JP3911475B2 - Optical pickup - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical recording / reproducing optical pickup device having aberration correction means.
[0002]
[Prior art]
Increase the numerical aperture (NA) of the objective lens of the optical pickup and shorten the wavelength of the light used to record information on the optical disc at high density and to reproduce information recorded on the optical disc at high density Has been proposed. In such an optical pickup, the change in the optical thickness of the cover layer of the optical disc (the change in the thickness of the cover layer due to manufacturing variations of the optical disc, the thickness of the cover layer that differs for each optical disc layer having a plurality of recording layers). There is a need for a spherical aberration corrector that corrects the spherical aberration caused by the change in thickness.
[0003]
Japanese Patent Application Laid-Open No. 2001-266392 discloses an optical pickup having a photodetector that receives laser light reflected from an optical disk and a spherical aberration correction device that uses a liquid crystal panel. In this pickup, a tracking error signal, a prepit detection signal, or a transmission substrate thickness error detection signal is generated from the electrical signal output from the photodetector, and the signal level is determined using any one of these signals. When the criterion is satisfied, it is determined that the spherical aberration correction is completed. For example, after applying the focus servo, the envelope level of the tracking error signal is detected, and the envelope level is compared with a predetermined reference level (referred to as REF). If the envelope level exceeds REF, it is determined that the spherical aberration correction has been completed.
[0004]
[Patent Document 1]
JP-A-2001-266392
[Problems to be solved by the invention]
FIG. 6 (FIG. 4 of Japanese Patent Laid-Open No. 2001-266392) shows the relationship between the envelope level of the tracking error signal and the error rate of the reproduction signal with respect to the thickness error of the cover layer of the optical disc. In FIG. 6, REF is a criterion and is an allowable range of error rate.
[0006]
However, the envelope of the tracking error signal is not only changed by spherical aberration, but also the material of the recording layer of the optical disc, the width and depth of the guide groove or signal pit engraved on the optical disc, the component accuracy of the optical pickup, the optical pickup Strongly depends on the assembly accuracy. As a result, regardless of the spherical aberration (thickness error of the cover layer of the optical disk), the tracking error signal envelope may not exceed REF, and it may not be possible to correctly determine that the spherical aberration correction has been completed. is there.
[0007]
Conversely, regardless of the spherical aberration (thickness error of the cover layer of the optical disc), when the envelope of the tracking error signal is large, the allowable thickness error range of the optical disc is widened when judged by REF (vertical length in FIG. 6). However, it may be determined that spherical aberration correction has been completed even though the error rate of the reproduction signal is not within the allowable range.
[0008]
Not only the tracking error signal but also a pre-pit detection signal and a transmission substrate thickness error detection signal are used, there is a similar problem.
[0009]
An object of the present invention is to provide an optical pickup capable of reliably correcting spherical aberration.
[0010]
[Means for Solving the Problems]
The optical pickup of the present invention includes a light source that emits a light beam, a beam splitter for separation that directs the light beam from the light source to the optical disc and separates the light beam that returns from the optical disc from the optical path of the light beam that goes to the optical disc, and an optical disc An objective lens for converging the light beam directed to the optical disk, a light detection unit for detecting at least a focus error signal based on the light beam returned from the optical disk separated from the optical path of the light beam directed to the optical disk by the separation beam splitter, and the optical disk A spherical aberration correction device that corrects the spherical aberration of the light beam converged on the light beam, a drive circuit that drives the spherical aberration correction device based on focus error information detected by the light detection unit, and the objective lens along the optical axis. And an actuator to be moved . The actuator vibrates the objective lens along the optical axis, and the drive circuit drives the spherical aberration correction device so that the focus error detection sensitivity of the focus error signal is maximized.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0014]
FIG. 1 shows the configuration of an optical pickup according to an embodiment of the present invention.
[0015]
The optical pickup 100 includes a light source 12 that emits a light beam, a collimator lens 14 that converts the divergent light beam emitted from the light source 12 into a parallel light beam, and directs the light beam from the light source 12 toward the optical disc 80 and from the optical disc 80. A separation beam splitter 16 that separates the returning light beam from the optical path of the light beam that travels toward the optical disk 80, and an objective lens 18 that converges the light beam that travels toward the optical disk 80 onto the optical disk 80.
