JP3904893B2 - Optical pickup device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an aberration correction optical system for recording and reproducing information on an optical information recording medium, and an optical pickup apparatus using an aberration detection system.
[0002]
[Prior art]
Technology using light has many features such as high frequency (high speed), spatial information processing, phase processing, etc., so research and research in various fields such as communication, measurement, and processing. Development / practical use is in progress. Among the techniques, a high-precision objective lens is used to narrow the light beam.
[0003]
In recent years, there has been a great demand for an image recording apparatus that uses light in particular, and a technology for increasing the capacity has become very important. In order to increase the capacity of optical information recording, it is necessary to reduce the diameter of the beam spot, that is, to sufficiently narrow the beam spot by the objective lens, in addition to the improvement of the recording medium. As is well known, the beam spot diameter is proportional to the wavelength of light and inversely proportional to the NA (Numerical Aperture) of the objective lens. In recent years, blue laser diodes and blue or green SHG lasers are being developed in terms of wavelength. On the other hand, the NA of the objective lens has been increased to NA 0.6 in the DVD (Digital Versatile Discs) compared to the NA 0.45 of the CD (Compact Disc). Further, an optical pickup device using two lenses in two groups and having an NA of 0.85 and aiming at higher density is disclosed in Japanese Patent Laid-Open No. 10-123410.
[0004]
In such an optical pickup device using an objective lens having a high NA, for example, for the purpose of correcting variations in the thickness of the light transmission layer of the medium and spherical aberration that occurs when performing multilayer recording, for example, JP-A-2001-2001 As disclosed in Japanese Patent No. 143303, a liquid crystal element for correcting spherical aberration is provided.
[0005]
As means for detecting spherical aberration, as disclosed in Japanese Patent Laid-Open No. 2000-1555979, the light beam incident on the light receiving unit is divided into a region close to the optical axis and a region far from the optical axis, and each focus position is detected.・ Spherical aberration is obtained by comparison.
[0006]
Furthermore, Japanese Patent Application Laid-Open No. 2001-34996 proposes an optical pickup using liquid crystal as aberration correction means. This is shown in FIG. Here, circularly polarized light is incident on the disk 101, and the reflected light beam is converted into linearly polarized light that forms an angle of 90 degrees with the quarter-wave plate 102, so that the total amount of light is reflected by the polarizing beam splitter 103, and the light receiving unit. 104 is made to enter.
[0007]
[Problems to be solved by the invention]
However, the liquid crystal element used for such aberration correction means has a configuration in which liquid crystal is sandwiched between electrodes formed on two glass substrates, and a voltage is applied to the electrodes in order to correct spherical aberration. The phase distribution is formed by changing the refractive index by changing the orientation of the liquid crystal. At this time, the refractive index can be changed only by linearly polarized light in one direction that becomes abnormal light with respect to the alignment direction of the liquid crystal element.
[0008]
In addition, because the phase distribution of the liquid crystal element has an inflection point, large aberrations occur due to misalignment with the objective lens, and the objective lens shift during tracking cannot be allowed. An objective lens, a quarter-wave plate, and a liquid crystal element were mounted on the tracking actuator.
[0009]
Therefore, since the polarization direction of the light beam transmitted through the liquid crystal element differs by 90 degrees on the return and the return, the light becomes abnormal light with respect to the liquid crystal element on the return and undergoes phase modulation. It will be.
[0010]
Therefore, when spherical aberration occurs when the light transmission layer thickness is different, by applying phase modulation with a liquid crystal element to correct the aberration, a good beam without spherical aberration can be obtained on the recording surface. Since the light beam reflected on the recording surface and returning to the light receiving portion is not subjected to phase modulation by the liquid crystal element, spherical aberration due to the thickness of the returning light transmission layer remains. When the aberration is detected, the aberration on the recording surface is significantly different from the aberration on the light receiving section, so it is impossible to grasp the spherical aberration information on the recording surface, and it is not possible to correctly feed back to the aberration correction optical system. There was a problem that correction would be difficult.
[0011]
[Means for Solving the Problems]
In order to solve the above problems, the present invention has the following configuration.
