JPH0943510A - Recording and reproducing optical system of optical information recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an information pickup device and an optical disk device capable of recording/reproducing an optical disk having the different thickness of a substrate by means of one optical pickup. SOLUTION: A recording/reproducing optical system has a laser beam source, a lens for converting the divergent angle of a divergent beam from the laser beam source to a small angle and an objective lens 6 of a positive refractive power for converging a transmitted light beam from the lens on an information recording surface through a transparent substrate of an information recording medium, the divergent angle converting lens is moved along the optical axis according to the thickness of a transparent substrate 7 and the system satisfies the following condition: d1 >d2 , where, d1 : a distance from the laser beam source to a point at which the surface of the divergent angle converting lens closest to the beam source intersects the optical axis, the distances are d1 , d2 when the thicknesses of the transparent substrate are t1 , t2 , respectively, and the relation: t1 <t2 holds.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for recording / reproducing optical information by irradiating an optical information recording medium with a light beam such as a laser beam.

[0002]

2. Description of the Related Art In FIG. 28, a light source 1 such as a semiconductor laser is provided.
The light beam emitted from the laser beam passes through the beam splitter 2, enters the collimator lens 3, becomes a parallel light beam, is limited to a predetermined light beam by the diaphragm 5, and enters the objective lens 6. When the parallel light flux enters, the objective lens 6 forms a substantially aberration-free light spot on the recording surface 8 through the substrate 7 having a predetermined thickness. The light flux modulated and reflected by the information pits on the recording surface 8 returns to the beam splitter 2 via the objective lens 6 and the collimator lens 3, where it is separated from the optical path from the laser light source 1 and enters the light receiving means 9. . The light receiving means 9 is a PIN photodiode which is multi-divided. Each element outputs a signal current proportional to the intensity of the incident light beam, and sends this signal current to a detection circuit system (not shown). Based on the detected information signal, focus error signal, and track error signal, the objective lens 6 is controlled by a two-dimensional actuator composed of a magnetic circuit and a coil, and the light spot position is always aligned on the track.

In such an information pickup, a large NA is used to reduce the light spot condensed by the objective lens 6.
Since it is (for example, NA 0.6), if the thickness of the substrate 7 placed in such a condensed light flux deviates from a predetermined thickness, large spherical aberration is generated. This will be explained with reference to FIG.
A0.6, wavelength 6 of laser light emitted from laser light source
With an objective lens optimized under the conditions of 35 nm, substrate thickness 0.6 mm, and substrate refractive index 1.58, when the substrate thickness is changed, the aberration increases by 0.01 λrms for every 0.01 mm deviation. Therefore, if the substrate thickness deviates by ± 0.07 mm, the aberration will be 0.07 λrms, and the Marechal limit value, which is a standard for normal reading, will be reached.

For this reason, when reproducing a substrate having a thickness of 1.2 mm instead of a substrate having a thickness of 0.6 mm, for example, the objective lens 1 corresponding to the thickness of 1.2 mm is used in the actuator section.
Playback is switched to 1 and aperture 10. Two sets of pickups for a 0.6 mm thick substrate and a 1.2 mm thick substrate are incorporated in the device. A hologram is provided in the information pickup, and each of the 0th-order light and the 1st-order light transmitted through the hologram is transferred to a 0.6 mm thick substrate and 1.2
A light spot corresponding to a mm-thick substrate is focused on the information recording surface. And other methods have been proposed.

However, as described above, in order to be able to reproduce optical disks having different substrate thicknesses with one optical disk device, as described above, the substrate thicknesses of the disk correspond to 0.6 mm and 1.2 mm, respectively. Two objective lenses are attached, and the substrate thickness of the disc is 0.6mm and 1.2mm.
In the method of incorporating two optical pickups for
The information pickup device and the optical disc device cannot be made compact and low cost. In addition, a hologram is provided in the information pickup, and the 0th-order light that passes through the hologram, 1
In the method of converging each of the next rays on the information recording surface as a light spot corresponding to a 0.6 mm thick substrate and a 1.2 mm thick substrate, two light fluxes are always emitted toward the information recording surface of the information recording medium. When information is read from a light spot by one light flux, the other light flux not only becomes unnecessary light that does not contribute to reading but also causes noise increase.
Also, since the laser light intensity will be divided and used,
The S / N ratio is reduced due to the decrease in the light amount, and the laser life is shortened when the light amount is increased. Further, when the recording method is adopted, the emission loss is increased, so that a laser having a higher output is required and the cost is increased.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to obtain an optical system for recording / reproducing an optical information recording medium which does not include the above-mentioned drawbacks. That is, it is possible to record and reproduce optical disks having different substrate thicknesses with one optical pickup, and to have compatibility with optical disks having different substrate thicknesses.
It is an object of the present invention to realize an information pickup device and an optical disc device which are simple and compact in structure.

