Spherical aberration detecting device and an optical pickup device including same

A photoreceiver includes: a first light-receiving section divided into two regions by a border extending in a radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the main beam from the polarization hologram, so as to detect a spherical aberration; a second light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the first sub-beam from the polarization hologram, so as to detect a spherical aberration; and a third light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the second sub-beam from the polarization hologram, so as to detect a spherical aberration. The second and third light-receiving sections are laid out such that an end of the second light-receiving section along the tangential direction and an opposite end of the third light-receiving section along the tangential direction are aligned on a straight line extending in the tangential direction across the first light-receiving section.

This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application Patent Application No. 2006/272243 filed in Japan on Oct. 3, 2006, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a spherical aberration detecting device for detecting a spherical aberration of an optical disk, and an optical pickup device including the spherical aberration detecting device.

BACKGROUND OF THE INVENTION

With the increase in density of optical disks in recent years, its information recording layer has higher linear recording density, and its tracks are provided at a smaller pitch. Such density increase in an optical disk also requires reduction in beam diameter of a light beam focused on the information recording layer of the optical disk. This can be achieved by an increase in numerical aperture of the light beam emitted from an objective lens serving as a light-harvesting optical system of the optical pickup device, or a decrease in wavelength of the light beam.

When the light beam passes through a cover glass of an optical disk, a spherical aberration is generated. The magnitude of the spherical aberration is generally proportional to a biquadrate of the numerical aperture, and therefore an error of the spherical aberration gives a certain influence to information recording when the objective lens has a high numerical aperture. For this reason, it is necessary to correct the spherical aberration when the objective lens has a high numerical aperture. The following describes some prior arts regarding detection of spherical aberration.

For example, Japanese Unexamined Patent Publication Tokukai 2002-55024 (published on Feb. 20, 2002) and Japanese Unexamined Patent Publication Tokukai 2000-171346 (published on Jun. 23, 2000) disclose a method of detecting a spherical aberration in a light-harvesting optical system using hologram diffraction ray. Also, Japanese Unexamined Patent Publication Tokukai 2002-157771 (published on May 31, 2002) discloses a method of detecting spherical aberration by appropriately dividing a light beam by a hologram. In this method, the difference between the positions where the spot diameters of the respective light beams are minimized is increased, which further increases the degrees of focal displacement of the respective light beams. In this manner, the spherical aberration can be detected at high sensitivity. Further, Japanese Unexamined Patent Publication Tokukai 2006-65935 (published on Mar. 9, 2006) discloses detection of a spherical aberration using a hologram with an optical pickup device having an optical integrated unit which is arranged such that the diameter of a light beam on a diffraction grating is increased to provide a longer light path between a diffraction element and a photoreceiver.

FIG. 11is a drawing showing a layout of optical system components in a conventional optical pickup device101. This optical pickup device101includes a semiconductor laser1, a beam splitter2, a polarization hologram3, a transmission grating4, a collimator lens5, an objective lens6and a photoreceiver8.FIG. 10shows a pattern of the polarization hologram3.FIG. 12shows a structure of the photoreceiver8. The light emitted from the semiconductor laser1(light source) passes through the beam splitter2, and is incident on the polarization hologram3. The incident light transmits through the polarization hologram3, and is divided into three beams by a transmission grating4in a tangential direction, before being condensed onto the surface of the optical disk7by the collimator lens5and the objective lens6. Passing through the collimator lens5and the objective lens6, the reflection light from the optical disk7is again incident on the polarization hologram3.

The polarizing directions of the light beams incident on the polarization hologram3, i.e., the light from the light source and the reflection light of the optical disk7has a 90° difference which is given by a wavelength plate (not shown). As a result, the light beam from the semiconductor laser1passes through the polarization hologram3, and the reflection light from the optical disk7is diffracted by the characteristic of the polarization hologram3, and the resulting light beams are respectively focused onto the five separate light-receiving sections PD1to PD5shown inFIG. 12in the regions3ato3cof the polarization hologram3shown inFIG. 10.FIG. 12shows the photoreceiver8and the light beams condensed thereon.

As shown inFIG. 10, the polarization hologram3has a circular shape, and includes regions3ato3c. Among them, the region3cis one of the semicircles divided in a radial direction by a center line. The regions3aand3bare included in the other one of the two semicircles of the polarization hologram3. The region3bis a semicircle smaller than the region3c, and the region3asurrounds the circular arc portion of the regions3b. The region3ais an area surrounded by a straight line in the radial direction orthogonal to the optical axis of the light beam, the first semicircle (the other one of the semicircles), and a second semicircle (the circular arc portion) which is concentric to a first semicircle and smaller in radius than the first semicircle.

FIG. 13shows the diffraction by the polarization hologram3. As shown inFIG. 13, the light-receiving sections PD1to PD3in the photoreceiver8are aligned at a predetermined interval along the tangential direction with the light-receiving section PD1in the center. Meanwhile, the light-receiving sections PD1, PD4and PD5are aligned at a predetermined interval along the radial direction with the light-receiving section PD1in the center.

