JP2922851B2 - Multifocal lens, optical head device, and optical information recording / reproducing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens used for an optical head such as a digital video disk, a digital audio disk, and an optical memory disk for a computer, and more particularly to correcting aberrations for a plurality of types of substrates having different thicknesses. To a multifocal objective lens, an optical head device and an optical information recording / reproducing device using the same.

[0002]

2. Description of the Related Art In general, in an optical head device for an optical disk, a single lens using an aspherical surface as an objective lens for condensing a diffraction-limited point image on an information medium surface and recording or reproducing information. Is often used. Hereinafter, a conventional objective lens will be described with reference to the drawings. FIG. 16 shows the relationship between a conventional objective lens and a disk (recording medium). In FIG. 16A, it is assumed that the aspheric objective lens 15 secures the performance within the diffraction limit with respect to the disk 16. Here, when light is focused on the disks 18 having different disk thicknesses by the same objective lens 15, the performance within the diffraction limit cannot be obtained due to the difference in disk thickness. Therefore, as shown in FIG. 16B, it is necessary to use different objective lenses 17 for disks 18 having different substrate thicknesses. That is,
When recording and reproducing a plurality of types of disks having different substrate thicknesses with the same optical head, a plurality of objective lenses must be prepared according to the type of substrate thickness.

On the other hand, there has been proposed a method of forming two focal points with one objective lens using a hologram. (For example, KOMMA et al .: OPTICAL REVIEW Vol.1,
No. 1 (1994) 27-29). FIG. 17 shows the configuration of a bifocal objective lens using a hologram. As shown in FIG. 17A, the hologram 19
The objective lens is designed such that when the next-order diffracted light 21 passes through the objective lens 20, a spot within the diffraction limit is collected. On the other hand, as shown in FIG. 17 (b), the hologram 19
The hologram 19 is designed so that when the next-order diffracted light 22 passes through the objective lens 20, a spot within the diffraction limit is collected. By designing the diffraction efficiency of the hologram so that the light quantity ratio between the 0th-order diffracted light and the 1st-order diffracted light of the hologram becomes 1: 1, two focal points are formed, and each focal point is directed to a disc having a different substrate thickness. Thus, the aberration can be corrected.

[0004]

In general, when an objective lens involves manufacturing errors such as decentering and tilting, coma may also be present on the axis. FIG. 18A shows a state in which coma aberration appears when the 0th-order diffracted light 21 of the hologram 19 is focused on the disk 16. In this case, if the objective lens 20 and the hologram 19 are simultaneously tilted with respect to the optical axis perpendicular to the disk,
As shown in (b), at a certain angle, coma is corrected. Next, when a disk 18 having a different substrate thickness is used, the first-order diffracted light 22 of the hologram 19 is condensed. However, the condition for correcting the coma aberration is different from that for the disk 16, so that FIG. In FIG. 18C, the coma aberration that had been corrected was not corrected, and a different coma aberration occurred.

The present invention has been made to solve the above-mentioned problems of the conventional example, and corrects coma aberration for a plurality of types of disks (recording media or substrates) having different thicknesses. It is an object of the present invention to provide a multifocal lens designed under substantially the same conditions, an optical head device and an optical information recording / reproducing device using the same.

[0006]

In order to achieve the above object, a multifocal lens according to the present invention includes a diffractive means, and focuses the zero-order diffracted light of the diffractive means on an information recording surface of a first substrate. The first-order diffracted light of the diffracting means is focused on the information recording surface of the second substrate having a thickness different from that of the first substrate, and the condition of (Equation 1) is satisfied.

Further, another multifocal lens of the present invention includes a diffractive means, condenses the zero-order diffracted light of the diffractive means on the information recording surface of the first substrate, and converts the first-order diffracted light of the diffractive means to the first substrate. The light is condensed on the information recording surface of the second substrate having a thickness different from that of the first substrate, and the residual coma aberration ZC satisfies the condition (Equation 2).

In each of the above structures, it is preferable that the diffraction means is formed on any one surface of the objective lens.

Preferably, the objective lens is a single lens having at least one aspheric surface.