[0016]
Although the light source 12 is comprised with a semiconductor laser, for example, it is not specifically limited to this. The semiconductor laser emits a light beam having a wavelength of 405 nm, for example. The objective lens 18 has a numerical aperture of 0.7 or more. Although the objective lens 18 is depicted as a single lens in FIG. 1, it may be composed of two lenses.
[0017]
The optical pickup 100 further includes a light detection unit that detects at least a focus error signal based on the light beam returning from the optical disk 80 separated from the optical path of the light beam directed to the optical disk 80 by the separation beam splitter 16. Although not limited to this, the light detection unit detects the focus error signal by, for example, a beam size method.
[0018]
For this reason, the light detection unit converges the light beam returning from the optical disc 80 and the convergent light beam from the convergent lens 22 into two light beams (a first convergent light beam and a second convergent light beam). ), A first photodetector 26 for detecting the first convergent light beam split by the split beam splitter 24, and a second split by the split beam splitter 24. A second photodetector for detecting the convergent light beam; and a focus error signal detection circuit 32 for detecting a focus error signal based on output signals of the first photodetector and the second photodetector. is doing.
[0019]
In the optical pickup for reproduction, the light detection unit further includes a reproduction signal detection circuit 34 that detects a reproduction signal based on the output signals of the first photodetector 26 and the second photodetector 28. . Further, although not shown, the light detection unit may have a tracking error signal detection circuit that detects a tracking error signal based on the output signals of the first light detector 26 and the second light detector 28. Good.
[0020]
The optical pickup 100 further includes an actuator 62 that moves the objective lens 18 along the optical axis, and a drive circuit 64 that drives the actuator 62 based on the focus error signal detected by the focus error signal detection circuit 32. Yes. The actuator 62 and the drive circuit 64 constitute, for example, a focus adjustment mechanism that adjusts the distance between the objective lens 18 and the optical disc 80 in order to focus the light beam on the recording layer of the optical disc. In the focused state, that is, the focused state, the spot diameter of the light beam formed on the recording layer of the optical disc 80 is minimum.
[0021]
The first photodetector 26 is positioned before the focal point of the first convergent light beam in the in-focus state, and the second photodetector 28 condenses the second converged light beam in the in-focus state. It is located behind the point. Further, the distance between the focal point of the first convergent light beam in the focused state and the first photodetector 26 is such that the focal point of the second converged light beam in the focused state and the second photodetector 28 are in the focused state. Equal to the interval. Therefore, in the in-focus state, the spot of the first convergent light beam formed on the first photodetector 26 and the spot of the second convergent light beam formed on the second photodetector 28 have the same diameter. have.
[0022]
The optical pickup 100 further includes a spherical aberration correction device 40 that corrects the spherical aberration of the light beam converged on the optical disc 80, and an amplitude detection circuit 52 that detects the amplitude of the focus error signal detected by the focus error signal detection circuit 32. And a drive circuit 54 for driving the spherical aberration corrector 40 based on information detected by the amplitude detection circuit 52.
[0023]
The spherical aberration correction device 40 is configured by, for example, a liquid crystal cell, and is disposed, for example, between the separating beam splitter 16 and the objective lens 18. For example, as shown in FIG. 5, the liquid crystal cell has three transparent electrodes 42, 44, and 46 divided by concentric circles, and the refractive index is set for each region of the transparent electrodes 42, 44, and 46. Can change.
[0024]
The divergent light beam emitted from the light source 12 is converted into a parallel light beam by the collimator lens 14, reflected by the separation beam splitter 16, transmitted through the liquid crystal cell 40, and converged on the recording layer of the optical disk 80 by the objective lens 18. The
[0025]
Since the objective lens 18 has a large numerical aperture, a large spherical aberration occurs in the spot of the light beam formed on the recording layer of the optical disc 80 even when the thickness of the cover layer of the optical disc 80 is slightly changed. To do. The change in the thickness of the cover layer of the optical disc 80 is caused by, for example, variations in the manufacturing process of the optical disc. Further, in the optical disc having a plurality of recording layers, the thickness of the cover layer for each recording layer is different, and this is a change in the thickness of the cover layer of the optical disc 80.
