JP2007080341A - Optical pickup apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that since a liquid crystal optical element is used as an aberration compensation optical element for compensating aberration due to optical axis deviation of an objective lens in a conventional optical head apparatus wherein recording and reproduction of information signals are performed to a multilayered optical recording medium, the element is expensive and a driving circuit and driving power are needed. <P>SOLUTION: When laser light is selectively condensed on information surfaces 1a and 1b of an optical disk 1 by using an objective lens 17 to perform recording or reproduction of information signals, the objective lens having a high NA has less influence on spherical aberration caused by difference of substrate thickness when special spherical aberration compensation is not performed and the objective lens having a low NA brings satisfactory recording and reproducing jitter value. Thereby, the range of the NA of the objective lens condensing the laser light on the optical disk 1 having a prescribed specification is specified to be ≤0.64 so that both spherical aberration and recording and reproducing jitter at the information recording surfaces 1a and 1b have low values having no problem on practical use. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical pickup device, and in particular, a laser beam emitted from a light source is selectively condensed on an information recording surface of a multilayer optical recording medium having two or more information recording surfaces by an objective lens, thereby generating an information signal. The present invention relates to an optical pickup device that records or reproduces.

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

  For example, CDs (Compact Discs) and DVDs (Digital Versatile Discs) are already commercially available as optical discs that serve as this type of disk-shaped optical recording media. Unlike a CD, a DVD has a recording capacity of 8.5 GB on one side and a read-only ROM with a recording capacity of 9.4 GB on each side for recording long-term or high-definition movie software. Read Only Memory). Recently, in order to further increase the density of recordable optical discs DVD-R, DVD + R, DVD-RW, and DVD + RW, the information recording surface of the optical disc is made to be two-layered to record information signals at an ultra-high density. Development of high-density optical discs that can be played back has been actively conducted.

  By the way, when the information recording surface of the optical disc is made into two layers, the disc substrate thickness from the disc surface to the first layer of the optical disc is different from the disc substrate thickness from the second layer. Since the amount of generation of spherical aberration increases, it is necessary to provide a spherical aberration correction mechanism in the optical pickup device to correct the spherical aberration in an optimum state.

  At this time, a method for correcting spherical aberration with respect to an optical disc having a double-layered information recording surface (hereinafter also simply referred to as a double-layer disc) changes the parallelism of the laser beam incident on the objective lens. A magnification error method for correcting spherical aberration by changing the imaging magnification and a wavefront correction method for correcting by correcting the aberration by generating a spherical aberration of the opposite polarity to the spherical aberration.

  Here, when the aberration correction optical element for correcting the spherical aberration with respect to the two-layer disc is provided in the optical pickup device, the aberration correction optical element is integrally provided in the lens holder that holds the objective lens. Since the weight of both the objective lens and the aberration correcting optical element is added to the lens holder, the overall weight becomes heavy, and there is a problem that the frequency characteristics when the objective lens is controlled in the tracking direction and the focus direction are lowered. The actuator that holds the objective lens inside the lens holder and that holds the tracking coil and focus coil outside the lens holder cannot be made thin, which hinders the reduction in size and thickness of the optical pickup device. is there.

  Therefore, a phase-variable aberration correction optical element using liquid crystal is fixedly installed in the optical system between the laser light source and the objective lens provided in the optical pickup device, and the substrate thickness variation of the optical disk and the laser light In some cases, spherical aberration that occurs due to the wavelength variation of the light is corrected by a phase-variable aberration correcting optical element using liquid crystal. However, in this case, another problem arises that coma occurs when decentration occurs between the objective lens and the aberration correcting optical element. Thus, in particular, an optical head device or an optical pickup device that can suppress the occurrence of aberrations even when there is an optical axis shift (lens shift) of the objective lens with respect to the aberration correction optical element is conventionally known. (For example, see Patent Documents 1 to 4).

