JP2004079123A - Optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress DC components in tracking error signals due to the shift of an objective lens and to enable highly accurate tracking servo. <P>SOLUTION: Defining the effective diameter of the objective lens as (a), the track pitch of a recording medium as d and the numerical aperture of the objective lens as NA, the width α in a radial direction when ±1st order diffracted lights are made incident on the objective lens satisfies α≤λ×a/(2d×NA), 0-order and ±1st order diffracted lights are converged on an optical disk through the objective lens and the return lights are detected on respective photodetectors having a division boundary in a direction orthogonal to a disk radial direction. Defining the differential signals of a 0-order light, +1st order light and a -1st order light as TE1, TE2 and TE3, sum output as A1, A2 and A3 and gain G2 as G2=A2/A3, the tracking error signals TES are defined as TES=TE1-G1×(TE2+G2×TE3). Also, at the time of defining an objective lens shift amount as b, the width α in the radial direction of the ±1st order diffracted lights satisfies 4b≤α. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical disc apparatus that generates a tracking error signal for positioning a light spot on a target track on an optical disc when recording or reproducing from a track on the optical disc.
[0002]
[Prior art]
In the optical disk device, a person who performs a process of reading information written on the optical disk is a laser spot as a minute light spot formed on the optical disk by the optical pickup. That is, the information is read from the optical disk by scanning the track on the optical disk with the minute laser spot.
[0003]
In this case, since the laser spot and the track are minute, the laser spot may deviate from the track due to vibration or the like, and as a result, a reading error may occur. Therefore, in order to accurately and continuously read data written on an optical disk, a servo technique for causing a laser spot to follow a track is indispensable. For this reason, in an optical disk device, a tracking servo that causes a laser spot to follow a track is generally performed based on a tracking error signal.
[0004]
In the tracking servo, when an objective lens shift or tilt occurs in the radial direction of the optical disc, a DC component due to the shift or tilt is added to the tracking error signal, and the spot scans a position different from the original track center, or the servo deviates. The problem arises. Therefore, assuming that the distance from the scanning point to the center of the track is set as an offset at this time, in order to obtain a stable tracking error signal, a technique for suppressing the offset due to an objective lens shift or the like to 0, that is, tracking without a DC component It is essential to develop a servo system that can obtain an error signal.
[0005]
As a conventional technique capable of suppressing a DC component in a tracking error signal, for example, a technique disclosed in Japanese Patent Application Laid-Open No. H10-162389 is known. In the method described in the publication, a circular diffraction grating having an area smaller than the cross-sectional area of a light beam is arranged in an optical path from a laser light source to an optical disk, and an incident light beam is composed of a 0th-order diffracted light and ± 1st-order diffracted light. The beam is divided into three beams, and the offset is canceled by using a differential signal obtained by each of these light beams. Hereinafter, the 0th-order diffracted light is referred to as a main beam, and the ± 1st-order diffracted lights are referred to as sub-beams.
[0006]
Specifically, the circular diffraction grating is:
λ: wavelength of light source
a: Effective diameter of objective lens
d: track pitch of the optical disk
NA: numerical aperture of the objective lens
When the sub-beam is incident on the objective lens, the radial width α of the optical beam of the sub-beam in the optical disc is a condition.
α ≦ λ × a / (2d × NA) (1)
Is to be satisfied.
[0007]
By using a diffraction grating that generates a light beam that satisfies the above expression (1), the light beam separated by the diffraction grating is received by a corresponding photodetector, so that a differential signal due to the main beam is obtained. Contains the position information of the track (push-pull component) and the information of the objective lens shift (DC component), and the differential signal by the sub-beam contains only the information of the objective lens shift (DC component).
[0008]
Here, assuming that the differential signal of the main beam is TE1 ′ and the two differential signals of the sub beam are TE2 ′ and TE3 ′, the tracking error signal TES ′ is
TES ′ = TE1′−G1 ′ · (TE2 ′ + G2 ′ × TE3 ′) (2)
It becomes.
[0009]
The calculation in the above equation (2) corresponds to canceling the DC component included in the differential signal by the main beam and the sub beam. In the equation, the gain G1 'is defined by the light amount ratio between the main beam and the sub-beam, and the gain G2' is defined by the light amount ratio between the sub-beams. Have a role to do.
[0010]
[Problems to be solved by the invention]
In recent years, optical discs have been increasingly densified, and with this densification, a servo system with high accuracy has been required. For example, a large capacity of 120 mm in diameter having a track pitch of 0.32 μm and a transparent cover layer of 0.1 mm, which is recorded and reproduced using a blue-violet semiconductor LD having a wavelength of 405 nm and a high NA objective lens having an NA of 0.85. Optical disks have been proposed. In order to perform a stable recording / reproducing operation on such an optical disk, an optical disk device allows only about 0.005 μm as the offset, and requires an extremely high-accuracy tracking servo.
[0011]
However, the technique described in the above-mentioned conventional publication does not sufficiently satisfy the above-mentioned demand, and is insufficient in the accuracy of tracking servo.
[0012]
Accordingly, it is an object of the present invention to provide an optical disk device that suppresses a DC component of a tracking error signal caused by a shift of an objective lens and enables high-accuracy tracking servo.
[0013]
[Means for Solving the Problems]
In order to solve the above problem, an optical disc device of the present invention includes a light source, an objective lens, and a diffractive optical element arranged between the light source and the objective lens,
λ: wavelength of light source
a: Effective diameter of objective lens
d: track pitch of the recording medium
NA: numerical aperture of the objective lens
When the ± 1st-order diffracted light diffracted by the first diffraction grating formed on the diffractive optical element enters the objective lens, a radial width α satisfies the following condition;
α ≦ λ × a / (2d × NA)
When the 0th-order diffracted light and ± 1st-order diffracted light are condensed on the optical disk by the objective lens, and the returned light is detected on each photodetector, the photodetectors are directed in a direction perpendicular to the radial direction of the recording medium. Has at least one dividing boundary line in a direction corresponding to
The differential signal of the photodetector that has received the zero-order light is TE1 and the sum output is A1.
