JP2011108359A - Device for recording and reproducing optical information - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for recording and reproducing optical information that reliably corrects aberrations at each recording layer of a multi-layer recording medium, and that stably records or reproduces the information. <P>SOLUTION: The device for recording and reproducing the optical information includes: a beam expander 6 which changes the divergence or convergent angle of light incident on an objective lens 10 and which corrects spherical aberrations generated at focusing light to a plurality of the recording layers of an optical disk 11; and a correction means which corrects coma aberrations generated by a tilt of the optical disk 11. The coma aberration correction means is driven by varying the magnitude of driving signals according to the recording layers of the optical disk 11. Thereby, no-coma aberrations are left in any recording layers even with the thicknesses of transmission layers various in the multi-layer recording medium. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical information recording / reproducing apparatus for recording or reproducing information on an optical information recording medium, and more particularly to a technique for correcting coma aberration when information is recorded or reproduced on an information recording medium having a plurality of recording layers. It is.

  In recent years, the demand for increasing the capacity of optical disks has been increasing. In order to improve the recording density of the optical disk, it is effective to reduce the beam spot diameter by increasing the NA (numerical aperture) of the objective lens, for example. In recent years, systems with an objective lens NA of 0.85 have come into practical use.

Also, increasing the recording layer of the optical disk is very effective for increasing the capacity because the capacity doubles without changing the optical system. However, since the amount of spherical aberration generated due to the difference in substrate thickness is proportional to NA ⁴ , the influence increases remarkably as the NA increases. For example, in the case of an optical system having a wavelength of 405 nm and NA of 0.85, the substrate thickness error is several μm and exceeds the diffraction limit. Therefore, it is necessary to correct spherical aberration even for a single-layer disc.

  When the recording layer of the optical disk is made multi-layered, the interlayer distance is required to be 10 μm or more, and the correction amount of the spherical aberration further increases. A beam expander that changes the divergence angle or the convergence angle of light incident on the objective lens has been put to practical use as a means for correcting spherical aberration.

  Furthermore, recently, in order to improve the system margin, it has become the mainstream to provide a coma aberration correcting mechanism including a recording DVD. In order to correct coma aberration, a method of tilting the entire optical pickup has been conventionally used. However, since the tilted portion is large and heavy, a space for providing the drive mechanism and drive source is required, and there is a problem that the apparatus becomes large.

  On the other hand, a method of tilting only the objective lens or a frame correction method using liquid crystal has been put into practical use. In the case of a system in which only the objective lens is tilted, the coma aberration is corrected by mounting the objective lens on the tilt actuator and tilting the objective lens according to the amount of disc tilt.

  In the case of a general objective lens and an infinite optical system, Japanese Patent Application Laid-Open No. 2005-108398 describes that the amount of coma generated by the tilt of the disk is substantially equal to the amount of coma generated by the tilt of the objective lens. (Patent Document 1). Therefore, if the disk and the objective lens are driven to be parallel, the coma aberration can be corrected almost optimally.

  An example of a frame correction method using liquid crystal is described in Japanese Patent Application Laid-Open No. 2000-090479 (Patent Document 2). In the publication, the liquid crystal element has a plurality of pattern electrodes having a shape corresponding to the shape of the distribution of coma aberration generated by the disk tilt, and the voltage applied to the liquid crystal element is changed according to the amount of the disk tilt. The coma with the opposite sign is generated to correct the coma.

  As described above, due to the recent trend toward higher NA of objective lenses and multilayered optical disks, it is expected that having a mechanism for simultaneously correcting coma and spherical aberration will become a necessary function for optical pickups. The

JP 2005-108398 A JP 2000-090479 A

  As described above, when spherical aberration is corrected by using non-parallel light by a beam expander or the like, the amount of coma generated by the tilt of the disc according to the thickness of the transmission layer of the disc and the tilt of the objective lens are generated. The coma aberration ratio changes.

  For this reason, when different transmission layer thickness conditions are used when reading from and writing to a multilayer disk, the optimum lens tilt amount or applied voltage to the liquid crystal element varies depending on the layer, and coma aberration depends on the same lens tilt amount or applied voltage to the liquid crystal element. When it is going to correct | amend, coma aberration will remain in either layer. For this reason, the imaging performance is lowered, and stable recording / reproduction becomes difficult.

