Method of controlling focus of optical disk recording and reproducing device

In an optical disk recording and reproducing device which uses an optical disk having at least a first recording layer and a second recording layer, a thickness of a spacer section between the first recording layer and the second recording layer is measured, an amount of defocus with respect to the second recording layer is set based on the measured spacer thickness, and a focus servo operation is performed for focusing the laser light irradiated from an optical pickup device on the second recording layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The entire disclosure of Japanese Patent Application Nos. 2004-210033 and 2004-217428, including specification, claims, drawings and abstract, is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for controlling focus in an optical disk recording and reproducing device which performs a reproducing operation of a signal recorded on an optical disk using laser light irradiated from a laser diode incorporated in an optical pickup device or a recording operation of a data signal to the optical disk.

2. Description of the Related Art

An optical disk recording and reproducing device has been made commercially available which performs a reproducing operation of a signal recorded on an optical disk using laser light irradiated from a laser diode and a recording operation of a signal to the optical disk. A typical optical disk recording and reproducing device uses an optical disk which is called a “CD” and another typical optical disk recording and reproducing device uses an optical disk which is called a “DVD”.

Because there is a strong demand for recording a larger amount of signals on an optical disk, currently, there is a shift from CD to DVD. Among DVD disks, an optical disk called a dual layer disk has been developed in which two recording layers are provided instead of one recording layer.

A technique has been developed in which laser light irradiated from an optical pickup device is focused on each signal layer in an optical disk recording and reproducing device which uses the dual layer disk.

The operation to control focus for focusing the laser light irradiated from the optical pickup device on two signal layers assuming that the placement of the recording layers of the optical disk is accurate, that is, the physical characteristics of the optical disk conform with the standard.

An optical disk of a DVD system is created by adhering two disks of a predetermined thickness (0.6 mm). As a result, although it is possible to achieve approximate uniform thickness for a cover layer provided between the surface of the optical disk and the first recording layer, the thickness of a spacer section which is the adhering portion of the two disks, that is, the spacer section provided between the first and second recording layers may not be uniform and may vary from one disk to another. As a result, there is a problem in that it is not possible to accurately perform a focus servo operation with an amount of defocus which is set with respect to the first and second recording layers.

In addition, in each optical disk, the thickness of the space section is not uniform over the optical disk. In particular, the thickness of the spacer section may vary significantly between an inner side and an outer side. Thus, there is a problem in that it is not possible to accurately perform a focus servo operation on the overall optical disk.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method of controlling focus in an optical disk recording and reproducing device which uses an optical disk having at least a first recording layer and a second recording layer and which performs recording and reproducing operations for a signal using laser light irradiated from a side of the first recording layer, wherein a thickness of a spacer section present between the first recording layer and the second recording layer is measured, an amount of defocus with respect to the second recording layer is set based on the measured spacer thickness, and a focus servo operation is performed to focus laser light irradiated from an optical pickup device on the second recording layer.

DESCRIPTION OF PREFERRED EMBODIMENTS

First Preferred Embodiment

In a first preferred embodiment of the present invention, an amount of defocus for performing the focus servo operation is set by measuring a thickness of a spacer section present between a first recording layer and a second recording layer.

FIG. 1is a block circuit diagram showing an optical disk recording and reproducing device according to the first preferred embodiment of the present invention.FIGS. 2A and 2Bare explanatory diagrams showing a relationship between an optical disk and laser light.FIGS. 3 and 4are signal waveform diagrams for explaining operations.

Referring toFIG. 1, an optical disk1is constructed to be rotationally driven by a spindle motor (not shown) so that the rotation is controlled such that, for example, the linear velocity is constant. As shown inFIG. 2, a first recording layer L1and a second recording layer L2are provided on the optical disk1. An optical pickup device2comprises a laser diode (not shown) which emits laser light, an objective lens3for focusing the laser light irradiated from the laser diode to the recording layer of the optical disk1, an optical detector4, comprising a quadrant sensor or the like, which receives the laser light reflected from the optical disk1and converts the laser light into an electrical signal, a tracking coil5for displacing the objective lens3along a radial direction of the optical disk1, and a focusing coil6for displacing the objective lens3in a direction perpendicular to a signal plane of the optical disk1.

A maximum distance position defining member7contacts a lens holder (not shown) which supports the objective lens3when the objective lens3is furthest away from the surface of the optical disk1due to a supply operation of a drive signal to the focusing coil6. The maximum distance position defining member7is provided to control a maximum distance position of the objective lens3with respect to the optical disk1. A minimum distance position defining member8contacts the lens holder which supports the objective lens3when the objective lens3approaches closest to the surface of the optical disk1due to a supply operation of a drive signal to the focusing coil6. The minimum distance position defining member8is provided to control the minimum distance position of the objective lens3with respect to the optical disk1.

