JP2006252770A - Objective lens for optical disk - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens for optical disk in accordance with a double-sided aspheric single lens having numerical aperture NA of ≥0.75 and superior in axial aberration, off-axis aberration, and eccentricity aberration between both sides. <P>SOLUTION: The objective lens for optical disk is the single lens having aspheric both sides and numerical aperture NA of ≥0.75, and has the optical characteristics in which an angle u1' defined by the optical axis and a light ray having the maximum height inside the lens satisfies expressions of (1-D)sin(K)<sin(u1')<(1+D)sin(K), K=(0.60866-0.11t/f-0.1272d/f)(0.83+0.2NA)NA/0.85, where f is the focal length of the lens, t is the thickness of the center of the lens, d is the thickness of a transmission layer of the optical disk which is irradiated with the light through the lens, and D is 0.06. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an objective lens for an optical disc having a high numerical aperture (NA) that realizes a large-capacity optical disc.

  Conventionally, a CD disk is read or written with a laser beam having a wavelength of about 780 nm using an objective lens having a numerical aperture (NA) of 0.45 to 0.5. In addition, a DVD disk is read or written with a laser beam having a wavelength of about 650 nm using an objective lens having a numerical aperture of about 0.6.

  By the way, in order to increase the capacity of the optical disk, development of a next-generation optical disk pickup system using a light source having a shorter wavelength and a lens having a higher numerical aperture has been advanced.

  As a laser having a shorter wavelength, a so-called blue laser having a wavelength of about 400 nm is considered.

  For example, the following system has been reported as the objective lens having a high numerical aperture.

(A) Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 978-979 M. Itonaga et al.
“Optical Disk System Using High-Numerical Aperture Single Objective Lens
and Blue LD ”.
(B) Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 937-942 I. Ichimura et al.
“Optical Disk Recording Using a GaN Blue-Violet Laser Diode”.
Here, (A) reports a system using a single lens with a numerical aperture of 0.7, and (B) reports a system using a two-group lens with a numerical aperture of 0.85.

  Another feature is that the thickness of the playback transmissive layer of the disc is reduced from 1.2 mm for CD to 0.6 mm for DVD in order to counter the decrease in system margin due to higher NA. is there. According to (A), it is 0.12 mm, and according to (B), it is 0.1 mm. Although it depends on how the system margin is distributed, it is desired that the transmission layer be thinner than about 0.3 mm.

  Although the system using the two-group lens of (B) has a larger numerical aperture than that of (A), it requires an assembly process and requires two lenses, so it is inferior in mass production and high in cost. Become.

  Therefore, an optical disk objective lens using a single lens having a numerical aperture of 0.7 or more is desired for the next generation system.

Japanese Patent Laid-Open No. 4-163510 discloses an objective lens using a single lens having a numerical aperture of about 0.6 to 0.8.
JP-A-4-163510 Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 978-979 M. Itonaga et al. “Optical Disk System Using High-Numerical Aperture Single Objective Lens and Blue LD”. Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 937-942 I. Ichimura et al. “Optical Disk Recording Using a GaN Blue-Violet Laser Diode”.

  It is conventionally known that a lens having a high numerical aperture can be designed. For example, “Research on Aspherical Aplanato Lenses with a Large Aperture Ratio” (Shotaro Yoshida, Tohoku University Research Institute for Scientific Research, March 1958) describes in detail how to design a double-sided aspherical lens with a high numerical aperture. It is written.

  However, a lens having a high numerical aperture cannot be manufactured simply by saying that the design is possible. In order to actually manufacture such a lens, it is necessary to have a design that ensures manufacturing tolerances. Furthermore, in order to reduce the influence when the wavelength of the light source fluctuates or when there is a width in the wavelength, the lens needs to be less affected by chromatic aberration.

  Here, in the case of a double-sided aspheric lens, the most severe and important manufacturing tolerance is the eccentricity between the surfaces (eccentricity between the surfaces). Therefore, it is necessary to satisfy the design tolerance of the objective lens represented by the on-axis aberration, which is an aberration in the case of normal incidence to the objective lens, and the off-axis aberration, which is an aberration in the case of oblique incidence, and manufacturing tolerances at the same time. .

  However, it is difficult to achieve both lens design performance and manufacturing tolerance, especially when the numerical aperture is higher than 0.75.

  In fact, in such a double-sided aspheric lens, the off-axis aberration is deteriorated as the numerical aperture is increased even when designed without considering the manufacturing tolerance described above, and becomes worse when the manufacturing tolerance is taken into consideration. That is, in order to ensure a large eccentricity tolerance, it is necessary to sacrifice on-axis aberration characteristics and off-axis aberration characteristics.

On-axis aberrations are only slightly degraded even when considering eccentricity tolerances, but off-axis aberrations can be manufactured for lenses with high numerical apertures that exceed 0.6. It will be considerably sacrificed if the tolerance of micron order is secured.
With respect to chromatic aberration, priority is given to the fact that the lens itself can be manufactured. Therefore, it is necessary to obtain a lens shape having as good a chromatic aberration characteristic as possible while satisfying manufacturing tolerances.

