Optical recording media objective lens and optical pickup device using it

An optical recording media objective lens is disclosed for converging either of two working lights of a first wavelength or a second wavelength. The working light of the first wavelength is converged at a first numerical aperture onto the first optical recording medium and the working light of the second wavelength is converged at a second numerical aperture onto the second optical recording medium. An aperture adjusting zonal part is included on at least one of the objective lens surfaces for apparently eliminating light at the periphery of a light flux having a wavelength λ1 while maintaining light at the periphery of a light flux having a wavelength λ2, where λ1 is one of the first and second wavelengths and λ2 is the other wavelength. The aperture adjusting zonal part is formed so as to satisfy two conditions.

BACKGROUND OF THE INVENTION

Recently, a variety of optical recording media have been developed and an optical pick-up device that may be shared by multiple types of optical recording media in order to record and reproduce signals have been manufactured. For example, it is known in the prior art to use a single optical pick-up device with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) in order to record and reproduce signals. For these two optical recording media, the DVD uses visible light having a wavelength of approximately 657 nm for improved recording densities while the CD is required to use infrared light having a wavelength of approximately 790 nm because some recording media are insensitive to visible light. The optical pick-up device shared by these two recording media uses illumination light of two different peak wavelengths.

The two optical recording media described above require different numerical apertures due to their different features. For example, the DVD is standardized to use a numerical aperture of 0.6 and the CD is standardized to use a numerical aperture in the range of 0.45-0.52. In prior art devices different numerical apertures are used depending on the optical recording media, and all aperture diaphragm, such as a liquid crystal shutter or a wavelength selective filter, may be used to achieve the different numerical apertures. Alternatively, multiple diaphragms may be interposed to achieve the different numerical apertures.

However, the prior art techniques as described above increase the size of the device, as well as increase its complexity and cost.

The inventors of the present application has previously disclosed in Japanese Patent Application 2002-156854 an objective lens that has a zonal part on one of lens surface at the outermost periphery thereof and that has a certain depth (or height) so as to apparently eliminate the light of one of tile wavelengths at the flux periphery and while maintaining the light of the outer wavelength. This objective lens eliminates the need for diaphragms, as provided in the prior art, and results in a downsized optical pickup device that can be produced at a reduced cost.

The objective lens as described in Japanese Patent Application 2002-156854 has a circumferential stepped part at the boundary between the outermost peripheral area and an area inside thereof on one surface (with a depth, for example, equal to (2n+1)λ/2) for one of the wavelengths λ, with n being an integer) so as to form a zonal part on one of the surfaces of the objective lens at the outermost peripheral area and thereby substantially reduce the numerical aperture at the periphery for one of the wavelengths. However, this structure limits the position of the stepped part to a point corresponding to a difference in numerical apertures between two optical recording media (i.e., to at a certain distance from the optical axis). In other words, in order to apparently eliminate the intensity of light at the periphery due to interference effects, the stepped part should be provided nearly at the center of the region that contributes light to cause the destructive interference.

The structure above does not give freedom of design with regard to the position at which the stepped part is formed. Therefore, it is difficult to design a lens having improved optical performance. Consequently, a beam profile corresponding to a required numerical aperture may in some cases not be obtainable. Prior art optical recording media objective lenses having three zonal parts are described for example in Japanese Laid-Open Patent Applications H09-145994, H09-145995, and H09-197108. However, the zonal parts are not for producing an interference effect and the two outer zonal parts converge light fluxes having different wavelengths from each other onto different predetermined points. Thus, the basic technical concepts differ from the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an optical recording media objective lens used with two different optical recording media that require different numerical apertures and working wavelengths, and enables efficient converging of the working lights on the corresponding optical recording media so as to record and/or reproduce information, and an optical pick-up device rising it. More precisely, the present invention relates to an optical recording media objective lens that uses interference effects so as to obtain the corresponding numerical apertures required for the two different optical recording media and an optical pick-up device using it.

DETAILED DESCRIPTION

The present invention relates to an optical recording media objective lens that, in a compact and inexpensive manner, does not make the structure of an optical pick-up device more complex and that allows for more freedom of optical design, utilizes optical interference effects to obtain two different numerical apertures corresponding to different optical recording media having different specifications, and an optical pick-up device using it.

The optical recording media objective lens of the present invention is for converging working light corresponding to a first numerical aperture and a first wavelength onto a first optical recording medium at a first predetermined point and for converging working light corresponding to a second numerical aperture and a second wavelength onto a second optical recording medium at a second predetermined point for recording or reproducing information.