An electrical signal obtained from the optical detector4is input to an optical output signal processor circuit9as an optical signal. The optical output signal processor circuit9is constructed to generate a tracking error signal which indicates a deviation of laser light from a signal track, a focus error signal which indicates a deviation in focus of the laser light with respect to the recording layer, and a replay signal which is a read signal of the signal recorded on the optical disk1. The operations to generate various signals in the optical output signal processor circuit9are performed by known circuits and will not be described in detail.

A tracking servo circuit10receives the tracking error signal generated by and output from the optical output signal processor circuit9. The tracking servo circuit10is constructed to perform a control operation of tracking by supplying a tracking coil drive signal to the tracking coil5based on the input tracking error signal. A focus servo circuit11receives the focus error signal generated by and output from the optical output signal processor circuit9. The focus servo circuit11is constructed to perform a control operation of the focus by supplying a focus coil drive signal to the focusing coil6based on the input focus error signal.

A digital signal processor circuit12receives a replay signal which is binarized by a binarization circuit provided within the optical output signal processor circuit9. The digital signal processor circuit12is constructed to apply digital signal processing to the input signal to decode various signals. A controller circuit13controls various operations of the optical disk recording and reproducing device. The controller circuit13is constructed to control a rotation control operation of the optical disk1by the spindle motor and processing operations of the replay signal and the recording signal using a synchronization signal generated by the digital signal processor circuit12and transmission/reception operations of signals to and from a host device such as a personal computer which is externally provided. The controller circuit13is formed by a microcomputer and is constructed to perform various control operations based on program software stored in a ROM or the like which is provided internally.

A comparator circuit14receives the focus error signal output from the optical output signal processor circuit9. The comparator circuit14is constructed to output, to the controller circuit13, a signal of H (high) level for a period in which the level of the focus error signal exceeds a threshold value VR. A graph (A) inFIG. 3shows a relationship between the focus error signal and the threshold value VR which is obtained when the objective lens3is moved to approach from a position distant from the recording layer provided on the optical disk1. As is known in the art, the focus error signal changes in an S-shape. When the optical disk1comprises two layers, two S-shaped changes occur. A graph (B) inFIG. 3shows a pulse signal output from the comparator circuit14, and a signal of H level is output for a period in which the level of the focus error signal exceeds the threshold value VR.

An operation of a counter circuit15is controlled by the controller circuit13. The counter circuit15is constructed to start a counting operation when a pulse signal is output from the comparator circuit14and to measure a period between a first pulse signal and a next pulse signal when the next pulse signal is output. For example, it is possible to provide a clock signal generating circuit which generates a clock signal of a predetermined frequency and measure the period using a number of clock signals which are counted between the pulse signals.

A spacer thickness measuring circuit16determines a thickness of a spacer section S provided between the first recording layer L1and the second recording layer L2based on a number of clock signals counted by the counter circuit15. Because the thickness of the spacer section S can be calculated from a relationship between the number of clock signals and the thickness which is determined in advance, the spacer thickness measuring circuit16can output data based on the calculated thickness to the controller circuit13.

An operation of a memory circuit17is controlled by the controller circuit13. The memory circuit17stores, as table data, data indicating an amount of defocus with respect to the thickness of the spacer section S. A control operation for focus of laser light irradiated from the optical pickup device2to the optical disk1will now be described. When a focus coil drive signal is supplied from the focus servo circuit11to the focusing coil6, the objective lens3is displaced in a direction perpendicular to the plane of the optical disk1and a focus error signal having an S-shape as shown in the graph (A) ofFIG. 3is generated by and output from the optical output signal processor circuit9. A zero-cross point F of the S-shaped focus error signal is the point at which it is determined that the laser light irradiated on the optical detector4is focused from the irradiated spot of the laser light. However, it is not possible to ensure that this point is an optimum focus point with respect to the recording layer of the optical disk1.

Specifically, the zero-cross point F of the S-shaped focus error signal may not be an optimum focus point with respect to the recording layer of the optical disk1because of a deviation in positional relationship between the optical disk1which is placed on a turntable (not shown) and the optical pickup device2or a difference in mounting position of various optical components in the optical pickup device2and the optical detector4. In order to improve this point, a setting operation is performed for shifting the focusing point to the best position for performing the focus servo operation by the focus servo circuit11, that is, a setting operation of an amount of defocus. The setting operation of the amount of defocus is performed by the controller circuit13controlling the focus servo circuit11.

FIG. 2Ais a diagram showing a state in which the focus control operation with respect to the first recording layer L1is performed. The laser light focused by the objective lens3is focused on the first recording layer L1through a cover layer C.FIG. 2Bshows a state in which a focus control operation with respect to the second recording layer L2is performed. The laser light focused by the objective lens3is focused on the second recording layer L2through the cover layer C, the first recording layer L1, and the spacer section S.