Alternatively, it is preferable that the diffraction means is formed on a flat substrate. Preferably, the diffraction means is a phase grating. Further, it is preferable that the diffraction means is a concentric phase grating.

In each of the above structures, the plurality of substrates having different thicknesses are of two types, and the two types of substrates can be condensed by using the 0th-order diffraction light and the 1st-order diffraction light of the diffraction means, respectively. preferable.

It is preferable that the substrate having the smaller thickness of the two types of substrates is condensed by zero-order diffracted light.

Alternatively, it is preferable that the thinner one of the two types of substrates is condensed by zero-order diffracted light.

Further, it is preferable that the above expression (3) is satisfied.

In any one of the above structures, it is preferable that diffraction means is integrated on at least one surface of the objective lens, and that the objective lens is formed by any one of glass molding and resin molding.

It is preferable that the flat substrate is formed by any one of glass molding and resin molding.

On the other hand, the optical head device of the present invention comprises a light source,
Light collecting means for condensing a light beam emitted from the light source on an information medium surface, a light beam separating means for separating a light beam modulated by the information medium, and receiving the light modulated by the information medium A light-receiving means is provided, and the light-collecting means is as described in any of the above configurations.

Further, the optical information recording / reproducing apparatus of the present invention comprises:
Using the optical head device, information is recorded on and reproduced from a plurality of types of recording media having different thicknesses.

[0019]

As described above, the multifocal lens according to the present invention includes a diffraction means whose aberration is corrected so that light beams having different diffraction orders are converged on a plurality of substrates having different thicknesses. When the entire lens is tilted to correct the axial coma of the lens, the tilt angle is substantially the same for a plurality of substrates having different thicknesses. That is, in assembling the optical head device, the entire lens is tilted in order to correct the on-axis coma of the lens. By making the inclination angles substantially the same, it is possible to obtain a good focused spot on a plurality of information recording media with one objective lens, and to record information or reproduce information with stable performance. it can.

Further, another multifocal lens of the present invention includes a diffractive means, condenses the 0th-order diffracted light of the diffractive means on the information recording surface of the first substrate, and converts the first-order diffracted light of the diffractive means. The light is condensed on the information recording surface of the second substrate having a thickness different from that of the first substrate, and the condition of (Equation 1) is satisfied. That is, the information recording medium is limited to two types.
The left side of (Equation 1) represents the inclination angle of the lens with respect to the first substrate, and the right side represents the inclination angle of the lens with respect to the second substrate.

Further, still another multifocal lens of the present invention includes a diffractive means, and converts the zero-order diffracted light of the diffractive means into a first-order diffracted light.
And the first-order diffracted light of the diffracting means is condensed on the information recording surface of a second substrate having a thickness different from that of the first substrate. The condition of (Equation 2) is satisfied. That is, conditions are set for a case where the residual coma of the lens can be tolerated to some extent. The above (Equation 3) shows the condition when the chromatic aberration is corrected.

Hereinafter, the multifocal objective lens of the present invention will be specifically described with reference to the drawings. FIG. 1 is an optical path diagram showing a configuration corresponding to the first to fourth embodiments of the multifocal objective lens of the present invention. In FIG. 1, an incident light 1 enters an objective lens 2. The objective lens 2 is a single lens having two aspheric surfaces, and a phase grating 4 is formed on a surface 2a on the incident side. The incident light beam 1 is focused on the information medium surface 3a of the disk 3. Here, when the thickness of the disk 3 is 1.2 mm and 0.6 mm, the first-order diffracted light of the phase grating 4
Use the next diffraction light properly.

FIG. 2 is an optical path diagram showing a configuration corresponding to the fifth embodiment of the multifocal objective lens of the present invention. In FIG. 2, an incident light beam 1 passes through a plane substrate 5 on which a phase grating is formed, then enters an objective lens 6 having an aspheric surface on both sides, and is condensed on the information medium surface of the disk 3. As in the case shown in FIG. 1, when the thickness of the disc 3 is 1.2 mm,
In the above case, the first-order diffracted light and the zero-order diffracted light of the phase grating on the plane substrate 5 are selectively used.

