JP2637739B2 - Magneto-optical head - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a configuration of a magneto-optical head for detecting a reproduction signal by a differential reproduction method.

[Conventional technology]

Conventional magneto-optical heads that provide an analyzer during convergent light
As described in Japanese Patent Application Laid-Open No. 62-8347, a telecentric optical system including a collimator lens that converts divergent light emitted from a semiconductor laser into parallel light and an objective lens that collects parallel light on an information recording surface is used. I have. The analyzer is arranged in a convergent light beam where the reflected light beam from the information recording surface is separated from the optical path from the semiconductor laser light source as a parallel light beam by a beam splitter and then narrowed down to a detector by a detection lens.

[Problems to be solved by the invention]

However, the configuration of the above-mentioned conventional magneto-optical head requires a semiconductor laser, a collimator lens, an objective lens, a beam splitter, a detection lens, an analyzer, and a detector.

Further, in the conventional technology, since the detection lens arranges the analyzer in the convergent light beam that narrows the parallel light beam to the light receiving area of the photodetector (generally, 1 mm ² or less), the incidence of light on the analyzer is reduced. There is a problem that the angle is large and the efficiency of the analyzer separating p-polarized light and s-polarized light is reduced. Also,
If the focal length of the detection lens is increased in order to increase the polarization separation efficiency of the analyzer, a problem arises in that the magneto-optical head becomes large because the optical path becomes longer.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magneto-optical head which has a small number of components, is small, and can separate and detect p-polarized light and s-polarized light with high efficiency.

[Means for solving the problem]

In order to achieve the above object, according to the present invention, a semiconductor laser light source, a diffused light beam emitted from the light source, a first lens having a diffused light beam having a smaller diffusion angle than the diffused light beam, A second lens that condenses the diffused light beam passing through the lens and irradiates the light onto the magneto-optical recording medium; and changes the optical path of the light beam reflected by the magneto-optical recording medium into an optical path of the light beam incident on the magneto-optical recording medium. A deflector that separates p-polarized light and s-polarized light of the reflected light beam deflected by the deflector; and a photodetector that detects the separated p-polarized light and s-polarized light, respectively. The deflector is disposed between the second lens and the first lens, and shifts the optical path of the reflected light flux passing through the second lens from the first lens to the second lens. Dividing from the optical path of the incident light beam Te, deflected toward the optical detector, said second lens converges the reflected light beam, the magneto-optical head, characterized in that for focusing onto said optical detector is provided.

As the above-described analyzer, one having a polarizing film that transmits one of p-polarized light and s-polarized light and reflects the other can be used.

The numerical aperture NAo _{(F) in} front of the second lens is 0.45 to
The first lens such that the numerical aperture NA _{C (B)} behind the first lens is equal to or greater than 0.15 and the numerical aperture NA _{O (B)} behind the second lens is equal to or less than 0.1 at 0.6. Focal length fc, and
It is desirable to determine the setting position.

[Action]

 The operation of the present invention will be described with reference to the drawings.

FIG. 2 schematically shows the optical system of the present invention. In FIG. 2, a coupling lens 3 as a first lens, an objective lens 7 as a second lens,
A beam splitter is disposed as a deflector, and a deflection beam splitter is disposed as an analyzer. A divergent light beam 2 emitted from a semiconductor laser light source 1 which is a linearly polarized light source has an object point and an image point at finite distances a and b, and is diverged by a coupling lens 3 which is a finite system lens having a focal length fc. _Is reduced to become a divergent light beam 4 having a diffusion angle θ _C2 . Here, the following relationship is established between the object point distance a, the image point distance b, and the focal length fc of the coupling lens 3.

If the diameter of the coupling lens 3 is D _C1 , D _C1 = 2 sin θ _C1 · a (2) D _C1 = 2 sin θ _C2 · b (3)

Further, the divergent light beam 4 has an object point and an image point having a finite distance d,
At c, it is narrowed down by an objective lens 7 which is a finite system lens having a focal length fo, and is irradiated onto an information recording surface 8a of a disk which is a magneto-optical information recording medium. Here, an object point distance d, an image point distance c, and a focal length fo of the objective lens 7 have the following relationship.

