Optical disc device

In an optical disc device of the present invention, time differences among the focus error signal corresponding to the surface of the optical disc, the focus error signal corresponding to the first reflection layer, and the focus error signal corresponding to the second reflection layer, all of which are obtained through the light receiving element when a relative distance between the objective lens and the optical disc is linearly changed in a focus search of the optical disc are measured. The thickness of the cover layer from the surface of the optical disc to the first reflection layer and the thickness of a cover layer from the surface of the optical disc to the second reflection layer are detected on the basis of these time differences.

The priority application No. 336787/2003 upon which this patent application is based is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc beneficial for reproducing, or recording and reproducing high-density, large-capacity optical discs.

2. Description of the Related Art

Currently prevailing optical discs include optical discs of the CD format such as CDs, CD-ROMs, CD-Rs and CD-RWs, and the DVD format such as DVDs, DVD-ROMs, DVD-Rs, DVD-RWs, DVD-RAMs, DVD+Rs, and DVD+RWs capable of recording and reproducing data with higher density and larger capacity by the use of red lasers. Particularly in recent years, the Blu-ray disc standard (the term “Blu-ray disc” is a trademark of SONY KABUSHIKI KAISHA CORPORATION JAPAN 7-35, Kitashinagawa 6-chome Shinagawa-ku, Tokyo JAPAN), the Advanced Optical Disc (AOD) standard and the like capable of recording and reproducing data with even higher density and larger capacity by the use of blue lasers have been introduced. Optical discs and disc drives according to such standards are being commercially developed.

As an example of a mechanism for reading out data (pickup) in such an optical disc device, a mechanism conventionally adopted for an optical disc device of the Blu-ray disc standard will be schematically described as referring toFIG. 9.

As shown inFIG. 9, an optical disc device of this kind usually comprises a pickup50which is a mechanism for reading a disc. The pickup50basically comprises a semiconductor laser oscillator (LD)150which oscillates the laser light, a 45-degree reflection mirror56which reflects the laser light, an objective lens2which converges the laser light to focus on a reflection layer (recording layer)10of the optical disc, and a polarizing beam splitter51which leads the reflection light from the optical disc and the 45-degree reflection mirror56to a light receiving element (PD)160. A lens actuator (not shown) capable of moving the objective lens2slightly upward and downward is provided for focusing as an auxiliary unit. The objective lens2must have high quality to converge a beam to the diffraction limit, and its numerical aperture (NA) is set as high as about 0.85 for example.

In this example, the semiconductor laser oscillator150used as a light source is typically a blue-violet laser diode which oscillates laser light with a wavelength of 405 nm. A collimator lens53for shaping incident laser light is provided so that collimated light is incident to the objective lens. A condenser lens57and a cylindrical lens58are provided for condensing reflection light of laser light reflected from the reflection layer10of the loaded optical disc.

More specifically, predetermined polarized components out of linear polarized laser light generated from the semiconductor laser oscillator150are transmitted toward the disc through the polarizing beam splitter51in order to be circularly polarized by a quarter wavelength plate52. Laser light collimated by the collimator lens53is reflected from the 45-degree reflection mirror56and irradiated to the reflection layer10after being converged by the objective lens2. The laser light reflected from the reflection layer10reaches the quarter wavelength plate52through the 45-degree reflection mirror56and the collimator lens53, to become linearly polarized light which has a phase difference of 90 degrees from the original polarizing direction. The polarizing beam splitter51only reflects polarized components different from the polarized components reflected as described above so that reflected light is condensed by the condenser lens57and the cylindrical lens58to be incident to the light receiving element160. The light receiving element160converts incident laser light into an electrical signal. The converted electrical signal is amplified and transmitted outside the pickup50to be demodulated in a well-known manner.

When focusing by a pickup, the distance between the disc surface and the objective lens is adjusted by driving the lens actuator to move the objective lens upward and downward.

