JP4463238B2 - Objective lens design method - Google Patents

Description

The present invention relates to a method for designing an objective lens used in an optical pickup that records and / or reproduces information on an optical recording medium such as an optical disk by using a light beam having a wavelength of about 405 nm.

  Conventionally, light for recording and / or reproducing information signals on an optical disc such as a CD (Compact Disc) using a light beam having a wavelength of about 785 nm and a DVD (Digital Versatile Disc) using a light beam having a wavelength of about 660 nm. There is a pickup.

  In recent years, with the demand for an increase in the capacity of an optical disk, the recording medium has been increased in density and the beam spot diameter on the signal recording surface of the optical disk has been reduced.

  The beam spot diameter is proportional to the wavelength of the light beam used, but inversely proportional to the numerical aperture NA of the objective lens. Therefore, recently, an objective lens having a NA exceeding 0.8 has been developed using a light source that emits a light beam having a wavelength of about 405 nm.

However, as the NA of the objective lens increases, the depth of focus becomes narrow, and a slight wavelength variation causes a problem that the image point moves and the beam spot is blurred. On the other hand, as shown in FIG. 3, the light beam emitted from the light source has a width of about ± 2 nm with respect to the center wavelength λ ₀ , and generally forms an objective lens as shown in FIG. When the refractive index of the material is around 405 nm, the amount of change in the refractive index with respect to the wavelength is large. Therefore, when there is a difference of about ± 2 nm in the light beam emitted from the light source, as shown in FIG. 5, the image point is moved by the objective lens, and as a result, chromatic aberration is generated, thereby deteriorating jitter. In order to suppress this chromatic aberration, it is necessary to provide a diffractive optical element. In FIG. 5, B ₀ represents a light beam having a center wavelength λ ₀ , and B ₁ and B ₂ represent light beams having a wavelength of about ± 2 nm from the center wavelength.

JP 2002-303787 A

It is an object of the present invention to reduce chromatic aberration and blur a beam spot in an optical pickup that records and / or reproduces information using an optical beam having a wavelength of about 405 nm with respect to an optical recording medium such as an optical disk. An object of the present invention is to provide a design method of an objective lens that can prevent this.

In order to achieve this object, the objective lens designing method according to the present invention is applied to an objective lens used for an optical pickup that records and reproduces an information signal with an optical beam having a wavelength of about 405 nm. any of the surface on the side and the exit side is aspheric, a method of designing a numerical aperture of 0.8 or more der Ru objective lens, the following equation (1), the following equation (2a), the following equation ( 2b), the following formula (2c), the following formula (2d), and the range defined by the following formula (2e) are selected .

However, in this Formula (1) , Formula (2a)-following Formula (2e) ,
Ng: Refractive index at the g-line of the material constituting the objective lens,
Nh: the refractive index of the material constituting the objective lens at the h-line,
Ni: Refractive index at the i-line of the material constituting the objective lens,
f: Focal length (mm) of the objective lens,
d: Thickness (mm) in the optical axis direction at the optical axis position of the objective lens,
n: refractive index at the wavelength used of the material constituting the objective lens,
r ₁ : radius of curvature (mm) of the incident side surface of the objective lens
It is.

Design method of the objective lens according to the present invention, the optical recording medium such as an optical disk, a method of designing the objective lens that is used in an optical pickup for recording and reproducing information by using a light beam having a wavelength of about 405 nm, By making the wavelength dependency of the refractive index within a predetermined range as the material constituting the objective lens, chromatic aberration can be reduced and the beam spot can be prevented from being blurred.

  Hereinafter, an objective lens to which the present invention is applied, an optical pickup 1 using the objective lens, and an optical disc apparatus will be described with reference to the drawings.