[0007]

A recording / reproducing optical system for an optical information recording medium according to the present invention comprises a laser light source, a lens for converting a divergence angle of diverging light from the laser light source to a small value, and a lens for converting the divergence angle. In a recording / reproducing optical system of an optical information recording medium having an objective lens having a positive refractive power for converging emitted light on an information recording surface through a transparent substrate of the optical information recording medium,
The divergence angle conversion lens moves along the optical axis according to the thickness of the transparent substrate, and the following condition is satisfied. d ₁ > d ₂ where d ₁ is the distance from the laser light source to the point where the surface of the divergence angle conversion lens closest to the light source intersects the optical axis. When the thickness of the transparent substrate is t ₁ and t ₂ , the distance is _They are d ₁ and d ₂ , respectively, and have a relationship of t ₁ <t ₂ .

The objective lens is a positive single lens having a convex surface facing the light source side, both surfaces facing the light source side and the information recording surface side are aspherical surfaces, and are made of a glass material or a plastic material.

Further, it is preferable that when the divergence angle conversion lens is located at the d ₁ , the light flux emitted from the divergence angle conversion lens and incident on the objective lens is substantially parallel.

Further,

[Equation 2] However, Δd = d ₁ −d ₂ Δt = t ₂ −t ₁ n: Refractive index of the transparent substrate of the optical information medium at the light source wavelength fo: Focal length of the objective lens fc: Focal length of the divergence angle conversion lens fT : It is desirable to satisfy the conditional expression of the focal length of the entire optical system at the time of the transparent substrate t ₁ .

[0011]

The present invention provides an optical system capable of recording / reproducing optical disks having different substrate thicknesses by changing the incident angle of a light beam incident on an objective lens according to the thickness of a transparent substrate of an optical information recording medium. Is what you are trying to do. FIG.
Is an optical path when a light beam is incident on the objective lens 6 whose aberration correction is optimized when a parallel light beam having a wavelength of 635 nm is incident under the conditions of NA 0.60, substrate thickness 0.60 mm, and substrate refractive index 1.58. It is a figure. A light beam from infinity passes through the diaphragm 5, the objective lens 6, and the substrate 7, and is condensed at a position corresponding to the recording surface 8. Such an objective lens 6 can easily have a wavefront aberration of 0.000λ by using an aspherical surface for the lens surface.
You can get ms.

FIG. 2 shows the substrate thickness of 0.6 in Example 1.
In an optical system in which aberration correction is optimized for mm,
The amount of spherical aberration generated when the divergence angle conversion lens is moved along the optical axis is shown. Here, the magnitude of the divergence angle is defined as + direction when the light flux is divergent light, −direction when the light flux is convergent light, and 0 when the light flux is parallel light. When the divergence angle conversion lens is moved from the predetermined position toward the light source along the optical axis, the emission angle of the light beam emitted from the divergence angle conversion lens becomes larger than the emission angle before the movement, and the optimum state for the objective lens. A light beam having a divergence angle larger than that of the light beam is incident, and under spherical aberration occurs. Further, when the divergence angle conversion lens is moved from the predetermined position toward the objective lens along the optical axis, the emission angle of the light flux emitted from the divergence angle conversion lens becomes smaller than the emission angle before the movement, and the objective lens In this case, a light beam with a divergence angle smaller than that in the optimum state is incident. As a result, excessive spherical aberration occurs.

From FIGS. 2 and 29, in the optical system optimized for a given substrate thickness, the thicker the substrate,
Further, the farther the divergence angle conversion lens is from the light source, the more spherical aberration becomes. On the contrary, it can be seen that the thinner the substrate is and the closer the divergence angle conversion lens is to the light source, the lower the spherical aberration is. Therefore, it is possible to cancel the excess spherical aberration that occurs when the substrate becomes thicker by bringing the divergence angle conversion lens closer to the light source and generating the under spherical aberration. On the contrary, it is possible to cancel the under spherical aberration that occurs when the substrate becomes thin by moving the divergence angle conversion lens away from the light source and generating the over spherical aberration.

