JPH11203711A - Optical system having large numerical aperture and optical information recording and reproducing device using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical system having a large numerical aperture and an optical information recording/reproducing device using the same capable of designing/manufacturing an objective lens and easily performing the performance inspection. SOLUTION: In an optical system converging a light beam emitted from a light source on the information surface of an information recording medium through a transparent protection substrate part 16, it is provided with an objective lens 4 individually having at least the image forming performance of a diffraction limit and an almost semispherical convex spherical lens 5 provided between the objective lens 4 and the transparent protection substrate part 16 of the recording medium and the planar part of the convex spherical lens 5 is arranged by separating it by a prescribed air gap from the transparent protection substrate part 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high numerical aperture 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, an optical information recording / reproducing apparatus using a recording medium such as an optical disk has been required to record information at a higher density. In order to respond to such a demand, for example, Japanese Patent Application Laid-Open No. 8-315404 proposes a technique relating to an optical system achieving a high NA. In this publication, a high NA optical system is realized by disposing a convex spherical lens, a so-called solid immersion lens (SIL), between an objective lens and a recording medium. Specifically, by optimizing the shape of the objective lens, 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, high NA optical Achieved system.

By the way, with respect to a solid immersion lens of a full hemispherical convex spherical lens, a light beam from an objective lens is perpendicularly incident on the spherical surface of the convex spherical lens and is imaged on the plane of the convex spherical lens. When the thickness of the convex spherical lens is changed, spherical aberration (positive polarity) occurs as the thickness increases. FIG. 9 shows the change value Δt (mm) of the thickness of the convex spherical lens and the spherical aberration W40 in this case.
(Λ _pv ). W40 represents a third-order spherical aberration. As is clear from FIG. 9, the spherical aberration W40 increases as Δt increases.

In the case where an air layer (air gap) is provided in a convex spherical lens, as the air layer becomes thicker with respect to the above-described reference state, spherical aberration (negative negative polarity) having a polarity opposite to that of the above case is obtained. Polarity). FIG. 10 shows the relationship between the thickness of the air layer air gap (μm) and the spherical aberration in this case. In the case of using a solid immersion lens as described above, spherical aberration easily occurs, which is a major obstacle, and a problem has been solved how to solve the problem.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-open No. Hei 8-
In the case of Japanese Patent Publication No. 315404, the configuration of the optical system is such that the thickness of the convex spherical lens is increased and the convex spherical lens is divided into two upper and lower parts via an air gap, one of which corresponds to the transparent protective substrate portion of the recording medium. And Therefore, the generated spherical aberration is a problem. However, in the case of the above conventional example, the shape of the objective lens is optimized with respect to the radius of curvature and thickness of the convex spherical lens, the thickness of the transparent protective substrate portion of the recording medium, and the distance between the convex spherical lens and the transparent protective substrate portion. I have. That is, the spherical aberration generated by increasing the thickness of the convex spherical lens and providing the air gap is offset by the shape of the objective lens.

Therefore, the objective lens alone has a large positive spherical aberration as shown in FIG. FIG. 11 is a spherical aberration curve diagram with the vertical axis indicating the height of incidence on the entrance pupil of the objective lens and the horizontal axis indicating spherical aberration. On the other hand, a spherical single lens generally has a negative spherical aberration, and in the case of the above-described conventional example, the characteristic shown in FIG. 11 is obtained by making the degree of the aspherical surface of the objective lens extremely large. However, when the degree of the aspherical surface of the objective lens is increased in this manner, there is a problem that designing and manufacturing are difficult and cost is increased.

In the conventional optical head device for an optical disk, an objective lens having an NA of 0.5 to 0.6 is used. In this case, the performance of the objective lens alone has negative spherical aberration as shown in FIG. have. This negative spherical aberration can be offset by inserting a flat glass plate corresponding to the transparent protective substrate of the optical disk. Therefore, when the performance inspection of the objective lens is performed by the conventional optical head device for an optical disk, the performance inspection can be easily performed by using the flat glass as described above and making the spherical aberration to be aberration-free. .

[0008] However, in the case of the above-mentioned conventional example, the aberration becomes astigmatism only by preparing a predetermined convex spherical lens and a plate glass corresponding to the transparent substrate portion of the recording medium and further providing a predetermined air gap. Inspection of the objective was never easy. Also, when actually inspecting the objective lens, the objective lens is inspected with a very large NA, so that a slight error in arranging a convex spherical lens, a flat glass, or the like is an error in the inspection result. As a result, the inspection could not be performed accurately.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, the present invention provides a high-numerical-aperture optical system and an optical information recording / reproducing apparatus using the same, which can easily design and manufacture an objective lens and can easily perform a performance test. The purpose is to provide.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical system for condensing a light beam emitted from a light source on an information surface of an information recording medium through a transparent protective substrate, at least independently of a diffraction-limited imaging performance. And a substantially hemispherical convex spherical lens provided between the objective lens and the transparent protective substrate portion of the recording medium, and the planar portion of the convex spherical lens is This is achieved by a high numerical aperture optical system characterized by being arranged with a predetermined air gap with respect to.

