Optical pickup, optical disk drive, light detecting apparatus, and signal generating method for optical pickup

An optical pickup includes a light source, an objective lens, light detectors, an optical system, and an error-signal generator. The light-receiving surface of at least one of the light detectors has a width extending in a first direction corresponding to a radial direction of an optical disk, and is formed by first to sixth photoreceptors arrayed along the first direction. The first and second photoreceptors, the third and fourth photoreceptors, and the fifth and sixth photoreceptors are disposed axisymmetrically with respect to a center line extending perpendicularly to the first direction. The error-signal generator generates a focus-error signal using a sum signal of detection signals output from the first and second photoreceptors or a sum signal of detection signals output from the first to fourth photoreceptors in a light spot size method, and generates a tracking-error signal using the other sum signal in a differential compensate push-pull method.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP 2004-305243 filed in the Japanese Patent Office on Oct. 20, 2004, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to optical pickups, optical disk drives, light detecting apparatuses, and signal generating methods for optical pickups.

In an optical pickup configured to record optical signals on an optical disk or to play back optical signals from an optical disk, light beams emitted from a light source are transmitted through an objective lens to irradiate the recording surface of the optical disk, the resulting reflected light beams are transmitted again through the objective lens and detected by light detectors to obtain detection signals, and a playback signal, a focus-error signal, and a tracking-error signal are generated on the basis of the detection signals.

For example, Japanese Unexamined Patent Application Publication No. 2003-248957 proposes an optical pickup having two light detectors each including four photoreceptors (photoreceptor segments).

In the optical pickup, reflected light beams form a single light spot on the light-receiving surface of each of the two light detectors. The focus-error signal is detected by a light spot size method, in which the difference between the sizes of the light spots formed on the respective light-receiving surfaces of the light detectors is detected. The tracking-error signal is detected by a differential compensate push-pull (DCPP) method, in which variation in the distribution of intensity due to interference between zeroth order light and positive and negative first-order lights constituting the reflected beams is detected and values obtained by the detection are corrected on the basis of the amount of shift in position between the objective lens and the two light detectors.

That is, both the focus-error-signal and the tracking-error signal are detected using the two light detectors.

SUMMARY OF THE INVENTION

In the optical pickup constructed as described above, in order to achieve optimal characteristics regarding both focus servo and tracking servo, i.e., in order to optimize characteristics of both the focus-error signal and the tracking-error signal, the sizes of the photoreceptors constituting the two light detectors should be chosen optimally.

However, in the optical pickup described above, since detection signals output from the photoreceptors of the two light detectors are used both for detection of the focus-error signal and detection of the tracking-error signal, when the sizes of the photoreceptors are chosen so that one of the focus-error signal and the tracking-error signal can be detected optimally, it becomes difficult to optimally detect the other one of the focus-error signal and the tracking-error signal.

Thus, it has been the case to inevitably select either choosing the sizes of the photoreceptors so as to optimize one of the focus-error signal and the tracking-error signal at the compromise of optimization of the other, or choosing the sizes of the photoreceptors so as to achieve fair characteristics for both the focus-error signal and the tracking-error signal.

It is desired that an optical pickup, an optical disk drive, a light detecting apparatus, and a signal generating method for an optical pickup that are favorable for achieving optimal characteristics for both a focus-error signal and a tracking-error signal be provided.

According to an embodiment of the present invention, there is provided an optical pickup including a light source configured to emit light beams; an objective lens configured to condense the light beams emitted from the light source so that an optical disk is irradiated with the condensed light beams; a first light detector and a second light detector having respective light-receiving surfaces; an optical system configured so that reflected light beams caused by reflection of the irradiating light beams by the optical disk are transmitted through the objective lens to form a single light spot on each of the light-receiving surfaces of the first and second light detectors; and error-signal generating means for generating a focus-error signal and a tracking-error signal on the basis of detection signals output from the first and second light detectors when the light spots are formed on the respective light-receiving surfaces. The light-receiving surface of at least one of the first and second light detectors has a width extending in a first direction corresponding to a radial direction of the optical disk. The light-receiving surface of the at least one light detector is formed by first to sixth photoreceptors arrayed along the first direction. The first and second photoreceptors are disposed axisymmetrically with respect to a center line passing through the center in the width direction and extending in a direction perpendicular to the first direction. The third and fourth photoreceptors are respectively disposed continuously outward from the first and second photoreceptors so as to be axisymmetric with respect to the center line. The fifth and sixth photoreceptors are respectively disposed continuously outward from the third and fourth photoreceptors so as to be axisymmetric with respect to the center line. The error-signal generating means is configured to generate the focus-error signal by using one of a sum signal of detection signals output from the first and second photoreceptors and a sum signal of detection signals output from the first to fourth photoreceptors as a detection signal in a light spot size method, and to generate the tracking-error signal by using the other sum signal as a correction signal in a differential compensate push-pull method.

According to another embodiment of the present invention, there is provided an optical disk drive including driving means for holding and rotating an optical disk; and an optical pickup configured to irradiate the optical disk rotated by the driving means with light beams for recording or playback, and to detect reflected light beams caused by reflection of the irradiating light beams by the optical disk. The optical pickup includes a light source configured to emit the light beams; an objective lens configured to condense the light beams emitted from the light source so that the optical disk is irradiated with the condensed light beams; a first light detector and a second light detector having respective light-receiving surfaces; an optical system configured so that reflected light beams caused by reflection of the irradiating light beams by the optical disk are transmitted through the objective lens to form a single light spot on each of the light-receiving surfaces of the first and second light detectors; and error-signal generating means for generating a focus-error signal and a tracking-error signal on the basis of detection signals output from the first and second light detectors when the light spots are formed on the respective light-receiving surfaces. The light-receiving surface of at least one of the first and second light detectors has a width extending in a first direction corresponding to a radial direction of the optical disk. The light-receiving surface of the at least one light detector is formed by first to sixth photoreceptors arrayed along the first direction. The first and second photoreceptors are disposed axisymmetrically with respect to a center line passing through the center in the width direction and extending in a direction perpendicular to the first direction. The third and fourth photoreceptors are respectively disposed continuously outward from the first and second photoreceptors so as to be axisymmetric with respect to the center line. The fifth and sixth photoreceptors are respectively disposed continuously outward from the third and fourth photoreceptors so as to be axisymmetric with respect to the center line. The error-signal generating means is configured to generate a focus-error signal by using one of a sum signal of detection signals output from the first and second photoreceptors and a sum signal of detection signals output from the first to fourth photoreceptors as a detection signal in a light spot size method, and to generate a tracking-error signal by using the other sum signal as a correction signal in a differential compensate push-pull method.

