KR20080017690A - Optical pick-up - Google Patents

Abstract

An optical pick-up is provided to remove interlayer noise in a multi-layer disk and to remove influence of a dead zone by diffracting some of the projected beams through first and second diffraction devices. An optical pick-up comprises a light source(110), an objective lens(170), a beam splitter(130), first and second diffraction devices, a sensor lens(180), and a beam detecting unit(190). The first diffraction device generates the second beam from the beam reflected from a beam storage medium. The beam detecting unit includes a first detecting unit for receiving the first beam, and two sub detecting units for receiving the second beam. The second diffraction device diffracts some of the beam reflected by another layer to prevent the beam from entering the beam detecting unit. The first and second diffraction devices include first and second areas for diffracting some of the projected beam.

Description

Optical pick-up {Optical pick-up}

1 illustrates the principle of detecting a tracking error signal by the DPP method,

2 shows a push-pull signal generated by 3 beam DPP,

FIG. 3 shows an example in which one objective lens is stockpiled away from the center axis of the disc when two objective lenses mounted on one actuator are arranged in the track direction,

4 illustrates a state in which noise light reflected from another layer flows into a photo detector,

5 illustrates an example in which a tracking error signal is distorted by interlayer interference in a disc having two or more recording layers.

FIG. 6 illustrates a phenomenon in which the offset of the DPP signal is increased and the interval in which the offset occurs at the record / unrecorded boundary is increased when the objective lens is non-stocked and the 3 beam DPP method is applied.

7 is a view for briefly comparing the 3 bema DPP method and the 1 beam DPP method,

8 illustrates an embodiment of an optical pickup to which 1 beam DPP is applied;

9 shows a form in which a beam incident on a recording disk is branched,

FIG. 10 exemplarily illustrates a baseball pattern formed by branching beams incident on a recording disc;

11 schematically illustrates the principle of obtaining a sub beam from which an ac component is removed from a beam reflected and branched from a disk,

FIG. 12 illustrates various embodiments of a diffraction grating for obtaining a sub beam from which an ac component has been removed.

FIG. 13 illustrates the result of removing only the dc component and the ac component from the push-pull signal of the sub-beam by the diffraction grating of FIG. 12.

Fig. 14 shows an example where a beam reflected from another layer enters the PDIC when recording or playing back the current layer of two or more optical discs,

FIG. 15 illustrates DOE shapes and positions when a general 3-beam DPP is used, and distribution of beams in a PD.

FIG. 16 illustrates various forms of DOE for removing interlayer interference when using a general 3 beam DPP.

FIG. 17 shows DOE shape and position when using 1 beam DPP, and beam distribution in PD.

18 illustrates a change in the offset of the tracking error signal according to the radial shift of the objective lens when the sub beam is positioned at the center of the subcell in the state where the objective lens is not radial shifted in 1 beam DPP.

19 illustrates a change in the offset of the tracking error signal according to the radial shift of the objective lens when the sub beam is not located at the center of the subcell even when the objective lens is not radially shifted in 1 beam DPP.

20 illustrates an embodiment in which the position of a sub beam in a PDIC is changed by a combination of grating for 1 beam DPP and DOE for interlayer interference cancellation.

FIG. 21 illustrates the direction of movement of a subbeam in a subcell in each embodiment in which the position of the subbeam in the PDIC is changed by a combination of grating for 1 beam DPP and DOE for interlayer interference cancellation,

22 illustrates various shapes of a DOE and a diffraction grating for 1 beam DPP for interlayer interference cancellation in a single plane;

FIG. 23 shows an example of implementation of a grating direction, a diffraction direction, and a blazed type HOE of the diffraction grating.

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

100, 200, 300, 400: optical pickup 110, 210, 310, 410: light source (LD)

120, 220, 320, 420: collimator lens 225: grating for 3 beam DPP

130, 230, 330, 430: beam splitter 340, 440: 1 beam grating for DPP

250, 350, 450: Diffraction optical element (DOE) 360: 1/4 wave plate (QWP)

170, 260, 370, 470: objective lens (OL) 180, 280, 380, 480: sensor lens

190, 290, 390, 490: light detecting means (PD)

TECHNICAL FIELD The present invention relates to an optical pickup, and more particularly, to an optical pickup that can suppress noise from entering a servo signal from another layer.

Since the launch of the Compact Disc (CD), an optical storage medium capable of storing 74 minutes of music and approximately 650 Mbytes of data, a DVD (Digital Versatile) that can hold 2 hours of standard definition (SD) movies Disc is widely commercialized, and a BD (Blu-ray Disc) or HD-DVD, which can store a high definition (HD) class movie, will soon appear in the market.

Optical storage media such as CD, DVD, BD, and the like are disc-type media for storing data using optical characteristics, and record data on or reproduce recorded data on the disc through optical pickup. On an optical storage medium, recording discs for playback in which data is already stored, and CD-R / RW, DVD-R / + R / -RW / + RW / RAM, BD-R / -RE, etc. that can be recorded or rewritten There is a disk.

The optical pickup writes data to or reads data from the disk, with the laser beam correctly focused on tracks in the disk rotating at high speed.

It is possible to obtain a servo signal such as a focusing error signal and a tracking error signal corresponding to a position error of an optical spot formed on the disk, and to correct a position error of the optical spot based on the servo signal, that is, to perform a servo operation. The optical pickup includes an optical lens such as an objective lens, a beam splitter, an instrument part such as an actuator, a base, a laser diode, a photo detector, and the like. And electrical components are arranged.

In general, astigmatism is used to detect a focusing error signal irrespective of the type of disc and the disc for recording / playback. Regarding the detection of the tracking error signal, a 3-beam method or a differential phase detection (DPD) method is used for a disc for playback, and a differential push-pull (DPP) method is used for a disc for recording. It is typically used.

1 illustrates the principle of detecting a tracking error signal by the DPP method.

The DPP method is an improvement on the conventional one-beam push-pull method. The DPP method can detect a stable tracking error signal by eliminating offset caused by movement in the radial direction of the objective lens or tilt of the disk. That's how it is.

In the DPP method, a laser beam output from a light source through a diffractive element called grating is divided into three, such as 0th-order diffraction light and +/- 1st diffraction light, and the main beam, which is the 0th-order diffraction light, is formed in the groove of the disc track. When placed in a groove, the sub-beams of the +/- 1st diffraction light are arranged in a land adjacent to the groove in which the main beam is placed (the sub beam is placed 1/2 track pitch away from the main beam). The grating is adjusted, and a tracking error signal is detected based on the difference signal on the left and right of each beam with respect to the radial direction.

