JP2008027569A - Optical information recording/reproducing device and objective lens for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens for an optical information recording/reproducing device suppressing the occurrence of aberrations in any of three types of optical disks different in standard by using a plurality of types of luminous fluxes, and achieving highly accurate information recording/reproducing by securing high diffraction efficiency. <P>SOLUTION: The objective lens for the optical information recording/reproducing device is a lens for a device which records or reproduces information by using three types of roughly parallel luminous fluxes different in wavelength for three types of optical disks having different standards. First and second optical members made of two types of materials satisfying predetermined conditions are formed by being joined at a joint surface having a phase shift structure. The phase shift structure is formed so that a numerical value defined by a refractive index in using light of each wavelength or diffraction orders maximizing diffraction efficiency satisfies predetermined conditions. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is mounted on a device that uses a plurality of types of light having different wavelengths, such as an optical information recording / reproducing device that records or reproduces information on a plurality of types of optical discs having different recording densities or protective layer thicknesses. Objective lens.

  Conventionally, there are a plurality of standards for optical disks, such as CD and DVD, which differ in recording density and protective layer thickness. In recent years, optical discs of a new standard having a higher recording density than that of DVDs are being put into practical use in order to realize a higher capacity for information recording. Examples of the new standard optical disc include HD DVD and BD (Blu-ray Disc). Such a new standard optical disc has a protective layer thickness equal to or less than the protective layer thickness of DVD. Since there are a plurality of optical discs with different standards in this way, in view of user convenience, in recent years, an optical information recording / reproducing device, more precisely, an objective lens provided in the device has been used for the above three types of optical discs. It is required to have compatibility. In the text, the term “optical information recording / reproducing apparatus” includes all of the information recording apparatus, the information reproduction apparatus, and the information recording / reproducing apparatus. “Compatible” means that recording or reproduction of information is guaranteed without changing parts even if the optical disk to be used is switched.

  In order for the device to be compatible with multiple optical discs with different standards, it can be used for recording or reproducing information while correcting spherical aberration that changes depending on the thickness of the protective layer when switching between optical discs with different standards. It is necessary to change the numerical aperture (NA) of the light to obtain a beam spot corresponding to the difference in recording density. Generally, the spot diameter can be made smaller as the wavelength is shorter. Therefore, conventionally, laser light having a plurality of wavelengths is used in the optical information recording / reproducing apparatus according to the recording density. For example, when using a DVD, laser light having a wavelength of about 660 nm, which is shorter than about 790 nm used when using a CD, is used. Further, when the optical disc of the new standard is used, light having a wavelength shorter than that used when recording or reproducing information on a DVD (for example, so-called blue laser light per 405 nm) is used because of its high recording density.

  Further, for each optical disk, as one means for converging to the recording surface position of each optical disk in a good state, an annular structure having a ring-shaped fine step on one surface of the objective lens is provided. A technique has been put into practical use that allows light of different wavelengths to converge well on the recording surface of the corresponding optical disk by the action of the band structure.

  As described above, for example, an objective lens that is compatible with three types of optical disks such as CD, DVD, and HD DVD is proposed in Patent Document 1 below, for example.

Japanese Patent Laid-Open No. 2004-247025

  The objective lens described in Patent Document 1 is compatible with three types of optical disks having different recording densities by correcting spherical aberration by making three types of parallel light beams having different wavelengths incident on an objective lens having a diffractive structure. Is given.

  However, in the conventional configuration as in Patent Document 1, when a CD is used, the normal diffraction order light used for recording or reproducing information is almost the same as the unnecessary diffraction order light not used for recording or reproducing information. The amount will be generated. Therefore, further improvement has been demanded so that information can be recorded or reproduced with respect to an optical disc having a relatively low recording density, such as a CD, with high accuracy.

  Therefore, in view of the above circumstances, the present invention is high even when information is recorded or reproduced on any of three types of optical discs having different standards by using any one of a plurality of types of light beams having different wavelengths. Highly accurate information recording or reproduction can be realized by ensuring diffraction efficiency, and furthermore, a good spot can be formed on the recording surface of each optical disk by suppressing various aberrations including spherical aberration. An object of the present invention is to provide an objective lens for an optical information recording / reproducing apparatus and the apparatus.

In order to solve the above problems, the objective lens for an optical information recording / reproducing apparatus of the present invention has three types of substantially parallel light beams having first to third wavelengths in order for the first to third optical disks having different recording densities. An objective lens used in an optical information recording / reproducing apparatus that records or reproduces information with respect to each optical disk by using a light beam. The first to third wavelengths are λ1, λ2, λ3 (units: nm), respectively. )
λ1 <λ2 <λ3
The thickness of the protective layer of the first optical disc on which information is recorded or reproduced using a light beam of the first wavelength is t1 (unit: mm), and information is recorded or reproduced using the light beam of the second wavelength. The thickness of the protective layer of the second optical disk to be recorded is t2 (unit: mm), and the thickness of the protective layer of the third optical disk on which information is recorded or reproduced using the light beam of the third wavelength is t3 (unit: mm). )
t1 ≦ t2 <t3
NA1 for the numerical aperture necessary for recording or reproducing information on the first optical disc, NA2 for the numerical aperture necessary for recording or reproducing information on the second optical disc, and recording or reproducing information on the third optical disc If the required numerical aperture is NA3,
NA1> NA3 and NA2> NA3
The objective lens is a cemented lens in which a first optical member and a second optical member made of different materials are joined at a joint surface, and the joint surface is continuously divided into a plurality of concentric shapes. At least a first phase shift structure composed of a refracting surface, and the first phase shift structure sequentially sets the diffraction order at which the diffraction efficiency is maximum for each light flux from the first wavelength to the third wavelength to m (Λ1), m (λ2), m (λ3), and the refractive index of the first optical member for each light flux from the first wavelength to the third wavelength is n1 (λ1), n1 (λ2), n1 ( λ3), and the refractive index of the second optical member for each light flux from the first wavelength to the third wavelength is n2 (λ1), n2 (λ2), and n2 (λ3) in order, the following two conditions ( 1), (2),
0.85 <Φ2 / Φ1 <1.15 (1)
0.10 <| (Φ3-Φ2) / Φ1 | <0.50 (2)
However, Φ1 = m (λ1) × (λ1 / (n2 (λ1) −n1 (λ1)))
Φ2 = m (λ2) × (λ2 / (n2 (λ2) −n1 (λ2)))
Φ3 = m (λ3) × (λ3 / (n2 (λ3) −n1 (λ3)))
It is characterized by satisfying.

According to the objective lens for an optical information recording / reproducing apparatus according to claim 1, a cemented lens having a phase shift structure on the cemented surface is used.
By satisfying the conditions (1) and (2), the spherical aberration can be corrected well even when a substantially parallel light beam is incident on the objective lens. This is effective regardless of which of the first to third optical discs with different standards for information recording or reproduction.

According to the objective lens for an optical information recording / reproducing apparatus according to claim 2, the first phase shift structure satisfies the following condition (3), that is, is provided by each step of the joint surface with respect to the light of the second wavelength. When the added optical path length is larger than the added optical path length provided by each step of the joint surface with respect to the light of the first wavelength, it is desirable to satisfy the following condition (4).
1.00 ≦ Φ2 / Φ1 <1.15 (3)
0.20 <| (Φ3-Φ2) / Φ1 | <0.50 (4)

According to the objective lens for an optical information recording / reproducing apparatus according to claim 3, the first phase shift structure further satisfies the following condition (5), so that the light used particularly for recording or reproducing information can be obtained. Spherical aberration that occurs when the wavelength changes slightly from the design wavelength can be satisfactorily suppressed.
0.20 <| (Φ3-Φ2) / Φ1 | <0.40 (5)

According to the objective lens for an optical information recording / reproducing apparatus according to claim 4, the first phase shift structure satisfies the following condition (6), that is, provided by each step of the joint surface with respect to the light of the first wavelength. When the added optical path length is larger than the added optical path length provided by each step of the joint surface with respect to the light of the second wavelength, it is desirable to satisfy the following condition (7).
0.85 <Φ2 / Φ1 <1.00 (6)
0.20 <| (Φ3-Φ2) / Φ1 | <0.35 (7)

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 5, any one of the two surfaces other than the cemented surface is formed of a plurality of concentric and continuously divided refractive surfaces. It is desirable that the optical path length given to the light beam of the first wavelength is approximately two wavelengths on the refracting surfaces adjacent to each other in the second phase shift structure. .

