Objective lens and optical pickup device Number:7,120,108 from the United States Patent and Trademark Office (PTO) owispatent An objective lens to converge a light flux having a working reference wavelength .lamda..sub.0 (380 nm.ltoreq..lamda..sub.0.ltoreq.450 nm) under a working reference temperature T.sub.0 onto an optical information recording medium equipped with a protective substrate having a thickness of 0.6 mm with almost no aberration. The diffractive structural section of the objective lens has a first compensating function to compensate a change amount .delta.SA1 of the third-order spherical aberration component of wavefront aberration caused by a fluctuation of a working wavelength, a second compensating function to compensate a deviation .delta.WD of a converged-light spot in an optical axis direction caused by a fluctuation of a working wavelength, and a third compensating function to compensate a change amount .delta.SA2 of the third-order spherical aberration component of wavefront aberration caused by a change of a refractive index of the lens body.<H Objective, optical, Objective, lens, , 7120108.html, Patent, pto, 7120108

Title: Objective lens and optical pickup device

Abstract: An objective lens to converge a light flux having a working reference wavelength .lamda..sub.0 (380 nm.ltoreq..lamda..sub.0.ltoreq.450 nm) under a working reference temperature T.sub.0 onto an optical information recording medium equipped with a protective substrate having a thickness of 0.6 mm with almost no aberration. The diffractive structural section of the objective lens has a first compensating function to compensate a change amount .delta.SA1 of the third-order spherical aberration component of wavefront aberration caused by a fluctuation of a working wavelength, a second compensating function to compensate a deviation .delta.WD of a converged-light spot in an optical axis direction caused by a fluctuation of a working wavelength, and a third compensating function to compensate a change amount .delta.SA2 of the third-order spherical aberration component of wavefront aberration caused by a change of a refractive index of the lens body.

Patent Number: 7,120,108 Issued on 10/10/2006 to Ota, et al.

Inventors: Ota; Kohei (Hachioji, JP), Mimori; Mitsuru (Kokubunji, JP)
Assignee: Konica Corporation (Tokyo, JP)

Appl. No.: 10/655,043

Filed: September 5, 2003

Foreign Application Priority Data


Sep 09, 2002 [JP]			2002-262934
Sep 10, 2002 [JP]			2002-264477

Current U.S. Class: 369/112.23 ; 369/112.01; 369/112.03; 369/112.08

Current International Class: G11B 7/00 (20060101)

Field of Search: 369/112.01,112.23,112.03,112.08,112.13,112.24,44.23,44.24

References Cited [Referenced By]

U.S. Patent Documents


6349000	February 2002	Yamagata et al.

Primary Examiner: Hindi; Nabil
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, L.L.P.

Claims

What is claimed is:

1. An objective lens for use in an optical pickup device for conducting reproducing and/or recording information for an optical information recording medium equipped with a protective substrate having a thickness of 0.6 mm and to converge a light flux having a working reference wavelength .lamda..sub.0 (380 nm.ltoreq..lamda..sub.0.ltoreq.450 nm) emitted from a laser light source under a working reference temperature T.sub.0 onto an information recording plane of the optical information recording medium with almost no aberration, comprising: a lens main body; and a diffractive structural section provided on at least one optical functional surface of the lens body; wherein the diffractive structural section has a first compensating function to compensate a change amount .delta.SA1 of the third-order spherical aberration component of wavefront aberration caused by a fluctuation of a working wavelength .lamda. (|.lamda. .lamda..sub.0|.ltoreq.10 nm) on the lens main body and the diffractive structural section to be less than a predetermined value, a second compensating function to compensate a deviation .delta.WD of a converged-light spot in an optical axis direction caused by a fluctuation of a working wavelength to be less than a predetermined value, and a third compensating function to compensate a change amount .delta.SA2 of the third-order spherical aberration component of wavefront aberration caused by a change of a refractive index of the lens body due to a fluctuation of a working temperature T (|T T.sub.0|.ltoreq.40.degree. C.) on the lens main body and the diffractive structural section to be less than a predetermined value.

2. The objective lens of claim 1, wherein when the working wavelength fluctuates by 5 nm, the change amount .delta.SA1 of the third-order spherical aberration component of wavefront aberration on the lens main body and the diffractive structural section is compensated within the following range by the first compensating function of the diffractive structural section: |.delta.SA1|.ltoreq.0.04 (.lamda..sub.0rms).

3. The objective lens of claim 1, wherein when the working wavelength fluctuates by 1 nm, the change amount .delta.SA1 of the third-order spherical aberration component of wavefront aberration on the lens main body and the diffractive structural section is compensated within the following range by the first compensating function of the diffractive structural section: -0.008 (.lamda..sub.0rms).ltoreq..delta.SA1.ltoreq.-0.003 (.lamda..sub.0rms).

4. The objective lens of claim 1, wherein when the working wavelength fluctuates by 1 nm, the deviation .delta.WD of a converged-light spot in an optical axis direction is compensated within the following range by the second compensating function of the diffractive structural section: |.delta.WD|.ltoreq.0.1 (.mu.m).

5. The objective lens of claim 1, wherein an paraxial power .phi.D of the diffractive structural section satisfies the following formula: 0.ltoreq..phi.D/.phi.D.sub.0.ltoreq.0.7 where .phi.D.sub.0 is a paraxial power of the diffractive structural section when chromatic aberration in a paraxial area of the lens main body is corrected completely by changing the power of the diffractive structural section without changing the power of the lens main body.

6. The objective lens of claim 1, wherein when the working temperature fluctuates by 40.degree. C., the change amount .delta.SA2 of the third-order spherical aberration component of wavefront aberration caused by a change of a refractive index of the lens body on the lens main body and the diffractive structural section is compensated within the following range by the third compensating function of the diffractive structural section: |.delta.SA2|.ltoreq.0.055 (.lamda..sub.0rms).

7. The objective lens of claim 1, wherein the diffractive structural section is formed by being divided into plural ring-shaped zones with plural concentric circles having the center on the optical axis and satisfies the following formula: 90.ltoreq.Lm (f.sup.1/2).ltoreq.300 where L is the number of ring-shaped zones, m is an order of a diffracted light ray having the maximum diffraction efficiency among diffracted light rays generated by the diffractive structural section when recording and/or reproducing information is conducted for an optical information recording medium, and f is a focal length (mm).