The main beam reflected from the disk is received by a 4-divided (a, b, c, d) main photodetector to the push-pull signal, Main Push-Pull (MPA) ((A + D)-(B + C)). Each sub-beam detected and reflected from the disc is received by a sub-photo detector divided into two (E1, E2) (F1, F2) in the radial direction, so that the SPP (Sub Push-Pull) ((E1-E2) + (F1-) F2)) is detected as a signal. When the main beam and the sub beam are arranged apart by a half track pitch, the phases of the MPP and the SPP are reversed as shown in FIG.

Since both the MPP and the SPP generate offset components in the same direction as the tilt or the objective lens moves in the radial direction, a push-pull signal in which the offset component is removed when calculated according to DPP = MPP-k ㅧ SPP (k is a proportional constant) You get In addition, by subtracting the SPP that is out of phase from the MPP, a larger amplitude push-pull signal can be obtained.

In general, in the DPP method, the ratio of light quantity between the main beam used for the MPP signal and the sub beam used for the SPP signal is about 1: 5: 1 to 1: 20: 1, so that the sub beam is 1/5 to 1 / Make it as small as 20 In the equation for DPP, the offset is adjusted by adjusting the proportional constant k (for example, 5 when the light ratio is 1: 10: 1).

As described above, in order to use the DPP method, it is necessary to adjust the angle of the sub-beam, which is +/- 1 order diffracted light while rotating the grating. However, the characteristics of the signal are affected by the degree of adjustment.

That is, if the main beam is in the groove (or land), the sub-beam should be in the land (or groove). Since the track pitch may be different for each type of disc, the sub beam cannot be applied simultaneously to discs having different track pitches.

With the advent of BD or HD-DVD, there is an increasing demand for an optical disc recording / reproducing apparatus capable of reproducing or recording all CDs, DVDs, and BDs (or HD-DVDs). However, since the numerical aperture of the objective lens for recording / reproducing CD, DVD, and BD varies greatly, it is almost impossible to reproduce all three types of discs with only one objective lens.

In response to this demand, optical pickups have recently been developed in which an actuator is equipped with two objective lenses, an objective lens for CD / DVD, and an objective lens for BD or HD-DVD. The two objective lenses can be arranged in the track (tangential) direction or the inner and outer circumferential (radial) direction of the disc. When the two objective lenses are arranged in the inner and outer circumferential direction, the two inner lenses are not easily accessible. Optical pickups arranged in track direction have been developed.

FIG. 3 shows an example in which one objective lens is stockpiled away from the center axis of the disc when two objective lenses mounted on one actuator are arranged in the track direction.

When two objective lenses are arranged in the track direction as shown in FIG. 3, at least one of the two objective lenses deviates from the axis connecting the inner and outer circumferences (the axis passing through the center of the disk). In an optical system using an objective lens positioned off the central axis of the disc, the relative position (angle of the sub beam) in the tracks of the main beam and the sub beam for DPP detection changes as the optical pickup moves from the inner circumference to the outer circumference, The angle adjustment of the sub beams becomes meaningless.

Meanwhile, a multi-layer structure in which two or more recording layers are formed to increase storage capacity has been adopted in the DVD and BD standards, and a multi-layer structure is expected to be generalized in the case of high density discs to be developed in the future. .

In order to increase the density of the disk, the wavelength of the laser beam is shortened and the numerical aperture (NA) of the objective lens is increasing. The laser diode and the objective lens of 780nm, NA = 0.45 for CD, 650nm for NA, 0.6 = NA, 405nm for BD, NA = 0.85 are adopted.

In multi-layer discs, the spacing between recording layers is determined approximately in proportion to the depth of focus of the light spot, which is proportional to the wavelength of the laser beam and inversely proportional to the square of the numerical aperture of the objective lens. The gap between them should also be narrow.

When performing a recording or reproducing operation for a multi-layer disc having a narrow gap between layers, as shown in FIG. 4, a beam reflected from a layer adjacent to the current recording layer, that is, noise light of another layer, is transferred to the photo detector. It is easily introduced.

The noise light of the other layer also flows into the main photo detector for receiving the main beam and the sub photo detector for receiving the sub beam, and affects not only the reproduction signal but also the servo signal. In particular, it has a greater influence on the servo signal obtained by using a sub beam having a relatively small amount of light.

As shown in Fig. 5, a problem arises in that the SPP signal is severely distorted by the noise light from another layer, and the DPP, that is, the tracking error signal, which is calculated based on the SPP signal, deteriorates. That is, when reproducing or recording a disc having two or more recording layers, as shown in Figs. 4 and 5, the beams reflected from the other layers generate noise in the reproduction signal and the servo signal, which causes the reproduction or recording performance. It may deteriorate and recording may become impossible.

As a solution to this, there is a method of removing a part of a beam received by using a special polarization diffraction element. This method uses a special diffraction element to completely remove the light reflected from the other layer. The special diffraction element not only removes the light reflected from the other layer, but also the incoming beam from the current layer where recording / playback proceeds. A portion of the beam is also diffracted so that a portion of the beam spot formed on the PDIC is removed or the intensity is significantly reduced.

The area that is removed or the intensity decreases is called a dead zone. When the objective lens is radial shifted, the dead zone changes the k value according to the radial shift period, and deteriorates the RF signal and playback / recording performance. .

Accordingly, the present invention has been made to solve the above problems, and an object of the present invention is to reduce interlayer interference in a multilayer disk to obtain a stable servo signal and to improve playback or recording performance.

Further, another object of the present invention is to provide a means for effectively coping with the positional change of the sub-beam generated by the interlayer movement, the correction of spherical aberration, the pitch variation of the diffraction element, or the focal length (magnification) of the lens. .

Optical pickup according to an embodiment of the present invention for achieving the above object, the light source for emitting a beam of a predetermined wavelength; An objective lens for condensing a beam emitted from the light source on an optical storage medium; A beam splitter for transmitting or reflecting a beam emitted from the light source or a beam reflected from the optical storage medium; A first diffraction element for generating, from the beam reflected in the optical storage medium, a second beam having no component corresponding to the position in the track of the beam bound to the optical storage medium; A second diffraction element; A sensor lens for generating astigmatism in the beam reflected from the optical storage medium; And light detecting means including first detection means for receiving the first beam passing through the first diffraction element and the second diffraction element and two sub detection means for receiving the second beam. Wherein the second diffractive element diffracts a part of the beam reflected from a layer other than the current layer of the optical storage medium in which recording or reproduction is in progress so as not to enter the light detecting means.

In one embodiment, the first diffractive element comprises a first region that diffracts a portion of the incident beam to produce the second beam, wherein the first region of the beams is reflected in a branched state from the optical storage medium. The beam diffracted by the optical storage medium and the beam diffracted do not include a region overlapping each other when passing through the first diffractive element.

Further, in one embodiment, the second diffractive element comprises a second region for diffracting a part of the incident beam, wherein the second region is a beam which will be bound to the light detecting means among the beams reflected from the other layer. Corresponds to the region passing through the second diffraction element.