By making the light source side surface of the objective lens convex and introducing an aspherical surface, the objective lens used in the present invention can be realized by a single lens, which contributes to cost reduction. At this time, it is possible to use either glass or resin as the material of the objective lens, but it is desirable to use resin in terms of contributing to cost reduction.

The spherical aberration generated by the change Δt in the substrate thickness is moved along the optical axis of the divergence angle conversion lens having the focal length fc to generate spherical aberration in the opposite direction, and the spherical surface is formed as the entire optical system. Corrects aberrations well, but at that time
The larger {(n ² −1) / n ³ } Δt, the larger the moving amount Δd of the divergence angle conversion lens, and the relationship is proportional. Also, the amount of spherical aberration that can be corrected by moving the divergence angle conversion lens is inversely proportional to √ (fo / fT) and (fo / fc) ² . From the above, the movement amount of the divergence angle conversion lens that moves to correct the spherical aberration caused by the change in the thickness of the transparent substrate of the optical information medium can be calculated by these {(n ²
−1) / n ³ } Δt, √ (fo / fT) and (fo / f
c) The value normalized by ² must be within the range of conditional expression (1). If the lens moves too much beyond the upper limit, spherical aberration will be too low and will exceed the Marechal limit, which is a measure of the diffraction limit. On the other hand, when the value goes below the lower limit, the excessive spherical aberration generated at Δt cannot be sufficiently corrected, and the Marechal limit is also exceeded.

[0016]

EXAMPLES Examples will be described below. In each example,
In the numerical example, the laser light source is the 0th surface, the radius of curvature of the i-th surface (including the diaphragm surface) is r _{1 in this order} , and the i-th surface and the (i + 1) th surface are on the optical axis. Thickness, spacing d
The refractive index at the wavelength of the laser light source of the medium between the i, i-th and (i + 1) -th is represented by ni. Further, the refractive index of air is 1. Further, when an aspherical surface is used for the lens surface, the aspherical surface shape has a vertex of the surface as an origin, and in a rectangular coordinate system with the optical axis direction as the X axis, κ is a conical coefficient and Ai is an aspherical surface coefficient. , Pi is a power of the aspherical surface,

(Equation 3) It is represented by

Example 1 Thickness of transparent substrate 0.6 mm 1.2 mm-Each surface spacing di di'-Aperture diameter φ4.08 φ4.08-Magnification of the whole optical system -1 / 7.9-1 / 6. 2 ・ NA on recording surface 0.60 0.59 Focal length of divergence angle conversion lens fc = 26.85 (mm) Focal length of objective lens fo = 3.40 (mm) Whole optical system at transparent substrate t ₁ Focal length fT = 3.53 (mm) i ri di di'ni 0 light source 0.55 0.55 1.0 1.0 1 ∞ 3.00 3.00 1.51455 2 ∞ 22.05 12.40 1.0 3 39.8985 2.00 2.00 1.80186 4 13.6343 3.00 3.00 1.58701 5-17.4703 3.00 12.83 1.06 Aperture (∞) 0.00 0. 00 1.0 7 2.0500 2.60 2.60 1. 9005 8 -5.1870 1.60 1.42 1.0 9 ∞ 0.60 1.20 1.58 10 recording surface (∞) seventh surface aspheric data κ = -0.46211 A ₁ = -0 .81009 × 10 ⁻³ , P ₁ = 4 A ₂ = −0.18077 × 10 ⁻³ , P ₂ = 6 A ₃ = −0.23378 × 10 ⁻⁴ , P ₃ = 8 A ₄ = −0.87100 × 10 ⁻⁵ , P ₄ = 10 Aspherical surface data of the eighth surface κ = −0.20447 × 10 ² A ₁ = + 0.91069 × 10 ⁻² , P ₁ = 4 A ₂ = −0.35507 × 10 ^{− 2} , P ₂ = 6 A ₃ = + 0.78952 × 10 ⁻³ , P ₃ = 8 A ₄ = −0.68843 × 10 ⁻⁴ , P ₄ = 10