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 6, any one of the two surfaces other than the cemented surface is constituted by a plurality of concentric and continuously divided refractive surfaces. It is desirable that the optical path length given to the light beam having the first wavelength is approximately 10 wavelengths on the refracting surfaces adjacent to each other in the second phase shift structure. .

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 7, the first phase shift structure has an optical axis for converging at least a third wavelength light beam on the recording surface of the third optical disc. It is arranged in the first region including.

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 8, the bonding surface allows the first wavelength light flux and the second wavelength light flux to be outside the first region, respectively, It is possible to have a second region that converges on each recording surface of the second optical disc and does not contribute to the convergence of the light beam having the third wavelength.

  Here, in the second region, by appropriately selecting the materials of the first optical member and the second optical member, the light beam having the first wavelength and the light beam having the second wavelength are respectively transmitted to the first optical disk and the second optical member. It is possible to provide an effect of converging on each recording surface of the second optical disc and not contributing to the convergence of the light beam having the third wavelength. Accordingly, the second region can be configured as a refractive surface. Further, the above-described operation may be more effectively realized by disposing a third phase shift structure including a plurality of concentrically divided refractive surfaces in the second region. Item 9).

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 10, the second phase shift structure includes an optical axis for converging at least a third wavelength light beam on a recording surface of the third optical disc. Arranged in the third region.

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 11, any one of the two surfaces other than the cemented surface has a fourth region outside the third region, The region can include a fourth phase shift structure that is configured by a plurality of concentric and continuously divided refractive surfaces and that provides an optical path length of approximately three wavelengths to the first wavelength light flux. .

  Further, the fourth phase shift structure may be provided with an optical path length of approximately 5 wavelengths with respect to the light beam having the first wavelength (claim 12). This makes it possible to satisfactorily suppress spherical aberration that occurs particularly when the wavelength of light used for recording or reproducing information changes minutely from the design wavelength, and provides an aperture limiting function for a third wavelength light beam. Can be granted.

According to the objective lens for an optical information recording / reproducing device according to claim 13, when the following condition (8) is satisfied, the phase shift structure has a first wavelength outside the second region. It is desirable to provide a fifth region that converges only the light beam and does not contribute to the convergence of the light beams of the second and third wavelengths.
f1 × NA1> f2 × NA2 (8)

According to the objective lens for an optical information recording / reproducing apparatus according to claim 14, when the following condition (9) is satisfied, the phase shift structure has a second wavelength outside the second region. It is desirable to provide a fifth region that converges only the luminous flux of, and does not contribute to the convergence of the luminous fluxes of the first and third wavelengths.
f1 × NA1 <f2 × NA2 (9)

  According to the objective lens for optical information recording / reproducing according to claim 15, the fifth region has a fifth difference having at least one optical path length difference with respect to an incident light beam at a boundary between adjacent refracting surfaces. The absolute value of the optical path length difference is different from the absolute value of at least one optical path length difference in the second region.

Further, according to the objective lens for an optical information recording / reproducing apparatus according to claim 16, when the following condition (8) is satisfied, the phase shift structure has a first wavelength outside the fourth region. It is desirable to provide a sixth region that converges only the light beam and does not contribute to the convergence of the light beams of the second and third wavelengths. The fifth region has a sixth phase shift structure having at least one kind of optical path length difference with respect to an incident light beam at a boundary between adjacent refracting surfaces, and an absolute value of the optical path length difference is It differs from the absolute value of at least one kind of optical path length difference in the fourth region.
f1 × NA1> f2 × NA2 (8)

According to the objective lens for an optical information recording / reproducing device according to claim 17, when the following condition (9) is satisfied, the phase shift structure has a second wavelength outside the fourth region. It is desirable to provide a sixth region that converges only the luminous flux of λ and does not contribute to the convergence of the luminous fluxes of the first and third wavelengths. The sixth region has a sixth phase shift structure having at least one kind of optical path length difference with respect to an incident light beam at a boundary between adjacent refractive surfaces, and an absolute value of the optical path length difference is It differs from the absolute value of at least one kind of optical path length difference in the fourth region.
f1 × NA1 <f2 × NA2 (9)

According to the objective lens for an optical information recording / reproducing apparatus according to claim 18, the center thickness of the first optical member is d1 (unit: mm), and the center thickness of the second optical member is d2 (unit: mm). Then, the following condition (10),
0.01 <d1 / d2 <0.20 (10)
It is desirable to satisfy. By satisfying the condition (10), the objective lens can be easily manufactured.

  From another point of view, an optical information recording / reproducing apparatus according to claim 19 is a light source that irradiates three types of substantially parallel light beams having first to third wavelengths different from each other, and the above-described claims 1 to 18. The optical information recording / reproducing apparatus objective lens according to any one of the above, and recording information on each optical disc by using one of three types of substantially parallel light beams for each of a plurality of optical discs having different recording densities It is characterized by performing reproduction.

According to the optical information recording / reproducing apparatus of claim 20, the protective layer thickness t1 of the first optical disc, the protective layer thickness t2 of the second optical disc, and the third protective layer thickness t3 are respectively defined as follows: Is done.
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm

  As described above, the objective lens according to the present invention is configured by joining two optical members having different optical performances to each other on the joining surface on which a predetermined phase shift structure is formed. As a result, even when any optical disc, particularly an optical disc such as a CD having a low recording density, is used, high diffraction efficiency can be ensured, and more accurate information recording or reproduction can be realized.

  Further, according to the present invention, it is possible to use a substantially parallel light beam when any optical disk is used. Thereby, it is possible to satisfactorily suppress not only spherical aberration but also aberration generated during tracking shift regardless of whether an existing optical disc or a new standard optical disc is used.

  The objective lens according to the embodiment of the present invention will be described below. The objective lens of this embodiment is mounted on an optical information recording / reproducing apparatus, and is compatible with three types of optical discs having different standards such as a protective layer thickness and a recording density.

  In the following, for convenience of explanation, among the above three types of optical discs, an optical disc having the highest recording density (for example, a new standard optical disc such as HD DVD or BD) is relatively compared to the first optical disc D1 and the first optical disc D1. The second optical disk D2 has a low recording density (for example, DVD or DVD-R), and the third optical disk D3 has the lowest recording density (for example, CD or CD-R).

Assuming that the protective layer thicknesses of the optical disks D1 to D3 are t1 to t3, the protective layer thicknesses have the following relationship.
t1 ≦ t2 <t3

Further, when information is recorded on or reproduced from each of the optical discs D1 to D3, it is necessary to change the required NA value so that a beam spot corresponding to the difference in recording density can be obtained. Here, assuming that the optimum design numerical apertures required for recording or reproducing information with respect to each of the optical discs D1 to D3 are NA1, NA2, and NA3, each NA has the following relationship.
NA1> NA3 and NA2> NA3

  That is, when recording or reproducing information with respect to the first optical disc D1 and the second optical disc D2 having a high recording density, formation of a spot with a smaller diameter is required, so that the necessary NA is increased. On the other hand, when recording or reproducing information on the third optical disc D3 having the lowest recording density, the required NA is relatively small. All optical disks are placed on a turntable (not shown) and rotated when information is recorded or reproduced.