The differential signal of the photodetector receiving the +1 order light is TE2, and the sum output is A2
The differential signal of the photodetector receiving the primary light is TE3, and the sum output is A3.
When the gain G2 is G2 = A2 / A3,
The tracking error signal TES is calculated using the gain G1.
TES = TE1-G1 × (TE2 + G2 × TE3)
Optical disc device,
b: Objective lens shift amount
Where the width α in the radial direction of the ± 1st-order diffracted light satisfies 4b ≦ α.
[0014]
In order to perform the objective lens shift compensation with high accuracy, the differential signal of the main beam (a luminous flux composed of the zero-order diffracted light by the diffraction grating and the transmitted light in a region other than the diffraction grating) is required for the shift of the objective lens. The ratio between the DC component and the DC component of the differential signal of the sub-beam (± 1st-order diffracted light by the diffraction grating) is constant, that is, both light beams straddle the dividing boundary on each photodetector. is necessary. On the other hand, when the amount of shift of the objective lens reaches the predetermined value b, the sub-beam crosses over the dividing boundary of the photodetector. Therefore, as described above, if 4b ≦ α, this situation can be prevented, and as a result, the residual offset after compensation for the shift of the objective lens that can occur on the optical disc is suppressed to within an allowable range. be able to.
[0015]
The above optical disk device may be configured to determine the gain G1 based on the RIM intensity of the 0th-order diffracted light and the ratio between the 0-time diffracted light and the width of the ± 1st-order diffracted light in the optical disc radial direction.
[0016]
According to the above configuration, the DC component of the signal TES is determined by determining the gain G1 based on the RIM intensity of the 0th-order diffracted light and the ratio between the 0-time diffracted light and the width of the ± 1st-order diffracted light in the radial direction of the optical disk. The gain G1 that can be suppressed to 0 can be set, and the G1 can be determined at the design stage. Furthermore, if the above-mentioned setting is performed for each optical disk device based on the optical parameters of each element to be used, it is possible to suppress variation in compensation accuracy occurring for each optical disk device.
[0017]
The above optical disc apparatus continuously changes the gain G1 while shifting the objective lens in a state where the focus servo is on and the radial servo is off, and the time when the DC component of the arithmetic signal TES becomes 0 is set as the best value of the gain G1. It may be configured to be determined.
[0018]
According to the above configuration, the DC component of the signal TES can be suppressed to 0 as much as possible in the radial servo ON state with respect to the shift of the objective lens satisfying the above α ≧ 4b. Thus, the residual offset remaining in the head can be more appropriately suppressed, and highly accurate tracking servo can be performed. That is, the DC component of the tracking error signal does not suddenly increase in response to the shift of the objective lens, and the residual offset amount remaining in the radial direction can be reduced by using a large-capacity optical disc system (a large-capacity optical disc and a recording / reproducing apparatus for this). System with a simple optical disk device) within the allowable range for stable operation.
[0019]
Further, according to the above configuration, the gain G1 can be set at the stage of adjusting the optical system, and by using the above-described method for each device, it is possible to suppress variation in compensation accuracy between devices.
[0020]
In the above optical disc device, the diffractive optical element may include a first diffraction grating in an area other than the area where the first diffraction grating is formed, at least in an area corresponding to a cross section of a light beam incident on the objective lens. A configuration in which a second diffraction grating having different diffraction directions and the same grating depth and duty ratio may be formed.
[0021]
According to the above configuration, in the 0th-order diffracted light (main beam) transmitted through the diffractive optical element, for example, the phase and amplitude of the light flux transmitted through the first diffraction grating and the light transmitted through the second diffraction grating are changed. No distribution occurs. Therefore, it is possible to suppress the deterioration of the S / N such as the crosstalk due to the generation of the side lobe of the focused spot on the optical disk.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below with reference to FIGS.
First, the configuration of the optical system according to the present embodiment will be described with reference to FIG. FIG. 2 shows an optical system in the pickup device of the optical disk device of the present embodiment.
[0023]
In this optical system, a semiconductor laser 1, a collimator lens 2, a diffractive optical element 3, a beam splitter 4, a beam splitter 4, an objective lens 5, and an optical disk 7 are arranged in an optical path until light from the light source reaches the optical disk. The centers of these optical elements are aligned on a straight line in a direction perpendicular to the surface of the optical disk 7. In an optical path perpendicular to the optical path starting from the beam splitter 4, a condenser lens 8, a cylindrical lens 9, and a photodetector 10 are arranged. The light emitted from the semiconductor laser 1 passes through the optical elements in the above configuration in order, and finally reaches the photodetector 10.
[0024]
Reference numeral 6a denotes an objective lens aperture (hereinafter, simply referred to as an aperture) formed on the aperture forming member 6. The aperture 6a is a circular hole, and the aperture forming member 6 holds the objective lens 5. The diameter of the aperture 6a corresponds to the effective diameter of the objective lens. Normally, the diameter of the aperture 6a is set smaller than the incident light beam, and has a function of blocking a portion of the incident light beam that is larger than the diameter of the aperture 6a. This is to produce a good circular beam when the cross section of the incident light beam is not perfectly circular. Another reason is that even when the shift of the objective lens 5 occurs, the light beam having passed through the aperture 6a enters the objective lens 5 in a circular state.