According to another embodiment of the present invention, there is provided a light detecting apparatus including a light source configured to emit light beams toward an optical disk; a first light detector and a second light detector having respective light-receiving surfaces; and a prism configured so that the light beams emitted from the light source are transmitted through an objective lens to irradiate the optical disk, and so that reflected light beams caused by reflection of the irradiating light beams by the optical disk are transmitted through the objective lens to form a single light spot on each of the light-receiving surfaces of the first and second light detectors. The light source, the first and second light detectors, and the prism are provided on the same substrate. The light-receiving surface of at least one of the first and second light detectors has a width extending in a first direction corresponding to a radial direction of the optical disk. The light-receiving surface of the at least one light detector is formed by first to sixth photoreceptors arrayed along the first direction. The first and second photoreceptors are disposed axisymmetrically with respect to a center line passing through the center in the width direction and extending in a direction perpendicular to the first direction. The third and fourth photoreceptors are respectively disposed continuously outward from the first and second photoreceptors so as to be axisymmetric with respect to the center line. The fifth and sixth photoreceptors are respectively disposed continuously outward from the third and fourth photoreceptors so as to be axisymmetric with respect to the center line.

According to another embodiment of the present invention, there is provided a signal generating method for an optical pickup, including the steps of condensing light beams emitted from a light source to irradiate an optical disk; irradiating respective light-receiving surfaces of first and second light detectors with reflected light beams caused by reflection of the irradiating light beams by the optical disk, thereby forming a single light spot on each of the light-receiving surfaces; and generating a focus-error signal and a tracking-error signal on the basis of detection signals output from the first and second light detectors. The light-receiving surface of at least one of the first and second light detectors has a width extending in a first direction corresponding to a radial direction of the optical disk. The light-receiving surface of the at least one light detector is formed by first to sixth photoreceptors arrayed along the first direction. The first and second photoreceptors are disposed axisymmetrically with respect to a center line passing through the center in the width direction and extending in a direction perpendicular to the first direction. The third and fourth photoreceptors are respectively disposed continuously outward from the first and second photoreceptors so as to be axisymmetric with respect to the center line. The fifth and sixth photoreceptors are respectively disposed continuously outward from the third and fourth photoreceptors so as to be axisymmetric with respect to the center line. The focus-error signal is generated by using one of a sum signal of detection signals output from the first and second photoreceptors and a sum signal of detection signals output from the first to fourth photoreceptors as a detection signal in a light spot size method, and the tracking-error signal is generated by using the other sum signal as a correction signal in a differential compensate push-pull method.

In the optical pickup, the optical disk drive, the light detecting apparatus, and the signal generating method for an optical pickup according to these embodiments, the light-receiving surface of at least one of the first and second light detectors is divided into the first to sixth photoreceptors. Thus, the widths of the first and second photoreceptors and the widths of the third and fourth photoreceptors can be chosen independently. Accordingly, an optimal focus-error signal FE and an optimal tracking-error signal TE can be obtained simultaneously. This is advantageous in achieving optimal characteristics regarding both the focus-error signal and the tracking-error signal.

DETAILED DESCRIPTION

To achieve optimal characteristics regarding both a focus-error signal and a tracking-error signal, the light-receiving surface of at least one of first and second light detectors is formed by first to sixth photoreceptors, and the focus-error signal is generated by using one of a sum signal of detection signals output from the first and second photoreceptors and a sum signal of detection signals output from the first to fourth photoreceptors as a detection signal used in a light spot size method, and a tracking-error signal is generated by using the other sum signal as a correction signal in a differential compensate push-pull method.

First Embodiment

FIG. 1is a block diagram showing the construction of an optical-disk drive including an optical pickup according to a first embodiment of the present invention.FIG. 2is a diagram showing the construction of an optical system of the optical pickup according to the first embodiment.FIG. 3Ais a plan view of a first light detector, andFIG. 3Bis a plan view of a second light detector. The optical disk drive shown inFIG. 1is an example of a recording and playback apparatus including the optical pickup described below.

Referring toFIG. 1, an optical disk drive101includes a spindle motor103configured to drive and rotate an optical disk102, which is an optical recording medium, such as a CD-R, a DVD±R, or a DVD-RAM, an optical pickup104, and a feed motor105for driving the optical pickup104. The spindle motor103is configured to rotate at a predetermined rotation rate under the control of a system controller107and a servo controller109.

A signal modem and ECC block108modulates or demodulates signals output from a signal processor120, and attaches error correcting codes (ECCs). The optical pickup104irradiates with light beams a signal-recording surface of the optical disk102rotating under the control of the system controller107and the servo controller109, whereby optical signals are recorded on or played back from the optical disk102.

The optical pickup104is configured to detect various types of light beams, which will be described later, on the basis of light beams reflected from the signal-recording surface of the optical disk102, and to supply signals corresponding to the light beams to the signal processor120.

The signal processor120is configured to generate servo control signals on the basis of detection signals corresponding to the light beams. The servo control signals include a focus-error signal, a tracking-error signal, an RF signal, a monitor signal used for running OPC (optimum power control) (hereinafter referred to as an R-OPC signal), and an ATIP (absolute time in pre-groove) signal used for controlling optical-disk rotation during recording. In this embodiment, the signal processor120includes a focus-error-signal generating circuit120A (FIG. 4) and a tracking-error-signal generating circuit120B (FIG. 5), which will be described later.

Furthermore, depending on the type of recording medium from which data is played back, the servo controller109, the signal modem and ECC block108, and other components execute specific processing on the basis of the above signals, such as demodulation and error correction.

For example, when signals obtained by demodulation of recorded signals by the signal modem and ECC block108are intended for data storage on a computer, the demodulated signals are output to an external computer130or the like via an interface111. Thus, the external computer130or the like can receive the signals recorded on the optical disk102as playback signals.

When signals obtained by demodulation of recorded signals by the signal modem and ECC block108are intended for an audio/visual application, the demodulated signals are converted from digital to analog by a D/A converter in a D/A-and-A/D converter112, and the resulting analog signals are fed to an audio/visual processor113. Then, the audio/visual processor113executes audio/video signal processing, and the resulting processed signals are transmitted to an external imaging or projecting device via an audio/visual signal input/output unit114.

The optical pickup104is connected to the feed motor105for moving the optical pickup104, for example, to a specific recording track of the optical disk102. The spindle motor103, the feed motor105, and the focusing direction and tracking direction of an actuator holding an objective lens of the optical pickup104are controlled by the servo controller109.

More specifically, the servo controller109controls the spindle motor103on the basis of the ATIP signal, and controls the actuator on the basis of the focus-error signal and the tracking-error signal.

Furthermore, a laser controller121controls a laser-beam source in the optical pickup104. In this embodiment, the laser controller121controls the power of laser beams emitted from the laser-beam source during recording and playback.

Next, the construction of the optical pickup104will be described.

Referring toFIG. 2, the optical pickup104includes a laser-beam source1, a collimating lens2, a polarizing beam splitter3, a quarter-wavelength plate4, an objective lens5, a condensing lens6, a prism7, a first light detector8, and a second light detector9. These components are mounted on a holder (not shown).

In front of the laser-beam source1, the collimating lens2, the polarizing beam splitter3, the quarter-wavelength plate4, and the objective lens5are arranged linearly in that order. The optical disk102is positioned in front of the objective lens5.