The invention is characterized by:

(a) the objective lens having an aperture adjusting zonal part on at least one of its lens surfaces for apparently eliminating the light at the periphery of a light flux having a wavelength λ1 and maintaining the light amount at the periphery of a light flux having a wavelength λ2, where λ1 is one of the first and second wavelengths and λ2 is the other wavelength; and(b) the zonal part is formed between two concentric stepped parts that are positioned about the optical axis of the objective lens in a manner such that light that passes through the area corresponding to the zonal part when the zonal part is actually absent versus the light that passes through the zonal part satisfy the following Conditions (1) and (2):
Δ1=(2n+1)·λ1/2+δ1Condition (1)
Δ2=m·λ2+δ2Condition (2)
whereΔ1is the difference in optical path length from the light source to the focus point of the optical recording media objective lens for light having the wavelength λ1that would pass through the area where the zonal part would be if the zonal part were to be removed from the optical recording media objective lens versus the light passing through the zonal part;Δ2is the difference in optical path length from the light source to the focus point of the optical recording media objective lens for light having the wavelength λ2that would pass through the area where the zonal part would be if the zonal part were to be removed from the optical recording media objective lens versus the light passing through the zonal part;m and n are integers,δ1 is a constant within the range |δ1|≦0.25 λ1, andδ2 is a constant within the range |δ2|≦0.25 λ2.

The two stepped parts have outer diameters that lie between the light flux diameters corresponding to the first and second numerical apertures, and it is desirable that an optical diffractive surface be provided on at least one surface.

It is more desirable that the constants δ1and δ2in Conditions (1) and (2) above lie within the following more narrow ranges:
|δ1|≦0.2 λ1
|δ2|≦0.2 λ2

It is also desirable that the following Condition (3) be satisfied:
0.95≦(a+c)/b≦1.05  Condition (3)
wherea is the distance in a direction orthogonal to the optical axis between the boundary of the smaller one of the first and second numerical apertures and the inner one of the two stepped parts,b is the distance in a direction orthogonal to the optical axis between the inner stepped part and tile outer stepped part, andc is the distance in a direction orthogonal to the optical axis between the outer stepped part and the boundary of the larger one of the first and second numerical apertures.

In addition to satisfying Condition (3) above, it is further desirable that the following Condition (4) be satisfied:
0.95≦a/c≦1.05  Condition (4)
where a and c are as defined above.

Also, it is further desired that the following Condition (5) is satisfied:
0.90≦(A+C)/B<1.10  Condition (5)
whereA is the area projected onto a plane that is orthogonal to the optical axis of the region between the boundary of the smaller one of the first and second numerical apertures and the inner one of the two stepped parts,B is the area projected onto a plane that is orthogonal to the optical axis of the region between the inner stepped part and the outer one of the two stepped parts, andC is the area projected onto a plane that is orthogonal to the optical axis of the region between the outer stepped part and the boundary of the larger one of the first and second numerical apertures.

It is further desired that the following Condition (6) be satisfied:
0.90≦A/C≦1.10  Condition (6)
whereA and C are as defined above.

It is also desirable, in the optical recording media objective lens described above, that the optical diffractive surface and the zonal part are provided on one and the same lens surface and that the optical diffractive surface produces a phase difference of an integral multiple of 2π radians at the two stepped parts.

The optical pick-up device of the present invention is characterized by comprising the optical recording media objective lens described above.

As described above, in the optical recording media objective lens of the present invention, the zonal part for substantially reducing the numerical aperture for one of the wavelengths is formed by two concentric stepped parts on a surface of the objective lens and positioned about the optical axis of the objective lens, and the two stepped parts have outer diameters between the diameter of the light flux at said surface corresponding to the first numerical aperture for one of the optical recording media and the diameter of the light flux at said surface corresponding to the second numerical aperture for the other optical recording

With the two stepped parts being formed in the area as discussed above so as to constitute a zonal part, the zonal part is located at a surface of the objective lens between the outer diameters of the incident light fluxes corresponding to the two numerical apertures. On the other hand, the zonal part can be at any position within the area discussed above, since it does not matter where the zonal part is formed within the area effective for the elimination so as to apparently eliminate, due to the interference effect, the light of a light flux in a region having a diameter between the two numerical apertures.

With the structure as described above, more freedom is given to the position at which the zonal part is formed, facilitating the designing of a lens having improved optical performance. Consequently, improved optical performance can be obtained while allowing for the two numerical apertures to be obtained for the two different incident wavelengths of light.

The optical recording media objective lens of the present invention has an inner part and an outer part on either side of the zonal part. Thus, it has a total of three zonal parts.

The invention will first be discussed in general terms with reference toFIG. 1which shows the geometry of the optical recording media objective lens of Embodiment 1 of the invention.FIG. 2shows an optical pick-up device using the optical recording media objective lens of Embodiment 1.

As shown inFIG. 2, a laser beam11that is emitted by a selected one of the semiconductor lasers3and4is reflected by a half mirror6, collimated by a collimator lens7, and converged by an optical recording media objective lens8so as to be incident onto a recording area10of an optical recording medium9. The semiconductor laser3emits an infrared laser beam having a wavelength of approximately 790 nm (λ1) for CDs such as CD-R (recordable optical recording media) (hereafter representatively termed CD-R). The semiconductor laser4is a light source that emits a visible laser beam having a wavelength of approximately 657 nm (λ2) for DVDs. The laser beam11that is emitted by either one of the semiconductor lasers3and4reaches a half mirror6formed within, for example, a beam splitter prism5. A selector switch2is provided between the power source1and semiconductor lasers3and4. The switch2is operated so as to selectively supply power to either one of the semiconductor lasers3and4but not to both simultaneously. Furthermore, a diaphragm12is provided on the light source side of the objective lens8.

