KR100657250B1 - Optical pickup apparatus - Google Patents

Abstract

Disclosed is an optical pickup apparatus for forming an optical disc with a main optical spot and an optical aberration having an optical aberration in a state where the optical disc is normally arranged, and correcting aberration by calculating a primary light and an auxiliary light received by the optical detector. have.

The disclosed optical pickup apparatus includes a light source for irradiating light; Optical path converting means for converting the advancing path of the incident light; An objective lens for focusing incident light to form an optical spot on the optical disk; By splitting the incident light into at least two lights including the first light and the second light so that the optical disk has at least two light spots including the main light spot and the sub light spot, the optical axes of the first light and the second light travel at an angle. Optical branch means for allowing; The first light and the second light is reflected by the optical disk and received through the optical path changing means. The first light is divided into a central part and a peripheral part, and receives a plurality of light receiving areas that independently photoelectrically convert the received light. A photodetector having a first light receiver having a first light receiver, and a second light receiver having a second light receiver divided into a central part and a peripheral part, and having a plurality of light receiver regions for photoelectric conversion independently of the received light; And a signal processing unit for correcting the aberration due to the inclination of the optical disk and the degraded signal due to crosstalk from the signal detected and photoelectrically converted by the first light receiving unit and the second light receiving unit.

Description

Optical pickup apparatus

1 is a schematic view showing a conventional optical disk aberration correction apparatus.

2 is a schematic view showing an optical arrangement of the optical pickup device according to an embodiment of the present invention.

3 is a schematic view showing a main light spot SP _m and a sub light spot SP _s formed in an optical disc.

4 is a view showing the shapes of the main light spot SP _m and the sub light spot SP _s when there is no inclination of the optical disc.

5 is a view showing the shapes of the main light spot SP _m and the sub light spot SP _s when the optical disc is disposed at an angle of 0.5 degrees.

6 is a schematic cross-sectional view of the hologram element of FIG.

7 is a schematic plan view of the hologram element of FIG.

8 is a schematic view illustrating the signal processor of FIG. 2.

9 is a schematic view illustrating another embodiment of the first light receiver of FIG. 2.

Fig. 10 is a graph showing the amount of correction jitter according to the values of the gains α, β, and γ when the optical disk has a 0.25 degree inclination.

FIG. 11 shows an example in which the secondary light spots SP _s are inclined with respect to both tangential and radial directions. FIG.

12 is a schematic view showing an optical arrangement of an optical pickup apparatus according to another embodiment of the present invention.

Fig. 13 is a graph for explaining the radial tilt correction effect of the optical disc.

Fig. 14 is a graph for explaining the tangential tilt correction effect of the optical disk;

<Explanation of symbols for the main parts of the drawings>

1 ... optical disk 11,51 ... light source 15 ... hologram element

17,65 ... polarized beam splitter 21,61 ... objective lens 25,70 ... photodetector

26, 71 First light-receiving unit 27, 73 Second-light-receiving unit 30, 80 Signal processing unit

53 ... polarization hologram element 59 ... beam splitter

According to the present invention, the aberration characteristics such as tilt and defocus can be improved by forming a main light spot and a second light spot on an optical disc and calculating a main light and an auxiliary light received by the photodetector. An optical pickup device for recording and reproducing high density using two light spots.

In general, the optical pickup apparatus is capable of recording information on the optical disc or reproducing the recorded information. As the optical disc becomes dense, the light irradiated from the light source is shortened and the numerical aperture NA of the light increases to a high number of apertures. Therefore, when recording / reproducing information on an optical disc through this optical pickup apparatus, when the optical disc is disposed at an inclination, that is, when the information surface of the optical disc is disposed at an inclination rather than being positioned in a direction perpendicular to the optical axis, There is a problem in that coma aberration occurs. In addition, there is a problem that other spherical aberration occurs in the optical disk thickness change and the wavelength fluctuation accompanying the short wavelength of the light source and the high aperture of the objective lens due to the high density of the optical disk, and the compatibility with the low density optical disk.

Here, since the coma aberration W ₃₁ has a proportional relationship as shown in Equation 1, the coma aberration due to the optical disk inclination increases rapidly in the optical pickup with a high numerical aperture compared to the relatively low numerical aperture for the same optical disk inclination.

과 대물렌즈의 개구수 NA에 의하여 수학식 2와 같이 결정된다. In addition, the recording capacity of the optical disk is the wavelength of light irradiated from the light source.

The numerical aperture NA of the objective lens is determined as in Equation 2.