8. The objective lens of claim 7, wherein the following formula is satisfied: 140.ltoreq.Lm (f.sup.1/2).ltoreq.220.

9. The objective lens of claim 1, wherein an image-side numerical aperture of the lens main body is 0.6 to 0.9.

10. The objective lens of claim 9, wherein the image-side numerical aperture of the lens main body is 0.65.

11. The objective lens of claim 1, wherein the lens main body and the diffractive structural section are made of a plastic.

12. An optical pickup apparatus for conducting reproducing and/or recording information for an optical information recording medium, comprising: a laser light source to emit a light flux; and the objective lens described in claim 1 and to converge the light flux on an information recording plane of the optical information recording medium.

13. An optical element for use in an optical pickup device for conducting reproducing and/or recording information for an optical information recording medium equipped with a protective substrate having a thickness of 0.6 mm and to converge a light flux having a working reference wavelength .lamda..sub.0 (380 nm.ltoreq..lamda..sub.0.ltoreq.450 nm) onto an information recording plane of the optical information recording medium, comprising: a diffractive structure provided on at least one optical surface of the optical element and including plural ring-shaped zones having a center on an optical axis, wherein the optical element satisfies the following formula: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300 where L is the number of ring-shaped zones, k is an order of a diffracted light ray having the maximum diffraction efficiency among diffracted light rays generated by the diffractive structural section when recording and/or reproducing information is conducted for an optical information recording medium, and f is a focal length (mm).

14. The optical element of claim 13, wherein the following formula is satisfied: 140.ltoreq.Lm (f.sup.1/2).ltoreq.220.

15. The optical element of claim 13, wherein an image-side numerical aperture of the optical element is 0.6 to 0.7.

16. The optical element of claim 15, wherein the image-side numerical aperture of the optical element is 0.65.

17. The optical element of claim 15, wherein the optical element is made of a plastic.

18. An optical pickup device for conducting reproducing and/or recording information for an optical information recording medium equipped with a protective substrate having a thickness of 0.6 mm and to converge a light flux having a working reference wavelength .lamda..sub.0 (380 nm.ltoreq..lamda..sub.0.ltoreq.450 nm) onto an information recording plane of the optical information recording medium, comprising: an optical element to converge the light flux and including a diffractive structure provided on at least one optical surface of the optical element and including plural ring-shaped zones having a center on an optical axis, wherein the following formula is satisfied: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300 where L is the number of ring-shaped zones, k is an order of a diffracted light ray having the maximum diffraction efficiency among diffracted light rays generated by the diffractive structural section when recording and/or reproducing information is conducted for an optical information recording medium, and f is a focal length (mm).

19. The optical pickup device of claim 18, wherein the following formula is satisfied: 140.ltoreq.Lm (f.sup.1/2).ltoreq.220.

Description

BACKGROUND OF THE INVENTION

The present invention relates to an optical pickup device for conducting recording and/or reproducing for an optical information recording medium and to an objective lens used in the optical pickup device.

In an optical system of an optical pickup device wherein DVD (digital versatile disk) representing an optical disk (optical information recording medium) is used as a medium, there is used a laser having a wavelength of about 650 nm (for example, see Patent Document 1).

In recent years, there has been advanced development of HD-DVD (high density DVD) representing an optical disk that is similar to DVD in terms of a size and is greater than DVD in terms of capacity. In the optical system of the optical pickup device employing this HD-DVD as a medium, a diameter of the light-converging spot is made small by using a violet laser with a short wavelength (380 450 nm approximately), for the purpose of achieving high density of recording signals or of reproducing high density recording signals.

(Patent Document)

1. TOKKAIHEI No. 11-337818 2. TOKKAIHEI No. 6-242373

SUMMARY OF THE INVENTION

However, in the optical system of the optical pickup device employing a laser with a wavelength that is as short as the foregoing, following two problems which have been insignificant in the optical system of the optical pickup device employing conventional DVD as a medium affect greatly.

Namely, one of them is a problem of changes in tertiary order spherical aberration components of spherical aberration of an objective lens caused by microscopic changes in used wavelength of a laser light source and of shifting of light-converging spot in the direction of an optical axis. In general, the shorter the used wavelength is, the greater the change of refractive index of an optical lens caused by microscopic changes of the used wavelength is, resulting in that changes in tertiary order spherical aberration components of wavefront aberration and shifting of light-converging spot are generated greatly, and recording and reproducing of information are disturbed.

Another problem is a change in tertiary order spherical aberration component of wavefront aberration of the objective lens caused by temperature changes. An objective lens made of plastic used generally in an optical pickup device tends to be deformed by temperature changes, and this deformation changes the refractive index of the objective lens, resulting in the change of the tertiary order spherical aberration of wavefront aberration. Amount of change .DELTA.SA (unit: .lamda.rms) of the tertiary order (third order) spherical aberration component of this wavefront aberration is one expressed roughly by the following expression; .DELTA.SA=kf (1-M).sup.4(NA).sup.4.DELTA.T/.lamda. (wherein, k represents a constant, f represents an image-side focal length, M represents a magnification, NA represents an image-side numerical aperture, .DELTA.T represents a temperature change and .lamda. represents a wavelength). Therefore, as is clear from this expression, the shorter the used wavelength is, the greater .DELTA.SA is. In the optical system of the optical pickup device employing a laser with a short wavelength, therefore, recording and reproducing of information are disturbed by changes of tertiary order spherical aberration components of wavefront aberration of the objective lens caused by temperature changes.

An object of the invention is to provide an optical pickup device capable of conducting recording and reproducing of information independently of changes in wavelengths and temperatures by using a laser light source with a wavelength of about 380 450 nm, and to provide an objective lens of the optical pickup device.

Incidentally, in the above patent document 2, an objective lens is disclosed wherein a ring-shaped structure having a plurality of ring-shaped zones is formed on the optical surface of the objective lens, and incident light passing through the ring-shaped structure is allowed to produce a predetermined difference of the light path, which compensates for the above chromatic aberration. By using such an objective lens, it is possible to control spherical aberration of the aspheric lens as well as to compensate for axial chromatic aberration, however, it is difficult to compensate for the before-mentioned thermal characteristic aberration, which is still a problem.