With the optical pick-up device of this embodiment, either one of a CD-R or DVD optical recording medium9is available for recording and reproducing of signals. The recording area10of the optical recording medium9has tracks of pits that carry signal information. The reflected light of the laser beam11from the recording area10carries signal information that enters the half mirror6via the objective lens8and collimator lens7. Light that is transmitted through the half mirror6then enters a four-quadrant photodiode13that is used to detect electrical signals in each of four quadrants. An operation means (not-showing) then obtains data signals as well as focusing and tracking en-or signals from the signals detected by the four-quadrant photodiode13.

The half mirror6is positioned in the return optical path from the optical recording medium9so that its surface makes an angle of about 45 degrees with the central rays of the incident light. Therefore, the half mirror6has the same effect as a cylindrical lens and the light beam transmitted through it has astigmatism. The magnitude of the focusing error is determined depending on the return beam, light spot profile on the four-quadrant photodiode13. The collimator lens7call be eliminated depending on the given situation. Also a grating can be positioned in all optical path between the semiconductor lasers3and4and the half mirror6so as to use three light beams for detecting the tracking error.

The objective lens8of this embodiment is characterized by a zonal part14being formed on a first surface16at the periphery which is on a different level from that of an inner part15aand an outer part15bas shown inFIGS. 1A and 2.FIG. 1Dis an enlarged cross-sectional view of the encircled part R shown inFIG. 1A, and shows a partial structure of the first surface16of the objective lens8.FIG. 1Dslows two stepped parts14aand14bthat form the boundaries of the zonal part14on the first surface16of the objective lens8. Here, the level differences are shown in an exaggerated manner in order to clearly illustrate the different levels among the zonal part14, the inner part15aand the outer part15b. The lens surfaces are defined using the following a spherical equation:
Z=[(C·Y2)/{1+(1−K·C2·Y2)1/2}]+ΣAi·Y2i+BEquation (A)
whereZ is the length (in mm) of a line drawn from a point on the a spheric lens surface at a distance Y from the optical axis to the tangential plane of the a spheric surface vertex,C is the curvature (=1/the radius of curvature, R) of the a spheric lens surface on the optical axis,Y is the distance (in mm) from the optical axis,K is the eccentricity,Aiis an a spheric coefficient, with the summation extending over i, andB is a constant.

As described above, the CD-R and DVD use light beams having different numerical apertures for recording/reproducing. The former uses a numerical aperture of approximately 0.45 and the latter uses a numerical aperture of approximately 0.60. The zonal part14ensures a proper numerical aperture for each optical recording medium9. The zonal part14is formed so that, among wavelengths of the laser beam11corresponding to the optical recording medium9, the light at the periphery of a light flux having one of the wavelengths is apparently eliminated due to destructive interference while light at the periphery of the light flux having the other wavelength is maintained. Light waves of wavelength 790 nm (λ1) passing inside and outside the zonal part14and through the zonal part14destructively interfere with each other, thereby apparently eliminating the light intensity at the periphery of the light flux. On the other hand, the zonal part14has the stepped parts14aand14bthat do not cause destructive interference of light having the wavelength 657 nm (λ2) between the light passing inside and outside the zonal part14and the light passing through the zonal parts14, thereby maintaining the light intensity of wavelength 657 nm at the periphery of the light flux.

A certain difference in optical path length between the light passing through the zonal part14and the light passing inside and outside the zonal part14leads to destructive interference for light of wavelength λ1that attenuates the light intensity in the periphery for this wavelength Thus, light having a wavelength λ1is subject to destructive interference due to there being a phase difference of an odd-number multiple of λ1/2, and the light having a wavelength λ2is subject to a phase difference of an integral multiple of λ2wavelengths. However, because the light spot has a Gaussian distribution, it is preferred that there be a phase difference margin of δ1and δ2, respectively, as noted above (i.e., with the values of δ1and δ2being 25% or, more desirably, 20% of the respective wavelength, as noted above). In other words, the stepped parts14aand14bshould produce phase differences that satisfy the above Conditions (1) and (2).

Assuming that the numerical aperture for the light having a wavelength of 790 nm (λ1) is 0.45 and the numerical aperture for the light having a wavelength of 657 nm (λ2) is 0.6, the zonal part14is formed of two concentric stepped parts14aand14babout the optical axis of the objective lens8. The two stepped parts14aand14bhave diameters on the lens surface that lie between the diameters that correspond to light fluxes having numerical apertures of 0.45 and 0.6 for the wavelengths 790 nm (λ1) and 657 nm (λ2), respectively.

As for the light having a wavelength of 790 nm (λ1), the light passing through the zonal part14and the light passing through the inner and outer parts15aand15binterfere with each other due to the phase difference being an odd-numbered multiple of λ1/2, which eliminates the intensity of light at the periphery of a light flux so as to form a light flux having a numerical aperture of 0.45. On the other hand, as for light having a wavelength of 657 nm (λ2), the light passing through the zonal part14and the light passing through the inner and outer parts15aand15bdo not undergo destructive interference, and thus the original numerical aperture of 0.6 for the objective lens8is maintained for the wavelength λ2.

