502, the pattern of the light beam incident thereon, and the calculation formulas for each signal. A 0th-order light beam that is not diffracted by the polarizing diffraction grating 2501 is incident on the light receiving portion A in the center. At this time, the position of the photodetector 2502 is adjusted so that the zero-order light is focused on the light receiving part A when the spot on the optical disk is focused. At this time, the diffracted light spreads without being condensed due to astigmatism, but the pattern of the polarizing diffraction grating 2501 in FIG. 26 has no condensing / diverging lens power, so that a minimum circle of confusion is formed on the detector. . Therefore, each of these diffracted lights is received by a four-divided light detection region, and a focus detection calculation is performed to obtain a difference in diagonal sum, and by adding each, a defocus signal (AF) can be obtained. However, the astigmatism of the diffraction grating is added in consideration of the polarity of the defocus signal because the sign is inverted between the + 1st order diffracted light and the −1st order diffracted light. Since the minimum circle of confusion due to astigmatism in the 45 ° direction is a distribution obtained by substantially rotating the distribution of the parallel light beam by 90 °, a diffraction pattern due to the guide groove of the disk appears in the tangential direction. In the figure, in order to show the polarity of the diffraction pattern, an example in which only one side of the diffraction pattern becomes dark assuming a state where there is a slight tracking deviation is shown. For example, if the guide groove pitch of the disk is 0.32 μm, the NA of the objective lens is 0.85, and the wavelength of light is 0.4 μm, the pitch is fine compared to the spot diameter (λ / NA = 0.47 μm). Therefore, there is a region without a diffraction pattern due to the guide groove at the center of the light beam. Therefore, by taking the light beam near the optical axis in a region where there is no diffraction pattern due to the guide groove, a tracking signal calculation in this region becomes a lens shift signal (LS). By multiplying this by an appropriate coefficient and subtracting it from the tracking signal of the outer light beam (TR), the problem of offset due to objective lens shift, which is a problem in push-pull tracking signal calculation, is solved. As described above, the spherical aberration can be detected by obtaining the defocus signal of the outer light beam and the defocus signal of the inner light beam and calculating the difference between them (SA).

と表せるため、もともとの球面収差に加えて、近似的にΔに比例したコマ収差を生じる。このコマ収差が許容範囲内であればよいが、補償する球面収差量が大きいか、ずれΔが大きく、発生するコマ収差が許容範囲以上の場合には、図２８に示したように、液晶素子を対物レンズ１４１１に搭載することにより、コマ収差の発生を抑えることができる。
FIG. 28 shows an embodiment in which the liquid crystal phase compensation element 1405 is mounted on the two-dimensional actuator 104 in the embodiment shown in FIG. In the embodiment of FIG. 25, since the objective lens 1411 is driven by the two-dimensional actuator 104 independently of the liquid crystal phase compensation element 1405, the spherical aberration applied by the liquid crystal phase compensation element 1405 is the objective by the drive amount by the two-dimensional actuator 104. Deviated from the optical axis of the lens 1411. Spherical aberration deviated by Δ from the optical axis is

(Formula 15)

Therefore, in addition to the original spherical aberration, coma aberration approximately proportional to Δ is generated. If the coma aberration is within the allowable range, but the amount of spherical aberration to be compensated is large or the shift Δ is large and the generated coma aberration is equal to or greater than the allowable range, as shown in FIG. Is mounted on the objective lens 1411, so that occurrence of coma aberration can be suppressed.

  FIG. 29 shows an embodiment with a coma aberration compensation function as another method for suppressing the occurrence of coma aberration. Here, as the liquid crystal phase compensation element, a liquid crystal phase compensation element 2901 capable of compensating for spherical aberration and simultaneously compensating coma is used. The coma aberration generated when the objective lens 1411 is displaced from the axis of the liquid crystal element 2901 by the two-dimensional actuator 104 due to the eccentricity of the disk 108, or the lens shift is calculated by the coma aberration circuit 2902 from the output of the photodetector 2502. Detected and fed back to the coma aberration driving electrode of the liquid crystal phase compensation element 2901. At this time, the photodetector 2502 described in FIG. 27 can be used. Further, the liquid crystal phase compensation element 2901 may be disposed between the semiconductor laser 101 and the beam splitter 2901 by designing a collimating lens in consideration of spherical aberration when diverging light is incident. Alternatively, when a non-polarizing beam splitter as shown in FIG. 24 is used, the liquid crystal phase compensation element 2901 is replaced with the non-polarizing diffraction grating with the liquid crystal phase compensation element 2901 in the arrangement shown in the figure, and the λ / 4 plate 1407 is replaced. Remove it.