  For the reasons described above, a search for a shape of a double-sided aspheric lens with good performance has been made conventionally, and various documents have been reported. JP-A-5-241069 and JP-A-4-163510 are examples thereof.

  Japanese Patent Application Laid-Open No. 4-163510 describes a shape range of a lens having good performance. However, this document does not mention securing an eccentricity tolerance. The lens of Example 2 having a numerical aperture of more than 0.75 (specification with a wavelength of 532 nm and a numerical aperture of 0.8) has a problem that a large aberration occurs even with a slight decentration. There is no description about chromatic aberration.

  Furthermore, the range shown by these prior documents is quite wide, and there is a problem that a good lens cannot be actually designed in these ranges.

  The present invention has been proposed in view of the above-described problems, and has a high numerical aperture for realizing a large-capacity optical disk, in particular, a numerical aperture NA of 0.75 or more, and NA on-axis aberration, off-axis aberration and inter-surface aberration. It is an object of the present invention to provide an objective lens for an optical disk using a single lens having a double-sided aspheric surface, which is excellent in decentration aberration and excellent in chromatic aberration characteristics.

In order to solve the above-described problems, the present invention provides an objective lens for an optical disc having the following configurations (1) to (4).
(1) In an optical disk objective lens applied to an optical disk having a transmission layer, the single lens having a numerical aperture NA of 0.75 or more whose both surfaces are aspheric surfaces,
An objective lens for an optical disc, characterized in that an angle u1 ′ formed with the optical axis of the light beam having the maximum height inside the lens has optical characteristics satisfying the following expression.
(1-D) sin (K) <sin (u1 ′) <(1 + D) sin (K)
K = (0.60866-0.11 · t / f-0.1272 · d / f)
(0.83 + 0.2 ・ NA) ・ NA / 0.85
Where f is the focal length of the lens, t is the center thickness of the lens,
d is the thickness of the transmission layer of the optical disc that is irradiated with light through the lens;
D is 0.06.
(2) The objective lens for an optical disk according to claim 1, wherein the center thickness t and the focal length f satisfy the following expression.
t> (1 + E) f
Here, E is a number of 0 or more.
(3) The objective lens for an optical disk according to claim 1 or 2, wherein the imaging magnification is 0.
(4) The objective lens for an optical disc according to any one of claims 1 to 3, wherein the wavelength of light irradiated to the optical disc through the lens is 450 nm or less.

  The present invention having the above-described means has a high numerical aperture for realizing a large-capacity optical disk, in particular, a numerical aperture NA of 0.75 or more, excellent NA axial aberration, off-axis aberration and decentering aberration between surfaces, and chromatic aberration. It is possible to provide an objective lens for an optical disk using a single lens having a double-sided aspherical surface that has excellent characteristics.

  Hereinafter, embodiments of an objective lens for an optical disc according to the present invention will be described in detail with reference to the drawings.

  First, prior to the description of the conditional expressions satisfied by the objective lens for an optical disk according to the present embodiment, basic balance of on-axis aberration characteristics, off-axis aberration characteristics, and eccentricity tolerance regarding the design of the lens according to the present embodiment will be described. To do. Here, the eccentricity tolerance is defined as an increase in wavefront aberration when there is an eccentricity.

  In the present embodiment, it is required to balance the following three conditions in order to ensure on-axis aberration, off-axis aberration, and eccentricity tolerance.

  (1) The spherical aberration of the lens is corrected to ensure axial aberration.

  (2) The lens satisfies the sine condition to ensure off-axis aberrations.

  (3) In order to ensure eccentricity tolerance, the second surface alone satisfies the sine condition.

  In addition, in order to prevent deterioration of chromatic aberration characteristics, it is required to satisfy the following conditions.

  (4) The increase in aberration of the best image plane at each wavelength when there is a wavelength error is small. This is called spherical aberration due to wavelength error.

  (5) In order to suppress an increase in aberration when the light source has a wavelength spread, it is required that the change in the focal position due to the wavelength change be small. Wavelength broadening occurs when the laser is multimoded by applying high frequency superposition to improve the noise characteristics of the semiconductor laser. Here, the fact that the change in the focal length is small due to the wavelength change means that the axial chromatic aberration is required to be small.

  In the following, a basic form of a lens that secures on-axis aberration and off-axis aberration will be described in detail, and then a lens system pair with good chromatic aberration characteristics will be described.

  The double-sided aspherical lens can simultaneously satisfy the two conditions (1) and (2) for ensuring on-axis aberration and off-axis aberration. A lens that satisfies the conditions (1) and (2) at the same time is called an aplanat.

  However, generally, if the conditions (1) and (2) are satisfied, the condition (3) for securing the eccentricity tolerance cannot be satisfied.

  By the way, when the condition (2) is satisfied and (3) is substantially satisfied, the entire lens satisfies the sine condition and the second surface also substantially satisfies the sine condition. The sine condition is almost satisfied in the relationship between the angle and the refraction angle.