Further, in recent years, an optical pick-up apparatus, having a 0.1 mm protecting base, was researched and developed, and technology for the optical pick-up apparatus capable of solving the above-stated problem was proposed.

However, concerning the optical element and the high density optical pick-up apparatus using that optical element with a protecting base 0.6 mm in thickness, being similar to that of the optical disk for normal DVDs, and with a 0.65 numerical aperture, technology which could solve the above problems was seldom proposed.

An object of the present invention is to solve the above-stated problems, that is, to propose an optical element and an optical pick-up apparatus using that optical element, which is employed for an optical information recording medium using working standard wavelengths of 380 450 nm, and having a protecting base of about 0.6 mm in thickness, and which can also compensate for axial chromatic aberration and spherical aberration caused by changes of temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the overall structure representing an example of an optical pickup device relating to the invention.

FIG. 2 is a side view showing an outline of an objective lens relating to the invention.

FIG. 3 is a front view showing an outline of an objective lens relating to the invention.

FIG. 4 is a diagram showing vertical spherical aberration of an objective lens in the occasion where a light flux having standard wavelength for use .lamda..sub.0 enters the objective lens at standard temperature for use T.sub.0.

FIG. 5 is a diagram showing vertical spherical aberration of an objective lens in the occasion where a light flux having standard wavelength for use .lamda..sub.0 enters the objective lens at temperature for use T (T=T.sub.0+40).

FIG. 6 is a diagram showing amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of an objective lens that is caused when wavelength for use .lamda. of a light flux is changed within a range of |.lamda. .lamda..sub.0|.ltoreq.10 nm.

FIG. 7 is a diagram showing shifting .delta.WD of a light-converging spot in the direction of the optical axis that is caused when wavelength for use .lamda. of a light flux is changed within a range of |.lamda. .lamda..sub.0|.ltoreq.1 nm.

FIG. 8 is a diagram showing amount of change .delta.SA2 of tertiary order spherical aberration component of wavefront aberration of an objective lens that is caused when temperature for use T is changed within a range of |T T.sub.0|.ltoreq.40.degree. C.

FIG. 9 is a schematic diagram showing another example of an optical pick-up apparatus and an optical element of the present embodiment.

FIG. 10 is a side view showing the structure of an objective lens.

FIG. 11 shows a longitudinal spherical aberration of an objective lens as a comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, the embodiment to achieve the above object of the present invention is described.

Item (1)

The structure described in Item (1) is objective lens 11 that is used in optical pickup device 1 which conducts recording of information and/or reproducing of recorded information for optical information recording medium 8 having a protective base board with a thickness of 0.6 mm, as shown, for example, in FIG. 1 FIG. 3, and converges a light flux with standard wavelength for use .lamda..sub.0 (380.ltoreq..lamda..sub.0.ltoreq.450) emitted from laser light source L1 at standard temperature for use T.sub.0 on information recording surface 8a of the optical information recording medium 8 with aberration which is almost zero, wherein lens main body 111 and diffractive structure portion 112 that is provided on at least one optical functional surface of the lens main body 111 are provided, and the diffractive structure portion 112 is provided with a first compensating function that compensates amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration for each of the lens main body 111 and the diffractive structure portion 112 caused by the change of wavelength for use .lamda. (|.lamda. .lamda..sub.0|.ltoreq.10 nm) to be a certain value or less, a second compensating function that compensates shifting .delta.WD of light-converging spot 7a in the direction of optical axis 6 caused by the change of wavelength for use .lamda. to be a certain value or less and with a third compensating function that compensates amount of change .delta.SA2 of tertiary order spherical aberration component of wavefront aberration for each of the lens main body 111 and the diffractive structure portion 112 caused by the refractive index of the lens main body 111 in the case of the change of temperature for use T (|T T.sub.0|.ltoreq.40.degree. C.) to be a certain value or less.

In this case, an optical information recording medium that has a protective base board whose thickness is 0.6 mm and uses standard wavelength .lamda..sub.0 of 380 450 nm includes, for example, HD-DVD (high density DVD).

Further, standard temperature for use T.sub.0 is a certain temperature within a range of temperature change in surroundings in which an optical pickup device is used, and it is a certain temperature in a normal temperature (about 10 40.degree. C.).

Standard wavelength for use .lamda..sub.0 is a wavelength of a light flux that is emitted usually at standard temperature for use T.sub.0 by a laser light source to be used. Wavelength for use .lamda. includes a wavelength of a light flux which is emitted by a laser light source at temperature for use T and a wavelength of a light flux in the case of mode hop. Incidentally, mode hop is a phenomenon wherein when power of a light flux emitted from a laser light source rises, a wavelength of the light flux becomes longer instantaneously.

Further, "to converge with aberration which is almost zero" mentioned above means that "a light flux is converged so that an absolute value of wavefront aberration may be 0.07 .lamda..sub.0rms or less".

In the structure described in Item (1), it is possible to compensate amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of a lens main body and the diffractive structure portion, namely, of an objective lens caused in the case of change in wavelength for use .lamda. of a laser light source, to be a certain value or less, because the diffractive structure portion is provided with a first compensating function. Incidentally, the change in wavelength for use .lamda. of a laser light source includes an occasion where wavelength for use .lamda. is changed by dispersion of oscillated wavelength between laser light source individuals, an occasion where a wavelength used is changed by instantaneous extension of a wavelength, namely, by the so-called mode hop in the course of data recording on an optical information recording medium, and an occasion where the wavelength used is changed by the change of temperature used T.

In addition, it is possible to compensate shifting .delta.WD of light-converging spot in the direction of optical axis caused by the change of wavelength for use .lamda. of a laser light source to be a certain value or less because the diffractive structure portion is provided with a second compensating function.

Further, it is possible to compensate amount of change .delta.SA2 of tertiary order spherical aberration component of wavefront aberration of a lens main body and the diffractive structure portion, namely, of an objective lens caused by changes in refractive index when the refractive index of the lens main body is changed by a change of temperature used T of an optical pickup device, to be a certain value or less, because the diffractive structure portion is provided with a third compensating function.

It is therefore possible to conduct recording and reproducing of information independently of the change in wavelength for use .lamda. and temperature for use T.