The main beam MB in the center of the three divided light beams is guided to the light-receiving section PD1as 0-th order diffraction ray which has passed through the polarization hologram3, and is condensed as a light spot SP1as shown inFIG. 12. Further, the sub-beams SB1and SB2on the both sides of the main beam MB among the three divided light beams are guided to the light-receiving sections PD2and PD3, and are condensed as light spots SP2and SP3. The main beam MB is guided to the light-receiving section PD4as −1st order diffraction ray which has been generated in the region3cof the polarization hologram3, and is condensed as a light spot SP4. The main beam MB is also guided to the light-receiving section PD5as a +1st order diffraction ray which has been generated in the region3aof the polarization hologram3, and is condensed on as a light spot SP5.

As shown inFIG. 12, the light-receiving section PD1has four divided light-receiving regions A to D, and detects the 0-th order diffraction ray of the main beam having been passed through the polarization hologram3. The light-receiving section PD2has two divided light-receiving regions E and F, and detects one of the sub-beams. The light-receiving section PD3has two divided light-receiving regions G and H, and detects the other of the sub-beams. The light-receiving sections PD2and PD3are used for generation of tracking servo signals. The light-receiving section PD4has two divided light-receiving regions I and J, and detects the −1st order diffraction ray. The light-receiving section PD4is used for detection of FES signals. The light-receiving section PD5has two divided light-receiving regions K and L, and detects the +1st order diffraction ray. The light-receiving section PD5is used for detection of spherical aberration signals (SA signals).

FIG. 12shows only the +1st order diffraction ray as the light diffracted by the region3a, and shows only the −1st order diffraction ray as the light diffracted by the region3c. The light diffracted by the region3bof the polarization hologram3is not discussed here, and therefore not shown in the figure.

When a spherical aberration occurs due to a thickness error of the optical disk7, as shown inFIGS. 14(a) and14(b), the light beams focused onto the light-receiving regions K and L of the light-receiving section PD5form light spots SP5aand SP5b. In each of them, the light-receiving area in the light-receiving region K or L is larger than the light-receiving area of the other light receiving region. Further, if a spherical aberration does not occur, the light beam focused onto the light-receiving regions K and L form a dot light spot SP5con the interface between the light-receiving regions K and L as shown inFIG. 14(c). It however should be noted that the influence of defocus is not taken into account. Here, expressing the electric signals generated in the light-receiving regions K and L respectively as Sk and Sl, the difference Sk−Sl of these electric signals is calculated. According to the calculation, the signal of Sk−Sl becomes 0 when a spherical aberration does not occur, but the signal of Sk−Sl becomes a positive value or a negative value when a spherical aberration occurs, and therefore a spherical aberration can be detected as a signal.

However, in the conventional optical pickup device101, the objective lens6is shifted in the radial direction (vertical to the track) so as to allow the light spot to follow a concentric or helical track formed on the optical disk7. With this shifting of the objective lens6, the spot SPH on the polarization hologram3derived from the reflection light of the optical disk7is also shifted in the radial direction from the state ofFIG. 15(b) into the state ofFIG. 15(a) or15(c). This affects the shapes of the light spot SP5dor the light spot SP5ediffracted onto the light-receiving regions K and L, as shown inFIG. 16(a) or16(c). This shifting of the objective lens6causes a change of shape of the optical light spot, thereby changing the electric signals in the light-receiving regions K and L. Therefore, such shifting of the objective lens6causes generation of positive or negative Sk−Sl signals, in contrast to the light spot SP5fofFIG. 16(b) where the objective lens6is not shifted. This is called an offset signal, hereinafter.

SUMMARY OF THE INVENTION

The present invention is made in view of the foregoing conventional problems, and an object is to provide a spherical aberration detecting device immune to influence of offset-signals and ensure accurate detection of a spherical aberration in the form of a signal. The present invention also provides an optical pickup device including the spherical aberration detecting device.

In order to attain the foregoing object, a first spherical aberration detecting device according to the present invention comprises: a transmission grating for dividing, in a tangential direction, a light beam emitted from a light source into a main beam, a first sub-beam and a second sub-beam; an optical element for focusing the three beams on an optical disk; a polarization hologram for diffracting a light beam reflected by the optical disk; and a photoreceiver for receiving a diffraction ray from the polarization hologram, wherein: the photoreceiver includes the first through third light-receiving sections adjacently aligned for respectively receiving +1st order diffraction rays of the main beam, the first sub-beam and the second sub-beam generated in the polarization hologram, the first light-receiving section is divided into two regions by a border extending in a radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the main beam from the polarization hologram, so as to detect a spherical aberration, the second light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the first sub-beam from the polarization hologram, so as to detect a spherical aberration, the third light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the second sub-beam from the polarization hologram, so as to detect a spherical aberration, and the second and third light-receiving sections are laid out such that an end of the second light-receiving section along the tangential direction and an opposite end of the third light-receiving section along the tangential direction are aligned on a straight line extending in the tangential direction across the first light-receiving section.