[0011] The present invention is also achieved by an optical information recording / reproducing apparatus having the above-mentioned high numerical aperture optical system.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram showing a configuration of an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a semiconductor laser used as a recording / reproducing light source. A laser beam emitted from the semiconductor laser 1 is collimated by a collimator lens 2, passes through a beam splitter 3 with beam shaping, and enters an objective lens 4. Further, a solid immersion lens 5 is provided between the objective lens 4 and a recording medium (such as an optical disk) 6, and a laser beam incident on the objective lens 4 is focused by the objective lens 4 and the solid immersion lens 5, and The light is condensed as a minute light spot on the recording layer.

The laser beam reflected from the recording medium 6 is separated from the incident light beam by the beam splitter 3 with beam shaping, and guided to the half-wave plate 7. Then, the polarization plane is rotated by approximately 45 degrees by the half-wave plate 7, and is incident on the sensor 10 via the condenser lens 8 and the polarization beam splitter 9. The output signal of the sensor 10 is sent to a servo error signal detection circuit (not shown), and a predetermined arithmetic process is performed to detect a focus error signal and a tracking error signal. Further, a focus actuator and a tracking actuator (not shown) are controlled based on a focus error signal and a tracking signal obtained by a servo control circuit (not shown), and a semiconductor laser is applied to an information track of the rotating recording medium 6. Focus control and tracking control are performed so that the light spot from No. 1 is focused and the light spot follows and scans.

When recording information on the recording medium 6, for example, the light intensity of the semiconductor laser 1 is set to a high intensity for recording, and scanning is performed on a track of the recording medium 6. On the other hand, a magnetic field modulated according to a recording signal is applied from the magnetic head 12 to the light beam irradiation site. By applying a modulating magnetic field while irradiating a light beam with a constant intensity in this manner, information can be recorded on the track of the recording medium 6. When reproducing the information recorded on the recording medium 6, the light intensity of the semiconductor laser 1 is set to a low intensity for reproduction and scanning is performed on the track of the optical disk 6. At this time, the reflected light from the recording medium 6 is detected by the sensor 10, and the sensor 10
The reproduction data is generated by performing a predetermined signal processing using the reproduction signal obtained from the output signal of FIG.

Here, in the present embodiment, the objective lens 4,
The structure including the solid immersion lens 5 and the transparent protective substrate 16 of the recording medium 6 is called an optical system 11, and the optical system 11 is shown in detail in FIG. In FIG.
Reference numeral denotes a transparent protective substrate portion of the recording medium 6, and a substantially hemispherical solid immersion lens 5 is provided between the objective lens 4 and the transparent protective substrate portion 16. The solid immersion lens 5 has a lower planar portion facing the transparent protective substrate portion 16 and an air gap at a predetermined interval therebetween. Solid immersion lens 5 and transparent protective substrate 1
The refractive indexes of 6 are set to be equal.

Further, as is apparent from FIG. 2, the light incident on the solid immersion lens 5 is refracted so that the relative angle with respect to the optical axis indicated by a substantially vertical or dashed line becomes large. Assuming that the refractive index is n, the numerical aperture NA of the optical system 11 is n times as large as the numerical aperture of the objective lens 4 alone, and a high NA can be achieved. When the objective lens 4 is a single unit, that is, when the solid immersion lens 5 and the transparent protective substrate 16 are not provided, the objective lens 4 has a diffraction-limited imaging performance. FIG. 3A shows the spherical aberration characteristics when the objective lens 4 is used alone. The vertical axis represents the height of incidence on the objective lens entrance pupil, and the horizontal axis represents spherical aberration. As is clear from FIG. 3A, it is understood that the spherical aberration is very well suppressed.

As described with reference to FIGS. 9 and 10, the polarity of the spherical aberration generated by the change value of the thickness of the solid immersion lens 5 (the amount of deviation from the case of a perfect hemispherical shape) Δt and the value of the air gap is opposite. Polarity. In the present embodiment, the generated spherical aberration is canceled by appropriately selecting the change value Δt of the thickness of the solid immersion lens 5 and the value of the air gap. If the solid immersion lens 5 is a perfect hemisphere, Δt corresponds to the thickness of the transparent protective substrate 16.

