JPH11202194A - High-numerical-aperture optical system and optical information recording and reproducing device using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-NA(numerical aperture) optical system with good image forming performance and the optical information recording and reproducing device using this optical system by suppressing small a spherical aberration generated owing to variation in the thickness of the transparent protection substrate part of a recording medium. SOLUTION: This high-numerical-aperture optical system converges the luminous flux from a light source on the information surface of the recording medium through the transparent protection substrate part 16 of the recording medium 6. The optical system has optical elements, such as a nearly hemispherical convex lens and an objective, arranged along the optical path, at a fine air interval on the transparent protection substrate part 16 and the respective optical elements of the optical system are so positioned that the spherical aberration generated by the optical system is less than a specific value against specific thickness variation of the transparent protection substrate part 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical system for condensing a light beam from a light source on an optical information recording medium, and an optical information recording / reproducing apparatus using the same.

[0002]

2. Description of the Related Art In recent years, researches for realizing a higher-density optical information recording / reproducing apparatus for recording / reproducing information by condensing a light beam from a light source onto an optical information recording medium have been actively conducted. It has been done.

For example, a high numerical aperture (High Numerical Apert)
ure: hereinafter referred to as high NA) is described in Japanese Patent Application Laid-Open No. 8-315404. Here, a convex spherical lens, a so-called solid immersion lens (SIL), is arranged between the objective lens and the recording medium to form a high NA optical system.

That is, the radius of curvature and thickness of the convex spherical lens, the thickness of the transparent protective substrate of the recording medium, and the distance between the convex spherical lens and the transparent protective substrate are set to desired values to optimize the shape of the objective lens. , A high NA optical system is realized.

[0005]

However, in general, an optical information recording / reproducing apparatus targets a plurality of types of recording media having different recording / reproducing methods, and the recording media themselves are manufactured with the same specifications. However, the thickness of the transparent protective substrate portion varies. Further, due to an error in the manufacturing process, the thickness of the transparent protective substrate may vary unacceptably even between one type of recording media.

FIG. 8 shows the relationship between the change in thickness of the transparent protective substrate of the recording medium and the generated spherical aberration in the case of the conventional example. Usually, the standard for the thickness of the transparent protective substrate portion of the recording medium is about ± 50 μm with respect to the center value.
Therefore, in the case of the conventional example, | W40 | = 0.4 to 0.5λp−v (where W
40 indicates a third-order spherical aberration, and λ indicates a used wavelength). That is, in the case of the above-described conventional example, there is a problem that a large spherical aberration occurs with a change in the thickness of the transparent protective substrate portion of the recording medium, and the imaging performance is significantly impaired.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to reduce spherical aberration caused by a change in thickness of a transparent protective substrate of a recording medium.
By keeping it small, high NA optical system with good imaging performance,
Another object of the present invention is to provide an optical information recording / reproducing apparatus using the optical system.

[0008]

Therefore, in the present invention,
A high-numerical-aperture optical system for condensing a light beam from a light source on an information surface of the recording medium through a transparent protective substrate portion of the recording medium. An optical element such as a substantially hemispherical convex spherical lens or an objective lens is arranged through a minute air gap, and a spherical surface generated in the optical system with respect to a predetermined thickness variation of the transparent protective substrate portion. Each optical element of the optical system is positioned so that aberration is within a predetermined value.

In this case, the predetermined thickness variation of the transparent protective substrate portion is ± 50 μm, and the predetermined value of the spherical aberration generated by each optical element of the optical system is ± 50 μm.
0.2λp−v (where λ is the wavelength of light used in the optical system), NA (numerical aperture) of the optical system is 0.80 or more and 0.96 or less, Is a, the radius of curvature of the spherical surface of the convex spherical lens is a, the thickness is Sd,
When the thickness of the transparent protective substrate portion of the recording medium is t,
The value of (Sd + ta) / a is greater than zero and 0.48
It is more effective as an embodiment of the present invention that the following is satisfied, and that the minute air gap in the optical system is set to a predetermined value of 0 to 32 μm.

According to the present invention, there is provided an optical information recording / reproducing apparatus for recording / reproducing information by condensing a light beam from a light source on an information surface of the recording medium through a transparent protective substrate portion of the recording medium. Characterized by using the high NA optical system described above.