[0012]
  In the optical pickup device of the present invention, a light source, an aberration detection system,TheIn an optical pickup device including an objective lens, a wave plate, an aberration correction optical system, and an optical path dividing unit using polarized light in an optical path from a light source to an optical recording medium,SaidWith aberration correction opticsSaidBetween wave platesSaidArranges optical path splitting means using polarized lightThe optical path splitting unit transmits only linearly polarized extraordinary light out of the reflected light from the recording medium, and the correction optical system imparts phase modulation to the extraordinary light as linearly polarized light emitted from the light source. At the same time, the abnormal light transmitted through the optical path dividing means is phase-modulated, and the aberration detection system receives the abnormal light that is reflected light from the recording medium and phase-modulated in the correction optical system.It is characterized by doing.
[0013]
The optical pickup device of the present invention is characterized in that an RF signal is detected by one of the light beams divided by the optical path dividing means by polarized light, and a track servo, focus servo, and spherical aberration are detected by the other light beam. .
[0014]
  In the optical pickup device of the present invention,The light beam incident on the optical recording medium is elliptically polarized light,The ellipticity of elliptically polarized light is 0.4 or more.
[0015]
Further, in the optical pickup device of the present invention, the aberration detection system has a light beam splitting unit that splits the light beam and makes it incident on a predetermined light receiving element, and outputs a signal obtained from at least one of the split light beams. Based on this, spherical aberration is detected.
[0016]
The optical pickup device of the present invention further includes a light beam splitting unit that forms a plurality of light spots on the optical recording medium, and the aberration detection system includes at least one reflected light of the split light spots. Based on the above, spherical aberration is detected.
[0017]
In the optical pickup device of the present invention, liquid crystal is used for the aberration correction optical system.
[0018]
In the optical pickup device of the present invention, the NA of the objective lens is 0.75 or more, and the aberration correction optical system includes a liquid crystal element.
[0019]
Further, in the optical pickup device of the present invention, the aberration correction optical system has a plurality of concentric electrodes, and the phase distribution curve applied when correcting the spherical aberration has a phase distribution curve throughout the aberration correction. The electrodes in the vicinity of the intermediate value are set to the same potential, and the potentials of the other portions of the electrode are varied.
[0020]
Further, in the optical pickup device of the present invention, the aberration correction optical system is characterized in that the phase distribution applied when correcting the spherical aberration is a Fresnel lens shape divided into a plurality of concentric regions. And
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Example 1
Examples of the present invention will be described below.
[0022]
FIG. 1 shows a configuration diagram of an optical pickup according to the first embodiment.
[0023]
Linearly polarized laser light emitted from the light source LD 1 is formed in a region larger than the effective light beam diameter on the side surface of the light source of the hologram element 2, and the light beam is divided into zero-order diffracted light, first-order diffracted light, −1 by the diffraction grating 3. Branched to the next diffracted light. Thereafter, the light beam passes through the hologram 4 formed on the surface opposite to the diffraction grating 3, becomes a parallel light beam by the collimator lens 5, and enters the shaping prism 6. The shaping prism 6 is for shaping the laser light emitted from the LD 1 into an almost circular shape because the intensity distribution of the laser light is elliptic.
[0024]
Thereafter, phase modulation corresponding to the spherical aberration due to the difference in the thickness of the light transmission layer is given by the liquid crystal element 7, transmitted through the polarization beam splitter 8, and converted into elliptically polarized light by the quarter wavelength plate 9.
[0025]
Here, the relationship between the angle θ formed by the optical axis of the quarter-wave plate 9 and the direction of incident linearly polarized light is shown in FIG. θ is set to about 30 degrees, and the light beam transmitted through the quarter-wave plate 9 is elliptically polarized light.
[0026]
The polarizing beam splitter 8 is designed to transmit 100% of p-polarized light and reflect 100% of s-polarized light. The light beam (outward path) emitted from the LD 1 is p-polarized light and transmits almost the entire amount of light through the polarization beam splitter.
[0027]
Next, after a light beam is raised by a 45 degree mirror (not shown), it is narrowed down by the objective lens 10 and reaches the optical recording medium 13.
[0028]
The configuration of the optical recording medium 13 is shown in FIG. The optical recording medium 13 includes a light transmission layer 11, a recording surface 12, and a substrate 14, and a beam spot is formed on the recording surface 12 through the light transmission layer 11. Here, the center value of the thickness of the light transmission layer 11 is 0.1 mm.