Next, specific numerical examples of the multifocal objective lens of the present invention will be shown. In addition, as common specifications in each of the following embodiments, the central wavelength of the design is 660 nm, the thickness of the first disk is 0.6 mm, the thickness of the second disk is 1.2 mm, and the refractive index of the disk is 1 .57815, first
The numerical aperture (NA) for the second disk was 0.60, and the numerical aperture (NA) for the second disk was 0.43. Further, in each of the following embodiments, the symbols shown in (Table 1) are common. The aspherical shape is given by the following (Equation 4).

[0025]

[Table 1]

[0026]

(Equation 4)

The phase grating was expressed by an ultra-high refractive index method (William C. Sweatt: Describing holographic op.
tical elements as lenses: Journal of Optical Soci
etyof America, Vol. 67, No. 6, June 1977). The aspherical shape representing the phase grating is represented by the same (Equation 4) as a normal aspherical shape, and each symbol is defined as shown in (Table 2). Also, parameters representing each performance of the lens are defined as in (Table 3).

[0028]

[Table 2]

[0029]

[Table 3]

The tilt angle difference DA and the residual coma ZC are defined by the following (Equation 5).

[0031]

(Equation 5)

Further, in Examples 4 and 6, the values shown in the following (Table 4) were also described.

[0033]

[Table 4]

[0034]

Embodiment 1 Specific numerical values of Embodiment 1 are shown in (Table 5).
In the first embodiment, the incident surface side (the first
A phase grating is provided on the surface side). The 0th-order diffracted light of the phase grating is applied to the first disk,
This is a configuration in which the second-order diffracted light is used. The parameters representing the phase grating are shown in Table 6 below. In addition, parameters indicating the performance of the lens are shown in the following (Table 7). As can be seen from Table 7, in a state in which the lens is tilted and adjusted until the coma aberration is completely corrected in the first disk, the coma aberration generated when the light passes through the second disk, that is, the residual coma aberration ZC is small. This is a very small value of 3.35 mλ. FIG. 3 is an aberration diagram for the first disk of Example 1, and FIG. 4 is an aberration diagram for the second disk.
Shown in In each of the aberration diagrams in FIGS. 3 to 12, the dotted line in the aberration diagram in FIG. 3A indicates spherical aberration, and the solid line indicates a sine condition. Also, the dotted line in the aberration diagram in (b) indicates astigmatism in the tangential direction, and the solid line indicates astigmatism in the sagittal direction.

[0035]

[Table 5]

[0036]

[Table 6]

[0037]

[Table 7]

[0038]

Embodiment 2 Specific numerical values of Embodiment 2 are shown in (Table 8).
In the second embodiment, the incident surface side of the double-sided aspheric objective lens (the first
A phase grating is provided on the surface side). The 0th-order diffracted light of the phase grating is applied to the first disk,
This is a configuration that uses second-order diffracted light. Table 9 shows parameters representing the phase grating. Table 10 shows parameters representing the performance of the lens. As can be seen from Table 10,
In a state where the lens is tilted and adjusted until the coma aberration is completely corrected by the first disk, the coma aberration generated when the light passes through the second disk, that is, the residual coma aberration ZC, is
This is a very small value of only 2.82 mλ. In the second embodiment, the off-axis coma aberration of the first disk and the off-axis coma aberration of the second disk are balanced, so that the off-axis performance of the objective lens can be improved for any substrate having any disk thickness. Consideration has been given to avoid extreme deterioration. FIG. 5 shows an aberration diagram for the first disk in Example 2, and FIG. 6 shows an aberration diagram for the second disk.

[0039]

[Table 8]

[0040]

[Table 9]

[0041]

[Table 10]

[0042]

Embodiment 3 Specific numerical values of Embodiment 3 are shown in (Table 11). In the third embodiment, a phase grating is provided on the incident surface side (first surface side) of the double-sided aspheric objective lens. In this configuration, the 0th-order diffracted light of the phase grating is used for the first disk, and the 1st-order diffracted light is used for the second disk. Table 12 shows parameters representing the phase grating. Table 13 shows parameters representing the performance of the lens. As can be seen from Table 13, in a state where the lens is tilted and adjusted until the coma aberration is completely corrected in the first disk, the coma aberration generated when the light passes through the second disk, that is, the residual coma aberration ZC is small. This is a very small value of 14.5 mλ. In the third embodiment, the difference between the focal position on the first disk and the focal position on the second disk is 0.47 m.
By setting it to m, the configuration is less susceptible to stray light such as 0-order diffracted light and diffracted light other than 1st-order diffracted light, for example, 2nd-order diffracted light and -1st-order diffracted light. FIG. 7 shows an aberration diagram for the first disk of Example 3, and FIG. 8 shows an aberration diagram for the second disk.

