KR100393059B1 - Objective lens apparatus for capable of compensating chromatic aberration and optical pickup adopting it - Google Patents

Abstract

Disclosed is an objective lens device comprising three lenses including a lens having negative power formed of a material having an Abbe number of 45 or less on a d-line, wherein at least one surface of the three lenses is aspheric. The objective lens device having such a configuration can correct chromatic aberration for blue light while achieving a high aperture of 0.70 or more, and therefore, optical pickup using blue light employing such an objective lens device can perform good recording and / or reproduction. Can be.

Description

Objective lens apparatus for capable of compensating chromatic aberration and optical pickup adopting it

The present invention relates to an objective lens device having a high number of apertures and an optical pickup employing the same, and more particularly, to a wavelength change and / or wavelength line width of light emitted from a light source having a high number of apertures to realize high-density light focusing. The present invention relates to an objective lens device capable of correcting chromatic aberration according to an increase of and an optical pickup employing the same.

In the optical recording / reproducing apparatus, the recording capacity is determined by the size of the optical spot formed on the optical disk by the objective lens of the optical pickup apparatus. The size S of the light spot is proportional to the wavelength [lambda] and inversely proportional to the numerical aperture NA of the objective lens.

Therefore, the next-generation DVD so-called HD-DVD optical pickup device (hereinafter, referred to as a high-density optical pickup device) currently being developed further reduces the size of the optical spot formed on the optical disk, thereby obtaining information obtained from a conventional CD or DVD-based optical disk. A light source emitting blue light and an objective lens having a numerical aperture of 0.6 or more will be employed to obtain a high information recording density compared to the recording density.

Optical materials such as glass and plastic used as the material of the objective lens show very sharp refractive index changes in the wavelength band shorter than 650 nm, as shown in Table 1.

Table 1 shows the change in refractive index according to the wavelength of M-BaCD5N of Hoya used as a glass material for objective lens molding.

Wavelength fluctuation Refractive Index Variation of Hoya's M-BaCD5N Glass 650 nm → 651 nm 0.000038 405 nm → 406 nm 0.000154

As can be seen from Table 1, the optical material has a refractive index that is about 4 times larger in the short blue wavelength band, for example, in the 405 nm wavelength band, compared to the 650 nm wavelength used in the optical pickup device for DVD, for a small wavelength change of about 1 nm. Indicates a change.

Such rapid refractive index change of the optical material with respect to blue light is a major cause of performance deterioration due to defocus in a recordable high-density optical recording / reproducing apparatus in which recording and reproduction using a blue wavelength light source are repeated.

That is, in the recordable optical recording / reproducing apparatus, different recording optical powers and reproduction optical powers are used. The wavelength fluctuations due to the optical output power fluctuations during recording / reproducing are about 0.5 to 1 nm, for example, for the blue light source. Usually, when the output of a light source is raised, the wavelength of the light emitted from the light source becomes long. Therefore, in the case of a high-density optical pickup device employing a blue light source, chromatic aberration due to a change in wavelength is greatly generated when switching recording / reproducing light output in an objective lens designed for a reference wavelength, causing defocus.

For example, as can be seen from Figs. 1 to 3, an objective lens device having a numerical aperture of 0.65 designed for 405 nm exhibits large wavefront aberration and defocus for minute wavelength changes on the order of 1 nm. 1 is a graph showing the intensity of an optical spot formed on an optical disc according to defocus due to a change in optical output power during recording / reproducing. FIGS. 2 and 3 are wavefronts of an objective lens device having a numerical aperture of 0.65 according to a wavelength change, respectively. It is a graph showing aberration and defocus amount.

Defocus due to this wavelength variation can be corrected by adjusting the objective lens apparatus, but it takes relatively long time to follow the wavelength variation by driving the objective lens apparatus with an actuator. Will degrade the quality. Defocus on increasing output for recording causes a lack of recording optical power, and defocus on reducing output for playback increases jitter.

That is, if the output of the light source is increased to record information on the optical disc, the wavelength of the light emitted from the light source is increased to 406 nm, for example, so that the optical spot formed on the optical disc is defocused and properly recorded until the actuator follows it. Can not. When the output of the light source is reduced for reproduction, the wavelength of the light source is shortened to, for example, 405 nm, and in this case, since the actuator follows the longer wavelength, defocus occurs again. This defocus increases jitter in the reproduction signal.

In addition, when the light source is driven at a high frequency (HF) to reduce the feed-back noise of the light source due to the light returning from the optical disk to the light source, the line width of the wavelength of the light source is widened, for example, about 1 nm, and thus chromatic aberration is reproduced. Deteriorates the signal.

