JP4443501B2 - Objective lens for optical information recording / reproducing device - Google Patents

Description

  The present invention provides, for example, an objective 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 disks having different recording densities and protective layer thicknesses. Related to lenses.

  Conventionally, there are a plurality of standards for optical disks, such as CD and DVD, which have different recording densities and protective layer thicknesses. 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 strictly, an objective optical system provided in the device has been used for the above three types of optical discs. And compatibility is required. 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 a plurality of optical discs with different standards, first, information is recorded or reproduced while correcting spherical aberration that changes depending on the thickness of the protective layer when switching between optical discs with different standards. Therefore, it is necessary to change the numerical aperture (NA) of the light used to obtain a beam spot corresponding to the difference in recording density. Generally, the spot diameter is 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 the wavelength used when recording or reproducing information on a DVD (for example, so-called blue laser light per 408 nm) is used because of the high recording density.

  Next, as one means for converging each optical disc to the recording surface position of each optical disc in a good state, the optical disc is rotated on an arbitrary surface of one or more optical elements (for example, objective lens) constituting the objective optical system. A technology has been put into practical use in which a ring-shaped structure having a band-shaped fine step is provided and light of different wavelengths is converged well on the recording surface of the corresponding optical disk by the action of the ring-shaped structure.

  In addition, the optical element preferably has an action of correcting spherical aberration that occurs when the wavelength of the laser beam to be used deviates from the design wavelength due to environmental differences such as individual differences of light sources and temperature changes. The design wavelength means the wavelength of each laser beam that is optimal for recording or reproducing information on each optical disc.

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

JP 2004-362626 A

  In order to provide compatibility with three types of optical discs having different recording densities, the objective lens described in Patent Document 1 is a ring zone for two types of light beams having different wavelengths. The spherical aberration is corrected by the structure, and for the remaining one wavelength, the spherical aberration is corrected by controlling the divergence of the light beam incident on the objective lens.

  Here, when using an HD DVD or DVD, which requires a high NA when recording or reproducing information, the tolerance for aberration is low. For this reason, in Patent Document 1, a substantially parallel light beam is incident on the objective lens when information is recorded or reproduced on the HD DVD or DVD having a relatively high recording density. When a CD that does not require a very high NA when recording or reproducing information is used, the diverging light is incident on the objective lens. Thus, an objective lens having compatibility with three types of optical disks having different recording densities is provided.

  However, when divergent light is incident on the objective lens, aberration such as coma aberration occurs during tracking. Therefore, there is a demand for an objective lens that can satisfactorily suppress these aberrations even when an optical disk such as a CD having a low tolerance for aberrations is used.

  In view of the circumstances described above, the present invention provides a recording surface of each optical disc even when information is recorded or reproduced on any of the three types of optical discs having different standards using a plurality of types of light beams having different wavelengths. It is an object of the present invention to provide an objective lens for an optical information recording / reproducing apparatus that can suppress spherical aberration to form a good spot and suppress occurrence of aberration even during tracking.

In order to solve the above-described problem, the objective lens for an optical information recording / reproducing apparatus of the present invention uses three types of substantially parallel light beams having first to third wavelengths for a plurality of optical disks having different recording densities. An objective lens used in an optical information recording / reproducing apparatus for recording or reproducing information with respect to each optical disc, wherein the first wavelength, which is the shortest of the first to third wavelengths, is λ1, which is greater than the first wavelength. If the long second wavelength is λ2 and the longest third wavelength is λ3,
λ1 <λ2 <λ3
1.9 <λ3 / λ1 <2.1
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, and the thickness of the second optical disk on which information is recorded or reproduced using the light beam of the second wavelength If the protective layer thickness is t2, and the protective layer thickness of the third optical disc on which information is recorded or reproduced using a light beam of the third wavelength is t3,
t1 ≦ t2 <t3
And
The numerical aperture necessary for recording or reproducing information on the first optical disk is NA1, the numerical aperture necessary for recording or reproducing information on the second optical disk is NA2, and the numerical aperture necessary for recording or reproducing information on the third optical disk. If the number is NA3,
NA1> NA3 and NA2> NA3
And having a phase shift structure composed of a plurality of concentric refracting surfaces on at least one surface, and the phase shift structure allows a third wavelength light beam to be projected onto the recording surface of the third optical disc. The first region has a first step that converges, and the first region has a step that provides at least two types of optical path length change amounts having different absolute values with respect to the light beam having the first wavelength at the boundary between adjacent refractive surfaces. The absolute values of the at least two kinds of optical path length variation amounts are (i _A + Δ _A ) times and (i _B + Δ _B ) times (i _A + Δ _B ) times (one i, _B is a natural number and i _A ≠ i _B , −0.5 <Δ _A <0.5, −0.5 <Δ _B <0.5), and i _A is i _A = 2k + 1 (provided that , K is a natural number), and the diffraction order at which the diffraction efficiency is maximum in the light flux of the third wavelength is k + 1) is the next, delta _A the following conditions (1),
0.000 ≦ Δ _A ≦ 0.384 (1)
It is characterized by satisfying.

According to the objective lens for an optical information recording / reproducing apparatus according to claim 1, at least one kind of optical path length variation i _A among the optical path length variations provided by the step in the first region is an odd value. By configuring in this way, it is possible to provide high compatibility with three types of optical discs having different standards. In addition, since almost parallel light beams are used when any of the optical disks is used, the occurrence of aberration can be suppressed during tracking shift, and a spot suitable for recording or reproducing information can be formed.