As shown inFIG. 1B, with the zonal part14provided on a surface of the objective lens8, the laser beam11having a wavelength of 657 nm (λ2) emitted from the semiconductor laser4is nearly collimated by the collimator lens7and enters the objective lens8when a DVD9ais placed at a certain position (on a turntable) for recording/reproducing. Then, the incident laser beam11is converged by the objective lens8on the recording area10aof the DVD9awith a numerical aperture of 0.6.

On the other hand, as shown inFIG. 1C, the laser beam11having a wavelength of 790 nm (λ1) that is emitted from the semiconductor laser3enters the objective lens8when a CD-R9bis placed at a certain position (on a turntable) for recording/reproducing. The incident laser beam11is then converged by the objective lens8onto the recording area10bof the CD-R9bwith a numerical aperture of 0.45 because destructive interference caused by the zonal part14eliminates the light flux at the periphery.

It is sufficient for this embodiment that the zonal part14is sized between the numerical apertures 0.45 and 0.6 for light having wavelengths of 790 nm (λ1) and 657 nm (λ2), respectively. Theoretically, it does not matter whether the zonal part14is positioned closer to the numerical aperture 0.45 or closer to the numerical aperture 0.6. However, it is desired in practice that the above Conditions (4) or (6) be satisfied, since more freedom is thereby given to the position of the zonal parts14. This facilitates the designing of a lens having improved optical performance and yields significant practical efficacy.

The zonal part14needs to cause destructive interference between the light passing inside and outside the zonal part14and the light passing through the zonal part14so as to substantially eliminate the light at the periphery of a light flux. Thus, it is desired that the zonal part14satisfy the above Condition (3) and, for improved optical performance, it is further desired that the zonal part satisfy the above Condition (4).

Excellent results were obtained experimentally by satisfying the above Conditions (3) and (4) in designing the zonal parts14. However, one may instead design the zonal part14so as to satisfy the above Condition (5). When designing the zonal part14using Condition (5), improved optical performance can be obtained by ensuring that the above Condition (6) is also satisfied.

When an optical diffraction surface is provided on the objective lens8, it is preferred that the optical diffractive surface and the zonal part14are formed on at least one and the same surface, and that the optical diffractive surface produces a phase difference of an integral multiple of 2π radians at the stepped parts14aand14bso as to not disturb the wavefront at these stepped parts even though the a spherical surface is discontinuous, as described above.

FIG. 40Aschematically illustrates the phase difference on the optical diffractive surface at the stepped part being an integral multiple of 2π radians.FIG. 40Bschematically illustrates the phase difference on the optical diffractive surface at the stepped part not being an integral multiple of 2π radians. InFIGS. 40A and 40B, the solid lines indicate the lens geometry, with the stepped parts having different depths in the direction Z′ that is parallel to the optical axis. For convenience of illustration, the surface profile of the Fresnel surface is spherical.

The optical recording medium9has a protective layer made of a PC (polycarbonate) for both CD-R and DVD recording media. A CD-R, including the protective layer, has a standardized geometric thickness of 1.2 mill. A DVD, including the protective layer, has a standardized geometric thickness of 0.6 mill. Due to a difference in thickness of the protective layer for these two optical recording media, the spherical aberrations that are generated differ in magnitude. To ensure the proper focusing, this requires different converging effects of the objective lens depending on the wavelength of light used. Hence, it is desired in the optical recording media objective lens of the present invention that an optical diffractive surface be provided on at least one of the lens surfaces so as to correct aberrations more efficiently in recording/reproducing on optical recording media. Needless to say, the optical recording media objective lens of the present invention can be formed without an optical diffractive surface. For example, this is possible where light from the light source is allowed to enter the objective lens in a slightly diverged state for one of the optical recording media, as in Embodiments 9 to 12 to be described in detail below. However, those embodiments in which an optical diffractive Surface is provided on the light source side of the optical recording media objective lens are the most preferred embodiments of the present invention, as in Embodiments 1 to 8 which will be described in detail below.

As shown inFIG. 1A, an optical diffractive surface consisting of a concentric grating integrally formed with the lens material and having a serrated cross section is provided on the light-source side surface (hereinafter termed the first surface) of the objective lens8so as to ensure excellent recording/reproducing on either CD-R or DVD optical recording media9. InFIG. 1A, as well as for the other figures that show a cross-sectional view, the serrated surface is shown in an exaggerated manner in order to clarify the optical diffractive surface.

The optical diffractive surface exhibits a high diffraction effect for light having the first wavelength and converges this light flux at a first predetermined position in conjunction with the refractive power of the objective lens8. On the other hand, the optical diffractive surface exhibits a low diffraction effect for light having the second wavelength and converges this light flux at a second predetermined position in conjunction with the refractive power of the objective lens. The low diffraction effect includes zero-order diffraction (i.e., 100% zero order diffracted light). In such a case, the light having the second wavelength converges at the second predetermined position due to the refractive power of the objective lens8.