  When using the optical discs D1 to D3 having different recording densities as described above, laser beams having different wavelengths are used in the optical information recording / reproducing apparatus so as to obtain beam spots corresponding to the recording densities. Specifically, when information is recorded on or reproduced from the first optical disc D1, in order to form the beam spot having the smallest diameter on the recording surface of the first optical disc D1, the shortest wavelength (the first wavelength) A laser beam (hereinafter, referred to as a first laser beam) that is one wavelength) is irradiated from a light source. Further, when recording or reproducing information on the third optical disc D3, in order to form a beam spot with the largest diameter on the recording surface of the third optical disc D3, A laser beam having a wavelength) (hereinafter referred to as a third laser beam) is irradiated from a light source. When recording or reproducing information on the second optical disc D2, a longer wavelength than that of the first laser beam is formed in order to form a relatively small-diameter spot on the recording surface of the second optical disc D2. In addition, a laser beam (hereinafter referred to as a second laser beam) having a shorter wavelength (second wavelength) than the third laser beam is irradiated from a light source.

  FIG. 1 is a schematic diagram showing a schematic configuration of an optical information recording / reproducing apparatus 100 having an objective lens 10 of the present embodiment. The optical information recording / reproducing apparatus 100 includes a light source 1A for irradiating a first laser beam, a light source 1B for irradiating a second laser beam, a light source 1C for irradiating a third laser beam, coupling lenses 2A, 2B, 2C, It has beam splitters 41 and 42, half mirrors 5A, 5B and 5C, and light receiving parts 6A, 6B and 6C. In the optical information recording / reproducing apparatus 100, it is necessary to cope with different NAs required when using each optical disk. Therefore, in the optical information recording / reproducing apparatus 100, although not shown, an aperture limiting element that defines the beam diameter of the third laser beam may be disposed between the light source 1C and the objective lens 10.

  As shown in FIG. 1, the first to third laser light beams emitted from the light sources 1A to 1C are deflected by the half mirrors 5A to 5C and converted into parallel light beams by the coupling lenses 2A to 2C. . That is, in this embodiment, each coupling lens 2A-2C functions as a collimating lens. The laser beams transmitted through the coupling lenses 2A to 2C are guided to a common optical path by the beam splitters 41 and 42 and enter the objective lens 10. Each light beam transmitted through the objective lens 10 converges in the vicinity of the recording surface of each of the optical discs D1 to D3 to be recorded or reproduced. Each laser beam reflected by the recording surface passes through the half mirrors 5A to 5C and is detected by the light receiving units 6A to 6C.

  As described above, by making each laser beam incident on the objective lens 10 into a parallel light beam, occurrence of off-axis aberrations such as coma aberration when the objective lens 10 is tracked can be suppressed.

  Strictly speaking, the light fluxes emitted from the coupling lenses 2A to 2C due to individual differences and installation positions of the light sources 1A to 1C and changes in the environment in which the optical information recording / reproducing apparatus 100 is placed. May not necessarily be a parallel light flux. However, since the divergence angle of the light beam for the above reason is very small, the aberration generated at the time of tracking shift is also small. Therefore, it can be said that there is no problem in actual use.

  2A to 2C are diagrams showing the objective lens 10 and the optical disks D1 to D3 separately for each optical path when each optical disk is used. 2A to 2C, the reference axis AX of the optical information recording / reproducing apparatus 100 is indicated by a one-dot chain line in the drawing. In the state shown in FIGS. 2A to 2C, the optical axis of the objective lens coincides with the reference axis AX of the optical system, but the optical axis of the objective lens is deviated from the reference axis AX of the optical system by a tracking operation or the like. There is also a state that comes off.

  As shown in FIG. 2, each of the optical discs D1 to D3 has a protective layer 21 and a recording surface 22. In the actual optical disks D1 to D3, the recording surface 22 is sandwiched between a protective layer 21 and a label layer (not shown).

FIG. 3 is an enlarged schematic diagram showing the objective lens 10. The objective lens 10 is a biconvex plastic cemented lens formed by joining two optical members 10A and 10B made of different materials at the joint surface 13. The objective lens 10 formed by bonding has a first surface 11 and a second surface 12 in order from the light source side in addition to the bonding surface 13. In the objective lens 10 of the present embodiment, the surfaces 11 to 13 are configured as aspheric surfaces. The shape of the aspheric surface is the distance (sag amount) from the tangential plane on the optical axis of the aspherical surface at the coordinate point on the aspherical surface where the height from the optical axis is h, and X (h). The above curvature (1 / r) is C, the conic coefficient is K, the fourth, sixth, eighth, tenth, twelve, etc. aspheric coefficients are A _2i (where i is an integer of 1 or more) Is expressed by the following formula.

  As in the optical information recording / reproducing apparatus 100, when using each of the optical disks D1 to D3, when using laser light of an appropriate wavelength, depending on the use of each optical disk, the change in the refractive index of the objective lens or the respective optical disks D1 to D3. Due to the difference in thickness of the protective layer 21, the spherical aberration changes.

  Therefore, in order to correct the spherical aberration generated when each of the three types of optical discs D1 to D3 is used and to have compatibility with each of the optical discs D1 to D3, at least the cemented surface 13 of the objective lens 10 of the present embodiment has A phase shift structure having a diffractive action that affects three types of light beams is provided. The phase shift structure provided on the joint surface 13 includes a plurality of concentric refracting surfaces centered on the optical axis AX and a plurality of minute steps formed between the refracting surfaces.

  The objective lens according to the present embodiment uses the diffraction action and the refraction action at the cemented surface 13 and the refraction action of the first surface 11 and the second surface 12 to respectively change the first to third light beams of the respective laser beams. The recording surface has a function of condensing while correcting the spherical aberration to be substantially zero.

  The optical path difference function that defines the phase shift structure represents the function of the objective lens 10 as a diffractive lens in the form of an additional optical path length at a height h from the optical axis. Assuming that the optical path difference function is φ (h), φ (h) is expressed by the following equation.

In the optical path difference function φ (h) shown in _{Equation 2} , P _2i (where i is an integer equal to or greater than 1) is a coefficient of second order, fourth order, sixth order,. m represents the diffraction order at which the diffraction efficiency of the laser beam used is maximized, and λ represents the design wavelength of the laser beam used. The same applies to the following conditions.

Further, in the objective lens 10 of the present embodiment, the phase shift structure provided on the cemented surface 13 satisfies the following conditions (1) and (2) in order to make the action of correcting spherical aberration more effective. Configured as follows.
0.85 <Φ2 / Φ1 <1.15 (1)
0.10 <| (Φ3-Φ2) / Φ1 | <0.50 (2)
Where Φ1 = m (λ1) × (λ1 / (n2 (λ1) −n1 (λ1))),
Φ2 = m (λ2) × (λ2 / (n2 (λ2) −n1 (λ2))),
Φ3 = m (λ3) × (λ3 / (n2 (λ3) −n1 (λ3))). More specifically, m (λ1), m (λ2), and m (λ3) are the diffraction orders at which the diffraction efficiency becomes maximum for each light flux from the first wavelength to the third wavelength in order, n1 (λ1), n1 (Λ2), n1 (λ3) are the refractive indices of the first optical member for the light beams of the first wavelength to the third wavelength in order, and n2 (λ1), n2 (λ2), and n2 (λ3) are The refractive indexes of the second optical member with respect to each light flux from the first wavelength to the third wavelength are shown in order, respectively.

  Φ1 to Φ3 correspond to the optical path length addition amount provided by each step for each of the first to third laser beams on the joint surface 13 having the phase shift structure defined by Equation 2. That is, the condition (1) is a condition regarding the ratio of the optical path length addition amount provided by each step of the bonding surface 13 to the first laser light and the second laser light. Condition (2) is a condition representing the relationship of the optical path length addition amount provided by each step of the bonding surface 13 with respect to each laser beam.

  Regarding the condition (1), when the numerical value exceeds the upper limit, correction for spherical aberration becomes excessive especially when the second optical disc D2 is used, and when the numerical value becomes lower than the lower limit, correction for spherical aberration particularly when the second optical disc D2 is used. Is not preferable because of lack of. Regarding the condition (2), when the numerical value exceeds the upper limit, correction for spherical aberration becomes excessive especially when the third optical disc D3 is used, and when the numerical value is lower than the lower limit, spherical aberration particularly when the third optical disc D3 is used. This is not preferable because there is insufficient correction.