Embodiment 2 FIG. Thickness of transparent substrate 0.6 mm 1.2 mm-Each surface spacing di di'-Aperture diameter φ4.08 φ4.08-Magnification of the entire optical system -1 / 8.0 -1 / 6.2-NA on the recording surface side 0.60 0.59 Focal length of divergence angle conversion lens fc = 26.85 (mm) Focal length of objective lens fo = 3.40 (mm) Focal length of optical system whole system at transparent substrate t ₁ fT = 3 .41 (mm) i ri di di 'ni 0 light source 0.55 0.55 1.0 1 ∞ 3.00 3.00 1.51455 2 ∞ 23.90 14.15 1.0 3 -156.00000 1 .20 1.20 1.82920 4 -195.5513 3.00 12.93 1.05 Aperture (∞) 0.00 0.00 1.0 1.0 6 2.0500 2.60 2.60 1.49 005 7 -5.1870 1.60 1.42 1.08 ∞ 0.60 .20 1.58 9 recording surface (∞) Aspherical surface data κ = -0.46211 sixth surface ^{_{A 1 = -0.81009 × 10 -3,}} P 1 = 4 A 2 = -0.18077 × 10 - ³ , P ₂ = 6 A ₃ = −0.23378 × 10 ⁻⁴ , P ₃ = 8 A ₄ = −0.87100 × 10 ⁻⁵ , P ₄ = 10 Aspherical surface data κ = −0. 20447 × 10 ² A ₁ = + 0.91069 × 10 ⁻² , P ₁ = 4 A ₂ = −0.35507 × 10 ⁻² , P ₂ = 6 A ₃ = + 0.78952 × 10 ⁻³ , P ₃ = 8 A ₄ = −0.68843 × 10 ⁻⁴ , P ₄ = 10

Example 3 Thickness of transparent substrate 0.6 mm 1.2 mm-Each surface spacing di di'-Aperture diameter φ4.08 φ4.08-Magnification of the entire optical system -1 / 8.0 -1/6. 1. NA 0.60 0.59 on recording surface side Focal length of divergence angle conversion lens fc = 26.85 (mm) Focal length of objective lens fo = 3.40 (mm) Overall system of transparent substrate t ₁ Focal length fT = 3.34 (mm) i ri di di'ni 0 light source 0.55 0.55 1.0 1.0 1 ∞ 3.00 3.00 1.51455 2 ∞ 23.90 14.51 1.0 3 -156.00000 1.20 1.20 1.822920 4 -195.5513 3.00 12.58 1.05 Aperture (∞) 0.00 0.00 1.0 6 2.4000 2.602 .60 1.694047 -78.2277 1.51 1.3 1.0 8 ∞ 0.60 1.20 1.58 9 recording surface (∞) first aspherical data of six surfaces _{κ = -0.22769 A 1 = -0.14079 ×} 10 -2, P 1 = 4 A ₂ = −0.18562 × 10 ⁻³ , P ₂ = 6 A ₃ = −0.22586 × 10 ⁻⁴ , P ₃ = 8 A ₄ = −0.24870 × 10 ⁻⁵ , P ₄ = 10 7th surface Aspherical surface data of κ = −0.21220 × 10 ² A ₁ = + 0.11864 × 10 ⁻¹ , P ₁ = 4 A ₂ = −0.32315 × 10 ⁻² , P ₂ = 6 A ₃ = + 0.89784 × 10 ^-3 , P ₃ = 8 A ₄ = -0.651119 × 10 ^-4 , P ₄ = 10

Example 4 Thickness of transparent substrate 0.6 mm 1.2 mm-Each surface spacing di di'-Aperture diameter φ4.08 φ4.08-Magnification of the whole optical system -1 / 11.8-1 / 8. 9 ・ NA on recording surface 0.60 0.59 Focal length of divergence angle conversion lens fc = 26.85 (mm) Focal length of objective lens fo = 3.40 (mm) Whole optical system at transparent substrate t ₁ Focal length fT = 3.31 (mm) i ri di di'ni 0 light source 0.55 0.55 1.0 1.0 1 ∞ 3.00 3.00 1.51455 2 ∞ 36.96 16.22 1.0 3 -339.33412 1.20 1.20 1.82920 4 -30.2635 3.00 23.92 1.05 Aperture (∞) 0.00 0.00 1.0 6 2.0500 2.602 .60 1.49005 7-5.1870 1.60 1.4 1.0 8 ∞ 0.60 1.20 1.58 9 recording surface (∞) sixth surface aspheric data _{κ = -0.46211 A 1 = -0.81009 ×} 10 -3, P 1 = 4 A ₂ = −0.18077 × 10 ⁻³ , P ₂ = 6 A ₃ = −0.23378 × 10 ⁻⁴ , P ₃ = 8 A ₄ = −0.87100 × 10 ⁻⁵ , P ₄ = 10 7th surface Aspherical surface data κ = −0.20447 × 10 ² A ₁ = + 0.91069 × 10 ⁻² , P ₁ = 4 A ₂ = −0.35507 × 10 ⁻² , P ₂ = 6 A ₃ = + 0.78952 × 10 ^-3 , P ₃ = 8 A ₄ = -0.68843 × 10 ^-4 , P ₄ = 10