Further, if the aperture of the objective lens 7 is D _O , D _O = 2 sin θ _O c (5) holds.

The divergent light beam from the information recording surface 8a is converted into a convergent light beam 10 by the objective lens 7, reflected by the reflection surface 5a of the beam splitter, and then P-polarized light (polarized light in a vibration direction parallel to the paper surface).
Transmitted through, after entering the polarization beam splitter is an analyzer having a polarization film 12a for reflecting S-polarized light (polarized light perpendicular oscillation direction to the paper surface), as convergent light angle theta _D with respect to the optical axis 100, converged The light flux 10 is split into a P-polarized light 10a and an S-polarized light 10b, and then enters the photodetector 14, respectively. Here, the aperture D _O of the objective lens 7 and the distance d between the objective lens 7 and the photodetector 14
And the angle θ _D have the following relationship:

D _O = 2 sin θ _D · d (6) The magneto-optical head of the present invention in which the relational expressions (1) to (6) are satisfied is a conventional magneto-optical head (see
No. 62-8347), the following conditions must be satisfied in order to achieve the same light utilization and light collection characteristics.

First, the coupling efficiency between the semiconductor laser light source 1 and the coupling lens 3 (i.e., the light utilization rate) is determined by arranging the semiconductor laser light source 1 at the position of the image point where the conventional point light source (object point) is at infinity. In order to make it equivalent to the case of using a collimating lens, which is an infinite system lens, the following equation must be satisfied, where f _col and numerical aperture NA _col are the collimating lens.

sin θ _c1 = NA _col (7) sin θ _c1 in the above equation (7) is the coupling lens 3
Is equivalent to the numerical aperture behind NAC _(B) .

Further, in order to be the same as the effective light flux D _col (= 2NA _col · f _col ) incident on the objective lens which is a conventional infinite lens, it is necessary to satisfy the following expression.

D _c2 = D _col (8) Further, the light-collecting characteristics (numerical aperture N _0bj ) and the light utilization rate (effective luminous flux diameter) are equivalent to those of the objective lens which is a conventional infinite lens.
D _0bj ) must satisfy the following equation.

sin θ _o = NA _0bj (9) D _O = D _0bj (10) sin θ _o in the above equation (9) is equivalent to the numerical aperture in front of the objective lens 7 being NA _{O (F)} .

On the other hand, as can be seen from FIG. 2, the following equation is obtained from the similar relationship.

Further, for the purpose of the present invention, the configuration must be such that the deterioration of the polarization characteristics of the polarization beam splitter is kept low. Therefore, it is necessary to suppress the opening angle θ _D of the convergent light beam 10 incident on the polarization beam splitter with respect to the optical axis 100 within the following equation.

The sin θ _D in the above equation (12) is equivalent to the numerical aperture behind the objective lens 7 being NA _{O (B)} .

As described above, the coupling lens 3 and the objective lens 7, which are finite lenses satisfying the above equations (1) to (12), are designed and installed. Also, the reflected light flux passing through the objective lens 7 is polarized by a beam splitter before passing through the coupling lens 3 and passed through the analyzer, so that the incident angle of the light flux entering the analyzer can be reduced. it can. As a result, it is possible to obtain the same light utilization rate and light condensing characteristics as those of a magneto-optical head using a conventional infinite system lens, and it is possible to reduce the size of the polarizing beam splitter while keeping the polarization characteristics from deteriorating.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram showing a first embodiment of a magneto-optical head according to the present invention. In FIG. 1, a divergent light beam 2 emitted from a semiconductor laser light source 1 which is a linearly polarized light source is
A divergent light beam 4 whose diffusion angle is reduced by the coupling lens 3 which is a finite system lens is obtained. The divergent light beam 4 transmits through the beam splitter 5, is reflected by the mirror 6, and is then narrowed down by the objective lens 7 which is a finite system lens, and the information recording surface 8a of the disk 8 which is a magneto-optical information recording medium.
Irradiated on top.

The reflected light beam 9 which is a divergent light beam from the disk 8 is converted into a convergent light beam 10 by the objective lens 7, reflected by the mirror 6, and reflected by the anti-slope surface 5a of the beam splitter 5. The reflected convergent light flux 10 is incident on the half-wave plate 11 having an optic axis direction of 22.5 degrees, and rotates the polarization direction of the emitted light by 45 degrees.
The convergent light beam 10 is transmitted to a polarization beam splitter, which is an analyzer.
It is incident on 26.