  Further, in the present embodiment, the conditions (1) and (2) for ensuring the on-axis aberration and the off-axis aberration and the condition (3) for ensuring the eccentricity tolerance are balanced and almost satisfied. By satisfying the satisfaction of the condition (3), it is possible to secure an eccentricity tolerance capable of manufacturing a lens while ensuring on-axis aberrations and off-axis aberrations.

  According to the above-mentioned “Research on Aspherical Aperon Lenses with a Large Aperture Ratio” (Shotaro Yoshida, Tohoku University Scientific Research Laboratory Report, March 1958) It has been clarified that when the radius is changed by bending, a lens satisfying the conditions (1) and (2) at the same time can be obtained in the range of a combination of considerably wide vertex radii.

  Furthermore, according to Yasuhiro Tanaka “Aplanatic Single Lens Design and Application to Disc Optical System”, Optics 27, 12 (1998) p720, a lens that is resistant to decentering between surfaces satisfies the condition (3). It is shown.

  Here, among the designs of the aspherical lenses that satisfy the conditions (1) and (2), if there is a lens that satisfies the condition (3), it can be said that the lens is strong against eccentricity tolerance. However, as described above, these cannot be completely satisfied at the same time. Furthermore, according to the analysis of the inventors of the present application, it has been found that the larger the numerical aperture, the greater the deviation from completeness for the conditions (1) to (3).

  In fact, if the conventional numerical aperture lens for DVD discs is 0.6 or the numerical aperture for CD discs is 0.45, the numerical aperture is so low that the apex radius can be set in a fairly wide range. Even if it is changed, the increase in aberration is small, and a balance between the on-axis aberration and the off-axis aberration can be easily obtained. That is, regardless of the radius, the eccentricity tolerance can be increased if the off-axis aberration or the on-axis aberration is slightly sacrificed.

  On the other hand, when the numerical aperture is increased and the wavelength is shortened, the aberration increases in inverse proportion to the wavelength, so there is no design margin. Therefore, it has been necessary to determine the shape (paraxial shape) more strictly for such a lens.

  Here, when the inventor of the present application determines the focal length of the lens, the thickness of the lens, and the thickness of the disk, in a lens having a large eccentricity tolerance, the light beam incident on the lens at the same height is It has been found that they have almost the same angle with respect to the optical axis inside the lens regardless of the refractive index of the lens. It has also been found that the angle depends on the thickness of the disc and the lens.

  In the present embodiment, such conditions are utilized to ensure each condition correspondingly while balancing the conditions (1) to (3).

  FIG. 1 is a diagram illustrating the form of a lens.

  The objective lens 11 refracts the incident light beam L1 and condenses it on the signal recording surface of the optical disc 21. The radius of curvature at the vertex of the first surface 1 of the objective lens 11 is R1, and the radius of curvature at the vertex of the second surface 2 is R2. The center thickness (also referred to as axial thickness) of the lens 11 is t, and the thickness of the transmission layer of the optical disc 21 is d. Further, the working distance of the lens 11 is DW.

  FIG. 2 is a diagram for explaining the angle of light rays inside the lens and the imaging magnification of the second surface.

  With respect to the light beam L1 incident on the objective lens 11 parallel to the optical axis, the light beam having the maximum height is refracted by the first surface 1 of the objective lens 11 to form an angle of u1 with the optical axis. Refracted at the surface 2 to form an angle of u2 with the optical axis.

  Here, an expression when the numerical aperture is 0.85 is shown. u1 is an angle formed by the light beam having the maximum height refracted on the first surface 1 and the optical axis, and the numerical aperture is 0 when the aspherical surface is formed by an planar that satisfies the conditions (1) and (2). It is also the angle that 85 rays make. As described above, this angle is determined by the center thickness t of the lens with respect to the focal length f of the lens and the optical disc thickness d in a lens having good characteristics, and does not depend on the refractive index of the lens. This equation is shown below.

sin (u1) = 0.60866-0.11t / f-0.1272d / f
... (6)
FIG. 3 is a diagram showing the relationship between the regression equation for obtaining u1 and the data used for obtaining the regression equation with respect to the lens thickness. The symbol ◆ in the figure represents the actual design value, and the straight line in the figure represents the value obtained from the regression equation.

  The actual design values were designed by changing the lens thickness t when the focal length f is 2 mm, the refractive index n of the lens glass material is 1.75, and the thickness d of the transmission layer of the optical disk is 0.1 mm. The value of things. The actual design value and the value obtained from the regression equation are in good agreement, indicating the validity of the regression equation.

  FIG. 4 is a diagram showing the relationship between the regression equation for obtaining u1 and the data used for obtaining the regression equation with respect to the disc thickness. The symbol ◆ in the figure represents the actual design value, and the straight line in the figure represents the value obtained from the regression equation.

  The actual design value is a value designed by changing the disc thickness d when the focal length f is 2 mm, the refractive index n of the glass material is 1.75, and the thickness t of the transmission layer of the lens is 3 mm. The actual design value and the value obtained from the regression equation are in good agreement, indicating the validity of the regression equation.

  Now, it is easy to use the paraxial formula in order to determine the lens constants using Equation (6).