When recording and reproducing data on such a high-density optical disc, a focal spot diameter of laser light oscillated from the semiconductor laser oscillator (laser light source) must be small on the disc. The spot diameter is basically calculated from the following formula.
Spot diameter=wavelength of laser light source λ/numerical aperture of objective lens
NA   (1)

As can be seen from this formula (1), the focal spot diameter of laser light is proportional to the wavelength of the laser light source λ, and is inversely proportional to the numerical aperture of the objective lens NA. Therefore, the focal spot diameter of laser light may be reduced by shortening the wavelength of the laser light source λ or by using the objective lens with higher numerical aperture. For example, the wavelength λ of the laser light source is 405 nm while the numerical aperture of the objective lens is 0.85 for the optical disc of the Blu-ray disc standard.

If the numerical aperture of the objective lens is this high, however, tolerance for disc tilt becomes stringent. Tolerance for disc tilt is calculated from the following formula.
Tolerance for disc tilt=wavelength of laser light source λ/(numerical aperture of
objective lens NA)3(2)

As can be seen from this formula (2), tolerance for disc tilt is proportional to the wavelength of the laser light source, and is reduced in inverse proportion to the 3rdpower of the numerical aperture of the objective lens. Therefore, the thickness of the disc cover layer must be particularly small in order to maintain tolerance for disc tilt for the optical disc of the Blu-ray disc standard which utilizes the objective lens with a high numerical aperture.

The optical disc of the Blu-ray disc standard has a tolerance for disc tilt which is one-fifth of that of the DVD standard (wavelength of laser light source λ: 650 nm, numerical aperture of objective lens NA: 0.6) Therefore, the optical disc of the Blu-ray disc standard must have a cover layer of approximately 100 μm in thickness as compared to a cover layer of 600 μm in thickness of the optical disc of the DVD standard.

Additionally, two reflection layers (recording layers) are supposed to be provided on one side of the high-density optical disc in order to increase data recording capacity. The first reflection layer (recording layer) and the second reflection layer (recording layer) must be distanced from each other as far as possible (for example, about 25 μm apart) so that reflection light from one layer does not affect reflection light from the other layer. Consequently, the thickness of the cover layer from the disc surface to the first reflection layer and the thickness of the cover layer from the disc surface to the second reflection layer are different. In the optical disc of the Blu-ray disc standard where the thickness of the cover layer is particularly thin, the ratio of each thickness deviation of the cover layer for the first reflection layer and the second reflection layer to the thickness 100 μm of the cover layer of the disc increases since the thickness of the cover layer of the optical disc is inherently thin.

On the other hand, although the objective lens is designed in consideration of the thickness of the cover layer of the disc, spherical aberration is generated on reflection layers of the optical disc if the thickness of the cover layer of the disc is out of the standard thickness of 100 μm.

Next, the relation between spherical aberration and the thickness of the cover layer of the disc will be described as referring toFIG. 10.FIGS. 10A,10B and10C enlarge and illustrate the relation between the cover layer of the disc and the focal position respectively when the cover layer of the disc is thinner than the standard (FIG. 10A), according to the standard (FIG. 10B) and thicker than the standard (FIG. 10C).

As shown inFIG. 10A, the focal position is recognized at a position a little more distant from the disc in a focus search when the thickness of the disc cover is thinner than the standard as compared to the case when the thickness of the cover layer is according to the standard. Therefore, a focus error9FE) signal of laser light reflected from the optical disc is recognized short of the reflection layer. The focal spot diameter of laser light becomes large on the surface of the reflection layer of the disc since rays of laser light intersect before the reflection layer of the disc, which causes spherical aberration. As shown inFIG. 10B, a focus error signal is recognized on the reflection layer of the disc in the focus search when the thickness of the disc cover is according to the standard, so that the focal position may fall on the reflection layer of the disc. On the other hand, when the thickness of the disc cover is thicker than the standard, the focal position of the objective lens is recognized at a position a little nearer to the disc in the focus search as compared to the case when the thickness of the cover layer is according to the standard, as shown inFIG. 10C. Therefore, laser light reflected from the optical disc is focused at a deeper position inside the disc away from the objective lens. The focal spot diameter of the laser light thus becomes large on the surface of the reflection layer of the disc since the focus error signal is recognized at the deeper position beyond the reflection layer, which causes spherical aberration.