Incidentally, since it is difficult to compensate completely amount of change .delta.SA1 and amount of change .delta.SA2 each of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion and shifting .delta.WD of a light-converging spot with the first, second and third compensating functions of the diffractive structure portion, it is preferable to keep amount of change .delta.SA1 and amount of change .delta.SA2 each of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion and shifting .delta.WD of a light-converging spot in an appropriate range by adjusting the first, second and third compensating functions relatively.

Item (2)

The structure described in Item (2) is the objective lens 11 according to the Item 1 as shown in FIGS. 1 3, for example, wherein amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of the lens main body 111 and the diffractive structure portion 112 caused when the wavelength for use .lamda. is changed by 5 nm is compensated by the first compensating function of the diffractive structure portion 112 to be within a range of the following expression. |.delta.SA1|.ltoreq.0.04 .lamda..sub.0rms

In this case, a value of .delta.SA1 which is negative means that the change of the tertiary order spherical aberration is on the under side, namely, the change is compensated toward a light source on the optical axis. On the contrary, a value of .delta.SA1 which is positive means that the change of the tertiary order spherical aberration is on the over side, namely, the change is compensated toward an image on the optical axis.

In the structure described in Item (2), amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion caused when the wavelength for use .lamda. is changed by 5 nm is compensated by the first compensating function of the diffractive structure portion to be within a range of the following expression, which makes it possible to conduct recording and reproducing of information surely independently of changes in wavelength for use .lamda.. |.delta.SA1|.ltoreq.0.04 .lamda..sub.0rms Item (3)

The structure described in Item (3) is the objective lens 11 according to either one of the Items 1 and 2 as shown in FIGS. 1 3, for example, wherein amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of the lens main body 111 and the diffractive structure portion 112 caused when the wavelength for use .lamda. is changed by 1 nm is compensated by the first compensating function of the diffractive structure portion 112 to be within a range of the following expression. -0.008 .lamda..sub.0rms.ltoreq..delta.SA1.ltoreq.-0.003 .lamda..sub.0rms

When a laser light source having a standard wavelength for use of 380 450 nm is used, a change in a used wavelength caused by a temperature change of 1.degree. C. is small, compared with a conventional occasion where a laser light source having a standard wavelength for use of 650 nm is used. To be concrete, for temperature rise of 1.degree. C., for example, an oscillated wavelength of a laser light source having a standard wavelength for use of 650 nm is lengthened by 0.2 nm, while, an oscillated wavelength of a laser light source having a standard wavelength for use of 405 nm is lengthened by 0.05 nm. Therefore, when a laser light source having a standard wavelength for use of 380 450 nm is used, an amount of wavelength change caused by temperature changes is smaller for an amount of wavelength change caused by reasons other than temperature changes, compared with an occasion where a laser light source having a standard wavelength for use of 650 nm is used. Therefore, when the third compensating function of the diffractive structure portion is strengthened by generating greatly the tertiary order spherical aberration component of wavefront aberration caused by wavelength changes, and the first compensating function is weakened on the contrary, when wavelength changes which are not caused by temperature changes are generated greatly, changes in tertiary order spherical aberration components of wavefront aberration caused by the wavelength changes are not compensated sufficiently by the first compensating function, and recording and reproducing of information are sometimes disturbed.

In the structure described in Item (3), amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion caused when the wavelength for use .lamda. is changed by 1 nm is compensated by the first compensating function of the diffractive structure portion within a range on the under side prescribed by -0.008 .lamda..sub.0rms.ltoreq..delta.SA1.ltoreq.-0.003 .lamda..sub.0rms. Therefore, amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration caused by changes in wavelength for use .lamda. resulting from changes of temperature for use T and amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration caused by wavelength changes which are not caused by changes in temperature for use T can be compensated by the first compensating function of the diffractive structure.

Now, the reason why amount of change .DELTA.SA1 of tertiary order spherical aberration is established to be equal to or higher than -0.008 .lamda..sub.0rms is that the change of tertiary order spherical aberration of wavefront aberration caused by wavelength change of 5 nm cannot be compensated sufficiently when the amount of change .DELTA.SA1 is less than -0.008 .lamda..sub.0rms, and recording and reproducing of information cannot be conducted. Further, the reason why amount of change .DELTA.SA1 of tertiary order spherical aberration is established to be equal to or lower than -0.003 .lamda..sub.0rms is that the change of tertiary order spherical aberration of wavefront aberration caused by a refractive index change of a lens main body caused by changes in used temperature T cannot be compensated sufficiently when the amount of change .DELTA.SA1 is greater than -0.003 .lamda..sub.0rms, and recording and reproducing of information cannot be conducted.

Item (4)

The structure described in Item (4) is the objective lens 11 according to either one of the Items 1 3 as shown in FIGS. 1 3, for example, wherein shifting .delta.WD of the light-converging spot 7a in the direction of an optical axis caused when the wavelength for use .lamda. is changed by 1 nm is compensated by the second compensating function of the diffractive structure portion 112 to be in the following range. |.delta.WD|.ltoreq.0.1 .mu.m

In the aforementioned expression, when a value of .delta.WD is negative, it indicates that the light-converging spot is shifted to the "over" side, namely, toward the "image" side on the optical axis.

Further, a width of a light-converging spot in the optical axis direction wherein a diameter of geometrical optical light-converging spot is not more than (.lamda..sub.0/2NA) is called a focal depth (.lamda..sub.0/2NA.sup.2). When a diameter of light-converging spot is not more than (.lamda..sub.0/2NA), light can be regarded to be converged on one point in terms of practical use. Therefore, if the light-converging spot is within a range of the focal depth, a diameter of the light-converging spot is not more than (.lamda..sub.0/2NA), which makes it possible to record and reproduce for an optical information recording medium. Therefore, if shifting .delta.WD of the light-converging spot in the direction of an optical axis caused when the wavelength for use .lamda. of a laser light source is changed from the standard wavelength for use .lamda..sub.0 is within a range of a half of the focal depth, recording and reproducing for an optical information recording medium can be conducted.

For example, in the optical pickup device employing HD-DVD as a medium, numerical aperture NA is about 0.65 0.85 and standard wavelength for use .lamda..sub.0 is about 380 450 nm. Therefore, the focal depth of this optical system is about 280 470 nm.