Further, when Φ2 / Φ1 satisfies the following condition (3), in detail, the optical path length addition amount provided by each step of the bonding surface 13 with respect to the second laser light is equal to that of the bonding surface 13 with respect to the first laser light. When it is larger than the optical path length addition amount given by each step, it is configured to satisfy the condition (4).
1.00 ≦ Φ2 / Φ1 <1.15 (3)
0.20 <| (Φ3-Φ2) / Φ1 | <0.50 (4)

The joint surface 13 satisfying the condition (3) further satisfies the following condition (5), so that the wavelength of light used for recording or reproducing information (for example, the first optical disk D1 when using the first optical disk D1) Spherical aberration that occurs when the laser light) changes slightly from the design wavelength can be satisfactorily suppressed.
0.20 <| (Φ3-Φ2) / Φ1 | <0.40 (5)

In addition, when Φ2 / Φ1 satisfies the following condition (6), in detail, the optical path length addition amount provided by each step of the bonding surface 13 with respect to the first laser light is equal to that of the bonding surface 13 with respect to the second laser light. When it is larger than the optical path length addition amount given by each step, it is configured to satisfy the condition (7).
0.85 <Φ2 / Φ1 <1.00 (6)
0.20 <| (Φ3-Φ2) / Φ1 | <0.35 (7)

  The critical significance regarding the conditions (6) and (7) is the same as the conditions (3) and (4) described above. As described above, by providing the phase shift structure on the joint surface 13, the refractive index difference between the two optical members also works more effectively than when the phase shift structure is provided at the interface with air. Thereby, higher diffraction efficiency can be obtained.

  The objective lens 10 shown in FIG. 3 has a single phase shift structure applied over the entire area of the bonding surface 13. On the other hand, the objective lens 10 according to the present invention divides the bonding surface 13 into a plurality of regions that give different actions to incident light in order to realize information recording or reproduction with respect to each optical disk with higher accuracy. It is also possible to do. FIG. 4 is a schematic diagram showing an objective lens 10 according to another aspect.

  As shown in FIG. 4, the joint surface 13 has a first region 13a including the optical axis AX and a second region 13b located outside the first region 13a. Furthermore, when the incident light beam diameter of the first laser light on the first surface 11 of the objective lens 10 is different from the effective light beam diameter of the second laser light, a fifth region 13c is formed outside the second region 13b. Have. The first region 13a is provided with a phase shift structure that contributes to convergence for any laser beam used during recording or reproduction of information with respect to each of the optical discs D1 to D3. In the present embodiment, the first region 13a has the above-described phase shift structure, that is, a phase shift structure that satisfies the conditions (1) to (7) (hereinafter referred to as the first phase shift structure for convenience). It is arranged.

  The second region 13b is configured as a region in which spherical aberration related to the first laser beam and the second laser beam is corrected well and each laser beam converges well on the recording surface of the corresponding optical disk. . Here, the objective lens 10 of the present embodiment includes two optical members 10A and 10B having different refractive indexes. Accordingly, if each of the optical members 10A and 10B is made of a material having a refractive index that can satisfactorily correct spherical aberration with respect to the first laser beam and the second laser beam, the second region 13b can be simply refracted. By simply designing it as a surface (aspherical surface), it becomes a region that suppresses the occurrence of spherical aberration and improves the light utilization efficiency for the first and second laser beams.

  In addition, the phase shift structure in which the first laser beam and the second laser beam converge well on the recording surface of the corresponding optical disk and do not contribute to the convergence of the third laser beam in the second region 13b. It is also possible to positively apply the third phase shift structure).

The fifth area 13c has the following condition (8), where f1 is the focal length when using the first optical disc D1, and f2 is the focal length when using the second optical disc D2.
f1 × NA1> f2 × NA2 (8)
That is, that is, when the first laser beam is incident, the effective beam diameter on the incident surface of the objective lens 10 is equal to the effective beam on the incident surface of the objective lens 10 when the second laser beam is incident. Provided when larger than diameter. The spherical aberration related to the first laser beam is corrected well, and each laser beam is configured as an area that converges well on the recording surface of the corresponding optical disc.

  Unlike the second region, the fifth region formed when the condition (8) is satisfied does not contribute to the convergence of the second laser beam. That is, the fifth region formed when the condition (8) is satisfied has an aperture limiting function for the second or third laser beam. For example, when a phase shift structure (fifth phase shift structure) is provided in the fifth region, the structure is such that the optical path length difference imparted at the boundary between the refracting surfaces adjacent to each other for the first laser light It is designed to be different from the optical path length difference for the first laser light in the region. Specifically, the fifth phase shift structure is blazed so that the diffraction efficiency with respect to the first laser beam is maximized.

On the other hand, the fifth region 13c has the following condition (9), where f1 is the focal length when using the first optical disc D1, and f2 is the focal length when using the second optical disc D2.
f1 × NA1 <f2 × NA2 (9)
That is, that is, the effective light beam diameter on the incident surface of the objective lens 10 when the second laser light is incident is the effective light beam on the incident surface of the objective lens 10 when the first laser light is incident. Provided when larger than diameter. Spherical aberration related to the second laser beam is corrected well, and each laser beam is configured as a region that converges well on the recording surface of the corresponding optical disk.

  Unlike the second region, the fifth region formed when the condition (9) is satisfied does not contribute to the convergence of the first laser beam. That is, the fifth region formed when the condition (9) is satisfied has an aperture limiting function for the first or third laser beam. For example, when a phase shift structure (fifth phase shift structure) is provided in the fifth region, the structure is such that the optical path length difference provided at the boundary between the refracting surfaces adjacent to each other for the second laser light is the fourth. It is designed to be different from the optical path length difference for the second laser light in the region. Specifically, the fifth phase shift structure is blazed so that the diffraction efficiency with respect to the second laser beam is maximized.

  Further, for the purpose of suppressing spherical aberration caused by wavelength change or providing the objective lens 10 with an aperture limiting function for the third laser light, the interface with the air in the objective lens 10, that is, the first surface 11 and the second surface 12 are formed. It is also possible to divide into a plurality of regions and provide different phase shift structures for each.

  In consideration of the possibility of being close to the optical disk placed on the turntable during the focusing operation, the phase shift structure is preferably provided on the first surface 11 as shown in FIG. The first surface 11 is divided into three regions 11 a, 11 b, and 11 c by a boundary having a height substantially the same as the height from the optical axis of the boundary of each region on the bonding surface 13. That is, the third region 11a corresponds to the first region 13a, the fourth region 11b corresponds to the second region 13b, and the sixth region 11c corresponds to the fifth region 13c. Therefore, when a single phase shift structure is applied to the entire joining surface 13 as shown in FIG. 3, in other words, when the joining surface 13 is composed of only the first region 13a, an interface with air (here, Only the third region 11a is present on the first surface 11).

  The third region 11a is provided with a phase shift structure (second phase shift structure) that imparts an additional optical path length for approximately two wavelengths to the first laser light. As a result, the diffraction order at which the diffraction efficiency becomes maximum in the third phase shift structure in each laser light is second order, first order, first order from the first laser light, and each laser light has high diffraction efficiency. Have. As a result, spherical aberration caused by the light emitted from each of the light sources 1A to 3A being slightly deviated from the design wavelength while maintaining high light utilization efficiency at the time of recording or reproducing information on any optical disk. It is possible to obtain an effect of correcting the above.

  Further, when the first laser light and the second laser light are incident on the fourth region 11b, each light is well converged on the recording surface of the corresponding optical disk D1, D2, and the third laser For light, a phase shift structure (fourth phase shift structure) having an action that does not contribute to convergence is provided. Thereby, the objective lens 10 is provided with an aperture limiting function for the third laser light. Therefore, when the fourth phase shift structure is provided, it is not necessary to provide the above aperture limiting element.