Further, in one embodiment, the lattice direction of the first region is either a tangential direction or a radial direction, and the lattice directions of the first region and the second region form 90 degrees.

Further, in one embodiment, the first region is a rectangular shape bordering an region in which the beam diffracted by the optical storage medium passes through the first diffractive element among the beams reflected from the optical storage medium, or The upper region and the lower region of the line connecting the two upper points among the four points that are reflected by the optical storage medium and non-diffraction beams reflected from the optical storage medium pass through the first diffractive element. It can be composed of the area below the line connecting the two points.

Further, in one embodiment, the second region is a beam that will be formed on the sub-detection means of the beam reflected from a layer other than the current layer of the optical storage medium in which recording or reproduction is in progress, passing through the second diffraction element. Corresponds to the region.

The first diffraction element and the second diffraction element may be composed of one element.

In one embodiment, the first diffraction element may be located between the beam splitter and the objective lens, between the beam splitter and the sensor lens, or between the sensor lens and the light detecting means. When the first diffraction element is located between the beam splitter and the objective lens, the optical pickup further includes a quarter wave plate for rotating the polarization of the beam between the first diffraction element and the objective lens. It is composed.

Further, in one embodiment, the second diffraction element may be located in any one of the beam splitter and the objective lens, between the beam splitter and the sensor lens, or between the sensor lens and the light detecting means. The first diffraction element or the second diffraction element, when positioned between the beam splitter and the objective lens, diffracts only the beam reflected from the optical storage medium without diffracting the beam emitted from the light source, so that the liquid crystal It can be prepared as.

Further, in one embodiment, the tracking error signal is detected by Mpp-k (Spp1 + Spp2), and k is calculated using the offset change of Mpp and the offset change of Spp1 and Spp2 according to the radial shift of the objective lens. .

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In the case of DPP using 3 beams, the sub beams are formed on the disc with a half track pitch deviating from the main beams. In the general case where the objective lens is located on the center axis of the disc, the relative position of the main beam and the sub beam does not change regardless of the inner and outer circumferences.

However, as shown in FIG. 3, in the case of a non-axis arrangement in which the objective lens is operated off the center axis of the disc, relative positions of the main beam and the sub beam in the track vary according to inner and outer circumferences. In this case, as shown in Fig. 6, the offset of the DPP signal is increased at the recorded / unrecorded boundary of the disk, and the length of the section in which the offset occurs increases.

The difference between the recorded and unrecorded areas is represented by the difference in reflectance. When the objective lens travels through the recorded and unrecorded areas in the DPP method using 3 beams, the level difference of the DPP signal appears in each area.

The offset of the DPP signal at this recorded / unrecorded boundary is caused by the time difference entering the recorded / unrecorded area when using 3 beams. In order to solve this problem, only one beam is incident on the disk, and a main diffraction grating generates a main beam and a sub beam from a beam reflected and branched by the track structure of the disk, and the main beam is used by using the sub beam. A method for removing an offset from a push-pull signal of A, a 1 beam DPP method has been proposed in another application by the applicant of the present invention, it will be described.

7 is a diagram comparing a 3 beam DPP method and a 1 beam DPP method. In the three beam DPP method, the beam emitted from the LD is divided into three beams through the grating, and the three beams are reflected from the disk to form the main and subcells of the PD. Only the main beam is incident and the incident beam is reflected in a branched state by the track structure of the disk, and becomes two sub beams in which the main beam and ac components are removed by a predetermined diffraction grating, thereby forming the main and sub cells of the PD. It is.

The main purpose of using a subbeam in DPP is to remove offsets generated in the main beam by radial shift of an objective lens, and so on. The increase is a secondary effect.

In the method proposed by the applicant of the present invention, the beam reflected and branched by the track structure of the disc (land / groove of the track serves as a diffraction grating) is passed through the diffraction grating as it is to become a main beam, and a predetermined shape in the diffraction grating. The beam diffracted by the pattern of becomes a sub beam.

The main beam passing through the diffraction grating as it contains both the ac component reflecting the position of the beam on the disk and the dc component reflecting the amount of radial shift of the objective lens. All dc components appear.

The pattern in the diffraction grating is shaped to diffract only the beam corresponding to the dc component (offset) except for the beam corresponding to the ac component among the beams reflected and branched by the track structure of the disk. Therefore, only the dc component appears in the push-pull signal of the sub-beam diffracted and generated by the pattern.

Therefore, by using the push-pull signal of the sub-beam, the offset corresponding to the radial shift of the objective lens appearing in the push-pull signal of the main beam passing through the diffraction grating can be effectively removed.

8 shows a configuration of an optical pickup to which the above method is applied.

The optical pickup 100 of FIG. 8 includes a light source 110, a collimator lens 120, a beam splitter 130, a diffraction grating 140, and a quarter wave plate (QWP). Wave plate 160, the objective lens 170, the sensor (Sensor) lens 180, and the optical detection means (or PD) 190 is configured to include. The disk is mounted on the side where the objective lens 170 is focused.

The light source 110 emits a laser beam, and the collimator lens 120 converts the beam emitted from the light source into parallel light. The beam splitter 130, which is an optical splitter, transmits the incident beam from the collimator lens 120 to the diffraction grating according to the polarization direction of the incident beam, and reflects the beam reflected from the disk to the sensor lens 170.

The quarter wave plate 160, which rotates the polarization of the beam, converts a beam of linearly polarized light emitted from the light source 110, for example a beam of p-polarized light into a beam of circularly polarized light, and also another circularly polarized light on the disk. Is converted into another linearly polarized beam, for example a beam of s polarized light.

The beam focused on the disk through the objective lens 170 is reflected and diffracted by the disk having a land / groove structure to become a zeroth order beam and a + 1st order beam and then directed to the objective lens 170. The beam of circularly polarized light reflected from the disk is converted into a parallel beam through the objective lens 170 and is converted into linearly polarized light while passing through the quarter wave plate 160. The sensor lens 180 reflects off the disk and generates a boiling point for the incident beam as the path is changed in the beam splitter 130 and transmits the boiling point to the light detecting means 190.

The diffraction grating 140 is a sub-pattern of the light detecting means 190 in the circular pattern region of the first-order beam and the circular pattern of the zero-order beam in the circular pattern of the diffraction beams generated while being reflected and branched onto the disk. The grating pattern is formed so that it is not received by the PD cell for the beam.

That is, the beam reflected and branched from the disk becomes the main beam and the sub beam through the diffraction grating 140. The main beam passing through the diffraction grating 140 as it is is the light detecting means 190, that is, the main cell of the light receiving element. Two sub-beams received by the main cell to form an MPP signal and diffracted by a pattern of a predetermined shape in the diffraction grating 140 are received by the sub-cells of the light detecting means 190, thereby receiving the first SPP (SPP1). ) Signal and the second SPP (SPP2) signal.