The cover layer C has an approximate predetermined thickness and an amount of defocus with respect to the first recording layer L1is set using a test disk which forms a standard during manufacturing of the optical disk recording and reproducing device. It is possible to set the amount of defocus with respect to the first recording layer L1by adjusting the level of the drive signal to be supplied to the focusing coil6so that a level of a replay signal obtained from the optical detector4to which the laser light reflected from the test disk is irradiated is maximized. This is because as the level of the laser light reflected from the test disk is increased, the level of the replay signal is increased and the level of the reflected laser light is maximized when the focus with respect to the first recording layer is in an optimum state. Similarly, it is also possible to set the amount of defocus with respect to the first recording layer L1by adjusting the level of the drive signal to be supplied to the focusing coil6so that a jitter value included in the replay signal obtained from the test disk is minimized. This is because the jitter value is minimized when the focus with respect to the first recording layer L1is in the optimum state.

When the focus characteristics with respect to the recording operation is to be considered in addition to the setting of the amount of defocus by the above-described method, it is possible to record test signals while changing the amount of defocus with respect to the first recording layer L1and to set, as an optimum amount of defocus with respect to the first recording layer L1, an amount of defocus which allows recording of a test signal maximizing the β value which is calculated based on a level of an RF signal obtained by reproducing the recorded signal. Specifically, the β value is calculated as β=(A1+A2)/(A1−A2) when the positive peak level of the RF signal which is the replay signal of the signal recorded on the first recording layer L1is A1and the negative peak level of the RF signal is A2. There is a tendency for the signal to be recorded with a better focus as the β value increases. Therefore, in order to obtain the best focus, it is possible to set the amount of defocus so that the β value is maximized.

A setting operation of the amount of defocus with respect to the second recording layer L2is performed in a similar manner to the setting operation of the amount of defocus with respect to the first recording layer L1. A relationship between the thickness of the spacer section S present between the first recording layer L1and the second recording layer L2and the amount of defocus is detected and an operation is performed to store data which indicates an amount of correction with respect to the thickness in the memory circuit17during manufacture of the optical disk recording and reproducing device.

The amount of defocus is set through the operation as described. Next, an operation to measure a thickness of the spacer section S will be described. A focus drive signal is supplied from the focus servo circuit11to the focusing coil6. The drive signal is supplied to move the objective lens3along a direction toward the surface of the optical disk1after a signal is supplied for temporarily moving the objective lens3in a direction moving away from the surface of the optical disk1.

When this operation is performed, the objective lens3is moved from a position distant from the surface of the optical disk1in a direction toward the optical disk1while the laser light generated by the laser diode continues to be irradiated. Every time the focus point of the laser light passes the first recording layer L1and the second recording layer L2, the focus error signal as shown in graph (A) ofFIG. 3is output from the optical output signal processor circuit9and a pulse signal as shown in graph (B) ofFIG. 3is output from the comparator circuit14.

FIG. 4shows a relationship of pulse signals output from the comparator circuit14when the objective lens3is displaced from a position distant from the optical disk1to a position near the optical disk1in order to measure the thickness of the spacer section S. A pulse signal P1corresponds to the first recording layer L1and a pulse signal P2corresponds to the second recording layer L2.

When the pulse signal P1is output from the comparator circuit14, a control operation of the counter circuit15by the controller circuit13is performed, and, as a result, the counter circuit15starts to count a number of clock signals output from the clock signal generating circuit. When the next pulse signal P2is output from the comparator circuit14while the counting operation is performed, a control operation of the counter circuit15by the controller circuit13is performed. Specifically, the counting operation of the clock signals by the counter circuit15is stopped and the number of clock signals that are counted from the time when the pulse signal P1is output from the comparator circuit14to the time when the pulse signal P2is output is output to the spacer thickness measuring circuit16.

The number of counted clock signals is a number of clocks obtained in a period shown by “T” inFIG. 4and the number is proportional to the thickness of the spacer section S. Therefore, by determining thicknesses of the spacer section S corresponding to numbers of clocks in advance, it is possible to recognize the thickness of the spacer section S as data. When the data indicating the thickness of the spacer section S is output from the spacer thickness measuring circuit16to the controller circuit13, the amount of defocus stored in the memory circuit17corresponding to the data is read and a setting operation is performed with respect to the focus servo circuit11for performing a focus servo operation based on the amount of defocus.

It is possible to accurately perform a focus control operation with respect to the second recording layer L2by performing the setting operation of the amount of defocus with respect to the focus servo circuit11. As a result, the reproducing operation of a signal recorded on the second recording layer L2and the recording operation of signals onto the second recording layer L2can be performed in an optimum state.

In the above-described structure, the thickness of the spacer section S is measured by displacing the objective lens3from a position distant from the surface of the optical disk1in a direction toward the optical disk1. Alternatively, it is also possible to measure the thickness of the spacer section S by displacing the objective lens3in the opposite direction, that is, from a position near the surface of the optical disk1in a direction moving away from the optical disk1. In this case, the pulse signal P2corresponding to the second recording layer L2is first output from the comparator circuit14and then, the pulse signal P1corresponding to the first recording layer L1is output.