  By the way, in this lens, the first surface and the second surface are independent and substantially satisfy the sine condition. Here, the relationship between u1 and u2 is determined by the imaging action on the second surface. By the way, the fact that the surface alone satisfies the sine condition means that the imaging magnification of the actual ray on the surface takes the same constant value as the paraxial magnification regardless of the height of the ray.

  That is, if the paraxial imaging magnification of the second surface is β, the following relationship is established. Here, up1 and up2 are inclinations of paraxial rays, and u1 and u2 are inclinations of real rays.

β = n · up1 / up2 = n · sin (u1) / sin (u2)
Here, since the numerical aperture of the light beam having the maximum height, that is, sin (u2) is 0.85, the following equation is obtained.

β = n · sin (u1) /0.85
Further, the inventor of the present application has found that u1 (and up1 and β) changes according to the numerical aperture.

  If conditions (1) and (2) are satisfied, a slight error remains in condition (3), but if condition (3) is satisfied at the outermost periphery of the lens where the refraction angle of the light beam is the largest. This is because it is strongest in eccentricity. For this reason, β depends on the numerical aperture.

  Β ′ obtained by generalizing β considering the numerical aperture is given by the following equation. The imaging magnification β ′ of the second surface takes into account the numerical aperture (NA).

β ′ = β (0.83 + 0.2 · NA)
FIG. 5 is a diagram showing the relationship between the data used for obtaining the change in the imaging magnification β ′ according to the numerical aperture and the regression equation. The symbol ◆ in the figure represents the actual design value, and the straight line in the figure represents the value obtained from the regression equation.

  The actual design value is a value designed by changing the numerical aperture when the focal length f is 2 mm, the refractive index n of the glass material is 1.75, and the lens thickness t is 2 mm. The actual design value and the value obtained from the regression equation are in good agreement, indicating the validity of the regression equation.

  Here, there is the following relationship between R1 and the magnification β ′.

R1 = f (n−1) / β ′
FIG. 6 is a diagram for explaining the derivation of this equation.

  A lens 111 shown in FIG. 6A has a refractive index n, a first surface 101 having a curvature R101, and a second surface 102 having an infinite curvature R102, and a light beam L101 parallel to the optical axis is incident thereon. . The second surface is a plane because the curvature R102 is infinite. In this case, the following relationship exists between the focal length f ′, the curvature R101 of the first surface 101, and the refractive index n.

f ′ = R / (n−1)
FIG. 6B shows a case where the image field (image space) 112 has a refractive index n. A light ray L101 parallel to the optical axis that is incident on the first surface 101 having the curvature R101 intersects the optical axis at a distance L from the apex of the first surface 101. In this case, the following relational expression holds.

f ′ = L / n
Therefore, the curvature R101 of the first surface 101 can be expressed as follows.

R101 = (n−1) f ′ = (n−1) / n · L
FIG. 6C is a diagram for explaining expansion to a biconvex lens. A light ray parallel to the optical axis having the maximum height h is incident on the double-sided convex lens 113. L in the figure corresponds to L indicated by B in FIG. By defining the magnification, the following equation is obtained.

β = n · u1 / u2 = n · f / L
That is, it is as follows.

L = n · f / β
Using this equation, the radius of curvature R101 of the first surface 101 can be expressed by the refractive index n, the focal length f, and the imaging magnification β.

R101 = (n−1) · f / β
In this way, the relational expression between R1 and β ′ is obtained. When R1 is obtained based on the above formula, the following formula is obtained.

R1 = B / C
B = 0.85f (n-1)
C = n (0.60866-0.11 · t / f-0.1272 · d / f) (0.83 + 0.2 · NA)
In the case of a lens having a numerical aperture of 0.75 or more, in order to design a lens that sufficiently satisfies the eccentricity tolerance, the curvature radius R1 is preferably 0.05, more preferably 0.04, and even more preferably. Is preferably within 0.03.

  Here, sin (u1) is inversely proportional to R1. As a characteristic of the sin function, since the change of sin (u1) is small compared to u1, the change of u1 is large. That is, the range allowed for u1 is wider than the range allowed for R1.

  If such a relationship is arranged, it can be expressed as the following condition for the curvature radius R1 of the first surface.

(1-D) A <R1 <(1 + D) A (7)
A = B / C
B = 0.85f (n-1)
C = n (0.60866-0.11 · t / f-0.1272 · d / f) (0.83 + 0.2 · NA)
However, D is a positive number, preferably 0.05, more preferably 0.04, and even more preferably 0.03.

  Incidentally, the influence of the transmission layer of the disk is relatively small, and there is no significant change in the refractive index range of 1.45 to 1.65.

  The width of 0.03 to 0.05 is a value including a slight difference caused by a difference in refractive index of the disk, strictly speaking, a refractive index of the lens.

  Since the margin increases when the refractive index of the lens is low and the numerical aperture is lower than 0.75, a generally good design can be obtained within 5% from this value.

  In summary, when the condition (7) regarding the radius of curvature R1 of the first surface of the lens is satisfied, the on-axis aberration characteristic, the off-axis aberration characteristic, and the eccentricity tolerance (due to increase in aberration) can be satisfied at the same time.