  An optical pickup 1 to which the present invention is applied performs information recording / reproduction with respect to an optical disc, a spindle motor as a driving means for rotating the optical disc, and a feed for moving the optical pickup 1 in the radial direction of the optical disc. An optical disk device is configured together with a motor and the like. The optical pickup 1 records and reproduces information with respect to the optical disc rotated by a spindle motor. The optical disk 8 used here is, for example, a high-density recording optical disk capable of high-density recording using a semiconductor laser having an emission wavelength of about 405 nm (blue-violet). The present invention is applicable not only to the above-described optical disc but also to an optical pickup and an optical disc apparatus that perform recording and / or reproduction on an optical recording medium that can be optically recorded and reproduced, and an objective lens used therefor. The

  As shown in FIG. 1, an optical pickup 1 to which the present invention is applied includes a light source unit 3 that emits a light beam having a wavelength of about 405 nm, and a diffractive optical element that divides the light beam emitted from the light source unit 3 into three beams. 4, the beam splitter 5 that reflects or transmits the light beam divided by the diffractive optical element 4 and the reflected light from the optical disc 8, the collimator lens 6, and the light beam emitted from the light source unit 3 is recorded on the optical disc 8. The objective lens 7 which condenses on a surface and the photodetector 9 which detects the return light beam reflected by the optical disk 8 are provided.

  The light source unit 3 includes a light emitting unit that emits a light beam having a wavelength of about 405 nm. The wavelength of the light beam emitted from the light source unit 3 is not limited to this. The diffractive optical element 4 is provided between the light source unit 3 and the beam splitter 5 and is provided with a diffractive unit that divides the light beam emitted from the light source unit 3 into three beams.

  The beam splitter 5 is disposed on the optical path between the diffractive optical element 4 and the collimator lens 6, and has a half mirror surface 5 a on the side close to the light source unit 3. The beam splitter 5 reflects the light beam that has been divided into three by the diffractive optical element 4 and emitted toward the optical disk 8 side by the half mirror surface 5a. Further, the beam splitter 5 transmits the return light beam reflected by the optical disc 8 and emits it to the photodetector 9 side. That is, the beam splitter 5 is an optical element that branches the optical path of the return light beam from the optical path of the forward light beam.

  The collimator lens 6 is disposed between the beam splitter 5 and the objective lens 7 and converts the light beam reflected by the beam splitter 5 into parallel light.

  The objective lens 7 focuses the light beam that has been made substantially parallel light by the collimator lens 6 on the signal recording surface of the optical disk 8. An aperture stop (not shown) is provided on the incident side of the objective lens 7, and this aperture stop limits the aperture so that the numerical aperture of the light beam incident on the objective lens 7 becomes a desired numerical aperture.

  The objective lens 7 is a single objective lens having a numerical aperture (NA) of 0.8 or more, and both the first surface 7a on the incident side and the second surface 7b on the emission side are aspherical. These are formed of a material having a wavelength dependency of the refractive index as will be described later. The objective lens 7 is formed of a material that satisfies a predetermined condition as described later, thereby reducing chromatic aberration and obtaining an appropriate beam spot.

  The photodetector 9 has a photodetector for receiving each of the light beams reflected by the signal recording surface of the optical disc 8, and detects various signals such as a tracking error signal and a focus error signal together with the information signal.

  The optical pickup 1 configured as described above drives the objective lens 7 on the basis of the focus servo signal and tracking servo signal generated by the return light detected by the photodetector 9, and performs focus servo and tracking. Servo is performed. When the objective lens 7 is driven, the objective lens 7 is moved to a focus position for focusing on the signal recording surface of the optical disc 8, and the light beam is focused on the recording surface of the optical disc 2, so Records or reproduces information.

  Here, the objective lens 7 to which the present invention used for the optical pickup 1 described above is applied will be described in more detail.

The radius of curvature of the first surface 7a is a surface on the light source side of the objective lens 7 is a single lens objective lens (incident side) and r _1, the curvature of the second face 7b radius is the surface of the optical disc 8 side (exit side) R ₂ , the thickness (center thickness) of the objective lens 7 in the optical axis direction at the optical axis position is d (mm), the focal length of the objective lens 7 is f (mm), and the objective lens 7 is configured. The following relational expression (3) is established, where n is the refractive index of the material to be used at the wavelength used (405 nm)

By differentiating both sides of the equation (3) by n, the further erased _{r 2,} the following equation (4) is obtained.

  Here, in order to collect the variables and organize the equations, “k” is set as in the following equation (5), and when this equation (5) is substituted into the equation (4), the following equation (6) is obtained. .

  Here, by setting d / f to 0.8 or more, the spherical aberration in the paraxial region can be reduced. Further, by setting d / f to 1.6 or less, coma aberration generated when the center axis shift between the first surface and the second surface of the objective lens 7 occurs can be reduced.