  Specifically, the fourth phase shift structure having the above-described action has a step structure that gives an additional optical path length of about 3 wavelengths or about 5 wavelengths to the first laser light. As a result, the diffraction efficiency of the first laser beam and the second laser beam is increased, and the spherical aberration caused by the slight deviation of the light emitted from each of the light sources 1A to 3A from the design wavelength is also corrected. can get.

The sixth area 11c has the following condition (8), where f1 is the focal length when using the first optical disc D1, and f2 is the focal length when using the second optical disc D2.
f1 × NA1> f2 × NA2 (8)
That is, that is, when the first laser beam is incident, the effective beam diameter on the incident surface of the objective lens 10 is equal to the effective beam on the incident surface of the objective lens 10 when the second laser beam is incident. Provided when larger than diameter.

  Unlike the fourth region, the sixth region formed when the condition (8) is satisfied has a phase shift structure (sixth phase shift structure) that does not contribute to the convergence of the second laser beam. . That is, the sixth region formed when the condition (8) is satisfied has an aperture limiting function for the second or third laser beam. Specifically, in the sixth phase shift structure, the optical path length difference given to the boundary between the refractive surfaces adjacent to each other for the first laser light is different from the optical path length difference for the first laser light in the fourth region. Designed to be different. At the time of the design, the sixth region is blazed so that the diffraction efficiency with respect to the first laser beam is maximized.

The sixth region 11c has the following condition (9), where f1 is the focal length when using the first optical disc D1, and f2 is the focal length when using the second optical disc D2.
f1 × NA1 <f2 × NA2 (9)
That is, that is, the effective light beam diameter on the incident surface of the objective lens 10 when the second laser light is incident is the effective light beam on the incident surface of the objective lens 10 when the first laser light is incident. Also provided when larger than the diameter.

  Unlike the fourth region, the sixth region formed when the condition (9) is satisfied has a phase shift structure (sixth phase shift structure) that does not contribute to the convergence of the first laser beam. . That is, the sixth region formed when the condition (9) is satisfied has an aperture limiting function for the first or third laser beam. Specifically, in the sixth phase shift structure, the optical path length difference given to the boundary between the refractive surfaces adjacent to each other for the second laser light is different from the optical path length difference for the second laser light in the fourth region. Designed to be different. At the time of the design, the sixth region is blazed so that the diffraction efficiency for the second laser beam is maximized.

  In the above description, the phase shift structure can be disposed in any of the first to sixth regions. However, in the objective lens 10 according to the present invention, as long as the first phase shift structure is disposed at least in the first region 13a, the other phase shift structures are not necessarily essential. . In other words, whether or not to provide the phase shift structure in the second to sixth regions can be arbitrarily determined by comprehensively considering the ease of design, the optical performance required for the objective lens 10, and the like. .

The objective lens 10 is configured to satisfy the following condition (10) in order to ensure ease of manufacture.
0.01 <d1 / d2 <0.20 (10)
However, d1 is the center thickness (unit: mm) of the first optical member 10A,
d2 represents the center thickness (unit: mm) of the second optical member 10B.

  Hereinafter, five specific examples of the objective lens 10 having the above-described features will be described. The schematic configuration of the objective lens 10 of Examples 1 and 2 is shown in FIG. 3, and the schematic configuration of the objective lens 10 of Examples 3 to 5 is shown in FIG. The optical disk used in each example is a first optical disk D1 having the highest recording density with a protective layer thickness of 0.6 mm, and a second optical disk having a protective layer thickness of 0.6 mm and a recording density lower than that of the first optical disk D1. Assume an optical disk D2 and a third optical disk D3 with a protective layer thickness of 1.2 mm and the lowest recording density.

  Specific specifications for each laser beam of the objective lens 10 of Example 1 are shown in Table 1.

  As shown in Table 1, the magnification value indicates that in Example 1, the laser light is incident on the objective lens 10 as a parallel light beam even when any of the optical disks D1 to D3 is used. Tables 2 to 4 show specific numerical configurations when the optical discs D1 to D3 of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 1 are used. However, in Tables 2 to 4, for the convenience of explanation, the numerical configuration relating to the members disposed between the light source and the objective lens is omitted. The same applies to tables relating to specific numerical configurations presented in each embodiment described later.

  In Tables 2 to 4, surface number 0 is the light source 1A to 1C, surface number 1 is the first surface 11 of the objective lens 10, surface number 2 is the cemented surface 13 of the objective lens 10, and surface number 3 is the first surface of the objective lens 10. Two surfaces 12 and surface numbers 4 and 5 respectively represent the protective layer 21 and the recording surface 22 in each of the optical disks D1 to D3. In Tables 2 to 4 and each table to be described later, r is a radius of curvature (unit: mm) of each lens surface, d is a lens thickness or lens interval (unit: mm), n ( Xnm) is the refractive index at the wavelength Xnm.

  The first surface 11, the cemented surface 13, and the second surface 12 (surface numbers 1, 2, and 3 in order) of the objective lens 10 are all aspherical surfaces. Table 5 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 to 13. In addition, the notation E in each table | surface represents the power which uses 10 as the radix and the number on the right of E is an exponent.

In addition, the objective lens 10 of Example 1 is provided with a phase shift structure (corresponding to the first phase shift structure described above) over the entire bonding surface 13. Table 6 shows the coefficient P _2i in the optical path difference function for defining the phase shift structure. Table 7 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

  From the above tables, Φ1 = 0.043, Φ2 = 0.044, and Φ3 = 0.054. Therefore, the objective lens 10 of Example 1 satisfies Φ2 / Φ1 = 1.024, | (Φ3-Φ2) /Φ1|=0.233, and satisfies all the conditions (1) to (5).

  Table 8 shows the diffraction efficiency of each laser beam when the optical disks D1 to D3 are used in the objective lens of Example 1 and the conventional single lens (Comparative Example 1).

  As shown in Table 8, in Example 1, the diffraction efficiency when using the optical discs D2 and D3 is higher than that in Comparative Example 1. That is, according to the first embodiment, high light utilization efficiency can be maintained even when any optical disk is used.

  FIGS. 5A to 5C are aberration diagrams showing spherical aberrations that occur when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 having the objective lens 10 according to the first embodiment. is there. 5A shows the spherical aberration generated when the first laser beam is used, FIG. 5B shows the spherical aberration generated when the second laser beam is used, and FIG. 5C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. 5A to 5C, the solid line represents the spherical aberration when the laser beam having the wavelength shown in Table 1 is incident, and the broken line represents the laser beam having changed by 5 nm from the wavelength illustrated in Table 1. Represents the spherical aberration. Note that the same notation is used in each aberration diagram presented in each example described later.

  As shown by the solid lines in FIGS. 5A to 5C, when the objective lens 10 of Example 1 that satisfies the conditions (1) to (5) is used, information recording or recording on any of the optical disks D1 to D3 is performed. It can be seen that even during reproduction, the spherical aberration is corrected well, and the laser beam of the corresponding design wavelength converges well on the recording surface 22 of each optical disc. Further, as shown by the broken lines in FIGS. 5A to 5C, when the objective lens 10 of Example 1 that satisfies the conditions (1) to (5) is used, the laser beams emitted from the light sources 1 </ b> A to 1 </ b> C. It can be seen that even when there is a minute wavelength shift in the light, a change in spherical aberration due to the wavelength shift hardly occurs and a good convergence state is maintained. Note that the deviation of the spherical aberration indicated by the broken lines in the horizontal axis direction is not a problem because it can be corrected by adjusting the relative positions of the optical disk and the objective lens in the optical axis direction.

  As shown in Tables 2 to 4, the center thickness d1 of the first optical member 10A is 0.10, and the center thickness d2 of the second optical member 10B is 2.20. Accordingly, since d1 / d2 is 0.045, the objective lens 10 satisfies the condition (10).

  Specific specifications for each laser beam of the objective lens 10 of Example 2 are shown in Table 9.