  FIG. 30 shows a liquid crystal phase compensation element that simultaneously compensates for coma and spherical aberration. Such an element is described in, for example, Japanese Patent Laid-Open No. 2001-84631. FIG. 30A shows a cross-sectional structure of the element. Transparent electrodes 3004a and 3004b are patterned on the surfaces of the glass substrates 3001a and 3001b, and insulating films 3005a and 3005b and alignment films 3006a and 3006b are stacked, and are sealed with a sealant 3002 with the liquid crystal 3003 interposed therebetween. The electrode 3004b can be wired from the substrate 3001a through a conductive film patterned on a sealing material. 30B is a plan view of the transparent electrode 3004a for compensating spherical aberration, and FIG. 30C is a plan view of the transparent electrode 3004b for compensating coma aberration. In order to reduce the aberration as in the wavefronts 3008a and 3008b by adding a phase shift to the wavefronts 3007a and 3007b having aberration as shown in FIGS. 30D and 30E, respectively, the input voltage V1, An alternating voltage is applied to V2, V3, and V4.

  FIG. 31 shows a drive circuit for the voltage applied to the liquid crystal of FIG. The AC voltage 3100 having a rectangular waveform of ± V0 and its inverted waveform are used as a reference for the applied voltage for spherical aberration and the applied voltage for coma aberration, respectively, and the amplitude is modulated by the spherical aberration signal and the coma aberration signal. The amplitudes of V1 and V2, and V3 and V4 are increased or decreased antisymmetrically by the spherical aberration signal and the coma aberration signal with respect to the reference voltage, respectively. By doing so, the compensation wavefront as shown in FIGS. 30D and 30E can always be realized.

  30 and 31 show an embodiment of a liquid crystal phase compensation element for simultaneously compensating for spherical aberration and coma, but when compensating only for spherical aberration, the transparent electrode for compensating coma is not uniformly divided. The driving voltage may be a ground level or an inverted waveform of a reference signal having a constant amplitude.

  FIG. 32 shows an embodiment in which the effective light flux system of the objective lens is 1 mm or less, and the semiconductor laser, the spherical aberration adding mechanism, the optical branching element, the objective lens, and the photodetector are integrated. The semiconductor laser chip 3201 is mounted on the photodetector substrate 3202, and the semiconductor laser light is raised vertically by a 45 ° reflection mirror formed by etching on the photodetector substrate. The photodetector substrate 3202 is fixed on the support substrate 3203, and the support substrate 3203 is bonded to the optical head housing 3214 so as to seal the semiconductor laser chip 3201. The optical head housing 3214 is made of glass or a metal having a hole formed at a position where light is transmitted, and the semiconductor laser light is transmitted through the optical head housing 3214 and reflected by the reflecting prism 3205, and is reflected by the concave lens 3206 and the convex lens 3207. Parallel light. Here, the concave lens 3206 is used to shorten the optical path length, and if there is a margin in length, only the convex lens 3207 may be used. The collimated light then passes through the liquid crystal phase compensation element 3208 and the polarizing diffraction grating 3209, is converted from linearly polarized light to circularly polarized light by the λ / 4 plate 3210, is reflected by the reflecting prism 3211, and is reflected by the objective lens 3212 to the optical disk 3213. Focused on top. The liquid crystal phase compensation element 3208 is larger in size than the beam diameter due to the size of the sealing material. The reflected light passes through the objective lens 3212 and the reflecting prism 3211, becomes a linearly polarized light whose polarization is rotated by 90 ° from the polarization direction of the outward light beam by the λ / 4 plate 3210, is diffracted by the polarizing diffraction grating 3209, and is liquid crystal phase compensation element 3208. And is collected on the photodetector substrate 3202. A signal line from the photodetector substrate 3202 is output from the signal terminal 3204 by a bonding wire. In the above optical system, the effective light beam diameter in the objective lens 3212 is set to 1 mm or less, so that the entire optical system is miniaturized, and the whole is integrated and mounted on the actuator arm 3215, whereby a liquid crystal phase compensation element accompanying a tracking operation is obtained. The effect of the optical axis deviation 3208 is eliminated. Furthermore, since the optical head can be reduced in size and thickness, it is effective for reducing the size of the entire optical disc apparatus.