  Further supplementally, the aspherical lens of the present embodiment is a toric lens whose aspherical shape is slightly changed depending on the direction even if it is a rotationally symmetric lens (coaxial optical system) with respect to the optical axis. ). Even in the case of a shape like a toric lens, it goes without saying that the radius of curvature R1 of the first surface in each direction needs to be within the aforementioned range.

  When the curvature radius R1 of the first surface is set in the range indicated by the condition (7), the curvature radius R2 of the second surface is automatically determined from the set focal length f by the following equation. This equation can be easily derived from the basic equation for calculating the paraxial focal length of a single lens given the radius and thickness of both surfaces.

R2 = G / H
G = f (n-1) (t (n-1) / n-R1)
H = (R1-f (n-1))
In this way, the radius of curvature R1 at the apex of the first surface and the radius of curvature R2 at the apex of the second surface are determined. If both surfaces are aspherical so as to satisfy the conditions (1) and (2) simultaneously based on these radii of curvature, the shape of the aspherical surface is uniquely determined. At this time, satisfaction with the sine condition of the condition (3) is increased, and a lens having a large eccentricity tolerance can be obtained.

  As described above, the condition (3) cannot be satisfied while the conditions (1) and (2) are completely satisfied. This is because the degree of design freedom of the lens is only two aspheric surfaces and the degree of design freedom is 2 for the three conditions. Therefore, the aspheric shape obtained as described above can be slightly changed to increase the eccentricity tolerance. In this case, deterioration of on-axis aberration or off-axis aberration is inevitable, but an important manufacturing tolerance can be ensured in order to obtain a practical lens.

  In other words, it can be said that the design is balanced so that the on-axis aberration and the off-axis aberration are appropriately deteriorated and the eccentricity tolerance is secured. In other words, it can be said that the work satisfies the three conditions (1) to (3).

  When searching for an aspherical shape in this way, if the radius of the spherical surface serving as the starting point does not satisfy the condition (6) or (7), the eccentricity tolerance, off-axis aberration, or on-axis A design that balances aberrations cannot be achieved because of increased aberrations.

 By the way, the lens described above does not satisfy a sufficient condition for ensuring chromatic aberration characteristics because the eccentricity tolerance is ensured and the conditions (4) and (5) are not considered. Next, the chromatic aberration characteristics will be described in detail.

  Here, the axial thickness of the lens and the focal length satisfy the following expression.

t> (1 + E) f
Here, f is the focal length, and t is the axial thickness of the lens. E is a number of 0 or more, preferably 0, more preferably 0.1, and even more preferably 0.2.

  When the above relationship is satisfied, the satisfaction level of the condition (4) and the condition (5) is increased.

  First, regarding the small increase in aberration of the best image plane at each wavelength when there is a wavelength error in the condition (4), the radius of the first lens surface (incident surface) is relatively large when the center thickness of the lens is thick. Because it can. More specifically, as the radius of curvature of the first surface increases, the incident angle θ (the angle formed between the normal of the lens surface and the light beam) of the light beam passing through the outer edge of the lens decreases. This is because the effect of refraction as a non-linear phenomenon is reduced, and as a result, an increase in spherical aberration is reduced when the wavelength is changed.

  FIG. 7 shows the relationship between the center thickness of the lens and the residual aberration due to the wavelength error (5 nm). Residual aberration is spherical aberration. In this figure, a large number of lenses having an NA of 0.85 and a focal length of 2.5 mm are designed and drawn. The glass material is OHARA LAM70. The lens design employs a design with a relatively large eccentricity tolerance.

  According to FIG. 7, it can be seen that when the lens thickness is thinner than the focal length, a large aberration of 0.04λ or more occurs. It can also be seen that the increase in aberration is large when the thickness is 3 mm or less, which is 1.2 times the focal length.

  Next, regarding the increase in aberration when there is a wavelength broadening of the condition (5), if there is a wavelength broadening, the best image plane of the central wavelength is used as the observation plane for the broadening, and the spherical surface described above at other wavelengths. In addition to aberrations, focus errors occur. Actually, the influence of the focus error is larger than that of the spherical aberration. In particular, when the wavelength is 0.45 μm or less, the dispersion of the refractive index of the glass becomes large, so the influence of the focus error is very large. Become.

  This focus error is caused by a change in the back focus distance of the lens when the wavelength is changed. The back focus distance fb of the lens can be obtained by a ray tracing formula based on paraxial approximation. That is R1, t, n and the following relational expression.

fb = f (1-t (n-1) / n / R1)
The difference in the value of fb when n is changed according to the dispersion of the glass becomes the focus error.

  FIG. 8 shows the amount of change in fb when the refractive index of glass is changed to 1.7486 in a lens having a focal length of 2 mm and a refractive index of 1.75. The amount of change in fb is axial chromatic aberration. This change in refractive index corresponds to a change in refractive index when a wavelength of about 5 nm is used when glass having an Abbe number of about 45 is used at a wavelength near 400 nm. The lens shape is a plano-convex lens, and R1 is 1.5 mm. The actual lens is not a plano-convex lens, but both surfaces are spherical. More precisely, although it is an aspherical surface, the paraxial quantities such as f and fb are determined by the radius of the apex, so there is no problem as a spherical lens. However, the change in fb changes the values of R1 and R2 while maintaining the focal length. The result is very similar to the case of the plano-convex lens without much influence from the lens bending. According to the figure, the axial chromatic aberration decreases in proportion to the lens thickness. Therefore, it is desirable that the thickness of the lens is as thick as possible.