The polarizing beam splitter3has a first surface3A facing the laser-beam source1, a second surface3B opposite to the first surface3A and facing the objective lens5, a third surface3C perpendicular to the first surface3A and the second surface3B, a fourth surface3D opposite to the third surface3C, and a polarizing beam splitter surface32substantially making an angle of 45 degrees with the first surface3A and the second surface3B.

In front of the fourth surface3D of the polarizing beam splitter3, the condensing lens6, the prism7, and the first light detector8are arranged linearly in that order.

The prism7has a first surface7A facing the condensing lens6, a second surface7B opposite to the first surface7A and facing the first light detector8, a third surface7C perpendicular to the first surface7A and the second surface7B, a fourth surface7D opposite to the third surface7C, and a half-mirror surface72substantially making an angle of 45 degrees with the first surface7A and the second surface7B.

The second light detector9is disposed so as to face the fourth surface7D of the prism7.

Let the focal length of the condensing lens6(the distance from the condensing lens6to the condensing point thereof) be denoted by L0, the length of the optical path from the condensing lens6to the light-receiving surface82of the first light detector8by L1, and the length of the optical path from the condensing lens6to the light-receiving surface92of the second light detector9by L2. Then, in this embodiment, in order that the size of a beam spot formed on the light-receiving surface82with respect to the radial direction is substantially the same as the size of a beam spot formed on the light-receiving surface92with respect to the same direction, the condensing lens6and the first and second light detectors8and9are configured so as to satisfy the following relationships:
L1=L0−ΔL(1)
L2=L0+ΔL(2)
L1<L0<L2  (3)
where ΔL is a predetermined length.

In the optical pickup104, light beams emitted from the laser-beam source1are made incident on the polarizing beam splitter3via the collimating lens2.

Part of the light beams made incident on the first surface3A of the polarizing beam splitter3is transmitted through the polarizing-beam-splitter surface32and the second surface3B so that the optical disk102is irradiated therewith via the quarter-wavelength plate4and the objective lens5, and the other part of the light beams made incident on the polarizing beam splitter3is reflected by the polarizing-beam-splitter surface32.

The light beams that reach the optical disk102are reflected by the recording surface of the optical disk102. The resulting reflected beams are made incident on the second surface3B of the polarizing beam splitter3via the quarter-wavelength plate4, reflected by the polarizing-beam-splitter surface32, and the reflected beams are transmitted from the fourth surface3D through the condensing lens6to reach the first surface7A of the prism7.

Part of the reflected beams made incident on the first surface7A of the prism7are transmitted through the half-mirror surface72and the second surface7B, and reach the light-receiving surface82of the first light detector8, whereby a single light spot is formed on the light-receiving surface82.

The other part of the reflected beams made incident on the prism7is reflected by the half-mirror surface72. The reflected beams are transmitted through the fourth surface7D and reach the light-receiving surface92of the second light detector9, whereby a single light spot is formed on the light-receiving surface92.

In this embodiment, an optical system is formed by the polarizing beam splitter3, the quarter-wavelength plate4, the condensing lens6, and the prism7.

Next, the first and second light detectors8and9will be described.

As shown inFIG. 3A, the light-receiving surface82of the first light detector8has a rectangular shape having a length and a width, the width direction coinciding with a first direction X corresponding to the radial direction of the optical disk102, and the length direction coinciding with a direction perpendicular to the first direction X. That is, the light beams irradiating the optical disk102are reflected, and the reflected beams are made incident on the light-receiving surface82of the first light detector8to form a light spot10. Of the light beams irradiating the optical disk102, a region of the light spot10corresponding to a region of the optical disk102extending along the radial direction thereof extends in the first direction X.

The light-receiving surface82of the first light detector8is formed by first to sixth rectangular photoreceptors8402,8404,8406,8408,8410, and8412arrayed along the width direction (i.e., the first direction X).

The first to sixth rectangular photoreceptors8402,8404,8406,8408,8410, and8412are disposed axisymmetrically with respect to a center line86passing through the center with respect to the width direction and extending in the length direction (i.e., the direction perpendicular to the first direction X). That is, the boundary between the first and second photoreceptors8402and8404coincides with the center line86. The first and second photoreceptors8402and8404have the same rectangular shape and size with the same length and width.

The third and fourth photoreceptors8406and8408are respectively disposed continuously outward from the first and second photoreceptors8402and8404so as to be axisymmetric with respect to the center line86. The third and fourth photoreceptors8406and8408have the same rectangular shape and size with the same length and width.

The fifth and sixth photoreceptors8410and8412are respectively disposed continuously outward from the third and fourth photoreceptors8406and8408so as to be axisymmetric with respect to the center line86. The fifth and sixth photoreceptors8410and8412have the same rectangular shape and size with the same length and width.

As shown inFIG. 3B, the light-receiving surface92of the second light detector9is formed by first to sixth rectangular photoreceptors9402,9404,9406,9408,9410, and9412arrayed along the width direction (the first direction X), similarly to the first light detector8described above.

FIG. 4is a block diagram showing the circuit configuration of the focus-error-signal generating circuit120A that generates a focus-error signal from detection signals output from the first and second light detectors8and9in the first embodiment.FIG. 5is a block diagram showing the circuit configuration of a tracking-error-signal generating circuit120B that generates a tracking-error signal from detection signals output from the first and second light detectors8and9in the first embodiment.

In this embodiment, the focus-error signal is generated by a light spot size method, and the tracking-error signal is generated by a differential compensate push-pull (DCPP) method.

Referring toFIGS. 4 and 5, let detection signals output from the first to sixth photoreceptors8402to8412of the first light detector8be denoted by B2, C1, B1, C2, A, and D, respectively, and detection signals output from the first to sixth photoreceptors9402to9412of the second light detector9by F2, C1, F1, G2, E, and H, respectively.

As shown inFIG. 4, a focus-error signal FE is generated using, for example, eight adders14and three subtractors16, according to equation (4) below:

As is apparent from equation (4), in this embodiment, of the light-receiving surface82of the first light detector8, a central region of the light spot10is detected by the first and second photoreceptors8402and8406near the center line86, and side regions of the light spot10with respect to the first direction X are detected by the third to sixth photoreceptors8406,8408,8410, and8412. Similarly, of the light-receiving surface92of the second light detector9, a central region of the light spot12is detected by the first and second photoreceptors9402and9404near the center line96, and side regions of the light spot12with respect to the first direction X are detected by the third to sixth photoreceptors9406,9408,9410, and9412.

Thus, in this embodiment, a sum signal (B1+C2) of the detection signals B1and C2output from the first and second photoreceptors8402and8404of the first light detector8and a sum signal (F2+G1) of the detection signals F2and G1output from the first and second photoreceptors9402and9404of the second light detector9are used as detection signals in the light spot size method.

As shown inFIG. 5, a tracking-error signal TE is generated using, for example, four adders14, three subtractors16, and one amplifier18, according to equation (5) below:

TE={(A+B⁢⁢1+B⁢⁢2)-(C⁢⁢1+C⁢⁢2+D)}-m⁢{(B⁢⁢1+B⁢⁢2)-(C⁢⁢1+C⁢⁢2)}(5)
where m denotes a correction coefficient for correcting for the effect of lens shift on the tracking-error signal TE.