The polarizing beam splitter 26 has a parallel transparent flat plate 26b sandwiching a polarizing film 26a that transmits P-polarized light (polarized light in a vibration direction parallel to the paper) and reflects S-polarized light (polarized light in a vibration direction perpendicular to the paper). And a parallel transparent flat plate 26c provided with a total reflection coating 26d. The convergent beam 10
The light enters the polarizing film 26a from above the transparent flat plate 26b. Polarizing film 26
The P-polarized light 10a that has passed through a is totally reflected by the total reflection coating 26d on the rear surface of the transparent flat plate 26c, passes through the polarizing film 26a again, and proceeds toward the right side of the transparent flat plate 26b. On the other hand, the S-polarized light 10b reflected by the polarizing film 26a also travels in the same direction as the P-polarized light 10a and enters the photodetector 14.

Further, a configuration may be adopted in which the cylindrical lens 13 is omitted by using the astigmatism generated by the polarization beam splitter 26 as it is.

As shown in FIG. 3, the photodetector 14 has a light receiving area 14a for receiving the P-polarized light 10a and a cross-shaped four-part light receiving area 14b, 14c, 14d, 14e for receiving the S-polarized light 10b. Light receiving area 14f
And each of the light receiving areas 14a to 14f is a six-segment light receiving element integrally formed on the same plane. 6 like this
Divided light receiving elements are widely used in compact disk players and the like, and are available at low cost.

Here, assuming that the incident angle of the convergent light beam 10 with respect to the polarization beam splitter 26 is θ ₁ , the refractive index of the parallel transparent flat plates 26 _b and 26 c is n, and the thickness of the parallel transparent flat plate 26 c is l ₂ , the P polarized light 10 a and the S polarized light
The interval e of 10b is given by the following equation.

For example, when θ ₁ ≒ 45 ゜ n ≒ 1.51 e mm0.36 mm, the thickness l of the transparent flat plate 26c is obtained from the equation (13).
₂ should be set to l ₂ ≒ 0.48mm. The distance d between the easily available six-segment light receiving elements is Is in the range of Should be within the range.

The output signal 16 of the light receiving regions 14b and 14d and the output signal 15 of the light receiving regions 14c and 14e are added by an adder 17 to become a signal 18. The output signal 19 and the signal 18 of the light receiving area 14a are subtracted by the subtractor 20 to detect the reproduced signal 21 by the differential reproducing method. On the other hand, the output signal 15 and the output signal 16 are subtracted by the subtractor 22, and the focus error signal 23 is detected. In addition, signal 18
And the output signal 19 are added by an adder 24, and a sum signal 25 is detected. A tracking error signal and the like are detected from the sum signal 25, but the description is omitted because it is not related to the gist of the present invention.

When the spot shape of the P-polarized light 10b on the photodetector 14 is circular, the spot shape of the S-polarized light 10a becomes slightly elliptical. This is because the length of the optical path of the P-polarized light 10a is longer than that of the S-polarized light 10b by the amount of traveling in the transparent flat plate 26c. The defocus δ by which the angle of converging light beam 10 is incident on the parallel transparent plate 26b theta _1, parallel transparent plate 26b, the refractive index of 26c n, the thickness of the parallel transparent plate 26c with l ₂ It is expressed. For example, when θ ₁ ≒ 45 ゜ n ≒ 1.51 e ≒ 0.48 mm, δ ≒ 0.72 mm, and the defocus δ is slight, indicating no problem. In this embodiment, as an analyzer, a parallel transparent flat plate 26b sandwiching a polarizing film 26a and a parallel transparent flat plate having a total reflection film 26d are used.
Although the polarization beam splitter 26 is formed by 26c, it is not limited to this. For example, FIG. 4 shows a polarizing beam splitter instead of the polarizing beam splitter 26.
Second Embodiment of the Magneto-Optical Head According to the Present Invention When Using No. 12
FIG. 3 is a configuration diagram showing an example of the embodiment. In FIG. 4, a divergent light beam 2 emitted from a semiconductor laser light source 1 which is a linearly polarized light source becomes a divergent light beam 4 whose diffusion angle is reduced by a coupling lens 3 which is a finite system lens. Divergent luminous flux 4
Is transmitted through a beam splitter 5 and reflected by a mirror 6, then narrowed down by an objective lens 7, which is a finite system lens, and radiated onto an information recording surface 8 a of a disk 8 which is a magneto-optical information recording medium. You.