Spherical aberration is basically calculated from the following formula.
Spherical aberration=(thickness deviation of cover layer Δ d/standard thickness of
cover layer d)×(numerical aperture of objective lens NA)4(3)

As can be seen from this formula (3), spherical aberration is proportional to the 4thpower of the numerical aperture of the objective lens NA.

Such spherical aberration hinders the appropriate focal spot diameter from falling on the reflection layer of the optical disc, degrading recording or reproducing function of the optical disc device.

In this connection, an optical disc device to detect spherical aberration by the use of a hologram element is conventionally disclosed in the Japanese Published Application 367197/2002, A, for example. In this optical disc device, a hologram element is used to separate light into a light flux passing through the outer circumference, which is away from the optical axis, of the objective lens and a light flux passing through the center, which is close to the optical axis, of the objective lens. Spherical aberration is detected by obtaining the difference in intensity of the two light fluxes.

In this way, spherical aberration may be corrected by providing means for detecting spherical aberration in an optical disc device to control an actuator for correcting spherical aberration with a feedback of the detected spherical aberration signal.

Providing means for detecting spherical aberration as described above for an optical disc deice, however, not only complicates the detection mechanism but also increases the number of parts. Furthermore, the number of manufacturing processes and manufacturing cost inevitably increase because of the necessity of adjustment work and so on.

The present invention was made in consideration of such conditions and its objective is to provide an optical disc device which may detect factors causing spherical aberration more easily and precisely without using complicated means for detecting spherical aberration.

SUMMARY OF THE INVENTION

In order to achieve the objective, an optical disc device according to the present invention comprises: an objective lens for condensing laser light emitted from a laser light source onto a reflection layer on which data is recorded of an optical disc through a light-transmitting cover layer covering the reflection layer; and a light receiving element for receiving reflection light from the reflection layer of the optical disc and converting the reflection light to an electrical signal in order to perform at least either data storage to the reflection layer or data reproduction from the same layer, wherein detection means is provided to detect a thickness of the cover layer on the basis of time difference between a focus error signal corresponding to a surface of the optical disc and a focus error signal corresponding to the reflection layer, both of which are obtained through the light receiving element when a relative distance between the objective lens and the optical disc is linearly changed in a focus search of the optical disc.

When the optical disc device structured like this, whether there is any influence of spherical aberration or not may be determined correctly, though indirectly, from the thickness of the cover layer detected on the basis of the above-described focus error signal. In other words, whether spherical aberration is generated or not, its level and so on may be determined without providing any special means to detect spherical aberration. As was already described, the thickness deviation of the cover layer causes spherical aberration. By correctly determining the thickness of the cover layer, it is possible to operate the optical disc device appropriately.

Further, the present invention is characterized by that in the above-described optical disc device, the optical disc has a dual layer structure with a first reflection layer and a second reflection layer laminated at a predetermined interval on one side, and detection means detects respectively a thickness of the cover layer from the surface of the optical disc to the first reflection layer and a thickness of the cover layer from the surface of the optical disc to the second reflection layer on the basis of time differences among the focus error signal corresponding to the surface of the optical disc, a focus error signal corresponding to the first reflection layer, and a focus error signal corresponding to the second reflection layer, all of which are obtained through the light receiving element when the relative distance between the objective lens and the optical disc is linearly changed in the focus search of the optical disc.

For example, the above-described optical disc of the Blu-ray disc standard has dual layer structure with a first reflection layer and a second reflection layer laminated at a predetermined interval on one side. At present, it is this optical disc of the Blu-ray disc standard that needs an objective lens with a high numerical aperture for its optical system (pickup). Therefore, the thickness deviation of the cover layer can not be ignored at a cause to generate spherical aberration. According to the above-described structure, the thickness of the cover layer from the optical disc surface to the first reflection layer and the thickness of the cover layer from the optical disc surface to the second reflection layer may be accurately detected. Consequently, it is possible to appropriately operate the optical disc device which reads out data from and writes data to the above-described optical disc of the Blu-ray disc standard for example.

Additionally, the present invention is characterized by that in each of the above-described optical disc devices, detection means comprises means for judging a movement speed of the objective lens on the basis of a drive voltage of an actuator which linearly changes the relative distance between the objective lens and the optical disc, and the thickness of the cover layer is detected by multiplying the time differences by the judged movement speed of the objective lens.