Accordingly, the repeatable high density optical pickup apparatus needs to have an optical system structure capable of suppressing or compensating for the occurrence of chromatic aberration even if the wavelength of light emitted from the light source changes due to the change in recording and reproduction output.

Conventionally, an objective lens device having two lenses having a function of correcting chromatic aberration has been proposed through Japanese Patent Application Laid-open No. Hei 10-123410.

Referring to FIG. 4, the conventional objective lens device is composed of first and second lenses (1) and (4) having a low dispersion glass having an Abbe number of 40 or more and formed so that at least one surface thereof is aspheric. Chromatic aberration correction and high aperture number can be realized for light having a thickness of nm. Chromatic aberration is corrected by the first lens 1 provided between the disk 6 and the second lens 4 that focuses light, and has a numerical aperture of 0.7 or more. In FIG. 4, reference numeral 2 denotes an aperture limiting a light incident area.

By the way, the conventional objective lens apparatus as described above has a two-group two-lens configuration and uses low dispersion glass having an Abbe number of 40 or more, so that chromatic aberration correction and high aperture number can be realized for light having a wavelength of 635 nm. For blue light, it is difficult to realize a high aperture number while correcting chromatic aberration.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described point, and an object thereof is to provide an objective lens device capable of correcting chromatic aberration with respect to blue light while achieving a high number of apertures, and an optical pickup employing the same.

1 is a graph showing the intensity of an optical spot formed on a recording medium according to a defocus due to variations in optical output power during recording / reproducing;

2 and 3 are graphs showing wavefront aberration and defocus amount of an objective lens device having a numerical aperture of 0.65 according to a wavelength change, respectively;

4 is a view showing an example of a conventional objective lens device;

5 is a view schematically showing an objective lens device according to an embodiment of the present invention;

6 is a view schematically showing an embodiment of an optical pickup employing an objective lens device according to the present invention;

7 is a view showing the structure of an objective lens device according to the present invention according to the first optical design example as shown in Table 2,

8 is an aberration diagram of the objective lens device according to the present invention designed with the optical data of Table 2,

9 is a view showing the structure of the objective lens device according to the present invention according to the second optical design example as shown in Table 3,

10 is an aberration diagram of the objective lens device according to the present invention designed with the optical data of Table 3,

11 is a view showing the structure of an objective lens device according to the present invention according to a third optical design example as shown in Table 4,

12 is an aberration diagram of the objective lens device according to the present invention designed with the optical data of Table 4,

13 is a view showing the structure of an objective lens device according to the present invention according to the fourth optical design example as shown in Table 5,

14 is an aberration diagram of the objective lens device according to the present invention designed with the optical data of Table 5. FIG.

<Description of the symbols for the main parts of the drawings>

10.Objective device 11, 12, 13 ... First to third lens

30 ... Recording medium 51 ... Light source

73.Photodetector

The objective lens device according to the present invention for achieving the above object is composed of three lenses, including a lens having a negative power formed of a material having an Abbe number of 45 or less in the d-line, at least of the three lenses One side is characterized in that the aspherical surface.

In this case, at least one of the three lenses has a positive power, it is preferable that the lens having a negative power has a two-group three-piece structure double bonded to one lens having a positive power.

The three lenses may include a first lens having positive power, a second lens having negative power, and a third lens having positive power from a side from which light is incident.

On the other hand, when the focal length of the lens having the negative power is fn and the total focal length is f, It is desirable to satisfy.

An optical pickup according to the present invention for achieving the above object is a light source for generating and emitting light; An objective lens device for condensing the light incident from the light source to form an optical spot of a recording medium; Optical path converting means disposed on an optical path between the light source and the objective lens device to convert a traveling path of incident light; And a photodetector for receiving light incident through the objective lens device and the optical path changing means after being reflected from the recording medium, wherein the objective lens device comprises a negative material formed of a material having an Abbe number of 45 or less on the d line. It is composed of three lenses including a lens having power, and at least one surface of the three lenses is aspheric.

Here, the light source emits light having a wavelength of approximately 400 nm to 420 nm, and the objective lens device preferably has a numerical aperture of 0.70 or more.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the chromatic aberration-correctable objective lens apparatus and the optical pickup employing the same.

Referring to FIG. 5, the objective lens device 10 according to the present invention includes first to third lenses 11, 13, and 15 from a light incident side, and the first to third lenses. At least one lens of (11) (13) (15) is formed of a glass or plastic material having an Abbe number of 45 or less, preferably 35 or less, on a line d, and has a negative power, wherein the first to third At least one surface of the lenses 11, 13 and 15, for example, the surface 15a on which the light of the third lens 15 is directed to the incident side is an aspheric surface.