  As shown in Table 9, the magnification value indicates that the laser light is incident on the objective lens 10 as a parallel light beam even in Example 2 and when any of the optical disks D1 to D3 is used. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 9 when the optical discs D1 to D3 are used are shown in Tables 10 to 12.

  In Tables 10 to 12, surface number 0 is the light sources 1A to 1C, surface number 1 is the first surface 11 of the objective lens 10, surface number 2 is the cemented surface 13 of the objective lens 10, and surface number 3 is the first surface of the objective lens 10. Two surfaces 12 and surface numbers 4 and 5 respectively represent the protective layer 21 and the recording surface 22 in each of the optical disks D1 to D3.

  The first surface 11, the cemented surface 13, and the second surface 12 (surface numbers 1, 2, and 3 in order) of the objective lens 10 are all aspherical surfaces. Table 13 shows the conical coefficient and the aspheric coefficient that define the shape of each aspheric surface of each of the surfaces 11 to 13.

In addition, the objective lens 10 of Example 2 is provided with a phase shift structure (corresponding to the first phase shift structure described above) over the entire bonding surface 13. Table 14 shows the coefficient P _2i in the optical path difference function for defining the phase shift structure. Table 15 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

  From the above tables, Φ1 = 0.072, Φ2 = 0.067, and Φ3 = 0.081. Therefore, in the objective lens 10 of Example 2, Φ2 / Φ1 = 0.931, | (Φ3-Φ2) /Φ1|=0.194, and the conditions (1), (2), (4) to (7) Meet.

  Table 16 shows the diffraction efficiencies of the laser beams when the optical disks D1 to D3 are used in the objective lens of Example 2 and the conventional single lens (Comparative Example 2).

  As shown in Table 16, in Example 2, the diffraction efficiency when using the optical disc D2 is slightly lower than that in Comparative Example 2, but the diffraction efficiency when using the optical disc D3 is very high, about twice or more. ing. Therefore, the objective lens of Example 2 is compatible with any of the optical discs D1 to D3, and is configured to maintain high light utilization efficiency when using all the optical discs.

  FIGS. 6A to 6C are aberration diagrams showing spherical aberrations that occur when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 having the objective lens 10 according to the first embodiment. is there.

  As shown by the solid lines in FIGS. 6A to 6C, when the objective lens 10 of Example 2 that satisfies the conditions (1), (2), and (4) to (7) is used, the optical disks D1 to D1 are used. It can be seen that the spherical aberration is satisfactorily corrected and the laser beam of the corresponding design wavelength is well converged on the recording surface 22 of each optical disc even when information is recorded or reproduced for any of D3. 6A to 6C, when the objective lens 10 of Example 2 is used, the laser light emitted from each of the light sources 1A to 1C has a slight wavelength shift. It can be seen that the spherical aberration occurring in the above is also suitably suppressed to such an extent that it does not affect actual use.

  As shown in Tables 10 to 12, in Example 2, the center thickness d1 of the first optical member 10A is 0.10, and the center thickness d2 of the second optical member 10B is 2.50. Accordingly, since d1 / d2 is 0.040, the objective lens 10 of Example 2 also satisfies the condition (8).

  The objective lens 10 of the third embodiment is a further improvement of the objective lens 10 of the second embodiment. Specifically, the objective lens 10 according to the third embodiment further improves performance by providing the first surface 11 with a diffractive surface, that is, a second phase shift structure. Specific specifications for each laser beam of the objective lens 10 of Example 3 are shown in Table 17. Tables 18 to 20 show specific numerical configurations when the optical discs D1 to D3 of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 17 are used. In addition, since the meaning of the surface number shown in Table 18-Table 20 is the same as said each Example 1, 2, description here is abbreviate | omitted.

  The first surface 11, the cemented surface 13, and the second surface 12 (surface numbers 1, 2, and 3 in order) of the objective lens 10 of Example 3 are all aspherical surfaces. Table 21 shows conic coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 to 13.

The objective lens 10 of Example 3 has a phase shift structure (corresponding to the second phase shift structure described above) over the entire first surface 11 and a phase shift structure (corresponding to the first phase shift described above) over the entire bonding surface 13. Equivalent to the structure). Table 22 shows the coefficient P _2i in the optical path difference function for defining each phase shift structure. Table 23 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

  From the above tables, Φ1 = 0.072, Φ2 = 0.067, and Φ3 = 0.081. Therefore, in the objective lens 10 of Example 3, Φ2 / Φ1 = 0.931, | (Φ3-Φ2) /Φ1|=0.194, and the conditions (1), (2), (4) to (7) Meet.

  Table 24 shows the diffraction efficiencies of the laser beams when the optical disks D1 to D3 are used in the objective lens of Example 3 and the conventional single lens (Comparative Example 2).

  As shown in Table 24, in Example 3, the diffraction efficiency when using the optical disc D2 is slightly lower than that in Comparative Example 2, but the diffraction efficiency when using the optical disc D3 is very high, about twice or more. ing. Therefore, the objective lens of Example 3 is compatible with any of the optical discs D1 to D3, and is configured to maintain high light utilization efficiency when all the optical discs are used.

  FIGS. 7A to 7C are aberration diagrams showing spherical aberrations that occur when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 having the objective lens 10 according to the third embodiment. is there.

  As shown by the solid lines in FIGS. 7A to 7C, when the objective lens 10 of Example 3 is used, spherical aberration is caused even when information is recorded or reproduced on any of the optical disks D1 to D3. It can be seen that the laser beam having the correct design wavelength is well converged on the recording surface 22 of each optical disk. Further, as shown by the broken lines in FIGS. 7A to 7C, when the objective lens 10 of Example 3 is used, the laser light emitted from each of the light sources 1A to 1C has a minute wavelength shift. Even so, it can be said that the spherical aberration caused by the wavelength shift is small and a good convergence state is maintained.

  As shown in Tables 18 to 20, the center thickness d1 of the first optical member 10A is 0.30, and the center thickness d2 of the second optical member 10B is 2.30. Therefore, since d1 / d2 is 0.130, the objective lens 10 of Example 3 also satisfies the condition (10).

  The objective lens 10 of Example 4 is based on the configuration of Examples 2 and 3, and further includes a third region having a second phase shift structure, and a fourth phase shift structure positioned outside the third region. And a fourth region having the first surface 11. Specific specifications for each laser beam of the objective lens 10 of Example 4 are shown in Table 25. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 25 when the optical discs D1 to D3 are used are shown in Tables 26 to 28.

  In Tables 26 to 28, surface number 0 is the light sources 1A to 1C, surface number 1 is the first surface 11 of the objective lens 10, surface number 2 is the cemented surface 13 of the objective lens 10, and surface number 3 is the first surface of the objective lens 10. Two surfaces 12 and surface numbers 4 and 5 respectively represent the protective layer 21 and the recording surface 22 in each of the optical disks D1 to D3. However, since the first surface 11 of the objective lens 10 of Example 4 has the third region and the fourth region as described above, they are shown separately.

  The first surface 11, the cemented surface 13, and the second surface 12 (surface numbers 1, 2, and 3 in order) of the objective lens 10 are all aspherical surfaces. Table 29 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 to 13.

In the objective lens 10 of Example 4, a phase shift structure (corresponding to the first phase shift structure described above) is provided on the entire surface of the cemented surface 13. Further, as described above, in the first surface, the second phase shift structure is provided in the region including the optical axis (third region), and the fourth region located outside the third region is provided in the fourth region. A phase shift structure is provided for each. Table 30 shows the coefficient P _2i in the optical path difference function for defining each phase shift structure. Table 31 shows the diffraction order m that maximizes the diffraction efficiency of each laser beam and the effective radius (unit: mm) of each surface (each region for the first surface 11).

  From the above tables, Φ1 = 0.072, Φ2 = 0.067, and Φ3 = 0.081. Therefore, in the objective lens 10 of Example 4, Φ2 / Φ1 = 0.931, | (Φ3-Φ2) /Φ1|=0.194, and the conditions (1), (2), (4) to (7) Meet.