With this structure, the second light-receiving section is disposed out of alignment with respect to the first light-receiving section in the radial direction vertical to the tangential direction, and the third light-receiving section is disposed out of alignment with respect to the first light-receiving section in the direction opposite to the second light-receiving section (see the light-receiving sections PD5to PD7ofFIG. 1). In this structure, the second light-receiving section receives a half of the +1st order diffraction ray identical in shape to the +1st order diffraction ray received by the first light-receiving section, while the third light-receiving section receives the other half. In this case, because of the shifting of the objective lens, the difference between the light detection signals of the two light-receiving sections is not 0 but identical to an error component (offset-signals) of the light detection signal (spherical aberration detection signal) of the first light-receiving section. Therefore, with the provision of the second and third light-receiving sections laid out in the foregoing manner, it becomes possible to correct the spherical aberration detection signal based on the difference between the light detection signals of the second and third light-receiving sections. Consequently, it becomes possible to detect spherical aberration signals without influence of offset-signals generated by the shifting of the objective lens in the radial direction.

In order to attain the foregoing object, a second spherical aberration detecting device according to the present invention comprises: a polarization hologram for diffracting a light beam reflected by an optical disk; and a photoreceiver for receiving a diffraction ray from the polarization hologram, the photoreceiver including a light-receiving section which is divided into four regions by a border extending in a radial direction and a border extending in a tangential direction.

With the foregoing structure in which the photoreceiver includes a light-receiving section divided into four regions, the light-receiving section is constituted of a combination of the first through third light-receiving sections of the first spherical aberration detecting device. With this structure, the second spherical aberration detecting device is capable of correcting spherical aberration detection signals as with the first spherical aberration detecting device. Consequently, it becomes possible to detect spherical aberration signals without influence of offset-signals generated by the shifting of the objective lens in the radial direction. Further, since the photoreceiver requires only a single light-receiving section for a spherical aberration, it has a simple structure. On this account, compensation of spherical aberration detection signals becomes possible without an increase in cost of the spherical aberration detecting device.

In order to attain the foregoing object, a third spherical aberration detecting device according to the present invention comprises: a polarization hologram for diffracting a light beam reflected by an optical disk; and a photoreceiver for receiving a diffraction ray from the polarization hologram, wherein: the photoreceiver includes the first to second light-receiving sections adjacently aligned in a tangential direction for respectively receiving a main beam and a sub-beam, the first light-receiving section is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the main beam from the polarization hologram, and the second light-receiving section is divided into four regions by a border extending in the radial direction and a border extending in the tangential direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the sub-beam from the polarization hologram.

With the foregoing structure in which the photoreceiver includes the second light-receiving section divided into four regions, the second light-receiving section is constituted of a combination of the second and third light-receiving sections of the first spherical aberration detecting device. With this structure, the third spherical aberration detecting device is capable of correcting spherical aberration detection signals as with the first spherical aberration detecting device. Consequently, it becomes possible to detect spherical aberration signals without influence of offset-signals generated by the shifting of the objective lens in the radial direction. Further, since the photoreceiver requires only two light-receiving sections for detecting a spherical aberration, it has a simple structure. On this account, compensation of spherical aberration detection signals becomes possible without an increase in cost of the spherical aberration detecting device.

Each of the optical pickup devices according to the present invention includes one of the spherical aberration detecting devices, and therefore obtains spherical aberration detection signals having been corrected to compensate offset-signals caused by the shifting of the objective lens. On this account, it is possible to accurately correct information signals read from the optical disk using the spherical aberration detection signals.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

The following describes one embodiment of the present invention with reference toFIG. 1toFIG. 7.FIG. 2shows a structure of the optical pickup device12according to the present embodiment.FIG. 1shows a structure of a photoreceiver21of the optical pickup device12.

As shown inFIG. 2, as with the conventional optical pickup device101ofFIG. 11, the optical pickup device12includes the semiconductor laser1, the beam splitter2, the polarization hologram3, the transmission grating4, the collimator lens5, and the objective lens6. The photoreceiver8is replaced with a photoreceiver21. This optical pickup device12further includes a calculation circuit9for calculating the SA signals (spherical aberration signals). As shown inFIG. 1, the photoreceiver21includes light-receiving sections PD6and PD7, in addition to the light-receiving sections PD1to PD5of the photoreceiver8of the conventional optical pickup device101shown inFIG. 12.

The light-receiving section PD6is provided to be adjacent to the light-receiving section PD2in the radial direction with a predetermined interval. The PD7is provided to be adjacent to the light-receiving section PD3in the radial direction with a predetermined interval. The light-receiving section PD6is placed with its right end (the end along the tangential direction) in line with the straight line CL (shown by a broken line ofFIG. 1) extending in the tangential direction crossing the light-receiving section PD5. Further, the light-receiving section PD7is placed with its left end (the other end along the tangential direction) in line with the straight line CL. The light-receiving section PD6includes two photodiodes, one of which has a light-receiving region O while the other has a light-receiving region P. The shapes of these regions are identical in size, and each of them corresponds to a half of the light-receiving section PD6. The light-receiving section PD7includes two photodiodes, one of which has a light-receiving region Q while the other has a light-receiving region R. The shapes of these regions are identical in size, and each of them corresponds to a half of the light-receiving section PD7. Further, the light-receiving sections PD5to PD7are laid out so that the interface between the light-receiving regions K and L, the interface between the light-receiving regions O and P, and the interface between the light-receiving regions Q and R extend in parallel in the radial direction.

Note that, the tangential direction designates a direction extending along the tangent line of the track of the optical disk7, vertical to the radial direction.

With this structure, it becomes possible to detect spherical aberration signals without influence of the Sk−Sl offset signals. The following mainly explains a spherical aberration detection method using this layout pattern of the light-receiving sections O to R of the photoreceiver21.