Here, the first wavelength λ1, corresponds to the wavelength of 790 nm for the CD-R and the second wavelength λ2corresponds to the wavelength of 657 nm for the DVD. An optical diffractive surface converges the first order diffracted light of these wavelengths at the corresponding recording areas in conjunction with the refractive power of the objective lens8. The optical diffractive surface adds a difference in optical path length equal to λ·Φ/(2π) to the diffracted light, where λ is the wavelength and Φ is the phase difference function of the optical diffractive surface. The phase difference function Φ is given by the following equation:
Φ=ΣWi·Y2iEquation (B)
whereY is distance from the optical axis; andWiis a phase difference coefficient, with the summation extending over i.

The grating pitch of the optical diffractive surface is determined by the phase difference function. The height of the serrated steps of the optical diffractive surface determines the percentage of light diffracted into each diffractive order. The largest diameter of the optical diffractive surface determines the numerical apertures and beam diameters of the incident laser beam11for the two wavelengths λ1and λ2.

The effect of the optical diffractive surface will now be described with reference toFIGS. 1B and 1C.FIGS. 1B and 1Crelate to the objective lens8of Embodiments 1 to 8, which are described later. As shown inFIG. 1B, with the DVD9abeing placed at a predetermined position (i.e., on a turntable) as the optical recording medium9for recording/reproducing, a laser beam11having a wavelength of 657 nm (λ2) that is emitted from the semiconductor laser4and is substantially collimated by the collimator lens7enters the objective lens8and is converged by it onto the recording area10aof the DVD9a.

On the other hand, as shown inFIG. 1C, with the CD-R9bbeing placed at a predetermined position (i.e., on the turntable) as the optical recording medium for recording/reproducing, a laser beam11having a wavelength of 790 nm (λ1) that is emitted from the semiconductor laser3and is substantially collimated by the collimator lens7enters the objective lens8. The incident laser beam11is converged by the objective lens8on the recording area10bof the CD-R9b.

The objective lens8having the optical diffractive surface on the first surface and a spherical surfaces on both surfaces satisfactorily corrects aberrations for either optical recording media, the CD-R9bor the DVD9athat is used, thus ensuring proper focusing and excellent recording/reproducing.

The laser beam11from the semiconductor laser3or4converges onto the recording area of the corresponding recording medium9a light spot having its aberrations corrected in either case due to the a spherical geometry formed on both Surfaces of the objective lens8and the effect of tile optical diffractive surface. The difference in magnitude of the spherical aberration is mainly due to tile difference in disk thickness for the two different types of optical recording media; however, the difference in wavelength of the incident light is also somewhat responsible. Both sources of aberrations are effectively corrected by the optical diffractive surface.

The optical recording media objective lens8described above has a certain zonal part14for adjusting the aperture on the first surface. This allows the different optical recording media having different recording specifications, namely, CD-R versus DVD, to be illuminated by a light beam having a proper numerical aperture in a compact and inexpensive manner without making the structure of the optical pick-up device more complex, such as when using an aperture diaphragm that is formed, for example, of a liquid crystal shutter and a wavelength selective filter that is interposed, or by using multiple diaphragms that are mechanically switched into the light path. Instead, in the present invention, an optical diffractive surface that is formed on the first surface16is used to optimize the spherical aberration for wavelengths corresponding to the different optical recording media having different specifications and thus ensures proper convergence of the illumination light for excellent recording/reproduction.

The optical recording media objective lens8described above has a certain optical diffractive surface on the first surface16and a certain zonal part14for adjusting the aperture on the first surface16. In other designs, both the optical diffractive surface and zonal part14can be formed on the surface on the optical recording media side, i.e., on the second surface. Alternatively, one of the optical diffractive surface and zonal part14can be formed on the light source side and the other on the optical recording media side. Theoretically, the same effect as obtained for this embodiment can be obtained with an appropriate geometrical design of the optical diffractive surface and zonal parts14.

The optical recording media objective lens8of the present invention will now be described in detail for several embodiments using Tables that define the construction and performance of the objective lens for each embodiment. In Embodiments 1-8, both surfaces of the objective lens are a spheric and an optical diffractive surface is also provided superimposed on the first surface16. In Embodiments 9-12, both surfaces of the objective lens8are a spheric, but there is no superimposed optical diffractive surface. Each objective lens8is suitable for use with two different optical recording media having different specifications, such as CD-R versus DVD recording/reproducing. In the case of using either recording medium, a laser beam halving a proper wavelength for the recording medium used is accurately converged by the objective lens8onto the recording area of the corresponding optical recording medium.

In the top portion of Tables 1, 4, 7, 10, 13, 16, 19, 22, 25, 27 29 and 31 that follow are listed the surface #, in order from the light source side, the surface type or radius of curvature, the on-axis distance (in mm) between surfaces for the two used wavelengths (λ=657 nm for the DVD9a, and 790 nm for a CD-R9b) and the refractive index at the two used wavelengths for each respective embodiment. In the bottom portion of each of these same tables is listed, for each used wavelength, the diaphragm diameter Φ (in mm), the focal length (in mm), the numerical aperture NA and the apparent light source position (as measured from the first surface).