In the structure described in Item (4), shifting .delta.WD of the light-converging spot caused when the wavelength for use .lamda. is changed by 1 nm is compensated by the second compensating function of the diffractive structure portion to be within a range of |.delta.WD|.ltoreq.0.1 .mu.m, and therefore, it can be within a range of a half of the focal depth in the optical pickup device employing HD-DVD as a medium, for example. Therefore, in the optical pickup device employing HD-DVD as a medium, recording and reproducing of information can be conducted surely, independently of changes in wavelength for use .lamda..

Item (5)

The structure described in Item (5) is the objective lens 11 according to either one of the Items 1 4 as shown in FIGS. 1 3, for example, wherein paraxial power .phi.D of the diffractive structure portion 112 satisfies the following expression; 0.ltoreq..phi.D/.phi.D.sub.0.ltoreq.0.7 (wherein, .phi.D.sub.0 is a paraxial power of the diffractive structure portion 112 shown when chromatic aberration in a paraxial area of the lens main body is corrected completely when power of the diffractive structure portion 112 is changed without changing power of the lens main body 111).

In this case, paraxial power .phi.D of the diffractive structure portion is power of a paraxial area of the diffractive structure portion. Shifting .delta.WD of the light-converging spot in the optical axis direction caused when the wavelength for use .lamda. of a light source is changed can be compensated by compensating spherical aberration of the lens main body toward the "under" side with a diffractive action of the diffractive structure portion or by making the paraxial power .phi.D of the diffractive structure portion to be a positive value.

In the structure described in Item (5),

shifting .delta.WD of the light-converging spot in the optical axis direction caused when the wavelength for use .lamda. of a light source is changed can be compensated because paraxial power .phi.D of the diffractive structure portion satisfies the expression 0.ltoreq..phi.D/.phi.D.sub.0.ltoreq.0.7. Therefore, Therefore, recording and reproducing of information can be conducted surely, independently of changes in wavelength for use .lamda..

In this case, the reason why the value obtained by dividing .phi.D by .phi.D.sub.0 in terms of paraxial power of the diffractive structure portion is established to be 0 or more is that the value of .phi.D/.phi.D.sub.0 which is less than zero makes it impossible to compensate the change of tertiary order spherical aberration component of wavefront aberration in a paraxial area of the lens main body and the diffractive structure portion caused when the wavelength for use .lamda. is changed, and to conduct recording and reproducing of information. Further, the reason why the value obtained by dividing .phi.D by .phi.D.sub.0 in terms of paraxial power of the diffractive structure portion is established to be 0.7 or less is that, when the value of .phi.D/.phi.D.sub.0 is greater than 0.7, amount of change .delta.SA2 of tertiary order spherical aberration of wavefront aberration caused by changes of temperature for use T is not compensated sufficiently, causing some occasions where recording and reproducing of information cannot be conducted, when amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration and shifting .delta.WD of light-converging spot are made by the first and second compensating functions of the diffractive structure portion to be capable of being compensated to be within ranges described in Items 2 4.

Item (6)

The structure described in Item (6) is the objective lens 11 according to either one of the Items 1 5 as shown in FIGS. 1 3, for example, wherein amount of change .delta.SA2 of tertiary order spherical aberration component of wavefront aberration of the lens main body 111 and the diffractive structure portion 112 caused by the change in refractive index of the lens main body 111 that is caused when the temperature for use T is changed by 40.degree. C., is compensated by the third compensating function of the diffractive structure portion 112 to be within the range of the following expression. |.delta.SA2|.ltoreq.0.055 .lamda..sub.0rms

In this case, a value of .delta.SA2 which is negative means that the change of the tertiary order spherical aberration is on the under side, namely, the change is compensated toward a light source on the optical axis. On the contrary, a value of .delta.SA2 which is positive means that the change of the tertiary order spherical aberration is on the over side, namely, the change is compensated toward an image on the optical axis.

In the structure described in Item (6), amount of change .delta.SA2 of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion caused by the change of refractive index of lens main body that is caused when temperature for use T is changed by 40.degree. C. is compensated by the third compensating function of the diffractive structure portion to be within a range of |.delta.SA2|.ltoreq.0.03 .lamda..sub.0rms, and therefore, recording and reproducing of information can be conducted surely, independently of changes in temperature for use T.

Item (7)

The structure described in Item (7) is the objective lens 11 according to either one of the Items 1 6 as shown in FIGS. 1 3, for example, wherein the diffractive structure portion 112 is formed to be divided into plural ring-shaped zones by concentric circles on the center of optical axis 6, and it satisfies the following expression; 90.ltoreq.Lm (f.sup.1/2).ltoreq.300 (wherein, L represents the number of ring-shaped zones, m represents the order of diffracted light having the greatest diffraction efficiency among diffracted light generated by the diffractive structure portion 112 when conducting recording of information and/or reproducing of information recorded for the optical information recording medium 8, and f represents a focal length (mm)).

In the structure described in Item (7), the same effects as in Items 1 6 are obtained by the objective lens composed of a diffractive structure portion having ring-shaped zones in number prescribed by the above expression and of a lens main body.

Item (8)

The structure described in Item (8) is the objective lens 11 according to either one of the Items 1 7 as shown in FIGS. 1 3, for example, wherein the diffractive structure portion 112 is formed to be divided into plural ring-shaped zones by concentric circles on the center of optical axis 6, and it satisfies the following expression; 140.ltoreq.Lm (f.sup.1/2).ltoreq.220 (wherein, L represents the number of ring-shaped zones, m represents the order of diffracted light having the greatest diffraction efficiency among diffracted light generated by the diffractive structure portion 112 when conducting recording of information and/or reproducing of information recorded for the optical information recording medium 8, and f represents a focal length mm).

In the structure described in Item (8), the same effects as in Items 1 7 are obtained by the objective lens composed of a diffractive structure portion having ring-shaped zones in number prescribed by the above expression and of a lens main body.

Item (9)

The structure described in Item (9) is the objective lens 11 according to either one of the Items 1 8 as shown in FIGS. 1 3, for example, wherein image-side numerical aperture of the objective lens main body 11 is not less than 0.60 and is not more than 0.90.

In the structure described in Item (9), it is possible to prevent that recording density of an optical information recording medium is lowered as an image-side numerical aperture of the objective lens becomes smaller, and to prevent that manufacture of the objective lens becomes difficult as an image-side numerical aperture grows greater.