Further, i _A desirably satisfies the following condition (2), more preferably condition (3).
0.020 ≦ Δ _A ≦ 0.324 (2) (Claim 2)
0.020 ≦ Δ _A ≦ 0.258 (3) (Claim 3)

Further, i _A preferably satisfies the following condition (4).
0.020 ≦ Δ _A ≦ 0.178 (4) (Claim 4)
As a result, the utilization efficiency of the light beam having the first wavelength can be set very high when information is recorded or reproduced on the first optical disk having the highest recording density.

The objective lens for an optical information recording / reproducing apparatus according to claim 5 uses three types of substantially parallel light beams having first to third wavelengths for a plurality of optical discs having different recording densities. An objective lens used in an optical information recording / reproducing apparatus for recording or reproducing information with respect to λ1, the shortest first wavelength among the first to third wavelengths being λ1, the second being longer than the first wavelength If the wavelength λ2 and the longest third wavelength is λ3,
λ1 <λ2 <λ3
1.9 <λ3 / λ1 <2.1
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, and the thickness of the second optical disk on which information is recorded or reproduced using the light beam of the second wavelength If the protective layer thickness is t2, and the protective layer thickness of the third optical disc on which information is recorded or reproduced using a light beam of the third wavelength is t3,
t1 ≦ t2 <t3
And
The numerical aperture necessary for recording or reproducing information on the first optical disk is NA1, the numerical aperture necessary for recording or reproducing information on the second optical disk is NA2, and the numerical aperture necessary for recording or reproducing information on the third optical disk. If the number is NA3,
NA1> NA3 and NA2> NA3
And
At least one surface has a phase shift structure composed of a plurality of concentric refracting surfaces, and the phase shift structure converges the light beam having the third wavelength on the recording surface of the third optical disk. The first region has a step that provides at least two types of optical path length variation amounts having different absolute values with respect to the light beam having the first wavelength at the boundary between adjacent refracting surfaces; The two types of optical path length variation are (i _A + Δ _A ) times the light flux of the first wavelength and (i _B + Δ _B ) (where i _A and i _B are natural numbers and i _A ≠ i _B , −0.5 <Δ _A <0.5, −0.5 <Δ _B <0.5), i _A is represented by i _A = 2k + 1 (where k is a natural number), and wherein the diffraction order at which the diffraction efficiency is maximized in the light flux of the third wavelength is k-th order, delta _a the following conditions ( ),
−0.384 ≦ Δ _A ≦ −0.070 (5)
It is characterized by satisfying.

Further delta _A preferably satisfies more preferably the condition (7) the following condition (6).
−0.324 ≦ Δ _A ≦ −0.070 (6) (Claim 6)
−0.258 ≦ Δ _A ≦ −0.070 (7) (Claim 7)

Further delta _A preferably satisfies the following condition (8).
−0.178 ≦ Δ _A ≦ −0.070 (8) (Claim 8)

For example, when there are two types of optical path length addition amount provided by the step of the first region, the combination of i _A and i _B of each step is 3 and 2 (Claim 9) or 5 and 2 (Claim). 10) is preferred. Further, when there are three types of optical path length addition amount provided by the step in the first region, the combination of i _A , i _B , and i _{C in} each step is 3, 2, 8 (Claim 11), or 3, 2, 10 (Claim 12), 5, 2, 8 (Claim 13), or 5, 2, 10 (Claim 14) are suitable.

  The objective lens for an optical information recording / reproducing apparatus according to the present invention is configured as a single lens.

  According to the objective lens for an optical information recording / reproducing apparatus according to claim 16, the phase shift structure has a first wavelength light beam and a second wavelength light beam outside the first region, respectively, A second region that converges on the recording surface of the second optical disk and does not contribute to the convergence of the light beam of the third wavelength; On the other hand, there is a step for providing at least one type of optical path length variation, and the absolute value of one type of optical path length variation is different from the absolute value of at least one type of optical path length variation in the first region.

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 second region. It is desirable to provide a third region that converges only the light beam and does not contribute to the convergence of the light beams of the first and third wavelengths. The third region has a step having at least one type of optical path length variation with respect to the second light flux at the boundary between adjacent refractive surfaces, and the absolute value of the optical path length variation is It differs from the absolute value of the optical path length variation in the two areas.
f1 × NA1 <f2 × NA2 (9)
However, f1 is the focal length when using the first optical disc,
f2 represents the focal length when the second optical disc is used.

According to the objective lens for an optical information recording / reproducing apparatus according to claim 18, when the following condition (10) is satisfied, the phase shift structure has a first wavelength outside the second region. It is desirable to provide a third 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 third region has a step having at least one kind of optical path length variation with respect to the first light flux at a boundary between adjacent refracting surfaces, and the absolute value of the optical path length variation is a second value. It differs from the absolute value of the optical path length change amount in the region.
f1 × NA1> f2 × NA2 (10)

  As described above, according to the present invention, by forming a predetermined phase shift structure on at least one surface, aberrations that occur on the recording surface of each optical disk are satisfactorily suppressed. According to the present invention, a substantially parallel light beam is used 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. That is, an optical information recording / reproducing objective lens capable of forming good spots on the recording surfaces of three types of optical discs having different recording densities and an optical information recording / reproducing apparatus equipped with the objective lens are provided.