In each of Tables 2, 5, 8, 11, 14, 17, 20, 23, 26, 28, 30, and 32 that follow are listed the Coefficients of the A spherical Equation for the surfaces indicated for the respective Embodiments 1-12. Coefficients not listed are zero. Where so indicated, a particular surface may have different a spherical coefficients in different regions thereon depending on the value of the distance Y from the optical axis. An “E” in the data indicates that the number following the “E” is the exponent to the base 10. For example, “1.0E-2” represents the number 1.0×10−2.

In each of Tables 3, 6, 9, 12, 15, 18, 21, and 24 that follow are listed the coefficients of the phase difference function Wifor the respective Embodiments 1-8. Coefficients not listed are zero. Once again, an “E” in the data indicates that the number following the “E” is the exponent to the base 10. For example, “1.0E-2” represents the number 1.0×10−2.

FIG. 3Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording arc a10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 3Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal parts14.

FIGS. 4A and 4Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 1 of the present invention, withFIG. 4Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 43Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 4A and 4B, the objective lens8of this embodiment yields accurately focused beam spots having, the required numerical apertures.

FIGS. 5A and 5Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 5Abeing for the incident illumination as shown inFIG. 3AandFIG. 5Bbeing for the incident illumination as shown inFIG. 3B. As is apparent fromFIGS. 5A and 5B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 5A) and the CD-R (FIG. 5B).

In this embodiment, the zonal part14(1.653≦Y<1.883) is recessed on the light source side and satisfies the above Conditions (3) and (4). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (3) and (4) of this embodiment are given in Table 33.

FIG. 6Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 6Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal parts14.

FIGS. 7A and 7Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 2 of the present invention, withFIG. 7Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 7Bbeing the light intensity profile of tile spot used for CD-R recording/reproducing. As is apparent fromFIGS. 7A and 7B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 8A and 5Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 8Abeing for the incident illumination as shown inFIG. 6AandFIG. 8Bbeing for the incident illumination as shown inFIG. 6B. As is apparent fromFIGS. 8A and 8B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 8A) and the CD-R (FIG. 8B).

In this embodiment, the zonal part14(1.676≦Y<1.884) protrudes on the light source side and satisfies the Conditions (3) and (4). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (3) and (4) for this embodiment are given in Table 33.

FIG. 9Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal part14.FIG. 9Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal part14.

FIGS. 10A and 10Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 3 of the present invention, withFIG. 10Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 10Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 10A and 10B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 11A and 11Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 11Abeing for the incident illumination as shown inFIG. 9AandFIG. 11Bbeing for the incident illumination as shown inFIG. 9B. As is apparent fromFIGS. 11A and 11B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 11A) and the CD-R (FIG. 11B).

In this embodiment, the zonal part14(1.643≦Y<1.892) is recessed on the light source side and satisfies Conditions (5) and (6). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of Conditions (5) and (6) for this embodiment are given in Table 33B.

FIG. 12Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal part14.FIG. 12Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal part14.

FIGS. 13A and 13Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 4 of the present invention, withFIG. 13Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 13Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 13A and 13B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 14A and 14Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 14Abeing for the incident illumination as shown inFIG. 12AandFIG. 14Bbeing for the incident illumination as shown inFIG. 12B. As is apparent fromFIGS. 14A and 14B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 14A) and the CD-R (FIG. 14B).

In this embodiment, the zonal part14(1.643≦Y<1.892) protrudes on the light source side and satisfies Conditions (5) and (6). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and values of Conditions (5) and (6) for this embodiment are given in Table 33B.

FIG. 15Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 15Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal parts14.

FIGS. 16A and 16Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 5 of the present invention, withFIG. 16Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 16Bbeing the light intensity profile of tile spot used for CD-R recording/reproducing. As is apparent fromFIGS. 16A and 16B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 17A and 17Bshow wavefront aberrations of tile light beams collected by the objective lens8of this embodiment, withFIG. 17Abeing for the incident illumination as shown inFIG. 15AandFIG. 17Bbeing for the incident illumination as shown inFIG. 15B. As is apparent fromFIGS. 17A and 17B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 17A) and the CD-R (FIG. 17B).

In this embodiment, tile zonal part14(1.767≦Y<1.922) is recessed on the light source side and satisfies the Conditions (5) and (6). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of Conditions (5) and (6) for this embodiment are given in Table 33B.

FIG. 18Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal part14.FIG. 18Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal parts14.

FIGS. 19A and 19Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 6 of the present invention, withFIG. 19Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 19Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 19A and 19B, the objective lens8of this embodiment yields accurately Focused beam spots having the required numerical apertures.

FIGS. 20A and 20Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 20Abeing for the incident illumination as shown inFIG. 18AandFIG. 20Bbeing for the incident illumination as shown inFIG. 18B. As is apparent fromFIGS. 20A and 20B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 20A) and the CD-R (FIG. 20B).

In this embodiment, the zonal part14(1.767≦Y<1.922) is recessed on the light source side and satisfies the Conditions (5) and (6). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of Conditions (5) and (6) for this embodiment are given in Table 33B.