[0026]
The light beam reflected by the optical disk 80 passes through the objective lens 18, the liquid crystal cell 40, and the separation beam splitter 16 and travels toward the detection unit. The light beam that has passed through the separating beam splitter 16 is converged by the converging lens 22 and divided by the dividing beam splitter 24 into a first convergent light beam and a second convergent light beam. The first convergent light beam and the second convergent light beam have the same amount of light, the first convergent light beam is incident on the first photodetector 26, and the second convergent light beam is the second light. The light enters the detector 28.
[0027]
FIG. 2 shows the correlation between the diameters of the light beam spots formed on the first photodetector 26 and the second photodetector 28 that change in accordance with the degree of focusing of the light beam on the optical disc.
[0028]
As shown in FIG. 2, the first photodetector 26 has three light receiving regions 26a, 26b, and 26c. The three light receiving regions 26a, 26b, and 26c all have a rectangular shape and are aligned in a row adjacent to each other. Similarly, the second photodetector 28 has three light receiving regions 28a, 28b and 28c. The three light receiving regions 28a, 28b, and 28c all have a rectangular shape and are aligned in a row adjacent to each other.
[0029]
In the focused state, the light beam reflected by the optical disk 80 is converted into a parallel light beam by the objective lens 18. Therefore, the light beam incident on the converging lens 22 is a parallel light beam and is converged to the focal position of the converging lens 22.
[0030]
When the distance between the objective lens 18 and the optical disk 80 is narrower than the distance between the objective lens 18 and the optical disk 80 in the focused state, the light beam incident on the converging lens 22 becomes a divergent light beam. It converges to a position farther than the position.
[0031]
On the contrary, when the distance between the objective lens 18 and the optical disk 80 is wider than the distance between the objective lens 18 and the optical disk 80 in the focused state, the light beam incident on the converging lens 22 becomes a convergent light beam. It converges to a position closer than the focal position of 22.
[0032]
FIG. 2C shows a spot when the distance between the objective lens 18 and the optical disk 80 is equal to the distance between the objective lens 18 and the optical disk 80 in a focused state. In the focused state, the spot S1 of the first convergent light beam formed on the first photodetector 26 and the spot S2 of the second convergent light beam formed on the second photodetector 28 have the same diameter. Have
[0033]
FIG. 2B shows the state of the spot when the distance between the objective lens 18 and the optical disk 80 is narrower than the distance between the objective lens 18 and the optical disk 80 in the focused state. In this state, since the convergent light beam is converged at a position far from the focal position of the convergent lens 22, the spot S1 of the first converged light beam formed on the first photodetector 26 is in an in-focus state. The spot S2 of the second convergent light beam formed on the second photodetector 28 has a smaller diameter than that in the focused state.
[0034]
FIG. 2A shows the state of the spot when the distance between the objective lens 18 and the optical disk 80 is further narrower than the distance between the objective lens 18 and the optical disk 80 in the state of FIG. In this state, since the convergence point of the first convergent light beam is further away from the first photodetector 26, the spot S1 of the first convergent light beam formed on the first photodetector 26 is shown in FIG. It has a large diameter compared with the state of b). On the other hand, the second convergent light beam incident on the second photodetector 28 is converged at a position behind the second photodetector 28, and is formed in the second photodetector 28. The spot S2 of the convergent light beam has a larger diameter than the state of FIG.
[0035]
FIG. 2D shows a spot when the distance between the objective lens 18 and the optical disk 80 is wider than the distance between the objective lens 18 and the optical disk 80 in the focused state. In this state, since all the convergent light beams are converged at a position closer to the focal position of the convergent lens 22, the spot S1 of the first convergent light beam formed on the first photodetector 26 is in an in-focus state. The spot S2 of the second convergent light beam formed on the second photodetector 28 has a larger diameter than that in the focused state.