The light emitted from the semiconductor laser1serving as a light source passes through the beam splitter2, and is incident on the polarization hologram3. The incident light transmits through the polarization hologram3, and is divided into three beams by a transmission grating4in a tangential direction, before being condensed onto the surface of the optical disk7by the collimator lens5and the objective lens6. Passing through the collimator lens5and the objective lens6, the reflection light from the optical disk7is again incident on the polarization hologram3.

The polarizing directions of the light beams incident on the polarization hologram3, i.e., the light from the light source and the reflection light of the optical disk7has a 90° difference which is given by a wavelength plate (not shown). As a result, the reflection light from the optical disk7is diffracted by the three regions3ato3cof the polarization hologram3shown inFIG. 10, and the resulting light beams are respectively focused onto the seven separate light-receiving sections PD1to PD7shown inFIG. 1.

FIG. 1shows the light-receiving sections PD1to PD7of the photoreceiver21, and the shapes of light focused thereon. As shown inFIG. 13, the light-receiving section PD1detects 0-th order diffraction ray of the main beam MB having been transmitted through the polarization hologram3. The light-receiving sections PD2and PD3detect sub-beams SB1and SB2, which are used for generation of tracking servo signals. The light-receiving section PD4detects −1st order diffraction ray DB1generated in the region3cof the polarization hologram3used for detection of FES signals. The light-receiving section PD5detects +1st order diffraction ray DB2of the main beam MB generated in the regions3aof the polarization hologram3used for detection of SA signals. In the light-receiving sections PD6and PD7, the two sub-beams (first and second sub-beams) in the tangential direction generated in the transmission grating4detect two +1st order diffraction rays generated in the region3aof the polarization hologram3. The +1st order diffraction rays detected by the light-receiving sections PD6and PD7are used for offset detection of SA signals (offset-signals).

Note that,FIG. 1shows only the +1st order diffraction ray as the light diffracted by the region3aofFIG. 8, and shows only the −1st order diffraction ray as the light diffracted by the region3cofFIG. 8. The light diffracted by the region3bof the polarization hologram3is not discussed here, and therefore not shown in the figure.

When a spherical aberration occurs due to a thickness error of the optical disk7, light spots SP5ato SP7aare formed on the light-receiving sections PD5to PD7as shown inFIG. 3(a), or light spots SP5band SP7bare formed as shown inFIG. 3(b). Further, if a spherical aberration does not occur, light spots SP5cto SP7care formed on the light-receiving sections PD5to PD7as shown inFIG. 3(c). Here, the difference Sk−Sl of the electric signals generated in the light-receiving regions K and L is calculated. According to the calculation, the signal of Sk−Sl becomes 0 when a spherical aberration does not occur, but the signal of Sk−Sl becomes either a positive value or a negative value when a spherical aberration occurs. In this way, a spherical aberration can be detected as a signal. Further, expressing the electric signals generated in the light-receiving region O, P, Q, R as So, Sp, Sq, Sr, respectively, signals of So−Sp and Sq−Sr are calculated. The calculation signals of So−Sp and Sq−Sr become 0 when a spherical aberration occurs, but the signals take either positive values or negative values when a spherical aberration does not occur.

The problem of prior art is generation of offset-signals of the Sk−Sl signals when the objective lens6following the track is shifted in the radial direction even though a spherical aberration does not occur. This is caused by associated shifting in the radial direction of the light spots formed by the reflection light of the optical disk7on the polarization hologram3, which further causes a change in shape of light spots on the photoreceiver8. The shapes of the light spots on the light-receiving sections K and L are changed in the radial direction by shifting of the objective lens6. Similarly, shapes of the light spots on the light-receiving sections O and P and the light spots on the light-receiving sections Q and R are also changed in the radial direction. In view of this problem, the following describes detection of spherical aberration signals using signals So−Sp and Sq−Sr to eliminate influences of the offset-signals generated by the shifting of the objective lens6.

The shifting of the objective lens6changes the shapes of the light spots on the photoreceiver21. The offset-signals of the Sk−Sl signals generated by the changes of the light spot shapes are found by approximate calculation according to: Soff={(So−Sp)−(Sq−Sr)}. For example,FIG. 3(a) shows generation of a spherical aberration when the objective lens6is not shifted. Soff becomes 0 in this case. Similarly, Soff is 0 also in the case ofFIG. 3(b).

When the objective lens6is shifted in the radial direction, the shapes of the light spots on the light-receiving section PD6and the light-receiving section PD7change in the radial direction, and Soff takes either a positive or a negative value. Soff is an offset-signal component derived from the shifting of the objective lens6, and its sign changes depending on whether the objective lens6is shifted toward inner or outer periphery of the optical disk7. For example, inFIG. 3(a), the light-receiving section PD6and the light-receiving section PD7are disposed such that they each receive a half of the light spot SP6aor SP7aidentical in shape to the light spot SP5aformed almost on the light-receiving region K of the light-receiving section PD5. Further, inFIG. 3(b), the light-receiving section PD6and the light-receiving section PD7are disposed such that they each receive a half of the light spot SP6bor SP7bidentical in shape to the light spot SP5bformed almost on the light-receiving region L of the light-receiving section PD5. Therefore, the magnitude relation of the (So−Sp) signal and (Sq−Sr) signal changes depending on the shifting direction of the objective lens6, and therefore the calculation for their difference in the subtraction circuit9gchanges the sign of Soff depending on the shifting direction of the objective lens6.