When the objective lens3is to be displaced from a position distant from the surface of the optical disk1in a direction toward the optical disk1in order to measure the thickness of the spacer section S, it is possible to determine a starting position of displacement for the objective lens3for starting measurement by displacing the objective lens3to a position where the lens holder contacts the maximum distance position defining member7. When, on the other hand, the objective lens3is to be displaced from a position near the surface of the optical disk1in a direction moving away from the optical disk1in order to measure the thickness of the spacer section S, it is possible to determine the displacement starting position of the objective lens3for measuring the spacer thickness by displacing the objective lens3to a position where the lens holder contacts the minimum distance position defining member8.

The measurement operation between the pulse signal P1and the pulse signal P2is performed in the above-described manner by the counter circuit15to measure the thickness of the spacer section S. A different method of measuring will now be described.

When the objective lens3is moved from a position defined by the maximum distance position defining member7in a direction toward the surface of the optical disk1in order to measure the thickness of the spacer section S, the pulse signal P1and the pulse signal P2are output from the comparator circuit14.

When the objective lens3starts to move from the maximum distance position, the count operation of the clock signal by the counter circuit15is started so as to count a number of clock signals that are counted until the pulse signal P1is output and count a number of clock signals until the pulse signal P2is output.

With the counting of the clock signals, it is possible to measure the number C1of clocks that are counted during a period T1in which the objective lens3is moved from the maximum distance position to the position of the first recording layer L1and the number C2of clocks that are counted during a period T2in which the objective lens3is moved to the position of the second recording layer L2. Therefore, it is possible to calculate the number of clocks that are counted during a period T between the first pulse signal P1and the second pulse signal P2from (C2-C1).

The number of clocks calculated in this manner is proportional to the thickness of the spacer section S. It is therefore possible to recognize the thickness of the spacer section S as data, by determining thicknesses of the spacer section S corresponding to numbers of clocks in advance. When the data indicating the thickness of the spacer section S is output from the spacer thickness measuring circuit16to the controller circuit13, an amount of defocus which is stored in the memory circuit17corresponding to the data is read so that a setting operation can be performed with respect to the focus servo circuit11for performing the focus servo operation based on the amount of defocus.

The thickness of the spacer section S can be measured by moving the objective lens3from a position defined by the maximum distance position defining member7in a direction toward the surface of the optical disk1. Alternatively, it is also possible to measure the thickness of the spacer section S by moving the objective lens3from a position defined by the minimum distance position defining member8in a direction moving away from the surface of the optical disk1.

In the present embodiment, an optical disk having two recording layers is exemplified, but the present embodiment is not limited to a two-layer structure and may be applied to an optical disk recording and reproducing device which uses an optical disk having three or more recording layers.

Second Preferred Embodiment

In a second preferred embodiment of the present invention, an amount of defocus for performing the focus servo operation is set by measuring a thickness of a spacer section present between a first recording layer and a second recording layer.

FIG. 5is a block circuit diagram showing an optical disk recording and reproducing device according to the second preferred embodiment of the present invention.

Referring toFIG. 5, an optical disk101is constructed to be rotationally driven by a spindle motor (not shown) so that the rotation is controlled such that, for example, the linear velocity is constant. As shown inFIG. 2, a first recording layer L1and a second recording layer L2are provided on the optical disk101. An optical pickup device102comprises a laser diode (not shown) which emits laser light, an objective lens3for focusing the laser light irradiated from the laser diode to the recording layer of the optical disk101, an optical detector104, comprising a quadrant sensor or the like, which receives the laser light reflected from the optical disk101and converts the laser light into an electrical signal, a tracking coil105for displacing the objective lens103along a radial direction of the optical disk101, and a focusing coil106for displacing the objective lens103in a direction perpendicular to a signal plane of the optical disk101.

A maximum distance position defining member107contacts a lens holder (not shown) which supports the objective lens103when the objective lens103is furthest away from the surface of the optical disk101due to a supply operation of a drive signal to the focusing coil106. The maximum distance position defining member107is provided to control a maximum distance position of the objective lens103with respect to the optical disk101. A minimum distance position defining member108contacts the lens holder which supports the objective lens103when the objective lens103approaches closest to the surface of the optical disk101due to a supply operation of a drive signal to the focusing coil106. The minimum distance position defining member108is provided to control the minimum distance position of the objective lens103with respect to the optical disk101.

A pickup feeding motor109moves the optical pickup device102along a radial direction of the optical disk101. The pickup feeding motor109is constructed to move the optical pickup device102by rotationally driving a feed shaft110having a feeding channel formed at its periphery.