  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. If the first wavelength is λ1 and the third wavelength is λ3,
1.9 <λ3 / λ1 <2.1
There is a relationship.

  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 that emits a first laser beam, a light source 2A that emits a second laser beam, a light source 3A that emits a third laser beam, coupling lenses 1B, 2B, 3B, Beam splitters 41 and 42, a half mirror 43, and a light receiving unit 44 are included. 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 light may be disposed between the light source 3A and the objective lens 10.

  As shown in FIG. 1, the first to third laser light beams emitted from the light sources 1A to 3A are converted into parallel light beams by the coupling lenses 1B to 3B. That is, in this embodiment, each coupling lens 1B-3B functions as a collimating lens. The laser beams transmitted through the coupling lenses 1B to 3B 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 is deflected by the half mirror 43 and detected by the light receiving unit 44.

  As described above, by making each laser beam incident on the objective lens 10 into a parallel light beam, it is possible to suppress the occurrence of aberrations such as coma aberration when the objective lens 10 is tracked.

  Strictly speaking, the light beams emitted from the coupling lenses 1B to 3B due to individual differences and installation positions of the light sources 1A to 3A, 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.

The objective lens 10 has a first surface 11 and a second surface 12 in order from the light source side. As shown in FIGS. 2A to 2C, the objective lens 10 is a biconvex plastic single lens in which the surfaces 11 and 12 are both aspherical. 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.

  Each of the optical disks D1 to D3 has a protective layer 21 and a recording surface 22 respectively. 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).

  As in the optical information recording / reproducing apparatus 100, when laser beams having different wavelengths are used when the optical disks D1 to D3 are used, the refractive index of the objective lens changes or the protective layer 21 of the optical disks D1 to D3 depends on the use of the optical disks. Due to the difference in thickness, the spherical aberration changes.

  Accordingly, in order to correct the spherical aberration generated when using the three types of optical discs D1 to D3 and to make the optical discs D1 to D3 compatible with each other, the objective lens 10 of the present embodiment has three types of light beams. A phase shift structure having a diffraction effect that affects the first surface 11 is provided on the first surface 11. The phase shift structure provided on the first surface 11 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 phase shift structure of the present embodiment has a diffractive action for controlling the spherical aberration due to the difference in the used wavelength in each of the first to third laser beams to be substantially zero. The diffractive action is such that each laser beam transmitted through the objective lens 10 can suppress the spherical aberration well on the recording surface 22 of the corresponding optical disc and form a spot suitable for recording or reproducing information. In other words, it can be said to be a three-wavelength compatible action.

  The objective lens 10 having the above phase shift structure is designed as follows, for example. First, at least two types of optical path difference functions, for example, a first optical path difference function and a second optical path difference function, which are different from each other in the ratio of the diffraction orders at which the diffraction efficiency becomes the maximum in each of the first to third laser light beams are calculated. .

  The optical path difference function represents the function of the objective lens 10 as a diffraction 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), P _2i (where i is an integer of 1 or more) is a second-order, fourth-order, sixth-order,... Coefficient. 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.

  Next, the shape of the phase shift structure is obtained by superimposing the calculated optical path difference functions. In this case, the ratios are different from each other as long as the diffraction orders at which the diffraction efficiency is maximum in at least one of the light beams to be used are different. Therefore, for example, when there are three types of light fluxes to be used, the ratio is different if the ratio in the first optical path difference function is 3: 2: 2 and the ratio in the second optical path difference function is 3: 2: 1. I can say that. In addition, although the combinations of orders in each optical path difference function are different, the ratio of the diffraction orders is the same, for example, the ratio in the first optical path difference function is 2: 1: 1, and the ratio in the second optical path difference function is 4: In the case of 2: 2, the diffractive action given by each optical path difference function is the same. Therefore, in the present invention, it is a requirement to define each optical path difference function that the combination of the orders is not different but the ratio of the diffraction orders is different.

As described above, the steps formed in the phase shift structure obtained based on two different optical path difference functions are two types of optical path length changes in which the absolute value of the optical path length change amount differs from that of the first laser beam. Bring. Note that the absolute value of the optical path length variation is different because the optical path length variation in the direction from the objective lens 10 toward each optical disk is defined as positive and the reverse direction is defined as negative. It is clarified that the amount of change in optical path length is not different. Here, two kinds of optical path length change amounts (unit: λ) are defined as (i _A + Δ _A ) times and (i _B + Δ _B ) times as much as one wavelength of the first laser beam. However, i _A and i _B are natural numbers different from each other, and in particular, i _A is represented by 2k + 1 (k is a natural number). Each Note delta _A and delta _B are, -0.5 <Δ _A <0.5, as defined by the scope of -0.5 <Δ _B <0.5. Δ _A satisfies the following condition (1) when the diffraction order at which the diffraction efficiency is highest in the third laser beam is set to (k + 1) th order, and satisfies the following condition (1), and when the diffraction order is set to kth order, Satisfy (5).
0.000 ≦ Δ _A ≦ 0.384 (1)
−0.384 ≦ Δ _A ≦ −0.070 (5)

  If the upper limit of condition (1) is exceeded or the lower limit of condition (5) is exceeded, the light utilization efficiency of the third laser light can be further increased, but the light utilization efficiency of the first laser light is particularly lowered. Therefore, it is not preferable. Further, exceeding the lower limit of the condition (1) or exceeding the upper limit of the condition (5) is not preferable because the light utilization efficiency of the third laser beam is particularly lowered.