により求めた。ここでｎはレンズ屈折率、ｔはレンズ厚、Ｒ１はレンズ第１面曲率半径、ｆは焦点距離、ｄはディスク基板厚、ｎｓは基板屈折率である。第１面曲率半径が光束径の１／２という条件は、幾何学的にはレンズが成立するためのギリギリの条件であるが、実際上は非球面形状であるため、この近傍であればレンズは成立する。これにより、ＮＡ０．８５、基板厚０．１ｍｍであっても、有効径１ｍｍでワーキングディスタンスが０．１ｍｍ程度は確保できることがわかる。これは図１１に示した有効径３ｍｍの２枚組レンズのワーキングディスタンス０．１３ｍｍとほぼ同等であり、十分実現可能である。 When the effective light beam diameter of the objective lens is 1 mm or less, the problem is how much the distance between the objective lens and the optical disk, that is, the working distance (working distance) can be secured. FIG. 33 shows a condition that NA is 0.85, the disk substrate thickness is 0.1 mm, the substrate refractive index is 1.62, the objective lens refractive index is 1.8, and the radius of curvature of the first surface of the single lens is 1/2 of the beam diameter. The calculation result of the working distance with respect to the effective diameter is shown. Here, based on Yoshiya Matsui's “Lens Design Method” (Kyoritsu Shuppan, 1989, first edition, 7th edition), working distance is set at a lens thickness where the value of spherical aberration derived analytically from aberration theory is minimized.

(Formula 16)

Determined by Here, n is the lens refractive index, t is the lens thickness, R1 is the radius of curvature of the first lens surface, f is the focal length, d is the disk substrate thickness, and ns is the substrate refractive index. The condition that the radius of curvature of the first surface is ½ of the light beam diameter is the last condition for geometrically forming a lens, but since it is actually an aspherical shape, if it is in this vicinity, the lens Holds. As a result, it can be seen that even with an NA of 0.85 and a substrate thickness of 0.1 mm, an effective diameter of 1 mm and a working distance of about 0.1 mm can be secured. This is almost equivalent to the working distance of 0.13 mm of the double-lens lens having an effective diameter of 3 mm shown in FIG.

  FIG. 34 shows an embodiment of a laser module in which a semiconductor laser chip 3201 is integrated on a photodetector substrate 3202. The diverging light emitted from the end face of the semiconductor laser chip 3201 is raised vertically from the substrate by a 45 ° mirror 3401 formed by etching. Since the silicon substrate is used as the photodetector substrate, if the 45 ° mirror is a substrate cut by shifting the crystal axis orientation by 9.7 °, an inclined surface of 45 ° appears by anisotropic etching.

FIG. 35 is a diagram showing a detector pattern and a signal calculation method in the laser module of FIG. 34, and FIG. 36 is a pattern of the polarizing diffraction grating 3209. FIG. 35 also shows a detection light pattern at the time of defocusing superimposed on the light detection region indicated by hatching. When using a blue semiconductor laser as a light source, considering the case where the light receiving sensitivity of the photodetector is lowered and the S / N ratio of the signal is deteriorated, all the + 1st order diffracted lights are connected to form one output, and RF Signal output. Thereby, an increase in amplifier noise can be suppressed. On the other hand, since the spherical aberration signal (SA signal) and the lens shift signal (LS signal) are obtained from the −1st order diffracted light with the smallest possible number of divisions, the focus error signal (AF signal) and SA signal are the upper half of the outer luminous flux. And the lower half of the inner luminous flux. The tracking error signal (TR signal) and the LS signal are detected by dividing the lower half of the outer light beam and the upper half of the inner light beam in the radial direction.