  By the way, k shown in the equation (5) has a relationship substantially dependent on d / f in order to perform a design in which the occurrence of coma in the paraxial region is suppressed, and d / f is expressed by the equation (7). When the relationship is satisfied and n satisfies the relationship of 1.5 ≦ n ≦ 1.75, k described above is expressed by the following equation (8). This range of the refractive index n is a range necessary for increasing the radius of curvature of the first surface, facilitating manufacture of the objective lens, and suppressing astigmatism occurring in off-axis optical performance.

  Here, assuming that the value of chromatic aberration is Δ [mm / nm], the relationship with the left side of the above equation (6) is calculated as in the following equation (9). When approximated to be small, the following equation (10) is obtained. Expression (6) described above using Expression (10) can be modified as shown in Expression (11) below. Here, the chromatic aberration Δ indicates the amount of change in focal length when the wavelength changes.

  In order to prevent the image point from moving due to slight wavelength fluctuations and blurring of the beam spot on the signal recording layer of the optical disc, it is necessary to reduce chromatic aberration, and it is desirable to satisfy the following equation (12). .

  Here, in order to rearrange the formula (11), the material constituting the objective lens 7 in each of g-line (about 435.8 nm), h-line (about 404.7 nm), and i-line (about 365.0 nm) The following formula (13) can be defined using these as refractive indexes of Ng, Nh, and Ni. In Equation (13), λg and λi are the wavelengths [nm] of the g-line and i-line, respectively, and λg−λi is 70.82 [nm].

  Substituting Equation (12) and Equation (13) into Equation (11) yields the following Equation (14), which can be further transformed into the following Equation (15).

  Here, the value of the fourth term on the right side of the equation (15) is Q as shown in the following equation (16). As shown in FIG. 2, Q changes depending on the value of d / f and the refractive index n. When the value of d / f is fixed, it increases monotonously as n increases. In order to increase the radius of curvature of the first surface of the objective lens 7 and facilitate the manufacture of the objective lens, 1.5 ≦ n is required. The value of Q at n = 1.5 is expressed by the following formula ( It can be approximated as shown in 17).

In FIG. 2, a curve L ₁ shows a change in the Q value accompanying a change in the refractive index n when d / f = 0.8, and a curve L ₂ shows a change when d / f = 1.027. The curve L ₃ shows the change of the Q value with the change of the refractive index n when d / f = 1.177, and the curve L ₄ shows the change of the Q value with the change of the refractive index n. The curve L ₅ shows the change of the Q value with the change of the refractive index n when d / f = 1.6. The curve L ₅ shows the change of the Q value with the change of the refractive index n when /f=1.364. It is shown.

  As shown in FIG. 2, the value of Q monotonically increases with respect to the increase in the refractive index n within the range shown in FIG. Is larger than the value obtained by the equation (17), and from this relationship and the above equation (15), the relationship of the following equation (18) and the following equation (19) is established.

  Further, in order to secure further excellent recording / reproducing performance, it is desirable that the chromatic aberration Δ satisfies the following expression (20).

  When this equation (20) is used to transform the equation in the same manner as described above, the following equations (21) and (22) are obtained.

As described above, the objective lens according to the present invention, the wavelength dependence of the refractive index as the material constituting the Rutotomoni objective lens is configured in shape so as to satisfy the above equation (18) and (19) By combining with a predetermined range, chromatic aberration can be reduced, and blurring of the beam spot can be reduced.

  In addition, the objective lens to which the present invention is applied can suppress the change in focal length when the wavelength of the light beam emitted from the light source fluctuates. In such a case, the chromatic aberration can be reduced. .

  Furthermore, the objective lens to which the present invention is applied can reduce the chromatic aberration and reduce the blur of the beam spot, and at the same time, enables a lens design that ensures image height characteristics and a wide eccentricity tolerance.

  In addition, the objective lens 7 to which the present invention is applied does not require a diffractive structure for suppressing chromatic aberration to be formed in the objective lens, so that the transmittance of the objective lens can be increased, and the objective is to suppress chromatic aberration. Since it is not necessary to provide the diffractive optical element in the optical pickup, the utilization efficiency of the light beam can be increased, the output of the light source unit can be reduced, and the cost of the light source unit can be reduced. Moreover, the objective lens to which the present invention is applied can design an objective lens having good aberration characteristics by keeping the lens thickness and focal length optimal.