Further, the objective lens device 10 according to the present invention, for example, when the focal length of the second lens 13 having negative power is fn, the total focal length of the objective lens device 10, f, It is formed to satisfy the equation (1).

FIG. 5 shows that the second lens 13 has a negative power, the first and third lenses 11 and 15 have a positive power, respectively, and the first lens 11 has a second power. A double junction structure with the lens 13 shows an example in which the objective lens device 10 according to the present invention is composed of two groups of three lenses. Here, reference numeral 30 denotes, for example, a recording medium of the next generation DVD series.

The objective lens device 10 according to the present invention configured as described above can correct chromatic aberration in the blue wavelength region while realizing a high aperture number of 0.70 or more, as can be seen in the following optical concrete design example.

6 is a view showing an embodiment of an optical pickup device employing the objective lens device 10 according to the present invention.

Referring to the drawings, the optical pickup apparatus according to an embodiment of the present invention, the optical path conversion means for converting the traveling path of the light source 51 and the incident light, and the light incident from the light source 51 to focus the recording medium ( An objective lens device 10 according to the present invention for forming a light spot at 30), and a photodetector 73 for receiving light incident on the optical path converting means after being reflected from the recording medium 30; It is composed.

The light source 51 includes a blue semiconductor laser that emits light having a wavelength of approximately 400 nm to 420 nm, and preferably has a wavelength of approximately 405 nm. The semiconductor laser includes an edge emitting laser and a vertical cavity surface emitting laser.

The optical path changing means is disposed on the optical path between the light source 51 and the objective lens device 10 to convert the traveling path of the incident light. As shown in FIG. 6, the optical path converting means includes a polarization beam splitter 57 for selectively transmitting or reflecting incident light according to polarization, and a quarter wave plate 59 for changing the phase of the incident light. desirable. Here, the optical path changing means may be provided with a beam splitter (not shown) for transmitting and reflecting incident light at a predetermined ratio.

The objective lens device 10 has a configuration as described above with reference to FIG. 5, and preferably 0.7 or more so as to form an optical spot capable of recording / reproducing a high density recording medium 30, such as a next generation DVD series recording medium. It has a numerical aperture of 0.85. In this case, the recording medium 30 is preferably a next-generation DVD recording medium. This recording medium may have a substrate thickness of, for example, 0.1 mm.

The photodetector 73 receives light reflected from the recording medium 30 to detect an information signal and an error signal.

It is preferable that the collimating lens 53 is further provided on the optical path between the light source 51 and the optical path changing means. The collimating lens 53 focuses divergent light emitted from the light source 51 to be parallel light. As shown in FIG. 7, when the collimating lens 53 is disposed on the optical path between the light source 51 and the optical path changing means, a focusing lens 71 is further provided between the optical path converting means and the photodetector 73. do.

On the other hand, when the edge light emitting laser is employed as the light source 51, the beam shaping prism 55 is placed on the optical path between the collimating lens 53 and the optical path changing means so that information can be recorded at a relatively low output. It is preferable to further provide. The beam shaping prism 55 shapes the elliptical beam emitted from the edge emitting laser to be a circular beam. The beam shaping prism 55 may be disposed between the light source 51 and the collimating lens 53. Here, in the case of employing a surface-emitting laser that emits a substantially circular beam as the light source 51, the beam shaping prism 55 is removed in the optical system structure of FIG.

Here, reference numeral 73 is a sensing lens. For example, when detecting a focus error signal by the astigmatism method, the sensing lens 73 is an astigmatism lens that causes astigmatism on incident light.

As described above, the optical pickup apparatus according to the present invention includes the objective lens apparatus 10 capable of realizing a high number of apertures and correcting chromatic aberration with respect to blue light, thereby recording and reproducing a next-generation DVD-based recording medium.

Therefore, the chromatic aberration due to the change in the wavelength of the light emitted from the light source 51 and / or the increase in the wavelength line width due to the driving of the light source 51 in HF according to the change in the light output amount during the reproduction and recording mode conversion is the objective lens. Since it is corrected in the apparatus 10, the optical pickup according to the present invention employing the objective lens apparatus 10 and the blue light source can perform high quality recording and / or reproduction of the next generation DVD series recording media.

6 shows an embodiment of the optical pickup device employing the objective lens device 10 according to the present invention, the optical pickup device according to the present invention is not limited to the optical configuration of FIG.