FIG. 37 shows an embodiment of a small optical disk device using the small optical head 3701 of FIG. FIG. 37 (a) is a plan view and FIG. 37 (b) is a side view. A small optical head 3701 is attached to an actuator arm 3215. The actuator arm 3215 can be finely moved in the optical axis direction of the objective lens of the optical head and the radial direction of the optical disk 3702 by a two-dimensional actuator 3707. Further, the actuator arm 3215 and the two-dimensional actuator 3707 are fixed to the swing arm 3703 together with the counterbalance 3705, and the swing arm 3703 drives the small optical head 3701 in the radial direction of the optical disc 3213 by the swing motor 3704. The optical disk 3213 is rotated by a spindle motor 3702. Signal input / output to the optical head is connected to the control circuit 3706 by a flexible plastic cable (not shown).

  FIG. 38 shows an embodiment of the optical disk apparatus in the case of the groove recording system or land recording system in which the track pitch of the optical disk is finer than that of the land / groove type disk. In a recordable optical disk, a guide groove that is periodic in the radial direction is generally used for tracking. The push-pull method is used for tracking detection by the guide groove. However, the push-pull method has a problem in that the light spot on the photodetector moves due to the tracking operation and an offset occurs. In order to alleviate this, DVD-RAM and the like employ a method in which a polarizing diffraction grating is mounted on an objective lens so that a line for dividing the light beam does not move with respect to the light beam. However, this method is not always perfect, and an offset caused by the movement of the center of intensity within the light beam due to tracking cannot be removed. This was not a problem with a land / groove type optical disc in which the amplitude of the push-pull signal can be increased. However, the amplitude of the push-pull signal is small in the groove recording method or the land recording method in which the pitch of the guide groove needs to be reduced. Therefore, an offset that increases relatively due to a deviation in intensity distribution cannot be ignored. There is a differential push-pull method as another method for reducing the offset of the push-pull tracking method. This is because the sub-spot is shifted from the main spot by 1/2 track on the disk surface, so that the polarity of the push-pull signal of the main spot and sub-spot obtained on the detector is inverted, and the differential output is taken. Thus, the offset mixed in the same phase in both the main spot and the sub spot is removed. According to this, it is possible to cancel the offset including an offset caused by a relatively increased intensity distribution in a disk having a fine track pitch. FIG. 38 shows an embodiment in which spherical aberration and lens shift are detected in an optical head using such a differential push-pull method.

  In FIG. 38, the light emitted from the semiconductor laser 101 passes through the liquid crystal phase compensation element 2901, the diffraction grating 3801, and the polarization beam splitter 1406, and is collimated by the collimator lens 102. In the diffraction grating 3801, diffracted light (not shown) is generated in order to form sub-spots on the disk surface. The collimated beam is further converted into circularly polarized light by the λ / 4 plate 1407 and condensed on the optical disk 108 by the objective lens 1411 mounted on the two-dimensional actuator 104. The reflected light is converted into linearly polarized light whose polarization direction is orthogonal to that of the incident light at the same λ / 4 plate 1407, reflected by the polarizing beam splitter 1406, given astigmatism by the cylindrical lens 111, and received by the photodetector 3802. Is done. An AF circuit 113, TR circuit 114, SA circuit 115, Coma circuit 2902, and RF signal are respectively calculated from an output signal from the photodetector 3802 by a defocus signal, tracking signal, spherical aberration signal, lens shift signal, and RF signal. The other than the RF signal is fed back to the two-dimensional actuator 104 and the liquid crystal phase compensation element 2901.

  FIG. 39 shows a grating pattern for the diffraction grating 3801 of FIG. With respect to the incident light beam indicated by the broken line, the linear grating is arranged in the orthogonal direction so that the light in the central part near the optical axis is diffracted in the substantially radial direction of the optical disk and the light in the peripheral part is diffracted in the substantially tangential direction. .

  FIG. 40 is a diagram showing a schematic shape and arrangement of a light spot formed on the optical disc 108 by the diffraction grating 3801 of FIG. Ambient light diffracted in a substantially tangential direction becomes spots 4002a and 4002b composed of relatively small main lobes and peripheral side lobes. At this time, the diffraction grating 3801 is adjusted so that the respective sub-spots are arranged on both sides of the main spot 4001 with a shift of ½ of the track pitch. The light beam in the vicinity of the optical axis diffracted in the substantially radial direction is separated on both sides in the radial direction to form a large spot where the structure of the guide groove cannot be resolved.

