JP2003114380A - Objective lens for optical head and optical head using the same - Google Patents

Abstract

(57) [Object] To provide an objective lens for an optical head, which is an objective lens used for applications of NA of 0.70 or more and is capable of satisfactorily correcting eccentric coma aberration while being a single lens. As an issue. An objective lens (20) is a single lens having a NA of 0.70 or more that forms a spot on a recording surface (12) via a transparent protective layer (11) of an optical disc (10) by converging an incident light beam. The objective lens 20 is a single lens in which both the first surface and the second surface are aspherical, and cancels each other on the axis when the first surface 21 and the second surface 22 have no eccentricity between the surfaces. A fifth-order eccentric coma is set to compensate for the deterioration of the wavefront aberration due to the third-order eccentric coma caused by the eccentricity of the lens surface in the direction perpendicular to the optical axis by adding a fifth-order spherical aberration. Have been.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens used in an optical head for recording and reproducing information on an optical recording medium such as an optical disk, and particularly applied to an optical recording medium having a high information recording density. It relates to an objective lens having an NA (numerical aperture) of 0.7 or more. The present invention also relates to an optical head using such an objective lens.

[0002]

2. Description of the Related Art The NA of an objective lens of this type is set according to the recording density of a target optical storage medium. The NA of the objective lens of an optical head using a CD (compact disc) as an optical disc is about 0.45, and the NA of the objective lens of an optical head using a DVD (digital versatile disc) having a higher recording density is about 0.60. .

A double-sided aspherical single lens is generally used as an objective lens for CDs and DVDs. Since the objective lens converges the incident light flux as a diffraction-limited spot, it is necessary to sufficiently correct spherical aberration. In addition, off-axis coma aberration also needs to be corrected in order to secure a margin of assembly error. Therefore, the objective lens, which is a conventional double-sided aspherical single lens, has spherical aberration corrected in a predetermined design reference state (generally, a state in which parallel light is incident on the objective lens) and satisfies the sine condition. Is designed to be.

On the other hand, an objective lens for an optical head is generally molded by using a mold. However, in manufacturing, the first surface of the objective lens (on the light source side) is formed in order to secure a clearance when the mold moves. ) And the second surface (optical disk side) between 0.001mm
An eccentricity of ~ 0.004 mm (relative displacement of the surface in the direction perpendicular to the optical axis) occurs. In conventional objective lenses for CDs and DVDs, the NA is low, so high-order aberrations are less likely to occur, and the degree of freedom such as the focal length and working distance (the distance between the final lens surface and the surface of the disc protective layer) is large. Therefore, it is possible to correct the coma aberration due to the eccentricity by the balance of the surface shape.

On the other hand, recently, an optical disc having a higher recording density than that of a DVD has been proposed, and in order to cope with such an optical disc, the NA of the objective lens needs to be 0.70 or more. However, if the focal length is shortened in order to increase the NA without increasing the lens diameter and a predetermined working distance is secured, the degree of freedom in designing the surface shape is reduced, and decentering coma aberration is corrected. Can not be. 0.004m for objectives with NA 0.70 or higher
The aberration generated by the decentering of the surface of m greatly exceeds the allowable range and cannot be used as an objective lens.

Therefore, Japanese Laid-Open Patent Publication No. 11-190818 discloses an objective lens having a high NA while suppressing decentering coma aberration and spherical aberration by forming a double lens.

[0007]

However, if two objective lenses are used, the weight and volume are larger than in the case of a single lens. Therefore, a fine actuator designed for a conventional single lens (the objective lens is an optical axis) is used. Mechanism for driving in the direction for focusing) cannot be used. In addition, since two lenses must be axially aligned and fixed to the frame, the number of assembling steps and the number of parts are increased. Further, the working distance (distance between the final lens surface and the surface of the disc protective layer) of the objective lens described in the above publication is 3.5 to 50 μm, which is extremely higher than that of a single objective lens having the same focal length. There is a problem of becoming smaller.

The present invention has been made in view of the above-mentioned problems of the prior art, and is an objective lens used for NA 0.70 or more, and is a single lens,
An object of the present invention is to provide an objective lens for an optical head, which can satisfactorily correct decentering coma.