An electrical signal obtained from the optical detector104is input to an optical output signal processor circuit111as an optical signal. The optical output signal processor circuit111is constructed to generate a tracking error signal which indicates a deviation of laser light from a signal track, a focus error signal which indicates a deviation in focus of the laser light with respect to the recording layer, and a replay signal which is a read signal of the signal recorded on the optical disk101. The operations to generate various signals in the optical output signal processor circuit111are performed by known circuits and will not be described in detail.

A tracking servo circuit112receives the tracking error signal generated by and output from the optical output signal processor circuit111. The tracking servo circuit112is constructed to perform a tracking servo operation by supplying a tracking coil drive signal to the tracking coil105based on the input tracking error signal. A focus servo circuit113receives the focus error signal generated by and output from the optical output signal processor circuit111. The focus servo circuit113is constructed to perform a focus servo operation by supplying a focus coil drive signal to the focusing coil106based on the input focus error signal.

A digital signal processor circuit114receives a replay signal which is binarized by a binarization circuit provided within the optical output signal processor circuit111. The digital signal processor circuit114performs a digital signal processing. The digital signal processor circuit114is constructed to decode various recorded signals such as the synchronization signal and positional information data. A controller circuit115controls various operations of the optical disk recording and reproducing device. The controller circuit115is constructed to control a rotation control operation of the optical disk101by the spindle motor and processing operations of the replay signal and the recording signal using a synchronization signal generated by the digital signal processor circuit114and transmission/reception operations of signals to and from a host device such as a personal computer which is externally provided. The controller circuit115is formed by a microcomputer and is constructed to perform various control operations based on program software stored in a flash ROM or the like which is provided internally.

A comparator circuit116receives the focus error signal output from the optical output signal processor circuit111. The comparator circuit116is constructed to output, to the controller circuit115, a signal of H (high) level for a period when the level of the focus error signal exceeds a threshold value VR. A graph (A) inFIG. 3shows a relationship between the focus error signal and the threshold value VR which is obtained when the objective lens103is moved to approach from a position distant from the recording layer provided on the optical disk101. As is known in the art, the focus error signal changes in an S-shape. When the optical disk101comprises two layers, two S-shaped changes occur. A graph (B) inFIG. 3shows a pulse signal output from the comparator circuit116, and a signal of H level is output for a period in which the level of the focus error signal exceeds the threshold value VR.

An operation of a counter circuit117is controlled by the controller circuit115. The counter circuit117is constructed to start a counting operation when a pulse signal is output from the comparator circuit116and to measure a period between a first pulse signal and a next pulse signal when the next pulse signal is output. For example, it is possible to provide a clock signal generating circuit which generates a clock signal of a predetermined frequency and measure the period using a number of clock signals which are counted between the pulse signals.

A spacer thickness measuring circuit118determines a thickness of a spacer section S provided between the first recording layer L1and the second recording layer L2based on a number of clock signals counted by the counter circuit117. The spacer thickness measuring circuit118can determine the spacer thickness from a relationship between the number of clock signals and the thickness which is determined in advance. Data indicating the determined spacer thickness is output to a spacer thickness memory circuit119in which the recording operation and reading operation of a signal is controlled by the controller circuit115. The spacer thickness memory circuit119is constructed to store data indicating the spacer thickness output from the spacer thickness measuring circuit118.

When the spacer thickness is measured by the spacer thickness measuring circuit118and the data is stored in the spacer thickness memory circuit119, the pickup feeding motor109is rotated to move the position of the optical pickup device102in, for example, a direction toward an outer periphery on the optical disk101to measure the thickness of the spacer section S. In this manner, data of the measured spacer thickness is input to the spacer thickness memory circuit119and stored in the spacer thickness memory circuit119.

An operation of a spacer thickness calculating circuit120is controlled by the controller circuit115. The spacer thickness calculating circuit120is constructed to calculate an average value of a plurality of spacer thickness data stored in the spacer thickness memory circuit119and output the value determined in the calculation to the controller circuit115as data indicating the thickness of the spacer section S present between the first recording layer L1and the second recording layer L2of the optical disk101.

A pickup feeding motor driving circuit121rotationally drives the pickup feeding motor109. The pickup feeding motor driving circuit121is constructed to perform an operation to very slightly move the body of the optical pickup device102by rotating the pickup feeding motor109in a predetermined manner when a level of a DC current of the tracking drive signal output from the tracking servo circuit112reaches a predetermined value and a drive operation to significantly move the body of the optical pickup device102such as a search operation.

An operation of a defocus data memory circuit122is controlled by the controller circuit115. The defocus data memory circuit122stores, as table data, data indicating an amount of defocus with respect to the thickness of the spacer section S. A control operation of focus of laser light irradiated from the optical pickup device102to the optical disk101will now be described. When a focus coil drive signal is supplied from the focus servo circuit113to the focusing coil106, the objective lens103is displaced in a direction perpendicular to the plane of the optical disk101and a focus error signal having an S-shape as shown in the graph (A) ofFIG. 3is generated by and output from the optical output signal processor circuit111. A zero-cross point F of the S-shaped focus error signal is the point where it is determined that the laser light irradiated on the optical detector104is focused from the irradiated spot of the laser light. However, it is not possible to ensure that this point is an optimum focus point with respect to the recording layer of the optical disk101.