Item (10)

The structure described in Item (10) is the objective lens 11 according to either one of the Items 1 9 as shown in FIGS. 1 3, for example, wherein an image-side numerical aperture of the lens main body 111 is 0.65.

In the structure described in Item (10), an image-side numerical aperture of the lens main body is 0.65 and it is within a range of numerical aperture 0.65 0.85 necessary for recording and reproducing of information for HD-DVD, thus, the same effects as those in Items 1 9 are obtained as an objective lens used in an optical pickup device employing HD-DVD as a medium.

Item (11)

The structure described in Item (11) is the objective lens 11 according to either one of the Items 1 10 as shown in FIGS. 1 3, for example, wherein the lens main body 111 and the diffractive structure portion 112 are made of plastic.

In the structure described in Item (11), molding of an objective lens is easy, and a material is lower in cost and is lighter compared with glass, which makes an optical pickup device to be light in weight and low in cost.

Item (12)

The structure described in Item (12) is optical pickup device as shown in FIGS. 1 3, for example, wherein objective lens 11 described in either one of Items 1 11 is provided, and a light flux emitted from laser light source L1 is converged on information recording surface 7a of optical information recording medium 8 for conducting recording and/or reproducing of information.

In the structure described in Item (12), it is possible to compensate amount of change .delta.SA1 of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion that is caused when wavelength for use .lamda. of a laser light source is changed, to a certain value or less, because the diffractive structure portion of the objective lens is provided with the first compensating function.

Further, it is possible to compensate shifting .delta.WD of the light-converging spot in the optical axis direction caused when the wavelength for use .lamda. of a laser light source is changed to a certain value or less, because the diffractive structure portion of the objective lens is provided with the second compensating function.

In addition, it is possible to compensate amount of change .delta.SA2 of tertiary order spherical aberration component of wavefront aberration of the lens main body and the diffractive structure portion that is caused when the diffractive index of the lens main body is changed when temperature for use T of the optical pickup device is changed, because the diffractive structure portion of the objective lens is provided with the third compensating function.

It is therefore possible to conduct recording and reproducing of information, independently of changes in wavelength for use .lamda. and temperature for use T.

Item 13.

An optical element (objective lens 4 for example), being employed in optical pick-up apparatus 1 which performs recording and/or reproducing of information on optical information recording medium 5 whose thickness of a protective substrate is approximately 0.6 mm, which converges the light flux of the working standard wavelengths of 380 450 nm onto information recording surface 7 of the above-mentioned optical information recording medium, and further provides a diffractive structure including L pieces of ring-shaped zones around optical axis LA on at least a single optical surface, wherein when the reproducing and/or recording of information onto the optical information recording medium is performed, with "k" (a positive integer) as the order of the diffracted light ray having maximum diffraction efficiency among diffracted light rays generated by the diffractive structure, and "f" mm as the focal length, the following formula holds.

Formula: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300

In the present specification, "protective substrate" means an optically transparent plane-parallel plate, formed on a light flux incident surface of the recording surface, which protects the information recording surface of the optical information recording medium, and "thickness of the protective substrate" means the thickness of the plane-parallel plate. The light fluxes emitted from a light source are brought to a focus by the objective lens, through the protective substrate, on the information recording surface of the information recording medium.

Further, "optical elements" mean members of which an optical system of the optical pick-up apparatus is composed, such as an objective lens, a coupling lens (collimator lens), a beam expander, a beam shaper, and a correcting plate.

The optical element is not limited to one which is composed of a single member, but can include a lens group composed of a plurality of lenses structured in the axial direction.

"Objective lens", in the narrow sense, means a lens having a focusing function, arranged nearest to the optical information recording medium, and facing that optical information recording medium, under the condition that the optical information recording medium is located in the optical pick-up apparatus, while, in the broad sense, the objective lens also means that lens which is able to move by an actuator at least in the axial direction of that lens.

Accordingly, in the present specification, an image side numerical aperture of the objective lens means a numerical aperture of the lens surface of the objective lens arranged at a position nearest to the optical information recording medium.

Further in the present specification, "necessary (and predetermined) numerical aperture" means the numerical aperture decided in the standard of the optical information recording medium, or it means the numerical aperture of the objective lens, having a diffraction marginal performance with which the diameter of beam spot can be obtained, being necessary for recording or regenerating information, in accordance with the wavelength of the light source used for the optical information recording medium.

Still further, "numerical aperture" means the numerical aperture which is defined as a result where the light flux for forming the beam spot on a best image point, is controlled by parts or members having a light stopping capability, such as a diaphragm or a filter, being provided in the optical pick-up apparatus, or is also controlled by a diffractive pattern arranged on the optical element, such as the objective lens.

"Working standard wavelength" means the wavelength of the light flux emitted from the light source used at a working standard temperature. On a contrary, "working wavelength" means the wavelength of the light flux emitted from the light source at the working temperature.

"Working standard temperature" means the ambient temperature (10 40.degree. C.), in which the temperature of the environment changes, when the optical element and the optical pick-up apparatus are used.

"Recording of information" means to record information onto the information recording surface of the information recording medium, while "reproducing of information" means to regenerate information which is recorded on the information recording surface of the optical information recording medium.

Further, the optical element of the present invention is one which performs only recording of information, or performs only reproducing of information, or one which performs both recording of information and reproducing of information. Specifically, the reproducing includes simply reading out of information.

"Optical surface (diffractive surface) having the diffractive structure", means the surface of the optical element such as the lens, on which a relief is provided to diffract the incident light flux, and further, when there are the area on which the diffraction occurs and the area on which the diffraction does not occur, on one optical surface, it means the area on which the diffraction occurs.

The relief shapes are such that the ring-shaped zones are nearly concentrically formed on the optical axis on the surface of the optical element, and its sectional view, cut by a plane including the optical axis, shows steps or a saw-tooth shape of each ring-shaped zone.

Generally, an infinite order of diffracted light fluxes, such as zero-ordered diffracted light, .+-. first ordered diffracted light, .+-. second ordered diffracted light, and so forth, are generated by the optical surface having the diffractive structure thereon. Concerning the diffractive surface with a relief whose meridian section is saw-toothed, it is possible to increase the diffractive efficiency of the specified ordered diffracted light to be greater than that of other ordered diffracted light, and further, it is possible to specifically produce the shape of the relief so that the diffractive efficiency of one of the specified ordered diffracted light rays (for example, + first ordered diffracted light) is increased to 100%.