Therefore, according to the configuration of the present invention,
By suppressing the spherical aberration generated due to the change in the thickness of the transparent protective substrate portion of the recording medium, it is possible to provide a high NA optical system having good imaging performance and an apparatus using the optical system.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be specifically described below with reference to an example in which the present invention is applied to an optical head for an optical disk device shown in FIG. Here, the beam from the semiconductor laser 1 is converted into a parallel beam by the collimator 2 and passed through the beam splitter 3 with beam shaping to the optical system 1.
In step 1, a spot is formed on the information recording surface in the recording medium 6.

The recording here is performed, for example, by increasing the intensity of the spot and modulating the magnetic head 12. The reflected light from the recording medium 6 is split by the beam splitter 3 with beam shaping, the polarization plane is rotated by approximately 45 degrees by the half-wave plate 7, and the condensing lens 8 passes through the polarization beam splitter 9 Then, the light is focused on the sensor 10. And the output from the sensor 10 is calculated by
Servo signals and information data signals.

As shown in FIG. 2, the optical system 11 includes an objective lens 4, a solid immersion lens 5,
In the configuration arranged in the order of the recording medium 6, a minute air gap (hereinafter referred to as an air gap) is formed between the solid immersion lens 5 and the transparent protective substrate 16 of the recording medium 6. The solid immersion lens 5 and the transparent protective substrate 16 of the recording medium 6 have the same refractive index. FIG. 2 shows this embodiment.

As can be understood from FIG. 2, the light incident on the solid immersion lens 5 is refracted so as to be substantially perpendicular or to increase the relative angle with respect to the optical axis indicated by the diagonal line. The transparent protective substrate 16 of the recording medium 6
Numerical aperture (NA) of the optical system 11, where n is the refractive index of
Is n times larger than the numerical aperture of the objective lens 4 alone, and a high N
A is achieved.

By the way, the wavefront aberration at which the performance of the diffraction limit can be obtained is within 0.072λrms in RMS value. The main aberrations are spherical aberration, coma, and astigmatism. When 0.072 λrms is allocated to each aberration, the spherical aberration is within 0.6 λp-v.

In the case of the above-described optical system, the spherical aberration is caused by a change in the specific air gap (AIR GAP) of the objective lens 4 or by a change in the thickness of the transparent protective substrate of the recording medium. Conceivable. Therefore, when the factors are divided into the factors, the change due to the change in the thickness of the transparent protective substrate portion of the recording medium is 0.2λp-v or less.

Here, FIGS. 3, 4 and 5 show the results of a study for suppressing the spherical aberration generated due to the thickness variation of the transparent protective substrate portion 16 to be low. The numerical aperture of the optical system 11 is NA, and the radius of curvature of the solid immersion lens 5 is a.
(If the solid immersion lens is a perfect hemisphere, the thickness is a).
t is the thickness of the transparent protective substrate), and t / a = r. Here, the air gap is represented by a, the thickness variation of the transparent protective substrate 16 is represented by Δt, and the refractive index of the transparent protective substrate 16 is represented by n.

FIG. 3A shows that r = 0.333 and n =
When the numerical aperture of the optical system 11 is set to NA at 1.58, the thickness variation Δt of the transparent protective substrate 16 is Δt = ± 50.
The air gap value when the spherical aberration generated at μm is minimum (solid line), and the generated spherical aberration is ± 0.2λp-v
The relative relationship between the maximum value of the air gap allowed within the range (dotted line) and the minimum value of the allowable air gap (dotted line) is shown.

FIG. 3B shows the numerical aperture, NA value and tolerance of the optical system 11 based on FIG. 3A (the air gap value when the generated spherical aberration is minimum, (The smaller of the differences from the maximum value or the minimum value of the air gap). This value is shown as an absolute value. Here, the shape of the objective lens 4 differs depending on a predetermined NA, r, n, and an arbitrary air gap, and the shape is optimized under individual conditions.
4 and 5 were obtained under the same conditions.

According to FIGS. 3A and 3B, when the NA is 0.
At 96, it can be seen that there is almost no tolerance. Here, the NA at which the tolerance disappears is slightly shifted to a smaller value when r and n are increased, and slightly shifted to a larger value when r and n are decreased. However, it can be generally represented by 0.96.

According to the present invention, it is considered that the effect of the high NA is that the recording capacity can be realized at least twice as large as the recording capacity of a conventional apparatus not using a solid immersion lens. Incidentally, in a conventional apparatus that does not use a solid immersion lens, the numerical aperture of the objective lens is about 0.50 to 0.60. Therefore, in order to achieve a recording capacity of twice or more, the spot size must be reduced to 1 / 1.4 or less, that is, the NA must be 1.4 times or more.