[0029]
The light reflected (modulated) by the recording surface 12 follows the path in reverse. Then, as shown in FIG. 1, the light is returned to linearly polarized light (p-polarized light and s-polarized light) by the quarter-wave plate 9. At this time, since the polarization direction is rotated 60 degrees in the forward and backward directions, the polarization beam splitter 8 Thus, s-polarized light, that is, ordinary light with respect to the liquid crystal element is bent at a substantially right angle, and p-polarized light is transmitted through the polarizing beam splitter 8. The s-polarized light enters the RF light receiving unit 16 through the condenser lens 15. The RF light receiving unit 16 uses 0th-order diffracted light.
[0030]
In a high-speed and large-capacity optical disk system, the RF signal is required to have a high bandwidth, and in order to satisfy this, it is necessary to reduce the area of the light receiving unit. In the present embodiment, the condensing lens 15 is disposed in front of the RF light receiving unit 16, and a small spot can be formed on the RF light receiving unit 16, and the light receiving unit is not affected by the objective lens shift or the like. The area may be a minimum, and in this embodiment, it is a square having a size of 50 μm.
[0031]
Even in such a small light receiving portion, since it is not shared with focus servo, track servo, spherical aberration detection, etc., positioning of the light receiving portion is easy and the band can be improved.
[0032]
On the other hand, p-polarized light, that is, extraordinary light with respect to the liquid crystal element 7 is transmitted through the polarization beam splitter 8, phase modulated by the liquid crystal element 7, and then diffracted by the hologram 4 through the collimator lens 5. 17 is incident. The servo signal light receiving unit 17 detects a focus error signal and a spherical aberration error signal using the 0th-order diffracted light, and detects a track error signal using the 1st-order diffracted light, the −1st-order diffracted light, and the 0th-order diffracted light.
[0033]
The spherical aberration amount is calculated by the aberration detection circuit from the optical signal detected by the servo signal light receiving unit 17, and the liquid crystal drive circuit drives the liquid crystal element 7 based on the signal. The light beam reaching the servo signal light receiving unit 17 is a liquid crystal that emits a laser beam in the liquid crystal element 7 and reaches the optical recording medium (outward path) and the light beam reflected by the optical recording medium and returned to the servo signal light receiving unit 17 (return path). Since the light has the same polarization direction with respect to the element 7, phase modulation corresponding to the amount of spherical aberration generated by the thickness of the light transmission layer is given, and the spherical aberration on the recording surface and the servo signal light receiving unit The spherical aberration at 17 is equivalent, detection and correction can be performed in a closed loop, and more accurate spherical aberration can be corrected.
[0034]
The focus error detection uses a hologram Foucault method, and the track error detection uses a phase difference push-pull method (Differential Push-Pull). Details of the aberration detection system will be described later.
[0035]
Furthermore, the objective lens 10 used here is fixed to a lens holder (not shown), and the lens holder is fixed to an optical pickup body (not shown) with four wires (not shown). The objective lens 10 is designed to have almost no aberration when a parallel light beam is incident at an NA of 0.85 (so-called infinite conjugate) and the thickness of the light transmission layer 11 is 0.1 mm. The one designed to have a diameter of 405 nm, an effective beam diameter of φ3, and a focal length of 1.76 mm is used.
[0036]
Although the configuration of the present embodiment has been described above, the reason why the aberration correction optical system, the quarter wavelength plate, and the polarization beam splitter have such an arrangement configuration will be described based on the light beam.
[0037]
First, the linearly polarized light beam emitted from the light source passes through the liquid crystal element, reflects off the disk, then passes through the liquid crystal element again, and returns to the light receiving section. As described above, phase modulation is applied to the liquid crystal element. In order to achieve this, it is necessary that the light beam incident on the liquid crystal element is abnormal light with respect to the liquid crystal element, that is, linearly polarized light parallel to the alignment direction of the liquid crystal is incident. The luminous flux emitted from the light source is arranged in this way with respect to the liquid crystal element. With respect to linearly polarized light in such a direction, the liquid crystal element continuously changes the angle of the liquid crystal molecules in the direction perpendicular to the glass plate in accordance with the applied voltage, and the incident light flux in the polarization direction. Thus, it has a refractive index distribution according to the direction of the liquid crystal molecules, and can thereby apply phase modulation.
