JP5049149B2 - Objective lens for optical pickup - Google Patents

Description

  The present invention relates to an objective lens for an optical pickup that collects a laser beam having a wavelength of about 400 nm and is used for recording and reproducing information on an optical disc such as a Blu-ray disc having a higher recording density than a conventional DVD.

  As this type of objective lens, for example, those described in Patent Documents 1 and 2 are known. All of the objective lenses described in these documents are single glass lenses. Further, in order to obtain a good spot with such an objective lens, it is necessary to increase the rim intensity (the intensity of light passing through the peripheral portion relative to the intensity of light passing near the optical axis of the objective lens). Patent Documents 3 and 4 disclose techniques for increasing the rim strength by applying a specific antireflection film to the lens surface of this type of objective lens.

JP 2003-114383 A JP 2005-156719 A JP 2004-39161 A JP 2005-11494 A

  However, since the objective lenses described in Patent Documents 1 to 3 are all glass lenses, there is a problem that they are heavy and expensive.

  In order to solve these problems, a plastic lens may be used. However, since plastic has a refractive index lower than that of glass, in order to obtain the same refractive power, the lens surface, particularly the first surface on the light source side is used. The radius of curvature is reduced. As a result, the surface slope of the first surface becomes steep at the peripheral portion, making manufacturing difficult. That is, the plastic lens is manufactured by injection molding using a mold, and the mold is formed by cutting with a cutting tool while rotating the mold using a lathe. At this time, if the surface inclination becomes steep, accurate machining becomes difficult. Also, when measuring a processed mold or molded lens, if a contact type probe is used, the change in sag with respect to the distance from the center will increase, resulting in slight errors in the radial probe. The amount of sag changes greatly, making accurate measurement difficult. Furthermore, even when coating is applied, if the inclination is steep, the adhesion of the vapor deposition material scattered from the vapor deposition source will deteriorate, the film thickness will not be uniform, or the refractive index will not be constant. Occurs. Furthermore, when the surface inclination is steep, there is a problem that the transmittance is lowered, and jitter is adversely affected.

  The antireflection film described in Patent Document 3 is a single layer because glass is assumed as the lens material. However, when plastic is used as the lens material, the difference in the refractive index between the lens material and the film. Therefore, a single layer cannot provide a sufficient antireflection effect, and the rim strength cannot be increased. Furthermore, the objective lens of Patent Document 4 has a maximum wavelength of 680 nm that minimizes the reflectance of light incident at an angle of 0 degrees. However, with such a setting, the film thickness of the peripheral portion is insufficient, and the transmittance is reduced. Since it cannot be increased, the rim strength cannot be increased.

  The present invention has been made in view of the above-described problems of the prior art, and is a high-NA objective lens for optical discs having a high recording density such as a Blu-ray disc. It is a first object to provide an objective lens that is easy to coat and the like and has little reduction in transmittance. A second object is to increase the rim strength of such an objective lens.

The objective lens for an optical pickup according to the present invention is a single lens made of plastic and having a convex first surface on the light source side and a second surface on the optical disk side made of plastic, in order to achieve the first object. NA of 0.75 or more, and the first surface has an optical axis as a central axis within an effective luminous flux diameter.
The rotationally symmetric surface includes an inflection point where the second derivative of the sag defined by the distance from the tangential plane to the lens surface on the optical axis with respect to the height from the optical axis is zero. .

The pupil height h (x) from the optical axis of the inflection point is the following condition (1):
0.94 <h (x) ≤ 0.99… (1)
It is desirable to satisfy.

Also, the focal length f is the following condition (2),
1.15 <f <1.45 (2)
It is desirable to satisfy.

Further, a value (SAG1) ′ / n obtained by dividing the first derivative (SAG1) ′ of the sag at the inflection point of the first surface by the refractive index n of the objective lens is the following condition (3):
1.20 <(SAG1) '/ n <1.65 (3)
It is desirable to satisfy.