In order to detect the MPP signal and the SPP signal, the main cell of the light detecting means 190 is divided into at least two parts in a direction corresponding to the radial direction and the tangential direction, and the subcells of the light detecting means 190 are At least two divisions are made in a direction corresponding to the radial direction.

9 exemplarily shows a form in which a beam incident on a recording disk is branched.

When the beam incident through the objective lens and incident on the recording disk is viewed from the focal length of the objective lens, as shown in FIG. 9, the beam is divided into a + 1st order beam, a 0th order beam, and a -1st order beam. This is due to the track (land / groove) structure of the recording disc having a predetermined track pitch TP, which is because lands and grooves of the recording disc have an irregular shape in cross section, which produces an effect such as a diffraction grating.

The 0th order beam (b2), the + 1st order beam (b1), and the -1st order beam (b3) reflected and branched from the disk form a circular pattern and directed to the objective lens, and the size of the circular pattern formed by the beams is the objective lens. It has the same value as the EPD (Entrance Pupil Diameter). If the focus of the objective lens is f and the numerical aperture is NA, the EPD is 2 ㅧ f ㅧ NA.

In addition, the + 1st order beam b1 and the -1st order beam b3 move left and right as shown in FIG. 9, and the movement amount is f ㅧ λ ㅧ when the track pitch of the disk is Tp and the wavelength of the laser beam is λ. Tp.

The circular pattern P2 of the center 0th order beam is partially overlapped with the circular pattern P1 by the + 1st order beam moving left and right by the track structure of the disk and the circular pattern P3 by the -1st order beam. It forms the same pattern as the one, which is called the baseball pattern because it resembles the shape of a baseball.

At this time, the size and the degree of overlap of the circular patterns (P1, P2, P3) are different depending on the type of disc. In the case of BD or DVD-RW, since the movement amount is large, the overlapping degree is relatively small, thereby forming a pattern as shown in FIG. . In the push-pull signal (MPP and SPP signal in the case of 3-beam DPP), the ac component is generated by the portion (hatched) where the 0th order beam and the +/- 1st order beam overlap in the baseball pattern of FIG.

FIG. 11 schematically illustrates the principle of obtaining a sub beam from which an ac component is removed from a beam reflected and branched from a disk.

The beam incident on the disk is diffracted into three beams by the disk track structure, wherein the center beam of the three diffracted beams is the 0th order beam, and the beams formed at the left and right sides of the 0th order beam are +/- 1. Is defined as the secondary beam. The three beams (0th order beam and +/- 1st order beam) formed separately from the beam incident on the disk form a baseball pattern as shown in FIG. 10 by overlapping some regions P7.

In this case, the diffraction grating illustrated as an embodiment in FIG. 11 diffracts only the 0th order beam P5 except for the portion where the 0th order beam and the +/- 1st order beam overlap in the baseball pattern, and receives the photodetector for the subbeam. And the pattern is designed such that the +/- 1st beams P4, P6, P7, including the portion where the 0th and +/- 1st beams overlap, are not received by the photo detector for the subbeam. .

However, the main beam passing through the diffraction grating as it is reflects the baseball pattern regardless of the pattern of the diffraction grating, so that both the ac component and the dc component (offset) appear in the push-pull signal (MPP) of the main beam.

Since the 0th order beam P5 except for the portion where the 0th order beam and the +/- 1st order beam overlap in the baseball pattern is diffracted by the pattern of the hatched area of the diffraction grating, it is received by the photo detector for the subbeam. The ac component does not appear in the push-pull signal SPP, but only the offset of the main beam.

Accordingly, by subtracting the push-pull signal SPP of the sub-beam from which ac has been removed from the push-pull signal MPP of the main beam, the tracking error signal from which the offset is removed without using the 3-beam method of attaching the 3 beams to the disk. Can be obtained.

12 illustrates various embodiments of a diffraction grating for obtaining a sub beam from which an ac component is removed, and the diffraction grating may be formed in various shapes of patterns.

As described above, the main beam passing through the diffraction grating of FIG. 12 as it is, reflects all of the baseball patterns, and both the ac component and the offset appear in the MPP. However, the beam reflected and branched from the disk is only partially diffracted by the first patterns A1, A3, and A5 corresponding to the use region of the diffraction grating to form a sub beam.

The diffraction grating of FIG. 12 (a) has a first pattern A1 formed in a vertically oriented gear ("I" shape) when the X axis is radial and the Y axis is tangential. And a second pattern A2 formed in the remaining areas except for the first pattern.

The first pattern A1 is formed of the circular pattern regions P1 and P3 of the first-order beams b1 and b3 and the circular pattern region P2 of the zero-order beam b2 and the first-order beams b1 and b3. The circular pattern regions P1 and P3 are formed in the region P5 except for the overlapping region P7.

In addition, the upper / lower end of the first pattern A1 (the rectangular portion except for the middle circular recessed portion) has a diffraction grating within the extent that it does not involve the circular pattern regions P4 and P6 of the primary beams b1 and b3. It may have a form extending to both sides of the.

The diffraction grating of FIG. 12 (b) comprises a first rectangular pattern A3 long and vertically long and a second pattern A4 formed on both of the first patterns A3. Here, the position and width of the first pattern (A3) is an important factor, the first pattern (A3) is a rectangular shape so that the circular pattern (P1, P3) of the first beam (b1, b3) is not included. That is, it is preferable that the position and width of the first pattern A3 in the x-direction are determined so as to contact the ends of the circular patterns P1 and P3 of the first-order beam lying in the circular pattern P2 of the zero-order beam. Do.

The diffraction grating of FIG. 12 (c) includes a first rectangular pattern A5 long in the center and a second pattern A6 formed above and below the first pattern A5. Here, among the four points where the boundary between the circular pattern P2 of the zeroth order beam b2 and the circular patterns P1 and P3 of the first order beams b1 and b3 meet, It is preferable that the position and width | variety of the y direction of 1st pattern A5 are determined by a line.

FIG. 13 shows the result of removing only the dc component and the ac component from the sub-pump push-pull signal by the diffraction grating of FIG. 12, and the influence of the push-pull signal of the main beam by the diffraction grating. On the other hand, the ac component of the push-pull (SPP) signal of the sub-beam can be confirmed through FIG. 13.

Here, the dc level (offset) of the SPP signal is closely related to the amount of shift of the objective lens, that is, the amount of radial shift indicating the movement of the objective lens from the center of the actuator to the inner and outer circumferential directions.

In general, since the offset (dc level) of the MPP signal and the SPP signal increases in proportion to the amount of radial shift of the objective lens, the offset of the push-pull signal is linear in proportion to the amount of radial shift of the objective lens. I can do it.