[0038]
In addition, when linearly polarized light that is polarized in the direction of normal light, that is, in the direction perpendicular to the alignment direction of the liquid crystal element is incident, the refractive index is constant regardless of the direction of the liquid crystal molecules. Since there is no distribution and phase modulation cannot be applied, spherical aberration cannot be corrected.
[0039]
On the other hand, the light beam reflected by the recording surface returns to linearly polarized light by the ¼ wavelength plate, but the polarizing beam splitter transmits only abnormal light to the liquid crystal element and reflects ordinary light. Only the extraordinary light is incident on the liquid crystal element, and the return path is also subjected to phase modulation. Therefore, when correcting the spherical aberration due to the difference in the thickness of the light transmission layer, the recording is performed by adjusting the aberration correction system, that is, the liquid crystal element in this embodiment so as to minimize the spherical aberration of the light beam incident on the light receiving portion. Spherical aberration on the surface can be minimized.
[0040]
Further, since the beam spot is configured to be elliptically polarized on the recording medium, it is preferable to effectively use the linearly polarized light component that does not contribute to the aberration correction optical system. Therefore, the light can be effectively used by configuring the quarter wavelength plate, the polarization beam splitter, and the aberration correction optical system (liquid crystal) in this order from the recording medium side. It can be said that the polarization component bent at 90 degrees by the polarization beam splitter is the most preferable form to be used for detecting the RF signal.
[0041]
Hereinafter, the aberration correction system, the aberration detection system, and the polarization state on the recording medium will be described in more detail.
[0042]
(Aberration correction system)
In this embodiment, a liquid crystal element is used as the aberration correction element. 4A and 4B are configuration diagrams of the liquid crystal element of this example.
[0043]
The liquid crystal element 7 includes a first glass plate 19, a liquid crystal 18 made of a nematic liquid crystal composition used for a display, a second glass plate 20, and a sealing material 28, and an optical axis of liquid crystal molecules. A material having a birefringence effect in which the refractive index differs between the direction and the direction perpendicular thereto. An electrode 21, an insulating layer 22, and an alignment layer 23 are formed on the first glass plate 19. Similarly, an electrode 25, an insulating layer 26, and an alignment layer 27 are formed on the second glass plate 20, and the electrode 25 is a circular common electrode.
[0044]
Here, it has a structure in which the liquid crystal 18 is sandwiched between transparent electrodes facing each other, for example, an ITO film deposited on a glass substrate, and the alignment state of the liquid crystal 18 is changed by adjusting the applied voltage between the transparent electrodes. In addition, when light incident from one transparent electrode side passes through the liquid crystal, birefringence change corresponding to the alignment state is given to the light, and the light is emitted to the other transparent electrode side.
[0045]
FIG. 4A shows a state before voltage application, and FIG. 4B shows a state where an appropriate voltage is applied. When no voltage is applied, the glass plate is oriented in one direction in the in-plane direction of the glass plate, so-called homogeneous orientation, and the liquid crystal molecules are oriented in a direction perpendicular to the glass plate by the voltage applied to the transparent electrode. Can be changed. Of course, when no voltage is applied, there may be a slight angle (so-called pretilt angle) in the direction perpendicular to the glass plate.
[0046]
Furthermore, a bias voltage may be given in advance in order to improve the response to the applied voltage or use a region where the relationship between the applied voltage and the refractive index is linear.
[0047]
Here, the light incident on the liquid crystal element 7 is linearly polarized light, and the polarization direction is a direction that becomes abnormal light with respect to the alignment method of the liquid crystal 18. As the liquid crystal material, nematic liquid crystal is taken as an example, but the same effect can be obtained if the liquid crystal has birefringence.
[0048]
FIG. 5 shows the shape of the electrode formed on the first glass plate 19. These are composed of three regions divided concentrically. FIG. 5A is a front view and FIG. 5B is a cross-sectional view. Further, FIG. 5C shows the electric field distribution corresponding to the cross section. Here, electrodes 21a, 21b, and 21c and a metal electrode 21d are provided, and lead wires 24a, 24b, and 24c are connected to each other. A combination of a high-resistance transparent electrode (ITO, etc.) and a low-resistance metal electrode (gold, aluminum, etc.) creates an electric field distribution from the center to the periphery, thereby causing a phase difference in the liquid crystal 18 and reducing aberrations. Can be removed. For example, by applying voltages of V1 = 4V, V2 = 2V, and V3 = 1V to the lead wires 24a, 24b, and 24c so that a low-resistance metal electrode is energized, the electric field distribution as shown in FIG. Thus, an aberration correction optical system can be obtained in which the refractive index is low at the center and gradually increases toward the periphery.