[0025]
The above-mentioned diffractive optical element 3 has a band-shaped diffraction grating 11 at the center as shown in FIG. The light beam incident on the diffractive optical element 3 includes a light beam (hereinafter, referred to as a main beam) composed of a zero-order diffracted light beam by the diffraction grating 11 and a transmitted light beam in a region other than the diffraction grating 11, and a ± 1st-order diffracted light beam (hereinafter, referred to as a main beam). (Hereinafter, referred to as a sub-beam). FIG. 4 shows the relationship between the main beam and the sub beam.
[0026]
In FIG. 3, reference numerals 12 and 13 denote incident light beams to the diffractive optical element 3, and the incident light beam 12 indicated by a circle having a small radius has a light beam diameter corresponding to the effective diameter of the objective lens 5. An incident light beam 13 indicated by a circle having a large radius is a light beam incident on the diffractive optical element 3 from the collimator lens 2. Note that the radial direction and the tangential direction indicated by arrows in the figure correspond to the radial direction and the tangential direction on the optical disk 7, respectively.
[0027]
Since the width of the diffraction grating 11 in the radial direction of the diffractive optical element 3 is, as shown in FIG. 2, the diffractive optical element 3 is arranged in a parallel light beam between the collimator lens 2 and the objective lens 5, When the ± 1st-order diffracted light diffracted by the grating 11 enters the objective lens 5 (on the entrance pupil plane of the objective lens 5), it matches the width α of the light beam.
[0028]
This width α is
λ: wavelength of light source
a: Effective diameter of objective lens
d: track pitch of the optical disk
NA: numerical aperture of the objective lens
And the condition
α ≦ λ × a / (2d × NA) (3)
Are satisfied.
[0029]
Next, the photodetector 10 will be described. The photodetector 10 includes three photodetectors 31 to 33, as shown in FIG. These photodetectors 31 to 33 have at least one dividing boundary line in a direction parallel to the tangential direction on the optical disk 7 in order to obtain a tracking error signal.
[0030]
A so-called astigmatism method is used for detecting the focus signal by the photodetector 10. Therefore, the photodetector 31 of the photodetectors 31 to 33 is divided into four for detecting the focus signal by the astigmatism method. As shown in FIG. 2, a cylindrical lens 9 is provided in front of the photodetector 10 for focus signal detection by the astigmatism method. Further, the cylindrical lens 9 is arranged such that the direction having the lens effect is inclined 45 degrees from the radial direction and the tangential direction on the optical disc 7. Therefore, the arrangement direction (in this case, the tangential direction) of the three laser beams on the optical disk 7 is different from the direction of the dividing line in the photodetectors 31 to 33 for obtaining the differential signal by 90 degrees.
[0031]
Further, the optical disk device includes an arithmetic circuit (arithmetic unit) 14 as shown in FIG. The arithmetic circuit 14 is for obtaining a tracking error signal having no DC component based on the differential signals output from the photodetectors 31 to 33 that have received the three laser beams.
[0032]
The arithmetic circuit 14 includes subtractors 50, 53, 54, 58, adders 51, 52, 56 and gain amplifiers 55, 57. The subtracter 50 obtains a differential signal from the outputs of the two divided areas a and b in the photodetector 32. The adders 51 and 52 convert the four signals output from the four divided areas c to f of the photodetector 31 into two signals. The subtractor 54 obtains a differential signal from the two signals. The subtractor 53 obtains a differential signal from the outputs of the two divided areas g and h of the photodetector 33. The gain amplifier 55 multiplies the output of the subtractor 53 by a gain G2. The adder 56 adds the output of the gain amplifier 55 and the output of the subtractor 50. The gain amplifier 57 multiplies the output of the adder 56 by a gain G1. The subtractor 58 outputs a differential signal between the output of the subtractor 54 and the output of the gain amplifier 57. Note that the directions in which differentials are taken in the photodetectors 31 to 33 are unified.
[0033]
In FIG. 1, each differential signal of the main beam and the sub beam is
TE1 (differential signal of main beam) = (c + d)-(e + f) (4)
TE2 (sub beam differential signal) = ab (5)
TE3 (sub beam differential signal) = gh (6)
Therefore, the tracking error signal TES is
TES = TE1−G1 · (TE2 + G2 × TE3) (7)
Is obtained.
[0034]
Here, if there is a concern that the objective lens 5 may be shifted by, for example, about b μm due to the eccentricity of the optical disk 7, for example, the light beam width α of the sub-beam is set to 4 b μm or more, in order to ensure the compensation up to the shift amount b μm. In equation (3)
α ≧ 4b (11)
It is necessary to add such a limitation. The reason will be described.
[0035]
In the present embodiment, the shift compensation of the objective lens 5 is performed by calculating the differential signals of the main beam and the sub beam as shown in the equations (4) to (6). In order to exhibit the performance of accuracy, the ratio of the DC component of the differential signal of the main beam and the DC component of the differential signal of the sub beam is constant with respect to a shift of a certain objective lens 5, that is, both light beams Is assumed to straddle the dividing boundary line on each of the photodetectors 31 to 33.
[0036]
However, when the shift amount of the objective lens 5 reaches a predetermined value (b μm), the sub-beam crosses over the dividing boundary of the photodetectors 32 and 33. This is because the width of the sub beam in the radial direction of the optical disk 7 is smaller than the width of the main beam, and the amount of movement of the sub beam on the photodetectors 31 to 33 is larger than the amount of movement of the main beam. . Here, the point that the movement amount of the sub beam is larger than that of the main beam will be described with reference to FIGS. 5 (a) to 5 (c).