[0009]

In order to achieve the above-mentioned object, an objective lens for an optical head according to the present invention converges an incident light beam to form a recording layer on a recording surface via a transparent protective layer of an optical recording medium. In the formation of spots,
A double-sided aspherical single lens with an NA of 0.70 or more, which cancels each other axially when there is no eccentricity between the first surface on the incident side and the second surface on the optical recording medium side. By compensating for higher-order spherical aberrations, the deterioration of the wavefront aberration due to the lower-order decentering coma aberration caused by the decentering of the lens surface in the direction perpendicular to the optical axis is compensated by the higher-order decentering coma aberration. It is characterized by doing so.

The degree of freedom in designing a double-sided aspherical single lens such as the objective lens of the present invention is determined by the paraxial radius of curvature r of the first surface.
1, paraxial radius of curvature r2 of second surface, thickness d, refractive index n, first
There are six aspherical surface shapes ASP1 and a second surface aspherical shape ASP2.

Of these, one degree of freedom is used to realize the focal length f required by the specifications. For example,
Here, it is assumed that the paraxial radius of curvature r2 of the second surface is set to a certain value for determining the focal length. Also, the refractive index n
Since it is determined by the type of material, it cannot be freely changed, and the thickness d can be changed within a range in which the thickness of the peripheral portion of the lens and the working distance can be secured. Therefore, the degree of freedom that can be easily changed is aspherical shapes APS1 and APS2 on both sides, and
There are three paraxial curvature radii r1 of the surface.

With these three degrees of freedom, spherical aberration, coma, and decentering coma cannot all be made zero.
That is, it is possible to correct spherical aberration including a high order in one of the aspherical shapes and correct coma aberration including a high order in the other aspherical shape. However, if the paraxial radius of curvature r1 of the remaining first surface is set, the amount of eccentric coma aberration generated can be changed by the degree of freedom in power distribution between the first surface and the second surface, but the third-order eccentric coma aberration can be changed. And high-order decentering coma cannot be reduced at the same time.

Therefore, the objective lens of the present invention is designed to prevent the deterioration of the wavefront aberration when decentering occurs on the lens surface by balancing the high-order decentering coma aberration and the low-order decentering coma aberration. Has been done. The high-order spherical aberrations added to both lens surfaces cancel each other out when there is no decentering between the surfaces, and therefore do not affect the wavefront aberration. When the lens surfaces are decentered, high-order spherical aberration is not canceled due to the displacement of both lens surfaces, and high-order decentered coma aberration is generated. Then, by adding spherical aberration, high-order eccentric coma aberration generated at the time of eccentricity and low-order eccentric coma aberration originally generated by eccentricity are canceled out, so that deterioration of wavefront aberration due to eccentricity can be suppressed to a small level. You can

If there is no decentering between the first and second surfaces of the objective lens, fifth-order spherical aberrations canceling each other on the axis are added, which causes decentering of the lens surface. The deterioration of the wavefront aberration due to the third-order decentering coma aberration
It is desirable to cancel by the wavefront aberration due to the fifth-order decentering coma aberration generated by the introduction of the spherical aberration.

Hereinafter, such a design will be specifically described. The aberration generated when the first surface and the second surface of the objective lens, which is a single lens, are decentered by Δh perpendicularly to the optical axis depends on the shape of the surface. Here, for the sake of simplicity, consider an aberration in a plane which passes through the optical axis and coincides with the direction of decentering of the lens surface.

The shape of the first surface which is a rotationally symmetric aspherical surface is
Letting g (h) be a function defined by the amount of sag in the optical axis direction at a point at a distance h from the optical axis, g ′ (h) obtained by differentiating g (h) by h is the surface inclination. The amount of deviation in the optical axis direction before and after the eccentricity of the surface is g ′ (h) Δh with respect to the amount of eccentricity Δh, and the optical path length of the ray incident on the point of height h is this deviation amount g ′ (h) Δh. Changes by the amount of.

In general, the addition of the optical path length received by a ray incident on a plane parallel plate having a refractive index n and a thickness t (thickness in the direction perpendicular to the surface) at an incident angle θ is t (−cos θ + √ (n ² −sin ² θ)). In the case of an objective lens for an optical head, in general, a light ray incident on the first surface is substantially parallel to the optical axis, so that if the incident angle on the first surface is θ, then tan θ = g ′
There is a relationship of (h).