Specifically, the zero-cross point F of the S-shaped focus error signal may not be an optimum focus point with respect to the recording layer of the optical disk101because of a deviation in positional relationship between the optical disk101which is placed on a turntable (not shown) and the optical pickup device102, or a difference in mounting position of various optical components in the optical pickup device102and the optical detector104. In order to improve this point, a setting operation for shifting the focusing point to the best position for performing the focus servo operation by the focus servo circuit113, that is, a setting operation of an amount of defocus, is performed. The setting operation of the amount of defocus is performed by the controller circuit115controlling the focus servo circuit113.

FIG. 2Ais a diagram showing a state in which the focus control operation with respect to the first recording layer L1is performed. The laser light focused by the objective lens103is focused on the first recording layer L1through a cover layer C.FIG. 2Bshows a state in which a focus control operation with respect to the second recording layer L2is performed. The laser light focused by the objective lens103is focused on the second recording layer L2through the cover layer C, the first recording layer L1, and the spacer section S.

The cover layer C has an approximate predetermined thickness and an amount of defocus with respect to the first recording layer L1is set using a test disk which forms a standard during manufacturing of the optical disk recording and reproducing device. It is possible to set the amount of defocus with respect to the first recording layer L1by adjusting the level of the drive signal to be supplied to the focusing coil106so that a level of a replay signal obtained from the optical detector104to which the laser light reflected from the test disk is irradiated is maximized. This is because as the level of the laser light reflected from the test disk is increased, the level of the replay signal is increased and the level of the reflected laser light is maximized when the focus with respect to the first recording layer is in an optimum state. Similarly, it is also possible to set the amount of defocus with respect to the first recording layer L1by adjusting the level of the drive signal to be supplied to the focusing coil106so that a jitter value included in the replay signal obtained from the test disk is minimized. This is because the jitter value is minimized when the focus with respect to the first recording layer L1is in the optimum state.

When the focus characteristics with respect to the recording operation are to be considered in addition to the setting of the amount of defocus by the above-described method, it is possible to record test signals while changing the amount of defocus with respect to the first recording layer L1and to set, as an optimum amount of defocus with respect to the first recording layer L1, an amount of defocus which allows recording of a test signal maximizing the β value which is calculated based on a level of an RF signal obtained by reproducing the recorded signal. Specifically, the β value is calculated as β=(A1+A2)/(A1−A2) when the positive peak level of the RF signal which is the replay signal of the signal recorded on the first recording layer L1is A1and the negative peak level of the RF signal is A2. There is a tendency for the signal to be recorded with a better focus as the β value increases. Therefore, in order to obtain the best focus, it is possible to set the amount of defocus so that the β value is maximized.

A setting operation of the amount of defocus with respect to the second recording layer L2is performed in a similar manner to the setting operation of the amount of defocus with respect to the first recording layer L1. A relationship between the thickness of the spacer section S present between the first recording layer L1and the second recording layer L2and the amount of defocus is detected and an operation to store, in the defocus data memory circuit122, data which indicates an amount of correction with respect to the thickness is performed during manufacture of the optical disk recording and reproducing device.

The setting operation of the amount of defocus is performed in a manner described above. Next, an operation for measuring the thickness of the spacer section S will be described. A first operation for measuring the thickness of the spacer section S is to rotationally drive the pickup feeding motor109to move the optical pickup device102into a read-in region provided at a radially inner side of the optical disk101. An operation to detect a position of the read-in region can be performed by reading positional information data recorded on the optical disk101. Here, it is also desirable to set the position for measuring the thickness of the spacer section based on a number of rotations of the pickup feeding motor109for performing the feeding operation of the optical pickup device along the radial direction.

After the optical pickup device102is moved to the read-in region, a focus driving signal for measuring the thickness of the spacer section S is supplied from the focus servo circuit113to the focusing coil106. The driving signal is supplied to move the objective lens103along a direction toward the surface of the optical disk101after a signal is supplied for temporarily moving the objective lens103in a direction moving away from the surface of the optical disk101.

When this operation is performed, the objective lens103is moved from a position distant from the surface of the optical disk101in a direction toward the optical disk101while the laser light generated by the laser diode continues to be irradiated. Every time the focus point of the laser light passes the first recording layer L1and the second recording layer L2, the focus error signal as shown in graph (A) ofFIG. 3is output from the optical output signal processor circuit111and a pulse signal as shown in graph (B) ofFIG. 3is output from the comparator circuit116. In this manner, the spacer thickness is measured by moving the objective lens103in a direction perpendicular to the plane of the optical disk101and measuring a period between pulse signals obtained by binarizing the focus error signal using a predetermined threshold value.