[0049]
As a result, an aberration correction optical system can be obtained in which there is no aberration deterioration due to misalignment between the liquid crystal element 7 and the objective lens 10 and the liquid crystal element need not be mounted on the focusing / tracking actuator.
[0050]
Examples of other electric field strength distributions of the liquid crystal element shown in FIG. 5 are shown in FIGS. 6 (a) to 6 (c). As shown in FIG. 6A, a liquid crystal element that is resistant to misalignment with the objective lens can also be obtained with the phase distribution divided into regions. The number of divisions and the region are appropriately determined from the amount of change in the refractive index of the liquid crystal, the thickness, the amount of spherical aberration correction, the number of electrodes, and the like.
[0051]
Further, in the case of a liquid crystal element, the maximum amount for modulating the phase is determined by the product of the change in refractive index of the liquid crystal and the thickness of the liquid crystal. For example, when the light transmission layer thickness fluctuates in the plus direction and the minus direction with respect to the reference value, a phase distribution that is inverted up and down is necessary, and the electric field distribution corresponding thereto is as shown in FIG. The same shape. At this time, the phase necessary for correction can be obtained by setting the same electric potential in the positive and negative directions of the light transmission layer thickness near the midpoint of the entire electric field (denoted as the same potential position in the figure). It is possible to suppress the minimum liquid crystal thickness or refractive index change amount with respect to the amount, and there is also an effect that a good correction element such as high-speed response and transmittance can be obtained. FIG. 6C shows another phase distribution, but the effect is the same as described above.
[0052]
As described above, the voltage distribution can be formed by controlling the voltage values of the electrodes 21a, 21b, and 21c formed on the first glass plate, and the refractive index distribution corresponding to the voltage distribution can be created. Therefore, it is possible to configure an aberration correction optical system for different light transmission layer thicknesses.
[0053]
For example, since the optical pickup device is compatible with a multi-layer recording medium, the optical transmission device 11 is configured to perform recording / reproducing with respect to a thickness of ± 0.015 mm with respect to a thickness of 0.1 mm, which is the central value. The Here, by changing the voltage applied to the electrodes 21a, 21b, and 21c of the liquid crystal element 7, the spherical aberration amount on the recording surface 12 can be reduced corresponding to the spherical aberration amount caused by the difference in the light transmission layer thickness 11. It can be minimized.
[0054]
(Aberration detection system)
The aberration detection system refers to a light receiving unit that detects spherical aberration of the servo signal light receiving unit and a spherical aberration detection unit that calculates spherical aberration based on the amount of received light. In spherical aberration detection, light beams near and around the optical axis are divided by holograms, and a focus error signal is detected by the hologram Foucault method for each of the light beams near and around the optical axis, and the focus position of each light beam is determined. To detect. Here, the spherical aberration is an aberration in which the in-focus positions of the light beam near the optical axis and the light beam in the peripheral portion are different, so that the in-focus position of the two light beams as described above minimizes the spherical aberration. become.
[0055]
FIG. 7 shows a schematic diagram of a hologram pattern, and FIG. 8 shows a layout of light receiving parts of the aberration detector.
[0056]
The hologram pattern is divided into four regions 4a, 4b, 4c, and 4d as shown in FIG. 7A. The portion 4a diffracts the light beam in the central region close to the optical axis, and the portion 4b diffracts the light beam in the peripheral region far from the optical axis so as to enter the light receiving portions 17a, 17b and 17c, 17d. Similarly, in the 4c portion and the 4d portion, the main beam is incident on 17e and 17f, and the sub beams are incident on 17g, 17h, 17i, and 17j, respectively.
[0057]
The spherical aberration error signal SAES can be calculated by the following arithmetic expression.
SAES = (17a−17b + 17c−17d) −α × (17c−17d)
Or
SAES = (17a-17b) −β × (17c-17d)
Here, both α and β are coefficients. This calculation is performed by the aberration detection circuit, and the liquid crystal driving circuit drives the liquid crystal element based on the signal. According to this hologram dividing method, the signal fluctuation is small with respect to the shift of the objective lens. FIG. 7B shows another hologram dividing method. The detection sensitivity of spherical aberration is improved by the amount divided into the circumference.