  Table 32 shows the diffraction efficiencies of the respective laser beams when the optical disks D1 to D3 are used in the objective lens of Example 4 and the conventional single lens (Comparative Example 2).

  As shown in Table 32, the diffraction efficiency when using the optical discs D2 and D3 is higher in Example 4 than in Comparative Example 2. That is, according to the fourth embodiment, high light utilization efficiency can be maintained regardless of which optical disk is used.

  FIGS. 8A to 8C are aberration diagrams showing spherical aberrations that occur when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 having the objective lens 10 according to the fourth embodiment. is there.

  As shown by the solid lines in FIGS. 8A to 8C, when the objective lens 10 of Example 4 is used, spherical aberration is caused even when information is recorded or reproduced on any of the optical disks D1 to D3. It can be seen that the laser beam having the correct design wavelength is well converged on the recording surface 22 of each optical disk. 8A to 8C, when the objective lens 10 of Example 4 is used, the laser light emitted from the light sources 1A to 1C has a slight wavelength shift. Even so, it can be said that the spherical aberration caused by the wavelength shift is small and a good convergence state is maintained.

  In addition, as described above, the objective lens 10 of Example 4 is designed so that the fourth region of the first surface 11 has the fifth diffraction order that maximizes the diffraction efficiency with respect to the first laser beam. ing. Therefore, the third laser light transmitted through the fourth region does not contribute to convergence. That is, it can be said that the objective lens 10 of Example 4 has an aperture limiting function with respect to the third laser beam.

  As shown in Tables 26 to 28, the center thickness d1 of the first optical member 10A is 0.10, and the center thickness d2 of the second optical member 10B is 2.50. Therefore, d1 / d2 is 0.040. Therefore, the objective lens 10 of Example 4 also satisfies the condition (10).

  The objective lens 10 of Example 5 has a configuration in which a sixth region having a sixth phase shift structure is formed on the first surface 11 based on the configuration of Example 4. Specific specifications for each laser beam of the objective lens 10 of Example 6 are shown in Table 25. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 25 when the optical discs D1 to D3 are used are shown in Tables 33 to 36.

  In Tables 33 to 36, the surface number 0 is the light source 1A to 1C, the surface number 1 is the first surface 11 of the objective lens 10, the surface number 2 is the cemented surface 13 of the objective lens 10, and the surface number 3 is the first surface of the objective lens 10. Two surfaces 12 and surface numbers 4 and 5 respectively represent the protective layer 21 and the recording surface 22 in each of the optical disks D1 to D3. However, since the first surface 11 of the objective lens 10 of Example 4 has the third, fourth, and sixth regions as described above, they are shown separately and independently.

  The first surface 11, the cemented surface 13, and the second surface 12 (surface numbers 1, 2, and 3 in order) of the objective lens 10 are all aspherical surfaces. Table 37 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 to 13.

In the objective lens 10 of Example 5, a phase shift structure (corresponding to the first phase shift structure described above) is provided on the entire surface of the cemented surface 13. Further, as described above, in the first surface, the second phase shift structure is provided in the region including the optical axis (third region), and the fourth region located outside the third region is provided in the fourth region. A sixth phase shift structure is provided in each of the sixth regions where the phase shift structure is located further outside the fourth region. The coefficient P _2i in the optical path difference function for defining each phase shift structure is shown in Table 38. Table 39 shows the diffraction order m that maximizes the diffraction efficiency of each laser beam and the effective radius (unit: mm) of each surface (each region for the first surface 11).

  From the above tables, Φ1 = 0.072, Φ2 = 0.067, and Φ3 = 0.081. Therefore, in the objective lens 10 of Example 5, Φ2 / Φ1 = 0.931, | (Φ3-Φ2) /Φ1|=0.194, and the conditions (1), (2), (4) to (7) Meet.

  Table 40 shows the diffraction efficiencies of the laser beams when the optical disks D1 to D3 are used in the objective lens of Example 5 and the conventional single lens (Comparative Example 2).

  As shown in Table 40, the diffraction efficiency when using the optical discs D2 and D3 is higher in Example 5 than in Comparative Example 2. That is, according to the fourth embodiment, high light utilization efficiency can be maintained regardless of which optical disk is used.

  FIGS. 9A to 9C are aberration diagrams showing spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 having the objective lens 10 according to the fifth embodiment. is there.

  As shown by the solid lines in FIGS. 9A to 9C, when the objective lens 10 of Example 5 is used, spherical aberration is caused even when information is recorded or reproduced on any of the optical disks D1 to D3. It can be seen that the laser beam having the correct design wavelength is well converged on the recording surface 22 of each optical disk. 9A to 9C, when the objective lens 10 of Example 5 is used, the laser light emitted from each of the light sources 1A to 1C has a slight wavelength shift. Even so, it can be said that the spherical aberration caused by the wavelength shift is small and a good convergence state is maintained.

  Further, as described above, the objective lens 10 of Example 5 has the fourth region and the sixth region on the first surface 11. The fourth region is designed such that the diffraction order that maximizes the diffraction efficiency for the first laser beam is the fifth order. As a result, the third laser light transmitted through the fourth region does not contribute to convergence. In addition, the objective lens of Example 5 satisfies the condition (9). Therefore, the sixth region is designed so that the number of diffraction characters that maximizes the diffraction efficiency with respect to the second laser light is first order. Thereby, the first laser light transmitted through the sixth region does not contribute to convergence. Thus, it can be said that the objective lens 10 of Example 5 has a preferable aperture limiting function.

  As shown in Tables 26 to 28, the center thickness d1 of the first optical member 10A is 0.10, and the center thickness d2 of the second optical member 10B is 2.50. Therefore, d1 / d2 is 0.040. Therefore, the objective lens 10 of Example 4 also satisfies the condition (10).

  The above is description of embodiment of this invention and the specific Example of this embodiment. The objective lens according to the present invention is not limited to the configuration of the above embodiment. For example, by dividing the bonding surface into two types of regions (first region and second region) having mutually different phase shift structures (first phase shift structure and third phase shift structure), It is possible to provide an aperture limiting function for the three laser beams.

It is a schematic diagram showing schematic structure of the optical information recording / reproducing apparatus of embodiment of this invention. 1 is a diagram showing an optical information recording / reproducing apparatus according to an embodiment of the present invention separately for each optical path when each optical disc is used. FIG. It is an enlarged view of the objective lens of embodiment of this invention. It is an enlarged view of the objective lens of embodiment of this invention. FIG. 6 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams are used in the objective lens according to the first exemplary embodiment. FIG. 10 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams are used in the objective lens according to Example 2; FIG. 10 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams are used in the objective lens according to Example 3; FIG. 10 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams are used in the objective lens according to Example 4; FIG. 10 is an aberration diagram illustrating spherical aberration that occurs when the first to third laser beams are used in the objective lens according to Example 5;

Explanation of symbols

1A, 2A, 3A Light source 10 Objective lens 13 Joint surface D1 to D3 Optical disc 100 Optical information recording / reproducing apparatus

Claims (20)