Usually, drive voltage for the above-described actuator is fairly stable. The movement speed of the objective lens when relative distance between the optical disc and the objective lens is linearly changed is almost directly proportional to this voltage. Therefore, the movement speed of the objective lens is detected by tabulating the relation between the drive voltage for the actuator and the movement speed of the objective lens, so that the thickness of the cover layer can be quite accurately detected.

The present invention is further characterized by any one of the above-described optical disc devices further comprising correction means to correct the way the objective lens condenses light in accordance with the thickness of the cover layer detected by the detection means.

With the optical disc device structured like this, the optical disc device itself may automatically correct spherical aberration caused by the thickness deviation of the cover layer. Such an additional function is particularly beneficial for the optical disc device which reads out data from and writes data to the optical disc of the Blu-ray disc standard.

Additionally, the present invention is characterized by that in the above-described optical disc device, correction means is a beam expander which may change a diameter of laser light incident to the objective lens by combining at least two lenses.

The above-described beam expander is generally adopted in an optical system (pickup) of such an optical disc device. Therefore, spherical aberration can be automatically corrected without increasing the number of parts according to the above-described structure using such a beam expander as correction means.

According to the present invention, whether there is any influence of spherical aberration or not may be determined correctly, though indirectly, from the thickness of the cover layer detected on the basis of the focus error signal. In other words, whether spherical aberration is generated or not, its level and so on may be determined without providing any special means to detect spherical aberration. By correctly determining the thickness of the cover layer, it is possible to operate the optical disc device appropriately.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when reviewed in conjunction with the accompanying drawings.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

One embodiment of an optical disc device according to the present invention will be described in detail as referring to drawings. In this embodiment, description is made for a device to reproduce data written in an optical disc of the Blu-ray disc standard which has dual layer structure with a first reflection layer and a second reflection layer for example.

First, a detection principle of detection means to detect the thickness of the cover layer of the optical disc according to this embodiment will be described, as referring toFIGS. 1A,1B and1C, andFIG. 2.

The concept of the optical disc1of the Blu-ray disc standard which has a dual layer structure with the first reflection layer11and the second reflection layer12is schematically illustrated inFIGS. 1A,1B and1C. According to the Blu-ray disc standard, the thickness of the cover layer between the optical disc surface13and the first reflection layer11is 100 μm while the thickness of the cover layer between the first reflection layer11and the second reflection layer12is 25 μm.

FIG. 1Aschematically illustrates the status where the focal position falls on the optical disc surface13as the objective lens2is being brought toward the optical disc1by driving the actuator of the objective lens2during the focus search (focusing). Likewise,FIGS. 1B and 1Cschematically illustrate statuses where the focal position falls on the first reflection layer11and the second reflection layer12respectively as the objective lens2is being brought toward the optical disc1by driving the actuator of the objective lens2during the focus search. The center position of the objective lens2when the focal position falls on the optical disc surface13, the first reflection layer11, and the second reflection layer12, respectively, is schematically illustrated by dotted lines as the first position, the second position and the third position in these Figs.

FIG. 2(a) illustrates the track of the objective lens2as the objective lens2(FIGS. 1A,1B and1C) is being linearly brought from a position distant from the optical disc1(FIGS. 1A,1B and1C) to a position nearer to the disc during the focus search. As can be seen from this track, the movement distance of the objective lens2may be determined from the drive voltage applied to the lens actuator and driving time of the actuator. The movement distance of the objective lens2may be determined by the movement speed of the actuator×driving time of the actuator. Besides, the movement speed of the actuator is proportional to the drive voltage of the actuator. Therefore, the relation between the drive voltage of the actuator and the movement speed of the actuator is tabulated on a conversion table in advance, so that the movement speed of the objective lens may be found from the table.