에 의해 발생하는 구면수차 W_40d는 수학식 3과 같은 관계를 가진다. Also, the thickness error of the optical disc

The spherical aberration W _40d generated by _에 has the same relationship as in Equation 3.

Where n is the refractive index of the substrate of the optical disc and d is the thickness of the substrate.

에 의하여 발생하는 구면수차 W_40d는 NA의 4승에 비례하므로 급격히 커지는 문제점이 있다. Therefore, in order to increase the recording density of the optical disk to approximately 15 GB or more, based on Equation 2, the light source for irradiating short wavelength light of approximately 410 nm region and the objective lens of NA 0.6 or more are essential. . On the other hand, when the numerical aperture of the objective lens is increased to increase the recording density of the optical disc in this way, the thickness error of the optical disc

Since the spherical aberration W _40d generated by is proportional to the fourth power of NA, there is a problem that increases rapidly.

Referring to FIG. 1, the conventional optical disk aberration correcting apparatus for correcting coma and spherical aberration as described above reconstructs the objective lens 3 for focusing incident light and the light focused by the objective lens 3 again. And a focusing lens 5 for focusing and converging on the optical disk 1.

Therefore, when the optical disc 1 is inclined in one direction, the focusing lens 5 is inclined in one direction to correct coma aberration.

The conventional optical disk aberration correcting apparatus configured as described above requires the objective lens and the focusing lens to be driven in the direction for tracking and focusing the optical spot and the tilting lens of the focusing lens, which makes the configuration of the actuator very complicated. There is this.

In addition, as the recording density of the optical disc is increased to 15 GB or more, there is a problem that large crosstalk occurs due to the influence of adjacent tracks when the information on one track is reproduced.

Accordingly, the present invention has been made in view of the above-described problems, so that at least two optical spots are formed on the optical disc, thereby improving the coma aberration correction and defocus characteristic due to the optical disc inclination and the photodetector. It is an object of the present invention to provide an optical pickup apparatus capable of reducing crosstalk by improving the structure of the present invention.

A compatible optical pickup device according to the present invention for achieving the above object, and a light source for irradiating light; Optical path converting means for converting the advancing path of the incident light; An objective lens for focusing incident light to form an optical spot on the optical disk; By splitting the incident light into at least two lights including a first light and a second light so that at least two light spots including a primary light spot and a secondary light spot are formed on the optical disc, the optical axes of the first and second light beams Optical branch means for proceeding at an angle; A plurality of light receivers for receiving the first light and the second light reflected from the optical disk and incident through the optical path converting means, receiving the first light into a central part and a peripheral part, and independently photoelectrically converting the received light. A photodetector having a first light receiving portion having an area, and a second light receiving portion having a plurality of light receiving regions for receiving and dividing the second light into a central portion and a peripheral portion and independently photoelectrically converting the received light; And a signal processor for correcting the aberration caused by the inclination of the optical disk and the signal degraded by the crosstalk from the signal detected and photoelectrically converted by the first light receiver and the second light receiver.

Hereinafter, an optical pickup apparatus according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings.

2, an optical pickup apparatus according to an exemplary embodiment of the present invention includes a light source 11, optical path converting means for converting a traveling path of incident light, an objective lens 21 for focusing incident light, and an optical disc 1. Optical splitter means for branching incident light such that at least two light spots are formed at the same time), an optical detector 25 for receiving the light reflected from the optical disk 1, and a signal processor for correcting the thickness deviation of the optical disk 1). 20 is configured to include.

Here, the light source 11 and the objective lens 21 is a light source for irradiating light having a short wavelength of approximately 410nm region and an objective lens of NA 0.6 or more in order to increase the recording density of the optical disk 1 to approximately 15GB or more. do.

The divergent light irradiated from the light source 11 becomes parallel light while focusing while passing through the collimating lens 13. The light irradiated from the light source 11 is split into at least two lights including first and second light (I) and (II) through the light branching means, and as shown in FIG. SP _m ) and the minor light spot SP _s are formed. Here, when the optical disc 1 is disposed without inclination, the first light I is incident on the optical disc 1 with its optical axis vertical, while the second light II is at an angle to the optical disc ( Incident on 1). Therefore, when there is no inclination of the optical disc 1, as shown in FIG. 4, the first light I forms the main light spot SP _m without aberration, and the second light II has a predetermined coma aberration. The secondary light spot SP _s having is formed.