  FIG. 3 is an enlarged view of the phase shift structure provided on the first surface 11. In FIG. 3, j indicates the jth refracting surface from the optical axis AX in the phase shift structure. Further, hj indicated by a two-dot chain line is the position of the step s formed between the jth refracting surface and the (j-1) th refracting surface, in other words, the boundary between the jth refracting surface and the (j-1) th refracting surface. Indicates the position. Here, the amount of change in the optical path length is, as shown in FIG. 3, an imaginary extension surface (AA ′ surface) obtained by extending the shape of the (j−1) th refracting surface in the direction away from the optical axis. The optical path length when evaluated up to the image plane obtained when refracting at the boundary position hj, and a virtual extended surface (BB ′ surface) obtained by extending the shape of the j-th refractive surface in the direction toward the optical axis Means the difference in optical path length when evaluated up to the image plane obtained when refracting at the boundary position hj.

  Note that, as shown in FIG. 3, the actual phase shift structure may be rounded because the vicinity of the boundary position (edge) of each refractive surface is bent at the time of formation. However, such a change in the shape of the edge has no problem in practical use.

4 to 7 show the objective lens 10 when i _A , i _B ,... (Collectively written as i) and Δ _A , Δ _B ,... (Collectively written as Δ) are variously set. It is a graph shown about the light utilization efficiency of each 1st-3rd laser beam inject | emitted from. FIG. 4 is a graph showing the light use efficiency of i = 2, that is, the second-order diffracted light of the first laser light, and the first-order diffracted light of the second and third laser lights. FIG. 5 is a graph showing the light utilization efficiency of i = 3, that is, the third-order diffracted light of the first laser light, the second-order diffracted light of the second laser light, the second-order diffracted light of the third laser light, and the first-order diffracted light. FIG. 6 is a graph showing the light utilization efficiency of i = 5, that is, the fifth-order diffracted light of the first laser light, the third-order diffracted light of the second laser light, the third-order diffracted light of the third laser light, and the second-order diffracted light. FIG. 7 is a graph showing the light utilization efficiency of i = 10, that is, the 10th-order diffracted light of the first laser light, the 6th-order diffracted light of the second laser light, and the fifth-order diffracted light of the third laser light.

  In each graph, the vertical axis indicates light utilization efficiency, and the horizontal axis indicates Δ. In each graph, the solid line is the first laser beam assuming the design wavelength of 408 nm, the broken line is the second laser beam assuming the design wavelength of 660 nm, and the one-dot chain line (and the two-dot chain line) is the design wavelength of 790 nm. Each assumed third laser beam is shown.

  As shown in FIGS. 4 to 7, the light utilization efficiency of the i-th order diffracted light of the first laser light becomes higher as the value of Δ approaches 0. If Δ is set to 0, the light utilization efficiency of the i-th order diffracted light of the first laser light is approximately 100%.

  On the other hand, high light utilization efficiency is not always obtained with other laser beams as long as Δ approaches 0. Generally, when recording or reproducing information on the first optical disc D1, a larger amount of light is required than when other optical discs are used. Therefore, it is more preferable that the light utilization efficiency is as high as possible in the second and third laser lights while maintaining the light utilization efficiency of the first laser light high.

Therefore, the diffraction efficiency in the third laser beam is set becomes highest diffraction orders the (k + 1) then, delta _A is at least the above conditions (1), further, the following condition (2) to (4) Is set to satisfy at least one of the following.
0.020 ≦ Δ _A ≦ 0.324 (2)
0.020 ≦ Δ _A ≦ 0.258 (3)
0.020 ≦ Δ _A ≦ 0.178 (4)

Further, when the diffraction order diffraction efficiency becomes highest in the third laser beam k then set, delta _A is at least the above conditions (5), furthermore, at least the following conditions (6) - (8) It is set to satisfy one.
−0.324 ≦ Δ _A ≦ −0.070 (6)
−0.258 ≦ Δ _A ≦ −0.070 (7)
−0.178 ≦ Δ _A ≦ −0.070 (8)

By delta _A satisfies the above condition (1), as shown in FIGS. 4 to 7, regardless of i, that light utilization efficiency of the first laser beam to ensure minimum of about 60% it can. In particular, as shown in FIG. 5 and FIG. 6, when i _A takes an odd value such that i _A = 2k + 1, the third laser beam has different diffraction orders with a light utilization efficiency of about 40%. Two types occur. Therefore, the diffraction order diffraction efficiency becomes highest in the third laser beam (k + 1) Next, the range of the condition (1) a delta _A, by setting respectively, the light utilization efficiency of the third laser beam Can be secured at least about 50%. Furthermore, it can be seen that if i = 3, the light utilization efficiency of the second laser light is approximately 100%.

By delta _A satisfies the condition (2), as shown in FIGS. 4 to 7, regardless of i, can be a light use efficiency of the first laser beam to ensure about 70%.

By delta _A satisfies the condition (3), as shown in FIGS. 4 to 7, regardless of i, can be a light use efficiency of the first laser beam to ensure about 80%.

By delta _A satisfies the condition (4), as shown in FIGS. 4 to 7, regardless of i, can be a light use efficiency of the first laser beam to ensure about 90%.

Further, when i _A is represented by i _A = 2k + 1, the diffraction order that gives the highest diffraction efficiency in the third laser light is set to k order, and Δ _A is set within the range of the condition (5). As a result, the light utilization efficiency of the third laser light can be secured at least about 50%.