[0037]
FIG. 5A shows the luminous flux of the main beam on the photodetector 31, FIG. 5B shows the luminous flux of the sub-beam on the photodetectors 32 and 33, and FIG. The positional relationship with the detectors 31 to 33 is shown. These figures show a state (objective lens 5b) shifted by OLs from a state where the objective lens 5 is at the neutral position (objective lens 5a).
[0038]
In this case, the center of the objective lens 5 moves by OLs from the neutral position 21a which coincides with the center of the light beam incident on the objective lens 5 as shown in FIG. Move to the position 21b indicated by the arrow. As shown in FIG. 5A, the center of the main beam moves by Ms1 from the neutral position 23a (main beam 24a) coinciding with the center of the photodetector 31, and moves to the position 23b (main beam 24b). I do. 24b1 indicates the center of the main beam 24b. Further, as shown in FIG. 5B, the sub beam moves by Ss1 from the neutral position 26a (sub beam 27a (28a)) coinciding with the centers of the photodetectors 32 and 33, and moves to the position 26b (sub beam 27b ( Move to 28b)). 27b1 (28b1) indicates the center of the sub beam 27b (28b).
[0039]
The ratio between the amount of movement of the main beam and the amount of movement of the sub-beam due to the shift of the objective lens 5 is determined by using the parameters shown in FIGS.
Ms1 / Ss1 = 1/2 (8)
Can be expressed as
[0040]
This relationship can be explained based on FIG. 5C as follows.
First, since the beam diameter of the main beam incident on the objective lens 5 is larger than the effective diameter of the objective lens 5, so-called shaking does not occur even when the objective lens 5 is shifted. Therefore, the center (position 21b) of the light beam immediately after passing through the aperture 6a after the shift of the objective lens 5 coincides with the center (position 21b) of the objective lens 5 (object lens 5b), and the light beam returning from the optical disk 7 Is the position 21b.
[0041]
On the other hand, since the light beam width of the sub beam in the radial direction of the optical disk is smaller than the effective diameter of the objective lens 5, the center (position 21 a) of the light beam immediately after passing through the aperture 6 a is shifted by the objective lens 5. It will deviate from the center (position 21b) of (objective lens 5b). Therefore, the center of the light beam returning from the optical disk 7 is located at the position 22.
[0042]
Note that, in the present specification, the above-mentioned “blur” refers to the relative positional relationship between the incident light beam and the aperture 6a of the objective lens 5 due to the shift of the objective lens 5 (the positional relationship in a plane perpendicular to the optical axis). As a result, the outermost part of the incident light beam passes through the inside of the aperture 6a. Specifically, as shown in FIG. 6A, when the “blur” does not occur, the light beam at the outermost periphery of the incident light beam B is incident on the aperture forming member 6. Thereby, the cross section of the light beam incident on the objective lens 5 becomes circular. On the other hand, as shown in FIG. 6B, when “blur” occurs, the light beam at the outermost periphery of the incident light beam B passes through the aperture 6a. As a result, the cross section of the light beam incident on the objective lens 5 has a shape lacking a circular shape, and consequently affects the condensed spot on the optical disk 7 (a minute spot is slightly blurred).
[0043]
For the above reasons, in FIGS. 5A and 5B, the luminous flux centers of the main beam (main beam 24b) and the sub beam (sub beam 27b (28b)) on the photodetectors 31 to 33 after the shift of the objective lens 5 are shown. Are at positions 23b and 26b, respectively, and the above equation (8) is established.
[0044]
FIGS. 9A and 9B show a state in which the sub beam has crossed over the dividing boundary of the photodetectors 32 and 33 due to the difference in the movement amount between the main beam and the sub beam. FIG. 9A shows the luminous flux of the main beam on the photodetector 31, and FIG. 9B shows the luminous flux of the sub-beams on the photodetectors 32 and 33. In this case, the center of the main beam moves by Ms2 from the neutral position 23a (main beam 24a) coinciding with the center of the photodetector 31, and as a result, moves to the position 23c (main beam 24c). 24c1 indicates the center of the main beam 24c. Further, the sub beam moves by Ss2 from the neutral position 26a (sub beam 27a (28a)) coinciding with the center of the photodetectors 32 and 33, and as a result, moves to the position 26c (sub beam 27c (28c)). Here, 27c1 (28c1) indicates the center of the sub beam 27c (28c). The light beam diameter of the main beam on the photodetector 31 matches the effective diameter of the objective lens 5, the width of the sub-beam in the radial direction of the optical disk 7 is α, and the shift amount of the objective lens 5 is b.
[0045]
In the above case, the relationship between the shift amount b of the objective lens 5 and the movement amount of the main beam due to the shift of the objective lens 5 is as follows.
Ms2 = b (12)
It is.
[0046]
When the objective lens 5 is shifted by the shift amount b, in FIG. 5C, the neutral position 21a of the center of the objective lens 5 and the center position of the light beam of the sub-beam returning from the optical disk 7 after the shift of the objective lens 5 22 is symmetrical with respect to the center position 21b of the objective lens 5 after the shift. Therefore, the relationship between the shift amount b of the objective lens 5 and the movement amount of the sub beam due to the shift of the objective lens 5 is as follows.
Ss2 = 2b (13)
It is. Also, as can be seen from FIG.
Ss2 = α / 2 (14)
It is.
[0047]
From the above equations (12) to (14), in order to ensure the compensation up to the shift amount b μm of the objective lens 5, the sub beam needs to straddle on the dividing boundary line. Therefore, the width α of the diffraction grating needs to be 4 bμm or more, and the condition at this time is the above-described equation (11).