  The optical head device disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2001-143309) includes a phase correction element for correcting coma aberration that occurs when a tilt occurs between the optical disk and the optical head device. Have. In this Patent Document 1, the occurrence of aberration caused by the optical axis shift of the objective lens by substantially displacing the electrode pattern of the electrode that drives the liquid crystal in the phase correction element according to the lens shift of the objective lens. An invention for suppressing the above is disclosed.

  In Patent Documents 2 to 4, a plurality of patterns corresponding to a plurality of aberrations are provided as electrode patterns of electrodes for driving liquid crystals, and aberrations caused by optical axis deviation of the objective lens are adaptively corrected. An enabled optical head device is disclosed.

  More specifically, the optical head device disclosed in Patent Document 2 (Japanese Patent Application Laid-Open No. 2002-133697) corrects spherical aberration with a phase correction element using liquid crystal, but at the same time, coma aberration is corrected. The phase correction element has a correctable electrode pattern. Here, when the optical axis of the objective lens is shifted in a state where the spherical aberration is corrected, coma aberration is generated in proportion to the shift of the optical axis of the objective lens. However, the conventional optical head device described in Patent Document 2 According to the amount of optical axis deviation of the objective lens, the coma aberration correction amount (coma aberration phase change amount) is changed by an appropriate amount to cancel the coma aberration due to the optical axis deviation of the objective lens.

  The optical head device disclosed in Patent Document 3 (Japanese Patent Application Laid-Open No. 2003-317298) corrects coma with a phase correction element using liquid crystal, but can simultaneously correct astigmatism. The phase correction element has an electrode pattern. Here, when the optical axis of the objective lens shifts in a state where the coma aberration is corrected, astigmatism occurs in proportion to the shift of the optical axis of the objective lens. In the conventional optical head device described in Patent Document 3, Depending on the amount of deviation of the optical axis of the objective lens, the amount of astigmatism correction (the amount of astigmatism phase change) is changed by an appropriate amount to cancel out the astigmatism caused by the optical axis deviation of the objective lens. ing.

  The optical head device disclosed in Patent Document 4 (Japanese Patent Application Laid-Open No. 2003-338070) corrects coma and spherical aberration with a phase correction element using liquid crystal, but at the same time astigmatism. The phase correction element has an electrode pattern capable of correcting the above. Here, when the optical axis of the objective lens shifts in a state where the coma aberration is corrected, astigmatism occurs in proportion to the shift of the optical axis of the objective lens. In the conventional optical head device described in Patent Document 4, Depending on the amount of deviation of the optical axis of the objective lens, the amount of astigmatism correction (the amount of astigmatism phase change) is changed by an appropriate amount to cancel out the astigmatism caused by the optical axis deviation of the objective lens. ing. Further, when the optical axis of the objective lens is deviated with the spherical aberration corrected, coma aberration is generated in proportion to the deviation of the optical axis of the objective lens. A coma aberration correction amount (a coma aberration phase change amount) is changed by an appropriate amount in accordance with the amount of deviation of the optical axis of the lens to cancel out coma aberration due to the optical axis deviation of the objective lens.

  Further, Patent Document 5 (Japanese Patent Laid-Open No. 2005-209297) discloses that the substrate thickness of the optical recording medium varies and the wavelength of the laser light even when the phase acquisition amount of the phase change type liquid crystal aberration correcting optical element is small. A conventional optical pickup device that can satisfactorily correct spherical aberration caused by fluctuation or the like is disclosed. That is, in the conventional optical pickup device described in Patent Document 5, when the change of the wavefront of the laser light transmitted through the phase-variable aberration correcting optical element is expressed by a phase function, the optical pickup device is in a region corresponding to the pupil diameter of the objective lens. With the exception of the central part where the optical axis passes through, the phase function does not have an extreme value, so that even if there is a lens shift of the objective lens when the optical recording medium is reproduced, the substrate thickness of the optical recording medium Spherical aberration that occurs due to variations and wavelength variation of laser light can be corrected satisfactorily.