Further, "diffraction efficiency" in the present specification means the ratio of the amount of diffracted light generated by the diffractive structure, whereby the total sum of the diffraction efficiencies of the total ordered diffracted light rays is equal to 1.

Still further, in the present invention, "order k of the diffracted light having the maximum diffraction efficiency" means the diffraction order which gives the greatest efficiency, compared with other orders, when light rays at working standard wavelengths of 380 450 nm enter the optical element.

In the structure described in Item 13, in the case that the recording and/or reproducing of information is performed onto and/or from the information recording surface of the optical information recording medium having the protective substrate of about 0.6 mm in thickness, through condensing the light flux of working standard wavelengths of 380 450 nm,

when the order of the diffracted light flux having the maximum diffraction efficiency is shown by k (a positive integer) among the diffracted light fluxes produced by the diffractive structure including L pieces of the ring-shaped zones around the optical axis,

and the focal distance is shown by f mm, it is possible to obtain an optical element which can compensate for axial chromatic aberration and spherical aberration caused by changes of temperature, by holding the following formula,

Formula: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300

Accordingly, when light flux having the working standard wavelengths of 380 450 nm, is used for the optical information recording medium having the protective substrate of about 0.6 mm in thickness, and even when the optical element is formed of plastic material whose refraction index and shape are easily changed by a change of temperature, it is possible to control the generation of thermal characteristic aberration, and it is further possible to control the generation of axial chromatic aberration, which occurs when the wavelength of the laser beam increases due to mode hop.

Further, it is possible to obtain the optical element which can increase the utilization efficiency of the laser beam, by focusing k-ordered diffracted light flux, which generates the maximum diffraction efficiency. Item 14.

The optical element described in Item 13, wherein the following formula applies.

Formula: 140.ltoreq.Lk (f.sup.1/2).ltoreq.220 Item 15.

The optical element described in Item 13 or 14, wherein the numerical aperture is equal to or greater than 0.60, and is equal to or smaller than 0.70.

Item 16.

The optical element described in any one of Items 13 15, wherein the numerical aperture is equal to 0.65.

Item 17.

The optical element described in any one of Items 13 16, wherein the optical element is formed of plastic.

By the structure described in Item 17, it is possible to obtain effects similar to those of Items 13 16, and it is also possible to reduce the raw material cost, still further it is possible to eliminate production cost of the optical element, because the optical elements having the diffractive ring-shaped zones can be produced on a large scale by inexpensive injection molding.

Item 18.

An optical pick-up apparatus which performs recording and/or reproducing of information on optical information recording medium, the protective substrate of which is approximately 0.6 mm in thickness, and further which converges light flux of standard wavelengths of 380 450 nm onto the information recording surface of optical information recording medium, wherein the optical element provides a diffractive structure including L pieces of ring-shaped zones around the optical axis, on at least a single optical surface, and wherein when reproducing from and/or recording of information onto the optical information recording medium is performed, with k (a positive integer) as the order of the diffracted light rays having the maximum diffraction efficiency, among the diffracted light rays generated by the diffractive structure, and with f mm as the focal length, the following formula holds.

Formula: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300

By the structure described in Item 18, when the recording and/or reproducing of information is performed onto and/or from the information recording surface of the optical information recording medium having protective substrate of about 0.6 mm in thickness, by condensing the light flux of working standard wavelengths of 380 450 nm, among the diffracted light fluxes produced by the diffractive structure including L pieces of the ring-shaped zones around the optical axis of the optical element, the order of the diffracted light flux having the maximum diffraction efficiency is shown by k (a positive integer), and the focal distance is f mm, the following formula holds:

Formula: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300 it is possible to obtain an optical pick-up apparatus which can compensate for axial chromatic aberration and spherical aberration caused by changes of temperature.

Further, it is possible to obtain the optical pick-up apparatus which can increase the utilization efficiency of laser beams, by focusing k-ordered diffracted light flux, which generates the maximum diffraction efficiency.

Item 19.

The optical pick-up apparatus described in Item 18, wherein the following formula holds:

Formula: 90.ltoreq.Lk (f.sup.1/2).ltoreq.300

An optical pickup device and an optical element of the invention will be explained as follows, referring to the drawings.

FIG. 1 is a schematic diagram of the overall structure of optical pickup device 1.

The optical pickup device 1 is one for conducting recording and reproducing of information by using HD-DVD 8 as a medium under the conditions of standard temperature for use T.sub.0=25.degree. C. and standard wavelength for use .lamda..sub.0=405 nm. To be more precise, the optical pickup device 1 is one that makes a light flux with standard wavelength for use .lamda..sub.0 emitted from laser light source L1 to pass through collimator lens and objective lens 11 to be converged on information recording surface 8a of HD-DVD 8 (optical information recording medium) on optical axis 6 to form light-converging spot 7a, and takes reflected light from the information recording surface 8a of HD-DVD 8 with beam splitter 13 to form a beam spot again on a light-receiving surface of detector L2. This optical pickup device 1 is arranged so that the objective lens 11 is moved by actuator 14 in the optical axis direction so that light-converging spot 7a may be formed on the information recording surface 8a of HD-DVD 8 under the conditions of each temperature for use T (-15.ltoreq.T.ltoreq.65.degree. C.) and each wavelength for use .lamda. (395.ltoreq..lamda..ltoreq.415). Incidentally, image-side numerical aperture NA and a focal length of the objective lens 11 where a light flux having standard wavelength for use .lamda..sub.0 enters at standard temperature for use T.sub.0 are designed respectively to be 0.65 and 3.0 mm.

FIG. 2 is a side view and FIG. 3 is a front view both showing an outline of objective lens 11. The objective lens 11 is made of plastic, and as shown in FIG. 2, the objective lens 11 is composed of lens main body 111 and diffractive structure portion 112 that is provided on an optical functional surface (hereinafter referred to as a base aspheric surface) of the lens main body 111 on an object side. Incidentally, the base aspheric surface is a surface shown with broken lines in FIG. 2.

The diffractive structure portion 112 is formed stepwise so that a thickness in the optical axis direction of each section may increase as the section becomes more distant from the optical axis 6, and it is divided into plural ring-shaped zones 101-n (n<101) which are in a form of concentric circles having their centers on the optical axis 6.