[0036]
FIG. 2E shows the state of the spot when the distance between the objective lens 18 and the optical disk 80 is further wider than the distance between the objective lens 18 and the optical disk 80 in the state of FIG. In this state, since the convergence point of the second convergent light beam is further away from the second photodetector 28, the spot S2 of the second convergent light beam formed on the second photodetector 28 is shown in FIG. It has a large diameter compared with the state of d). On the other hand, the first convergent light beam incident on the first photodetector 26 is converged at a position before the first photodetector 26, and is formed in the first photodetector 26. The spot S1 of the convergent light beam has a larger diameter than the state of FIG.
[0037]
The focus error signal detection circuit 32 obtains the focus error signal FES according to the calculation expressed by FES = (I26b−I26a−I26c) − (I28b−I28a−I28c). Here, I26a, I26b, and I26c are the outputs of the light receiving regions 26a, 26b, and 26c of the first photodetector 26, respectively, and I28a, I28b, and I28c are the light receiving regions 28a, 28b, and 28c of the second photodetector 28, respectively. Output. Further, when I26 = I26b-I26a-I26c and I28 = I28b-I28a-I28c, it can be expressed as FES = I26-I28.
[0038]
FIG. 3 shows changes in FES, I26, and I28 with respect to the distance (WD) between the objective lens 18 and the optical disk 80 in a state where no spherical aberration occurs. FES is a so-called S-shaped curve, and I26 and I28 are mountain-shaped curves.
[0039]
As shown in FIG. 2D, when the first convergent light beam is focused on the light receiving region 26b of the first photodetector 26, I26a = I26c = 0, and I26 = I26b = constant. (I26max portion in FIG. 3). When the diameter of the spot S1 is slightly larger than the width of the light receiving area 26b, I26 decreases rapidly, and when the diameter of the spot S1 is sufficiently larger than the width of the light receiving area 26b, The effect is relatively small and the change in I26 is small.
[0040]
As shown in FIG. 2B, when the second convergent light beam is focused on the light receiving region 28b of the second photodetector 28, I28a = I28c = 0, and I28 = I28b. = Constant (I28max portion in FIG. 3). When the diameter of the spot S2 is slightly larger than the width of the light receiving area 28b, I28 decreases rapidly. When the diameter of the spot S2 is sufficiently larger than the width of the light receiving area 28b, The effect is relatively small and the change in I28 is small.
[0041]
When spherical aberration is generated in the spot of the light beam converged on the recording layer of the optical disc 80 due to a slight change in the thickness of the cover layer of the optical disc 80, the first photodetector 26 and the second photodetector are detected. Spherical aberration also occurs on the light receiving area of the instrument 28. In the state where the spherical aberration is generated, the diameter of the spot formed on the light receiving region of the first photodetector 26 or the second photodetector 28 is not sufficiently small, and the light receiving region 26b or the light receiving region 28b. No longer concentrate on.
[0042]
FIG. 4 shows changes in FES, I26, and I28 with respect to the distance (WD) between the objective lens 18 and the optical disk 80 in a state where spherical aberration occurs. In the state where spherical aberration is generated, the spot diameters of the first convergent light beam and the second convergent light beam are not sufficiently small and do not concentrate on the light receiving region 26b and the light receiving region 28b. As shown in FIG. 3, the peaks of I26 and I28 are lower and the amplitude T of the FES is smaller than the characteristic in the state where no spherical aberration occurs (see FIG. 3).
[0043]
The amplitude detection circuit 52 detects the amplitude T of the focus error signal detected by the focus error signal detection circuit 32, and the drive circuit 54 makes the amplitude of the focus error signal detected by the amplitude detection circuit 52 maximum. The liquid crystal cell 40 is driven. That is, the drive circuit 54 applies an appropriate voltage to the transparent electrodes 42, 44 and 46 of the liquid crystal cell 40 shown in FIG. 5 to adjust the refractive index for each region of the transparent electrodes 42, 44 and 46.
[0044]
In a state where no spherical aberration occurs, the diameter of the spot of the first convergent light beam or the second convergent light beam is sufficiently small, and the amplitude T of the focus error signal shows a large value. Accordingly, the spherical aberration is corrected by driving the liquid crystal cell 40 so that the amplitude T of the focus error signal is maximized.