The reflected light beam 9 which is a divergent light beam from the disk 8 is converted into a convergent light beam 10 by the objective lens 7 and reflected by the mirror 6. The reflected convergent light flux 10 is incident on the half-wave plate 11 having an optic axis direction of 22.5 degrees, and rotates the polarization direction of the emitted light by 45 degrees. This convergent light beam 10 enters a polarizing beam splitter 12 which is an analyzer.

The polarizing beam splitter 12 has an isosceles right-angled triangular prism sandwiching a polarizing film 12a that transmits P-polarized light (polarized light in a vibration direction parallel to the paper surface) and reflects S-polarized light (polarized light in a vibration direction perpendicular to the paper surface). 12b and a parallel transparent flat plate 12c. The convergent light beam 10 enters the polarizing film 12a from above the triangular prism 12b. P-polarized light 10a transmitted through the polarizing film 12a
Is totally reflected by the rear flat surface 12d of the transparent flat plate 12c and is again a polarizing film.
The light passes through 12a and proceeds toward the right side of the triangular prism 12b. On the other hand, the S-polarized light 10b reflected by the polarizing film 12a also travels in the same direction as the P-polarized light 10a, passes through the triangular prism 12b, and is incident on the cylindrical lens 13 arranged at a rotation of approximately 45 degrees,
Astigmatism is provided. P-polarized light transmitted through cylindrical lens 13
The 10a and the S-polarized light 10b enter the photodetector 14, respectively.

As shown in FIG. 3, similarly to the first embodiment, the photodetector 14 includes a light receiving region 14a for receiving the P-polarized light 10a and an S-polarized light 10b.
-Shaped quadrant light receiving areas 14b, 14c, 14d, 14e
And unused light receiving areas 14f, and each light receiving area 14a to
f is a six-segment light receiving element integrally formed on the same plane. Such a six-segment light receiving element is widely used in compact disk players and the like, and is available at low cost. For example, when the distance e between the P-polarized light 10a and the S-polarized light 10b is e ≒ 0.36 mm, and the thickness of the transparent flat plate is t, Holds. Therefore, from equation (15), the thickness t of the transparent flat plate 12c is obtained.
Should be set to t ≒ 0.25 mm. The distance d between the easily available six-segment light receiving elements is Is in the range of Should be within the range.

The output signal 16 of the light receiving regions 14b and 14d and the output signal 15 of the light receiving regions 14c and 14e are added by an adder 17 to become a signal 18. The output signal 19 and the signal 18 of the light receiving area 14a are subtracted by the subtractor 20 to detect the reproduced signal 21 by the differential reproducing method. On the other hand, the output signal 15 and the output signal 16 are subtracted by the subtractor 22, and the focus error signal 23 is detected. In addition, signal 18
And the output signal 19 are added by an adder 24, and a sum signal 25 is detected. A tracking error signal and the like are detected from the sum signal 25, but the description is omitted because it is not related to the gist of the present invention.

When the spot shape of the P-polarized light 10b on the photodetector 14 is circular, the spot shape of the S-polarized light 10a becomes slightly elliptical. This is because the length of the optical path of the P-polarized light 10a is longer than that of the S-polarized light 10b by the amount of traveling in the transparent flat plate 12c. The defocus δ due to this is calculated using the thickness t and the refractive index n. It is expressed. For example, when t = 0.25 mm and n = 1.765, δ ≒ 0.20 mm, and the defocus δ is slight, indicating that there is no problem.

The method of detecting the signal from the photodetector 14 is the same as in the first embodiment, and a description thereof will be omitted.

In the first embodiment and the second embodiment, the constituent optical parts are bonded and combined for the purpose of further reducing the size of the magneto-optical head.

In the first embodiment shown in FIG. 1, a beam splitter 5, a mirror 6, a half-wave plate 11, and a cylindrical lens 13 are joined.