FIG. 21Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal part14.FIG. 21Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal parts14.

FIGS. 22A and 22Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 7 of the present invention, withFIG. 22Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 22Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 22A and 22B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 23A and 23Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 23Abeing for the incident illumination as shown inFIG. 21AandFIG. 23Bbeing for the incident illumination as shown inFIG. 21B. As is apparent fromFIGS. 23A and 23B, the objective lens8causes a certain phase difference due to the zonal part14of the lens Surface for both the DVD (FIG. 23A) and the CD-R (FIG. 23B).

In this embodiment, the zonal part14(1.752≦Y<1.912) is recessed on the light source side and satisfies the Conditions (3) and (4). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (3) and (4) of this embodiment are given in Table 33.

FIG. 24Ashows that a substantially collimated laser beam having a wavelength of 657 nm (λ2) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 24Bshows that a substantially collimated laser beam having a wavelength of 790 nm (λ1) enters the objective lens8and the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect from the zonal parts14.

FIGS. 25A and 25Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 8 of the present invention, withFIG. 25Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 25Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 25A and 25B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 26A and 26Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 26Abeing for the incident illumination as shown inFIG. 24AandFIG. 26Bbeing for the incident illumination as shown inFIG. 24B. As is apparent fromFIGS. 26A and 26B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 26A) and the CD-R (FIG. 26B).

In this embodiment, the zonal part14(1.752≦Y<1.912) protrudes on the light source side and satisfies the Conditions (3) and (4). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (3) and (4) of this Embodiment are given in Table 33.

As mentioned previously, starting with this embodiment, the optical diffraction surface having a phase difference function is omitted, but the zonal part14is retained.

FIG. 27Ashows that, when a substantially collimated laser beam having a wavelength of 650 nm (λ2) enters the objective lens8, the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 27Bshows that, when a slightly diverging laser beam having a wavelength of 780 nm (λ1) enters the objective lens 8, the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect of the zonal parts14.

FIGS. 28A and 28Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 9 of the present invention, withFIG. 28Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 28Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 28A and 28B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 29A and 29Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 29Abeing for the incident illumination as shown inFIG. 27AandFIG. 29Bbeing for the incident illumination as shown inFIG. 27B. As is apparent fromFIGS. 29A and 29B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 29A) and the CD-R (FIG. 29B).

In this embodiment, the zonal part14(1.554≦Y<1.743) is recessed on the light source side and satisfies the Conditions (5) and (6). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (5) and (6) of this embodiment are given in Table 33B.

As mentioned previously, the optical diffraction surface having a phase difference function is omitted from this embodiment, but the zonal part14is retained.

FIG. 30Ashows that, when a substantially collimated laser beam having a wavelength of 650 nm (λ2) enters the objective lens8, the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 30Bshows that, when a slightly diverging laser beam having a wavelength of 780 nm (λ1) enters the objective lens8, the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect of the zonal part14.

FIGS. 31A and 31Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 10 of the present invention, withFIG. 31Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 31Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 31A and 31B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 32A and 32Bshow wavefront aberrations of the light beams collected by the objective lens8for this embodiment, withFIG. 32Abeing for the incident illumination as shown inFIG. 30AandFIG. 32Bbeing for the incident illumination as shown inFIG. 30B. As is apparent fromFIGS. 32A and 32B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 32A) and the CD-R (FIG. 32B).

In this embodiment, the zonal part14(1.554≦Y<1.743) protrudes on the light source side and satisfies the Conditions (5) and (6). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (5) and (6) for this Embodiment are given in Table 33B.

As mentioned previously, the optical diffraction surface having a phase difference function is omitted from this embodiment, but the zonal part14is retained.

FIG. 33Ashows that, when a substantially collimated laser beam having a wavelength of 650 nm (λ2) enters the objective lens8, the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal parts14.FIG. 33Bshows that, when a slightly diverging laser beam having a wavelength of 780 nm (λ1) enters the objective lens8, the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect of the zonal parts14.

FIGS. 34A and 34Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 11 of the present invention, withFIG. 34Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 34Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 34A and 34B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 35A and 35Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 35Abeing for the incident illumination as shown inFIG. 33AandFIG. 35Bbeing for the incident illumination as shown inFIG. 33B. As is apparent fromFIGS. 35A and 35B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 35A) and the CD-R (FIG. 35B).

In this Embodiment, the zonal part14(1.546≦Y<1.735) is recessed on the light source side and satisfies the Conditions (3) and (4). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (3) and (4) of this Embodiment are given in Table 33.

As mentioned previously, the optical diffraction surface having a phase difference function is omitted from this embodiment, but the zonal part14is retained.

FIG. 36Ashows that, when a substantially collimated laser beam having a wavelength of 650 nm (λ2) enters the objective lens8, the incident laser beam is converged by the objective lens8onto the recording area10aof the DVD9aunder no substantial influence of the zonal part14.FIG. 36Bshows that, when a slightly diverging laser beam having a wavelength of 780 nm (λ1) enters the objective lens 8, the incident laser beam is converged by the objective lens8onto the recording area10bof the CD-R9bwith a smaller numerical aperture after the light flux at the periphery is eliminated due to an interference effect of the zonal part14.