Therefore, at DPP = MPP-k ㅧ (SPP1 + SPP2), the slope of the dc level of the MPP signal relative to the radial shift amount of the objective lens is a, and the slope of the dc level of the SPP signal relative to the radial shift amount of the objective lens is b. When the proportional constant k = 1 / 2b is used, the offset of the MPP signal caused by the movement of the objective lens can be eliminated.

Optical disks of two or more layers form a transflective layer for reproducing different recording layers. The transmittance varies from disk to disk. As shown in Fig. 14, when the recording layer near the incidence plane of the optical disk is L1 and the far recording layer is L0, Fig. 14A shows the reproduction or recording of the L1 layer. The case where the L0 layer is played back or recorded is shown.

In Fig. 14, it is shown that the interlayer movement is performed by the positional variation of the objective lens constituting the optical system without changing the positions of the disk and the PDIC. In FIG. 14A, light reflected from the layer L0, which is a layer other than the current layer L1, on which playback or recording is in progress is focused on a side closer to the PDIC, and in FIG. 14B, the light is reflected from the L1 layer. Light is focused farther away from the PDIC.

5 illustrates the influence of dual layer noise by the aperture limiting scheme. 5 (a) is a single layer (b) is a dual layer, the MPP signal is good, but the SPP signal has a lot of noise, the DPP signal is distorted.

That is, noise is generated in the SPP due to interference caused by beams coming from different layers and sub-beams of the reproduction signal. Therefore, it is possible to prevent deterioration of the DPP signal due to interlayer interference by preventing the beam coming from another layer from entering the region of the sub beam (or up to the region of the main beam), that is, the sub cell receiving the sub beam.

과

라고 하면, 빛과 빛의 양을 다음과 같이 정의할 수 있다.Looking at this analytically, the light formed in the sub-cells (sub 1 (E1 / E2) and sub 2 (F1 / F2)) of FIG.

and

In this case, the light and the amount of light can be defined as follows.

에서 0은 L0 층에서 반사된 빛이 회절 격자에 의해서 +1차로 회절되는 빛을 나타내고,

는 위상을 나타내고, V는 visibility를 나타낸다. 주요 인자만을 정리하면 다음과 같다.Here, the first subscript is the layer and the second subscript represents the diffraction order.

Where 0 represents the light reflected by the diffraction grating in the +1 order by the diffraction grating,

Denotes a phase and V denotes visibility. The major factors are summarized as follows.

을 제거하면 dual layer의 노이즈가 없어진다. 다시 말해, 다른 층에서 들어오는 0차 광이 가장 큰 노이즈이기 때문에, 이 빛을 적절히 감소 또는 제거하는 것에 의해 노이즈의 원인을 제거할 수 있다.In other words,

If you remove the noise of the dual layer. In other words, since zero-order light coming from another layer is the largest noise, the cause of the noise can be eliminated by appropriately reducing or eliminating this light.

In order to remove the zero-order light coming from the other layer, two methods can be considered, one of which is applied to a disc using a diffractive optical element (DOE) such as a diffraction grating or a holographic optical element (HOE). A part of the light is diffracted to another place to reduce or eliminate the amount of light, and the other is to apply the same method to the part of the light reflected by the disk and received by the PDIC.

The first method has a problem that the signal at the time of recording / reproducing is very deteriorated because it greatly affects the quality of the beam incident on the disk. Thus, the first method is not appropriate, and the second method, ie, how DOE is used in the optical path reflected from the disk and received by the PDIC, is justified.

In the present invention, the concept of DOE is used to generically refer to devices using diffraction gratings such as diffraction gratings and HOE.

First, a method of reducing interlayer interference by combining a general DPP method using 3 beams and a DOE will be described.

FIG. 15 shows the DOE shape and position when using a general 3-beam DPP, and the distribution of beams in the PD.

The optical pickup 200 using the general three beam DPP as shown in FIG. 15B includes a light source 210, a collimator lens 220, a grating 225, a beam splitter 230, a DOE 250, and an object. It comprises a lens 270, a sensor lens 280, and the light detecting means 290. In an optical system of 3 beam DPP, a grating 225 for generating a sub beam for DPP is located in the optical path between the light source 210 and the objective lens 270, and in general, the grating 225 for 3 beam is It lies between the collimator lens 220 and the beam splitter 230.

In the optical system using a general three beam DPP as shown in FIG. 15B, the optical path of the reflected light from the disk to the PD 290, that is, the light reflected from the disk, is transferred to the objective lens 270 and the beam splitter 230. When the DOE 250 having the pattern as shown in FIG. 15A is inserted into the optical path through the sensor lens 280 to the PD 290, the PD 290 as shown in FIG. 15C. Some of the zero light blocking coming in from the other layers in the phase diverges elsewhere by the DOE pattern.

In FIG. 15A, the DOE pattern of the DOE 250 has a lattice direction formed in a vertical (vertical) direction, and as shown in FIG. 15C, a beam passing through the DOE 250. Some are diffracted by the DOE pattern (+/- 1st order diffraction beam) and arranged to the left and right of the 0th order beam passing through the DOE pattern as it is. Here, the 0th order beam and the +/- 1st order diffraction beam indicate whether the diffraction is based on the DOE pattern.

Since the beam reflected from the current layer in which recording or reproduction is in progress is focused on the PD 290, the beam size corresponds to the cell size of the PD 290 and the beam intensity is strong ((c)). In dark colors).

The main beam and two sub-beams (beams branched by the grating 225) (B _{CL, 0} , B _{CL, + 1} , B _{CL, -1} ) reflected at the current layer are MPP to the PD 290 It is formed in the main cell and the subcell for the SPP, which is diffracted by the DOE pattern to form the left and right sides of the main cell and the subcell (B _{CL, 0} ^{+/- 1} , B _{CL, + 1} ^{+/- 1} , B _{CL, -1} ^{+/- 1} ), and thus the main and subcells have the dead zone (B _{CL, 0} ⁰ , B _{CL, + 1} ⁰ , B _{CL, -1} ⁰ ) described above. Is formed.

The beam reflected from a layer other than the current layer has a large size of the beam formed on the PD 290 and a weak beam intensity ((c) because the focus is formed before or after the PD 290). Expressed in color).

The beam reflected from the other layer (exactly the zeroth order beam not diffracted by the grating 225 or the uneven structure of the disc track) passes through the DOE 250 and the large circle B _{OL in} FIG. 15 (c). ) And left and right rectangles (B _OL ⁺¹ , B _OL ^-1 ), and the beams passing through the DOE pattern are elongated rectangles (B _OL ⁰ ) and left and right in a longitudinal direction inside the middle large circle (B _OL ). Form a square of the same shape (B _OL ⁺¹ , B _OL ^-1 ).