  The object of the present invention is to enable good aberration correction in all layers and realize stable recording or reproduction even when aberration is corrected by tilting the objective lens with respect to each recording layer of the multilayer recording medium. An object is to provide an optical information recording / reproducing apparatus.

In order to achieve the above object, the present invention records information on the plurality of recording layers or reproduces the recorded information by condensing the light emitted from the light source onto the plurality of recording layers of the information recording medium by the objective lens. In the optical information recording / reproducing apparatus, spherical aberration correcting means for correcting spherical aberration generated when the emitted light is focused on a plurality of recording layers of the recording medium, and coma aberration generated due to the inclination of the recording medium. In order to generate coma aberration to be corrected, there is a means for generating coma aberration by tilting the objective lens, or a liquid crystal element having an electrode pattern having a shape similar to coma aberration generated by tilting the recording medium. A coma aberration correcting means comprising means for generating coma aberration by changing a voltage applied to the electrode; A driving unit that drives the coma aberration correcting unit according to a layer by changing a magnitude of a driving signal; a unit that detects a driving signal of the coma aberration correcting unit for each recording layer of the recording medium; and the recording Storing a drive signal for each recording layer of the medium, and
The driving unit drives the coma aberration correcting unit for each recording layer of the recording medium by a driving signal stored in the storage unit, and the detecting unit corrects spherical aberration by the spherical aberration correcting unit. Then, while changing the drive signal of the coma aberration correction unit, measure a predetermined index correlated with the tilt amount of the objective lens or the voltage applied to the electrode of the liquid crystal element of the coma aberration correction unit, A drive signal for each recording layer of the recording medium is detected based on a measurement result of a predetermined index.

  According to the present invention, by changing the magnitude of the drive signal of the coma aberration correcting means according to the recording layer of the recording medium, it is possible to perform good aberration correction in all the recording layers of the recording medium, and to stabilize Recording or reproduction can be realized.

1 is a block diagram showing a first embodiment of an optical information recording / reproducing apparatus according to the present invention. It is a figure which shows the light-receiving surface of RF and a servo sensor. It is a figure which shows the change of the aberration with respect to a disk tilt at the time of condensing a light beam to the 1st layer of an optical disk, and optimizing a spherical aberration with a beam expander. It is a figure which shows the change of the aberration with respect to the objective lens tilt at the time of condensing a light beam to the 1st layer of an optical disk, and optimizing spherical aberration with a beam expander. It is a figure which shows the change of the aberration with respect to a disc tilt at the time of condensing a light beam to the 2nd layer of an optical disk, and optimizing a spherical aberration with a beam expander. It is a figure which shows the change of the aberration with respect to the objective lens tilt at the time of condensing a light beam to the 2nd layer of an optical disk, and optimizing spherical aberration with a beam expander. It is a figure which shows the change of the aberration when a light beam is condensed on the 2nd layer of an optical disk, and the objective lens tilt of the same amount as a disk tilt is given when spherical aberration is optimized by a beam expander. It is a figure which shows the change of an aberration at the time of condensing a light beam to the 2nd layer of an optical disk, and optimizing a spherical aberration with a beam expander. It is a block diagram which shows the 4th Embodiment of this invention. It is a figure which shows the electrode pattern of a liquid crystal coma correction | amendment element. It is a figure which shows the phase difference which a wavefront aberration by a coma aberration and a liquid crystal coma correction element generate | occur | produce. It is a figure which shows the wavefront aberration after correction | amendment by a liquid crystal coma correction element.

  Next, the best mode for carrying out the invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing a first embodiment of an optical information recording / reproducing apparatus according to the present invention. In the figure, reference numeral 1 denotes a semiconductor laser as a light source, for example, the wavelength is 405 nm. Reference numeral 11 denotes an optical disc as an information recording medium, which is a two-layer disc having recording layers of a first layer 11a and a second layer 11b as shown in FIG. The second layer 11b is on the light incident side, and the transmission layer thickness is 70 μm. The second layer 11a is a recording layer opposite to the light incident side, and the transmission thick layer is 100 μm.