[0043]

[Table 11]

[0044]

[Table 12]

[0045]

[Table 13]

[0046]

Embodiment 4 Specific numerical values of Embodiment 4 are shown in (Table 14). In the fourth embodiment, a phase grating is provided on the incident surface side (first surface side) of the double-sided aspheric objective lens. In this configuration, the first-order diffracted light of the phase grating is used for the first disc, and the zero-order diffracted light is used for the second disc. Table 15 shows parameters representing the phase grating. Table 16 shows parameters representing the performance of the lens. As can be seen from Table 16, in a state in which the lens is tilted and adjusted until the coma aberration is completely corrected in the first disk, the coma aberration generated when the light passes through the second disk, that is, the residual coma aberration ZC is small. It is a very small value of 5.08 mλ.

In the fourth embodiment, a positive power is given to the diffraction element. Therefore, the chromatic aberration of the lens can be corrected by the combination with the refractive element. In a rewritable optical disk, the output of the semiconductor laser at the time of reproduction and at the time of writing are significantly different, so that wavelength fluctuation occurs.
As in the fourth embodiment, the chromatic aberration of the first disk using the diffracted light is reduced to zero in the vicinity of the used wavelength,
It is possible to realize a lens having no defocus due to a wavelength change of the semiconductor laser. FIG. 9 shows an aberration diagram for the first disk in Example 4, and FIG. 10 shows an aberration diagram for the second disk.

[0048]

[Table 14]

[0049]

[Table 15]

[0050]

[Table 16]

[0051]

Fifth Embodiment Specific numerical values of the fifth embodiment are shown in (Table 17). In the fifth embodiment, as shown in FIG. 2, a phase grating is provided on the second surface side of a planar substrate separate from the objective lens.
The 0th-order diffracted light of the phase grating with respect to the first disk is
Is a configuration in which the first-order diffracted light is used for the disk of FIG.
Table 18 shows parameters representing the phase grating. Regarding the thickness and the refractive index of the flat substrate, since parallel rays are incident, there is no influence on the design, so that they are omitted here. Table 19 shows parameters representing the phase grating. As can be seen from Table 19, the coma which occurs when the first disk is transmitted through the second disk with the lens tilted and adjusted until the coma is completely corrected, that is, the residual coma, is only 5. It is a very small value of 69 mλ. FIG. 11 shows an aberration diagram for the first disk of Example 5, and FIG. 12 shows an aberration diagram for the second disk.

[0052]

[Table 17]

[0053]

[Table 18]

[0054]

[Table 19]

[0055]

Embodiment 6 Specific numerical values of Embodiment 6 are shown in (Table 20). In the sixth embodiment, a phase grating is provided on the incident surface side (first surface side) of the double-sided aspheric objective lens. In this configuration, the first-order diffracted light of the phase grating is used for the first disc, and the zero-order diffracted light is used for the second disc. Table 21 shows parameters representing the phase grating. Table 22 shows parameters representing the lens performance. As can be seen from Table 22, in a state in which the lens is tilted and adjusted until the coma aberration is completely corrected in the first disk, the coma aberration generated when the light passes through the second disk, that is, the residual coma aberration ZC is small. This is a very small value of 7.8 mλ.