  In addition, the objective lens to which the present invention is applied is configured to satisfy the above formulas (21) and (22), so that the chromatic aberration can be further reduced and the blur of the beam spot is further reduced. In addition, an appropriate spot shape can be obtained.

  In addition, the optical pickup and the optical disc apparatus to which the present invention is applied use such an objective lens to reduce chromatic aberration and blur the beam spot without providing a diffractive optical element for reducing chromatic aberration. This makes it possible to obtain good recording and / or reproduction characteristics.

Examples 1 to 4 of the objective lens to which the present invention is applied will be described below with specific numerical values.

  In the following embodiments, the first surface of the objective lens is the light source side surface, and the second surface is the optical disk side surface. The optical disk is a parallel plate having a thickness of 87.5 um. Further, the design wavelength is 405 nm, and the refractive index of each optical disk is 1.61692. F represents the focal length (mm) of the objective lens, R represents the radius of curvature (mm) of the objective lens, d represents the thickness (mm) of the objective lens at the optical axis position, and n represents The refractive index at the working wavelength (405 nm) of the objective lens is shown, and NA denotes the numerical aperture of the objective lens when the light beam is focused on the optical disk.

The aspherical shapes of the first surface and the second surface of the objective lens 7 are given by the following equation (23). In Expression (23), h represents a distance (mm) from the optical axis, and Z (h) represents a distance (mm) from the tangent plane of the surface vertex at a position where the distance from the optical axis is h. Where K represents the estimated number of circles and A _i represents the i-th order aspheric coefficient.

"Example 1 "
In the objective lens of Example 1 , f, n, d, and NA are as follows.
f: 2.2 [mm]
n: 1.5966
d: 2.59 [mm]
NA: 0.85

In addition, R, K, and A _i indicating the shape of the first surface on the incident side of the objective lens of Example 1 are as follows.
<First surface shape>
R: 1.50042 [mm]
K: -0.70037
A ₄ : 1.04045 × 10 ⁻²
A ₆ : −7.19398 × 10 ⁻⁴
A ₈ : 2.99815 × 10 ⁻³
A ₁₀ : −1.66248 × 10 ⁻³
A ₁₂ : 3.28269 × 10 ⁻⁴
A ₁₄ : 2.20028 × 10 ⁻⁴
A ₁₆ : -1.556511 × 10 ⁻⁴
A ₁₈ : 3.94403 × 10 ⁻⁵
A _20: -3.72388 × ^{10 -6}

Further, R, K, and A _i indicating the shape of the second surface on the emission side of the objective lens of Example 1 are as follows.
<Second surface shape>
R: -3.71882 [mm]
K: -86.0664
A ₄ : 8.3268 × 10 ⁻²
A ₆ : −8.884698 × 10 ⁻²
A ₈ : 7.27801 × 10 ⁻²
A ₁₀ : -4.16242 × 10 ⁻²
A ₁₂ : 1.276658 × 10 ⁻²
A _14: -1.36042 × ^{10 -3}
A ₁₆ : -4.20916 × 10 ⁻⁵
A ₁₈ : -4.29466 × 10 ⁻⁵
A ₂₀ : 1.06655 × 10 ⁻⁵

  At this time, d / f = 1.177, k shown in the above equation (5) is k = 1.122, the on-axis wavefront aberration is 2.9 mλrms, and the off-axis at an angle of view of 0.5 °. Wavefront aberration is also kept low at 23.9 mλrms. Further, if the glass material has a refractive index Ng = 1.5925 for g-line, a refractive index Nh = 1.5966 for h-line, and a refractive index Ni = 1.6036 for i-line, (Nh / (Ni−Ng)) = 143.8, which satisfies the expressions (18) and (19), and the expressions (21) and (22). Actually, the chromatic aberration is 0.44 um / nm, and blurring of the beam spot due to wavelength variation can be suppressed.