By delta _A satisfies the condition (6), as shown in FIGS. 4 to 7, regardless of i, can be a light use efficiency of the first laser beam to ensure about 70%.

By delta _A satisfies the condition (7), as shown in FIGS. 4 to 7, regardless of i, can be a light use efficiency of the first laser beam to ensure about 80%.

By delta _A satisfies the condition (8), as shown in FIGS. 4 to 7, regardless of i, can be a light use efficiency of the first laser beam to ensure about 90%.

In addition, in order to give a three-wavelength compatible action to the phase shift structure, one of the two types of optical path length variation amounts i _A is set to an odd value. This makes it possible to satisfactorily correct spherical aberration that occurs when the third laser light is used, while maintaining high light utilization efficiency of the first laser light. By making the other optical path length variation i _B an even value, it is possible to increase both the light utilization efficiency of the first laser beam and the third laser beam. That is, the three-wavelength compatibility effect is imparted to the phase shift structure. Specifically, the objective lens 10 receives a parallel light beam having a design wavelength corresponding to each of the optical discs D1 to D3, and suppresses spherical aberration on the recording surface 22 of each of the optical discs D1 to D3. Spots can be formed.

  The above three-wavelength compatibility action is obtained by the interaction between the first optical path difference function and the second optical path difference function. For example, one optical path difference function is related to two specific types of laser light. It does not give compatibility.

  The objective lens 10 of the present embodiment is formed with a step (hereinafter referred to as a special step for convenience) in which the optical path length variation with respect to the first laser beam appears as the sum or difference of the two types of optical path length variations. There is also a case.

  Further, for example, it is assumed that the laser light used when using the first optical disc D1 slightly changes from the first wavelength, which is the design wavelength for the optical disc D1, due to individual differences in light sources or changes with time. The same applies when other optical disks D1 to D3 are used. Thus, spherical aberration occurs when the wavelength of the laser beam used deviates from the design wavelength.

  Therefore, in the objective lens 10 of the present embodiment, the phase shift structure provided on the first surface 11 causes the wavelength of the light beam used when using each optical disk to slightly deviate from the design wavelength due to temperature changes and individual differences. A diffraction effect that suppresses spherical aberration can also be imparted. Even if the wavelength of each laser beam incident on the objective lens 10 deviates from the design wavelength, the diffraction action is not affected by the wavelength deviation, and information is recorded on the recording surface 22 of the corresponding optical disc. Or, it is a diffraction action that forms a spot suitable for reproduction, that is, a wavelength shift compensation action.

  In the case where the phase shift structure is also provided with a diffractive action for suppressing spherical aberration due to a slight shift in wavelength, it is necessary to further increase the degree of freedom in designing the phase shift structure. For this purpose, in addition to the first and second optical path difference functions described above, a third optical path difference function is also calculated, and the three types of optical path difference functions are superimposed to provide a three-wavelength compatibility action and a wavelength shift compensation action. The shape of the phase shift structure having both of the above is obtained. Therefore, the amount of change in the optical path length given to the first laser beam by the formed step becomes three types. Here, the three-wavelength compatibility action and the wavelength shift compensation action are obtained by the first to third optical path difference functions interacting with each other, and any one of the optical path difference functions is only for a specific action. It does not contribute.

The third type of optical path length variation (unit: λ) is defined as (i _C + Δ _C ) times the wavelength of the first laser beam. However, i _C is a natural number different from i _A and i _B. The delta _C is defined in the range of -0.5 <Δ _C <0.5.

  The phase shift structure described above does not necessarily have to be provided over the entire area of the first surface 11, and is the innermost area including the optical axis and contributes to the convergence of the third laser beam, that is, any laser beam. As long as it is provided in a region that contributes to the convergence of the first (hereinafter referred to as the first region).

  The objective lens 10 of the present embodiment has a second region outside the first region, and in some cases outside the second region, due to the difference in effective beam diameter for ensuring the NA necessary for recording or reproducing information. Furthermore, a third region is provided.

  The phase shift structure of the second region causes the first and second laser beams, which are generally considered to have a larger effective light beam diameter than the third laser beam, to converge well on the recording surfaces 22 of the corresponding optical disks D1 and D2. Because of its dual wavelength compatibility. The phase shift structure in the second region also has a wavelength shift compensation function for the first laser light and the second laser light as necessary. When the wavelength shift compensation function is also given to the phase shift structure of the second region, the phase shift structure is formed by calculating and superposing two kinds of optical path difference functions as in the first region.

  The phase shift structure of the second region has a level difference that does not contribute to the convergence of the third laser beam. That is, at least one type of the optical path length change amount in the step of the second region is different from at least one type of optical path length change amount in the first region.

  In addition, when the phase shift structure of the second region is formed using two kinds of optical path difference functions, a special step may be formed also in the second region.

  The third region is provided 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.

First, when the focal length when using the first optical disc D1 is f1, and the focal length when using the second optical disc D2 is f2, the following condition (9):
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. When the diameter is larger than the diameter, a third region having a phase shift structure is formed on the first surface 11 so that the second laser beam converges satisfactorily with almost no aberration on the recording surface of the second optical disc D2. Unlike the second region, the third region formed when the condition (9) is satisfied does not contribute to the convergence of the first laser beam. That is, the third region formed when the condition (9) is satisfied has an aperture limiting function for the first laser beam. For this reason, the phase shift structure is such that the optical path length change amount provided at the boundary between adjacent refractive surfaces for the second laser light is different from the optical path length change amount for the second laser light in the second region. Designed to. At the time of the design, the third region is blazed so that the diffraction efficiency for the second laser beam is maximized.