The rectangle B _OL ⁰ formed by the DOE pattern region of FIG. 15A corresponds to the cell shape of the PD 290 so as not to interfere with the beam reflected by the current layer and formed on the PD 290. do. That is, it is possible to reduce the occurrence of noise due to the noise light reflected from the other layer to the MPP signal and the SPP signal.

This is the shape and location of the main and subcells of the PD (for beams formed by different layers) and / or the size and intensity of dead zones in the main and subcells (beams formed by the current layer). The shape of the DOE pattern and the DOE pattern area is designed.

The DOE 250 may have various shapes as shown in FIG. 16, and there is little difference in performance of removing noise. In particular, it is important to be designed so that the beam reflected from another layer does not enter the subcell for SPP generation. The region other than the DOE pattern may be appropriately adjusted in thickness so that a phase difference does not occur with the DOE pattern region.

Further, the DOE pattern direction (lattice direction) does not necessarily have to be perpendicular to the lattice direction of the three-beam branching grating 225, but it is advantageous to maintain the right angle to each other. In Figure 15, the grating 225 is because the grid is formed in the horizontal direction, the sub-beam 15 as shown in (c) (B _{CL, + 1, CL B, -1),} the main beam (B _{CL , 0} ) above and below. In addition, the lattice pitch of the DOE pattern is advantageously designed so that the beams branched by the DOE pattern and formed on the PD are separated from each other without overlapping.

The DOE may be placed at the positions (A) to (C), as in FIG. 15B. As in (A), when DOE is placed in a path through which a beam entering the disk passes and a path through a beam reflected from the disk, the DOE should be made of a polarization dependent device such as a liquid crystal. In addition, when the DOE is placed at the position of (A), it is advantageous to be mounted on the actuator so as to move together with the objective lens, so as to reduce the DOE area.

When placed in the position of (B), the DOE area should be designed larger than in the case of (A) so that no noise of another layer that is not removed by the radial shift of the objective lens does not enter. Although the case of position (C) is possible, as in the case of position (B), the DOE area is equal to the diameter of the incident beam, but it is more disadvantageous than the case of (B) in consideration of assembly tolerances and the like.

Typically, DOE patterns with 0-order diffraction efficiencies in the range of 0 to 60 (%) are used, where 60 (%) of 0-order diffraction efficiencies pass through 60 (%) of the beam passing through the DOE pattern and the remaining 40 ( %) Beam means diffraction.

In Figure 15 (a) closer to the diffraction efficiency of 0 (%) of the DOE pattern 15 of (c) in temperature may cause problems intensity into 0 order light to the B _OL ⁰ territorial because of, on the PD main cell and sub-cell Less beams are reflected from other layers, which means less noise to the MPP and SPP signals.

However, the DOE pattern does not only remove or reduce the noise of the MPP and SPP signals, so that part of the light used for the reproduction signal (B _{CL, 0} ⁰ , B _{CL, + 1} ⁰ , B _{CL, -1} ⁰ ) is also used. It is called dead zone for this reason. The less the 0th order diffracted light entering the dead zone, i.e., the closer the diffraction efficiency of the DOE pattern is to 0 (%), the worse the reproduction signals such as RF signals, servo signals, and the like.

In addition, such a dead zone has a disadvantage in that when the objective lens is radially shifted, its value changes in each section according to the amount of radial shift. This change in k value causes an offset in the DPP signal, which degrades the performance of the tracking servo.

On the other hand, as shown in Fig. 15 (c), among the beams incident and diffracted in the elongated rectangular DOE pattern region in Fig. 15 (a), the beam B is reflected on the current layer and formed on the PD plane (B). _{CL, 0} ⁰ , B _{CL, + 1} ⁰ , B _{CL, -1} ⁰ , B _{CL, 0} ^{+/- 1} , B _{CL, + 1} ^{+/- 1} , B _{CL, -1} ^{+/- 1} ) (The elongated rectangular shape in the horizontal direction) and the beams (B _OL ⁰ , B _OL ⁺¹ , B _OL ^-1 ) formed on the PD plane reflected from the other layers are different from each other.

In general, a sensor lens that generates astigmatism to obtain a focus error signal uses a cylindrical lens to change the focal position of the two axes, for example, the x-axis and the y-axis, with respect to the incident beam. Make the focusing point the intermediate position of the focal point on the axis and y axis.

The lens that generates astigmatism serves to rotate the shape (light distribution) of the beam by 90 degrees while maintaining the geometric position of the beam with respect to the incident beam so as to focus between two focal points. It does not rotate the shape of the beam with respect to the incident beam so that it focuses outside the focal point of the two axes.

That is, the beam reflected from the current layer in focus for recording or playback passes through the sensor lens and the light quantity distribution is rotated 90 degrees, and the beam reflected from another layer out of focus is passed through the sensor lens, even though it passes through the sensor lens. maintain.

Thus, the beam reflected from the current layer is diffracted by the DOE pattern area of the elongated rectangular shape in Fig. 15 (a), and then passes through the sensor lens that generates astigmatism, B _CL of Fig. 15 (c). _{, 0} ⁰ , B _{CL, + 1} ⁰ , B _{CL, -1} ⁰ , B _{CL, 0} ^{+/- 1} , B _{CL, + 1} ^{+/- 1} , B _{CL, -1} ^{+/- 1} Changed into an elongated rectangular shape and formed on PD surface.

On the other hand, even if the beam reflected from the other layer through the sensor lens for generating astigmatism and then diffracted by the DOE pattern area of the elongated square shape, the vertical direction in Fig. 15 (a), Figure 15 (c) of B _OL It is formed on the PD plane while maintaining the shape of an elongated rectangle in the vertical direction such as ⁰ , B _OL ⁺¹ and B _OL ^-1 .

Next, a method of reducing interlayer interference by combining the 1 beam DPP method and the DOE will be described.

FIG. 17 shows DOE shape and position when using 1 beam DPP, and beam distribution in PD.

In FIG. 17B, the optical pickups 300 and 400 using 1 beam DPP include light sources 310 and 410, collimator lenses 320 and 420, beam splitters 330 and 430, and gratings 340 and 440. , DOE 350, 450, objective lenses 370, 470, sensor lenses 380, 480, and light detection means 390, 490, depending on the location of the gratings 340, 440. A quarter wave plate (QWP) 360 may be added.

The gratings 340 and 440 are completely different from the grating 225 of FIG. 15 (b). The grating 225 of FIG. 15 (b) is a lattice to generate 3 beams, but the grating 225 of FIG. The grades 340 and 440 are the ac components in the beam reflected in a branched state by the uneven structure of the disc track, as shown in Fig. 7C or Figs. 12A, 12B and 12C. Except for the beam corresponding to the lattice shape, only the beam corresponding to the dc component (offset) is diffracted.