FIG. 4shows a relationship of pulse signals output from the comparator circuit116when the objective lens103is displaced from a position distant from the optical disk101to a position near the optical disk101in order to measure the thickness of the spacer section S. A pulse signal P1corresponds to the first recording layer L1and a pulse signal P2corresponds to the second recording layer L2.

When the pulse signal P1is output from the comparator circuit116, a control operation of the counter circuit117by the controller circuit115is performed, and as a result, the counter circuit117starts to count a number of clock signals output from the clock signal generating circuit. When the next pulse signal P2is output from the comparator circuit116while the counting operation is performed, a control operation of the counter circuit117by the controller circuit115is performed. Specifically, the counting operation of the clock signals by the counter circuit117is stopped and the number of clock signals that are counted from the time when the pulse signal P1is output from the comparator circuit116to the time when the pulse signal P2is output is output to the spacer thickness measuring circuit118.

The counted number of clocks corresponds to a number of clocks obtained in a period shown by “T” inFIG. 4and is proportional to the thickness of the spacer section S. It is therefore possible to recognize the thickness of the spacer section S as data by determining thicknesses of the spacer section S corresponding to numbers of clocks in advance. Data indicating the thickness of the spacer section S is output from a spacer thickness measuring circuit118to the spacer thickness memory circuit119.

When the data indicating the thickness of the spacer section S is output from the spacer thickness measuring circuit118and is input to the spacer thickness memory circuit119, the spacer thickness memory circuit119performs an operation to store, as first data, the position where the thickness is measured and the data indicating the thickness.

When the first data is stored in the spacer thickness memory circuit119, the optical pickup device102is moved by a rotational driving operation of the pickup feeding motor109to a read-out region provided at a radially outer position of the optical disk101. A position detecting operation of the read-out region can be performed by reading positional information data recorded on the optical disk101.

When the optical pickup device102is moved to the read-out region, a focus drive signal for measuring the thickness of the spacer section S is supplied from the focus servo circuit111to the focusing coil106. In this case also, the drive signal is supplied to move the objective lens103along a direction toward the surface of the optical disk101after a signal is supplied for temporarily moving the objective lens103in a direction departing from the surface of the optical disk101.

When this operation is performed, the objective lens103is moved from a position distant from the surface of the optical disk101in a direction toward the optical disk101while the laser light generated by the laser diode continues to be irradiated. Similar to the above-described operations, every time the focus point of the laser light passes the first recording layer L1and the second recording layer L2, the focus error signal as shown in graph (A) ofFIG. 3is output from the optical output signal processor circuit111and a pulse signal as shown in graph (B) ofFIG. 3is output from the comparator circuit116.

As shown inFIG. 4, a pulse signal P1corresponding to the first recording layer L1and a pulse signal P2corresponding to the second recording layer L2are output from the comparator circuit116. When the pulse signal P1and the pulse signal P2are output from the comparator circuit116, the counter circuit117performs a counting operation to count a number of clock signals between the pulse signal P1and the pulse signal P2, that is, the period shown by “T” inFIG. 4.

The number of clock signals thus counted constitutes data indicating the thickness of the spacer section S and the data indicating the thickness is output from the spacer thickness measuring circuit118to the spacer thickness memory circuit119. When the data indicating the thickness of the spacer section S is output from the spacer thickness measuring circuit118and is input to the spacer thickness memory circuit119, the spacer thickness memory circuit119stores, as second data, the data indicating the position where the thickness is measured, and the thickness.

When the first data, which is the thickness data of the read-in region, and the second data, which is the thickness data of the read-out region, are stored in the spacer thickness memory circuit119, the spacer thickness calculating circuit120calculates an average value. When the spacer thickness calculating circuit120performs the calculation process operation, the average value obtained from the first data and the second data is output to the controller circuit115as the data indicating the thickness of the spacer section S.

When the data indicating the thickness of the spacer section S is input to the controller circuit115, an amount of defocus stored in the defocus data memory circuit122is read corresponding to the input data, and a setting operation is performed with respect to the focus servo circuit113for a focus servo operation based on the amount of defocus.

It is possible to accurately perform a focus control operation with respect to the second recording layer L2by performing the setting operation of the amount of defocus with respect to the focus servo circuit113. As a result, the reproducing operation of a signal recorded on the second recording layer L2and the recording operation of signals onto the second recording layer L2can be performed in an optimum state.

In the above-described structure, the thickness of the spacer section S in the read-in region or in the read-out region is measured by displacing the objective lens103from a position distant from the surface of the optical disk101in a direction toward the optical disk101. Alternatively, it is also possible to measure the thickness of the spacer section S by displacing the objective lens103in the opposite direction, that is, from a position near the surface of the optical disk101in a direction moving away from the optical disk101. In this case, the pulse signal P2corresponding to the second recording layer L2is first output from the comparator circuit116, and then the pulse signal P1corresponding to the first recording layer L1is output.