  By the way, within the range of the formula of this specification, there is a first surface radius that minimizes an increase in aberration due to decentration in a complete aplanato lens. Here, in order to balance aberrations as described above, it is not always necessary to use a radius that minimizes decentration aberration. It can be used as the radius of the apex of the shape to balance on-axis aberrations and off-axis aberrations.

  Aberration balance is the introduction of imperfections in a perfect aplanat lens, which can be said to be a degree of design freedom, and the radius of the lens itself can be balanced by adding the degree of freedom. Become.

  Of course, if it is changed beyond the conditional expression, it becomes difficult to balance the aberrations, so it is necessary to keep the conditional expression.

  In the above description, the change in the characteristics due to the numerical aperture has been described by paying attention to the change in the imaging magnification β. However, here, the change due to the numerical aperture at the angle u1 between the light beam having the highest height and the optical axis inside the lens is described. Pay attention to the explanation.

  First, when the numerical aperture changes, u1 changes approximately in proportion to the numerical aperture. In addition to this, for the same reason as described for the change in β, there is a slight deviation from the proportional change. To be precise, the change in β is due to the change in u1.

  Considering this, the internal angle when NA is other than 0.85 is set to u1 ′.

  u1 ′ is expressed by the following equation.

K = (0.60866-0.11 · t / f-0.1272 · d / f) (0.83 + 0.2 · NA) · NA / 0.85
Here, f is the focal length, t is the center thickness of the disk, d is the thickness of the transmission layer of the optical disk, and NA is the numerical aperture of the lens.

  In the case of a lens having a numerical aperture of 0.75 or more, in order to design a lens having a sufficient eccentricity tolerance, from the internal angle u1 ′, preferably 0.06, more preferably 0.05, More preferably, it is in the range of 0.04.

  If such a relationship is arranged, it can be arranged as the following conditions for the internal angle u1 ′.

(1-D) sin (K) <sin (u1 ′) <(1 + D) sin (K) (8)
K = (0.60866-0.11 · t / f-0.1272 · d / f) (0.83 + 0.2 · NA) · NA / 0.85
However, D is a positive number, preferably 0.06, more preferably 0.05, and still more preferably 0.04.

  Incidentally, the influence of the transmission layer of the disk is relatively small, and there is no significant change in the refractive index range of 1.45 to 1.65.

  The width of 0.04 to 0.06 described above is a value including a difference in the refractive index of the disk, strictly speaking, including a slight difference caused by the refractive index of the lens.

  Further, when the refractive index of the lens is low and the numerical aperture is lower than 0.75, the margin increases, so that a generally good value can be obtained at an angle within 6% from this value.

  Here, the axial thickness of the lens and the focal length satisfy the following relational expression.

t> (1 + E) f
Here, f is the focal length, t is the axial thickness of the lens, and E is a number of 0 or more, preferably 0, more preferably 0.1, and even more preferably 0.2.

  In summary, when the condition (4) is satisfied with respect to the angle u1 ′ formed with the optical axis of the light beam having the maximum height inside the lens, the on-axis aberration characteristic, the off-axis aberration characteristic, and the eccentricity tolerance (aberration due to the aberration) Increase) can be satisfied at the same time.

  In addition, even if this aspherical lens is a rotationally symmetric lens (coaxial optical system) with respect to the optical axis, it has a shape like a touic lens with the aspherical shape slightly changed depending on the direction. There may be. In the latter case as well, it goes without saying that the radius of curvature and the thickness of the first surface in each direction must be within the above-described ranges.

  Examples of the objective lens for optical disc according to the present invention will be described below.

  In the embodiment, an aspheric surface is represented by using the following polynomial.

Z = Ch ² / (1+ (1− (1 + K) C ² h ² ) ^0.5 ) + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ + A ₁₀ h ¹⁰ + A ₁₂ h ¹² + A ₁₄ h ¹⁴
Here, Z is the distance from the vertex of the surface, h is the height from the optical axis, K is the conic constant, C is the curvature (= 1 / R), and A _{4 to} A ₁₄ are the 4th to 14th aspherical surfaces. It is a coefficient. For example, A ₄ corresponds to the fourth power of h.

<Example 1>
FIG. 9 is a cross-sectional view of the objective lens of Example 1. FIG.

  The light beam L incident on the objective lens 11 is refracted by the first surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disk 21, and is condensed on the signal recording surface.

  The lens specifications are as shown in Table 1.

  The lens design values are shown in Table 2. The unit of radius and thickness is mm. The same applies to the following.

  The aspherical coefficients of the first surface are as shown in Table 3.

  The aspherical coefficients of the second surface are as shown in Table 4.