In addition, the following condition (10),
f1 × NA1> f2 × NA2 (10)
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. When the diameter is larger than the diameter, a third region having a phase shift structure is formed on the first surface 11 so that the first laser beam converges satisfactorily with almost no aberration on the recording surface of the first optical disc D1. Unlike the second region, the third region formed when the condition (10) is satisfied does not contribute to the convergence of the second laser beam. That is, the third region formed when the condition (10) is satisfied has an aperture limiting function for the second laser beam. Therefore, the structure is designed so that the optical path length variation applied at the boundary between adjacent refractive surfaces for the first laser beam is different from the optical path length variation for the first laser beam in the second region. Is done. At the time of the design, the third region is blazed so that the diffraction efficiency with respect to the first laser beam is maximized.

  Six specific examples (Examples 1 to 6) of the optical information recording / reproducing apparatus 100 using the objective lens 10 designed by the design method of the first embodiment described above as an objective optical system are shown. Examples 1 to 5 are configuration examples in which the diffraction order that gives the highest diffraction efficiency in the third laser light is set to (k + 1) th order. Example 6 is a configuration example in which the diffraction order at which the diffraction efficiency is highest in the third laser light is set to kth order.

  An optical information recording / reproducing apparatus 100 on which the objective lens 10 of each of Examples 1 to 6 is mounted is shown in FIG. 1 and FIGS. 2 (A) to (C). As for the first, second, fourth and sixth embodiments, when using the third optical disc D3, an aperture limiting element (not shown) is used to obtain a numerical aperture suitable for recording or reproducing information. The beam diameter is specified. Therefore, as shown in FIGS. 2A to 2C, when the third optical disk D3 is used, the effective light beam diameter is smaller than when the first and second optical disks D1 and D2 are used.

  Of the embodiments, the optical disc used in the embodiment of the optical information recording / reproducing apparatus 100 having compatibility with three types of optical discs is the first optical disc D1 having the protective layer thickness of 0.6 mm and the highest recording density. Assume a second optical disk D2 having a layer thickness of 0.6 mm and a recording density lower than that of the first optical disk D1, and a third optical disk D3 having a protective layer thickness of 1.2 mm and the lowest recording density.

  The objective lens 10 of the first embodiment has a phase shift structure on the first surface 11 that gives two types of optical path length variation. Specific specifications 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 the specific numerical configurations of the following embodiments.

  In Tables 2 to 4, surface number 0 is the light sources 1A to 3A, surface number 1 is the first surface 11 of the objective lens 10, surface number 2 is the second surface 12 of the objective lens 10, and surface numbers 3 and 4 are each. The protective layer 21 and the recording surface 22 in the optical discs D1 to D3 are respectively shown. r is the radius of curvature (unit: mm) of each lens surface, d is the lens thickness or lens interval (unit: mm) during information recording or reproduction, and n (Xnm) is the refractive index at the wavelength Xnm. The same applies to each table in Example 2 and Example 3 to be described later.

  Both surfaces 11 and 12 (surface numbers 1 and 2) of the objective lens 10 are aspherical surfaces. Table 5 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 and 12. 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.

The coefficient P _2i in the first optical path difference function and the second optical path difference function for defining the phase shift structure to be formed on the first surface 11 of the objective lens 10 of Example 1 is shown in Table 6. It is. Table 7 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized. As shown in Table 7 and FIGS. 3 to 6, i, that is, the diffraction order that maximizes the diffraction efficiency of the first laser beam is different from the diffraction order that maximizes the diffraction efficiency of each of the other laser beams. In Example 1 and Examples 2 to 5 described later, the diffraction order m that maximizes the diffraction efficiency in the third laser light is represented by (k + 1).

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 1 is specifically shown in Table 8. Table 8 gives the range of each annular zone (corresponding to the above-mentioned refractive surface) formed on the first surface 11 of the objective lens 10 of Example 1 and the first laser beam passing through each annular zone. It is the table | surface which showed the amount of optical path length changes. The range of each annular zone is represented by heights hmin to hmax from the optical axis AX.

As shown in Table 8, it can be seen that the amount of change in the optical path length to which the first laser beam is applied by the step between the annular zones is -3 or -2 wavelengths. That is, i _A = 3, i _B = 2 and Δ _A = Δ _B = 0. The steps formed between the annular zones of the annular zones 17 and 18 and between the annular zones of the annular zones 24 and 25 have an optical path length variation of -5 wavelengths, that is, two types of optical path length variations. It is a special step found as the sum of

  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 first embodiment. is there. 8A shows the spherical aberration generated when the first laser beam is used, FIG. 8B shows the spherical aberration generated when the second laser beam is used, and FIG. 8C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. In each aberration diagram, the spherical aberration when each laser beam has a design wavelength is indicated by a solid line. In each aberration diagram, the spherical aberration when the wavelength of each laser beam is deviated by +5 nm from the design wavelength is indicated by a broken line. The same applies to the aberration diagrams shown in the following examples.