Further, the minimum value ((SAG2) '/ n) _MIN obtained by dividing the first derivative of the sag of the second surface (SAG2)' by the refractive index n of the objective lens (SAG2) '/ n is as follows: 4),
−0.60 <((SAG2) '/ n) _MIN ≦ −0.10… (4)
It is desirable to satisfy.

Further, assuming that the lens thickness is d, the focal length is f and the refractive index is n, the following condition (5):
0.77 <d / (f · n) <1.10 (5)
It is desirable to satisfy.

Furthermore, assuming that the curvature radius of the first surface is r1 and the curvature radius of the second surface is r2, the following condition (6):
−1.50 <r1 / r2 <−0.60 (6)
It is desirable to satisfy.

In order to achieve the second object described above, at least a first surface is provided with an antireflection film, and a transmittance Tc in a circular central region inside 10% of the effective diameter from the center of the first surface, The transmittance Tp in the ring-shaped peripheral region outside 90% of the effective diameter of the surface is the following condition (7),
1.00 <Tp / Tc <1.40 (7)
And the antireflection film applied to the first surface has a wavelength λ0 at which the reflectivity with respect to the normal incident light is minimum and the shortest wavelength λmin among the laser beams to be used, according to the following condition (8):
1.75 <λ0 / λmin <2.00 (8)
It is desirable to satisfy.

  In addition, it is desirable that the refractive index with respect to the shortest wavelength λmin of the laser light to be used is smaller than 1.58. It is desirable to apply an antireflection film.

  The optical pickup optical system of the present invention is characterized by using any of the objective lenses for optical pickups described above.

  According to the configuration of the present invention described above, the surface inclination of the peripheral portion of the first surface of the lens can be relaxed, and a high NA objective lens for an optical disc having a high recording density such as a Blu-ray disc, which is a plastic lens However, it is possible to provide an objective lens that is easy to process, measure, coat, and the like and has little reduction in transmittance. Further, by applying an antireflection film that satisfies the conditions (7) and (8) on the first surface, the rim strength of such an objective lens can be increased.

  Hereinafter, embodiments of the objective lens for an optical pickup according to the present invention will be described. FIG. 1 shows an optical system of an optical pickup using the objective lens 10 of the embodiment. The objective lens 10 has a function of forming spots by converging laser light emitted from the semiconductor laser 1 and collimated by the collimator lens 2 onto a recording surface of an optical disc 20 such as a Blu-ray disc. A half mirror 3 is arranged between the semiconductor laser 1 and the collimating lens 2, and a part of the reflected light reflected by the optical disk 20 and transmitted through the objective lens 10 and the collimating lens 2 is directed to the photodetector 4 side. To reflect. The light detector 4 has a known light receiving area pattern, reproduces a signal recorded on the optical disc, and detects a tracking error signal and a focusing error signal.

  The objective lens 10 of the embodiment has an NA of 0.75 or more. For example, as shown in FIG. 2 (Example 1), the first surface 11 on the light source side and the second surface on the optical disc 20 side are made of plastic. Reference numeral 12 denotes a single lens having a convex shape. Further, as shown in FIG. 2 and the like, the first surface 11 of the objective lens 10 includes an inflection point where the second derivative of the sag becomes 0 within the effective light beam diameter.

  By having an inflection point on the first surface 11, it is possible to prevent the surface inclination of the first surface from becoming steep in the peripheral portion. For this reason, manufacture, measurement, coating, etc. become easy, and the bad influence to the jitter by the light quantity fall can be reduced by suppressing the fall of the transmittance | permeability.