Meanwhile, referring to FIG. 5, when the optical disc is disposed at an angle, the main light spot is distorted, and the minor light spot SP _s increases or decreases the amount of distortion depending on the tilting direction of the disc. The distortion of the Figure 5 illustrates a case in which is disposed inclined is also 0.5, part a light spot (SP _s) of the same direction as the coma aberration for the case dark optical disc 1 inclined portion optical spot (SP _s) added to the Increased results are shown. On the other hand, when the optical disc 1 is inclined in the opposite direction, the amount of coma aberration of the secondary light spot SP _s decreases.

As described above, the coma aberration according to the degree of inclination of the optical disk can be corrected by the signal processing unit described later by the relation between the main light spot and the sub light spot SP _s formed.

As described above, in order for the main light spot SP _m and the sub light spot SP _s to be formed, the optical splitter means has the main light spot SP _m in a state where the optical disc 1 is disposed without tilting. ) Splits the incident light into the first light (I), which is the zero-order diffraction light, and the second light (II), which is the first-order diffracted light, so that the light becomes an aberration-free light and the secondary light spot SP _s is a light having coma aberration. In addition, a hologram element for generating a predetermined coma aberration with respect to the second light (II) so that the first light (I) is an aberration light and the second light (II) is a light having a predetermined amount of coma aberration. It is preferable to have 15). The optical axis of the second light II travels obliquely with respect to the optical axis of the first light by coma aberration so that the secondary light spot SP _{s is} formed on the optical disk at a predetermined slope.

The hologram element 15 is a substrate in which the hologram pattern is formed of an isotropic material, and has a hologram pattern 15a as shown in FIGS. 6 and 7.

6 and 7, the hologram pattern 15a is formed in a plurality of distorted stripe shapes in which a gap is widened toward a center through which the first optical axis I passes, and a gap is narrowed toward an outside.

The optical path changing means is provided on the optical path between the hologram element 15 and the objective lens 21, and converts the traveling path of the incident light. That is, the light incident from the light source 11 is directed toward the objective lens 21, and the light incident from the objective lens 21 is directed toward the photodetector 25. The optical path converting means preferably includes a beam splitter 17 for dividing the incident light into a predetermined light quantity ratio to transmit or reflect the light to change the path of the light.

The objective lens 21 focuses each of the first and second light beams I and II branched through the hologram element 15 to be formed at the same track position of the optical disc 1. Here, the second light (II) is formed on the same track as the track where the first light (I) is focused.

Each of the first and second light beams I and II reflected from the optical disk 1 passes through the objective lens 21 and the beam splitter 17 and is then focused by the condenser lens 23. The photodetector 25 receives light.

The photodetector 25 includes first and second light receiving portions 26 and 27 for receiving each of the first and second light (I) and (II).

The first light receiver 26 receives the first light by dividing the first light into a central part and a peripheral part, and has a plurality of light receiving areas for photoelectric conversion of the received light independently. The second light receiving unit 27 divides the second light into a central portion and a peripheral portion to receive the second light, and has a plurality of light receiving regions for photoelectric conversion of the received light independently.

Referring to FIG. 8, the first light receiving unit 26 may include a first light receiving region 26a for detecting a center signal of the first light I and a direction corresponding to a radial direction of the optical disc 1. It is preferable to include the second and third light receiving areas 26b and 26c provided on both sides of the first light receiving area 26a to detect peripheral signals of the first light I. Here, the peripheral signal of the first light I refers to the peripheral signal in a direction corresponding to the radial direction of the optical disk 1.

The second light receiving unit 27 may include a fourth light receiving region 27a for detecting a center signal of the second light II and the fourth light receiving region in a direction corresponding to the radial direction of the optical disc 1. And fifth and sixth light receiving regions 27b and 27c provided on both sides of the 27a to detect peripheral signals of the second light II.

As described above, the electrical signals detected through the divided first and second light receiving units 26 and 27 are processed by the signal processing unit 30 to be described later to correct the aberration due to the inclination of the optical disc and the deteriorated signal due to crosstalk. do.

Here, the first light receiving unit may be divided as shown in FIG. 9 in order to detect the track error signal. Referring to FIG. 9, the first light receiving portion 26 ′ is divided into eight divided regions a ₁ , .. a ₈ having a 4 × 2 array. That is, it is divided into four in the direction corresponding to the radial direction of the optical disc 1, and is divided into two in the direction corresponding to the tangential direction of the optical disc 1. Here, the four-divided areas a ₂ , a ₃ , a ₆ , a ₇ in the center part receive the central light of the incident first light I, and the remaining four-divided areas a ₁ , a ₄ , a ₅ , a ₈ ) receives the peripheral light of the first light I.