That is, in the optical system 11 using the solid immersion lens 5, its NA is set to 0.50 to 0.60.
About 1.4 times, in other words, about 0.80 or more. If this is expressed by an equation, 0.80 ≦ NA ≦ 0.96 (1) is desirable.

FIG. 4A shows that NA = 0.87 and n =
At 1.58, the value of r (= t / a) and the transparent protective substrate 1
6, when the thickness variation Δt is Δt = ± 50 μm, the air gap value when the spherical aberration is the minimum (solid line), and the maximum value of the air gap allowed when the generated spherical aberration is within ± 0.2λp-v (Dotted line) and the minimum value of the allowable air gap (dotted line).

FIG. 4B shows the relationship between the value of r (= t / a) and the tolerance based on FIG. 4A.
This value is shown as an absolute value. From FIGS. 4A and 4B, it can be seen that the tolerance substantially disappears when r is 0.48, and that the tolerance increases monotonously as the value of r decreases. If this is expressed by an equation, 0 <r ≦ 0.48 (2) is desirable.

FIG. 5 shows that NA = 0.87 and r = 0.333.
The air gap value when the refractive index n of the transparent protective substrate section 16 and the thickness variation Δt of the transparent protective substrate section 16 are Δt = ± 50 μm is the minimum (solid line), the generated spherical aberration. Indicates the relationship between the maximum value of the air gap allowed within ± 0.2λp-v (dotted line) and the minimum value of the allowed air gap (dotted line).

Usually, the material of the transparent protective substrate portion of the recording medium is polycarbonate, and the refractive index is about 1.58. When the material is glass, it is usually about 1.5. Therefore, a value having a lower limit of about 1.50 and an upper limit of about 1.66 was adopted with 1.58 as the central value.

From FIG. 5, n = 1.50 to 1.58-1.
At 66, the allowable air gap value is ± 15
It can be seen that there is a change of about%. 3, 4, and 5 that the value of r is between 0.2 and 0.3 when the NA is small, or that the smaller the refractive index n, the larger the allowable air gap. . That is, in the equation (1), NA = 0.80, r = 0.2-0.3, n = 1.
At 50, it is near the maximum value.

Actually, NA = 0.80, r ≒ 0.26, n
When = 1.50, the air gap is 18 μm when the spherical aberration generated when the thickness variation Δt of the transparent protective substrate 16 of the recording medium is Δt = ± 50 μm is minimum. Taking tolerance into account, NA = 0.80, r = 0.22, n =
At 1.50, the maximum air gap value is 32 μm. Therefore, the allowable air gap is below zero and up to about 32 μm. If this is expressed by an equation, 0 <a ≦ 32 μm (3) FIG. 6 shows a graph obtained as an approximate function based on the numerical values obtained from the above study. Here, the X axis is r (= t
/ A), the Y axis represents the numerical aperture NA of the optical system 11, and the Z axis represents the air gap value a. In the figure, α is n
= 1.58, β indicates a curved surface when n = 1.50. Each curved surface has a tolerance in a direction in which the air gap value increases. Therefore, also from FIG. 6, the allowable air gap is larger than zero at the bottom and 32 μm
Is confirmed.

[0030]

Next, specific numerical values of the embodiment will be described.
Table 1 shows conditions satisfying the expressions (1) to (3). Here, the numerical aperture NA of the optical system 11 is 0.87,
The solid immersion lens 5 is a perfect hemisphere, the radius of curvature a = 1.2 mm, the thickness of the transparent protective substrate portion is 0.4 mm,
On the other hand, the air gap is 12 μm. In addition,
The substrate is polycarbonate and n = 1.58.
The objective lens 4 is an aspherical lens, and the aspherical coefficient is as shown in Table 2. The shape of the objective lens 4 is represented by a light ray height h.
It is expressed by the following equation.

[0031]

(Equation 1)

[0032]

[Table 1]

[0033]

[Table 2] FIG. 7 shows the thickness change Δt of the transparent protective substrate portion of the recording medium and the generated spherical aberration ΔW4 in the combinations shown in Table 3.
0 is shown. The above numerical examples correspond to A in Table 3 (A in FIG. 7).