  From this lens specification, the recommended value of R1 calculated, that is, the value of A in Equation (7) is 1.731695 mm. The difference between the recommended value and the actual design value is 1.25%.

  The characteristic of this lens is an applanate that substantially satisfies the conditions (1) and (2) and has a slight error in the condition (3).

  In this lens, the wavefront aberration on the axis is as small as 0.002λ, which is practically no aberration. The wavefront aberration for an incident light beam with an off-axis angle of 0.5 degrees is 0.023λ, which is a good characteristic. Further, the decentering between the planes important in the manufacturing process has a very good value of 0.036λ wavefront aberration when the decentering is 3 μm.

  The sine of the angle within the lens of the highest ray of this lens is sin (u1 ′) = 0.46. On the other hand, the recommended value of sin (u1 ′) calculated from this lens specification, that is, sin (K) in equation (8) is 0.4511. The deviation of this recommended value from the actual design value is 1.97%.

  FIG. 10 is a longitudinal aberration diagram, FIG. 11 is a diagram showing an unsatisfactory sine condition, and FIG. 12 is an astigmatism diagram.

  FIG. 13 shows an increase in aberration when the eccentricity is 3 μm in a lens designed by slightly changing the curvature radius R1 of the first surface while maintaining the same refractive index and thickness as the lens of Example 1. FIG.

  The increase in aberration is shown when the numerical aperture indicated by □ is 0.75 and when the numerical aperture indicated by ♦ is 0.85.

  In each numerical aperture, the value of A in the equation (7) is 1.767 mm when the numerical aperture is 0.75, and 1.732 mm when the numerical aperture is 0.85.

  From the figure, it is understood that when the limit of aberration at the time of decentering is set to 0.04λ, it is desirable to set the curvature radius R1 of the first surface within at least 5% of the recommended value A. Further, desirably, it can be reliably set to 0.04λ or less within 4%.

  Further, when the numerical aperture is increased, it is further recommended to be within 3% in consideration of the increase in aberration when R1 changes.

  By the way, when the numerical aperture is 0.85, the value theoretically given from the aberration best point and A is deviated by about 1%. This is due to an error in the regression equation. This is a value that takes into account.

  In the case of this lens specification, an aberration of 0.04λ is realized with an eccentricity of 3 μm. However, depending on the specification, only a considerably large value may be realized. Even in such a case, it is needless to say that it is necessary to suppress the aberration within the above-described range in order to reduce the aberration.

  The lens thickness is 1.375 times the focal length. The glass material of this lens is designed with a fixed refractive index, but the aberration at the best image plane is 0 when the refractive index is 1.8486 as a refractive index change corresponding to a change in wavelength of 5 nm. It is suppressed to a small value of .01λ. The amount of axial chromatic aberration is 2.17 μm, which is kept low.

<Example 2>
FIG. 14 is a cross-sectional view of the objective lens of Example 2.

  The light beam L incident on the objective lens 11 is refracted by the first surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disk 21, and is condensed on the signal recording surface.

  The lens specifications are as shown in Table 5.

  Table 6 shows the lens design values.

  Table 7 shows the aspheric coefficients of the first surface.

  Table 8 shows the aspherical coefficients of the second surface.

  From this lens specification, the recommended value of R1, which is calculated, that is, the value of A in Equation (7) is 1.4495 mm. The difference between the recommended value and the actual design value is 0.3%. The characteristic of this lens is an aplanato with a slight error in the condition (3) that substantially satisfies the conditions (1) and (2).

  The wavefront aberration on the axis is as very small as 0.001λ, and it can be said that there is no aberration in practical use. The wavefront aberration with respect to an incident light beam with an off-axis angle of 0.5 degrees is as good as 0.013λ. Further, the decentering between the planes, which is important in terms of manufacturing tolerance, has a very good value of wavefront aberration of 0.023λ when the decentering is 3 μm.

  The sine of the angle u1 ′ inside the lens of the highest ray of this lens is sin (u1 ′) = 0.421. On the other hand, the recommended value of sin (u1 ′) calculated from this lens specification, that is, sin (K) in equation (8) is 0.44. The difference between the recommended value and the actual design value is 1.7%.

  15 is a longitudinal aberration diagram, FIG. 16 is a diagram showing an unsatisfactory sine condition, and FIG. 17 is an astigmatism diagram.

  The lens thickness is 1.429 times the focal length. The glass material of this lens is designed with a fixed refractive index, but the aberration at the best image plane is 0 when the refractive index is 1.7486 as a refractive index change corresponding to a change in wavelength of 5 nm. It is suppressed to a small value of .01λ. The amount of axial chromatic aberration is 2.10 μm, which is kept low.

<Example 3>
FIG. 18 is a sectional view of the objective lens of Example 3.

  The light beam L incident on the objective lens 11 is refracted by the first surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disc 21, and is condensed on the signal recording surface.

  The lens specifications are as shown in Table 9.

  Table 10 shows the lens design values.

  Table 11 shows the aspherical coefficients of the first surface.

  Table 12 shows the aspheric coefficients of the second surface.

  The refractive index of each glass material is as shown in Table 13.