When the sub beams B _{CL, + 1} , B _{CL, -1} of FIG. 17C are used with the gratings 340 and 440 having the pattern of FIG. 7C or FIG. 12A, FIG. Shape for. When the pattern of FIG. 12 (a) is used for the gratings 340 and 440, since the lattice is formed in the horizontal direction of the first pattern A1, the region A1 is formed as shown in FIG. 17 (c). The sub-beams B _{CL, + 1} , B _{CL, -1} , which are the +/- primary beams to be diffracted, are above and below the main beam B _{CL, 0,} which is the zero-order beam passing through the gratings 340, 440. Are arranged.

The DOEs 350 and 450 may be placed at positions (A) to (C), as shown in FIG. 15B, and the features of each case are the same as those described in the 3 beam DPP case. In addition, the gratings 340 and 440 and the DOE 350 and 450 may be integrally formed. The grating for DOE or 1 beam DPP is reflected in the disk without diffracting the beam incident on the disk when placed in the path through which the beam entering the disk passes and the path through which the beam reflected from the disk passes, as in (A). Since it is preferable for the light efficiency to be made to be able to diffract only the beam to be made, it should be made of a polarization dependent element such as liquid crystal.

As before, the lattice direction of the DOE patterns in the DOEs 350 and 450 is not necessarily perpendicular to the lattice direction of the gratings 340 and 440 for 1 beam DPP. That is, the DOE pattern may be designed so that beams diffracted by the DOEs 350 and 450 do not form in the main cell and the subcell of the PDIC. For example, even when the DOE patterns in the DOE 350 and 450 and the lattice directions of the 1 beam DPP gratings 340 and 440 coincide with each other, the beam diffracts by the DOE pattern by increasing the diffraction angle of the DOE pattern. The main cell and the sub cell of the PDIC may not be formed.

However, it is more advantageous that the lattice direction of the DOE pattern and the 1 beam DPP gratings 340 and 440 are at right angles. In the example of FIG. 17, since the gratings 340 and 440 have a lattice formed in a horizontal direction (radial direction), the sub beams B _{CL, + 1} , B _{CL, -1, as shown in} FIG. It is formed above and below this main beam B _{CL, 0} . In addition, the lattice pitch of the DOE pattern and the lattice pitch of the grating are advantageously designed such that the beams formed on the PD do not overlap and are far from each other.

As described above, the shape of the DOE pattern and the DOE pattern region is designed in consideration of the shape and position of the main and subcells of the PD and / or the size and intensity of the dead zone in the main and subcells.

That is, the DOE pattern area in the DOE 350 and 450 corresponds to an area in which beams that may affect the main and subcells of the PD pass through the DOE 350 and 450 among the beams reflected from other layers. If possible, the outline shape is designed. In particular, it is important that the DOE pattern region be designed so that the subcells of the PD are less affected by beams reflected from other layers. Accordingly, the DOEs 350 and 450 may have various shapes as shown in FIG. 16.

When the DOE pattern region is the same as in the case of 3 beam DPP of FIG. 15A as in FIG. 17A, dead zones B _{CL, 0} ⁰ , B _{CL, + 1} ⁰ , B _{CL, -1} Degradation of the tracking servo and the reproduction / recording signal by ⁰ ) occurs similarly.

Note the change in k value by the dead zone. If the sub-beam is exactly at the center of the subcell when the objective lens is not radial shifted, the k value in the section including the dead zone and the section not including the dead zone when the objective lens is radial shifted (in the graph It can be seen from FIG. 18B that the slopes are different from each other. Although the MPP offset and the SPP offset change, they do not matter much because they change in the same direction.

However, as shown in FIG. 19 (a), even when the objective lens is not radial shifted, the sub beam is often disposed slightly away from the center of the subcell due to the interlayer movement, the pitch error of the diffraction element, and the variation of the optical magnification. do. Even if there is no other error, the position of the sub-beams when reproducing or recording L0 and when reproducing or recording L1 is inevitably changed due to the interlayer movement.

In this case, as shown in Fig. 19 (b), the MPP offset and the SPP offset are changed in different directions although they are partial and not large, and through k, the offset of the MPP signal generated when the objective lens is shifted radially is obtained. Problems can occur that can't be offset.

In the case of 1 beam DPP, it is possible to change the position of the subcell, that is, the position where the subbeams are formed. FIG. 20A illustrates the embodiment of FIG. 17. As shown in FIG. 20A, the lattice direction of the pattern for generating the sub beams in the gratings 340 and 440 for 1 beam DPP is horizontal. Formed, the sub beams B _{CL, + 1} , B _{CL, -1} are arranged above and below the main beam B _{CL, 0} .

The DOE pattern regions of the DOEs 350 and 450 for eliminating interlayer interference in FIG. 20A have a lattice direction in the vertical direction, but are not shown in FIG. Beams diffracted by the DOE pattern are formed on the left and right sides of the PDIC.

On the other hand, in FIG. 20B, the lattice direction of the pattern for generating the sub beams in the gratings 340 and 440 for 1 beam DPP and the lattice direction of the DOE pattern region in the DOEs 350 and 450 are shown in FIG. Each 90 degrees is designed differently from that of a), so the position of the subcell of the PDIC must also be rotated 90 degrees.

The subcells should be divided into at least two in a direction corresponding to the radial direction, wherein the direction corresponding to the radial direction is determined irrespective of the lattice direction of the pattern for generating the sub beam in the grating for 1 beam DPP. In FIG. 20, since the horizontal direction is the radial direction and the beam reflected from the current layer is rotated by 90 degrees by the sensor lens, the radial direction in the subcell of the PDIC becomes the vertical direction. Therefore, as shown in (a) and (b) of FIG. 20, the subcells should be divided up and down in the vertical direction.

In the case of FIG. 20B, the dead zone lies in the horizontal direction. In addition, when the objective lens is radially shifted, the main beam and the sub beam move in the radial direction. As described above, in the PDIC of FIG. 20B, the radial direction becomes the vertical direction. Therefore, a change in k value due to the radial shift of the objective lens does not occur.

In addition, as described above, even when the objective lens is located in the center, the sub-beams are often disposed slightly away from the center of the sub-cells due to the interlayer movement, the pitch error of the diffraction element, and the variation in the optical magnification. In the case of the embodiment of FIG. 20B, the sub beam moves in the same direction as the line dividing the subcells as shown in FIG. 21B. Therefore, the signal is not affected even by the movement of the sub beam due to various causes.

FIG. 22 illustrates various shapes of a DOE and a diffraction grating for 1 beam DPP for interlayer interference cancellation in one plane. When the zero-order transmittance is as high as 50 to 60 (%), a general grating can be used without using an HOE, and a grating pattern can be formed on one plane as shown in FIG. 22. As in FIG. 22, a combination of the DOE pattern shape and each diffraction grating of FIG. 12 is possible.