[0021]

As described above, according to the present invention, optical information recording media having different substrate thicknesses can be recorded / reproduced by one information pickup device, and compatible with a plurality of substrate thicknesses, and the structure is simple and compact. Thus, a low-cost information pickup device and optical disc device were obtained. Also, since there is only one light spot used for reading and writing,
There is no power loss, the S / N ratio is improved in reading, and writing can be performed with lower laser power in the method of recording. Furthermore, it becomes possible to easily cope with an arbitrary thickness of the substrate and to correct a variation in thickness of each substrate. It should be noted that although the working distance slightly changes as the divergence angle of the incident light on the objective lens changes,
It is within the working range of the focusing actuator and this need not be taken into account.

[Brief description of drawings]

FIG. 1 is an optical path diagram of an objective lens in which aberration correction is optimized.

FIG. 2 is a graph showing the relationship between the amount of divergence angle conversion lens movement and spherical aberration in Embodiment 1 of the present invention.

FIG. 3 is an optical path diagram of Example 1 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 0.6 mm.

FIG. 4 is an optical path diagram of Example 1 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 1.2 mm.

FIG. 5 is an optical path diagram of Example 2 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 0.6 mm.

FIG. 6 is an optical path diagram when a transparent substrate thickness is 1.2 mm in Example 2 of the information recording / reproducing optical system of the present invention.

FIG. 7 is an optical path diagram of Example 3 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 0.6 mm.

FIG. 8 is an optical path diagram of Example 3 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 1.2 mm.

FIG. 9 is an optical path diagram of Example 4 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 0.6 mm.

FIG. 10 is an optical path diagram of Example 4 of the information recording / reproducing optical system of the present invention when the transparent substrate has a thickness of 1.2 mm.

FIG. 11 shows a transparent substrate having a thickness of 0.
It is a spherical-aberration figure at 6 mm.

FIG. 12 is a spherical aberration diagram in the case where a transparent substrate having a thickness of 1.2 mm is inserted in the optical system of Example 1 while the divergence angle converting lens is still arranged for the transparent substrate having a thickness of 0.6 mm.

FIG. 13 shows an optical system of Example 1, in which the divergence angle conversion lens has a thickness of 0.
It is a spherical-aberration figure at the time of inserting a 6-mm transparent substrate.

FIG. 14 shows a transparent substrate having a thickness of 1.
It is a spherical-aberration figure at the time of adjusting to 2 mm.

FIG. 15 shows a transparent substrate having a thickness of 0.
It is a spherical-aberration figure at 6 mm.

FIG. 16 is a spherical aberration diagram in the case where a transparent substrate having a thickness of 1.2 mm is inserted in the optical system of Example 2 while the divergence angle converting lens is still arranged for the transparent substrate having a thickness of 0.6 mm.

FIG. 17 is an optical system of Example 2 in which the divergence angle conversion lens has a thickness of 0.
It is a spherical-aberration figure at the time of inserting a 6-mm transparent substrate.

FIG. 18 shows a transparent substrate having a thickness of 1.
It is a spherical-aberration figure at the time of adjusting to 2 mm.

FIG. 19 shows a transparent substrate having a thickness of 0.
It is a spherical-aberration figure at 6 mm.

FIG. 20 is a spherical aberration diagram in the case where a transparent substrate having a thickness of 1.2 mm is inserted in the optical system of Example 3 while the divergence angle converting lens is still arranged for the transparent substrate having a thickness of 0.6 mm.

FIG. 21 is an optical system of Example 3 in which the divergence angle conversion lens has a thickness of 0.
It is a spherical-aberration figure at the time of inserting a 6-mm transparent substrate.