[0058]
According to such an aberration detection method, spherical aberration can be corrected in real time during signal reproduction, and correction can be performed even during recording.
[0059]
For example, a method of correcting spherical aberration while detecting the envelope, jitter value, etc. of the RF signal is conceivable. However, since this detection method uses the signal being reproduced, the amount and direction of the spherical aberration cannot be known. Since it is a hill-climbing correction, it is difficult to adjust in real time. Further, since it cannot be detected during recording, the method of dividing and detecting the return light beam described above is advantageous.
[0060]
In other words, in the conventional configuration that only undergoes phase modulation only in the forward direction, the difference in the thickness of the light transmission layer and the spherical aberration remaining in the optical components in the optical pickup are corrected, and a good aberration is achieved on the optical recording medium. Even when the beam spot is formed, the phase modulation by the liquid crystal element is not performed in the return path, so that the spherical aberration corresponding to the return path remains on the light receiving portion, and correct aberration detection cannot be performed. Further, even in the servo system for driving the aberration correction element, the aberrations on the aberration detection unit and the optical recording medium are different, so that a closed loop cannot be formed and accurate feedback correction cannot be performed.
[0061]
However, according to the present invention, the above-described problems can be solved, and the amount of spherical aberration on the optical recording medium can be correctly detected and corrected, and thus correction during recording and real-time correction during reproduction can be performed. .
[0062]
In addition, based on the above principle of aberration detection, it is preferable to detect the spherical aberration by the servo light receiving unit from the viewpoints of the light use efficiency and the common use of parts by the integrated formation of the light receiving elements.
[0063]
Furthermore, since the liquid crystal element undergoes phase modulation in a reciprocating manner, the focus on the light receiving unit that detects the focus error signal does not change when correcting the spherical aberration. Therefore, there is also an effect that a focus offset caused by the occurrence does not occur.
[0064]
(Polarization state on optical recording medium)
In this embodiment, the optical axis of the quarter-wave plate has an angle of about 30 degrees with the direction of incident linearly polarized light, and the light beam transmitted through the quarter-wave plate has an ellipticity of 0.58. It becomes elliptically polarized light. When connecting spots on the optical recording medium, the ellipticity of this polarization has an important meaning.
[0065]
FIG. 9A shows the relationship between the ellipticity of the polarized light and the roundness of the spot diameter (ratio of major axis to minor axis), and FIG. 9B shows the ellipticity of the polarized light and the intensity ratio of each polarization direction to ¼. The relationship between the optical axis of the wave plate and the angle θ of the polarization direction of the light beam is shown.
[0066]
If the spot diameter is not a circle but an ellipse, the signal jitter characteristics, crosstalk characteristics, etc. are affected when recording / reproducing is performed. If the deviation of the roundness of the spot diameter is 10% or less, the jitter does not exceed 10% and the crosstalk is small and good. For this purpose, the ellipticity of the polarized light needs to be 0.4 or more. At that time, the angle θ (see FIG. 3) between the optical axis of the quarter-wave plate and the polarization direction of the light beam is 22 degrees. Between 68 and 68 degrees.
[0067]
On the other hand, the ellipticity is also related to the light quantity distribution between the RF light receiving part and the servo signal light receiving part. The amount of light required for the RF light receiver varies depending on the sensitivity and response band of the light receiver, but at least 10% of the total can minimize signal quality degradation. Also, the servo can be detected as long as it is 10% of the minimum total light amount. Accordingly, the ellipticity needs to be 0.15 to 0.75, that is, the angle θ is between 10 degrees and 37 degrees or between 53 degrees and 80 degrees.
[0068]
As described above, in order to satisfy both the spot diameter and the light quantity distribution, the ellipticity is 0.4 or more and 0.75 or less, and the angle θ between the optical axis of the quarter-wave plate and the polarization direction of the light beam is 22 <θ <37 or 53 <θ <68 (degrees).
[0069]
Here, the ellipticity is the ratio of the minor axis to the major axis of the ellipse polarization (minor axis / major axis), where 1 indicates circularly polarized light and 0 indicates linearly polarized light. The intensity ratio indicates the ratio of the intensity of the light beam reflected by the polarization beam splitter and directed to the RF light receiving unit to the entire return light.