In summary, the addition of the wavefront aberration due to the decentering of the lens surface with respect to the ray incident on the first surface of the objective lens at the distance h from the optical axis is expressed by the following expression (A). Δhg '(h) (-1 + √ (n ² + (n ² -1) ((g' (h)) ² ))) / (1+ (g '(h)) ² ) ... (A) Equation (A) includes a paraxial first-order component of the wavefront aberration, but this is not decentering aberration of the wavefront, so it must be subtracted. The deduction is h (n-1) c, where c is the value on the optical axis (paraxial curvature) of the function obtained by performing the second-order differentiation of g (h). Therefore, the actual wavefront aberration caused by the eccentricity of the lens surface is given by the following expression (B). Δhg ′ (h) (− 1 + √ (n ² + (n ² −1) ((g ′ (h)) ² ))) / (1+ (g ′ (h)) ² ) − h (n−1) c)… (B)

When the unit of wavefront aberration is λ, it may be divided by the wavelength. Assuming that the sensitivity Sw of the wavefront aberration generated by the eccentricity of the lens surface is 1 mm, Sw is given by the following expression (C). Sw = g '(h) ( - 1 + √ (n 2 + (n 2 -1) ((g' (h)) 2))) / (1+ (g '(h)) 2) -h (n- 1) c) (C) When the differential value g ′ (h) is set to X for simplicity, the above equation (C) is given by the following equation (D), and the wavefront aberration ΔOPD generated by the eccentricity Δh is given by It will be as shown in (E). Sw = X / (1 + X ² ) × (−1 + √ (n ² + (n ² −1) X ² )) − h (n−1) c ... (D) ΔOPD = Sw (h) × Δh. E) Formula (E) is, for example, when decentering of 1 μm occurs on the lens surface, if the value of Sw at a certain height h from the optical axis is 1, the light flux incident at that height h This means that a wavefront aberration of 1 μm occurs.

The objective lens for an optical head according to the present invention is as described above.
Sw is the maximum effective luminous flux radius h of the first surface_maxAs 0
<H ≦ h_maxHas a positive maximum in the range of 0, and 0 <h
≤h_{ma x}The following condition (1) is satisfied in the entire range of
To do. -0.10 <Sw <0.10 (1)

When a high NA lens is designed by a conventional design method that corrects spherical aberration and satisfies the sine condition, the sensitivity S
w increases as a cubic function from the center toward the periphery. In comparison with this, when the sensitivity Sw has a positive maximum value as in the present invention, the curvature becomes tight in the intermediate region in the range of 0 <h ≦ h _max , and becomes gentler in the peripheral region.

When the value exceeds the upper limit of the condition (1), the sensitivity of decentering coma aberration increases, and the allowable error range for decentering at the time of manufacturing decreases, so that the manufacturing yield decreases. When the value goes below the lower limit of the condition (1), high-order off-axis coma aberration becomes large, the image circle becomes narrow, and a method of moving only the objective lens at the time of tracking ensures a sufficient tracking movement range. Difficult to do.

Further, it is desirable to satisfy the following condition (2). -0.01 <Sw <0.05 (2) When the upper limit of the condition (2) is satisfied, the sensitivity of the wavefront aberration generation to the eccentricity decreases, so that the allowable error range for the eccentricity at the time of manufacturing becomes large and the manufacturing becomes easy. Become. By satisfying the lower limit, it is possible to further reduce the off-axis coma aberration and widen the image circle.

On the other hand, the optical head according to the present invention comprises a light source section for emitting a laser beam and the above objective for forming a spot on the recording surface of the laser beam from the light source section through the transparent protective layer of the optical recording medium. And a lens.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an objective lens for an optical head according to the present invention and an optical head including the objective lens will be described below. First, the configuration of the optical head of the embodiment will be described with reference to FIG. The optical head of the embodiment is D
It is used for an optical disc having a higher density than VD, the NA of the objective lens is 0.70 or more, and the laser light used is blue light having a wavelength of 405 nm.

As shown in FIG. 1, the optical head of the embodiment has a semiconductor laser 1 for emitting a laser beam, a collimating lens 2 for making a divergent light emitted from the semiconductor laser a substantially parallel beam, and an optical disk 10 for the laser beam. Objective lens 20 that converges on the recording surface 12 via the transparent protective layer 11, a beam splitter 3 that separates the return light from the optical disk 10, a condenser lens 4 that collects the separated return light, and a condenser lens 4. It is provided with a light receiving element 5 that receives the returned light that has been emitted and outputs various signals. The semiconductor laser 1 and the collimator lens 2 form a light source unit that emits laser light.