As is apparent from equation (5), in this embodiment, of the light-receiving surface82of the first light detector8, a region of the light spot10on one side of the center line86with respect to the first direction X is detected by the first, third, and fifth photoreceptors8402,8406, and8410located on that side of the center line86, and a region of the light spot10on the other side of the center line86with respect to the first direction X is detected by the second, fourth, and sixth photoreceptors8404,8408, and8412located on that side of the center line86. Furthermore, a central region of the light spot10is detected by the first, second, third, and fourth photoreceptors8402,8404,8406, and8408.

Thus, in this embodiment, a difference signal (B1+B2)−(C1+C2) between a sum signal of the detection signals B1and B2output from the first and second photoreceptors8402and8404of the first light detector8and a sum signal of the detection signals C1and C2output from the third and fourth photoreceptors8406and8408of the first light detector8is used as a correction signal in the differential compensate push-pull method.

In this embodiment, the focus-error-signal generating circuit120A and the tracking-error-signal generating circuit120B constitute error-signal generating means.

Let the respective widths of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408be denoted by D1, D2, D3, and D4, and the respective widths of the first, second, third, and fourth photoreceptors9402,9404,9406, and9408by D1′, D2′, D3′, and D4′. Then, according to the embodiment described above, the following two conditions can be satisfied simultaneously.

(1) The widths D1and D2of the first and second photoreceptors8402and8404of the first light detector8and the widths D1′ and D2′ of the first and second photoreceptors9402and9404of the second light detector9are chosen so that an optimal focus-error signal FE will be obtained.
(2) The widths D1, D2, D3, and D4of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8are chosen so that an optimal tracking-error signal TE will be obtained, i.e., so that a correction signal that enables the obtainment of an optimal tracking-error signal will be obtained.

That is, since the light-receiving surface82of the first light detector8is divided into the six photoreceptors8402to8412, the widths D1and D2of the first and second photoreceptors8402and8404and the widths D3and D4of the third and fourth photoreceptors8406and8408can be chosen independently. Thus, it is possible to obtain an optimal focus-error signal FE and an optimal tracking-error signal TE simultaneously. This is advantageous in achieving optimal characteristics regarding both the focus-error signal and the tracking-error signal.

Second Embodiment

The second embodiment differs from the first embodiment regarding equations used for calculating the focus-error signal FE and the tracking-error signal TE.

FIG. 6is a block diagram showing the circuit configuration of a focus-error-signal generating circuit120A that generates a focus-error signal from detection signals of the first and second light detectors8and9in the second embodiment.FIG. 7is a block diagram showing the circuit configuration of a tracking-error-signal generating circuit120B that generates a tracking-error signal from detection signals of the first and second light detectors8and9in the second embodiment.

As shown inFIG. 6, the focus-error signal FE is generated, for example, using eight adders14and three subtractors16, according to equation (6) below:

As is apparent from equation (6), in the second embodiment, of the light-receiving surface82of the first light detector8, a central region of the light spot10is detected by the first, second, third, and fourth photoreceptors8402,8404,8406, and8408near the center line86, and side regions of the light spot10with respect to the first direction X are detected by the fifth and sixth photoreceptors8410and8412. Similarly, of the light-receiving surface92of the second light detector9, a central region of the light spot12is detected by the first, second, third, and fourth photoreceptors9402,9404,9406, and9408near the center line96, and side regions of the light spot12with respect to the first direction X are detected by the fifth and sixth photoreceptors9410and9412.

Thus, in the second embodiment, a sum signal (B1+B2+C1+C2) of the detection signals B1, B2, C1, and C2of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8and a sum signal (F1+F2+G1+G2) of the detection signals F1, F2, G1, and G2of the first, second, third, and fourth photoreceptors9402,9404,9406, and9408are used as detection signals in the light spot size method.

As shown inFIG. 7, the tracking-error signal TE is generated using, for example, four adders14, three subtractors16, and one amplifier18, according to equation (7) below:
TE={(A+B1+B2)−(C1+C2+D)}−m(B2−C1)  (7)

As is apparent from equation (7), in the second embodiment, similarly to the first embodiment, of the light-receiving surface82of the first light detector8, a region of the light spot10on one side of the center line86with respect to the first direction X is detected by the first, third, and fifth photoreceptors8402,8406, and8410located on that side of the center line86, and a region of the light spot10on the other side of the center line86with respect to the first direction X is detected by the second, fourth, and sixth photoreceptors8404,8408, and8412located on that side of the center line86. As opposed to the first embodiment, a central region of the light spot10is detected by the first and second photoreceptors8402and8404.

Thus, in the second embodiment, a difference signal (B2−C1) of the detection signals B2and C1of the first and second photoreceptors8402and8404is used as a correction signal in the differential compensate push-pull method.

According to the second embodiment, the following two conditions can be satisfied simultaneously.

(1) The widths D1, D2, D3, and D4of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8and the widths D1′, D2′, D3′, and D4′ of the first, second, third, and fourth photoreceptors9402,9404,9406, and9408of the second light detector9are chosen so that an optimal focus-error signal FE will be obtained.
(2) The widths D1and D2of the first and second photoreceptors8402and8404of the first light detector8are chosen so that an optimal tracking-error signal TE will be obtained, i.e., so that a correction signal that enables the obtainment of an optimal tracking-error signal will be obtained.

That is, similarly to the first embodiment, since the light-receiving surface82of the first light detector8is divided into the six photoreceptors8402to8412, the widths D1and D2of the first and second photoreceptors8402and8404and the widths D3and D4of the third and fourth photoreceptors8406and8408can be chosen independently. Thus, it is possible to obtain an optimal focus-error signal FE and an optimal tracking-error signal TE simultaneously. This is advantageous in achieving optimal characteristics regarding both the focus-error signal and the tracking-error signal.

Third Embodiment

The third embodiment differs from the first and second embodiments in that, as opposed to the first and second embodiments in which each of the first and second light detectors8and9is implemented using six photoreceptors, in the third embodiment, the first light detector8is implemented using six photoreceptors and the second light detector9is implemented using three photoreceptors.

FIG. 8is a block diagram showing the circuit configuration of a focus-error-signal generating circuit120A that generates a focus-error signal from detection signals output from the first and second light detectors8and9in the third embodiment.FIG. 9is a block diagram showing the circuit configuration of a tracking-error-signal generating circuit120B that generates a tracking-error signal from detection signals output from the first and second light detectors8and9in the third embodiment.

Next, the first and second light detectors8and9will be described.

Referring toFIG. 8, the light-receiving surface82of the first light detector8is configured the same as in the first embodiment. That is, the light-receiving surface82of the first light detector8is formed by first to sixth rectangular photoreceptors8402,8404,8406,8408,8410, and8412arrayed along the width direction (the first direction X).

The light-receiving surface92of the second light detector9is formed by first to third photoreceptors9402,9404, and9406arrayed along the width direction (the first direction X).