[0070]
As described above, in this embodiment, a liquid crystal element is used as an aberration correction element. However, any other element can be used as long as the phase distribution can be obtained. For example, the refractive index can be made variable by voltage. The effect is the same in the element using the material and other elements. In addition, the spherical aberration amount is set to ± 15 μm as an example of the light transmission layer thickness, but may be other than that, and is determined according to the spherical aberration that needs to be corrected in each system.
[0071]
In addition, an optical system having an objective lens with an NA of 0.85 is taken as an example. However, in a pickup using an objective lens with an NA of 0.75 or more, spherical aberration with respect to a change in the thickness of the light transmission layer such as two-layer recording Is large and the effect of aberration correction by this aberration correction element is great. The third-order spherical aberration coefficient W40 is expressed by the following equation.
W40≈ (t / 8) × {(n²-1) / n^Three} × (NA)^Four
In other words, NA0.75 is more than twice as much as spherical aberration with respect to the same change in light transmission layer thickness as compared with NA0.6. In a two-layer recording (reproducing) medium, the thickness between the layers is determined by thermal interference between recording layers, interference of a focus servo signal, the thickness limit due to the manufacturing method of the interlayer, and the like. It is about 20 μm.
[0072]
Due to this difference in thickness, the allowable aberration value exceeds 0.03λrms at NA 0.75. Accordingly, in such an optical system with a high NA, the aberration detection and aberration correction elements are more important and have a great effect on improving the characteristics of the optical pickup.
[0073]
Further, a beam splitter may be used instead of the hologram 4 in addition to the configuration described above. The beam splitter may be a half mirror that transmits about 70% and reflects the rest.
[0074]
As described above, various configurations that do not depart from the gist of the present invention are conceivable and are not limited to the configurations of the present invention.
[0075]
According to the above description, the optical pickup device according to the present invention detects spherical aberration using the return light from the optical recording medium and can detect and correct in real time. The present invention can be applied to a multi-layer recording medium, and has an effect that good jitter characteristics can be obtained.
Example 2
In the above-described first embodiment, the configuration of the hologram 4 in the optical pickup device in FIG. 1 has been described with reference to FIG. 7 and the configuration of the servo signal light receiving unit 17 with reference to FIG. The configuration of the hologram 34 and the servo signal light receiving unit 43 is shown below.
[0076]
In addition, the diffraction grating 33 has a shape as shown in FIG. 10, and only the light beam in the inner peripheral portion near the optical axis among all the transmitted light beams is branched into the + 1st order diffracted light and the −1st order diffracted light. That is, for these sub-beams, an inner peripheral portion close to the optical axis, that is, a beam having a small NA is obtained. The diffraction grating is formed in a region corresponding to about 70% of the diameter of the total luminous flux. The spherical aberration can be detected by detecting and comparing the focus errors of the reflected light of the + 1st order diffracted light and the −1st order diffracted light from the optical recording medium and the reflected light of the 0th order light. 70% of the total luminous flux is a division ratio that provides the highest sensitivity for detecting spherical aberration.
[0077]
In order to be oriented to the servo signal light receiving portion 43, the hologram 34 is divided into three regions as shown in FIG. Further, as shown in FIG. 11B, it may be divided into four. In the case of a quadrant hologram, the arrangement of the light receiving parts corresponding to the hologram is determined separately.
[0078]
Here, the case of a three-division hologram will be described. Each light receiving part of the servo signal light receiving part 43 is shown in FIG. The main beam focus is 43a and 43b, the sub beam focus is 43c and 43d, 43e and 43f, the main beam push pull is 43g and 43h, and the sub beam push pull is 43i and 43j, 43k and 43m. Detect with. The light beam diffracted by the hologram 34a is incident on the light receiving portions 43a, 43b, 43c, 43d, 43e, and 43f, and the light beam diffracted by the hologram 34b is diffracted by the light receiving portions 43i, 43g, and 43k by the hologram 34c. The light beam enters the light receiving portions 43j, 43h, and 43m.
[0079]
The spherical aberration error signal SAES is expressed by the following equation.
SAES = (43a−43b) −α × (43c−43d) + β × (43e−43f)
Here, α and β are coefficients. This calculation is performed by the aberration detection circuit, and the liquid crystal driving circuit drives the liquid crystal element based on the signal.