FIGS. 37A and 37Billustrate intensity profiles of the light after being imaged to a spot by the optical recording media objective lens according to Embodiment 12 of the present invention, withFIG. 37Abeing the light intensity profile of the spot used for DVD recording/reproducing andFIG. 37Bbeing the light intensity profile of the spot used for CD-R recording/reproducing. As is apparent fromFIGS. 37A and 37B, the objective lens8of this embodiment yields accurately focused beam spots having the required numerical apertures.

FIGS. 38A and 38Bshow wavefront aberrations of the light beams collected by the objective lens8of this embodiment, withFIG. 38Abeing for the incident illumination as shown inFIG. 36AandFIG. 38Bbeing for the incident illumination as shown inFIG. 36B. As is apparent fromFIGS. 38A and 38B, the objective lens8causes a certain phase difference due to the zonal part14of the lens surface for both the DVD (FIG. 38A) and the CD-R (FIG. 38B).

In this embodiment, the zonal part14(1.546≦Y<1.735) protrudes on the light source side and satisfies the Conditions (3) and (4). The aperture diameter of the smaller numerical aperture, the aperture diameter of the larger numerical aperture, the inner stepped part position, the outer stepped part position, and the values of the Conditions (3) and (4) for this embodiment are given in Table 33.

Table 33 below lists the values of the aperture diameter (in mm) of the smaller numerical aperture ΦS, the aperture diameter (in mm) of the larger numerical aperture ΦL, the inner stepped part position Y1(in mm) as measured from the optical axis, the outer stepped part position YO(in mm) as measured from the optical axis, and the various values a, b, c, (a+c)/b, and a/c listed in Conditions (3) and (4) for Embodiments 1, 2, 7, 8, 11 and 12.

Table 34 below lists the values of the aperture diameter (in mm) of the smaller numerical aperture ΦS, the aperture diameter (in mm) of the larger numerical aperture ΦL, the inner stepped part position Y1(in mm) as measured from the optical axis, the outer stepped part position YO(in mm) as measured from the optical axis, and the various values of A, B, C, (A+C)/B, and A/C listed in Conditions (5) and (6) for Embodiments 3, 4, 5, 6, 9 and 10.

For comparison with the beam profiles in the embodiments described above, the numerical apertures and beam profiles for various wavelengths of a conventional optical recording media objective lens are shown inFIGS. 39A-39G.

The zonal part can be positioned at any point within the range described above. In order to apparently eliminate the light amount of a light flux having the diameter between the two numerical apertures due to an interference effect, the zonal part can be positioned at a convenient point for optical design within the area effective for the elimination.

With the structure above, more freedom of design is given to the zonal part position, facilitating the designing of a lens having improved optical performance. The optical recording media objective lens of the present invention and the optical pick-up device using it allow for the corresponding numerical apertures to two optical recording media having different specifications in a compact and inexpensive manner with excellent optical performance and with more freedom of design without making the structure of an optical pick-up device more complex.

In the structure described above, a certain optical diffractive surface is formed on at least one of the lens surfaces, desirably on the light source side surface so that all light fluxes are collimated and excellently converged for multiple optical recording media for recording or reproducing.

The invention being thus described, it will be obvious that the same may be varied in many ways. For example, the optical recording media objective lens of the present invention is not restricted to the embodiments described above and various modifications can be made thereto. For example, the optical pick-up device of the present invention is not restricted to those that use a DVD or CD-R as the optical recording media for recording/reproducing. Instead, the present invention is applicable to an optical pick-up device shared by two different optical recording media having different working wavelength ranges and numerical apertures for recording/reproducing. In addition, the present invention may be applicable to two optical recording media that have different specifications, but the same disk thickness. The optical diffractive surface and zonal part of the objective lens8are configured based on the required specifications for each optical recording media.

The objective lens8can be made of plastic materials, which reduce the weight and cost of the lens. The objective lens of the embodiments described above is a spherical on both surfaces in order to improve correction of aberrations. However, instead of using a spherical surfaces, a spherical lens can be used. Alternatively, only one of the surfaces may be a spherical.

Also, the zonal part14of the objective lens8can protrude towards the light source. Although the optical pick-up device of the embodiments described above has two light sources that emit different wavelengths, and a selected light source is used depending on the optical recording medium used, one light source that selectively emits light of two different wavelengths, depending on the optical recording medium used, can be provided.

In the embodiments described above, the objective lens8has the optical diffractive surface on the first surface16. Therefore, the light from the light source is nearly collimated before it enters the objective lens8for both optical recording media. As described above, it is unnecessary to provide the optical diffractive surface on the objective lens8where the objective lens8is allowed to receive nearly collimated light for one of the optical recording media (for example, the DVD) and slightly diverged light from the light source for the other optical recording medium (for example the CD-R). Nevertheless, the optical diffractive surface can be formed in such a case. Such variations are not to be regarded as a departure from the spirit and scope of the invention. Rather, the scope of the invention shall be defined as set forth in the following claims and their legal equivalents. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.