  FIG. 41 is a schematic diagram of a light receiving surface pattern and a detected light beam of the photodetector 3802 in FIG. 38, and a diagram illustrating a signal calculation method. Diffracted light from the guide groove overlaps the detected light beam to form an interference pattern. In the figure, for the sake of easy understanding of the polarity of the TR signal calculation, a case where the main spot on the disk surface is slightly off-tracked for the sake of convenience is shown, and the case where an imbalance occurs in the interference pattern is shown. The distribution of the light beam on the detector indicates the position of the minimum circle of confusion of the focused spot affected by the astigmatism by the cylindrical lens 111. For this reason, the distribution of the light beam is a distribution obtained by rotating the intensity distribution of the parallel light beam by 90 degrees. The AF signal is calculated by the astigmatism method using the outputs of the four diffracted lights 4102a, 4102b, 4103a and 4103b. The TR signal is obtained by calculating the difference between the push-pull signal of the main spot 4101 and the push-pull signal of the four diffracted lights so as to absorb the light amount difference by the gain coefficient G1. The SA signal is obtained by taking the difference between the sum of the defocus signals of the outer light beams 4102a and 4102b and the sum of the defocus signals of the inner light beams 4103a and 4103b. The RF signal is obtained from the sum of the main spots 4101. The LS signal is obtained from the sum of the push-pull signals of the inner light beams 4103a and 4103b. As shown in the figure, by selecting the boundary between the inner and outer light beams so that the diffracted light from the disk guide groove does not overlap the inner light beam, only the intensity distribution unbalance due to lens shift is detected without disturbance by the guide groove. Can do. In addition, since the diffracted light from the disk guide groove does not overlap the inner luminous flux and no disturbance occurs, the spot due to the inner luminous flux does not need to be placed on the same track as the outer luminous flux and is easy to detect separately from the outer luminous flux, It can be arranged in the radial direction with a small angle of view from the objective lens. When performing these signal calculations, it is not necessary to detect all the output signals from each light receiving area independently, and by always connecting the outputs to be added and subtracted with the same polarity on the photodetector board in advance, the output The number of signal lines can be reduced to ten. In this embodiment, the defocus signal is obtained from the sub-spot. This is because the number of signal lines in the external calculation when obtaining the RF signal is reduced as much as possible. When output from the light receiving surface is performed at the photocurrent stage, noise from the addition amplifier is not mixed. However, when adding and subtracting separately output signals using the addition amplifier, each output signal is converted to current and voltage in advance. It is necessary to keep amplifier noise in each. In order to obtain a defocus signal from the main spot 4101, it is necessary to divide the light receiving area of the main spot into four instead of two, and the amplifier noise is doubled. If the S / N ratio of the signal can be ensured so that this is within the allowable range, it is desirable to obtain the AF signal from the 0th order light from the viewpoint of the accuracy and stability of the focus control.

The problem of the offset of the tracking signal accompanying such tracking control occurs when the position of the objective lens moves relative to the semiconductor laser or the photodetector. Therefore, such a problem does not occur in the case of the optical head in which the semiconductor laser, the photodetector, and the objective lens described in FIG.

  FIG. 42 shows an embodiment in which the diffraction grating 3801 of the embodiment of FIG. 38 is arranged in the detection optical system as another example of the present invention. Since the diffraction grating 4201 is arranged in the detection optical system, only the main spot is formed on the optical disk 108. Light incident on the detection optical system is diffracted by the diffraction grating 4201 having the same pattern as in FIG. 39, and the inner light beam and the outer light beam are separated in the diffracted light.

FIG. 43 shows a light receiving pattern of the photodetector 4202. Since the light beam is separated in the detection optical system, the interference pattern of the main spot and the sub spot is the same. Therefore, since the differential push-pull method cannot be used, the offset of the tracking signal is compensated by multiplying the LS signal by a constant gain and subtracting it from the push-pull signal of the main spot 4301.

As still another embodiment, another photodetector pattern in the embodiment of FIG. 38 is shown in FIG. Here, the diffraction grating 3801 is a linear grating without division used in the conventional differential push-pull method, and two sub-spots are arranged on the optical disc with a shift of 1/2 track on both sides of the main spot. As a result, three spots 4401, 4402a, 4402b in which the interference pattern by the guide groove is reversed are formed on the photodetector. For the defocus signal, the sub-spots 4402a and 4402b are used to perform normal astigmatism focus detection calculation, and the tracking signal is also subjected to normal differential push-pull calculation. The lens shift is obtained by the sum of two push-pull signals with reversed polarities. However, since the inner side and the outer side of the light beam described in the present invention are not separated, it is difficult to detect spherical aberration. Therefore, spherical aberration is detected from the defocus characteristic of the push-pull signal.