The objective lens 20 is mounted on the objective lens actuator 6 for tracking and focusing. The objective lens 20 is a single lens having an NA of 0.70 or more, and a first surface 21 on the incident side and a second surface 22 on the optical disk 10 side, both of which are aspherical surfaces.
If there is no eccentricity between the surfaces, a high-order spherical aberration that cancels each other on the axis is added to both the first and second surfaces 22,
The deterioration of the wavefront aberration due to the low-order decentering coma aberration caused by the decentering of the lens surface in the direction perpendicular to the optical axis is set to be corrected by the high-order decentering coma aberration. With the above setting, even if the first surface 21 and the second surface 22 are decentered in the direction perpendicular to the optical axis, the deterioration of the wavefront aberration can be prevented.

The correction state of the decentering coma aberration as described above is obtained.
Therefore, the objective lens 20 of the embodiment emits light when decentered.
The sensitivity Sw of the resulting wavefront aberration is the maximum effective luminous flux half of the first surface.
Diameter h _maxAs 0 <h ≤ h_maxIf we have a maximum in the range of
Both 0 <h ≦ h_maxThe following conditions (1) in the entire range of -0.10 <Sw <0.10 (1) Is designed so that it also satisfies the above condition (2)
Has been done. -0.01 <Sw <0.05 (2) The sensitivity Sw is Sw = X / (1 + X²) × (−1 + √ (n²+ (N²-1) X²)) − H (n−
1) c Is defined as Here, X is from the optical axis of the first surface 21.
The sag amount in the optical axis direction at the distance h is calculated by the function g (h) at the distance h.
The divided function, c is the function of X differentiated by h on the optical axis
The value n is the refractive index of the objective lens.

Next, three concrete examples based on the above-described embodiment will be presented. In both cases, the thickness of the protective layer is 0.1.
This is an objective lens for an optical head that is applied to an optical disc 10 of mm.

[0030]

Example 1 FIG. 2 is an objective lens 20 according to Example 1.
FIG. 3 is a lens diagram showing a protective layer 11 of the optical disc 10. Table 1 shows specific numerical configurations of the objective lens of Example 1. In the table, f is the focal length, NA is the numerical aperture, r is the radius of curvature of the surface (unit: mm), d is the distance between the surfaces on the optical axis (unit: mm), and n is the refractive index at the working wavelength of 405 nm. is there. Surface numbers 1 and 2 indicate the first surface 21 and the second surface 22 of the objective lens 20, respectively, and surface numbers 3 and 4 indicate both surfaces of the protective layer 11 of the optical disk 10.

Both the first surface 21 and the second surface 22 are rotationally symmetric aspherical surfaces around the optical axis. The shape of the aspherical surface is g (h), which is the distance (sag amount) from the tangent plane on the optical axis of the aspherical surface of the aspherical point whose distance from the optical axis is h to the optical axis of the aspherical surface. C is the curvature (1 / r) above, K is the conic coefficient, 4th, 6th, 8
The following tenth and twelfth aspherical coefficients are represented by the following equations, where A ₄ , A ₆ , A ₈ , A ₁₀ and A ₁₂ are used. These coefficients are shown in Table 2. g (h) = Ch ² / (1 + √ (1- (1 + K) C ² h ² )) + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸
+ A ₁₀ h ¹⁰ + A ₁₂ h ¹²

[0032]

[Table 1]

[0033]

[Table 2]

FIG. 3 shows the spherical aberration SA in the reference state of the objective lens 20 of Example 1 (the state in which there is no eccentricity between the first surface and the second surface of the objective lens), and the sine condition violation amount (hereinafter ,
In the figure, SC is shown simply as the sine condition. The horizontal axis of the graph shows the amount of aberration (unit: mm), and the vertical axis shows the numerical aperture N.
A is shown.

FIG. 4 shows the wavefront aberration of the objective lens of Example 1, where (A) is the reference state and (B) the first surface 21 and the second surface 22 are relatively decentered by 0.001 mm. The case is shown. In the graph of wavefront aberration, the vertical axis represents the amount of aberration generation (unit: wavelength), and the horizontal axis represents the distance from the optical axis. 0.001
The wavefront aberration when decentered by mm is 0.016λ in rms value, which is suppressed to a level that can be sufficiently used as an objective lens for optical recording.

[0036]

Second Embodiment FIG. 5 is an objective lens 20 according to the first embodiment.
FIG. 3 is a lens diagram showing a protective layer 11 of the optical disc 10. Table 3 shows the basic numerical configuration of the objective lens of Example 2.
Table 4 shows the coefficients relating to the aspherical surface.