The objective lens 10 is designed to satisfy the following conditions (1) to (6).
0.94 <h (x) ≤ 0.99… (1)
1.15 <f <1.45 (2)
1.20 <(SAG1) '/ n <1.65 (3)
−0.60 <((SAG2) '/ n) _MIN ≦ −0.10… (4)
0.77 <d / (f · n) <1.10 (5)
−1.50 <r1 / r2 <−0.60 (6)
However,
h (x) is the pupil height from the optical axis of the inflection point,
f is the focal length,
(SAG1) '/ n is a value obtained by dividing the first derivative of the sag at the inflection point of the first surface (SAG1)' by the refractive index n,
((SAG2) '/ n) _MIN is the minimum value of (SAG2)' / n obtained by dividing the first derivative of the sag at the inflection point of the second surface (SAG2) 'by the refractive index n,
d is the lens thickness,
n is the refractive index,
r1 is the radius of curvature of the first surface,
r2 is the radius of curvature of the second surface.

  Condition (1) defines the pupil height from the optical axis of the inflection point. By satisfying this condition, the inflection point can be set at an appropriate position in the peripheral portion, and it is possible to prevent the surface inclination from becoming steep while obtaining a predetermined power. If the upper limit of condition (1) is exceeded, the inflection point will be outside the effective diameter, and if the lower limit is not reached, the inflection point will be close to the lens center, making it difficult to maintain a predetermined power.

  Condition (2) defines the focal length of the objective lens 10. By satisfying this condition, it is possible to prevent deterioration of aberration due to temperature change while securing a working distance necessary for the optical pickup. In other words, the plastic lens has a larger refractive index change due to temperature than a glass lens, and the amount of spherical aberration that occurs is proportional to the focal length. Can be suppressed. When the upper limit of the condition (2) is exceeded, the amount of spherical aberration generated due to temperature change increases. On the other hand, below the lower limit, the working distance cannot be secured.

  Condition (3) defines a value relating to the first derivative of the sag at the inflection point of the first surface, that is, the inclination of the surface at the inflection point. Since the plastic lens has a lower refractive index than the glass lens, the surface inclination tends to be steep in order to obtain a predetermined power. By satisfying the condition (3), it is possible to prevent the inclination from becoming excessively steep, and it is possible to prevent the lens thickness from increasing because the edge thickness of the lens can be secured. When the upper limit of the condition (3) is exceeded, the inclination at the inflection point becomes too large and the lens thickness becomes large. On the other hand, below the lower limit, the inclination at the inflection point becomes too small, and the predetermined power cannot be secured.

  Condition (4) defines the minimum value related to the first derivative of the sag of the second surface, that is, the minimum value of the slope. Since the sign of the convex sag is negative on the second surface, the minimum value of the inclination indicates a value at a position where the inclination is steepest. By satisfying this condition (4), it is possible to prevent the inclination of the second surface from becoming excessively steep. If the upper limit of the condition (4) is exceeded, a predetermined power cannot be obtained, and if the lower limit is not reached, the surface inclination of the second surface becomes steep and the edge thickness cannot be secured.

  Condition (5) defines the lens thickness. Since the lens thickness is scaled by the focal length, and the required lens thickness increases as the refractive index decreases, the denominator includes the focal length and the refractive index. By satisfying this condition, it is possible to secure the thickness (edge thickness) of the lens periphery while securing the working distance necessary for the optical pickup. If the upper limit of the condition (5) is exceeded, the lens thickness becomes too large, and it becomes difficult to ensure the working distance. On the other hand, below the lower limit, the lens thickness becomes too small and the edge thickness cannot be secured.

  Condition (6) defines the power distribution between the first surface 11 and the second surface 12. By satisfying this condition, it is possible to balance off-axis characteristics of the objective lens and decentration coma. The off-axis characteristic is coma aberration that occurs when incident light is tilted with respect to the lens optical axis. Since the focal length is determined by the condition (2), if the curvature radius of the first surface is determined, the curvature radius of the second surface is also uniquely determined. Therefore, there is only one degree of freedom of the radius of curvature, and off-axis characteristics and decentration coma cannot be corrected simultaneously. Therefore, the radius of curvature is determined so as to balance these. If the condition (6) is not satisfied, it is difficult to achieve both off-axis characteristics and correction of decentration coma.