A form and a refractive index of each of a refracting interface of objective lens 11, namely, the surface of the diffractive structure portion 112 (hereinafter referred to as First surface 11A) and optical functional surface (hereinafter referred to as Second surface 11B) of lens main body 111 on the image side, are established as follows.

Table 1 shows lens data of the objective lens 11.

TABLE-US-00001 TABLE 1 Lens data Surface No. R d n Object point .infin. 1 Described 1.500 1.5246 (Aspheric surface, below diffractive surface) 2 Described 1.173 (Aspheric surface) below 3 .infin. 0.60 1.6187 (Cover glass) 4 .infin. f = 2.4 mm NA0.65

In Table 1, Surface Nos. 1 and 2 are respectively First surface 11A and Second surface 11B both of objective lens 11. Surface Nos. 3 and 4 are respectively the surface of HD-DVD 8 and an information recording surface.

A base aspheric surface of the First surface 11A and the Second surface 11B both of the objective lens 11 are formed to be aspheric surfaces which are symmetrical about the optical axis 6. These First surface 11A and the Second surface 11B are prescribed by the expression of an aspheric surface form wherein a coefficient shown in Table 2 is substituted for the following expression.

.kappa..times..times..times..times..times..times..times..times..times..tim- es. ##EQU00001##

In the above expression, A.sub.2i represents an aspheric surface coefficient and h represents a height (mm) from the optical axis.

TABLE-US-00002 TABLE 2 First surface Aspheric surface coefficient .kappa. -0.83899 R 1.5518 A.sub.0 0.0 A.sub.2 0.0 A.sub.4 0.99748 .times. 10.sup.-2 A.sub.6 -0.67103 .times. 10.sup.-4 A.sub.8 0.14401 .times. 10.sup.-2 A.sub.10 -0.71063 .times. 10.sup.-3 A.sub.12 0.27069 .times. 10.sup.-3 A.sub.14 -0.65903 .times. 10.sup.-4

TABLE-US-00003 TABLE 3 Second surface Aspheric surface coefficient .kappa. -50.0000 R -6.2256 A.sub.0 0.0 A.sub.2 0.0 A.sub.4 0.93157 .times. 10.sup.-2 A.sub.6 0.48983 .times. 10.sup.-2 A.sub.8 -0.60555 .times. 10.sup.-2 A.sub.10 0.19105 .times. 10.sup.-2 A.sub.12 -0.25287 .times. 10.sup.-3 A.sub.14 0.58014 .times. 10.sup.-5

In this case, X represents a length (advancing direction of light is assumed to be positive) in the optical axis direction, h represents a height from the optical axis, R represents a paraxial radius of curvature, .kappa. represents constant of the cone and A represents an aspheric surface coefficient.

The number of ring-shaped zones of the diffractive structure portion 112 is determined to satisfy 140.ltoreq.Lm (f.sup.1/2).ltoreq.220. Here, L represents the number of ring-shaped zones. The symbol m represents the order of diffracted light having the greatest diffraction efficiency among diffracted light generated by the diffractive structure portion 112 when conducting recording of information and/or reproducing of recorded information for the optical information recording medium 8. Further, f represents a focal length (mm).

A pitch of ring-shaped zones 101-n of the diffractive structure portion 112 is determined by the optical path difference function wherein a coefficient shown in Table 4 is substituted for the following expression.

.PHI..function..times..times..times..times..times..times..times..times..ti- mes. ##EQU00002##

Here, B.sub.2i represents a coefficient of the optical path difference function.

TABLE-US-00004 TABLE 4 Coefficient of optical path difference function Standard wavelength 405 nm Order of diffraction 3 B.sub.0 0.0 B.sub.2 -0.37934 .times. 10.sup.-2 B.sub.4 0.52430 .times. 10.sup.-3 B.sub.6 -0.22084 .times. 10.sup.-3 B.sub.8 -0.71200 .times. 10.sup.-4 B.sub.10 0.17193 .times. 10.sup.-4

In the aforesaid expression, B.sub.2 represents a coefficient of the optical path difference function, and h represents a height (mm) from the optical axis.

Further, a difference in level between two adjoining ring-shaped zones, namely, a displacement amount in the direction of optical axis 6 between adjoining ring-shaped zones is established so that an optical path difference in an amount equivalent to a multiple of an integer of standard wavelength for use .lamda..sub.0 may be generated between a beam passing through a certain ring-shaped zone and a beam passing through its adjoining ring-shaped zone, and shifting of wavefront may not be generated. To be more concrete, the difference in level between two adjoining ring-shaped zones, namely, the displacement amount in the direction of the optical axis 6 between adjoining ring-shaped zones is determined so that a blazed wavelength of the diffractive structure portion 112 may agree with a standard wavelength for use. Incidentally, a blazed wavelength is a wavelength for which the diffraction efficiency of diffracted light generated by the diffractive structure portion 112 is the greatest.

With respect to paraxial power .phi.D of the diffractive structure portion 112, .phi.D/.phi.D.sub.0=0.54 holds. Here, .phi.D.sub.0 is a paraxial power of the diffractive structure portion 112 in the case where paraxial chromatic aberration is corrected completely when power of the diffractive structure portion 112 is changed without changing power of the lens main body 111.

Vertical spherical aberrations under the conditions of standard temperature for use T.sub.0 and standard wavelength for use .lamda..sub.0 (405 nm) for the objective lens 11 designed in the aforesaid way are shown in FIG. 4.

In FIG. 4, the vertical axis represents a numerical aperture of objective lens 11, and the horizontal axis represents an amount of tertiary order vertical spherical aberration (mm) of the objective lens 11. In FIG. 4, an amount of tertiary order vertical spherical aberration in the case when a light flux having a wavelength of 400 nm enters and that in the case when a light flux having a wavelength of 410 nm enters are also illustrated.

As is clear from this figure, the tertiary order vertical spherical aberration is made to be smaller than 0.07 .lamda..sub.0rms in the objective lens 11 so that a light flux may be converged under the condition of substantially no aberration.

Further, vertical spherical aberration of the objective lens 11 for wavelength 405 nm in the case where temperature for use T is standard temperature for use T.sub.0+40.degree. C. is shown in FIG. 5.

In FIG. 5, the vertical axis represents a numerical aperture of objective le