The first, second, and third photoreceptors9402,9404, and9406are disposed axisymmetrically with respect to a center line96passing through the center of the direction of array and extending in the length direction (the direction perpendicular to the first direction X).

The first photoreceptor9402is disposed with its center located at the center line96.

The second and third photoreceptors9404and9406are disposed continuously outward from the first photoreceptor9402. The second and third photoreceptors9404and9406have the same rectangular shape and size with the same length and width.

In the third embodiment, the first photoreceptor9402of the second light detector9corresponds to a combination of the first and second photoreceptors9402and9404of the second light detector9in the first embodiment. That is, the width D1′ of the first photoreceptor9402of the second light detector9in the third embodiment is equivalent to the sum of the widths D1′ and D2′ of the first and second photoreceptors9402and9404of the second light detector9in the first embodiment.

Let detection signals output from the first to sixth photoreceptors8402to8412of the first light detector8be denoted by B2, C1, B1, C2, A, and D, respectively, and detection signals output from the first, second, and third photoreceptors9402,9404, and9406of the second light detector9by E, F, and G, respectively.

As shown inFIG. 8, the focus-error signal is generated using, for example, six adders14and two subtractors16, according to equation (8) below:
FE={(B2+C1)−(A+B1+C2+D)}+{−E+(F+G)}  (8)

As is apparent from equation (8), in the third embodiment, of the light-receiving surface82of the first light detector8, a central region of the light spot10is detected by the first and second photoreceptors8402and8404near the center line86, and side regions of the light spot10with respect to the first direction X are detected by the third, fourth, fifth, and sixth photoreceptors8406,8408,8410, and8412. Furthermore, of the light-receiving surface92of the second light detector9, a central region of the light spot12is detected by the first photoreceptor9402near the center line96, and side regions of the light spot12with respect to the first direction X are detected by the second and third photoreceptors9404and9406.

Thus, in the third embodiment, a sum signal (B1+C2) of the detection signals B1and C2output from the first and second photoreceptors8402and8404of the first light detector8, and the detection signal E output from the first photoreceptor9402of the second light detector9, are used as detection signal in the light spot size method.

As shown inFIG. 5, the tracking-error signal TE is generated according to equation (5) given earlier, in the same manner as in the first embodiment.

Thus, in the third embodiment, a difference signal (B1+B2)−(C1+C2) between a sum signal of the detection signals B1and B2output from the first and second photoreceptors8402and8404of the first light detector8and a sum signal of the detection signals C1and C2of the third and fourth photoreceptors8406and8408of the first light detector8is used as a correction signal in the differential compensate push-pull method.

According to the third embodiment described above, letting the widths of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8be denoted by D1, D2, D3, and D4, respectively, and the widths of the first, second, and third photoreceptors9402,9404, and9406by D1′, D2′, and D3′, respectively, the following two conditions can be satisfied simultaneously.

(1) The widths D1and D2of the first and second photoreceptors8402and8404of the first light detector8and the width D1′ of the first photoreceptor9402of the second light detector9are chosen so that an optimal focus-error signal FE will be obtained.
(2) The widths D1, D2, D3, and D4of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8are chosen so that an optimal tracking-error signal TE will be obtained, i.e., so that a correction signal that enables the obtainment of an optimal tracking-error signal will be obtained.

That is, also in the third embodiment, similarly to the first and second embodiments, since the light-receiving surface82of the first light detector8is divided into the six photoreceptors8402to8412, the widths D1and D2of the first and second photoreceptors8402and8404and the widths D3and D4of the third and fourth photoreceptors8406and8408can be chosen independently. Thus, it is possible to obtain an optimal focus-error signal FE and an optimal tracking-error signal TE simultaneously. This is advantageous in achieving optimal characteristics regarding both the focus-error signal and the tracking-error signal.

Furthermore, in the third embodiment, since the light-receiving surface92of the second light detector9is divided into three photoreceptors, i.e., the first, second, and third photoreceptors9402,9404, and9406, compared with the case where the light-receiving surface92is divided into six photoreceptors, the construction of the second light detector9and error-signal generating means (the focus-error-signal generating circuit120A) can be simplified. This is advantageous in reducing cost. Furthermore, since the number of photoreceptors is small, amp noise can be reduced. This serves to improve quality of signals.

Fourth Embodiment

The fourth embodiment differs from the third embodiment regarding equations used for calculating the focus-error signal FE and the tracking-error signal TE.

FIG. 10is a block diagram showing the circuit configuration of a focus-error-signal generating circuit120A that generates a focus-error signal from detection signals output from the first and second light detectors8and9in the fourth embodiment.FIG. 11is a block diagram showing the circuit configuration of a tracking-error-signal generating circuit120B that generates a tracking-error signal from detection signals output from the first and second light detectors8and9in the fourth embodiment.

Now, the first and second light detectors8and9will be described.

As shown inFIG. 10, the light-receiving surface82of the first light detector8is configured the same as in the second embodiment. That is, the light-receiving surface82of the first light detector8is formed by first to sixth rectangular photoreceptors8402,8404,8406,8408,8410, and8412arrayed along the width direction (the first direction X).

The light-receiving surface92of the second light detector9is configured the same as in the third embodiment. That is, the light-receiving surface92is formed by first, second, and third rectangular photoreceptors9402,9404, and9406arrayed along the width direction (the first direction X).

In the fourth embodiment, the first photoreceptor9402of the second light detector9corresponds to a combination of the first, second, third, and fourth photoreceptors9402,9404,9406, and9408of the second light detector9in the first embodiment. That is, the width D1′ of the first photoreceptor9402of the second light detector9in the fourth embodiment is equivalent to the sum of the widths D1′, D2′, D3′, and D4′ of the first, second, third, and fourth photoreceptors9402,9404,9406, and9408of the second light detector9in the first embodiment.

Let detection signals output from the first to sixth photoreceptors8402to8412of the first light detector8be denoted by B2, C1, B1, C2, A, and D, respectively, and detection signals output from the first, second, and third photoreceptors9402,9404, and9406of the second light detector9by E, F, and G, respectively.

As shown inFIG. 10, similarly to the third embodiment, the focus-error signal FE is generated using, for example, six adders14and two subtractors16, according to equation (8) given earlier. In the fourth embodiment, similarly to the third embodiment, of the light-receiving surface82of the first light detector8, a central region of the light spot10is detected by the first, second, third, and fourth photoreceptors8402,8404,8406, and8408near the center line86, and side regions of the light spot10with respect to the first direction X are detected by the fifth and sixth photoreceptors8410and8412. Furthermore, of the light-receiving surface92of the second light detector9, a central region of the light spot12is detected by the first photoreceptor9402near the center line96, and side regions of the light spot12with respect to the first direction X are detected by the second and third photoreceptors9404and9406.

Thus, in the fourth embodiment, a sum signal (B1+B2+C1+C2) of the detection signals B1, B2, C1, C2output from the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8, and the detection signal E output from the first photoreceptor of the second light detector9, are used as detection signals in the light spot size method.

As shown inFIG. 11, the tracking-error signal TE is generated in the same manner as in the second embodiment, according to equation (5) given earlier.