  Reference numeral 2 denotes a diffraction grating, 3 denotes a polarizing beam splitter, 4 denotes a front monitor sensor, and 5 denotes a collimator lens. Reference numeral 6 denotes a beam expander for correcting spherical aberration, and includes a concave lens 6a and a convex lens 6b.

  Of the two lenses, the convex lens 6b is movable in the optical axis direction by driving of the drive mechanism 19, and the lens interval between the concave lens 6a and the convex lens 6b can be varied. The other concave lens 6a may be used as the lens movable in the optical axis direction.

  The drive mechanism 19 includes a drive source such as a stepping motor and a gear mechanism that converts the drive force into a linear motion of the convex lens 6b in the optical axis direction. The controller 21 controls the SA drive circuit 20 and adjusts the position of the convex lens 6 b in the optical axis direction by driving the drive mechanism 19 to correct spherical aberration in each recording layer of the optical disc 11.

  7 is a quarter wavelength plate, 8 is a flip-up mirror, 9 is an objective lens actuator, and 10 is an objective lens. The NA of the objective lens 10 is 0.85, and the objective lens 10 is designed to have no aberration in an infinite system when the transmission layer thickness is 100 μm.

  The objective lens actuator 9 is an actuator having a driving mechanism of three axes in total, which is a focus direction, two-axis translation in the radial direction of the optical disk 11, and tilting in the radial direction of the optical disk 11. It is mounted on the lens actuator 9. A drive signal is supplied from the tilt drive circuit 22 to the tilt coil of the objective lens actuator 9 based on the control of the controller 21, and the coma aberration is corrected by tilting the objective lens 10.

  Reference numeral 12 denotes a sensor lens, and 13 denotes an RF / servo sensor. The RF generation circuit 23 generates an RF reproduction signal from the output of the RF / servo sensor 13 as will be described later. Similarly, the servo error generation circuit 24 generates a focus error signal and a tracking error signal from the output of the RF / servo sensor 13.

  The controller 21 is a control circuit that controls each unit in the apparatus, and performs control for recording information on each recording layer of the optical disc 11 or reading information. Further, as will be described in detail later, spherical aberration correction control or coma aberration correction control is performed. The memory 25 stores an optimum drive signal value for correcting coma for each recording layer of the optical disc 11.

  Furthermore, the controller 21 drives the focus actuator and tracking actuator of the objective lens actuator 9 based on the focus error signal and tracking error signal described above, and performs focus control and tracking control. In FIG. 1, the detailed configuration of these servo controls is omitted.

  Further, in FIG. 1, a modulation circuit that modulates recording data by a predetermined modulation method, a laser driving circuit that drives the semiconductor laser 1 in accordance with a recording signal from the modulation circuit, or a demodulation circuit that performs demodulation processing is omitted. ing. Further, a mechanism such as a spindle motor for rotating the optical disk 11 is also omitted.

  The light beam emitted from the semiconductor laser 1 passes through the diffraction grating 2 and is divided into three beams and enters the polarization beam splitter 3. A part of the light beam incident on the polarization beam splitter 3 is reflected and enters the front monitor sensor 4. APC control of the output of the semiconductor laser 1 is performed based on the output of the front monitor sensor 4.

  The light beam that has passed through the polarization beam splitter 3 is converted into a parallel light beam by the collimator lens 5, and the parallel light beam that has passed through the collimator lens 5 is incident on the beam expander 6 including the concave lens 6 a and the convex lens 6 b. In the beam expander 6, the convex lens 6b is movable in the optical axis direction as described above, and the divergence angle or convergence angle of the incident light beam to the objective lens 10 can be changed.

  The light beam that has passed through the beam expander 6 passes through the quarter-wave plate 7, is reflected by the flip-up mirror 8, and is further collected on the information recording surface of the optical disk 11 by the objective lens 10. Reflected light from the optical disk 11 enters the polarization beam splitter 3 via the objective lens 10, the quarter-wave plate 7, the flip-up mirror 8, and the beam expander 6.