In the sixth embodiment, a positive power is given to the diffraction element. Therefore, the chromatic aberration of the lens can be corrected by the combination with the refractive element. In a rewritable optical disk, the output of the semiconductor laser at the time of reproduction and at the time of writing are significantly different, so that wavelength fluctuation occurs.
As in the sixth embodiment, the chromatic aberration of the first disk using the diffracted light is reduced to zero in the vicinity of the used wavelength.
It is possible to realize a lens having no defocus due to a wavelength change of the semiconductor laser. In the sixth embodiment, the lens is designed so that the performance deterioration when the first surface and the second surface of the lens are decentered is suppressed as much as possible. Therefore, the lens has a loose manufacturing tolerance and is easy to manufacture. FIG. 13 shows an aberration diagram for the first disk of Example 6, and FIG. 14 shows an aberration diagram for the second disk.

[0057]

[Table 20]

[0058]

[Table 21]

[0059]

[Table 22]

The objective lens described in each of the above embodiments is preferably formed by glass molding or resin molding. That is, by processing the diffraction grating in the mold, it becomes possible to mass-produce lenses having the same shape and performance in large quantities at low cost. Also, the diffraction grating of the flat substrate shown in the fifth embodiment is preferably formed by glass molding or resin molding.

Next, FIG. 15 shows a configuration diagram of an optical head device and an optical information recording / reproducing device using the multifocal objective lens of the present invention. In FIG. 15, light emitted from a semiconductor laser 7 is reflected by a half mirror 8, the direction of the optical path is changed by a bending mirror 9, and focused on an information medium surface 12 of a disk 11 by an objective lens 10. Here, the objective lens 10 has the configuration shown in FIG. 1 or FIG. 2, and converges on the disk 11 using either the 0th-order diffracted light or the 1st-order diffracted light of the phase grating. The condensed spot is diffracted by the irregularities formed on the information medium surface 12. The laser light reflected and diffracted by the information medium surface 12 passes through the half mirror 8 and is condensed on the photodetector 14 by the detection lens 13. A change in the amount of light modulated on the information medium surface 12 is detected by an electric signal of the photodetector 14 and data is read.

The objective lens 10 may have coma due to a manufacturing error generated during processing. In this case, coma is corrected by tilting the optical axis of the objective lens 10 with respect to the optical axis of the disk 11. Next, when the disk 11 is changed to a disk having a different thickness, diffracted light having a different order from the 0th-order diffracted light or the 1st-order diffracted light of the phase grating, which is different from the diffracted light condensed on the disk 11, is converted to a light beam having a different thickness. Focus on the disc. Here, since the objective lens 10 has the performance shown in Examples 1 to 5, almost no coma aberration occurs even if the substrate thickness changes. Therefore, different tilt adjustments are not required for different substrate thicknesses, and a signal can be recorded and reproduced in an optimum light-condensing state at all times. In addition, it is possible to obtain an optical recording / reproducing apparatus capable of recording / reproducing in good condition by recording / reproducing such optical disk media having different substrate thicknesses using the multifocal objective lens or the optical head device of the present invention. Can be.

In each of the above embodiments, the case where parallel light is made incident on the objective lens for an optical disk has been described. However, the light of the semiconductor laser is directly condensed by one lens, or the parallel light is collimated by a collimating lens. The lens may be a finite-magnification lens that emits divergent light or converged light instead of light. Although the objective lens for an optical disc is a single lens having a double-sided aspherical surface, it may be a single-sided aspherical surface or a spherical group lens, or may be a composite element thereof.

Although the phase grating is provided on the flat substrate in the fifth embodiment, the phase grating may be provided not on a plane but on a spherical surface or an aspherical surface. Further, the position of the hologram may be on the incident surface side or the emission surface side. Further, the order of the diffracted light may be other than the zero-order diffracted light and the first-order diffracted light.
It may be a first-order diffracted light or a second-order diffracted light. Further, three or more different orders of diffracted light may be used to reproduce three or more different substrate thicknesses. When integrally molded with the objective lens, a diffraction grating may be formed on the front surface or the rear surface.

[0065]

As described above, the multifocal objective lens according to the present invention includes diffraction means which is aberration-corrected so that light beams having different diffraction orders are focused on a plurality of substrates having different thicknesses. When tilting the entire objective lens in order to correct the axial coma aberration of the objective lens, the tilt angle is configured to be substantially the same for a plurality of different substrate thicknesses.
Even if optical discs having different substrate thicknesses are reproduced, a good focused spot can be obtained for each disc. In particular, when coma occurs on the axis due to manufacturing errors, etc., when correcting by tilting the entire objective lens, the correction angle is always constant even when using disks with different thicknesses. There is no need to readjust the tilt angle of the objective lens.