[0037]

[Table 3]

[0038]

[Table 4]

FIG. 6 shows the spherical aberration SA and the sine condition violation amount SC in the reference state of the objective lens 20 of the second embodiment. 7A and 7B show the wavefront aberration of the objective lens 20 of Example 2, where FIG. 7A is the reference state, and FIG. 7B is the first surface 21 and the second surface.
The case where the surface 22 is relatively eccentric by 0.001 mm is shown. The wavefront aberration when decentered by 0.001 mm is 0.0 in rms value.
It is 26λ, which is suppressed to such an extent that it can be sufficiently used as an objective lens for optical recording.

[0040]

Third Embodiment FIG. 8 is an objective lens 20 according to a third embodiment.
FIG. 3 is a lens diagram showing a protective layer 11 of the optical disc 10. Table 5 shows the basic numerical configuration of the objective lens of Example 3.
Table 6 shows the coefficients relating to the aspherical surface.

[0041]

[Table 5]

[0042]

[Table 6]

FIG. 9 shows the spherical aberration SA and the sine condition violation amount SC in the reference state of the objective lens 20 of Example 3. 10A and 10B show the wavefront aberration of the objective lens 20 of Example 3, where FIG. 10A shows the reference state and FIG. 10B shows the case where the first surface 21 and the second surface 22 are relatively decentered by 0.001 mm. Shows. The wavefront aberration when decentered by 0.001 mm is 0.0 in rms value.
It is 23λ, which is suppressed to such an extent that it can be sufficiently used as an objective lens for optical recording.

Next, what value the sensitivity Sw of the wavefront aberration generated by the eccentricity of the lens surface shown in the above conditions (1) and (2) has for each distance h in each embodiment will be described. This will be explained with reference to Table 7 below. The pupil coordinates in the table are ratios obtained by h / h _max , where h _{max is} the distance from the outermost optical axis of the objective lens pupil and h is the distance from the actual optical axis through which the light ray passes. Yes, Table 7 shows the sensitivity to light rays passing through each pupil coordinate.

[0045]

[Table 7] Pupil coordinates Example 1 Example 2 Example 3 1.00 -0.0017 0.0032 -0.0054 0.90 0.0248 0.0396 0.0210 0.80 0.0268 0.0420 0.0236 0.70 0.0209 0.0328 0.0186 0.60 0.0139 0.0219 0.0125 0.50 0.0082 0.0129 0.0075 0.40 0.0042 0.0066 0.0039 0.30 0.0018 0.0028 0.0016 0.20 0.0005 0.0008 0.0005 0.10 0.0001 0.0001 0.0001 0.00 0.0000 0.0000 0.0000

As shown in Table 7, in each of the examples, the condition (1) and the stricter condition (2) are satisfied for all distances, and when relative eccentricity occurs on the lens surface of the objective lens. Also, the amount of wavefront aberration generated can be suppressed to a low level. FIG. 11 is a graph of the numerical values in Table 7. From this graph, it can be seen that the sensitivity Sw has a positive maximum value at the position of the pupil coordinate 0.8 in each of the examples. Recognize.

[0047]

As described above, according to the present invention, the low-order eccentric coma aberration caused by the eccentricity of the lens surface is canceled out by the high-order eccentric coma aberration, so that the single lens causes the eccentricity. Generation of wavefront aberration can be suppressed to a small level. Therefore, the allowable error range for the eccentricity at the time of manufacturing can be increased, and the manufacturing yield can be improved. In addition, the weight and volume of the lens are reduced, and the load on the fine actuator is reduced, as compared with the conventional two-element objective lens.
It becomes possible to secure a sufficient working distance.

[Brief description of drawings]

FIG. 1 is an explanatory diagram conceptually showing the structure of an optical head according to an embodiment.

FIG. 2 is a lens diagram showing an objective lens according to Example 1 and a protective layer of an optical disc.

FIG. 3 is a graph showing spherical aberration and sine conditions in the reference state of the objective lens according to the example 1.

4A and 4B are graphs showing wavefront aberration of the objective lens of Example 1, where FIG. 4A is a reference state and FIG. 4B is 0.001 mm on the lens surface.
The aberrations when the eccentricity occurs are shown.

FIG. 5 is a lens diagram showing an objective lens according to a second example and a protective layer of an optical disc.