When the lens actuator is driven to move the objective lens2(FIGS. 1A,1B and1C) toward the optical disc1(FIGS. 1A,1B and1C), the laser light reflected at the vicinity of the focal point is detected, and an S-curved error signal is generated as the focus error (FE) signal.FIG. 2(b) illustrates generation mode of the focus error (FE) signal in accordance with the track of the objective lens2as shown inFIG. 2(a). Each focal position (position of focal point) of the lens appears in accordance with the timing when the FE signal is on the zero-crossing. For example, the first S-curved FE signal as shown inFIG. 2(b) appears at the point A in the track ofFIG. 2(a) corresponding to the optical disc surface13inFIG. 1A. The second S-curved FE signal as shown inFIG. 2(b) appears at the point B in the track ofFIG. 2(a) corresponding to the first reflection layer11inFIG. 1B. This FE signal appears with a larger amplitude of vibration than that of the FE signal at the point A, since this FE signal appears corresponding to the reflection layer (recording layer). Additionally, the third FE signal as shown inFIG. 2(b) appears at the point C in the track ofFIG. 2(a) corresponding to the second reflection layer12inFIG. 1C. This FE signal appears with amplitude of vibration comparable to that of the FE signal at the point B, since this FE signal also appears corresponding to the reflection layer (recording layer). The present movement distance of the objective lens2, namely, the thickness of the cover layer of the optical disc1is calculated based on generation intervals (time) of the FE signal and drive voltage (movement speed of the objective lens2) of the actuator, as disclosed below.

(i) The movement distance of the objective lens from the point A on the disc surface to the point B on the first reflection layer [M1]=(driving time of the actuator from the point A to the point B)×(the movement speed of the objective lens)=the thickness of the cover layer from the point A on the disc surface to the point B on the first reflection layer.

(ii) The movement distance of the objective lens from the point B on the first reflection layer to the point C on the second reflection layer [M2]=(driving time of the actuator from the point B to the point C)×(the movement speed of the objective lens)=the thickness of the cover layer from the point B on the first reflection layer to the point C on the second reflection layer.

(iii) The thickness of the cover layer from the point A on the disc surface to the point C on the second reflection layer=M1+M2.

Next, brief explanation is made for the optical disc device according to this embodiment structured on the basis of these principles, as referring toFIGS. 3,4and5.

In the optical disc device as shown inFIG. 3, a pickup (optical pickup)100reads out data stored in an optical disc1by laser light in order to covert into an electrical signal. Inside the pickup100are provided a semiconductor laser oscillator150as a light source, an optical system comprising a plurality of lenses such as an objective lens2, a mirror, a beam splitter and the like, an actuator110for driving the objective lens, correction means for spherical aberration120which is a beam expander for example, and a light receiving element160. The pickup100irradiates laser light to the reflection layer of the optical disc1as well as receives laser light reflected from the reflection layer by the light receiving element160in order to convert into the electrical signal. The converted electrical signal is properly amplified and transmitted to a computing circuit200as the FE signal.

The computing circuit200computes the electrical signal transmitted from the pickup100during the focus search by a well-known computing method in order to generate the FE signal. The FE signal generated in this manner is outputted to a control circuit300as shown inFIG. 3.

The control circuit300shown inFIG. 3generally controls each function of the optical disc device on the basis of each control program stored in a built-in memory or the like, as well as commands transmitted from an external processor unit (such as a personal computer, player, recorder), which is not shown through an interface. An LD (semiconductor laser oscillator) drive circuit400, which will be described later, also carries out necessary processes based on commands from the control circuit300. Specifically, the control circuit300drives the semiconductor laser oscillator150of the pickup100by outputting an LD drive signal to an LD drive circuit. The above-described focus search in the pickup100is made possible by an actuator drive signal outputted from the control circuit300to the actuator110of the objective lens.

Additionally, the focus position (focal point) is recognized by the control circuit300from the FE signal computed in the above-described manner. The thickness deviation of the cover layer is determined from the thickness of the cover layer of the disc detected by the above-described detection means and the standard thickness of the cover layer of the optical disc of the Blu-ray disc standard (the thickness of the cover layer between the optical disc surface and the first reflection layer is 100 μm while the thickness of the cover layer between the first reflection layer and the second reflection layer is 25 μm). After that, spherical aberration is corrected by outputting a required drive signal to the beam expander which is a correction means for spherical aberration120on the basis of the determination of the thickness of the cover layer of the disc.