4 to 6 show the numerical aperture of the optical system 11 as NA,
Let the radius of curvature of the solid immersion lens 5 be a (the thickness is a if it is a full hemisphere) and the change in the thickness of the lens 5 be Δ
t (Δt is the thickness of the transparent protective substrate portion 16 in the case of a full hemisphere), r = Δt / a, and shows characteristics when the air gap between the plane portion of the lens 5 and the transparent protective substrate portion 16 is air gap. ing. Here, assuming that the thickness of the lens 5 is Sd and the thickness of the transparent protective substrate 16 is t, including the case where the solid immersion lens 5 is an incomplete hemisphere, Δt = (Sd + t) −a (1) r = (Sd + ta) / a (2)

First, FIG. 4 shows the numerical aperture NA and the wavefront aberration W. of the optical system 11 when the air gaps airgap and r are set so as to best offset the generated spherical aberration. A.
6 is a graph showing the relationship of (λrms). A is r = 0.
9 and B are graphs when r = 0.5 and C is r = 0.25. Generally, in an optical disc device, the W.F. A. The allowable limit of the rms value is 1 / 14λrm
s. Therefore, since the deterioration component of the optical system 11 is divided into two components by the initial state of the other optical elements and the component dynamically generated in the entire optical head, the deterioration component of the optical system 11 is 1 / 20λr.
ms. Thus 1 / 20λ
Assuming that it is allowable up to rms, as shown in FIG.
It can be seen that the numerical aperture NA of the optical system 11 when = 0.9 is allowed up to about 0.92.

In this embodiment, since the numerical aperture is high, a recording capacity twice or more as compared with a conventional apparatus not using a solid immersion lens can be realized. In a conventional apparatus that does not use a solid immersion lens, the numerical aperture of the objective lens is about 0.5 to 0.6.
It is necessary to reduce it to 1.4 or less (NA is 1.4 times or more). Therefore, the NA of the optical system 11 using the solid immersion lens must be about 0.8 or more, which is about 1.4 times 0.5 to 0.6. Since the allowable value of the numerical aperture NA of the optical system 11 is about 0.92 as described above, it is necessary that the NA of the optical system 11 satisfies 0.80 ≦ NA ≦ 0.92 (3) .

FIG. 5 is a graph showing the relationship between the numerical aperture NA of the optical system 11 and the air gap air gap when r = 0.3. It can be seen from the graph of FIG. 5 that the smaller the numerical aperture NA, the larger the air gap. According to the knowledge obtained in FIG. 5, since the value of the air gap is large when the NA of the optical system 11 is 0.8 in the range of Expression (3), r and air when the NA of the optical system 11 is 0.8 are large. The relationship of the gap is shown in the graph of FIG. In the graph of FIG. 6, since the value of the air gap is saturated at about r = 0.9, increasing r further than this means that cancellation of spherical aberration in the air gap cannot be expected. Therefore, it is desirable that the range of r is 0.1 <r ≦ 0.9 (4).

In the graph of FIG. 6, r = 0.
9 indicates that the refractive index n of the solid immersion lens 5 and the transparent protective substrate 16 of the recording medium 6 is about 1.5.
Is the value of the air gap, and β is the refractive index n of about 1.
66 shows the value of the air gap in the case of 66. Usually, polycarbonate is used as the material of the transparent protective substrate portion 16 of the recording medium 6, and the refractive index is about 1.58. When glass is used, the refractive index is usually about 1.5. Therefore, the lower limit of the refractive index may be considered to be about 1.5 with 1.58 as the center value. The value of β when the upper limit of the refractive index is 1.66 is shown as a reference value.

Here, in FIG. 6, the value of α is 20 μm
It is weak. Since the value of the air gap saturates at r = 0.9, the range of the air gap is desirably 0 <air gap ≦ 20 μm (5). The above description is for the case where the objective lens 4 is a lens having a diffraction-limited imaging performance by itself. Conversely, the expressions (3) to (5) indicate that the objective lens 4 which is a component of the optical system 11 has the diffraction limit. This is a condition that allows the lens to have the imaging performance of

Table 1 shows specific numerical values of the optical system 11 according to the present embodiment. First, the wavelength of the semiconductor laser 1 is 650 nm, the diameter of the entrance pupil is 3.48 mm, and the numerical aperture NA of the optical system 11 is 0.87. The solid immersion lens 5 is a perfect hemisphere, and the thickness (r5) of the transparent protective substrate 16 is 0.4 mm, and the air gap (r4)
Is 10 μm. The objective lens 4 is an aspheric lens, and the aspheric coefficients are shown in Table 2.

[0025]

[Table 1]

[0026]

[Table 2] FIG. 3B is a spherical aberration curve diagram of the optical system 11, and it can be seen that the spherical aberration is well canceled. In this manner, design, manufacture, and performance inspection are easy, and a high NA can be realized by an optical system using an objective lens having a diffraction-limited imaging performance by itself. The optical system 11 can cope with a change in the thickness of the transparent protective substrate 16 of the recording medium 6 by changing the thickness of the solid immersion lens 5 according to the thickness. Further, the advantage of constructing a high NA optical system by using an objective lens having a diffraction-limited imaging performance by itself is that such an objective lens has another high N
It can be easily diverted to the A optical system.
That is, it is possible to easily use the objective lens for another optical system by changing the thickness of the air gap and the thickness of the transparent protective substrate.