When the first light receiving unit 26 'is to detect a track error signal, to perform the same function as a conventional light detector with a substantially four slices, and slices a ₁ + a _2, a ₃ + a ₄ , a ₅ + a ₆ , a ₇ + a _{8 The} track error signal is detected by dividing into four areas. On the other hand, if the reduction of crosstalk and characters are the signals from the pane _{a 1 + a 4, a 2} + a 6 + a 3 + a 7, a 4 + a signal processing unit 30 to be described later divided into _eight three areas Process.

Referring to FIG. 8, the signal processing unit 30 performs optical aberration and crosstalk due to the inclination of the optical disc 1 from signals detected and photoelectrically converted through the first light receiving unit 26 and the second light receiving unit 27, respectively. Calibrate

The signal processing unit 30 calculates a coma aberration according to the change of the inclination of the optical disk by calculating a reproduction signal by the following equation (4).

Here, S _m is a main reproducing signal by the main light spot received and photoelectrically converted by the first light receiving unit 26, and S _sub is a _sub -light spot received and photoelectrically converted by the second light receiving unit 27. Is a negative reproduction signal, and α represents a gain factor.

Here, the main reproduction signal S _m detected by the first light receiving unit 26 is the central signal value of the first light I detected through the first light receiving region 26a and the second and third light receiving regions 26b. It is a sum signal obtained by multiplying a peripheral signal of the first light I detected through 26c by a gain factor β that is adjusted to minimize crosstalk by adjacent tracks of the optical disc 1.

The _sub reproduction signal S _sub detected by the second light receiving unit 27 is the central signal value of the second light II detected through the fourth light receiving area 27a, and the fourth and sixth light receiving areas 27b and 27c. Is a sum signal obtained by multiplying a peripheral signal of the second light II through a gain factor γ which is adjusted to minimize crosstalk by an adjacent track of the optical disc 1.

As described above, the first and second light receiving units 26 and 27 divide the center signal and the peripheral signal to detect a signal, and give the peripheral signal a predetermined gain β or γ so that the adjacent track of the narrow track optical disc 1 Can reduce the influence of crosstalk.

The delay shown in FIG. 8 is a signal having an earlier phase so as to match a phase between two signals when a phase difference between signals received through the first and second light receivers 26 and 27 occurs. Delays the phase. Therefore, the delay may be eliminated when there is no phase delay between the two signals.

The reproduction signal correction operation by the signal processing unit 30 described above will be described in detail with reference to FIG. FIG. 9 is a graph showing the amount of jitter corrected according to the gain a value when the optical disk has a 0.25 degree tilt. The graph shows an example of the case where the numerical aperture NA of the objective lens is 0.6, the wavelength of the light source is 400 nm, the track pitch is 0.37 μm, the minimum mark length is 0.25 μm, and the modulation code EFM + is used. In addition, both gain ratios (beta) and (gamma) were set to 2.0.

Referring to FIG. 9, when comparing the case of setting α = 0 and setting the gains β and γ to 0 and 2.0, the jitter amount is 15.5% and the jitter amount is about 5%. It can be seen that it is corrected. In addition, it can be seen that when the gain ratio α is optimized (α = 2.0), the jitter amount before correction is improved from 13.5% to 8.8% after correction. On the other hand, the gain ratio? = 0.5 is set even when crosstalk correction is performed only on light received on the first light receiving unit (β = 2.0, γ = 1.0) without performing crosstalk correction on the light received on the second light receiving unit. In one case, the jitter amount is improved to 10.2% compared to before the correction.

The result of FIG. 10 shows an example in which the optical disc is inclined in the tangential direction. Compensation for the case where the optical disc is inclined in the radial direction is possible by determining the pattern of the hologram element so as to have radial coma aberration in the secondary optical pot.

In addition, referring to FIG. 11, when the secondary light spot SP _s is disposed to be inclined with respect to both the tangential direction and the radial direction, coma aberration according to the radial and tangential inclination of the optical disc 1 may be simultaneously corrected. Can be.

Referring to FIG. 12, an optical pickup apparatus according to another exemplary embodiment of the present invention includes a light source 51, optical path changing means for converting a traveling path of incident light, an objective lens 61 for focusing incident light, and an optical disc 1. Optical branch means for branching incident light such that at least two light spots form at the same time), a photodetector 70 for receiving light reflected from the optical disk 1, and a tilt deviation of the optical disk 1 It is configured to include a signal processor 80. Here, since the light source 51 and the objective lens 61 are substantially the same as those shown in FIG. 3, detailed description thereof will be omitted.