  From this lens specification, the recommended value of R1 calculated, that is, the value of A in Equation (7) is 1.81581 mm. The difference between the recommended value and the actual design value is 0.2%. The characteristics of this lens almost satisfied the condition (1), and (2) left some dissatisfaction, and the lens of the lens of the first embodiment has a smaller increase in aberrations when decentered. In condition (3), the lens is very close to an aplanat with a slight error.

  The wavefront aberration on the axis is as very small as 0.006λ, and it can be said that there is no aberration in practical use. The wavefront aberration for an incident light beam with an off-axis angle of 0.5 degrees is 0.069λ, which is a good characteristic. Further, the decentering between the planes, which is important in terms of manufacturing tolerance, has a very good value of wavefront aberration of 0.034λ when the decentering is 5 μm.

  The sine of the angle u1 ′ inside the lens of the highest ray of this lens is sin (u1 ′) = 0.466. On the other hand, the recommended value of sin (u1 ′) calculated from this lens specification, that is, sin (K) in equation (8) is 0.464. The deviation of the recommended value from the actual design value is 0.43%.

  19 is a longitudinal aberration diagram, FIG. 20 is a diagram showing an unsatisfactory sine condition, and FIG. 21 is an astigmatism diagram.

  The lens thickness is 1.411 times the focal length. When the wavelength is changed to 5 nm by 410 nm, the aberration on the best image plane is suppressed to a small value of 0.029λ. The amount of axial chromatic aberration is 2.21 μm, which is kept low.

  In the present embodiment, the optical disk objective lens has been described using specific numerical values, but the present invention is not limited to these numerical values. The present invention can be applied to various types of objective lenses for optical discs without departing from the present invention.

  As a specific example of numerical values, in the present embodiment, an optical disk having a transmission layer with a thickness in the range of 0.01 to 0.3 mm can be used. For the objective lens, for example, an optical glass such as NBF1, for example, having a refractive index in the range of 1.5 to 2.0 can be used.

It is a figure explaining the form of an objective lens. It is a figure explaining the angle of the light ray inside an objective lens, and the imaging magnification of a 2nd surface. It is the figure which showed the relationship between the data used for calculating | requiring the regression equation which calculates | requires u1, and the regression equation regarding lens thickness. It is the figure which showed the relationship between the data used for calculating | requiring the regression equation which calculates | requires u1, and the regression equation regarding disk thickness. It is a figure which shows the relationship between the data used in order to obtain | require the change of the coupling magnification (beta) by numerical aperture, and a regression equation. It is a figure explaining derivation | leading-out of the relational expression of R1 and (beta) '. It is a figure which shows the relationship between the center thickness of a lens, and the residual aberration by a wavelength error (5 nm). In the lens having a focal length of 2 mm and a refractive index of 1.75, the amount of change in fb when the refractive index of the glass is changed to 1.7486 is shown. FIG. 3 is a cross-sectional view of the objective lens of Example 1. FIG. 3 is a longitudinal aberration diagram of the objective lens according to Example 1. It is a figure which shows the sine condition dissatisfaction amount of the objective lens of Example 1. 2 is an astigmatism diagram of the objective lens according to Example 1. FIG. It is a figure which shows the mode of the increase in the aberration in the lens designed by changing R1 slightly, maintaining the same refractive index and thickness as the lens of Example 1. FIG. 6 is a cross-sectional view of an objective lens according to Example 2. FIG. FIG. 6 is a longitudinal aberration diagram of the objective lens according to Example 2. It is a figure which shows the sine condition unsatisfactory amount of the objective lens of Example 2. 6 is an astigmatism diagram of the objective lens according to Example 2. FIG. 6 is a cross-sectional view of an objective lens according to Example 3. FIG. FIG. 6 is a longitudinal aberration diagram of the objective lens according to Example 3. It is a figure which shows the sine condition unsatisfactory amount of the objective lens of Example 3. 6 is an astigmatism diagram of the objective lens according to Example 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st surface 2 2nd surface 11 Objective lens 21 Optical disk

Claims (4)

In the objective lens for an optical disc applied to an optical disc having a transmission layer, both surfaces are aspheric and a numerical aperture NA is 0.75 or more,
An objective lens for an optical disc, characterized in that an angle u1 ′ formed with the optical axis of the light beam having the maximum height inside the lens has optical characteristics satisfying the following expression.
(1-D) sin (K) <sin (u1 ′) <(1 + D) sin (K)
K = (0.60866-0.11 · t / f-0.1272 · d / f)
(0.83 + 0.2 ・ NA) ・ NA / 0.85
Where f is the focal length of the lens, t is the center thickness of the lens,
d is the thickness of the transmission layer of the optical disc that is irradiated with light through the lens;
D is 0.06. The objective lens for an optical disk according to claim 1, wherein the center thickness t and the focal length f satisfy the following expression.
t> (1 + E) f
Here, E is a number of 0 or more. 3. The objective lens for an optical disk according to claim 1, wherein the imaging magnification is 0. The objective lens for an optical disk according to any one of claims 1 to 3, wherein the wavelength of light irradiated to the optical disk through the lens is 450 nm or less.

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