[0048]
FIG. 10 shows the shift amount of the objective lens when the ± 1st-order diffracted light diffracted by the band-shaped diffraction gratings 11 having different widths enters the objective lens 5 and the tracking without being completely compensated by the arithmetic circuit (Equation (7)). The simulation result about the relationship with the track offset (= residual offset) generated as a result of the DC component remaining in the error signal TES is shown. Here, as an example, the case where the radial width of the optical disk 7 in the band-shaped diffraction grating 11 is 2.1 mm and the case where it is 1.2 mm were examined. As parameters for the simulation, parameters other than the width of the light beam (equivalent to the width of the diffraction grating 11 in the radial direction of the optical disk 7) are the same as those in the above-described simulation (FIG. 8).
[0049]
According to the above equation (11), the width of the light beam must be 1.2 mm or more to ensure the shift compensation of the objective lens 5 up to 300 μm. In this case, the widths of the light beams 1.2 mm and 2.1 mm satisfy Expression (11). Therefore, when these light beams are used, as shown in FIG. 10, the offset after compensation for the shift of the objective lens 5 up to 300 μm is suppressed to be small, and the shift for the objective lens 5 is surely compensated. .
[0050]
On the other hand, in order to ensure the shift compensation of the objective lens 5 up to 525 μm, the width of the light beam must be 2.1 mm or more. If this condition is erroneously set and the width of the light beam is set to 1.2 mm, which is narrower than 2.1 mm, the offset after compensation becomes sharply large when the objective lens 5 is shifted to 300 μm or more. For this reason, it can be seen that the shift of the objective lens 5 within 525 μm cannot be handled. From the above, it can be said that when the shift amount that can be reliably compensated for as the shift compensation mechanism of the objective lens 5 is known, the width of the light beam needs to be a design value according to the equation (11).
[0051]
In FIG. 10, the residual offset is accurately suppressed to around 0 within the range of the objective lens shift of b μm. This is because the gains G1 and G2 in the equation (7) are set correctly, and the DC component in the signal TES is canceled. Because it is done. Here, the gains G1 and G2 in the present invention will be described.
[0052]
The gain G1 is a coefficient for canceling each DC component of the main beam differential signal and the sub beam differential signal by shifting the objective lens. From the equation (7), it can be seen that the difference between the main beam differential signal and the sub beam when the DC component of the signal TES in the radial servo OFF state becomes 0 when the objective lens shift satisfying the equation (11) occurs. This is the ratio of the motion signals. The gain G2 is for eliminating unevenness in the amount of light between the sub beams.
[0053]
Next, a method for determining the gains G1 and G2 will be described.
The gain G1 depends on various parameters of the optical system and should be determined based on them. From this point, it is possible to determine the gain G1 at the design stage. In the prior art (Japanese Patent Laid-Open No. 10-162383), only the light amount ratio between the main beam and the sub beam is described as a determining factor of the gain G1, and the gain is set based on the ratio. However, this is insufficient. is there. According to the simulation performed in the present invention, it is known that the gain G1 depends not only on the light amount ratio but also on the RIM (peripheral portion) intensity of the incident light beam and the ratio of the width of the main beam and the sub beam in the radial direction.
[0054]
The simulation performed here will be described. The contents of the simulation are as follows: a model is created according to the actual optical system, and the differential signal of each of the main beam and the sub-beam when the objective lens shift satisfying the expression (11) is obtained. In this state, the gain G1 at which the DC component of the operation signal TES becomes 0 is calculated. Although G2 will be described later, G2 is set to 1 here. The simulation conditions are
Wavelength: 405 nm
NA of objective lens: 0.85
Track pitch: 0.32 μm
Thickness of cover layer on optical disc: 0.1 mm
Refractive index of cover layer on optical disk: 1.6
Effective diameter of objective lens: φ3mm
Light intensity ratio (main beam light intensity / sub beam light intensity): 6
The ratio of the width of the main beam and the sub beam in the radial direction and the RIM intensity were calculated as parameters. The results are shown in FIG. From this figure, the gain G1 that can suppress the DC component of the signal TES to 0 depends at least on the ratio of the width of the main beam and the sub beam in the radial direction and the RIM intensity. Therefore, the gain G1 can be obtained only by the light amount ratio (= 6). It turns out that it cannot be specified.
[0055]
From the above, to determine the gain G1 in the design stage, a simulation is performed based on the optical parameters of the optical system to be used, such as the ratio of the width of the main beam and the sub beam in the radial direction and the RIM intensity, and the radial servo is turned off. The gain G1 at which the DC component of the signal TES becomes 0 may be calculated backward. Here, for example, the RIM intensity is determined by the radiation angle of the semiconductor laser, and considering that there is a variation in the performance of each element, a simulation is performed based on the measured optical parameters of all the optical elements used. It is appropriate to perform the reverse calculation of the gain G1.
[0056]
Further, from the viewpoint that the gain G1 when the operation signal TES becomes 0 in the state of the radial servo OFF is the best value, the gain G1 can be determined at the stage of adjusting the optical system. In the adjustment stage after setting up the optical system, the value of the gain G1 is continuously changed while the objective lens is shifted so as to satisfy Expression (11) in a state where the radial servo is turned off. The gain G1 can be determined using the time when the operation signal becomes 0 as the best value of the gain G1. As described above, in consideration of the variation in the characteristics of each element, it is effective to use this adjustment method for each device to suppress the variation between the devices of the same device and, as a result, the variation in the compensation system. However, at this time, it should be noted that the focus servo is in the ON state, and the shift amount of the objective lens is within the range of Expression (11).
[0057]
Further, the gain G2 is determined by the light amount ratio between the sub beams. Therefore,
G2 = A2 / A3 (10)
As described above, it can be determined by normalizing the light amount of one sub-beam (in this case, the + 1st-order diffracted light) by the light amount of the other sub-beam. Note that the sub beam to be normalized may be + 1st-order diffracted light.