By satisfying either of the above (Equation 1) and (Equation 2), the objective lens is composed of an aspherical single lens, and the diffraction grating is used as a phase grating.
The objective lens can be integrally formed on the surface of the aspheric surface, is lightweight, has a long working distance, and has high diffraction efficiency. Further, by satisfying the condition of the above (Equation 3), it is possible to use the chromatic aberration correction function of the diffraction grating, and it is possible to realize an objective lens in which the chromatic aberration is corrected without changing the focal position with respect to wavelength fluctuation. Becomes Further, by manufacturing these lenses by glass molding or resin molding, it becomes possible to mass-produce low-cost and large quantities of the same performance.

Further, according to the optical head device and the optical information recording / reproducing apparatus of the present invention, since the correction amount of the coma aberration is substantially the same for different substrate thicknesses, it is necessary to adjust the tilt for each substrate thickness. In addition, an inexpensive optical head and an optical information recording / reproducing apparatus can be realized, and a good spot having no coma aberration can be focused on any substrate, so that good recording, reproducing, and erasing performance can be obtained.

[Brief description of the drawings]

FIG. 1 is an optical path diagram showing one configuration example of a multifocal objective lens of the present invention.

FIG. 2 is an optical path diagram showing another configuration example of the multifocal objective lens of the present invention.

FIG. 3 is an aberrational diagram with respect to a first disk in the first embodiment of the multifocal objective lens of the present invention.

FIG. 4 is an aberration diagram of the multifocal objective lens of the present invention with respect to the second disk in the first embodiment.

FIG. 5 is an aberration diagram for a first disc in a multifocal objective lens according to a second embodiment of the present invention.

FIG. 6 is an aberration diagram with respect to a second disk in the multifocal objective lens according to the second embodiment of the present invention.

FIG. 7 is an aberration diagram for a first disc in a multifocal objective lens according to a third embodiment of the present invention.

FIG. 8 is an aberrational diagram of the multifocal objective lens according to the third embodiment of the present invention with respect to the second disk.

FIG. 9 is an aberration diagram of the multifocal objective lens according to the fourth embodiment of the present invention with respect to the first disk.

FIG. 10 is an aberrational diagram of the multifocal objective lens of the present invention with respect to the second disk in the fourth embodiment.

FIG. 11 is an aberration diagram of the multifocal objective lens according to the fifth embodiment of the present invention with respect to the first disk.

FIG. 12 is an aberration diagram for a second disc in the multifocal objective lens according to the fifth embodiment of the present invention.

FIG. 13 is an aberration diagram for the first disc in the multifocal objective lens according to the sixth embodiment of the present invention.

FIG. 14 is an aberration diagram with respect to a second disk in the sixth embodiment of the multifocal objective lens of the present invention.

FIG. 15 is a perspective view showing a configuration of an embodiment of an optical head device and an optical information recording / reproducing device of the present invention.

FIG. 16 is an optical path diagram showing a configuration of an optical system of a conventional optical head device.

FIG. 17 is an optical path diagram showing a configuration of a bifocal objective lens using a conventional hologram.

FIG. 18 is an optical path diagram showing a tilt adjustment of an objective lens in a conventional bifocal objective lens using a hologram.

[Explanation of symbols]

 1: incident light 2: objective lens 3: disk 4: phase grating

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Sadao Mizuno 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. (56) References JP-A-7-98431 (JP, A) JP-A-7-98 294707 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02B 13/00 G02B 3/10 G02B 5/32 G02B 13/18 G11B 7/135

Claims (15)