  The incident light is reflected by the polarization beam splitter 3 and condensed on the RF / servo sensor 13 via the sensor lens 12. As described above, an RF reproduction signal, a focus error signal, a tracking error signal, and the like are obtained from the output of the RF / servo sensor 13.

FIG. 2 shows a light receiving surface of the RF / servo sensor 13. The RF / servo sensor 13 includes a central quadrant sensor and two split sensors on both sides thereof. When the light receiving surfaces of the RF / servo sensor 13 are a to h and the outputs from the respective light receiving surfaces are A to H, the focus error signal FE is obtained by the astigmatism method.
FE = (A + C) − (B + D) (1)
Is obtained by the following calculation.

The tracking error signal TE is obtained by a differential push-pull method.
TE = {(A + D)-(B + C)}-k {(FE) + (HG)} (2)
Is obtained by the following calculation. These focus error signal and tracking error signal are generated by the servo error generation circuit 24 and output to the controller 21. The tracking error signal of equation (2) is called a push-pull signal, and coma aberration is corrected in each recording layer of the optical disc 11 based on the push-pull signal amplitude.

Furthermore, the RF reproduction signal is the sum of the four-divided sensors,
RF reproduction signal = A + B + C + D (3)
Is obtained by the following calculation. This RF reproduction signal is generated by the RF generation circuit 23 as described above, and is output to the controller 21.

  As will be described later in detail, the controller 21 detects the amplitude of the push-pull signal during the tilt amount optimizing operation such as when the power is turned on, and searches for the optimum drive signal that maximizes the push-pull signal amplitude in each recording layer of the optical disc 11. . Then, at the time of recording or reproduction, the coma aberration is corrected by applying an optimum drive signal to the tilt coil in each recording layer.

  As will be described later, the controller 21 detects the RF reproduction signal amplitude and adjusts the position of the convex lens 6b so as to maximize the RF reproduction signal amplitude, thereby correcting the spherical aberration in each recording layer of the optical disc 11.

  Here, when the light beam from the semiconductor laser 1 is condensed on the first layer 11a of the optical disk 11 and the spherical aberration is optimized by the beam expander 6, the change of the aberration with respect to the disk tilt and the change of the aberration with respect to the objective lens tilt. Are shown in FIGS.

  In the legend, rms is the least square error value of the wavefront aberration, and COMA and AS are the least square error values of the third-order coma and astigmatism components, respectively. 3 and 4, an amount substantially equal to the disc tilt amount is the optimum objective lens tilt amount.

  Similarly, when the light beam from the semiconductor laser 1 is focused on the second layer 11b of the optical disk 11 and the spherical aberration is optimized by the expander lens 6, the change of the aberration with respect to the disk tilt and the change of the aberration with respect to the objective lens tilt. Are shown in FIGS.

  According to FIGS. 5 and 6, for example, when the objective lens is tilted by 0.55 degrees, coma aberration of 81 mλrms is generated, but when the optical disk 11 is tilted by 0.5 degrees, only coma aberration of 37 mλrms is generated. I understand that I don't.

  FIG. 7 shows a change in aberration when an objective lens tilt of the same amount as the disc tilt is given to the second layer 11 b of the optical disc 11. As can be seen from FIG. 7, in the second layer 11b of the optical disc 11, when a disc tilt of 0.5 degrees is corrected with the same amount of objective lens tilt, a coma aberration of 44 mλ rms remains.

  Next, FIG. 8 shows a change in aberration when an objective lens tilt of 0.45 times the tilt of the optical disk 11 is given. As can be seen from FIG. 8, the coma aberration can be corrected almost completely by setting the amount of tilt of the objective lens 10 in the second layer 11b to 0.45 times that of the first layer 11a. Even when tilted, it is within 8 mλrms. In this embodiment, the coma aberration is corrected by tilting the objective lens 10.

Thus, in the present invention, the objective lens tilt amount is optimized by making the tilt amount of the objective lens 10 in the second layer 11b of the optical disk 11 0.45 times that of the first layer 11a.