[0058]
Next, the compensation accuracy of the objective lens shift compensation in the above optical disk device will be described.
[0059]
FIG. 7 shows a conventional technique, that is, a compensation mechanism using a circular diffraction grating and a gain G1 as a light amount ratio. FIG. 8 shows this embodiment, that is, a band-shaped diffraction grating using an optical system. 12 is a simulation result when using a compensation mechanism calculated backward by a simulation based on the parameter of FIG. This is a simulation result of the residual offset after compensation when the objective lens is shifted so as to satisfy Expression (11) in a state where the radial servo is ON.
[0060]
The simulation conditions are
Wavelength: 405nm
NA of objective lens: 0.85
Track pitch: 0.32 μm
Thickness of cover layer on optical disc: 0.1 mm
Refractive index of cover layer on optical disk: 1.6
Effective diameter of objective lens: φ3mm
Radial width α: 2.1 mm when ± 1st-order diffracted light diffracted by the band-shaped diffraction grating enters the objective lens
Diameter when the ± 1st-order diffracted light diffracted by the circular diffraction grating enters the objective lens: 2.1 mm
And
[0061]
From FIGS. 7 and 8, it is clear that the residual offset amount after compensation when the objective lens is shifted in the range defined by the equation (11) is smaller in the present embodiment than in the related art. In particular, considering that the allowable offset in the high-density optical disk system defined by the above conditions is about 0.005 μm, the present embodiment provides a larger shift of the objective lens 5 than the conventional technique. This is effective because it can be handled and the offset can be suppressed smaller.
[0062]
Since the light is actually condensed on the photodetector 10 using the condensing optical system, the beam diameters on the photodetector 10 are different from those described above, however, the expression (11) is used. The condition of ()) does not change.
[0063]
Further, the diffraction grating 11 has a length such that the diffraction direction is the tangential direction of the optical disk 7 and the light diffracted at the longitudinal end of the band is not incident on the objective lens 5 (γ in FIG. 3). It has become. By setting as described above, the state in which the light flux after passing through the aperture is cut off (in the cross section of the diffracted light, the light beam at the outermost periphery does not enter the aperture forming member 6 but enters the aperture 6a) It can be avoided and can enter the objective lens while keeping the cross section of the light beam circular, and a minute spot can be formed.
[0064]
Next, the arrangement of the main and sub beams on the optical disk 7 is shown in FIG. As shown in the figure, the focused spots 42 and 43 of the sub beam are arranged in the tangential direction on the optical disk 7 with respect to the focused spot 41 of the main beam. Since the push-pull signal indicating the position information of the track 44 is not generated in the differential signal obtained from the sub-beam, the arrangement of the condensed spots 42 and 43 of the sub-beam on the track 44 may be arbitrary. For example, it may be arranged on a track 44 adjacent to the track 44 on which the condensing spot 41 of the main beam is located. In other words, it means that it is not necessary to precisely adjust the positions of the condensed spots 42 and 43 of the sub-beams on the track 44.
[0065]
However, when the converging spots 42 and 43 of the sub beam are arranged in the radial direction (the direction orthogonal to the direction of the track 44 in FIG. 12) with respect to the converging spot 41 of the main beam, the innermost track of the optical disc 7 When the focused spot 41 of the main beam is moved to the innermost track 44 in order to read / write to / from the 44, the focused spot 41 of the sub-beam on the inner peripheral side (focus Either of the spots 42 and 43) comes off the track 44. Therefore, it is desirable that the direction in which the condensing spots 41 to 43 are arranged is as parallel as possible to the tangential direction.
[0066]
Next, another example of the diffractive optical element will be described with reference to FIG.
The diffractive optical element 51 shown in the figure has two types of diffractive gratings 11 and 52, and has a band-shaped diffractive grating 11 at the center similarly to the diffractive optical element 3. Band-shaped diffraction gratings 52 are provided on both sides, that is, on both sides of the optical disk 7 in the radial direction with respect to the diffraction grating 11. The diffraction grating 52 is formed at least in a region corresponding to a cross section of the light beam 13 incident on the objective lens 5. In other words, the diffraction grating 52 corresponds to a portion of the diffractive optical element 51 other than the region where the diffraction grating 11 is formed, excluding at least the portion of the light beam incident on the objective lens 5 that is incident on the diffraction grating 11. It is formed so as to have a region to be formed. Thus, the incident light beam can enter without diffusing from the diffractive optical element 51.
[0067]
The diffraction grating 52 has a configuration in which the grating depth and the duty ratio are different from those of the diffraction grating 11, but the diffraction directions are different, and the light diffracted by the diffraction grating 52 does not affect the tracking error signal.
[0068]
Here, the duty has the following meaning. That is, the cross section of the diffraction grating has a periodic structure in which peaks and valleys are repeated, and the duty ratio means the ratio of the width of one peak and one valley. It is considered that the amount of phase difference that the 0th-order diffracted light receives from the diffraction grating changes depending on the duty ratio. Therefore, in the diffractive optical element 51 shown in FIG. 11, if the duty ratios of the diffraction grating 11 and the diffraction grating 52 are the same, no phase difference occurs between the 0th-order diffracted lights generated in the respective regions, It is considered that a phase difference occurs if the duty ratios are different. Here, it is desirable that no phase difference occurs.
[0069]
With the above configuration, in the 0th-order diffracted light (main beam) transmitted through the diffractive optical element 51, a distribution of phase and amplitude occurs between the light transmitted through the diffraction grating 11 and the light transmitted through the diffraction grating 52. Therefore, deterioration of S / N such as crosstalk due to generation of side lobes of the focused spot on the optical disk 7 can be suppressed.