The LD drive circuit400also shown inFIG. 3controls oscillation of the semiconductor laser oscillator150in the pickup100on the basis of the LD drive signal fed from the control circuit300.

Additionally, the optical device of the present embodiment comprises an encoder/decoder (not shown) which conducts encoding (modulation) and decoding (demodulation) processes in cooperation with the above-described control circuit300.

For example, when data is recorded (written) in the optical disc, the control circuit300encodes the data transferred from the external processor unit through the interface in accordance with the specification of the optical disc fed from the control circuit300. In addition, the control circuit300controls the pickup100by transferring a signal to the LD drive circuit400to control oscillation of laser as well as writes the above encoded data in the reflection layer of the optical disc.

On the other hand, when reproducing the data from the optical disc, the control circuit300decodes the data on the basis of a data signal (RF signal) in reading the disc inputted from the computing circuit200in accordance with the specification of the optical disc fed from the control circuit300. The decoded data is forwarded to the external processor unit through the interface. Circuits according to standards like AT Attachment Packet Interface (ATAPI), Small Computer System Interface (SCSI) and so on may be properly employed as the interface.

FIGS. 4 and 5show a more detailed internal structure of the pickup100, according to which will be described in more details the structure of the pickup100. The pickup100of the present embodiment has basically the same structure as the one shown inFIG. 9but a beam expander is provided anew as correction means for spherical aberration120.

The pickup100comprises the semiconductor laser oscillator150which oscillates laser light, a 45-degree reflection mirror156which reflects laser light, the objective lens2which converges laser light to focus on the reflection layer10of the optical disc, and a polarizing beam splitter151which leads reflection light from the optical disc and the 45-degree reflection mirror156to the light receiving element160, as shown inFIG. 4. A lens actuator (not shown) which may move the objective lens2slightly upward and downward is provided for focusing as an auxiliary unit. The objective lens2must have high quality to converge a beam to the diffraction limit, and its numerical aperture (NA) is set as high as about 0.85.

In addition, the beam expander with dual lens structure of a concave lens154and a convex lens155is provided as a correction means for spherical aberration between a collimator lens153and the objective lens2.

In this embodiment, the semiconductor laser oscillator150used as the light source is a blue-violet laser diode which oscillates laser light with a wavelength of 405 nm. The collimator lens153is provided for shaping incident laser light so that collimated light is incident to the objective lens, and a condenser lens157and a cylindrical lens158are provided for condensing reflection light of laser light reflected from the reflection layer10of the loaded optical disc.

More specifically, predetermined polarized components out of linear polarized laser light generated from the semiconductor laser oscillator150are transmitted toward the disc through the polarizing beam splitter151in order to be circularly polarized light by a quarter wavelength plate152. The concave lens154which is included in the beam expander turns laser light collimated by the collimator lens153into diffused light, and the convex lens155which is also included in the beam expander turns the diffused light into converging light. The laser light turned into converging light is reflected from the 45-degree reflection mirror156to be irradiated to the reflection layer10after being converged by the objective lens2. The laser light reflected from the reflection layer10reaches the quarter wavelength plate152through the 45-degree reflection mirror156and the collimator lens153to become linearly polarized light which has different phase by 90 degree from the original polarizing direction. The polarizing beam splitter151only reflects polarized components different from the polarized components reflected as above so that reflected light is condensed by the condenser lens157and the cylindrical lens158to be incident to the light receiving element160. The light receiving element160converts incident laser light into an electrical signal. The converted electrical signal is amplified and transmitted outside the pickup100to be demodulated in a well-known manner.

During focusing by the pickup100, the distance between the disc surface and the objective lens is adjusted by driving the lens actuator to move the objective lens2upward and downward.

FIGS. 5A and 5Bshow the structure of the pickup100on the XY-plane and the XZ-plane, respectively.

Next, recording and reproducing action by the optical disc device of the present embodiment which is performed in parallel with correcting spherical aberration on the basis of the thickness of the cover layer determined as above will be described in detail as referring toFIGS. 6 and 7.