  As shown in FIGS. 8A to 8C, when the objective lens 10 of Example 1 is used, a laser having a corresponding design wavelength is used at the time of recording or reproducing information on any of the optical disks D1 to D3. It can be seen that the light converges well on the recording surface 22 of each optical disk without causing spherical aberration. Further, it can be seen that the change in spherical aberration due to the laser beam deviating from the design wavelength is suppressed to the extent that there is no problem in practical use. Note that the deviation in the optical axis direction of the convergence position due to the laser beam deviating from the design wavelength can be corrected by driving the objective lens 10 in the optical axis AX direction using an actuator or the like (not shown). The correction using the actuator or the like is similarly applied to the following embodiments.

  The objective lens 10 of Example 2 has a phase shift structure on the first surface 11 that gives three types of optical path length variation. Specific specifications of the objective lens 10 of Example 2 are shown in Table 9.

  As shown in Table 9, the magnification value indicates that in Example 2, each laser beam when using any one of the optical disks D1 to D3 is incident on the objective lens 10 as a parallel light flux. 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.

  Both surfaces 11 and 12 (surface numbers 1 and 2) of the objective lens 10 are aspherical surfaces as in the first embodiment. Table 13 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 and 12.

Table 14 shows coefficients P _2i in the first to third optical path difference functions for defining the phase shift structure to be formed on the first surface 11 of the objective lens 10 of the second embodiment. Table 15 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 2 is specifically shown in Table 16. Table 16 shows the range of each annular zone formed on the first surface 11 of the objective lens 10 of Example 2, and the optical path length change amount given by the first laser beam passing through the steps between the annular zones. It is the table | surface which showed.

As shown in Table 16, it can be seen that the amount of change in the optical path length to which the first laser light is applied by the step between the annular zones is −3, −2 or 10 wavelengths. That is, i _A = 3, i _B = 2 and i _C = 10, and Δ _A = Δ _B = Δ _c = 0. In addition, between each ring zone of ring numbers 22 and 23, between ring zones of ring zone numbers 26 and 27, between ring zones of ring zone numbers 28 and 29, and between ring zones of ring numbers 31 and 32, respectively. Each step formed in is a special step determined as the sum of three types of optical path length variation.

  FIGS. 9A to 9C 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 second embodiment. is there. 9A shows the spherical aberration generated when the first laser beam is used, FIG. 9B shows the spherical aberration generated when the second laser beam is used, and FIG. 9C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented.

  As shown in FIGS. 9A to 9C, when the objective lens 10 according to the second embodiment is used, a laser having a corresponding design wavelength can be used for recording or reproducing information on any of the optical disks D1 to D3. It can be seen that the light converges well on the recording surface 22 of each optical disk without causing spherical aberration. Furthermore, the objective lens 10 of Example 2 is formed with a phase shift structure having a level difference that gives three types of optical path length variation. That is, the phase shift structure formed in the objective lens 10 of Example 2 has both a three-wavelength compatibility effect and a wavelength shift compensation effect. Therefore, as shown in FIGS. 9A to 9C, even when the first to third laser beams deviate from the design wavelength, the change in spherical aberration can be suppressed very well, and the spherical surface is always spherical. It can be seen that the aberration is corrected.

  Specific specifications of the objective lens 10 of the objective lens 10 of Example 3 are shown in Table 17.

  As shown in Table 17, the magnification values indicate that in Example 3, each laser beam is incident on the objective lens 10 as a parallel light beam when any of the optical disks D1 to D3 is used. 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.

According to Table 17, f1 × NA1 is 2.015, and f2 × NA2 is 2.079. That is, the optical pickup device 100 of Example 3 satisfies the condition (10). Accordingly, the first surface 11 of the objective lens 10 of Example 3 has a first region having a phase shift structure that gives two types of optical path length variation and a phase shift structure having an aperture limiting function for the third laser light. A second region having a third region having a phase shift structure having an aperture limiting function for the first laser beam is formed. In addition, when the range of each area | region in the 1st surface 11 is represented by the height h from the optical axis AX,
1st area | region ... h <= 1.640,
Second region ... 1.640 <h ≦ 2.015,
Third region: 2.015 <h ≦ 2.080.

  As described above, the third region contributes only to the convergence of the second laser beam and does not contribute to the convergence of the first laser beam. Therefore, in Example 3, the optical path length change amount in the third region is expressed as (i + Δ) times as much as one wavelength of the second laser beam.

  Both surfaces 11 and 12 (surface numbers 1 and 2) of the objective lens 10 of Example 3 are aspherical surfaces as in Examples 1 and 2. Table 21 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 and 12. As shown in Table 21, the aspherical shape of the first surface 11 is different in each of the first to third regions.

Table 22 shows the coefficient P _2i in the optical path difference function for defining the phase shift structure to be formed on the first surface 11 of the objective lens 10 of Example 3. Table 23 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

  As shown in Table 23, the diffraction order m is set to a different value for each laser beam used and for each region where the phase shift structure is provided. Specifically, the first region has a phase shift structure that contributes to the convergence of the first to third laser beams, although the diffraction orders at which the diffraction efficiency is maximized are different. The second region has a phase shift structure that contributes to the convergence of the first laser beam and the second laser beam. The third region has a phase shift structure that contributes only to the convergence of the second laser beam.

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 3 is specifically shown in Table 24. Table 24 is a table showing the range of each annular zone formed on the first surface 11 of the objective lens 10 of Example 3 and the optical path length variation.