Hereinafter, optical first to fourth design examples of the objective lens device 10 according to the present invention will be described in order to confirm the effect of correcting chromatic aberration on the blue light of the objective lens device 10 according to the present invention. In the following optical first to fourth design examples, the objective lens device 10 according to the present invention uses the first lens 11 having positive power, the second lens 13 having negative power, and the positive power. A third lens 15 having an aspherical surface, and having a reference wavelength of 405 nm and a substrate thickness of 0.1 mm on the recording medium 51. This is for the case where parallel light having a total focal length of 1.765 mm and an incident pupil diameter of 3.0 mm is incident and has a numerical aperture of 0.70 or more.

Table 2 and FIG. 7 show an optical first design example of the objective lens apparatus 10 according to the present invention, and FIG. 8 is an aberration diagram of the objective lens apparatus 10 according to the present invention having the optical data of Table 2. Table 3 and Fig. 9 are examples of the second optical design of the objective lens apparatus 10 according to the present invention, and Fig. 10 is an aberration diagram of the objective lens apparatus 10 according to the present invention having the optical data of Table 3. In addition, Table 4 and FIG. 11 show an optical third design example of the objective lens apparatus 10 according to the present invention, and FIG. 12 is an aberration diagram of the objective lens apparatus 10 according to the present invention having the optical data of Table 4. .

if Bending Radius [mm] Thickness / spacing [mm] Material (glass) Refractive index Abbe number at d line S11 3.003070 1.2000000 E-BaF8_HOYA 1.646734 47.1 S12 -3.003070 0.500000 E-FD4_HOYA 1.806295 27.5 S13 ∞ 1.707467 S14 (Aspherical) 0.911822 1.250000 M-Lac130_HOYA 1.715566 53.2 K: -0.699640A: 0.917676E-01 B: 0.462801E-01 C: 0.317180E-01 D: 0.114090E + 00E:-. 182547E + 00 F:-. 198125E-09 G:-. 406463E-10 S15 ∞ 0.100000 S16 ∞ 0.100000 'CG' 1.621462 31.0 S17 ∞ 0.000000

if Bending Radius [mm] Thickness / spacing [mm] Material (glass) Refractive index Abbe number at d line S21 2.823244 1.3500000 E-BaF8_HOYA 1.646734 47.1 S22 -2.798572 0.500000 E-FD4_HOYA 1.806295 27.5 S23 148.526158 1.020972 S24 (Aspherical) 0.903599 1.300000 M-BaCD5N_HOYA 1.605183 61.3 K: -0.596430A: 0.807580E-01 B:-. 827319E-02 C: 0.165423E + 00 D:-. 912224E-01 S25 ∞ 0.150000 S26 ∞ 0.100000 'CG' 1.621462 31.0 S27 ∞ 0.000000

if Bending Radius [mm] Thickness / spacing [mm] Material (glass) Refractive index Abbe number at d line S31 2.591625 1.5000000 E-BaF8_HOYA 1.646734 47.1 S32 -2.636949 0.500000 E-FD4_HOYA 1.806295 27.5 S33 13.932416 1.649604 S34 (Aspherical) 0.795351 1.000000 M-Lac130_HOYA 1.715566 53.2 K: -0.350912A: 0.333620E-01 B:-. 767010E-01 C: 0.337696E + 00 D:-. 546842E + 00 S35 ∞ 0.150000 S36 ∞ 0.100000 'CG' 1.621462 31.0 S37 ∞ 0.000000

As shown in Tables 2 to 4, in the optical first to third design examples of the objective lens device 10 according to the present invention, the second lens 13 having negative power is formed of a glass material having an Abbe number of 27.5. The focal lengths of the second lens 13 were designed to be -3.725 mm, -3.402 mm, and -2.713 mm, respectively.

As shown in FIGS. 8, 10, and 12 showing aberration diagrams of the objective lens apparatus 10 having the optical data of Tables 2 to 4, in the objective lens apparatus 10, the light source 51 Aberration hardly occurs even when the outgoing light wavelength changes from the reference wavelength of 405 nm to 406 nm.

Table 5 and FIG. 13 show an optical fourth design example of the objective lens apparatus 10 according to the present invention, and FIG. 14 is an aberration diagram of the objective lens apparatus 10 according to the present invention having the optical data of Table 5.

if Bending Radius [mm] Thickness / spacing [mm] Material (glass) Refractive index Abbe number at d line S41 3.017331 1.2000000 M-BaCD12_HOYA 1.599581 59.5 S42 -2.869224 0.500000 E-FD8_HOYA 1.729488 31.2 S43 ∞ 1.872786 S44 (Aspherical) 0.888542 1.250000 M-Lac130_HOYA 1.715566 53.2 K: -0.654060A: 0.895216E-01 B: 0.422347E-01 C: 0.559695E-01 D: 0.930347E-01E:-. 182547E + 00 F:-. 193469E-09 S45 ∞ 0.100000 S46 ∞ 0.100000 'CG' 1.621462 31.0 S47 ∞ 0.000000

As shown in Table 5, in the fourth optical design example of the objective lens device 10 according to the present invention, the second lens 13 having negative power was formed of a glass material having an Abbe number of 31.2 at d line. The focal length of the second lens 13 is designed to be -3.933 mm.