However, offset-signals of the Sk−Sl due to the shifting of the objective lens6are always same values regardless of the shifting direction of the objective lens6, and therefore an absolute value of Soff needs to be added to the spherical aberration signals Sk−Sl. Further, as shown inFIGS. 3(a) and3(b), the shapes of the light spots SP5aand SP5bon the light-receiving sections K and L change depending on whether the spherical aberration is a positive value or a negative value, and the sign of the Sk−Sl also changes. As a result, the Sk−Sl signal also takes an absolute value, and a spherical aberration signal SA is found by the following formula.

In the formula, α is a coefficient. Since the light spots on the light-receiving section PD6and light-receiving section PD7are both +1st order diffraction ray of the polarization hologram3, which is a sub beam having a small light intensity, it is necessary to multiple the value by an appropriate coefficient for adjustment.

FIG. 4shows a structure of a calculation circuit9for calculating SA signals. As shown in the figure, the calculation circuit9includes a current/voltage conversion circuits (shown as I/V in the figure)9ato9f, subtraction circuits9gto9j, a multiplication circuit9kand an addition circuit9m.

The current/voltage conversion circuits9ato9fserve to convert output signals (light-receiving signals) of the light-receiving regions K, L, O, P, Q, and R into voltage signals Sk, Sl, So, Sp, Sq, and Sr, respectively. The subtraction circuit9gserves to subtract a signal Sl from a signal Sk, the subtraction circuit9hserves to subtract a signal Sq from a signal So, and the subtraction circuit9iserves to subtract a signal Sr from the signal Sq. The subtraction circuit9jsubtracts Sq−Sr outputted from the subtraction circuit9ifrom So−Sp outputted from the subtraction circuit9h, thereby outputting Soff. The multiplication circuit9kserves to multiply the output of the subtraction circuit9jby the coefficient α. The addition circuit9mserves to add Sk−Sl outputted from the subtraction circuit9gto αSoff outputted from the multiplication circuit9k. The current/voltage conversion circuits9ato9f, the subtraction circuits9gto9j, the multiplication circuits9kand addition circuit9mare general analog circuits each mainly constituted of an operational amplifier, and therefore explanation of the details of these circuits is omitted here.

As described, the optical pickup device12includes the photoreceiver21including the light-receiving sections PD6and PD7disposed in the radial direction with respect to the light-receiving section PD5; and the offset calculation circuit9. With this structure, the optical pickup device12is capable of calculating a SA signal which includes Soff found by the outputs of the light-receiving sections PD6and PD7. In this way, the decrease of Sk−Sl is corrected by Soff even with the shifting of the objective lens6, and therefore the influence of the shifting of the objective lens6to the SA signals can be greatly reduced.

Second Embodiment

Another embodiment of the present invention is described below with reference toFIG. 5throughFIG. 7. Note that, only the difference from First Embodiment is described in the following embodiment. For ease of explanation, materials having the equivalent functions as those shown in the drawings pertaining to the foregoing First Embodiment will be given the same reference symbols, and explanation thereof will be omitted here.

FIG. 5shows a structure of an optical pickup device13according to the present embodiment.FIG. 6shows a structure of a photoreceiver22of the optical pickup device13.

As shown inFIG. 5, as with the optical pickup device12of First Embodiment shown inFIG. 1, the optical pickup device13includes the semiconductor laser1, the beam splitter2, the polarization hologram3, the transmission grating4, the collimator lens5, and the objective lens6. The photoreceiver21is replaced with a photoreceiver22. The calculation circuit9is replaced with a calculation circuit10. As shown inFIG. 6, the photoreceiver22includes the light-receiving sections PD1to PD4as with the photoreceiver8of the conventional optical pickup device101shown inFIG. 12. The light-receiving section PD5is replaced with a light-receiving section PD8.

The light-receiving section PD8has four-divisional light-receiving regions O, P, Q, and R. In the light-receiving section PD8, the light-receiving regions O and P of the light-receiving section PD6of First Embodiment, and the light-receiving regions Q and R of the light-receiving section PD7of First Embodiment are combined. The light-receiving regions O and Q are aligned in the radial direction, and the light-receiving regions P and R are aligned in the radial direction. Further, the light-receiving regions O and P are aligned in the tangential direction, and the light-receiving sections Q and R are aligned in the tangential direction.

Assuming that the electronic signals generated in the four light-receiving regions O, P, Q, and R are expressed as So, Sp, Sq, and Sr, the Sk and Sl are expressed as follows.
Sk=So+Sq(3)
Sl=Sp+Sr(4)

With this calculation, in addition to the prior art, the +1st order diffraction ray of the main beam generated in the region3aof the polarization hologram3is focused onto the light-receiving section PD8as a light spot SP8, and is detected in the light-receiving regions O, P, Q, and R in the light-receiving section PD8. It is thus possible to detect spherical aberration signals without influence of the offset signals due to shifting of the objective lens6.