In the embodiment, the control circuit300(FIG. 3) corrects spherical aberration after determining the thickness of the disc by the focus search, the whole process of which is shown inFIG. 6.

Specifically, the focus search as illustrated inFIGS. 1A,1B and1C, andFIG. 2always precedes recording and reproduction of the optical disc (step S101).

Next, in step S102, after the focus positions (focal points) on the disc surface, on the first reflection layer, and on the second reflection layer are recognized from the FE signal detected during the focus search, movement distance of the objective lens2is detected by detection means as described above. The thickness of the cover layer of the disc is determined from the detected movement distance of the objective lens2. The thickness deviation of the cover layer is determined from the detected thickness of the cover layer of the disc and the standard thickness of the cover layer of the optical disc of the Blu-ray disc standard.

Next, in step S103, a lens (the convex lens155in this example) of the beam expander which is correction means for spherical aberration120is moved forward or backward appropriately by an actuator (not shown) on the basis of the thickness deviation of the cover layer determined in step S102.

FIGS. 7A,7B and7C show examples of a process to correct spherical aberration through the beam expander on the basis of the determination of the thickness of the cover layer of the disc.

FIGS. 7A,7B and7C show examples for operating the beam expander with a dual lens structure of the concave lens154and the convex lens155provided as a correction means for spherical aberration120between the collimator lens153and the objective lens2, as described inFIGS. 4 and 5. Laser light incident to the concave lens154is, as described inFIG. 4, circularly polarized collimated light.

When the thickness of the cover layer of the disc is determined to be thinner than the standard in step S102for example, relative distance between the concave lens154and the convex lens155is increased so that light diffused by the concave lens154is further diffused to be incident to the convex lens155, as shown inFIG. 7A. The incident light whose diameter is enlarged toward the outer circumference of the convex lens155is turned into converging light by a diffraction property of the convex lens155, and the converging light is incident to the objective lens2. The focal position of laser light incident to the objective lens2as converging light falls on a shallower position inside the disc than the usual position (focal position of light incident as collimated light) because of a diffraction property of the objective lens2. The focal position may be made to fall on the reflection layer of the disc by moving the objective lens2nearer to the disc.

On the other hand, when the thickness of the cover layer of the disc is determined to be equal to the standard in step S102, the relative distance between the concave lens154and the convex lens155is maintained without any change, as shown inFIG. 7B. Light diffused by the concave lens154is turned into collimated light by diffraction properties of the convex lens155, and the collimated light is incident to the objective lens2. Therefore, the focal position may fall on the usual position.

When the thickness of the cover layer of the disc is determined to be thicker than the standard in step S102, relative distance between the concave lens154and the convex lens155is decreased so that light diffused by the concave lens154is further diffused by the convex lens155to be incident to the objective lens2, as shown inFIG. 7C. The focal position of laser light incident to the objective lens2as diffused light falls on a deeper position inside the disc than the usual position (focal position of light incident as collimated light) because of the diffraction property of the objective lens2. The focal position may be made to fall on the reflection layer of the disc by moving the objective lens2away from the disc.

If other conditions (for example, laser power of the semiconductor laser oscillator150(FIG. 3) and so on) necessary for recording and reproduction of the disc are optimal in step S104(YES), recording and reproduction is made possible, so begins actual recording and reproduction (step S105).

On the other hand, if other conditions necessary for recording and reproduction of the disc are not optimal (NO in step S104), recording condition such as above-described laser power are changed (step S106) and whether the conditions are optimal or not is judged again (step S104). This process is repeated until the conditions are optimal. Actual recording and reproduction starts when the conditions become satisfactory (step S105).

As described hereinbefore in detail, the optical disc device according to this embodiment has excellent advantages cited hereinafter.

(1) An optical disc1of the Blu-ray disc standard has dual layer structure with a first reflection layer11and a second reflection layer12laminated at a predetermined interval on one side. At present, it is this optical disc of the Blu-ray disc standard that needs an objective lens with a high numerical aperture for its optical system (pickup). Therefore, the thickness deviation of the cover layer can not be ignored as a cause to generate spherical aberration. According to the above-described structure, the thickness of the cover layer from the optical disc surface13to the first reflection layer11and the thickness of the cover layer from optical disc surface13to the second reflection layer12may be accurately detected in the optical disc of dual layer structure on one side. In other words, the cause of spherical aberration (the thickness deviation of the cover layer of the disc) may be accurately detected without providing any special means to detect spherical aberration.