Thus, in the fourth embodiment, a difference signal (B2−C1) of detection signals B2and C1output from the first and second photoreceptors8402and8404of the first light detector8is used as a correction signal in the differential compensate push-pull method.

According to the fourth embodiment described above, letting the widths of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8be denoted by D1, D2, D3, and D4, respectively, and the widths of the first, second, and third photoreceptors9402,9404, and9406of the second light detector9by D1′, D2′, and D3′, respectively, the following two conditions can be satisfied simultaneously.

(1) The widths D1, D2, D3, and D4of the first, second, third, and fourth photoreceptors8402,8404,8406, and8408of the first light detector8and the width D1′ of the first photoreceptors9402of the second light detector9are chosen so that an optimal focus-error signal FE will be obtained.
(2) The widths D1and D2of the first and second photoreceptors8402and8404of the first light detector8are chosen so that an optimal tracking-error signal TE will be obtained, i.e., so that a correction signal that serves to obtain an optimal tracking-error signal will be obtained.

That is, also in the fourth embodiment, similarly to the first, second, and third embodiments, since the light-receiving surface82of the first light detector8is divided into the six photoreceptors8402to8412, the widths D1and D2of the first and second photoreceptors8402and8404and the widths D3and D4of the third and fourth photoreceptors8406and8408can be chosen independently. Thus, it is possible to obtain an optimal focus-error signal FE and an optimal tracking-error signal TE simultaneously. This is advantageous in achieving optimal characteristics regarding both the focus-error signal and the tracking-error signal.

Furthermore, in the fourth embodiment, similarly to the third embodiment, since the light-receiving surface92of the second light detector9is divided into three photoreceptors, i.e., the first, second, and third photoreceptors9402,9404, and9406, compared with the case where the light-receiving surface92is divided into six photoreceptors, the construction of the second light detector9and error-signal generating means (the focus-error-signal generating circuit120A) can be simplified. This is advantageous in reducing cost. Furthermore, since the number of photoreceptors is small, amp noise can be reduced. This serves to improve quality of signals.

Fifth Embodiment

Next, a fifth embodiment of the present invention will be described.

The fifth embodiment differs from the first to fourth embodiments in that, as opposed to the first to fourth embodiments in which reflected light beams are lead to the first and second light detectors8and9by the half-mirror surface72of the prism7, in the fifth embodiment, reflected light beams are lead to the first and second light detectors8and9by a holographic optical element (HOE).

FIG. 12is a diagram showing the construction of an optical system of an optical pickup according to the fifth embodiment. In the following description, parts corresponding to those in the first to fourth embodiments are designated by the same numerals, and descriptions thereof will be omitted.

As shown inFIG. 12, in front of the fourth surface3D of the polarizing beam splitter3, the condensing lens6, an HOE20, and a light-receiving substrate22are arranged linearly in that order.

The light-receiving substrate22has a plate-shaped insulating substrate2202with an upper surface (one surface with respect to the thickness direction) thereof facing the HOE20. On the upper surface, first and second optical elements8and9are provided with a gap therebetween.

The HOE20diffracts reflected light beams transmitted through the condensing lens6, emitting two diffracted lights, i.e., a positive first-order light and a negative first-order light. Furthermore, the HOE20generates a defocusing aberration. Since a positive defocusing aberration occurs in the positive first-order diffracted light beams condensed at the first light detector8, the condensing point L1is behind the light-receiving surface82of the first light detector8. On the other hand, the negative first-order diffracted light beams condensed at the second light detector9has a negative defocusing aberration, which is opposite to the case of the positive first-order diffracted light beams, so that the condensing point L2is before the light-receiving surface92of the second light detector9. Of the two diffracted lights, one is lead to the first light detector8, and the other is lead to the second light detector9.

Letting the focal length of the condensing lens6(the distance from the condensing lens6to the condensing point) be denoted by L0, the amount of shift of focus due to the condensing power of the HOE20by ΔL, the condensing point of the positive first-order diffracted light beams condensed at the first light detector8by L1, and the negative first-order diffracted light beams condensed at the second light detector9by L2, in order that the size of a beam spot on the light-receiving surface82with respect to the radial direction be substantially the same as the size of a beam spot on the light-receiving surface92with respect to the same direction, the amount ΔL of defocusing of the HOE20, the length L from the condensing lens6to the first light detector8and the second light detector9, the length L1from the condensing lens6to the condensing point of the positive first-order diffracted light, and the length L2from the condensing lens6to the condensing point of the negative first-order diffracted light are chosen so as to satisfy the following relationships:
L=L0  (1)
L1=L0+ΔL(2)
L2=L0−ΔL(3)
L1<L0<L2  (4)
where ΔL is a predetermined length.

In the optical pickup104, light beams emitted from the laser beam source1are made incident on the polarizing beam splitter3via the collimating lens2.

Part of the light beams made incident on the first surface3A of the polarizing beam splitter3are transmitted through the polarizing-beam-splitter surface32and the second surface3B so that the optical disk102is irradiated via the quarter-wavelength plate4and the objective lens5. The other part of the light beams made incident on the polarizing beam splitter3is reflected by the polarizing-beam-splitter surface32.

The light beams that reach the optical disk102are reflected by the recording surface of the optical disk102. The resulting reflected light beams are made incident on the second surface3B of the polarizing beam splitter3via the objective lens5and the quarter-wavelength plate4and reflected by the polarizing-beam-splitter surface32. The resulting reflected light beams are transmitted from the fourth surface3D through the condensing lens6and are made incident on the HOE20.

The reflected light beams made incident on the HOE20are separated into two diffracted lights. One of the diffracted lights is made incident on the light-receiving surface82of the first light detector8to form a single light spot thereon, and the other diffracted light is made incident on the light-receiving surface92of the second light detector9to form a single light spot thereon.

In this embodiment, an optical system is formed by the polarizing beam splitter3, the quarter-wavelength plate4, the condensing lens6, and the HOE20.

In the fifth embodiment constructed as described above, with the same configuration of the signal processing circuits120as in the first to fourth embodiments, the same operation and advantages as in the first to fourth embodiments can be achieved.

Sixth Embodiment

Next, a sixth embodiment of the present invention will be described.

The sixth embodiment differs from the first to fifth embodiments in that, as opposed to the first to fifth embodiments in which optical elements, such as the laser beam source1, the collimating lens2, the polarizing beam splitter3, and the first and second light detectors8and9, are separate, in the sixth embodiment, these optical elements are integrated in a light detecting apparatus.

FIG. 13is a diagram showing the construction of an optical system of an optical pickup according to the sixth embodiment.

Referring toFIG. 13, the optical pickup104includes the collimating lens2, the quarter-wavelength plate4, the objective lens5, and a light detecting apparatus24, these components being mounted on a holder (not shown).

In front of the light detecting apparatus24, the quarter-wavelength plate4, the collimating lens2, and the objective lens5are arranged linearly in that order, and the optical disk102is positioned in front of the objective lens5.