Further, the first surface 11 and the second surface 12 of the objective lens 10 are provided with antireflection films, and the transmittance Tc in a circular central region inside 10% of the effective diameter from the center of the first surface 11. And the transmittance Tp in the ring-shaped peripheral region outside 90% of the effective diameter of the first surface is the following condition (7):
1.00 <Tp / Tc <1.40 (7)
Meet. Furthermore, the antireflection film formed on the first surface 11 has a wavelength λ0 that minimizes the reflectance with respect to the normal incident light and the shortest wavelength λmin of the laser light to be used, under the following condition (8):
1.75 <λ0 / λmin <2.00 (8)
It is desirable to satisfy.

  By satisfying these conditions, the transmittance of the peripheral region is larger than that of the central region, and the rim strength can be increased. If the lower limit of the conditions (7) and (8) is not reached, the transmittance in the peripheral use area is small and the rim strength cannot be increased. When the upper limit is exceeded, the overall transmittance is reduced, and the light utilization efficiency is lowered.

  The refractive index of the objective lens 10 is set to be smaller than 1.58 with respect to the shortest wavelength λmin of the laser light to be used. Further, the first surface 11 is provided with an antireflection film having a two-layer structure, and the second surface 12 is provided with an antireflection film having a two to four-layer structure. Since the refractive index of the plastic lens is smaller than that of the glass lens, a difference in refractive index from the film material is small, and a single layer cannot provide a sufficient antireflection effect, so that a film having two or more layers is used. Since the first surface 11 is more inclined than the second surface 12, it is difficult to apply a multilayer film, and if a multilayer structure is used, stability during mass production cannot be maintained, so a two-layer structure is adopted. Since the surface slope of the second surface 12 is gentler than that of the first surface 11, it is possible to keep the reflectance lower by adopting a 2-4 layer structure. However, if the number of layers is greater than four, it is difficult to apply even if the second surface 12 is inclined, and stability during mass production cannot be maintained.

  Next, five specific examples of the objective lens 10 based on the above-described embodiment will be presented.

  FIG. 2 is a lens diagram illustrating the objective lens 10 for optical pickup and the protective layer of the optical disc 20 according to the first embodiment. The specific numerical configuration of the objective lens 10 of Example 1 is shown in Table 1. The symbol f in the table is the focal length, NA is the numerical aperture, m is the magnification, r is the radius of curvature of the surface (unit: mm), d is the distance on the optical axis between the surfaces (unit: mm), and n is the design wavelength. Is the refractive index. Surface numbers 1 and 2 indicate the first surface 11 and the second surface 12 of the objective lens 10, and surface numbers 3 and 4 indicate both surfaces of the protective layer of the optical disc 20.

Both the first surface 11 and the second surface 12 are rotationally symmetric aspheric surfaces. The radius of curvature in Table 1 indicates the value on the axis. A rotationally symmetric aspherical surface is the distance (sag amount) from the tangential plane on the optical surface of the aspherical surface at the coordinate point on the aspherical surface where the height from the optical axis is h. The curvature (1 / r) on the axis is C, the conic coefficient is κ, and the aspherical coefficients of the _{4th to 22nd} even order are A _{4 to} A ₂₂ , and are expressed by the following formula (9).
H (h) = Ch ² / (1 + √ (1- (1 + κ) C ² h ² )) + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ +… + A ₂₂ h ²² … ( 9)
Table 2 shows the conical coefficient κ and the aspherical coefficients A _{4 to} A ₂₂ of the objective lens 10 of Example 1.