[0045]
In one example, correction of spherical aberration is performed only once when an optical disc is loaded. In this example, immediately after loading the optical disk, the optical disk is rotated without applying focus servo. While the optical disk is rotating in this way, the focus error signal as shown in FIG. The amplitude detection circuit 52 detects the amplitude T of the focus error signal, and the drive circuit 54 drives the liquid crystal cell 40 so as to maximize the amplitude T of the focus error signal detected by the amplitude detection circuit 52.
[0046]
As a result, in this state where the optical disk is rotating, correction is made so that the spherical aberration is minimized. After that, focus servo is applied so that the focus adjustment mechanism constituted by the actuator 62 and the drive circuit 64 always has FES = 0.
[0047]
In another example, the spherical aberration is always corrected while the objective lens 18 is vibrated with a constant amplitude within the depth of focus along the optical axis. In this example, focus servo is applied so that FES = 0 always immediately after the optical disk is loaded, and the objective lens 18 is vibrated with a constant amplitude within the depth of focus along the optical axis by the actuator 62. Since the objective lens 18 vibrates within the depth of focus, the focus error signal also vibrates around FES = 0. The amplitude of the focus error signal that vibrates in this way is determined by the detection sensitivity of the focus error signal, and the detection sensitivity depends on spherical aberration.
[0048]
In the state where spherical aberration is occurring, as described above, the amplitude of the focus error signal is small, and therefore, the FES slope (ie, the FES value) in the vicinity of the in-focus state (in the vicinity of FES = 0 in FIGS. 3 and 4). Detection sensitivity is also reduced. That is, the amplitude of the vibration of the focus error signal generated when the objective lens 18 is vibrated is proportional to the FES detection sensitivity. The amplitude detection circuit 52 detects the amplitude (detection sensitivity) of the vibration of the focus error signal, and the drive circuit 54 maximizes the detection sensitivity of the focus error signal detected by the amplitude detection circuit 52. Drive. As a result, the spherical aberration is always corrected to the minimum with the focus servo applied.
[0049]
In this example, since the spherical aberration can be corrected with the focus servo applied, the spherical aberration can always be optimally corrected even during the recording or reproducing operation of the optical disk 80.
[0050]
In a state where spherical aberration has occurred, the focused beam diameter is larger than the width of the light receiving region 26b and the light receiving region 28b, and the amplitude of the focus error signal is reduced. Since the light beam converged by the converging lens 22 is converged to a size determined by the light wavelength λ and the numerical aperture NA of the converged light beam, the widths of the light receiving region 26b and the light receiving region 28b are not more than λ / NA. It is desirable that When λ = 0.4 μm and NA = 0.05, 8 μm or less is desirable.
[0051]
Although the embodiments of the present invention have been described above with reference to the drawings, the present invention is not limited to these embodiments, and various modifications and changes can be made without departing from the scope of the present invention. May be.
[0052]
For example, in the present embodiment, the split beam splitter 24 splits the first convergent light beam and the second convergent light beam. However, instead of the split beam splitter 24, a hologram is used to split the light beam. May be performed. Further, the photodetector may have two light receiving areas located concentrically instead of having three light receiving areas.
[0053]
The light detection unit detects the focus error signal by the beam size method, but may detect the focus error signal by other methods such as a knife edge method and an astigmatism method.
[0054]
The knife edge method utilizes the fact that the spot of the light beam returning from the optical disk moves on the light receiving surface of a photodetector having two light receiving areas. When spherical aberration occurs and the return light beam spot increases, the amount of movement becomes relatively small, so that the amplitude of the focus error signal decreases and the detection sensitivity also decreases.
[0055]
The astigmatism method utilizes the fact that the spot of the light beam returning from the optical disk is deformed into an ellipse on the light receiving surface of a photodetector having four light receiving regions arranged in a lattice pattern. When spherical aberration occurs and the return light beam spot increases, the deformation becomes relatively small, so that the amplitude of the focus error signal decreases and the detection sensitivity also decreases.