The divergent light irradiated from the light source 51 is split into at least two lights including first and second lights I 'and II' through the optical branching means. Here, the first and second lights I 'and II' are focused on the optical disk 1 via the objective lens 61, and the first light I 'is used to detect the main light spot of the aberration. And the second light II 'forms a secondary light spot having a predetermined coma aberration.

To this end, the first light I 'is made to be an unpolarized light of one polarization by the optical branching means, and the second light II' is made to be light having a spherical aberration of another polarized light. It is preferable to include a polarization hologram element 57 for generating a predetermined coma aberration for the? The hologram pattern formed on the polarization hologram element 57 is substantially the same as the pattern of the hologram element 15 described with reference to FIGS. 5 and 6. The polarization hologram element 57 is a substrate in which a hologram pattern is formed of an anisotropic material, and each of the branched first and second lights I 'and II' has polarization characteristics.

The optical path converting means is provided on the optical path between the polarization hologram element 57 and the objective lens 61, and converts the traveling path of the incident light. The optical path changing means is provided on a beam splitter 59 for transmitting or reflecting incident light at a predetermined light quantity ratio and converting a path of light, and on an optical path between the beam splitter 59 and the photodetector 70. It includes a polarizing beam splitter 65 that is reflected at and transmitted or reflected light incident through the beam splitter 59 according to the polarization. Here, a condenser lens 63 for condensing incident light may be further provided on the optical path between the beam splitter 59 and the polarization beam splitter 65.

Each of the first and second light receivers 71 and 73 of the photodetector 70 may receive the first and second light beams I ′ and II ′ branched from the polarization beam splitter 65. Are arranged to be. Accordingly, the first light I ′ of one polarization branched from the polarization beam splitter 65 is received by the first light receiving unit 71, and the second light II ′ of another polarization is second light receiving unit 73. Is received. Here, the first and second light receivers 71 and 73 are divided into a plurality of divided regions, like the two light receivers 26 and 27 according to an exemplary embodiment. Since the signal processing and crosstalk correction operation of the light received in each divided area are substantially the same as described above, detailed description thereof will be omitted.

The signal processing unit 80 corrects aberrations due to the inclination of the optical disc 1 from signals detected and photoelectrically converted through the first light receiving unit 71 and the second light receiving unit 73, respectively. Since the signal processing unit 80 is substantially the same as described in the embodiment, detailed description thereof will be omitted.

As described above, the effects of crosstalk correction and aberration correction when the optical pickup device is configured will be described in detail with reference to FIGS. 13 and 14.

12 and 13 are graphs for explaining the jitter amount improvement effect when the tangential and radial inclinations of the optical disc occur. 12 and 13, reference numeral A shows the result when only the main light spot is used. Reference numeral B denotes the aberration of the optical disc using the sub-light spot, and shows the result when the signal processing unit does not apply the crosstalk correction operation. Further, reference numeral C denotes correction of aberration of the optical disc by using the sub-light spot, crosstalk correction operation is applied to the signal received on the first light receiving unit, and crosstalk correction association on the signal received on the second light receiving unit. It shows the result when not applied. Reference numeral D denotes the aberration of the optical disk using the sub-light spot, and shows the result when the crosstalk correction operation is applied to both the signals received by the first and second light receiving units.

Referring to FIGS. 13 and 14, as indicated by reference numeral D, aberration correction of the optical disc is performed using two light spots, and the first and second light receiving units are divided into a plurality of divided regions to detect the center of the detected signal. When the signal is divided into a peripheral signal and a predetermined gain is given to the peripheral signal, it can be seen that the jitter characteristic according to the tangential direction and the radial inclination of the optical disc is greatly improved.

As described above, the primary light spot and the secondary light spot are formed on the optical disc by using the first light having the aberration and the second light having the aberration, and the light received at each of the first and second light receiving parts of the photodetector. Detects the playback signal through an operation as shown in Equation 4 and performs crosstalk correction by dividing the first and second light receiving sections into a plurality of light receiving regions, thereby causing coma aberration caused by the tilt of the optical disk and the thickness of the optical disk. Spherical aberration correction and crosstalk due to the deviation can be reduced, so that the jitter characteristic of the reproduced signal can be significantly improved.