  The first surface 11 and the second surface 12 are both provided with an antireflection film having a two-layer structure. The specific numerical configuration of the antireflection film applied to each surface is shown in Table 3 below. In the table, the first layer is a layer in contact with the lens surface, and the second layer is a layer applied on the first layer. The first layer uses a medium with a refractive index of 2.07 on both sides, and the second layer uses a medium with a refractive index of 1.46 on both sides. However, the film thickness differs on each surface. As a result, the wavelength λ 0 at which the normal incidence reflectance is minimized is different on each surface.

  Subsequently, the performance of the objective lens of Example 1 is shown in FIGS. 3 shows axial wavefront aberration, FIG. 4 shows spherical aberration (SA) and sine condition violation amount (SC), and FIG. 5 shows transmittance distribution characteristics of the objective lens when the antireflection film shown in Table 3 is applied. Indicates. The transmittance Tc in the circular center region inside 10% of the effective diameter from the center is about 60%, and the transmittance Tp in the ring-shaped peripheral region outside 90% of the effective diameter of the first surface is about 72%. Tp / Tc = 1.20, which satisfies the above condition (7). The antireflection film on the first surface satisfies λ0 / λmin = 1.80, which satisfies the above condition (8). As a result, an objective lens having a large rim strength can be provided, and the spots can be converged satisfactorily.

  FIG. 6 is a lens diagram illustrating the objective lens 10 for optical pickup and the protective layer of the optical disc 20 according to the second embodiment. The specific numerical configuration of the objective lens 10 of Example 2 is shown in Table 4, and the cone coefficient and the aspheric coefficient are shown in Table 5. FIG. 7 shows the wavefront aberration on the axis of the objective lens 10 of Example 2, and FIG. 8 shows the spherical aberration (SA) and the sine condition violation amount (SC). In Examples 2 to 5, an antireflection film having the same configuration as the antireflection film shown in Example 1 is applied.

  FIG. 9 is a lens diagram showing the optical pickup objective lens 10 of Example 3 and the protective layer of the optical disc 20. The specific numerical configuration of the objective lens 10 of Example 3 is shown in Table 6, the conic coefficient, and the aspheric coefficient are shown in Table 7. FIG. 10 shows the wavefront aberration on the axis of the objective lens 10 of Example 3, and FIG. 11 shows the spherical aberration (SA) and the sine condition violation amount (SC).

  FIG. 12 is a lens diagram illustrating the optical pickup objective lens 10 and the protective layer of the optical disc 20 according to the fourth embodiment. The specific numerical configuration of the objective lens 10 of Example 4 is shown in Table 8, the conic coefficient, and the aspheric coefficient are shown in Table 9. FIG. 13 shows the on-axis wavefront aberration of the objective lens 10 of Example 4, and FIG. 14 shows the spherical aberration (SA) and the sine condition violation amount (SC).

  FIG. 15 is a lens diagram showing the optical pickup objective lens 10 of Example 5 and the protective layer of the optical disk 20. The specific numerical configuration of the objective lens 10 of Example 5 is shown in Table 10, and the conic coefficient and the aspheric coefficient are shown in Table 11. FIG. 16 shows the on-axis wavefront aberration of the objective lens 10 of Example 5, and FIG. 17 shows the spherical aberration (SA) and the sine condition violation amount (SC).

  Next, Table 12 below shows the relationship between the objective lenses of the above five examples and the conditional expressions (1) to (6). All the examples satisfy all the conditional expressions (1) to (6).