"Example 2 "
In the objective lens of Example 2 , f, n, d, and NA are as follows.
f: 1.765 [mm]
n: 1.5965
d: 2.078 [mm]
NA: 0.85

Further, R, K, and A _i indicating the shape of the first surface on the incident side of the objective lens of Example 2 are as follows.
<First surface shape>
R: 1.20743 [mm]
K: -0.63742
A ₄ : 1.63026 × 10 ⁻²
A ₆ : 6.09412 × 10 ⁻⁴
A ₈ : 3.82809 × 10 ⁻³
A ₁₀ : −1.53316 × 10 ⁻³
A ₁₂ : 4.004485 × 10 ⁻⁴
A ₁₄ : 2.54213 × 10 ⁻⁴
A ₁₆ : -1.39315 × 10 ⁻⁴
A ₁₈ : 4.33184 × 10 ⁻⁵
A ₂₀ : −6.30045 × 10 ⁻⁶

In addition, R, K, and A _i indicating the shape of the second surface on the emission side of the objective lens of Example 2 are as follows.
<Second surface shape>
R: -2.993725 [mm]
K: -121.9082
A ₄ : 1.03086 × 10 ⁻¹
A ₆ : -9.15242 × 10 ⁻²
A ₈ : 6.80729 × 10 ⁻²
A ₁₀ : -4.227514 × 10 ⁻²
A ₁₂ : 1.32586 × 10 ⁻²
A ₁₄ : −3.6774 × 10 ⁻⁴
A ₁₆ : 8.89052 × 10 ⁻⁴
A ₁₈ : -4.008131 × 10 ⁻⁴
A ₂₀ : −2.15241 × 10 ⁻⁴

  At this time, d / f = 1.177, k shown in the above equation (5) is k = 1.125, the on-axis wavefront aberration is 4.3 mλrms, and the off-axis at an angle of view of 0.5 °. Wavefront aberration is also kept as low as 30.7 mλrms. If the glass material has a refractive index Ng = 1.5914 for g-line, a refractive index Nh = 1.5966 for h-line, and a refractive index Ni = 1.6054 for i-line, (Nh / (Ni−Ng)) = 114.0, which satisfies the expressions (18) and (19), and the expressions (21) and (22). Blur of the beam spot due to wavelength variation can be suppressed. Actually, the chromatic aberration is 0.45 um / nm, and blurring of the beam spot due to wavelength variation can be suppressed.

"Example 3 "
In the objective lens of Example 3 , f, n, d, and NA are as follows.
f: 2.2 [mm]
n: 1.6037
d: 2.59 [mm]
NA: 0.85

In addition, R, K, and A _i indicating the shape of the first surface on the incident side of the objective lens of Example 3 are as follows.
<First surface shape>
R: 1.50936 [mm]
K: -0.66148
A ₄ : 8.1256 × 10 ⁻³
A ₆ : -2.49464 × 10 ⁻⁴
A ₈ : 2.33274 × 10 ⁻³
A ₁₀ : −1.3762 × 10 ⁻³
A ₁₂ : 2.70202 × 10 ⁻⁴
A ₁₄ : 2.33322 × 10 ⁻⁴
A ₁₆ : -1.669667 × 10 ⁻⁴
A ₁₈ : 4.44355 × 10 ⁻⁵
A ₂₀ : -4.36009 × 10 ⁻⁶

In addition, R, K, and A _i indicating the shape of the second surface on the emission side of the objective lens of Example 3 are as follows.
<Second surface shape>
R: -3.991425 [mm]
K: -75.17455
A ₄ : 9.21876 × 10 ⁻²
A ₆ : −9.889535 × 10 ⁻²
A ₈ : 7.33139 × 10 ⁻²
A ₁₀ : 3.8594 × 10 ⁻²
A ₁₂ : 1.15928 × 10 ⁻²
A ₁₄ : −1.4702 × 10 ⁻³

  At this time, d / f = 1.177, k shown in the above equation (5) is k = 1.122, the on-axis wavefront aberration is 2.2 mλrms, and the off-axis at an angle of view of 0.5 °. Wavefront aberration is also kept low at 25.5 mλrms. Further, if the glass material has a refractive index Ng = 1.5996 for g-line, a refractive index Nh = 1.6037 for h-line, and a refractive index Ni = 1.6108 for i-line, (Nh / (Ni−Ng)) = 143.2, which satisfies the expressions (18) and (19), and the expressions (21) and (22). Blur of the beam spot due to wavelength variation can be suppressed. Actually, the chromatic aberration is 0.44 um / nm, and blurring of the beam spot due to wavelength variation can be suppressed.