In the objective lens device 10 according to the present invention having the optical data as shown in Table 5, as shown in FIG. Almost no aberration occurs.

Aspherical formulas for the aspherical surfaces S14, S24, S34, and S44 may be expressed by Equation 2 when Z is a depth from an aspherical vertex.

In Tables 2 to 5 and Equation 2, K is a conical constant of aspherical surfaces S14, S24, S34, and S44 of the third lens 15, and A to J are aspherical coefficients. H is the height from the optical axis and c is the curvature.

As confirmed through the above optical design examples, the objective lens device 10 according to the present invention has a chromatic aberration correction function for blue light.

Therefore, when the objective lens device 10 according to the present invention is employed in an optical pickup device for a next-generation DVD series recording medium using a blue light source, good recording and / or reproduction signals can be obtained.

The objective lens device according to the present invention as described above is composed of three lenses including a lens having a negative power formed of a material having an Abbe number of 45 or less on the d line, and at least one surface of the three lenses is Since it is formed as an aspherical surface, chromatic aberration can be corrected with respect to blue light while achieving a high number of apertures.

Therefore, by using the optical pickup employing such an objective lens device according to the present invention, good recording and / or reproduction is possible.

Claims (14)

An objective lens device capable of correcting chromatic aberration, comprising three lenses including a lens having a negative power formed of a material having an Abbe number of 45 or less on a d-line, and at least one surface of the three lenses being aspheric. The method of claim 1, wherein at least one of the three lenses has positive power, and the negative power lens has a two-group three-piece structure in which a double lens is bonded to one lens having positive power. An objective lens device capable of correcting chromatic aberration. The objective lens device of claim 1, wherein the objective lens device has a numerical aperture of 0.70 or more. The objective lens device of claim 1, wherein the negative power lens is formed of a glass or plastic material having an Abbe number of 45 or less on a d line. The lens of any one of claims 1 to 4, wherein the three lenses have a first lens having positive power, a second lens having negative power, and a positive power from the side from which light is incident. An objective lens device capable of correcting chromatic aberration, characterized in that the third lens. The method of claim 5, wherein the focal length of the second lens having the negative power is fn and the total focal length is f. Chromatic aberration correctable objective lens device characterized in that to satisfy. The method according to any one of claims 1 to 4, wherein the focal length of the lens having the negative power is fn and the total focal length is f. Chromatic aberration correction objective lens device characterized in that the satisfactory. A light source for generating and emitting light; An objective lens device for condensing the light incident from the light source to form an optical spot of a recording medium; Optical path converting means disposed on an optical path between the light source and the objective lens device to convert a traveling path of incident light; And a photodetector for reflecting light from the recording medium and receiving the incident light through the objective lens device and the optical path changing means. The objective lens device, It consists of three lenses, including a lens having a negative power formed of a material having an Abbe number of 45 or less in the d-line, at least one surface of the three lenses is aspherical, characterized in that chromatic aberration correction is possible Optical pickup. The method of claim 8, wherein at least one of the three lenses of the objective lens device has positive power, and the negative power lens has a double junction structure bonded to one lens having positive power. Optical pickup characterized in that it became. The method of claim 8, wherein the light source emits light of approximately 400 nm to 420 nm wavelength, And said objective lens device has a numerical aperture of 0.70 or more. The optical pickup of claim 8, wherein the negative power lens is formed of a glass or plastic material having an Abbe number of 45 or less at the d line. 12. The objective lens device according to any one of claims 8 to 11, wherein the objective lens device comprises a first lens having positive power, a second lens having negative power, and a positive power having a positive power from a side from which light is incident. Optical pickup, characterized in that consisting of three lenses. The objective lens apparatus of claim 12, wherein the objective lens apparatus is expressed as fn and a total focal length is fn of the second lens having the negative power. Optical pickup, characterized in that to satisfy. The said objective lens apparatus is a formula of any one of Claims 8-11, when the focal length of the lens having the negative power is fn, and the total focal length is f. Optical pickup, characterized in that to satisfy.