  FIG. 45 shows the calculation result of the defocus characteristic of the push-pull signal when the NA is 0.85, the wavelength is 0.405 μm, and the track pitch is 0.32 μm. It can be seen that changing the magnitude of the spherical aberration shifts the peak position. Therefore, by using this, the amplitude of the push-pull signal when the focal point is moved back and forth from the just focus position is obtained, and a signal proportional to the spherical aberration can be obtained by obtaining the difference.

FIG. 46 shows the spherical aberration signal obtained as described above. It can be seen that a signal proportional to the spherical aberration can be obtained by changing the amount of defocus to ± 0.25 μm, ± 0.5 μm, and ± 0.75 μm and determining the push-pull signal amplitude difference. However, with this method, spherical aberration signals cannot be obtained steadily, so when jumping between layers on a multilayer disc, etc., spherical aberration is learned before recording and playback, and is constantly constant in the same layer. It can be used to compensate for spherical aberration by value. Further, this method is effective for detecting only a change in spherical aberration due to an interlayer jump in which astigmatism does not change because signals are similarly obtained not only in spherical aberration but also in astigmatism.

  FIG. 50 shows another embodiment of the laser module in the embodiment of the small optical head of FIG. The polarization diffraction grating 5101 shown in FIG. 51 is used as the polarization diffraction grating 3209. The same astigmatism is given in the direction of 45 ° with respect to the radial direction of the optical disk on the inner side and the outer side of the light beam. The light is incident on a different light receiving area. The light spots 5001a, 5001b, 5002a, and 5002b on the photodetector are added to the + 1st order diffracted light (5001a and 5002a) and the −1st order diffracted light (5001b and 5002b) at the same time as the astigmatism having different signs is added. When focusing, a minimum circle of confusion due to astigmatism is formed. For this reason, the interference pattern by the guide groove of the optical disk is rotated by 90 ° in the opposite direction between the + 1st order light and the −1st order light. In the photodetection area, + 1st-order diffracted light is received by one uniform light-receiving area on both the inner and outer sides, connected on the photodetector, and output as an RF signal. The first-order diffracted light is received by the four-divided light detection region on the inner side and the outer side, and the sum of the defocus signals by the astigmatism method is used as the AF signal and the difference is used as the SA signal. The TR signal is obtained by subtracting the LS signal by the push-pull signal of the inner light beam by a constant gain from the push-pull signal of the outer light beam. This makes it possible to compensate for the lens shift offset when the objective lens is not integrated with the laser module. However, in the embodiment of FIG. 32, since these are integrated, the LS signal is substantially always 0, and there is no difference whether or not such an operation is performed. When such an operation is performed, the number of output signal lines including the monitor output is ten.

  When spherical aberration occurs in the light spot on the disk surface due to the substrate thickness error, if the spherical aberration is compensated only for the light beam directed to the disk using the liquid crystal phase compensation element as described above, the light beam returning to the photodetector is Remains spherical aberration. FIG. 52 shows the calculation result of this influence on the defocus signal. The focus detection method is assumed to be an astigmatism method, an objective lens NA of 0.85, a light source wavelength of 405 nm, an objective lens effective light beam diameter of 4 mm, a detection system condensing lens focal length of 20 mm, and a focus confinement range of ± 3 μm. It was found that when the spherical aberration due to the substrate thickness deviation was given in the range of ± 1.5λ (corresponding to the substrate thickness deviation of about ± 11 μm) only in the detection system, the focal position was shifted by about ± 0.7 μm. This is a quantity that cannot be ignored. FIG. 53 shows the relationship between the defocus signal at the focus position (defocus 0) and the spherical aberration amount at this time. It can be seen that there is almost a proportional relationship.