In the second embodiment shown in FIG. 4, a beam splitter 5, a mirror 6, a half-wave plate 11, a polarizing beam splitter 1
2. Further, a cylindrical lens 13 was joined.

In the first and second embodiments, the mirror 6 is joined using internal reflection. It should be noted here that in internal reflection, a phase difference may be generated in polarized light, and therefore, a coating of a reflective film that does not cause a phase difference or a phase difference correcting means outside is required.

In the first embodiment and the second embodiment,
In order to detect a focus error signal in addition to a reproduction signal, an astigmatism optical unit such as a cylindrical lens 13 and a four-divided light receiving area are provided, but the invention is not limited to this.

FIG. 5 is a modification of the second embodiment of the magneto-optical head according to the present invention. Convergent light beam 10 is incident on the half-wave plate,
The operation is the same as that of the second embodiment until the direction of the polarization of the emitted light is rotated by 45 degrees. The convergent beam 10 is polarized by a polarizing beam splitter 27.
Incident on.

The polarizing beam splitter 27 sandwiches the polarizing film 27a,
It consists by isosceles right triangular prisms 27b and, sixth thicknesses different two parallel transparent plate 27c as shown in FIG. _1, 27c _2. The two transparent flat plates 27c ₁ and 27c ₂ are joined, and the joint line 27e is located at the center of the convergent light flux 10. The convergent light beam 10 enters the polarizing film 27a from above the triangular prism 27b. P-polarized light 10a transmitted through the polarizing film 27a
It is totally reflected to 2 divided by the back plane 27d ₁ 27d ₂ of the transparent flat plate 27c _1, 27c _2, P-polarized light 10a _1, 10a _2, and the re-polarizing film 27a
And travels toward the right side of the triangular prism 27b. On the other hand, the S-polarized light 10b reflected by the polarizing film 27a is also a P-polarized light.
It travels in the same direction as 7a ₁ and 7a ₂ and enters the photodetector 14.

As shown in FIG. 7, the same photodetector 14 as in the first embodiment can be used. For example, P polarized light 10a ₁ and S polarized light
10b the spacing e ₁ of the spacing e ₂ are the respective P-polarized light 10a ₂ and the S-polarized light 10b, when e ₁ ≒ 0.72 mm e ₂ ≒ 0.36 mm is (15) the thickness of the transparent plate 27c _1, 27c ₂ from equation T ₁ , t
₂ should be set to t ₁ ≒ 0.50 mm t ₂ ≒ 0.25 mm

The output signal 29 of the light receiving areas 14b and 14e and the output signal 28 of the light receiving areas 14c and 14d are subtracted by a subtractor 30 to detect a focus error signal 31. In addition, signal 28 and signal 29 are summed
The signal is added by 32 and a signal 33 is output. The signal 33 and the output signal 34 of the light receiving area 14a are subtracted by a subtractor 35 to detect a tracking error signal 36. Further, the signal 33 and the signal 34 are added by an adder 37, and a signal 38 is output. The output signal 39 and the signal 38 of the light receiving area 14f are subtracted by a subtractor 40, and a reproduced signal 41 by a differential reproducing method is detected. Further, the signal 39 and the signal 38 are added by the adder 42 and the sum signal 43 is output.

This embodiment is an example of the configuration of the focus error detection by the wedge method, and is not limited to this, and various modifications are possible.

FIG. 8 shows another modification of the first embodiment of the magneto-optical head according to the present invention. A convergent light beam 10 passes through a half-wave plate 11, and is further rotated by approximately 45 degrees.
Is the same as that of the first embodiment until astigmatism is given. The convergent light beam 10 enters a polarizing beam splitter 44, which is an analyzer.

As shown in FIG. 9, the polarization beam splitter 44 is composed of a wedge-shaped transmission flat plate 44b having a polarization film 44 and a total reflection film 44c. Here, assuming that the thickness of the transparent flat plate is t and the wedge angle of the wedge-shaped transparent flat plate 44b is β, β is the angle at which the convergent light flux 10 is incident on the polarizing film 44a in order to improve the polarization characteristics of the polarizing film 44a. through the alpha ₁ and the polarizing film 44a, P-polarized light 10a that is totally reflected by the total reflection film 44c is selected so that the angle alpha ₂ and matches that reenters the polarizing film 44a. Assuming that the same photodetector 14 as that shown in FIG. 3 is used as the plate thickness t, the P-polarized light 10a and the SH-polarized light 10b separated by the polarization beam splitter 44 It is selected so as to be incident on the respective light receiving regions separated by the upper distance d.