JP 2001-143309 A Japanese Patent Laid-Open No. 2002-133697 JP 2003-317298 A JP 2003-338070 A JP 2005-209297 A

  However, in the conventional optical head devices described in Patent Documents 1 to 4, since the phase correction element using liquid crystal is fixedly installed in the optical system of the optical head device, the phase correction element is arranged in the optical axis direction. Various aberrations can be corrected without moving, and the optical head device can be reduced in size and weight, but each has the following problems.

  First, in the conventional optical head device of Patent Document 1, when the electrode pattern of the electrode that drives the liquid crystal is subdivided to increase the aberration correction power, the electrode pattern becomes extremely complicated and substantially phase correction is performed. It becomes impossible to make an element.

  Further, the conventional optical head devices described in Patent Documents 2 to 4 have the following common problems. That is, (a) the correction of aberration due to the optical axis deviation of the objective lens is proportional to the original aberration correction amount and the correction optical axis deviation amount, and thus it is necessary to accurately detect and control each of them. In particular, accurate detection of the amount of optical axis deviation of the objective lens requires an extra detection element and is disadvantageous in terms of size and cost. Further, when the optical axis deviation of the objective lens is caused by the eccentricity of the optical disk, it is difficult to follow the response speed of the liquid crystal because the frequency at which the optical axis deviation of the objective lens changes is high.

  (B) In order to increase the correction amount of aberration, it is necessary to increase the number of divisions of the electrodes that drive the liquid crystal or to realize a smoother phase distribution. In this case, providing an electrode pattern corresponding to a plurality of aberrations causes complication of the aberration correction element.

  In addition, since the conventional optical head device described in Patent Document 3 handles a plurality of aberrations at the same time, for example, when correcting spherical aberration, the optical axis shift of the objective lens occurs to correct coma aberration. In this case, the control in the case where coma aberration due to tilt occurs at the same time is quite complicated, and it is difficult to ensure the necessary correction accuracy.

  Therefore, the conventional optical pickup device described in Patent Document 5 is not an adaptive method using the detection value of the lens shift of the objective lens, but essentially generates secondary aberrations due to the optical axis shift of the objective lens. Although there is no phase variable aberration correction optical element, a liquid crystal optical element is used as the aberration correction optical element as in the conventional optical head device described in Patent Documents 1 to 4, and the element is expensive. Furthermore, there is a problem that a driving circuit and driving power are required.

  The present invention has been made in view of the above points, and spherical aberration generated due to variations in the substrate thickness of a multilayer optical recording medium having two or more layers and differences in the total substrate thickness due to differences in recording or reproducing layers. It is an object of the present invention to provide an optical pickup device that is less susceptible to the influence of the above and can perform good recording and reproduction.

  Another object of the present invention is to provide an optical pickup device having a small size and a light weight without including an aberration correction optical element for correcting spherical aberration.

In order to achieve the above object, according to the present invention, a laser beam having a predetermined wavelength emitted from a light source is formed by laminating at least a first information recording surface and a second information recording surface in the optical axis direction of the laser beam. In an optical pickup device for recording or reproducing by condensing and irradiating the first information recording surface or the second information recording surface of an optical recording medium having two or more layers with an objective lens,
Optical recording media
The substrate thickness minimum value from the laser light incident side surface to the first information recording surface is 0.56 mm, the substrate thickness maximum value from the laser light incident side surface to the second information recording surface is 0.64 mm, the first The distance between the information recording surface and the second information recording surface is within 0.055 ± 0.005 mm, the transmittance of the first information recording surface is 40 to 60%, the reflectance is 5 to 12%, the second A phase change recording medium having a reflectivity of 5 to 12% on an information recording surface, and a groove for information signal recording and reproduction in a spiral shape or a concentric shape on the first information recording surface and the second information recording surface And a land is formed between two adjacent groove grooves, and at least address information is formed as pre-pits at predetermined intervals in the land, and the side wall of the boundary between the groove groove and the land has a single frequency. Meandering at 140.6 ± 42.2kHz,
The effective numerical aperture of the objective lens for condensing the laser light is set to 0.64 or less, and the first information recording surface or the second information recording is performed by the laser light condensed by the objective lens without providing spherical aberration correcting means. An information signal is recorded and reproduced with a channel bit length of 146.7 ± 1.5 nm in the groove groove of the surface.