FIG. 7 shows an example in which the objective lens 4 used in the embodiment of FIG. 1 is used in another optical system. Reference numeral 15 denotes a solid immersion lens. Also,
Table 3 shows specific numerical values in the case of the embodiment of FIG.

[0028]

[Table 3] In the present embodiment, the wavelength of the semiconductor laser 1 is 650 nm,
The entrance pupil diameter is 3.70 mm, and the numerical aperture NA of the optical system 11 is 0.87. The solid immersion lens 15 is a perfect hemisphere having a radius of curvature (r3) of 1.6, the thickness (r5) of the transparent protective substrate is 0.4 mm, and the air gap (r4) is 9 μm. The numerical aperture N
In order to make A uniform, the diameter of the entrance pupil is set to 3.7 mm, which is different from Table 1. When the entrance pupil diameter is not changed, the air gap is 10 μm and the numerical aperture NA of the optical system is about 0.82. Here, in this embodiment, since the entrance pupil diameter changes, a spherical aberration curve diagram of the objective lens 4 alone in that case is shown in FIG. 8A. The lens alone has a diffraction-limited imaging performance without any problem. FIG. 8B is a spherical aberration curve diagram of the optical system of Table 3. FIG. 8B shows that the spherical aberration is well canceled.

[0029]

As described above, according to the present invention, an objective lens having a diffraction-limited imaging performance by itself is used, and a substantially hemispherical convex spherical lens is formed between the objective lens and the transparent protective substrate of the recording medium. Since it is configured to be arranged with an air gap with respect to this substrate part, the design of the objective lens,
It is easy to manufacture and can be manufactured at low cost, and performance inspection of the objective lens can be easily and accurately performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing an optical system 11 of the embodiment of FIG. 1 in detail.

FIG. 3 is a diagram showing the spherical aberration of the objective lens 4 and the optical system 11;

FIG. 4 is a diagram illustrating a relationship between a numerical aperture NA of the optical system 11 and a wavefront aberration.

FIG. 5 is a diagram showing the relationship between the numerical aperture NA of the optical system 11 and the air gap when r = 0.3.

FIG. 6 is a diagram showing the relationship between r and the air gap when the numerical aperture NA of the optical system 11 is 0.8.

FIG. 7 is a diagram showing another embodiment of the present invention.

FIG. 8 is a diagram illustrating spherical aberration of the objective lens and the optical system according to the embodiment of FIG. 7;

FIG. 9 is a diagram showing a relationship between a change value Δt in thickness of a convex spherical lens and spherical aberration.

FIG. 10 is a diagram illustrating a relationship between an air gap and a spherical aberration when an air gap is provided in a convex spherical lens.

FIG. 11 is a spherical aberration curve diagram of a conventional objective lens alone.

FIG. 12 is a spherical aberration curve diagram of a single objective lens used in a general optical disk device.

[Explanation of symbols]

 Reference Signs List 1 semiconductor laser 4 objective lens 5, 15 solid immersion lens 6 recording medium 11 optical system 16 transparent protective substrate

Claims (5)

[Claims] In an optical system for condensing a light beam emitted from a light source on an information surface of an information recording medium through a transparent protective substrate, at least one objective lens having diffraction-limited imaging performance alone, and the objective lens A substantially hemispherical convex spherical lens provided between the transparent protective substrate portion of the recording medium and the convex spherical lens with a predetermined air gap between the planar portion and the transparent protective substrate portion. A high numerical aperture optical system characterized by being arranged. 2. A numerical aperture of an optical system including the objective lens, the convex spherical lens, and a transparent protective substrate of the recording medium has a numerical aperture of:
2. The high numerical aperture optical system according to claim 1, wherein the value is 0.8 or more and 0.92 or less. 3. When the radius of curvature of the spherical surface of the convex spherical lens is a, the thickness of the convex spherical lens is Sd, and the thickness of the transparent protective substrate portion of the recording medium is t, (Sd + ta) / a.
The high numerical aperture optical system according to claim 1, wherein the value of is larger than 0 and equal to or smaller than 0.9. 4. The high numerical aperture optical system according to claim 1, wherein an interval between a plane portion of the convex spherical lens and the transparent protective substrate portion is set to 0 to 20 μm. 5. An optical information recording / reproducing apparatus for recording information or reproducing recorded information by condensing a light beam emitted from a light source on an information surface of an information recording medium through a transparent protective substrate. An optical information recording / reproducing apparatus, comprising: a high numerical aperture optical system.