  Therefore, an optical disc apparatus as shown in FIG. 54 is configured, and an offset proportional to the spherical aberration signal is given to the defocus signal. The configuration of the optical system is based on the optical disk apparatus shown in FIG. 17 as an example. However, the spherical aberration due to the substrate thickness deviation is compensated only for the forward beam and is effective for all configurations remaining in the detected beam. Here, first, the defocus signal of the inner luminous flux and the defocus signal of the outer luminous flux are obtained from the output from the photodetector 1421 using the arithmetic circuits 5401 and 5402, respectively. When these difference signals are obtained by the differential amplifier 5403, this becomes a spherical aberration signal reflecting the spherical aberration on the detector. The configuration for compensating the spherical aberration of the one-way optical path from this output is the same as in FIG. On the other hand, a sum signal of the defocus signal of the inner light beam and the defocus signal of the outer light beam is obtained by an adder 5404. From this signal, the spherical aberration signal on the detector is multiplied by a constant by an amplifier 5405 and subtracted from the differential amplifier 5406. Thus, the defocus signal is obtained, and the lens actuator 1410 is driven by this. In the above circuit configuration, the AF circuit 113 and the SA circuit 115 are effectively complicated.

The calculation as described above is effectively equivalent to changing the gain distribution of the addition of the defocus signal of the inner luminous flux and the defocus signal of the outer luminous flux in the defocus signal. The spherical aberration signal is (Expression 17) SA = AF _in −AF _out
Therefore, when the amplification gain of the spherical aberration signal when the spherical aberration signal is subtracted is k, the defocus signal in this case is (Expression 18) AF = AF _in + AF _out −k · SA
= AF _in + AF _out -k · (AF _in -AF _out )
= (1-k) AF _in + (1 + k) AF _out
Given in. That is, it is equivalent to changing the gain of addition of the defocus signal of the inner light beam and the defocus signal of the outer light beam.
FIG. 55 shows the result of subtracting 1.5 times the spherical aberration amount from the defocus signal from FIG. The offset of the defocus signal is kept sufficiently small.

  The present application can be applied to optical recording and reproduction of information.

A basic embodiment of an optical disc device according to the invention, 1 is a schematic diagram of a pattern of a light separation hologram 109 in the embodiment of FIG. The light receiving surface pattern and circuit calculation method of the photodetector 112 in the embodiment of FIG. The figure explaining the detection principle of spherical aberration, The separation beam defocus signal simulation of the present invention due to spherical aberration, Spherical aberration signal simulation according to the invention, Defocused beam defocusing signal simulation of the conventional example due to spherical aberration, Spherical aberration signal simulation by conventional example, Spherical aberration signal simulation according to the invention, A model of a two-lens calculation example using a conventional example, Conventional two-lens shape, Changes in spherical aberration due to the distance between the two lenses, An embodiment in which two optical branching elements are integrated; Other embodiments according to the invention, The figure explaining the diffraction grating of FIG. The figure explaining the photodetector of FIG. The figure explaining the spherical aberration compensation system of FIG. Embodiment in which the spherical aberration compensation mechanism compensates for the amount of aberration of incident light, Control block diagram of the embodiment, A circuit diagram of the spherical aberration correction circuit in the embodiment, Transfer frequency characteristics in the control system part Frequency characteristics of phase lead compensation circuit, Control system open-loop transfer function characteristics, An embodiment using a non-polarizing beam splitter, An embodiment that suppresses the occurrence of spherical aberration in a one-way optical path, Polarizing diffraction grating pattern in the embodiment of FIG. The detector pattern and signal calculation method in the embodiment of FIG. Embodiment in which a liquid crystal phase compensation element is mounted on an actuator, Embodiment with coma aberration compensation function, Structure diagram of liquid crystal phase compensation element, Liquid crystal drive circuit, Embodiment of a small optical head, Relationship between working distance of objective lens and effective diameter, Laser module perspective view, Laser module detector pattern and signal calculation method, Polarizing diffraction grating pattern for small optical heads, An embodiment of an optical disk device with a small optical head, Embodiments of the present invention with a fine track pitch disc, Diffraction grating pattern diagram in the embodiment of FIG. FIG. 38 is a schematic diagram of spot arrangement on the optical disc in the embodiment of FIG. Photodetector light receiving pattern and signal calculation method in the embodiment of FIG. Embodiment in which the diffraction grating of the embodiment of FIG. 42. The photodetector pattern and signal calculation method in the embodiment of FIG. Photodetector light receiving pattern and signal calculation method in another detection method in the embodiment of FIG. Push-pull signal defocus characteristics when there is spherical aberration, Calculation result of spherical aberration signal due to push-pull signal difference when focus offset is added, Block diagram of control compensation system, Block diagram of control compensation system with low-pass filter inserted, Block diagram of a control system embodiment that compensates for spherical aberration only in the one-way optical path, Another embodiment of the laser module used in the embodiment of FIG. A diffraction grating pattern used in the embodiment of FIG. Defocus signal calculation result due to spherical aberration on detector due to substrate thickness deviation, Defocus signal offset amount for spherical aberration in FIG. An embodiment of an optical disc device that compensates for a defocus signal offset when correcting only one-way spherical aberration, FIG. 6 is a diagram illustrating a defocus signal calculation result after compensating for a defocus signal offset when spherical aberration remains.