  When the information signal is recorded or reproduced by selectively focusing on a plurality of information recording surfaces of an optical recording medium having two or more layers, when special spherical aberration correction is not performed, it is caused by a difference in substrate thickness. The larger the numerical aperture (NA) of the objective lens, the smaller the influence of the spherical aberration generated in this manner. On the other hand, as for the recording / reproducing jitter, the smaller the NA of the objective lens, the better the jitter value. Therefore, in the present invention, when information signals are recorded or reproduced on an optical recording medium having two or more layers, both the spherical aberration and the recording / reproducing jitter on each information recording surface are small values that have no practical problem. In addition, by setting the NA range of the objective lens to 0.64 or less, it is possible to record and reproduce information signals on each information recording surface satisfactorily without providing special spherical aberration correction means.

  According to the present invention, by setting the numerical aperture (NA) of an objective lens for condensing laser light to a predetermined value or less for each information recording surface of an optical recording medium having two or more layers having a predetermined specification. Even without special spherical aberration correction means, due to variations in the substrate thickness of two or more optical recording media and differences in the total substrate thickness due to differences in the information recording surface (recording layer) to be recorded or reproduced It is difficult to be affected by the generated spherical aberration, and recording and reproduction can be performed satisfactorily.

  Therefore, according to the present invention, it is not necessary to provide an aberration correction optical element for correcting spherical aberration as in the prior art, the configuration of the optical pickup can be simplified, and the size and weight can be reduced. Moreover, it is possible to reduce material cost, assembly, and adjustment cost.

  Next, the best mode for carrying out the invention will be described with reference to the drawings. FIG. 1 shows a configuration diagram of an embodiment of an optical pickup device according to the present invention. In the figure, laser light emitted from a semiconductor laser 11 as a light source is converted into parallel light by a collimator lens 12, and when a known three-beam method or a differential push-pull (DPP) method is used as a tracking method, Divided into three beams by the diffraction grating 13. Since the polarization direction of the three-beam laser light is P-polarized with respect to the reflecting surface of the polarizing beam splitter (PBS) 14, most of the laser light passes through the reflecting surface of the PBS 14, and a part of the laser light is PBS 14 The light is reflected by the reflecting surface of the semiconductor laser 11 and is incident on the front monitor 15 for detecting and controlling the power of the semiconductor laser 11.

  The laser light that has passed through the reflection surface of the PBS 14 is converted into circularly polarized light by the quarter-wave plate 16 and then enters the objective lens 17 to enter the first layer having a predetermined substrate thickness, predetermined reflectance, and transmittance. The light is condensed on the optical disk 1 which is a disk-shaped phase change optical recording medium having the information recording surface of the second layer to form three spots. Of these three spots, the main spot in the middle with respect to the track scanning direction scans on the track center line, and the remaining two spots are positions where one precedes and the other follows the main spot, and This is a sub-spot that scans the main spot at a position shifted by about 1/2 track pitch in the disk radial direction.

  The three spots on the optical disk 1 are reflected by the optical disk 1, and are converted into linearly polarized light whose 90 ° polarization plane is changed with respect to the incident light by passing again through the objective lens 17 and the quarter wavelength plate 16 as reflected light. The Since the reflected light that has passed through the quarter-wave plate 16 becomes S-polarized light with respect to the reflective surface of the PBS 14, it is reflected by the reflective surface of the PBS 14 and then collected through the detection lens 18 to be non-focusing. When the point aberration method is used, the light passes through the cylindrical lens 19 and enters the photodetector 20 having a known configuration, is photoelectrically converted, and a signal is detected by a known method.

  Thus, in the present embodiment, no aberration correction optical element for correcting spherical aberration is provided, and the numerical aperture (NA) of the objective lens 17 is set to an optimum value as will be described later. Thus, the effect of spherical aberration is suppressed as much as possible without using an aberration correction optical element, and recording and reproduction on the two-layered optical disk 1 are favorably performed.