The spherical aberration detection method using the layout pattern of the photoreceiver22according to the present embodiment differs from the spherical aberration detection method using the layout pattern of the photoreceiver21according to First Embodiment only in the layout and pattern of the photoreceiver22and the way of calculation in the detection of spherical aberration signals. Therefore, the following describes only the difference.

The light-receiving signal So of the spherical aberration detection method using the layout pattern of the photoreceiver21according to First Embodiment differs from the light-receiving signal So of the spherical aberration detection method using the layout pattern of the photoreceiver22according to the present embodiment in its output due to the difference between the sub-beam and the main-beam. However, the shape of light spot to be detected is identical. The same can be said for the signals Sp, Sq, and Sr. Therefore, in the calculation formula of the spherical aberration detection of the present embodiment, the following formulas (3) and (4) are substituted into the calculation formula according to the spherical aberration detection method of First Embodiment using the layout pattern of the photoreceiver21, as shown below.

Since the signal of the main beam is used for Soff, the correction coefficient α is not necessary.

FIG. 7shows a structure of the calculation circuit10for calculating the SA signals. As shown inFIG. 7, the calculation circuit10includes current/voltage conversion circuits (denoted by “I/V” in the figure)10ato10d, addition circuits10eto10g, and subtraction circuits10hto10k.

The current/voltage conversion circuits10ato10dare circuits for converting respective output currents (light-receiving currents) of the light-receiving regions O, P, Q, and R into voltage signals So, Sp, Sq, and Sr. The addition circuit10eadds signals So and Sq, and the addition circuit10fadds signals Sp and Sr. The subtraction circuit10hsubtracts a signal Sr from a signal Sq, and the subtraction circuit10isubtracts a signal Sp from So. The subtraction circuit10jsubtracts Sp+Sr outputted from the addition circuit10ffrom So+Sq outputted from the addition circuit10e. The subtraction circuit10ksubtracts So−Sp outputted from the subtraction circuit10ifrom Sq−Sr outputted from the subtraction circuit10h. The addition circuit10moutputs a SA signal, after adding the outputs of the subtraction circuits10jand10kthereto.

Since each of the current/voltage conversion circuits10ato10d, the addition circuits10eto10gand the subtraction circuits10hto10kare general analog circuits each mainly constituted of an operational amplifier, explanation of the details of these circuits is omitted here.

As described, the optical pickup device13includes the photoreceiver22including the light-receiving section PD8having four light-receiving regions O, P, Q and R; and the calculation circuit10. With this structure, the optical pickup device13is capable of calculating SA signals including Soff. In this way, the decrease of Sk−Sl is corrected by Soff even with the shifting of the objective lens6, and therefore the influence of the shifting of the objective lens6to the SA signals can be greatly reduced. Further, since the photoreceiver22requires only a single light-receiving section PD8for detecting SA signals, it has a simpler structure than the photoreceiver21of First Embodiment. On this account, compensation of offset of SA signals becomes possible without an increase in cost of the optical pickup device13.

Third Embodiment

Another embodiment of the present invention is described below with reference toFIG. 8andFIG. 9. Note that, only the difference from First Embodiment is described in the following embodiment. For ease of explanation, materials having the equivalent functions as those shown in the drawings pertaining to the foregoing First and Second Embodiments will be given the same reference symbols, and explanation thereof will be omitted here.

FIG. 8shows a structure of an optical pickup device14according to the present embodiment.FIG. 9shows a structure of a photoreceiver23of the optical pickup device14.

As shown inFIG. 8, as with the optical pickup device12of First Embodiment shown inFIG. 1, the optical pickup device14includes the semiconductor laser1, the beam splitter2, the polarization hologram3, the transmission grating4, the collimator lens5, and the objective lens6. The photoreceiver21is replaced with the photoreceiver22. The calculation circuit9is replaced with a calculation circuit11.

As shown inFIG. 9, the photoreceiver23includes the light-receiving sections PD1to PD5, as with the photoreceiver8of the optical pickup101shown inFIG. 12, but further includes a light-receiving section PD8. The light-receiving section PD8is provided to be adjacent to the light-receiving section PD5in the tangential direction with a predetermined interval. The light-receiving section PD8is also disposed to be adjacent to the light-receiving section PD2in the radial direction with a predetermined interval. More specifically, the light-receiving section PD8is disposed with its center in the tangential direction concentric to the center of the light-receiving section PD5in the tangential direction on a same straight line (shown by a broken line ofFIG. 9).

In such an optical pickup device14, detection of a spherical aberration is carried out using a sub-beam SB1and a main beam MB in the tangential direction generated in the transmission grating4, as shown inFIG. 13. The +1st order diffraction rays of the sub-beam SB1generated in the polarization hologram3are received by the four divided light-receiving regions O, P, Q and R shown inFIG. 7, and the +1st order diffraction ray of the main beam MB is received by the two-divided light-receiving regions K and L, so as to detect spherical aberration signals.

The spherical aberration detection method according to the present embodiment differs from the spherical aberration detection method using the layout pattern of the photoreceiver21according to First Embodiment only in the layout and pattern of the photoreceiver and the way of calculation in the detection of spherical aberration signals. Therefore, the following describes only the difference.