"Example 4 "
In the objective lens of Example 4 , f, n, d, and NA are as follows.
f: 2.2 [mm]
n: 1.6459
d: 2.59 [mm]
NA: 0.85

Further, R, K, and A _i indicating the shape of the first surface on the incident side of the objective lens of Example 4 are as follows.
<First surface shape>
R: 1.55744 [mm]
K: -0.66079
A ₄ : 7.88222 × 10 ⁻³
A ₆ : −1.85348 × 10 ⁻⁴
A ₈ : 2.22664 × 10 ⁻³
A ^_10: -1.38148 × ₁₀ ^-3
A ₁₂ : 2.84263 × 10 ⁻⁴
A ₁₄ : 2.33304 × 10 ⁻⁴
A ₁₆ : -1.71022 × 10 ⁻⁴
A ₁₈ : 4.4247 × 10 ⁻⁵
A _20: -4.21752 × ^{10 -6}

Further, R, K, and A _i indicating the shape of the second surface on the emission side of the objective lens of Example 4 are as follows.
<Second surface shape>
R: -5.63388 [mm]
K: -143.222731
A ₄ : 9.60818 × 10 ⁻²
A ₆ : −1.03370 × 10 ⁻¹
A ₈ : 7.42406 × 10 ⁻²
A ₁₀ : -3.88704 × 10 ⁻²
A ₁₂ : 1.16908 × 10 ⁻²
A ₁₄ : -1.504 × 10 ⁻³

  At this time, d / f = 1.177, k shown in the above formula (5) is k = 1.122, the on-axis wavefront aberration is 3.1 mλrms, and the off-axis at an angle of view of 0.5 °. Wavefront aberration is also kept low at 25.4 mλrms. If the glass material has a refractive index Ng = 1.6417 for g-line, a refractive index Nh = 1.6459 for h-line, and a refractive index Ni = 1.6532 for i-line, (Nh / (Ni−Ng)) = 143.1, which satisfies the expressions (18) and (19), and the expressions (21) and (22). Blur of the beam spot due to wavelength variation can be suppressed. Actually, the chromatic aberration is 0.44 um / nm, and blurring of the beam spot due to wavelength variation can be suppressed.

Thus, according to the objective lenses of Examples 1 to 4 described above, when used in an optical pickup that uses a wavelength near 405 nm as a light source, a glass material having a wavelength dependency of the refractive index within the above range is used. By using it, chromatic aberration can be reduced and the beam spot can be prevented from blurring.

1 is an optical path diagram showing an optical system of an optical pickup to which the present invention is applied. It is a figure which shows the d / f value of Q value, and the dependence of a refractive index, and is a figure which shows the change of Q value with respect to the change of the refractive index for every d / f value. It is a wavelength characteristic figure of a light source part. It is a figure which shows that a refractive index changes according to the wavelength of the material which comprises an objective lens. It is a figure which shows that a depth of focus changes according to the wavelength of the light beam condensed with an objective lens.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical pick-up, 3 Light source part, 4 Diffractive optical element, 5 Beam splitter, 6 Collimator lens, 7 Objective lens, 8 Optical disk, 9 Optical detector

Claims (1)

Used for an optical pickup that records and reproduces an information signal with an optical beam having a wavelength of about 405 nm with respect to an optical recording medium,
Both the incident side and emission side surfaces are aspherical,
Numerical aperture (NA) is a method of designing der Ru objective lens at least 0.8,
An objective lens design method for selecting a range defined by the following formula (1), the following formula (2), the following formula (3), the following formula (4), the following formula (5), and the following formula (6).

However,
Ng: Refractive index at the g-line of the material constituting the objective lens,
Nh: the refractive index of the material constituting the objective lens at the h-line,
Ni: Refractive index at the i-line of the material constituting the objective lens,
f: Focal length (mm) of the objective lens,
d: Thickness (mm) in the optical axis direction at the optical axis position of the objective lens,
n: refractive index at the wavelength used of the material constituting the objective lens,
r ₁ : radius of curvature (mm) of the incident side surface of the objective lens
It is.

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