[0056]
Furthermore, the spherical aberration correction device 40 is not limited to the liquid crystal cell, and may be formed of a variable shape mirror, for example. Further, the method for correcting the spherical aberration is not limited to a method using a liquid crystal cell or a deformable mirror. For example, the spherical aberration may be corrected by configuring the objective lens 18 with two lenses and adjusting the distance between the two lenses. In this case, the spherical aberration correction device 40 is configured by a mechanism that adjusts the distance between the two lenses constituting the objective lens.
[0057]
【The invention's effect】
According to the present invention, an optical pickup capable of reliably correcting spherical aberration is provided.
[Brief description of the drawings]
FIG. 1 shows a configuration of an optical pickup according to an embodiment of the present invention.
FIG. 2 shows a correlation between the diameters of the light beam spots formed on the first photodetector and the second photodetector, which change in accordance with the degree of focusing of the light beam on the optical disc.
FIG. 3 shows changes in FES, I26, and I28 with respect to the distance (WD) between the objective lens and the optical disk in a state where no spherical aberration occurs.
FIG. 4 shows changes in FES, I26, and I28 with respect to the distance (WD) between the objective lens and the optical disk in a state where spherical aberration occurs.
FIG. 5 shows a liquid crystal cell which is a specific example of a spherical aberration corrector.
FIG. 6 shows the relationship between the envelope level of the tracking error signal and the error rate of the reproduction signal with respect to the thickness error of the cover layer of the optical disc.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 12 Light source 14 Collimating lens 16 Separating beam splitter 18 Objective lens 22 Converging lens 24 Dividing beam splitter 26 First photo detector 28 Second photo detector 32 Focus error signal detection circuit 40 Spherical aberration correction device 52 Amplitude detection circuit 54 Drive circuit 100 Optical pickup

Claims (5)

A light source that emits a light beam;
A beam splitter for separating the light beam from the light source toward the optical disc and separating the light beam returning from the optical disc from the optical path of the light beam toward the optical disc;
An objective lens for converging the optical beam directed to the optical disc to the optical disc;
A light detection unit for detecting at least a focus error signal based on a light beam returning from the optical disk separated from the optical path of the light beam directed to the optical disk by the separating beam splitter;
A spherical aberration corrector for correcting the spherical aberration of the light beam converged on the optical disc;
A drive circuit for driving the spherical aberration correction device based on focus error information detected by the light detection unit ;
An actuator that moves the objective lens along the optical axis, vibrates the objective lens along the optical axis, and the drive circuit drives the spherical aberration correction device so that the focus error detection sensitivity of the focus error signal is maximized. And have an optical pickup.   2. The optical pickup according to claim 1, wherein the drive circuit drives the spherical aberration correction device so that the amplitude of the focus error signal is maximized. 3. The optical pickup according to claim 1, wherein the light detection unit detects a focus error signal by a beam size method, an astigmatism method, or a knife edge method. 3. The converging lens for converging the light beam returning from the optical disc, and the beam splitter for splitting the converging light beam from the converging lens into a first converging light beam and a second converging light beam. A first photodetector for detecting the first convergent light beam split by the splitting beam splitter, and a second photodetector for detecting the second convergent light beam split by the splitting beam splitter And a focus error signal detection circuit for detecting a focus error signal based on an output signal of the first photodetector and the second photodetector, and the first photodetector is converged on the optical disc. The second light detector is positioned in front of the focal point of the first convergent light beam when the light beam is in focus. Second yield It is located behind the condensing point of the light beam, and the converging point of the first converging light beam and the first photodetector when the light beam converged on the optical disk is in focus. The optical pickup having an interval equal to the interval between the focal point of the second convergent light beam and the second photodetector when the light beam converged on the optical disk is in focus. In Claim 4 , each of the first photodetector and the second photodetector has three rectangular light receiving areas that are adjacent to each other and aligned in a row, and the central light receiving area is Each of the optical pickups has a width having a dimension equal to or smaller than a value obtained by dividing the wavelength of the light beam emitted from the light source by the numerical aperture of the convergent light beam.

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