The light-receiving signal So of the spherical aberration detection method using the layout pattern of the photoreceiver21according to First Embodiment and the light-receiving signal So of the spherical aberration detection method using the layout pattern of the photoreceiver23according to the present embodiment are identical in shape of the light spot to be detected. The same can be said for the signal Sp. As to the signals Sq and Sr, there is a difference between the sub-beam SB1and the main-beam SB2, but the shape of light spot to be detected is identical. Therefore, the calculation formula for detection is the same as that of the spherical aberration detection according to First Embodiment using the layout pattern of the photoreceiver21, as shown below.

In this manner, the calculation of SA signals can be performed by a calculation circuit11having the same structure as that of the calculation circuit9. Therefore, the calculation circuit11is not shown in the figure.

In the formula, α is a coefficient. Since the light spots on the light-receiving regions O, P, Q and R are +1st order diffraction rays of the sub beam SB1generated in the polarization hologram3, the light intensity is small. Therefore, it is necessary to multiple the value by an appropriate coefficient for adjustment.

As described, the optical pickup device14includes the photoreceiver23including the light-receiving section PD5and the light-receiving section PD8having four light-receiving regions O, P, Q and R; and the calculation circuit11. With this structure, the optical pickup device14is capable of calculating SA signals including Soff found from the output of the light-receiving section PD8. In this way, the decrease of Sk−Sl is corrected by Soff even with the shifting of the objective lens6, and therefore the influence of the shifting of the objective lens6to the SA signals can be greatly reduced. Further, since the photoreceiver23requires only two single light-receiving sections PD5and PD8for detecting SA signals, it has a simpler structure than the photoreceiver21of First Embodiment. On this account, compensation of offset of SA signals becomes possible without an increase in cost of the optical pickup device14.

Summary of Embodiments

As described, a first spherical aberration detecting device according to the present invention comprises: a transmission grating4for dividing, in a tangential direction, a light beam emitted from a semiconductor laser1into a main beam, a first sub-beam and a second sub-beam; a collimator lens5and an objective lens6for focusing the three beams on an optical disk7; a polarization hologram3for diffracting a light beam reflected by the optical disk7; and a photoreceiver21for receiving a diffraction ray from the polarization hologram3, wherein: the photoreceiver21includes the light-receiving sections PD5to PD7adjacently aligned for respectively receiving +1st order diffraction rays of the main beam, the first sub-beam and the second sub-beam generated in the polarization hologram3, the light-receiving section PD5is divided into two regions by a border extending in a radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the main beam from the polarization hologram3, so as to detect a spherical aberration, the light-receiving section PD6is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the first sub-beam from the polarization hologram3, so as to detect a spherical aberration, the light-receiving section PD7is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the second sub-beam from the polarization hologram3, so as to detect a spherical aberration, and the light-receiving sections PD6and PD7are laid out such that an end of the light-receiving section PD6along the tangential direction and an opposite end of the light-receiving section PD7along the tangential direction are aligned on a straight line extending in the tangential direction across the light-receiving section PD5.

With this structure, the light-receiving section PD6is disposed out of alignment with respect to the light-receiving section PD5in the radial direction vertical to the tangential direction, and the light-receiving section PD7is disposed out of alignment with respect to the light-receiving section PD5in the direction opposite to the light-receiving section PD6(see the light-receiving sections PD5to PD7ofFIG. 1). In this structure, the light-receiving section PD6receives a half of the +1st order diffraction ray identical in shape to the +1st order diffraction ray received by the light-receiving section PD5, while the light-receiving section PD7receives the other half. In this case, because of the shifting of the objective lens, the difference between the light detection signals of the two light-receiving sections is not 0 but identical to an error component (offset-signals) of the light detection signal (spherical aberration detection signal) of the light-receiving section PD5. Therefore, with the provision of the light-receiving sections PD6and PD7laid out in the foregoing manner, it becomes possible to correct the spherical aberration detection signal based on the difference between the light detection signals of the light-receiving sections PD6and PD7. Consequently, it becomes possible to detect spherical aberration signals without influence of offset-signals generated by the shifting of the objective lens in the radial direction.

The first spherical aberration detecting device is preferably arranged so that the polarization hologram3includes a region3asurrounded by a straight line in the radial direction orthogonal to an optical axis of the light beam, a first semicircle, and a second semicircle which is concentric to a first semicircle and smaller in radius than the first semicircle, the region3aemitting the +1st order diffraction rays of the main beam, the first sub-beam and the second sub-beam. In this way, it becomes possible to detect a highly-accurate spherical aberration without influence of the shifting of the objective lens.

Further, the first spherical aberration detecting device preferably further comprises: a subtraction circuit9gfor calculating a difference between light detection signals outputted from the two light-receiving regions K and L constituting the light-receiving section PD5; a subtraction circuit9hfor calculating a difference between light detection signals outputted from the two light-receiving regions O and P constituting the light-receiving section PD6; a subtraction circuit9ifor calculating a difference between light detection signals outputted from the two light-receiving regions Q and R constituting the light-receiving section PD7; a subtraction circuit9jfor calculating a difference between an output of the subtraction circuit9hand an output of the subtraction circuit9i; and an addition circuit9mfor calculating a sum of an output of the subtraction circuit9gand an output of the subtraction circuit9j. With this structure, it becomes possible to calculate spherical aberration signals immune to offset-signals.