  Next, the operation of this embodiment will be described. The objective lens tilt amount is optimized when the power is turned on or the disk is replaced. First, the controller 21 controls the SA drive circuit 20, aligns the position of the convex lens 6b of the beam expander 6 so as to correct the spherical aberration in the first layer 11a of the optical disc 11, and pulls the focus into the first layer 11a.

  Specifically, the controller 21 controls the drive circuit 20 to move the convex lens 6b in the direction of the optical axis by driving the drive mechanism 19, fetches the RF reproduction signal at that time from the RF generation circuit 23, and sets the RF reproduction signal amplitude. To detect. Then, the controller 21 adjusts the position of the convex lens 6b so that the RF reproduction signal amplitude becomes maximum, and corrects the spherical aberration in the first layer 11a. In the following description, correction of spherical aberration in each recording layer of the optical disc 11 is performed by the same method.

  The controller 21 controls the tilt drive circuit 22 to apply a drive voltage to the tilt coil of the objective lens actuator 9. Then, the push-pull signal is fetched from the servo error generation circuit 24 while changing the drive voltage and the tilt amount is changed, and the push-pull signal amplitude is detected to search for the optimum drive voltage. That is, the controller 21 detects the drive voltage value when the push-pull signal amplitude becomes maximum, and sets it as the optimum drive signal value.

  Next, the controller 21 aligns the position of the convex lens 6b of the beam expander 6 with the second layer 11b of the optical disc 11, and draws the focus into the second layer 11b. Also in this case, the position of the convex lens 6b is adjusted so that the RF reproduction signal amplitude value is maximized.

  Similarly, the controller 21 controls the tilt drive circuit 22, detects the push-pull signal amplitude while changing the tilt amount, and detects the drive voltage when the push-pull signal amplitude is maximum as the optimum drive signal. The optimum drive signal values of the first layer 11 a and the second layer 11 b of the optical disc 11 are stored in the memory 25.

  Next, when data is written to or read from the optical disc 11, the optimum drive signal stored in the memory 25 is applied to the tilt coil of the objective lens actuator 9 in accordance with the recording layer to be focused on the optical disc 11.

  First, when reading from and writing to the first layer 11 a of the optical disc 11, the controller 21 controls the tilt drive circuit 22 to send the optimum drive signal corresponding to the first layer 11 a stored in the memory 25 to the objective lens actuator 9. Apply to the tilt coil. Subsequently, the controller 21 adjusts the position of the convex lens 6b of the beam expander 6 in order to correct the spherical aberration with respect to the first layer 11a of the optical disc 11, and brings the focus into the first layer 11a. Thereafter, tracking control is performed on the target track, and data is read and written.

  Next, when reading from and writing to the second layer 11 b of the optical disk 11, the optimum drive signal corresponding to the second layer 11 b stored in the memory 25 is similarly sent from the tilt drive circuit 22 to the tilt coil of the objective lens actuator 9. Apply. Subsequently, the position of the convex lens 6b of the beam expander 6 is adjusted in order to correct spherical aberration with respect to the second layer 11b of the optical disc 11, and the focus is drawn into the second layer 11b. Also, tracking control is performed on the target track, and data is read and written.

  In this embodiment, the push-pull signal is used as an index for optimizing the tilt. However, as long as the signal has a correlation with the tilt amount, another index such as an RF reproduction signal amplitude may be used. Further, although a two-layer medium is used, the present invention can be used even in the case of a multilayer medium having three or more layers. Further, although a beam expander is used to correct spherical aberration, a finite system is used, but the effect is the same when other methods such as a liquid crystal element are used.

  In this embodiment, different objective lens tilt amounts are given to the plurality of recording layers of the optical disc 11, so that even when correcting large spherical aberration, the multi-layer medium is not affected by fluctuations in the amount of coma generated due to finite light beam incidence. Stable recording / reproduction is possible for all the layers.

( Second Embodiment )
FIG. 9 is a block diagram showing the configuration of the second exemplary embodiment of the present invention. In FIG. 9, the same parts as those in FIG. This embodiment is different from FIG. 1 in that a liquid crystal coma correction element 17 is provided between the beam expander 6 and the flip-up mirror 8. Reference numeral 27 denotes a liquid crystal driving circuit for applying a driving voltage to the liquid crystal coma correction element 17.