[0080]
Even with the configuration of the diffraction grating 33, the hologram 34, and the servo signal light receiving unit 43 as described above, the same effects as those of the configuration described in the first embodiment can be obtained.
[0081]
【The invention's effect】
The optical pickup device according to the present invention can detect spherical aberration using return light from the optical recording medium, and can detect and correct in real time. The present invention is also applicable to recording media, and has an effect that good jitter characteristics can be obtained.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical pickup device of the present invention.
FIG. 2 is a diagram showing a relationship between an optical axis direction and a polarization direction of a quarter-wave plate of the present invention.
FIG. 3 is a configuration diagram of a recording medium of the present invention.
FIG. 4 is a configuration diagram of the liquid crystal element of the present invention before and after voltage application.
FIGS. 5A and 5B are a configuration diagram and an electric field strength distribution of an electrode arrangement of a liquid crystal element of the present invention. FIGS.
FIG. 6 is a diagram showing another electric field intensity distribution of the liquid crystal element of the present invention.
FIG. 7 is a plan view of the hologram of the present invention.
FIG. 8 is a layout view of a servo signal light receiving unit of the present invention.
FIG. 9 is a graph showing the relationship between the ellipticity of polarized light and the spot diameter according to the present invention.
FIG. 10 is a plan view of a diffraction grating according to a second embodiment of the present invention.
FIG. 11 is a plan view of a hologram according to the second embodiment of the present invention.
FIG. 12 is a layout diagram of servo signal light receiving units according to the second embodiment of the present invention.
FIG. 13 is a configuration diagram of an optical pickup device showing a conventional example.
[Explanation of symbols]
1 LD
2 Hologram element
3 Diffraction grating
4 Hologram
5 Collimator lens
6 Shaping prism
7 Liquid crystal elements
8 Polarizing beam splitter
9 1/4 wave plate
10 Objective lens
13 Optical recording media
16 RF receiver
17 Servo signal receiver

Claims (9)

Light source and the aberration detection system, in an optical pickup device including the optical path splitting means in the optical path of the objective lens and the wave plate and the aberration correcting optical system according to polarized light reaches the optical recording medium from said light source,
An optical path splitting means by the polarized light disposed on the wavelength plates and the aberration correcting optical system,
The optical path splitting unit transmits only linearly polarized extraordinary light out of the reflected light from the recording medium,
The correction optical system imparts phase modulation to extraordinary light as linearly polarized light emitted from the light source, and imparts phase modulation to extraordinary light transmitted through the optical path splitting unit,
The optical pickup device, wherein the aberration detection system receives reflected light from the recording medium and abnormal light that has undergone phase modulation in the correction optical system. 2. The optical pickup device according to claim 1, wherein an RF signal is detected by one of the light beams divided by the optical path dividing means based on the polarized light, and a track servo, a focus servo, and a spherical aberration are detected by the other light beam. Optical pickup device. 3. The optical pickup device according to claim 1, wherein a light beam incident on the optical recording medium is elliptically polarized light, and an ellipticity of the elliptically polarized light is 0.4 or more. . 4. The optical pickup apparatus according to claim 1, wherein the aberration detection system includes a light beam splitting unit that splits a light beam and makes it incident on a predetermined light receiving element, and is obtained from at least one of the split light beams. An optical pickup device that detects spherical aberration based on a received signal. 4. The optical pickup device according to claim 1, further comprising a light beam splitting unit that forms a plurality of light spots on an optical recording medium, wherein the aberration detection system is at least one of the split light spots. An optical pickup device that detects spherical aberration based on two reflected lights. 6. The optical pickup device according to claim 1, wherein a liquid crystal is used for the aberration correction optical system. 7. The optical pickup device according to claim 1, wherein the NA of the objective lens is 0.75 or more, and the aberration correction optical system includes a liquid crystal element. 8. The optical pickup device according to claim 1, wherein the aberration correction optical system includes a plurality of concentric electrodes, and a phase distribution applied when correcting spherical aberration is adjusted during phase correction. An optical pickup device characterized in that electrodes in the vicinity of an intermediate value of a distribution curve are set to the same potential, and the potentials of the other portions of the electrode are varied. 8. The optical pickup device according to claim 1, wherein the aberration correction optical system has a Fresnel lens shape in which a phase distribution applied when correcting spherical aberration is divided into a plurality of concentric regions. An optical pickup device characterized by the above.

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