The light detecting apparatus24is implemented by providing the laser beam source1, the first and second light detectors8and9, and a prism28on the same substrate26composed of, for example, semiconductor.

The prism28is configured to lead laser beams emitted from the laser beam source1to the optical disk102, and to lead reflected light beams from the optical disk102to the first and second light detectors8and9.

More specifically, the prism28has a bottom surface2802that is placed over the top surface of the insulating substrate26, a top surface2804that is opposite and in parallel to the bottom surface2802with a gap, and a slanted surface2806substantially making an angle of 45 degrees with the bottom surface2802and facing the laser beam source1.

A half-mirror film2802A is provided in a region where the bottom surface2802faces the first light detector8, a reflecting film (mirror coat)2804A is provided on the top surface2804, and a polarizing-beam-splitter film2806A is provided on the slanted surface2806.

Let the focal length of the condensing lens6(the distance from the condensing lens6to the condensing point thereof) be denoted by L0, the length of the optical path from the condensing lens6to the light-receiving surface82of the first light detector8via the slanted surface2806by L1, and the length of the optical path from the condensing lens6to the light-receiving surface92of the second light detector9via the slanted surface2806, the bottom surface2802, and the top surface2804by L2. Then, in the sixth embodiment, in order that the size of a beam spot formed on the light-receiving surface82with respect to the radial direction is substantially the same as the size of a beam spot formed on the light-receiving surface92with respect to the same direction, the condensing lens6, the prism28, and the first and second light detectors8and9are configured so as to satisfy the following relationships:
L1=L0−ΔL(1)
L2=L0+ΔL(2)
L1<L0<L2  (3)
where ΔL is a predetermined length.

In the optical pickup104, light beams emitted from the laser beam source1are reflected by the polarizing-beam-splitter film2806A of the slanted surface2806of the prism28so that the optical disk102is irradiated with the reflected light beams via the quarter-wavelength plate4, the collimating lens2, and the objective lens5.

The light beams that reach the optical disk102are reflected by the recording surface of the optical disk102. The reflected light beams are transmitted through the polarizing-beam-splitter film2806A of the slanted surface2806of the prism28via the objective lens5, the collimating lens2, and the quarter-wavelength plate4, and are made incident on the half-mirror surface2802A of the bottom surface2802of the prism28.

Part of the reflected light beams made incident on the half-mirror surface2802A is transmitted through the half-mirror surface2802A to irradiate the light-receiving surface82of the first light detector8, whereby a single light spot is formed on the light-receiving surface82.

The other part of the reflected light beams made incident on the half-mirror surface2802A is reflected by the half-mirror surface2802A, and the reflected light beams are made incident on the light-receiving surface92of the second light detector9from the bottom surface2802, whereby a single light spot is formed on the light-receiving surface92.

In this embodiment, an optical system is formed by the collimating lens2, the quarter-wavelength plate4, and the prism28.

In the sixth embodiment constructed as described above, with the same configuration of the signal processing circuit120as in the first to fourth embodiments, the same operation and advantages as in the first to fourth embodiments can be achieved.

Furthermore, the signal processing circuit120(error-signal generating means) configured the same as in the first to fourth embodiments may be provided on the substrate26of the light detecting apparatus24.

The embodiments have been described in the context of cases where the tracking-error signal TE is generated on the basis of detection signals output from the photoreceptors of the first light detector8. However, it is to be understood that the tracking-error signal TE may be generated on the basis of detection signals output from the photoreceptors of the second light detector9.

Furthermore, the embodiments have been described in the context of cases where at least one of the first and second light detectors8and9is implemented using six photoreceptors. However, for example, it is possible to implement at least one of the first and second light detectors8and9using an even number of photoreceptors that is greater than or equal to 8. Also in this case, by independently choosing the widths of the photoreceptors, an optimal focus-error signal FE and an optimal tracking-error signal TE can be obtained simultaneously. This is advantageous in achieving optimal characteristics for both the focus-error signal and the tracking-error signal.

Next, comparison between a related art and the embodiments will be described.

FIG. 14is a plan view of first and second detectors according to a related art.FIG. 15is a diagram for explaining detection of a focus-error signal by a light spot size method.FIG. 16is a diagram for explaining the focus-error signal.FIGS. 17 and 18are diagrams for explaining problems that occur with the focus-error signal according to the related art.

FIG. 19is a diagram for explaining detection of a tracking-error signal by a push-pull method.FIG. 20is a diagram for explaining detection of a tracking-error signal according to the related art.FIG. 21is a diagram for explaining the principles of occurrence of a problem with the tracking-error signal according to the related art.FIG. 22is a diagram for explaining the problem with the tracking-error signal.FIG. 23is a diagram for explaining the principles of a differential compensate push-pull method.

First, detection of a focus-error signal by a spot size method, and problems that occur in the related art will be described.

Referring toFIG. 14, also in the related art, a first light detector8′ and a second light detector9′ are provided, and light spots10and12are formed on respective light-receiving surfaces82′ and92′. The light spots10and12have elliptic shapes since astigmatism occurs due to transmission of converging reflected light beams through the prism28.

According to the related art, the light-receiving surfaces82′ and92′ are each formed by four photoreceptors. Detection signals output from the respective photoreceptors of the first light detector8′ will be denoted by A′, B′, C′, and D′, and detection signals output from the respective photoreceptors of the second light detector9′ by E′, F′, G′, and H′.

In the first light detector8′, regarding the widths of the photoreceptors with respect to the first direction X, the widths of the two middle photoreceptors8402′ and8404′ are the same, and the widths of the two side photoreceptors8406′ and8408′ are the same.

Similarly, in the second light detector9′, regarding the widths of the photoreceptors with respect to the first direction X, the widths of the two middle photoreceptors9402′ and9404′ are the same, and the widths of the two side photoreceptors9406′ and9408′ are the same.

As shown inFIG. 15, when the objective lens5comes in focus, light spots10and12indicated by solid lines are formed. When the objective lens5moves nearer or further to the optical disk102from the position in focus, light spots10and12indicated by broken lines are formed. That is, when the objective lens5goes out of focus, the diameter of one of the light spots10and12becomes larger than that indicated by the solid lines, and the diameter of the other becomes smaller than that indicated by the solid lines.

According to the light spot size method, the focus-error signal FE can be expressed by equation (10) below:
FE={(B′+C′)−(A′+D′)}+{−(F′+G′)+(E′+H′)}  (10)

InFIG. 16, the horizontal axis represents the amount d of defocusing of the objective lens5(the amount of shift from the position in focus), and the vertical axis represents the signal level L.

A first difference signal S1corresponds to the first term in equation (10), which is generated on the basis of the detection signals A′, B′, C′, and D′ output from the photoreceptors of the first light detector8′.

A second difference signal S2corresponds to the second term in equation (10), which is generated on the basis of the detection signals E′, F′, G′, and H′ output from the photoreceptors of the second light detector9′.

Thus, the focus-error signal FE can be expressed as S1to S2, which is indicated by a broken line. The focus-error signal TE indicated by the broken line is referred to as an S-shaped signal (S-curve signal).