  The objective lens actuator 18 has a biaxial translation drive mechanism in the focus direction and the radial direction of the optical disk 11. The objective lens 10 is mounted on the objective lens actuator 18.

  The wavelength of the semiconductor laser 1 is 405 nm, and the transmission layer thickness of the optical disk 11 is 100 μm for the first layer 11 a and 70 μm for the second layer 11 b. The NA of the objective lens 10 is 0.85, and the objective lens 10 is designed to have no aberration in an infinite system when the transmission layer thickness is 100 μm.

  An electrode pattern of the liquid crystal coma correction element 17 is shown in FIG. The electrode pattern has a shape similar to coma generated by disc tilt, and a phase difference can be generated according to the voltage applied to the electrode.

  FIG. 11 shows the wavefront aberration due to the ma aberration and the phase difference generated by the liquid crystal coma correction element 17 when the disc radial direction is taken as an axis. Similarly, FIG. 12 shows the wavefront aberration after correction. It is possible to correct the coma aberration by giving the phase difference of the opposite sign of the generated coma aberration. The liquid crystal coma correction element 17 is described in detail in, for example, Japanese Patent Application Laid-Open Nos. 2000-090479 and 2000-3526.

  Next, a case where the optical axis incident on the objective lens 10 is tilted by 0.5 degrees due to an adjustment error will be described. The inclination of the objective lens 10 is adjusted to be parallel to the disk surface.

  First, a case where there is no disc tilt will be described. The light beam from the semiconductor laser 1 is focused on the first layer 11a of the optical disc 11, and the spherical aberration is optimized by the beam expander 6 and is focused on the second layer 11b, and the spherical aberration is optimized by the beam expander 6. Compare in the same state. In this case, the amount of coma generated in the first layer 11a is almost zero, whereas a coma of 44 mλrms is generated in the second layer 11b.

  Next, a case where the disc tilt is 0.3 degrees will be described. Similarly, the light beam is focused on the first layer 11a of the optical disc 11 and the spherical aberration is optimized by the beam expander 6, and the light beam is focused on the second layer 11b and the spherical aberration is optimized by the beam expander 6. Compare in a state that In this case, coma aberration of 32 mλrms occurs in the first layer 11a, whereas coma aberration of 66 mλrms occurs in the second layer 11b.

  Next, the operation of this embodiment will be described. Similarly, an operation for optimizing the voltage applied to the liquid crystal coma correction element 17 is performed when the power is turned on or the disk is replaced. First, the controller 21 aligns the position of the convex lens 6b of the beam expander 6 so as to correct the spherical aberration in the first layer 11a of the optical disc 11, and draws the focus into the first layer 11a. Also in this case, the position of the convex lens 6b is adjusted so that the RF reproduction signal amplitude becomes maximum as described above.

  The controller 21 controls the liquid crystal drive circuit 27 that drives the liquid crystal coma correction element 17 to detect the push-pull signal amplitude while changing the voltage applied to the liquid crystal coma correction element 17. Then, the drive voltage to the liquid crystal coma correction element 17 when the push-pull signal amplitude becomes maximum is detected as the optimum drive signal of the first layer 11a.

  Next, the position of the convex lens 6b of the beam expander 6 is adjusted with respect to the second layer 11b of the optical disc 11, and the focus is drawn into the second layer 11b. Similarly, the push-pull signal amplitude is detected while changing the voltage applied to the liquid crystal coma correction element 17, and the applied voltage to the liquid crystal coma correction element 17 when the push-pull signal amplitude becomes maximum is applied to the second layer 11b. It is detected as the optimum drive signal. The optimum drive signals for the first layer and the second layer are stored in the memory 25.

  For example, when there is almost no disc tilt, the applied voltage on the first layer 11a is almost zero, and the applied voltage on the second layer 11b is optimized to correct the coma aberration of 44 mλrms. When the disc tilt is 0.3 degrees, the applied voltage at the first layer 11a is optimized to correct the coma aberration of 32 mλrms, and the applied voltage at the second layer 11b has a coma aberration of 66 mλ. Optimized to correct.