(2) Usually, drive voltage for the actuator used for the above-described focus search is fairly stable. The movement speed of the objective lens2when relative distance between the optical disc1and the objective lens2is linearly changed is almost directly proportional to this voltage. The movement speed of the objective lens is detected by tabulating the relation between the drive voltage for the actuator and the movement speed of the objective lens, so that the thickness of the cover layer can be easily and quite accurately detected.

(3) Spherical aberration caused by the thickness deviation of the cover layer of the optical disc1is automatically corrected by a beam expander for example. Such an additional function is particularly beneficial for the optical disc device which reads out data from and writes data to an optical disc of the Blu-ray disc standard.

(4) The above-described beam expander is generally adopted in an optical system (pickup) of such an optical disc device. Therefore, spherical aberration can be automatically corrected without increasing the number of parts by the above-described structure using such a beam expander as correction means.

The optical disc device according to the present invention is not limited to the above-described embodiment. The embodiment may be properly modified as described hereinbelow.

Although in the above-described embodiment, the focus search is conducted by moving the objective lens from a position away from the disc to a position nearer to the disc, the focus search may also be conducted by reversely moving the objective lens2from a position nearer to the disc to a position away from the disc as shown inFIG. 8. InFIG. 8(a), the point D corresponds toFIG. 1Cwhere the focal position falls on the second reflection layer12, the point E corresponds toFIG. 1Bwhere the focal position falls on the first reflection layer11, and the point F corresponds toFIG. 1Awhere the focal position falls on the disc surface13. Every time the focal position falls on each of these points, an FE signal in opposite phase to the FE signal shown inFIG. 2(b) appears as shown inFIG. 8(b). Therefore, the thickness of the cover layer from the first reflection layer11to the second reflection layer12can be determined corresponding to movement distance of the objective lens2[M3], and the thickness of the cover layer from the disc surface13to the first reflection layer11can be determined corresponding to movement distance of the objective lens2[M4] even when the focus search is conducted in such a manner. Also in this case, M3+M4represents the thickness of the cover layer from the disc surface13to the second reflection layer12including the first reflection layer11.

Although in the above-described embodiment, the beam expander has dual lens structure with the concave lens154and the convex lens155, the structure of the beam expander is not limited to this. For example, the beam expander may consist of two convex lenses, or more than three lenses. In other words, physical structure of the beam expander is optional.

Although in the above-described embodiment, the beam expander is used as correction means for spherical aberration, correction means for spherical aberration is not limited to the beam expander. The point is to variably control incident light to the objective lens to be collimated light/converging light/diffused light in accordance with the thickness of the cover layer, and the physical structure of correction means for spherical aberration is optional. Additionally, although in the above-described embodiment, correction means for spherical aberration is disposed between the collimator lens153and the objective lens2, the layout is optional so long as incident light to the objective lens may be variably controlled to be collimated light/converging light/diffused light in accordance with the thickness of the cover layer.

Although in the above-described embodiment, the movement speed of the objective lens is detected by tabulating the relation between the movement speed of the objective lens and the drive voltage of the actuator, the movement speed of the objective lens may be detected otherwise. For example, the movement speed of the objective lens may be determined by maintaining the movement speed of the objective lens at a constant level.

Although in the above-described embodiment, the optical disc is an optical disc of the Blu-ray disc standard which has dual layer structure with a first reflection layer11and a second reflection layer12laminated at a predetermined interval on one side, the intended optical disc is optional. In other words, the present invention may be applicable to optical discs of the CD, DVD or AOD standards regardless of single or dual layer structure.

Although the present invention has been described and illustrated in detail, it should be clearly understood that the description discloses examples of different embodiments of the invention and is not intended to be limited to the examples or illustrations provided. Any changes or modifications within the spirit and scope of the present invention are intended to be included, the invention being limited only by the terms of the appended claims.