  FIG. 2 is an explanatory diagram showing an example of a recording / reproducing method for the optical disc 1. As shown in FIG. 2, the optical disc 1 has a substrate 1c formed on the lower surface on the incident light side, and a first information recording surface 1a made of a translucent reflective film is formed on the upper surface of the substrate 1c. A disc-shaped phase change recording medium having a two-layer structure in which an intermediate layer 1d is formed on the upper surface of one information recording surface 1a, and a second information recording surface 1b made of a reflective film is formed on the upper surface of the intermediate layer 1d. It is.

  2, when an information signal is recorded on or reproduced from the first information recording surface 1a of the two-layer optical disc 1, the objective lens 17 shown in FIG. Then, the laser beam 22 is focused on the first information recording surface 1a. On the other hand, when an information signal is recorded on or reproduced from the second information recording surface 1b of the two-layer optical disc 1, the objective lens 17 shown in FIG. To focus the laser beam 23 on the second information recording surface 1b.

  The objective lens 17 is designed so that the spherical aberration is minimized in consideration of the substrate thickness of the optical disk 1. In the two-layer optical disc 1, the substrate thickness of the first information recording surface 1a and the substrate thickness of the second information recording surface 1b are different, so that the objective lens 17 is optimal for each of the information recording surfaces 1a and 1b. In the case of design, the design specifications are different. When recording / reproduction is performed on each information recording surface 1a, 1b with the same objective lens 17, spherical aberration occurs, and it is difficult to perform good recording / reproduction on each information recording surface 1a, 1b.

  In the conventional optical pickup device, the spherical aberration that occurs when recording / reproducing is performed on the two-layer optical disc 1 is newly provided with an aberration correction optical element as described above. An attempt is being made to satisfactorily perform recording and reproduction on the optical disc 1. However, the configuration of the optical pickup is complicated, which hinders downsizing and weight reduction, and increases material costs, assembly, and adjustment costs.

  Therefore, in the present embodiment, without providing an aberration correction optical element for correcting the spherical aberration, the NA of the objective lens 17 is set to an optimum value as described below, thereby reducing the influence of the spherical aberration. It is kept as small as possible, and recording / reproduction on the two-layered optical disk 1 is favorably performed.

  In the present embodiment, for example, a two-layer optical disc 1 is a DVD-RW, and the optical disc 1 has a minimum substrate thickness of 0.56 mm from the surface to the first information recording surface 1a in FIG. The maximum substrate thickness up to the second information recording surface 1b in FIG. 2 is 0.64 mm, and the distance between the first information recording surface 1a and the second information recording surface 1b of the optical disc 1 is 0.055 ± 0.005 mm. The transmittance of the first information recording surface 1a is 40 to 60%, the reflectance is 5 to 12% (preferably 7 to 12%), and the reflectance of the second information recording surface 1b is 5 to 12% ( Preferably, it is a 5 to 7%) two-layer phase change recording medium.

  The reflectance of the information recording surface 1b is the ratio of the amount of light incident on the information recording surface 1a, reflected by the information recording surface 1b, and transmitted again through the information recording surface 1a to the incident light. Increasing the transmittance of the recording surface 1a increases the reflectance of the information recording surface 1b.

  The optical disk 1 has spiral or concentric information signal recording / reproducing track grooves (groove grooves) formed at a predetermined track pitch in the radial direction of the disk, and lands are formed between adjacent groove grooves. The side wall at the boundary between the groove groove and the land is slightly meandered (wobbled) at a single frequency (for example, 140.6 ± 42.2 kHz), and prepits on which address information is placed at predetermined intervals are landed. This is the so-called Wobble & LPP format recorded at the disc manufacturing stage.

  Further, the optical disc 1 has a channel bit length of 146.7 ± 1.5 nm and a recording capacity of 8.54 GB, and a channel bit length of 133.3 ± 1.4 nm. Thus, an optical disk having a recording capacity of 9.4 GB is targeted. Moreover, the wavelength of the laser beam to be used has a center value of 650 nm in the range of 635 to 660 nm.