[0070]
In the above embodiment, the diffractive optical elements 3 and 51 are arranged in a parallel light beam between the collimator lens 2 and the objective lens 5, and the light beams converge in the optical path from the diffractive optical elements 3 and 51 to the objective lens 5. -It does not diverge. Therefore, the width of the band-shaped diffraction grating 11 in the radial direction of the optical disk 7 and the ± 1st-order diffracted light diffracted by the diffraction grating 11 are incident on the objective lens 5 (on the entrance pupil plane of the objective lens 5). The width α of the light beam in the radial direction matches.
[0071]
However, the effect obtained by the present invention is not limited to such a configuration. For example, the diffractive optical elements 3 and 51 are arranged in the divergent light beam between the light source (semiconductor laser 1) and the collimator lens 2. It doesn't matter. Alternatively, a beam diameter conversion optical system such as a beam expander combining a convex lens and a concave lens may be arranged between the diffractive optical elements 3 and 51 and the objective lens 5.
[0072]
In these configurations, the width of the band-shaped diffraction grating 11 and the width α of the light flux when the ± 1st-order diffracted light (sub-beam) diffracted by the diffraction grating 11 enters the objective lens (on the entrance pupil plane of the objective lens) Will be different. In this case, when the ± first-order diffracted light (sub-beam) diffracted by the diffraction grating 11 enters the objective lens 5 (on the entrance pupil plane of the objective lens 5), the width α of the light beam satisfies the condition of Expression (2). What is necessary is just to set the width of the diffraction grating 11 to satisfy it.
[0073]
As described above, the tracking error signal generating method of the present invention generates a main beam and a sub-beam by diffracting a beam light, and converges both beams on a track of a recording medium by an objective lens. In the tracking error signal generating method for generating a tracking error signal from the output of the main beam and the output of the sub-beam of the photodetector that has received the return light, the gain G1 for determining the compensation accuracy is taken into account in the design stage in consideration of optical parameters. The configuration is determined and corrected based on a method of performing a simulation or an adjustment method of continuously changing the gain G1 so that the DC component of the signal TES becomes 0 in the adjustment stage.
[0074]
In the above optical disk device, when the diffraction grating of the diffractive optical element is a first diffraction grating, at least the area other than the area where the first diffraction grating is formed is incident on the objective lens. A second diffraction grating may be formed in a region corresponding to the cross section of the light beam, and the second diffraction grating and the first diffraction grating may be configured to have the same grating depth and duty ratio. As a result, it is possible to suppress the deterioration of S / N such as crosstalk due to generation of side lobes of the beam spot on the optical disk.
[0075]
【The invention's effect】
As described above, the optical disc device of the present invention includes a light source, an objective lens, and a diffractive optical element arranged between the light source and the objective lens.
λ: wavelength of light source
a: Effective diameter of objective lens
d: track pitch of the recording medium
NA: numerical aperture of the objective lens
When the ± 1st-order diffracted light diffracted by the first diffraction grating formed on the diffractive optical element enters the objective lens, a radial width α satisfies the following condition;
α ≦ λ × a / (2d × NA)
When the 0th-order diffracted light and ± 1st-order diffracted light are condensed on the optical disk by the objective lens, and the returned light is detected on each photodetector, the photodetectors are directed in a direction perpendicular to the radial direction of the recording medium. Has at least one dividing boundary line in a direction corresponding to
The differential signal of the photodetector that has received the zero-order light is TE1 and the sum output is A1.
The differential signal of the photodetector receiving the +1 order light is TE2, and the sum output is A2
The differential signal of the photodetector receiving the primary light is TE3, and the sum output is A3.
When the gain G2 is G2 = A2 / A3,
The tracking error signal TES is calculated using the gain G1.
TES = TE1-G1 × (TE2 + G2 × TE3)
Optical disc device,
b: Objective lens shift amount
In this case, the width α of the ± 1st-order diffracted light in the radial direction satisfies 4b ≦ α.
[0076]
This makes it possible to suppress the residual offset after compensation for the shift of the objective lens that may occur in the optical disc within an allowable range.
[0077]
The above optical disk device may be configured to determine the gain G1 based on the RIM intensity of the 0th-order diffracted light and the ratio between the 0-time diffracted light and the width of the ± 1st-order diffracted light in the optical disc radial direction.
[0078]
Thus, the gain G1 that can suppress the DC component of the signal TES to 0 can be set, and the G1 can be determined at the design stage. Furthermore, if the above-mentioned setting is performed for each optical disk device based on the optical parameters of each element to be used, it is possible to suppress variation in compensation accuracy occurring for each optical disk device.
[0079]
The above optical disc apparatus continuously changes the gain G1 while shifting the objective lens in a state where the focus servo is on and the radial servo is off, and the time when the DC component of the arithmetic signal TES becomes 0 is set as the best value of the gain G1. It may be configured to be determined.
According to the above configuration, the DC component of the signal TES can be suppressed to 0 as much as possible in the radial servo ON state with respect to the shift of the objective lens satisfying the above α ≧ 4b. Thus, the residual offset remaining in the head can be more appropriately suppressed, and highly accurate tracking servo can be performed. That is, the DC component of the tracking error signal does not suddenly increase in response to the shift of the objective lens, and the residual offset amount remaining in the radial direction can be reduced by using a large-capacity optical disc system (a large-capacity optical disc and a recording / reproducing apparatus for this). System with a simple optical disk device) within the allowable range for stable operation.
[0080]
Further, according to the above configuration, the gain G1 can be set at the stage of adjusting the optical system, and by using the above-described method for each device, it is possible to suppress variation in compensation accuracy between devices.