As shown in Figs. 23A and 23B, the beams incident on the diffraction grating are the 0th order beams passing through as they are and the +/- 1st order beams diffracted by the diffraction gratings. The amount of light of the beam and the +/- 1st beam can be adjusted by a combination of pitch of the unevenness, depth of the valley, duty ratio of the floor and the valley, and the slope of the floor and the valley.

Most of the light passing through the DOE pattern needs to be diffracted in order for the DOE light transmittance of the DOE to approach zero, which may be realized through a blazed type HOE as shown in FIG. When the DOE is formed by dividing it into several steps, the smaller the step, the closer to the blazed type HOE, and the transmittance of the zero-order light is closer to zero.

When the DOE is arranged to move with the objective lens, it must respond only to the polarization of the light reflected from the disk, for which liquid crystals are used. The liquid crystal is arranged so that the refractive index is different only in a specific polarization component to generate a phase difference equivalent to that of a general diffraction grating or HOE.

The optical pickups 300 and 400 to which the DOE and the diffraction grating for 1 beam DPP according to the present invention are applied together can be applied to a part of the optical pickup capable of recording and / or playing both CD, DVD, and BD.

For example, when the optical system for the CD and DVD and the optical system for the BD are implemented in one pickup, two objective lenses should be mounted on one actuator, and the objective lens of the BD optical system is mainly shown in FIG. Is stockpiled.

Therefore, the optical pickups 300 and 400 to which the DOE and the diffraction grating for one beam DPP according to the present invention are applied together are applied to the optical system for the BD, and thus the problems of the three beam DPP that may be caused by the non-axis arrangement of the objective lens. Can be solved.

In addition, the optical pick-ups 300 and 400 to which the DOE and the 1-beam DPP diffraction grating are applied together can remove noise generated in the reproduction / recording signal due to interlayer interference, thereby providing a plurality of recording layers. It is advantageous for recording and reproduction of optical storage media for recording, for example, DVD-RAM, DVD-RW, DVD + RW, DVD-R, DVD + R, BD-R, BD-RE and the like.

Preferred embodiments of the present invention described above have been disclosed for the purpose of illustration, and those skilled in the art will improve, change, and substitute various other embodiments within the technical spirit and the technical scope of the present invention disclosed in the appended claims below. Or addition may be possible.

The optical pickup device according to the present invention can remove the offset of the push-pull signal generated by the stockpile arrangement of the objective lens, can effectively remove the interlayer noise in the multilayer disk, and is generated when the interlayer noise is removed. By eliminating the influence of dead zones, the offset of the push-pull signal due to the radial shift of the objective lens can be effectively removed and the influence of the movement of the sub beam can be eliminated, so that a stable tracking error signal is obtained and the reproduction or recording performance is improved. It is effective.

Claims (18)

A light source emitting a beam of a predetermined wavelength; An objective lens for condensing a beam emitted from the light source on an optical storage medium; A beam splitter for transmitting or reflecting a beam emitted from the light source or a beam reflected from the optical storage medium; A first diffraction element for generating, from the beam reflected in the optical storage medium, a second beam having no component corresponding to the position in the track of the beam bound to the optical storage medium; A second diffraction element; A sensor lens for generating astigmatism in the beam reflected from the optical storage medium; And And a first detecting means for receiving a first beam passing through the first and second diffractive elements as it is, and a light detecting means comprising two sub detecting means for receiving the second beam. , Wherein the second diffractive element diffracts a portion of the beam reflected from a layer other than the current layer of the optical storage medium in which recording or reproduction is in progress so as not to enter the light detecting means. The method of claim 1, The first diffractive element includes a first region that diffracts a portion of an incident beam to produce the second beam, wherein the first region is in the optical storage medium of the beams reflected in a branched state from the optical storage medium. And the non-diffraction beams do not include regions overlapping each other when passing through the first diffractive element. The method of claim 1, The second diffractive element comprises a second region for diffracting a part of the incident beam, the second region of which the beam to be bound to the light detecting means is passed through the second diffractive element among the beams reflected from the other layer. And an optical pickup corresponding to the area. The method according to any one of claims 1 to 3, And the lattice direction of the first region is either a tangential direction or a radial direction. The method of claim 4, wherein And the grating direction of the first region and the second region is 90 degrees. The method of claim 2, And the first region has a rectangular shape bordering a region in which the beam diffracted by the optical storage medium passes through the first diffractive element among the beams reflected from the optical storage medium. The method of claim 6, The first region connects the upper two points among the four points that meet when the beam diffracted by the optical storage medium and the non-diffraction beam among the beams reflected by the optical storage medium pass through the first diffractive element. An optical pickup, comprising an area below the line connecting the top and bottom points of the line. The method of claim 3, wherein The second area corresponds to an area where a beam to be formed on the sub-detection means passes through the second diffraction element among beams reflected from a layer other than the current layer of the optical storage medium in which recording or reproduction is in progress. Optical pickup made. The method according to any one of claims 1 to 4, And said first diffraction element and said second diffraction element comprise one element. The method according to any one of claims 1 to 4, And the first diffractive element is positioned between the beam splitter and the objective lens, between the beam splitter and the sensor lens, or between the sensor lens and the light detecting means. The method of claim 10, And when the first diffraction element is positioned between the beam splitter and the objective lens, a quarter wave plate for rotating the polarization of the beam is further placed between the first diffraction element and the objective lens. The method of claim 11, And the first diffractive element diffracts only the beam reflected from the optical storage medium without diffracting the beam emitted from the light source. The method according to any one of claims 1 to 4, The second diffraction element is located between any one of the beam splitter and the objective lens, between the beam splitter and the sensor lens, or between the sensor lens and the light detecting means. The method of claim 13, And the second diffractive element diffracts only the beam reflected from the optical storage medium without diffracting the beam emitted from the light source when positioned between the beam splitter and the objective lens. The method of claim 1, And the first and second diffractive elements are made of liquid crystal. The method according to any one of claims 1 to 4, The main detecting means is divided into at least two in a direction corresponding to the radial direction and a tangential direction, and the sub detecting means is divided into at least two in a direction corresponding to the radial direction, respectively, When the push-pull signal detected by the main detecting means is Mpp and the two push-pull signals detected by the sub detecting means are Spp1 and Spp2, the tracking error signal is detected by Mpp-k (Spp1 + Spp2). Wherein k is calculated based on offset change of Mpp and offset change of Spp1 and Spp2 according to the radial shift of the objective lens. The method according to any one of claims 1 to 4, The optical storage medium is at least one of CD, DVD-RAM, DVD-RW, DVD + RW, DVD-R, DVD + R, BD-R, and BD-RE. The method according to any one of claims 1 to 4, And the light source emits a beam of blue wavelengths and the objective lens is off the central axis of the optical storage medium.

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