FIG. 6 is a graph showing spherical aberration and sine conditions in the reference state of the objective lens according to the example 2;

7A and 7B are graphs showing the wavefront aberration of the objective lens of Example 2, where FIG. 7A is a reference state and FIG. 7B is 0.001 mm on the lens surface.
The aberrations when the eccentricity occurs are shown.

FIG. 8 is a lens diagram showing an objective lens according to a third example and a protective layer of an optical disc.

FIG. 9 is a graph showing spherical aberration and sine conditions in the reference state of the objective lens according to the example 3;

10A and 10B are graphs showing wavefront aberration of the objective lens of Example 3, where (A) is a reference state and (B) is 0.001 on the lens surface.
Aberrations when decentering of mm occurs.

FIG. 11 is a graph showing a change in sensitivity with respect to decentering of the objective lens of each example.

[Explanation of symbols]

1 Semiconductor laser 2 Collimating lens 3 beam splitter 6 Collimating lens actuator 7 Objective lens actuator 10 optical disc 11 Protective layer 12 Recording surface 20 Objective lens 21 First side 22 Second side

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H087 KA13 LA01 PA01 PA17 PB01                       QA02 QA06 QA07 QA12 QA14                       QA32 QA34 RA05 RA12 RA13                       RA42                 5D119 AA22 BA01 BB01 BB04 EC04                       EC05 JA09 JA43 JB02 JB03                 5D789 AA22 BA01 BB01 BB04 EC04                       EC05 JA09 JA43 JB02 JB03

Claims (7)

[Claims] 1. An objective lens for an optical head, which forms a spot on a recording surface through a transparent protective layer of an optical recording medium by converging an incident light beam, wherein a single lens having a double-sided aspherical surface of NA 0.70 or more is used. Then, on both sides of the first surface on the incident side and the second surface on the optical recording medium side,
If there is no eccentricity between the surfaces, a high-order spherical aberration that cancels each other on the axis is added, and a wavefront due to a low-order eccentric coma aberration generated by the eccentricity of the lens surface in the direction perpendicular to the optical axis. An objective lens for an optical head, characterized in that deterioration of aberration is compensated by high-order decentering coma aberration. 2. The high-order spherical aberration added to the lens surface is a fifth-order spherical aberration.
The objective lens for an optical head as described in 1. 3. A function obtained by differentiating a function g (h) representing the sag amount in the optical axis direction at a distance h from the optical axis of the first surface by the distance h is expressed as X, and X is differentiated by h. Let c be the value of the function on the optical axis and n be the refractive index of the objective lens. Sw = X / (1 + X ² ) × (−1 + √ (n ² + (n ² −1) X ² )) − h ( n−
1) When c is defined, the Sw has a positive maximum value in the range of 0 <h ≦ h _max as the maximum effective luminous flux radius h _max of the first surface, and the total of 0 <h ≦ h _max . The following conditions in the range
(1), -0.10 <Sw <0.10 ... (1) The objective lens for an optical head according to claim 1 or 2, characterized in that: 4. The objective lens for an optical head according to claim 3, wherein the Sw further satisfies the following condition (2): −0.01 <Sw <0.05 (2). 5. In an objective lens for an optical head, which forms a spot on a recording surface through a transparent protective layer of an optical recording medium by converging an incident light beam, the first surface on the incident side and the optical recording medium. A single lens having an NA of 0.70 or more, in which both surfaces of the second surface which is the side are aspherical surfaces, and a function g (h) representing a sag amount in the optical axis direction at a distance h from the optical axis of the first surface. The function obtained by differentiating with respect to the distance h is expressed as X, and X
Let c be the value on the optical axis of the function obtained by differentiating with respect to h and n be the refractive index of the objective lens. Sw = X / (1 + X ² ) × (−1 + √ (n ² + (n ² −1) X ² )) − H (n−
1) When c is defined, Sw has a maximum value within the range of 0 <h ≦ h _max as the maximum effective luminous flux radius h _max of the first surface, and within the entire range of 0 <h ≦ h _max. An objective lens for an optical head, which satisfies the following condition (1), -0.10 <Sw <0.10 (1). 6. The objective lens for an optical head according to claim 5, wherein the Sw further satisfies the following condition (2), −0.01 <Sw <0.05 (2). 7. The light source section which emits a laser beam, and the laser beam from the light source section forms a spot on a recording surface through a transparent protective layer of an optical recording medium. And an objective lens for the optical head.