Explanation of symbols

  102: Semiconductor laser, 102: Collimating lens, 103: Beam splitter, 104: Two-dimensional actuator, 105: Actuator for correcting spherical aberration, 106, 107: Objective lens, 108: Optical disk, 109: Light separation hologram, 110: Condensing Lens: 111: Cylindrical lens, 112: Photo detector, 113: Defocus signal detection circuit, 114: Tracking signal detection circuit, 115: Spherical aberration signal detection circuit, 116: Reproduction signal detection circuit

Claims (6)

A semiconductor laser;
An objective lens for irradiating light from the semiconductor laser on the optical disc;
A variable focus mechanism for changing the focal position of the irradiated light;
A first light branching element for branching reflected light from the optical disc;
A spherical aberration adding mechanism provided between the semiconductor laser and the first optical branching element, which adds variable spherical aberration to the irradiated light;
A lens for collecting the branched reflected light;
A light receiving element that receives light collected by the lens and converts it into an electrical signal;
The reflected light to be branched is further divided into a first light flux near the optical axis of the reflected light and a second light flux in the peripheral portion and branched so as to be condensed on the light receiving element. 2 optical branching elements
An optical head added to the luminous flux;
An arithmetic circuit that obtains a reproduction signal and a defocus signal from an electrical signal derived from the light receiving element ;
First and second defocus signals are detected for each of the first light flux and the second light flux, and the spherical aberration adding mechanism is substantially controlled by the difference signal between them, and the variable is made by a sum signal. Control the focus mechanism,
The spherical aberration adding mechanism does not add aberration to the reflected light , but adds aberration to the incident light to the optical disc ,
Further, the first and second defocus signals are detected so as to compensate for the spherical aberration on the optical disk surface, and the spherical aberration error substantially obtained from the difference signal is converted into the spherical aberration adding mechanism. An optical disc apparatus having a feedback system for feeding back to the optical disc.   2. The optical disk apparatus according to claim 1, further comprising a loop for feeding back a drive signal for electrically driving the spherical aberration adding mechanism to a system for amplifying the spherical aberration error.   2. The optical disk apparatus according to claim 1, wherein the spherical aberration error is multiplied by an appropriate coefficient and added to a drive signal of the variable focus mechanism to drive the variable focus mechanism. A belt tracking control Organization to drive the objective lens in the radial direction of the optical disc,
An arithmetic circuit that obtains a reproduction signal, a defocus signal, and a tracking error signal from the electrical signal derived from the light receiving element;
A coma aberration adding mechanism;
It has a previous SL controls the tracking control mechanism by a tracking error signal, through a means for detecting the amount of movement of the tracking control at the time of the objective lens, to have a means for driving the coma additional mechanism in accordance with the amount of movement The optical disc apparatus according to claim 1. The optical head has means for independently detecting the first light flux by dividing it by a diameter in a direction tangential to the optical disc;
The means for detecting the amount of movement of the objective lens has means for calculating a difference between two detected light amounts of the first light flux detected by being divided.
5. The optical disk apparatus according to claim 4, wherein A tracking control Organization for driving the objective lens in the radial direction of the optical disc,
An arithmetic circuit that obtains a reproduction signal, a defocus signal, and a tracking error signal from the electrical signal derived from the light receiving element;
A coma aberration adding mechanism;
You have to control the tracking control mechanism by prior Symbol tracking error signal, through a means for detecting the coma aberration of the light converging spot on the optical disc, having a means for driving the coma additional mechanism in response to said detection signal 2. The optical disk apparatus according to claim 1, wherein

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