[0081]
In the above optical disc device, the diffractive optical element may include a first diffraction grating in an area other than the area where the first diffraction grating is formed, at least in an area corresponding to a cross section of a light beam incident on the objective lens. A configuration in which a second diffraction grating having different diffraction directions and the same grating depth and duty ratio may be formed.
[0082]
According to the above configuration, in the 0th-order diffracted light (main beam) transmitted through the diffractive optical element, for example, the phase and amplitude of the light flux transmitted through the first diffraction grating and the light transmitted through the second diffraction grating are changed. No distribution occurs. Therefore, it is possible to suppress the deterioration of the S / N such as the crosstalk due to the generation of the side lobe of the focused spot on the optical disk.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of a photodetector included in an optical disc device according to an embodiment of the present invention, and an arithmetic circuit for obtaining a tracking error signal from an output of the photodetector.
FIG. 2 is a schematic diagram illustrating a configuration of an optical system of an optical pickup device included in the optical disc device according to the embodiment of the present invention.
FIG. 3 is a front view of the diffractive optical element shown in FIG.
FIG. 4 is an explanatory diagram showing a relationship between a main beam and a sub beam generated by diffracting an incident light beam by the diffractive optical element shown in FIG.
5A is a schematic diagram illustrating movement of a main beam on a photodetector when a shift of an objective lens occurs, and FIG. 5B is a schematic diagram illustrating movement of the sub beam. FIG. 5C is a schematic diagram illustrating the difference in the amount of movement between the main beam and the sub beam in the above movement.
FIG. 6A is an explanatory view showing a case where a light beam at an outermost peripheral portion of an incident light beam is incident on an aperture forming member, and FIG. It is explanatory drawing which shows the case where it passes through inside.
FIG. 7 is a graph showing a simulation result of a relationship between a shift amount of an objective lens and an offset amount generated in a tracking error signal in a conventional compensation mechanism for a shift of an objective lens.
FIG. 8 is a graph showing a simulation result of a relationship between a shift amount of the objective lens and an offset amount generated in the tracking error signal in the compensation mechanism according to the embodiment of the present invention with respect to the shift of the objective lens.
FIG. 9A is a schematic diagram illustrating movement of a main beam on a photodetector when a sub-beam crosses over a division boundary of the photodetector due to a shift of an objective lens; (B) is a schematic diagram for explaining the movement of the sub-beam on the photodetector in the above case.
FIG. 10 shows the compensation mechanism according to the embodiment of the present invention for the shift of the objective lens when the radial width of the optical disk of the ± 1st-order diffracted light beam is 2.1 mm or 1.2 mm. 11 is a graph showing a simulation result on a relationship between an objective lens shift amount and a remaining track offset amount (residual offset).
FIG. 11 is a simulation of the relationship between the width ratio of the main beam and the sub beam and the gain G1 (when the DC component of the signal TES becomes 0) in the compensation mechanism according to the embodiment of the present invention for the shift of the objective lens. It is a graph which shows a result.
FIG. 12 is a schematic diagram showing a positional relationship between condensed spots of a main beam and a sub beam on an optical disk in the optical disk device according to the embodiment of the present invention.
FIG. 13 is a front view showing another example of the diffractive optical element shown in FIG.
[Explanation of symbols]
1 Semiconductor laser
3 Diffractive optical element
5 Objective lens
6a Objective lens aperture
7 Optical disk
10 Photodetector
11 diffraction grating
12 Effective diameter of objective lens
13 Diameter of light beam incident on diffraction grating
14 Arithmetic circuit (arithmetic unit)
31-33 Photodetector
51 Diffractive optical element
52 diffraction grating
55,57 gain amplifier

Claims (4)

A light source,
An objective lens,
With a diffractive optical element arranged between these light sources and the objective lens,
λ: wavelength of light source a: effective diameter of objective lens d: track pitch of recording medium NA: numerical aperture of objective lens ± 1st time diffracted by the first diffraction grating formed in the diffractive optical element The width α in the radial direction when the folded light is incident on the objective lens satisfies the following conditions,
α ≦ λ × a / (2d × NA)
When the 0th-order diffracted light and ± 1st-order diffracted light are condensed on the optical disk by the objective lens, and the returned light is detected on each photodetector, the photodetectors are directed in a direction perpendicular to the radial direction of the recording medium. Has at least one dividing boundary line in a direction corresponding to
The differential signal of the photodetector that has received the zero-order light is TE1 and the sum output is A1.
The differential signal of the photodetector receiving the +1 order light is TE2, and the sum output is A2
The differential signal of the photodetector receiving the primary light is TE3, and the sum output is A3.
When the gain G2 is G2 = A2 / A3
And when
Using the gain G1, the tracking error signal TES is calculated as TES = TE1−G1 × (TE2 + G2 × TE3)
Optical disc device,
b: When the shift amount of the objective lens is set, the width α in the radial direction of the ± first-order diffracted light is
4b ≦ α
An optical disk device characterized by satisfying the following. 2. The optical disk device according to claim 1, wherein the gain G1 is determined based on the RIM intensity of the 0th-order diffracted light and the ratio of the width of the 0th-order diffracted light and the ± 1st-order diffracted light in the radial direction of the optical disc. The gain G1 is continuously changed while shifting the objective lens with the focus servo ON and the radial servo OFF, and the time when the DC component of the arithmetic signal TES becomes 0 is determined as the best value of the gain G1. The optical disk device according to claim 1, wherein The diffractive optical element has a diffraction direction different from that of the first diffraction grating, at least in a region other than the region where the first diffraction grating is formed, corresponding to a cross section of a light beam incident on the objective lens. 2. The optical disk device according to claim 1, wherein a second diffraction grating having the same depth and the same duty ratio is formed.

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