A second spherical aberration detecting device according to the present invention comprises: a polarization hologram3for diffracting a light beam reflected by an optical disk7; and a photoreceiver22for receiving a diffraction ray from the polarization hologram3, the photoreceiver22including a light-receiving section which is divided into four regions by a border extending in a radial direction and a border extending in a tangential direction.

With the foregoing structure in which the photoreceiver22includes a light-receiving section PD8divided into four regions, the light-receiving section PD8is constituted of a combination of the light-receiving sections PD5to PD7of the first spherical aberration detecting device. With this structure, the second spherical aberration detecting device is capable of correcting spherical aberration detection signals as with the first spherical aberration detecting device. Consequently, it becomes possible to detect spherical aberration signals without influence of offset-signals generated by the shifting of the objective lens in the radial direction. Further, since the photoreceiver22requires only a single light-receiving section PD8for a spherical aberration, it has a simple structure. On this account, compensation of spherical aberration detection signals becomes possible without an increase in cost of the spherical aberration detecting device.

Further, the second spherical aberration detecting device is preferably arranged so that the polarization hologram3includes a region3asurrounded by a straight line in the radial direction orthogonal to an optical axis of the light beam, a first semicircle, and a second semicircle which is concentric to a first semicircle and smaller in radius than the first semicircle, the region3aemitting the diffraction rays to the light-receiving section PD8. In this way, it becomes possible to detect a highly-accurate spherical aberration.

The second spherical aberration detecting device according to the present invention preferably further comprises: an addition circuit10efor calculating a sum of a light detection signal outputted from the light-receiving region O and a light detection signal outputted from the light-receiving region Q; an addition circuit10ffor calculating a sum of a light detection signal outputted from the light-receiving region P and a light detection signal outputted from the light-receiving region R; a subtraction circuit10hfor calculating a difference between the light detection signal outputted from the light-receiving region Q and the light detection signal outputted from the light-receiving region R; a subtraction circuit10ifor calculating a difference between the light detection signal outputted from the light-receiving region O and the light detection signal outputted from the light-receiving region P; a subtraction circuit10jfor calculating a difference between an output of the addition circuit10eand an output of the addition circuit10f; a subtraction circuit10kfor calculating a difference between an output of the subtraction circuit10hand an output of the subtraction circuit10i; and an addition circuit10gfor calculating a sum of the addition circuits10jand10k. With this structure, it becomes possible to calculate spherical aberration signals immune to offset-signals.

A third spherical aberration detecting device according to the present invention comprises:

a polarization hologram3for diffracting a light beam reflected by an optical disk7; and a photoreceiver23for receiving a diffraction ray from the polarization hologram3, wherein: the photoreceiver23includes light-receiving sections PD5and PD8adjacently aligned in a tangential direction for respectively receiving a main beam and a sub-beam, the light-receiving section PD5is divided into two regions by a border extending in the radial direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the main beam from the polarization hologram3, and the light-receiving section PD8is divided into four regions by a border extending in the radial direction and a border extending in the tangential direction, and is disposed in a portion where it can receive a +1st order diffraction ray of the sub-beam from the polarization hologram3.

With the foregoing structure in which the photoreceiver23includes a light-receiving section PD8divided into four regions, the light-receiving section PD8is constituted of a combination of the light-receiving sections PD6and PD7of the first spherical aberration detecting device. With this structure, the third spherical aberration detecting device is capable of correcting spherical aberration detection signals as with the first spherical aberration detecting device. Consequently, it becomes possible to detect spherical aberration signals without influence of offset-signals generated by the shifting of the objective lens in the radial direction. Further, since the photoreceiver23requires only two light-receiving sections PD5and PD8for detecting a spherical aberration, it has a simple structure. On this account, compensation of spherical aberration detection signals becomes possible without an increase in cost of the spherical aberration detecting device.

Further, the third spherical aberration detecting device is preferably arranged so that the polarization hologram3includes a region3asurrounded by a straight line in the radial direction orthogonal to an optical axis of the light beam, a first semicircle, and a second semicircle which is concentric to a first semicircle and smaller in radius than the first semicircle, the region3aemitting the +1st order diffraction rays of the main beam, the sub-beams. In this way, it becomes possible to detect a highly-accurate spherical aberration.

Further, the third spherical aberration detecting device preferably further comprises: a subtraction circuit9gfor calculating a difference between light detection signals outputted from the two light-receiving regions K and L constituting the light-receiving section PD5; a subtraction circuit9hfor calculating a difference between light detection signals outputted from the light-receiving regions O and P; a subtraction circuit9ifor calculating a difference between light detection signals outputted from the light-receiving regions Q and R, a subtraction circuit9jfor calculating a difference between an output of the subtraction circuit9hand an output of the subtraction circuit9i; and an addition circuit9mfor calculating a sum of an output of the subtraction circuit9gand an output of the subtraction circuit9i. With this structure, it becomes possible to calculate spherical aberration signals immune to offset-signals.

Each of the optical pickup devices12to14according to the present embodiment includes one of the spherical aberration detecting devices, and therefore obtains spherical aberration detection signals having been corrected to compensate offset-signals caused by the shifting of the objective lens. On this account, it is possible to accurately correct information signals read from the optical disk7using the spherical aberration detection signals.