(57) [Claims] 1. A diffraction device, comprising: a diffracting means;
The folded light is focused on the information recording surface of the first substrate, and the diffraction
The first-order diffracted light of the step is converted into a second light having a thickness different from that of the first substrate.
Focus on the information recording surface of the substrate and satisfy the condition of (Equation 1)
Multi-focus lens. [Number 1] _{L 1 / (J 1 + D} 1) ≒ L 2 / (J 2 + D 2) J 1: coma per unit angle of off-axis with respect to the first substrate (mλ) J _2: second substrate Coma per unit angle off-axis per unit angle (mλ) D ₁ : Coma aberration per unit angle generated when the first substrate is tilted (mλ) D ₂ : Unit angle generated when the second substrate is tilted Coma per unit (mλ) L ₁ : on-axis coma of the lens with respect to the first substrate (m
λ) L ₂ : axial coma of the lens with respect to the second substrate (m
λ) A diffracting means for converging the 0th-order diffracted light of the diffracting means on an information recording surface of a first substrate, wherein the first-order diffracted light of the diffracting means has a thickness different from that of the first substrate. The light is condensed on the information recording surface of the second substrate, and the residual coma aberration ZC is
A multifocal lens that satisfies condition 2) . [Number 2] _{ZC = | L 1 (J 2} + D 2) / (J 1 + D 1) -L 2 | <20 J 1: coma per unit angle of off-axis with respect to the first substrate (m [lambda) J ₂ : Off-axis coma aberration per unit angle (mλ) with respect to the second substrate D ₁ : Coma aberration per unit angle (mλ) generated when the first substrate is tilted D ₂ : Second substrate tilted Coma per unit angle (mλ) L ₁ : on-axis coma (m m) of the lens with respect to the first substrate
λ) L ₂ : axial coma of the lens with respect to the second substrate (m
λ) 3. The diffraction means according to claim 1, wherein
The multifocal lens according to claim 1, wherein the multifocal lens is formed on one surface . 4. The objective lens according to claim 1, wherein at least one surface of the objective lens is non-conductive.
The multifocal lens according to claim 3, which is a spherical single lens. 5. A process in which said diffraction means is formed on a flat substrate.
3. The multifocal lens according to claim 1 or 2 . 6. The apparatus according to claim 1, wherein said diffraction means is a phase grating.
6. The multifocal lens according to any one of items 1 to 5 , 7. The multifocal lens according to claim 3 , wherein said diffraction means is a concentric phase grating. 8. The plurality of substrates having different thicknesses are of two types.
And the diffraction means for each of the two types of substrates.
The light is condensed by using the 0th-order diffracted light and the 1st-order diffracted light.
Is a multifocal lens according to 2. 9. A substrate having a larger thickness among the two types of substrates.
The multifocal lens according to claim 8, wherein the plate is condensed by the 0th-order diffracted light . Wherein said two multi-focal lens according to claim 8, wherein condensing the thinner substrate thicknesses 0-order diffracted light of the substrate. 11. The multifocal lens according to claim 1, wherein the following expression (3) is satisfied . Equation 3] ^{| fa 2 {1 / (fd} · Vd) + 1 / (fs · Vs)} | <0.0025 Vs = (nλ-1) / (nλ (-) - nλ (+)) Vd = λ / (Λ (−) − λ (+)) λ: design wavelength λ (+): wavelength longer than the design wavelength by 10 nm λ (−): wavelength shorter than the design wavelength by 10 nm nλ: refractive index nλ (+) of the lens at the design wavelength ): At 10 nm longer than the design wavelength
Lens refractive index nλ (-): laser at a wavelength 10 nm shorter than the design wavelength.
Lens refractive index fs: focal length fd of the lens defined by the zero-order diffracted light fd: focal length fa of the lens only by the diffracting means fa: focal length of the lens defined by the first-order diffracted light 12. At least one side twice of the objective lens
Integrate folding means and select from glass molding and resin molding
The multifocal lens according to any one of claims 1 to 11 , wherein the multifocal lens is formed by any one of the above methods . 13. The flat substrate is formed by glass molding and resin molding.
Claims created by any method selected from the form
5. The multifocal lens according to 5 . 14. A light source and a light beam emitted from the light source
Light collecting means for collecting light on the information medium surface;
A light beam separating means for separating the modulated light beam;
Comprising light receiving means for receiving light modulated by the information medium,
14. The light collecting means according to claim 1.
Optical head device . 15. An optical head device according to claim 14,
Record and reproduce information on multiple types of recording media with different thicknesses
Optical information recording and reproducing device .