  Next, when data is written to or read from the optical disk 11, the optimum drive voltage stored in the memory 25 is applied from the liquid crystal drive circuit 27 to the liquid crystal coma correction element 17 in accordance with the recording layer to be recorded on or reproduced from the optical disk 11. To do.

  First, a case where data is read from and written to the first layer 11a of the optical disc 11 will be described. The controller 21 controls the liquid crystal drive circuit 27 and applies an optimum drive signal corresponding to the first layer 11 a stored in the memory 25 to the liquid crystal coma correction element 17. Subsequently, the position of the convex lens 6b of the beam expander 6 is adjusted with respect to the first layer 11a of the optical disc 11, the focus is drawn into the first layer 11a, and information is read and written.

  In addition, when reading / writing from / to the second layer 11 b of the optical disc 11, an optimum drive signal corresponding to the second layer 11 b stored in the memory 25 is applied from the liquid crystal drive circuit 27 to the liquid crystal coma correction element 17. Subsequently, the position of the convex lens 6b of the beam expander 6 is adjusted with respect to the second layer 11b, the focus is drawn into the second layer 11b, and information is read and written.

  In this embodiment, the push-pull signal is used as an index for optimizing the tilt. However, any other index such as an RF signal amplitude may be used as long as the signal has a correlation with the tilt amount. Further, although a two-layer medium is used, the present invention can be applied to a multilayer medium having three or more layers. Furthermore, in order to correct spherical aberration, an expander is used as a finite system. However, the effect is the same even if other methods such as a liquid crystal element are used as long as the system corrects by changing the power of light incident on the objective lens. .

  As described above, according to the present embodiment, different driving voltages are applied to the liquid crystal coma correction elements for the recording layers for reading and writing on the optical disc. Therefore, even when correcting coma aberration with a liquid crystal element, stable recording and reproduction can be performed on all layers of a multilayer medium without being affected by variations in the amount of coma generated due to the incidence of a finite light beam when correcting large spherical aberration. It becomes possible.

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Diffraction grating 3 Polarizing beam splitter 4 Front monitor sensor 5 Collimator lens 6 Beam expander 6a Concave lens 6b Convex lens 7 1/4 wavelength plate 8 Bounce mirror 9 Objective lens actuator 10 Objective lens 11 Optical disk 11a First layer 11b First 2-layer 12 sensor lens 13 servo / RF sensor 17 liquid crystal coma correction element 18 biaxial objective lens actuator 19 drive mechanism 20 SA drive circuit 21 controller 22 tilt drive circuit 23 RF generation circuit 24 servo error generation circuit 25 memory 27 liquid crystal drive circuit

Claims (1)

In the optical information recording / reproducing apparatus for recording information on the plurality of recording layers or reproducing the recorded information by condensing the emitted light from the light source onto the plurality of recording layers of the information recording medium by the objective lens,
Spherical aberration correction means for correcting spherical aberration generated when the emitted light is focused on a plurality of recording layers of the recording medium;
Means for generating coma by tilting the objective lens to generate coma for correcting coma generated by tilt of the recording medium, or similar to coma generated by tilting of the recording medium A coma aberration correcting means comprising a liquid crystal element having an electrode pattern of the following shape, and means for generating coma aberration by changing a voltage applied to the electrode;
Driving means for driving the coma aberration correcting means by varying the magnitude of the driving signal according to the recording layer of the recording medium;
Means for detecting a drive signal of the coma aberration correcting means for each recording layer of the recording medium in advance;
A drive means for each recording layer of the recording medium, and storage means,
The drive means drives the coma aberration correcting means by a drive signal stored in the storage means for each recording layer of the recording medium,
The detecting means changes the amount of tilt of the objective lens or the liquid crystal element of the coma aberration correcting means while changing the drive signal of the coma aberration correcting means after the spherical aberration is corrected by the spherical aberration correcting means. An optical information recording / reproducing apparatus, wherein a predetermined index correlated with a voltage applied to an electrode is measured, and a drive signal for each recording layer of the recording medium is detected based on a measurement result of the predetermined index.

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