It is explanatory drawing which shows the optical system of the optical pick-up using the objective lens of embodiment of this invention. It is sectional drawing of the objective lens for optical pick-ups which concerns on Example 1 of this invention. FIG. 4 is a wavefront aberration diagram of the objective lens according to Example 1. 6 is a graph showing spherical aberration and sine condition violation amount of the objective lens of Example 1; 3 is a graph showing transmittance distribution characteristics of the objective lens of Example 1. FIG. It is sectional drawing of the objective lens for optical pick-ups which concerns on Example 2 of this invention. FIG. 6 is a wavefront aberration diagram of the objective lens according to Example 2. 6 is a graph showing spherical aberration and sine condition violation amount of the objective lens of Example 2. It is sectional drawing of the objective lens for optical pick-ups which concerns on Example 3 of this invention. FIG. 6 is a wavefront aberration diagram of the objective lens according to Example 3. 10 is a graph showing spherical aberration and sine condition violation amount of the objective lens of Example 3. It is sectional drawing of the objective lens for optical pick-ups which concerns on Example 4 of this invention. FIG. 10 is a wavefront aberration diagram of the objective lens according to Example 4; It is a graph which shows the spherical aberration of the objective lens of Example 4, and the sine condition violation amount. It is sectional drawing of the objective lens for optical pick-ups which concerns on Example 5 of this invention. FIG. 6 is a wavefront aberration diagram of the objective lens according to Example 5. 10 is a graph showing spherical aberration and sine condition violation amount of the objective lens of Example 5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Collimating lens 3 Half mirror 4 Optical detector 10 Objective lens 11 1st surface 12 2nd surface 20 Optical disk

Claims (11)

The first surface on the light source side and the second surface on the optical disk side made of plastic are both convex single lenses having NA of 0.75 or more, and the first surface has an optical axis within the effective beam diameter. Is the central axis
A rotationally symmetric surface including an inflection point at which a second derivative of a sag defined by a distance from a tangential plane to the lens surface on the optical axis with respect to a height from the optical axis is zero. Objective lens for optical pickup. The pupil height h (x) from the optical axis of the inflection point is the following condition (1):
0.94 <h (x) ≤ 0.99… (1)
The objective lens for an optical pickup according to claim 1, wherein: The focal length f is the following condition (2):
1.15 <f <1.45 (2)
The objective lens for an optical pickup according to claim 1, wherein: A value (SAG1) ′ / n obtained by dividing the first derivative (SAG1) ′ of the sag at the inflection point of the first surface by the refractive index n of the objective lens is the following condition (3):
1.20 <(SAG1) '/ n <1.65 (3)
The objective lens for an optical pickup according to claim 1, wherein: The minimum value ((SAG2) '/ n) MIN of (SAG2)' / n obtained by dividing the first derivative (SAG2) 'of the second surface by the refractive index n of the objective lens satisfies the following condition (4 ),
−0.60 <((SAG2) '/ n) MIN ≦ −0.10… (4)
The objective lens for an optical pickup according to claim 1, wherein: When the lens thickness is d, the focal length is f, and the refractive index is n, the following condition (5):
0.77 <d / (f · n) <1.10 (5)
The objective lens for an optical pickup according to claim 1, wherein: When the radius of curvature of the first surface is r1 and the radius of curvature of the second surface is r2, the following condition (6):
−1.50 <r1 / r2 <−0.60 (6)
The objective lens for an optical pickup according to claim 1, wherein: An antireflection film is applied to at least the first surface, and the transmittance Tc in a circular central region inside 10% of the effective diameter from the center of the first surface and outside of 90% of the effective diameter of the first surface The transmittance Tp in the ring-shaped peripheral region of the following condition (7),
1.00 <Tp / Tc <1.40 (7)
The antireflection film applied to the first surface has a wavelength λ0 at which the reflectance with respect to the normal incident light is minimum and the shortest wavelength λmin among the laser beams to be used, under the following condition (8):
1.75 <λ0 / λmin <2.00 (8)
The objective lens for an optical pickup according to claim 1, wherein: 9. The objective lens for an optical pickup according to claim 8, wherein a refractive index with respect to the shortest wavelength [lambda] min among laser beams to be used is smaller than 1.58.   An antireflection film is applied to the first surface and the second surface, the antireflection film applied to the first surface has a two-layer structure, and the antireflection films applied to the second surface are 2 to 4 The objective lens for an optical pickup according to claim 8, wherein the objective lens has a layer structure.   An optical pickup optical system using the optical pickup objective lens according to claim 1.