Optical information recording / reproduction for recording / reproducing information on / from each optical disc by using three types of substantially parallel light beams having first to third wavelengths in order for the first to third optical discs having different recording densities. An objective lens used in the apparatus,
When the first to third wavelengths are λ1, λ2, and λ3 (each unit: nm),
λ1 <λ2 <λ3
And
The thickness of the protective layer of the first optical disk on which information is recorded or reproduced using the light beam of the first wavelength is t1 (unit: mm), and information is recorded or reproduced using the light beam of the second wavelength. The thickness of the protective layer of the second optical disc to be performed is t2 (unit: mm), and the thickness of the protective layer of the third optical disc on which information is recorded or reproduced using the light beam of the third wavelength is t3 (unit: mm). )
t1 ≦ t2 <t3
And
The numerical aperture required for recording or reproducing information on the first optical disc is NA1, the numerical aperture required for recording or reproducing information on the second optical disc is NA2, and the information is recorded or reproduced on the third optical disc. If the required numerical aperture is NA3,
NA1> NA3 and NA2> NA3
And
The objective lens is a cemented lens in which a first optical member and a second optical member made of different materials are joined at a joint surface,
The joint surface has at least a first phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
The first phase shift structure has m (λ1), m (λ2), and m (λ3) in order as diffraction orders at which diffraction efficiency is maximized in each light flux from the first wavelength to the third wavelength, The refractive index of the first optical member for each light flux from the first wavelength to the third wavelength is set to n1 (λ1), n1 (λ2), n1 (λ3) in order, and the first wavelength to the third wavelength. Assuming that the refractive index of the second optical member with respect to each light beam is n2 (λ1), n2 (λ2), and n2 (λ3) in order, the following two conditions (1), (2),
0.85 <Φ2 / Φ1 <1.15 (1)
0.10 <| (Φ3-Φ2) / Φ1 | <0.50 (2)
However, Φ1 = m (λ1) × (λ1 / (n2 (λ1) −n1 (λ1)))
Φ2 = m (λ2) × (λ2 / (n2 (λ2) −n1 (λ2)))
Φ3 = m (λ3) × (λ3 / (n2 (λ3) −n1 (λ3)))
An objective lens for an optical information recording / reproducing apparatus, characterized in that: The objective lens for an optical information recording / reproducing apparatus according to claim 1,
The first phase shift structure further includes the following two conditions (3), (4),
1.00 ≦ Φ2 / Φ1 <1.15 (3)
0.20 <| (Φ3-Φ2) / Φ1 | <0.50 (4)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: The objective lens for an optical information recording / reproducing apparatus according to claim 1 or 2,
The first phase shift structure further includes the following condition (5):
0.20 <| (Φ3-Φ2) / Φ1 | <0.40 (5)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: The objective lens for an optical information recording / reproducing apparatus according to claim 1,
The first phase shift structure further includes the following two conditions (6), (7),
0.85 <Φ2 / Φ1 <1.00 (6)
0.20 <| (Φ3-Φ2) / Φ1 | <0.35 (7)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: In the objective lens for optical information recording / reproducing apparatuses according to any one of claims 1 to 4,
Either one of the two surfaces other than the joint surface has a second phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
In the optical information recording / reproducing apparatus, the second phase shift structure has an optical path length applied to the light beam having the first wavelength on the refracting surfaces adjacent to each other for approximately two wavelengths. Objective lens. In the objective lens for optical information recording / reproducing apparatuses according to any one of claims 1 to 4,
Either one of the two surfaces other than the joint surface has a second phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
In the optical information recording / reproducing apparatus, the second phase shift structure has an optical path length of about 10 wavelengths applied to the light flux having the first wavelength on the refracting surfaces adjacent to each other. Objective lens. The objective lens for an optical information recording / reproducing apparatus according to any one of claims 1 to 6,
The first phase shift structure is disposed in a first region including an optical axis for converging at least the light beam having the third wavelength on the recording surface of the third optical disc. Objective lens for information recording / reproducing device. The objective lens for an optical information recording / reproducing apparatus according to claim 7,
The joining surface converges the light flux of the first wavelength and the light flux of the second wavelength on the recording surfaces of the first optical disc and the second optical disc, respectively, outside the first region. And having a second region that does not contribute to the convergence of the light flux of the third wavelength,
The objective lens for optical information recording / reproducing, wherein the second region is configured as a refractive surface. The objective lens for an optical information recording / reproducing apparatus according to claim 7,
The joining surface converges the light flux of the first wavelength and the light flux of the second wavelength on the recording surfaces of the first optical disc and the second optical disc, respectively, outside the first region. And having a second region that does not contribute to the convergence of the light flux of the third wavelength,
The optical information recording / reproducing objective lens, wherein the second region has a third phase shift structure composed of a plurality of concentric and continuously refracting surfaces. The objective lens for an optical information recording / reproducing apparatus according to any one of claims 5 to 9,
The second phase shift structure is disposed in a third region including an optical axis for converging at least the light beam having the third wavelength on the recording surface of the third optical disc. Objective lens for information recording / reproducing device. The objective lens for optical information recording / reproducing according to claim 10,
Either one of the two surfaces other than the bonding surface has a fourth region outside the third region,
The fourth region has a fourth phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
The optical information recording / reproducing objective lens according to the fourth phase shift structure, wherein an optical path length given to the light flux having the first wavelength is approximately three wavelengths. The objective lens for an optical information recording / reproducing apparatus according to claim 10,
Either one of the two surfaces other than the bonding surface has a fourth region outside the third region,
The fourth region has a fourth phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
The optical information recording / reproducing objective lens according to the fourth phase shift structure, wherein an optical path length given to the light beam having the first wavelength is about five wavelengths. In the objective lens for optical information recording / reproducing apparatus according to claim 8 or 9,
The following condition (8),
f1 × NA1> f2 × NA2 (8)
The filling,
The joint surface is configured to focus only the light beam having the first wavelength on each recording surface of the first optical disc and to converge the light beams having the second and third wavelengths outside the second region. An optical information recording / reproducing objective lens, characterized in that it has a fifth region which does not contribute to. In the objective lens for optical information recording / reproducing apparatus according to claim 8 or 9,
The following condition (9),
f1 × NA1 <f2 × NA2 (9)
The filling,
The joint surface is configured to converge only the light beam having the second wavelength on each recording surface of the second optical disc outside the second region, and converge the light beam having the first and third wavelengths. An optical information recording / reproducing objective lens, characterized in that it has a fifth region which does not contribute to. The objective lens for optical information recording / reproducing according to claim 13 or 14,
The fifth region has a fifth phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
An optical information recording / reproducing objective characterized in that the absolute value of the optical path length difference provided by the fifth phase shift structure is different from the absolute value of the optical path length difference provided by the step of the second region. lens. The objective lens for an optical information recording / reproducing apparatus according to claim 11 or 12,
The following condition (8),
f1 × NA1> f2 × NA2 (8)
The filling,
Either one of the two surfaces other than the bonding surface is configured to converge only the light beam having the first wavelength on each recording surface of the first optical disc outside the fourth region, and Having a sixth region that does not contribute to the convergence of the light fluxes of the second and third wavelengths;
The sixth region has a sixth phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
An optical information recording / reproducing objective characterized in that the absolute value of the optical path length difference provided by the sixth phase shift structure is different from the absolute value of the optical path length difference provided by the step of the fourth region. lens. The objective lens for an optical information recording / reproducing apparatus according to claim 11 or 12,
The following condition (9),
f1 × NA1 <f2 × NA2 (9)
The filling,
Either one of the two surfaces other than the bonding surface is configured to focus only the light beam having the second wavelength on the recording surface of the second optical disc outside the fourth region, and Having a sixth region that does not contribute to the convergence of the luminous flux of the first and third wavelengths,
The sixth region has a sixth phase shift structure composed of a plurality of concentric and continuously divided refractive surfaces,
An optical information recording / reproducing objective characterized in that the absolute value of the optical path length difference provided by the sixth phase shift structure is different from the absolute value of the optical path length difference provided by the step of the fourth region. lens. The objective lens for an optical information recording / reproducing apparatus according to any one of claims 1 to 17,
When the center thickness of the first optical member is d1 (unit: mm) and the center thickness of the second optical member is d2 (unit: mm), the following condition (10):
0.01 <d1 / d2 <0.20 (10)
An optical information recording / reproducing objective lens characterized by satisfying A light source that irradiates three types of substantially parallel light beams having different first to third wavelengths;
An objective lens for an optical information recording / reproducing apparatus according to any one of claims 1 to 18,
An optical information recording / reproducing apparatus which records or reproduces information on each optical disk by using any one of the three types of substantially parallel light beams for each of a plurality of optical disks having different recording densities. The optical information recording / reproducing apparatus according to claim 19,
The protective layer thickness t1 of the first optical disc, the protective layer thickness t2 of the second optical disc, and the third protective layer thickness t3 are respectively
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
An optical information recording / reproducing apparatus.

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