Regarding the focus-error signal FE, the linearity of the portion between the positive peak and the negative peak of the S-shaped signal should be achieved, preferably with a wide peak-to-peak gap.

FIG. 17Ashows the first signal S1, the second signal S2, and the focus-error signal FE in a case where the widths of the middle two photoreceptors of each of the first and second light detectors8′ and9′ are chosen to be large.

When the widths of the middle two photoreceptors of each of the first and second light detectors8′ and9′ are chosen to be large, the gradient of the S-shaped signal is small, and linearity is deteriorated. This is disadvantageous in achieving favorable focus-servo characteristics. Furthermore, when AC components caused by variation of the distribution of the intensities of diffracted lights in the light spots10and12formed on the light-receiving surfaces82′ and92′ are superposed as noise on the focus-error signal FE as will be described later, characteristics are susceptible to the effect of the noise.

FIG. 17Bshows the first signal S1, the second signal S2, and the focus-error signal FE in a case where the widths of the middle two photoreceptors of each of the first and second light detectors8′ and9′ are chosen to be small.

When the widths of the middle two photoreceptors of each of the first and second light detectors8′ and9′ are chosen to be small, the gradient of the S-shaped signal is increased, and linearity is improved. However, the range of the S-shaped signal (the portion between the positive peak and the negative peak of the focus-error signal FE) becomes smaller, so that the range where focus servo can be exercised becomes smaller. This is disadvantageous in achieving stable focus-servo operation. Furthermore, the focus-error signal FE becomes susceptible to the effect of variation in the relative position (with respect to the first direction) between the first light detectors8′ and9′ and the optical system that leads reflected light beams to the first light detectors8′ and9′.

Thus, in the related art described above, optimal values should be chosen for the widths of the photoreceptors of the first light detectors8′ and9′ in order to achieve favorable characteristics regarding the focus-error signal FE and to thereby achieve stable focus-servo characteristics.

Next, detection of a tracking-error signal by a differential compensate push-pull method, and problems that occur in the related art, will be described.

As shown inFIG. 19, on the recording surface of the optical disk102, a groove102A and lands102B are formed. When the center of the groove102A is irradiated with a light beam spot, a zeroth order diffracted light, a positive first-order diffracted light, and a negative first-order diffracted light occur due to the difference in height between the groove102A and the lands102B, and the zeroth order diffracted light, the positive first-order diffracted light, and the negative first-order diffracted light form reflected light beams.

As shown inFIG. 20, a light spot formed by the reflected light beams on the light-receiving surface82′ of the first light detector8′ includes a central region10A formed by the zeroth order diffracted light, and side regions10B and10C where the zeroth order diffracted light is interfered with by the positive and negative first-order diffracted lights. The side regions10B and10C are on both sides of the central region10A with respect to the first direction X, and the light intensities of lights in the side regions10B and10C is either larger or smaller than the light intensity in the central region10A.

When the light beam spot is located at the center of the groove102A, the light intensities in the side regions10B and10C are the same. When the light beam spot is off the center of the groove102A, the light intensity of one of the side regions10B and10C becomes larger and that of the other becomes smaller.

In an ordinary push-pull method, the tracking-error signal TE can be expressed by equation (11) below:
TE=(A+B)−(C+D)  (11)

As shown inFIG. 21, when the objective lens5is at the position indicated by a solid line and a light spot is formed at the center of the groove102A of the optical disk102, the light spot formed by reflected light beams is formed on the light-receiving surface82′ of the first light detector8′, as also indicated by a solid line.

When a seek operation of the optical pickup (a move operation in the radial direction of the optical pickup102) is performed, the objective lens5is moved, with a time delay, in the tracking direction by a tracking-servo operation in accordance with the seek operation.

This is equivalent to the objective lens5being shifted in the tracking direction, as indicated by a broken line inFIG. 21. As indicated by (a) inFIG. 22, an offset occurs with the tracking-error signal TE, so that an incorrect tracking signal with an error corresponding to a DC offset with respect to a true track position is detected. That is, as indicated by (b) inFIG. 22, when the offset is zero, the true track position is detected reliably.

Thus, in the differential compensate push-pull method, such an offset is canceled.

More specifically, as shown inFIG. 20, regarding an optical spot relating to the two middle photoreceptors8402′ and8404′ of the first light detector8′, the two middle photoreceptors8402′ and8404′ are irradiated only with the zeroth order diffracted light, so that the distribution of light intensity exhibits a Gaussian distribution, as indicated by (a) inFIG. 20. If a portion including the peak of the Gaussian distribution is uniformly distributed between the two photoreceptors8402′ and8404′, the amount of shift of the objective lens5(the amount of shift in the radial direction of the optical disk102) is zero.

Thus, as shown inFIG. 23, a difference signal (B−C) between detection signals output from the two middle photoreceptors8402′ and8404′ is proportional to the amount of shift of the objective lens5, similarly to the amount Soff of offset of the tracking-error signal TE.

Thus, it is possible to cancel the amount Soff of offset by multiplying the difference signal (B−C) by a correction coefficient.

That is, in the differential compensate push-pull method, the tracking-error signal TE can be expressed by equation (12) below:
TE={(A+B)−(C+D)}−m(B−C)  (12)

The portion of the Gaussian distribution detected can be increased by increasing the widths of the two middle photoreceptors8402′ and8404′, thereby increasing the amount of change in the difference signal (B−C) in relation to the amount of lens shift. This is advantageous in accurately canceling the amount Soff of offset of the tracking-error signal TE.

However, when the widths of the two middle photoreceptors8402′ and8404′ are increased excessively, the two middle photoreceptors8402′ and8404′ are irradiated with the positive and negative first-order diffracted lights on both sides of the zeroth-order diffracted light. Thus, for example, when a seek operation of the optical pickup is performed, the components of the positive and negative first-order diffracted lights affect the difference signal (B−C) as AC components, so that the level of the tracking-error signal TE expressed by equation (12) is increased or decreased. This causes detection of an incorrect tracking signal and therefore occurrence of a track offset (AC offset).

Thus, in the related art described above, as described earlier, optimal values should be chosen for the widths of the photoreceptors8402′,8404′,9402′, and9404′ of the first and second light detectors8′ and9′. Furthermore, in order to achieve favorable characteristics regarding the tracking-error signal and to thereby achieve stable tracking-servo characteristics, optimal values should be chosen for the widths of the photoreceptors8402′ and8404′ of the first light detector8′.

Thus, it has been the case to inevitably select either choosing the widths of the photoreceptors8402′ and8404′ so as to optimize one of the focus-error signal and the tracking-error signal at the compromise of optimization of the other, or choosing the widths of the photoreceptors8402′ and8404′ so as to achieve fair characteristics for both the focus-error signal and the tracking-error signal.

In contrast, according to the embodiments, since the light-receiving surface of the first light detector8(or the second light detector9) is formed by six photoreceptors, an optimal focus-error signal FE and an optimal tracking-error signal TE can be obtained simultaneously. This is advantageous in achieving favorable characteristics regarding both focus-error signal and tracking-error signal.