FIG. 22 is a diagram showing a transparent substrate thickness 1.
It is a spherical-aberration figure at the time of adjusting to 2 mm.

FIG. 23 is a diagram illustrating a transparent substrate thickness of 0.
It is a spherical-aberration figure at 6 mm.

FIG. 24 is a spherical aberration diagram in the case where a transparent substrate having a thickness of 1.2 mm is inserted in the optical system of Example 4 while the divergence angle converting lens is still arranged for the transparent substrate having a thickness of 0.6 mm.

FIG. 25 is an optical system of Example 4, in which the divergence angle conversion lens has a thickness of 0.
It is a spherical-aberration figure at the time of inserting a 6-mm transparent substrate.

FIG. 26 shows a transparent substrate having a thickness of 1.
It is a spherical-aberration figure at the time of adjusting to 2 mm.

FIG. 27 is an optical layout diagram showing an example of a recording / reproducing optical system of an optical information recording medium in a conventional technique.

FIG. 28 is a graph showing the relationship between the thickness of the substrate of the optical information recording medium and spherical aberration.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 laser light source 2 beam splitter 3 divergence angle conversion lens 5,10 diaphragm 6,11 objective lens 7 transparent substrate 8 information recording surface 9 light detection means

Claims (13)

[Claims] 1. A laser light source, a lens for converting a divergence angle of divergent light from the laser light source to a small value, and light emitted from the lens is collected on an information recording surface via a transparent substrate of an optical information recording medium. In a recording / reproducing optical system of an optical information recording medium having an objective lens having a positive refracting power to emit light, the divergence angle converting lens moves along the optical axis according to the thickness of the transparent substrate, and the following conditions are satisfied. An optical system for recording / reproducing an optical information recording medium characterized by satisfying. d ₁ > d ₂ where d ₁ is the distance from the laser light source to the point where the surface of the divergence angle conversion lens closest to the light source intersects the optical axis. When the thickness of the transparent substrate is t ₁ and t ₂ , the distance is _They are d ₁ and d ₂ , respectively, and have a relationship of t ₁ <t ₂ . 2. The optical information according to claim 1, wherein the objective lens is a positive single lens having a convex surface facing the light source side, and both surfaces facing the light source side and the information recording surface side are aspherical surfaces. Optical system for recording and reproducing recording media. 3. The recording / reproducing optical system for an optical information recording medium according to claim 2, wherein the objective lens is made of a glass material. 4. The recording / reproducing optical system for an optical information recording medium according to claim 2, wherein the objective lens is made of a plastic material. 5. The light according to claim 1, wherein when the divergence angle conversion lens is located at the d ₁ , the light flux emitted from the divergence angle conversion lens and incident on the objective lens are substantially parallel. Optical system for recording / reproducing information recording medium. 6. The optical information according to claim 5, wherein the objective lens is a positive single lens having a convex surface facing the light source side, and both surfaces facing the light source side and the information recording surface side are aspherical surfaces. Optical system for recording and reproducing recording media. 7. The recording / reproducing optical system for an optical information recording medium according to claim 6, wherein the objective lens is made of a glass material. 8. The recording / reproducing optical system for an optical information recording medium according to claim 6, wherein the objective lens is made of a plastic material. 9. The recording / reproducing optical system for an optical information recording medium according to claim 1, wherein the following conditional expression is satisfied. [Equation 1] However, Δd = d ₁ −d ₂ Δt = t ₂ −t ₁ n: Refractive index of the transparent substrate of the optical information medium at the light source wavelength fo: Focal length of the objective lens fc: Focal length of the divergence angle conversion lens fT : Focal length of the whole optical system at transparent substrate t ₁ 10. The light according to claim 9, wherein when the divergence angle conversion lens is located at the d ₁ , the light flux emitted from the divergence angle conversion lens and incident on the objective lens are substantially parallel. Optical system for recording / reproducing information recording medium. 11. The optical information according to claim 10, wherein the objective lens is a positive single lens having a convex surface facing the light source side, and both surfaces facing the light source side and the information recording surface side are aspherical surfaces. Optical system for recording and reproducing recording media. 12. The recording / reproducing optical system for an optical information recording medium according to claim 11, wherein the objective lens is made of a glass material. 13. The recording / reproducing optical system for an optical information recording medium according to claim 11, wherein the objective lens is made of a plastic material.