  FIG. 3 shows the relationship between the NA of the objective lens 17, wavefront aberration, and substrate thickness obtained by optical simulation. The optical system is designed with a single-layer substrate thickness of 0.6 mm. When the information recording surface of the two-layer optical disc is set to 0.63 mm (t0.63) 30 μm thick and 0.57 mm (t0.57) 30 μm thinner than the substrate thickness 0.6 mm, FIG. As shown by II, the wavefront aberration is about 30 mλ to about 40 mλ when the NA of the objective lens 17 is in the range of 0.6 to 0.65, but the difference between them is about 5 mλ. As a result, it increases almost equally.

  Similarly, when the thickness is set to 0.64 mm (t0.64) which is 40 μm thick and 0.56 mm (t0.56) which is 40 μm thinner than the above substrate thickness of 0.6 mm, as shown by III and IV in FIG. The wavefront aberration is about 40 mλ to about 55 mλ when the NA of the objective lens 17 is in the range of 0.6 to 0.65, but the difference between the two is about 5 mλ. To do.

  Therefore, in the present embodiment, the objective lens 17 has a substrate thickness intermediate between the layers so that the spherical aberration of the first information recording surface 1a and the second information recording surface 1b is uniformly reduced with the same objective lens. The objective lens 17 is designed at 0.6 mm.

  As can be seen from FIG. 3, when the substrate thickness deviates from the design value of 0.6 mm, the spherical aberration deteriorates with the difference. It can also be seen that the NA of the objective lens 17 is less affected by the difference in substrate thickness when the NA is smaller, and the spherical aberration is smaller.

  On the other hand, FIG. 4 shows the result of measuring the recording / reproducing jitter when the NA of the objective lens 17 is changed. However, FIG. 4 shows a case where a signal is recorded on and reproduced from a single-layer optical disk when the channel bit length is 133.3 ± 1.4 nm, and the reproduced signal is measured with a jitter meter. At this time, NAs of 0.60, 0.625, and 0.65 are switched and used.

  3 that the smaller NA of the objective lens 17 is less affected by the difference in substrate thickness and the spherical aberration is smaller. However, from FIG. 4, when the NA of the objective lens 17 is small, the recording surface of the optical disc 1 is obtained. It can be seen that the spot diameter focused on the surface increases, the resolution during recording and reproduction deteriorates, and the jitter deteriorates.

  FIG. 5 shows the maximum intensity of the focused spot and the spot diameters in the X and Y directions (full width at half maximum) when the NA of the objective lens 17 is changed by optical simulation. FIG. 5 shows that the spot diameter increases and the maximum intensity decreases as the NA of the objective lens 17 decreases. For this reason, when the NA of the objective lens 17 is reduced, the resolution of the focused spot is lowered and the recording / reproducing jitter is deteriorated.

  As described above, the smaller the NA of the objective lens 17 is, the less affected by the difference in substrate thickness, the smaller the spherical aberration, and conversely, if the NA is too small, the recording / reproducing jitter deteriorates. Therefore, in the present embodiment, the first information recording surface (first layer) 1a and the second information recording surface (second layer) 1b are hardly affected by spherical aberration, and can be recorded and reproduced satisfactorily. Therefore, from the relationship between the NA of the objective lens 17 in FIG. 3 and the spherical aberration and the substrate thickness, the objective lens is set so that the spherical aberration is 55 mλ or less on the first information recording surface 1a and the second information recording surface 1b. 17 NA is defined as 0.64 or less.

  However, as shown in FIG. 4, when the NA of the objective lens 17 is small, the jitter deteriorates. Therefore, if the above NA is 0.64 or less, it is possible to perform better recording and reproduction by setting the NA as large as possible. This is desirable.