Further, a configuration may be adopted in which the cylindrical lens 13 is omitted by using astigmatism generated by the polarization beam splitter 44 as it is. However, astigmatism occurs only in the P-polarized light 10a.

The method of detecting a signal from the photodetector 14 is the same as in the first and second embodiments, and a description thereof will be omitted.

FIG. 10 shows a fifth embodiment of the magneto-optical head according to the present invention. The feature of this embodiment is that the coupling lens 3 for reducing the diffusion angle of the divergent light beam 2 emitted from the semiconductor laser light source 1 in the first to fourth embodiments is removed. As a result, the semiconductor laser light source 1 is installed at the object point position shown in FIG. 2 of the objective lens 7 which is a finite system lens. The light detection method is the same as that of the first embodiment, and a description thereof will be omitted.

〔The invention's effect〕

According to the present invention, a converging light is incident on the polarization beam splitter at a small angle without the need for a detection system lens, and the polarization characteristics of the polarization beam splitter can be prevented from deteriorating. Therefore, the magneto-optical head can be reduced in size and cost. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a magneto-optical head according to a first embodiment of the present invention, FIG. 2 is a diagram illustrating the principle of the present invention, FIG. 3 is a configuration diagram of the photodetector and signal processing of FIG. FIG. 5 is a diagram showing a configuration of a magneto-optical head according to a second embodiment of the present invention, FIG. 5 is a diagram showing a configuration of a magneto-optical head according to a third embodiment of the present invention, and FIG. FIG. 7 is a configuration diagram of the photodetector and signal processing of FIG. 5, FIG. 8 is a configuration diagram of a magneto-optical head according to a fourth embodiment of the present invention, and FIG. 9 is a polarization beam of FIG. FIG. 10 is a side view of a splitter, and FIG. 10 is a configuration diagram of a magneto-optical head according to a fifth embodiment of the present invention. 3 Coupling lens 7 Objective lens 12 Polarizing beam splitter 10a P-polarized light 10b S-polarized light 14 Photodetector

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Norihiro Katase 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Home Appliances Research Laboratory, Hitachi, Ltd. (56) References JP-A-60-38740 (JP, A) JP-A Sho 62-8347 (JP, A) JP-A-61-261837 (JP, A) JP-A-59-186156 (JP, A) JP-A-61-13457 (JP, A)

Claims (3)

(57) [Claims] A semiconductor laser light source; a first lens that converts a diffused light beam emitted from the light source into a diffused light beam having a smaller diffusion angle than the diffused light beam; and a diffused light beam that has passed through the first lens. A second lens for condensing light and irradiating the magneto-optical recording medium, and an optical path of a light beam reflected by the magneto-optical recording medium,
A deflector that diverges from the optical path of the light beam incident on the magneto-optical recording medium; an analyzer that separates p-polarized light and s-polarized light of the reflected light beam deflected by the deflector; and the separated p-polarized light and s-polarized light And a photodetector that detects each of the following: the deflector is disposed between the second lens and the first lens, and the optical path of the reflected light flux that has passed through the second lens, The light beam incident from the first lens to the second lens is branched from the optical path and deflected toward the photodetector. The second lens converges the reflected light beam, and the second lens converges on the photodetector. A magneto-optical head characterized in that light is condensed on the surface. 2. The magnetic device according to claim 1, wherein the analyzer includes a polarizing film that transmits one of the p-polarized light and the s-polarized light and reflects the other. Optical head. 3. The focal length of the first lens and the distance between the first lens and the second lens are such that the numerical aperture on the light source side of the second lens is 0.45 or more and 0.6 or less, The numerical aperture of the second lens on the magneto-optical recording medium side is 0.1
2. The magneto-optical head according to claim 1, wherein a numerical aperture on the magneto-optical recording medium side of the first lens is determined to be 0.15 or more.