When the objective lens103is to be displaced from a position distant from the surface of the optical disk101in a direction toward the optical disk101in order to measure the thickness of the spacer section S, it is possible to determine a starting position of displacement for the objective lens103for starting measurement by displacing the objective lens103to a position where the lens holder contacts the maximum distance position defining member7. When, on the other hand, the objective lens103is to be displaced from a position near the surface of the optical disk101in a direction moving away from the optical disk101in order to measure the thickness of the spacer section S, it is possible to determine the displacement starting position of the objective lens103for measuring the spacer thickness by displacing the objective lens103to a position where the lens holder contacts the minimum distance position defining member108.

The measurement operation between the pulse signal P1and the pulse signal P2is performed in the above-described manner by the counter circuit117to measure the thickness of the spacer section S. A different method of measuring will now be described.

When the objective lens103is moved from a position defined by the maximum distance position defining member107in a direction toward the surface of the optical disk101in order to measure the thickness of the spacer section S, the pulse signal P1and the pulse signal P2are output from the comparator circuit116.

When the objective lens103starts to move from the maximum distance position, the count operation of the clock signal by the counter circuit117is started to count a number of clock signals until the pulse signal P1is output and a number of clock signals until the pulse signal P2is output.

With the counting of the clock signals, it is possible to measure the number C1of clocks that are counted during a period T1in which the objective lens103is moved from the maximum distance position to the position of the first recording layer L1and the number C2of clocks that are counted during a period T2in which the objective lens103is moved to the position of the second recording layer L2. It is therefore possible to calculate the number of clocks that are counted during a period T between the first pulse signal P1and the second pulse signal P2from (C2-C1).

The number of clocks calculated in this manner is proportional to the thickness of the spacer section S. It is therefore possible to recognize the thickness of the spacer section S as data, by setting thicknesses of the spacer section S corresponding to numbers of clocks in advance. When the data indicating the thickness of the spacer section S is output from the spacer thickness measuring circuit118to the spacer thickness memory circuit119, the first data and the second data are stored in the spacer thickness memory circuit119.

When the first data and the second data are stored in the spacer thickness memory circuit119, the spacer thickness calculating circuit120performs a calculation and the calculated average value is output to the controller circuit115as the thickness data of the spacer section S. As a result, the controller circuit115reads the amount of defocus stored in the defocus data memory circuit122corresponding to the input data and can perform a setting operation of the focus servo circuit113in order to perform a focus servo operation based on the amount of defocus.

The thickness of the spacer section S can be measured by moving the objective lens103from a position defined by the maximum distance position defining member107in a direction toward the surface of the optical disk110. Alternatively, it is also possible to measure the thickness of the spacer section S by moving the objective lens103from a position defined by the minimum distance position defining member108in a direction departing from the surface of the optical disk101.

In the present embodiment, the thickness of the spacer section S is measured in the read-in region and in the read-out region. The positions for measuring the thickness are not limited to this configuration and may be arbitrarily set as long as the positions are at a radially inner position and at a radially outer position. In addition, although measurement of the thickness of the spacer section S at two positions, one at a radially inner position and another at a radially outer position, is described in the embodiment, it is also possible to measure the thickness of the spacer section S at three or more positions. In this case, it is possible to perform various operations as the calculation process using the spacer thickness calculating circuit120such as, for example, a process of summing all data and calculating the average value and a process of determining an average using a maximum value and a minimum value.

In the present embodiment, the displacement operation of the optical pickup device102to a position for measuring the thickness of the spacer section S is performed using the positional information data obtained from the optical disk101. Alternatively, it is also possible to determine a displacement distance of the optical pickup device102from, for example, an output time of the drive signal output from the pickup feeding motor driving circuit121and a number of rotation pulse signals obtained from the pickup feeding motor109and to measure the thickness of the spacer section S based on the determined displacement distance.

In the present embodiment, an optical disk having two recording layers is exemplified, but the present embodiment is not limited to a two-layer structure and may be applied to an optical disk recording and reproducing device which uses an optical disk having three or more recording layers.

As described, in the present embodiment, a thickness of the spacer section between the first recording layer and the second recording layer is measured at two or more locations, the spacer thickness is determined from an average value of the plurality of measured spacer thicknesses, the amount of defocus with respect to the second recording layer is set based on the determined spacer thickness, and the focus servo operation is performed for focusing the laser light irradiated from the optical pickup device on the second recording layer. Because of this structure, it is possible to accurately perform a focus control operation.

In addition, in the present embodiment, the objective lens is moved in a direction perpendicular to the plane of the optical disk, the focus error signal generated from the optical detector is binarized using a predetermined threshold value to obtain pulse signals, and a period between the pulse signals is measured to determine the spacer thickness. Because of this structure, it is possible to easily measure the thickness of the spacer section.