  Here, the objective lens 17 having the NA of 0.64 or less will be described in detail. For example, by mounting an objective lens having an NA of 0.65 on a lens holder (also called a bobbin), the hole diameter (= aperture) of the lens holder is made smaller than the effective diameter of the objective lens having an NA of 0.65. , An objective lens having an optical characteristic equivalent to that of an objective lens having an NA of 0.64 or less can be obtained. In other words, an objective lens having an effective NA of 0.64 or less can be obtained using an objective lens having an NA of 0.65.

  When attaching the objective lens to the lens holder, it is necessary to align the center of the hole with the center (optical axis) of the lens, but it is effective even if the center of the hole is deviated from the optical axis of the lens. NA becomes small. In the above description, the case where an objective lens having an NA of 0.65 is used in order to obtain an objective lens having an effective NA of 0.64 or less has been described. It goes without saying that an objective lens having an NA greater than 0.65, for example, NA of 0.70 can be used if an aperture limit is given so that the NA of the lens is effectively 0.64 or less. Nor.

  Thus, according to the present embodiment, by selecting the NA of the objective lens 17 within a predetermined range, there is no need for an aberration correction optical element for correcting spherical aberration as in the conventional example. It becomes difficult to be affected by the spherical aberration caused by the difference in the substrate thickness of each layer of the layered optical disc 1, and it becomes possible to perform recording and reproduction with respect to each layer satisfactorily.

  In the above embodiment, when the channel bit length is 133.3 ± 1.4 nm, an information signal is recorded and reproduced with respect to a two-layer optical disc (recording capacity 8.54 GB). The present invention is not limited to this, and can be applied to a two-layer optical disc (recording capacity 9.4 GB) when the channel bit length is 133.3 ± 1.4 nm.

It is a block diagram of one embodiment of the optical pickup device of the present invention. It is a figure which shows typically the position of the objective lens at the time of the recording / reproducing with respect to a 2 layer optical disk. It is a figure which shows an example of the relationship between NA of an objective lens, spherical aberration, and board | substrate thickness. It is a figure which shows an example of the relationship between NA of an objective lens, and a recording / reproducing jitter. It is a figure which shows an example of the relationship between NA of an objective lens, the maximum intensity | strength of a condensing spot, and a spot diameter.

Explanation of symbols

1 Double-layer optical disc 1a First information recording surface (first layer)
1b Second information recording surface (second layer)
1c substrate 1d intermediate layer 11 semiconductor laser 12 collimator lens 13 diffraction grating 14 polarization beam splitter (PBS)
15 Front monitor 16 1/4 wavelength plate 17 Objective lens 18 Detection lens 19 Cylindrical lens 20 Photo detector

Claims (1)

Laser light having a predetermined wavelength emitted from a light source is a first optical recording medium having two or more layers in which at least a first information recording surface and a second information recording surface are stacked in the optical axis direction of the laser light. In the optical pickup device for recording or reproducing the information recording surface or the second information recording surface by focusing and irradiating with an objective lens,
The optical recording medium is
The substrate thickness minimum value from the laser beam incident side surface to the first information recording surface is 0.56 mm, and the substrate thickness maximum value from the laser beam incident side surface to the second information recording surface is 0.5 mm. 64 mm, the distance between the first information recording surface and the second information recording surface is within 0.055 ± 0.005 mm, the transmittance of the first information recording surface is 40 to 60%, and the reflectance is 5 A phase change recording medium having a reflectivity of 5 to 12% of the second information recording surface of -12%,
On the first information recording surface and the second information recording surface, groove grooves for information signal recording / reproduction are formed in a spiral shape or concentric circle shape, and a land is formed between two adjacent groove grooves. And at least address information is formed as pre-pits at predetermined intervals on the land, and a side wall at the boundary between the groove and the land is meandered at a single frequency of 140.6 ± 42.2 kHz,
The effective numerical aperture of the objective lens for condensing the laser light is set to 0.64 or less, and the first information recording surface or the first information recording surface by the laser light condensed by the objective lens without providing spherical aberration correction means. An optical pickup device for recording and reproducing information signals with a channel bit length of 146.7 ± 1.5 nm in the groove groove of the second information recording surface.

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