Claims (8)

delete delete A light source for irradiating light; Optical path converting means for converting the advancing path of the incident light; An objective lens for focusing incident light to form an optical spot on the optical disk; By splitting the incident light into at least two lights including a first light and a second light such that at least two light spots including a primary light spot and a secondary light spot are formed on the optical disc, any one of the first and second light beams may be formed. One is an optical branch means for converting the light with optical aberration; A plurality of light receivers for receiving the first light and the second light reflected from the optical disk and incident through the optical path converting means, receiving the first light into a central part and a peripheral part, and independently photoelectrically converting the received light. A photodetector having a first light receiving portion having an area, and a second light receiving portion having a plurality of light receiving regions for receiving and dividing the second light into a central portion and a peripheral portion and independently photoelectrically converting the received light; And a signal processor for correcting aberrations caused by tilts of the optical disk, aberrations caused by optical disk thickness changes, and deteriorated signals due to crosstalk from signals detected and photoelectrically converted by the first and second light receivers. S _m for the main reproduction signal by the main light spot received and photoelectrically converted by the first light receiving unit S _sub , and a sub reproduction signal by the sub light spot received and photoelectrically converted by the second light receiving unit S _sub , When the factor) is α, the signal processing unit calculates a reproduction signal according to the following equation so that the aberration according to the change of the inclination of the optical disc can be corrected. The main reproduction signal S _m detected by the first light receiving unit is crosstalk caused by an adjacent track of the optical disk to a peripheral signal of the first light detected through the light receiving region different from the central signal value of the first light detected through the one light receiving region. Is a sum signal of a value multiplied by a constant β adjusted to be a minimum. Equation

The method of claim 3, wherein the first light receiving unit, A first light receiving area for detecting a center signal of the first light; And second and third light receiving areas provided at both sides of the first light receiving area in a direction corresponding to a radial direction of the optical disc to detect peripheral signals of the first light. A light source for irradiating light; Optical path converting means for converting the advancing path of the incident light; An objective lens for focusing incident light to form an optical spot on the optical disk; By splitting the incident light into at least two lights including a first light and a second light such that at least two light spots including a primary light spot and a secondary light spot are formed on the optical disc, any one of the first and second light beams may be formed. One is an optical branch means for converting the light with optical aberration; A plurality of light receivers for receiving the first light and the second light reflected from the optical disk and incident through the optical path converting means, receiving the first light into a central part and a peripheral part, and independently photoelectrically converting the received light. A photodetector having a first light receiving portion having an area, and a second light receiving portion having a plurality of light receiving regions for receiving and dividing the second light into a central portion and a peripheral portion and independently photoelectrically converting the received light; And a signal processor for correcting aberrations caused by tilts of the optical disk, aberrations caused by optical disk thickness changes, and deteriorated signals due to crosstalk from signals detected and photoelectrically converted by the first and second light receivers. S _m for the main reproduction signal by the main light spot received and photoelectrically converted by the first light receiving unit S _sub , and a sub reproduction signal by the sub light spot received and photoelectrically converted by the second light receiving unit S _sub , When the factor) is α, the signal processing unit calculates a reproduction signal according to the following equation so that the aberration according to the change of the inclination of the optical disc can be corrected. The _sub reproduction signal S _sub detected by the second light receiving unit cross-talks by adjacent tracks of the optical disc to peripheral signals of the second light detected through the light receiving region different from the central signal value of the second light detected through the one light receiving region. And a sum signal of a value multiplied by a constant γ adjusted to be a minimum. Equation

The method of claim 5, wherein the second light receiving unit, A fourth light receiving area for detecting a center signal of the second light; And fifth and sixth light-receiving areas provided at both sides of the fourth light-receiving area in a direction corresponding to the radial direction of the optical disc to detect peripheral signals of the second light. The optical branch means according to any one of claims 3 to 6, And a hologram element for generating a predetermined coma aberration for the second light such that the main light spot becomes light without aberration and the sub light spot becomes light with coma aberration in a normal state of the optical disk. Optical pickup device. The optical branch means according to any one of claims 3 to 6, A polarization hologram element for generating a predetermined coma aberration for the second light such that the first light is an unpolarized light of one polarization and the second light is a light having a coma aberration of another polarization, The optical path converting means includes a beam splitter for converting a path of light by transmitting or reflecting incident light at a predetermined light quantity ratio, and an optical beam provided on the optical path between the beam splitter and the photodetector and reflected from the optical disk and incident through the beam splitter. It includes a polarizing beam splitter for transmitting or reflecting according to the polarization, And each of the first and second light receiving units of the photodetector is arranged to receive each of the first and second light beams branched from the polarization beam splitter.

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