[0034]

[Table 3] Each combination in Table 3 satisfies Expressions (1) to (3), and the substrate is polycarbonate, and n ≒ 1.58. As can be seen from FIG. 7, in each of the combinations in Table 3, the thickness change Δt of the transparent protective substrate portion of the recording medium is ± 5.
0 μm or less, the generated spherical aberration ΔW4
0 is ± 0.2λp−v. Therefore, in each combination, the occurrence of spherical aberration is sufficiently suppressed.

As described above, a high NA optical system in which the spherical aberration generated due to the change in the thickness of the transparent protective substrate portion of the recording medium was suppressed was constituted.

[0036]

As described above, in the present invention,
An optical system for condensing a light beam from the light source on the information surface of the recording medium through the transparent protective substrate portion of the recording medium, an objective lens;
A convex spherical lens having a substantially hemispherical shape, the transparent protective substrate portion being positioned with a small air gap therebetween, and a spherical aberration generated by the optical system with respect to a predetermined thickness variation of the transparent protective substrate portion is predetermined. Since the optical system is arranged so as to be within the range, it is possible to provide a high NA optical system in which spherical aberration generated due to a change in the thickness of the transparent protective substrate of the recording medium is suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing an optical head for an optical disk device to which an optical system according to the present invention is applied.

FIG. 2 is a diagram showing an embodiment of the optical system of the present invention.

FIG. 3 shows the relationship between the numerical aperture of the optical system according to the present invention and the value of the air gap in which the spherical aberration generated when the thickness variation of the transparent protective substrate is Δt = ± 50 μm is within ± 0.2λp-v. FIG.

FIG. 4 shows the value of r (t = t / a) according to the present invention and the value of the air gap where the spherical aberration generated when the thickness variation of the transparent protective substrate portion is Δt = ± 50 μm is within ± 0.2λp-v. It is a graph showing the relationship with a value.

FIG. 5 is a graph showing the relationship between the refractive index of the transparent protective substrate according to the present invention and the value of an air gap in which the spherical aberration generated when the thickness variation of the transparent protective substrate is Δt = ± 50 μm is within ± 0.2λp-v. It is a graph showing a relationship.

FIG. 6 relates to the present invention, the numerical aperture of the optical system, r (= t /
Relationship between the value of a), the refractive index of the transparent protective substrate portion, and the value of the air gap in which the spherical aberration generated when the thickness variation Δt of the transparent protective substrate portion is ± 50 μm is within ± 0.2λp-v. Is a graph that approximately represents.

FIG. 7 is a graph showing a relationship between a variation in thickness of a transparent protective substrate and a generated spherical aberration in an example of the present invention.

FIG. 8 shows a variation in thickness of a transparent protective substrate in a conventional example,
6 is a graph showing a relationship with generated spherical aberration.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 4 Objective lens 5 Solid immersion lens 6 Recording medium 11 Optical system 16 Transparent protective substrate part

Claims (6)

[Claims] 1. A high-numerical-aperture optical system for condensing a light beam from a light source on an information surface of a recording medium through a transparent protective substrate portion of the recording medium, wherein the optical system includes the transparent optical system on its optical path. Optical elements such as a substantially hemispherical convex spherical lens and an objective lens are arranged via a minute air gap with respect to the protective substrate portion. An optical system, wherein each optical element of the optical system is positioned so that spherical aberration generated in the system is within a predetermined value. 2. The method according to claim 1, wherein the predetermined thickness variation of the transparent protective substrate is ± 50 μm, and the predetermined value of the spherical aberration generated by each optical element of the optical system is ± 0.2λ.
2. The optical system according to claim 1, wherein pv (where λ is the wavelength of light used in the optical system). 3. An optical system having an NA (numerical aperture) of 0.80.
The optical system according to claim 1, wherein the optical system is at least 0.96. 4. When the radius of curvature of the spherical surface of the convex spherical lens in the optical system is a, the thickness is Sd, and the thickness of the transparent protective substrate portion of the recording medium is t, the value of (Sd + ta) / a is The optical system according to claim 1, wherein the optical system is larger than zero and equal to or smaller than 0.48. 5. A minute air gap in the optical system,
5. The optical system according to claim 1, wherein the optical system is set to a predetermined value of 0 to 32 [mu] m. 6. An optical information recording / reproducing apparatus for recording / reproducing information by condensing a light beam from a light source onto an information surface of the recording medium through a transparent protective substrate portion of the recording medium. An optical information recording / reproducing apparatus using the optical system according to any one of the above.