  As described above, in the objective lens 10 according to the third embodiment, different phase shift structures are formed on the first surface 11 for each region. Therefore, as shown in Table 24, the amount of change in the optical path length to which the first laser beam is applied due to the step between the annular zones varies from region to region.

That is, as shown in Table 24, it can be seen that in the first region, the amount of change in the optical path length to which the first laser light is applied by the step between the annular zones is −5 or 2 wavelengths. That is, i _A = 5, i _B = 2, and Δ _A = Δ _B = 0. In the second region, it can be seen that the amount of change in the optical path length is ± 5 or −3 wavelengths. That is, i _A = 5, i _B = 3, and Δ _A = Δ _B = 0. In the third region, the amount of change in the optical path length is -1 wavelength.

  Each step formed between the annular zones of the annular numbers 20 and 21 is a special step obtained as the sum of two kinds of optical path length variation amounts in the second region.

  FIGS. 10A to 10C 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. 10A shows the spherical aberration generated when the first laser beam is used, FIG. 10B shows the spherical aberration generated when the second laser beam is used, and FIG. 10C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented.

  As shown in FIGS. 10A to 10C, when the objective lens 10 of the third embodiment is used, a laser having a corresponding design wavelength is used at the time of recording or reproducing information on any of the optical disks D1 to D3. It can be seen that the light converges well on the recording surface 22 of each optical disk without causing spherical aberration. It can also be seen that the change in spherical aberration due to the laser beam deviating from the design wavelength is suppressed to the extent that there is no problem in practical use. In particular, according to the objective lens 10 of Example 3, as shown in FIG. 10C, when the third optical disk D3 is used, even if the third laser beam deviates from the design wavelength, it is spherical. It can be seen that the change in aberration is very well suppressed and the spherical aberration is always corrected.

  Similar to the second embodiment, the objective lens 10 according to the fourth embodiment has a phase shift structure that gives three kinds of optical path length variation amounts on the first surface 11. Specific specifications of the objective lens 10 of Example 4 are shown in Table 25.

  As shown in Table 25, the magnification value indicates that in Example 4 as well, each laser beam when using any one of the optical discs D1 to D3 enters the objective lens 10 as a parallel light flux. 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.

  Both surfaces 11 and 12 (surface numbers 1 and 2) of the objective lens 10 are aspherical surfaces as in the above embodiments. Table 29 shows conic coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 and 12.

Table 30 shows the coefficient P _2i in the first to third optical path difference functions for defining the phase shift structure to be formed on the first surface 11 of the objective lens 10 of Example 4. Table 31 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 4 is specifically shown in Table 32. Table 32 shows the range of each annular zone formed on the first surface 11 of the objective lens 10 of Example 4, and the optical path length change amount given by the first laser beam passing through the step between the annular zones. It is the table | surface which showed.

As shown in Table 32, it can be seen that the amount of change in the optical path length to which the first laser light is applied by the step between the annular zones is −3, −2 or 8 wavelengths. That is, i _A = 3, i _B = 2 and i _C = 8, Δ _A = 0.22, and Δ _B = Δ _c = 0. Therefore, the objective lens 10 of Example 4 satisfies the above conditions (1) to (3). The step formed between the annular zones of the annular numbers 27 and 28 is a special step obtained as the sum of two types of optical path length variations of i _A = 3 and i _C = 8.

  FIGS. 11A to 11C 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. FIG. 11A shows the spherical aberration generated when the first laser beam is used, FIG. 11B shows the spherical aberration generated when the second laser beam is used, and FIG. 11C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented.

  As shown in FIGS. 11A to 11C, when the objective lens 10 according to the fourth embodiment is used, a laser having a corresponding design wavelength is used at the time of recording or reproducing information on any of the optical disks D1 to D3. It can be seen that the light converges well on the recording surface 22 of each optical disk without causing spherical aberration. Furthermore, the objective lens 10 of Example 4 is formed with a phase shift structure having a level difference that gives three types of optical path length variation. That is, the phase shift structure formed in the objective lens 10 of Example 4 has both a three-wavelength compatibility effect and a wavelength shift compensation effect. Accordingly, as shown in FIGS. 11A to 11C, even when the first to third laser beams deviate from the design wavelength, the change in spherical aberration is suppressed extremely well, and the spherical surface is always spherical. It can be seen that the aberration is corrected.

  Specific specifications of the objective lens 10 of the objective lens 10 of Example 5 are shown in Table 34.

  As shown in Table 33, the magnification value indicates that in Example 5 as well, each laser beam when using any one of the optical discs D1 to D3 is incident on the objective lens 10 as a parallel light beam. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 33 when the optical discs D1 to D3 are used are shown in Tables 34 to 36.

The first surface 11 of the objective lens 10 according to the fifth embodiment has a first region having a phase shift structure that imparts three types of optical path length variations, and a third laser beam that imparts three types of optical path length variations. A second region having a phase shift structure having an aperture limiting function with respect to is formed. In addition, when the range of each area | region in the 1st surface 11 is represented by the height h from the optical axis AX,
1st area | region ... h <= 1.580,
Second region: 1.580 <h ≦ 1.950.

  Both surfaces 11 and 12 (surface numbers 1 and 2) of the objective lens 10 of Example 5 are aspherical surfaces as in the above examples. Table 37 shows conical coefficients and aspheric coefficients that define the shape of each aspheric surface of each of the surfaces 11 and 12. As shown in Table 37, the aspherical shape of the first surface 11 is different between the first region and the second region.

Table 38 shows the coefficient P _2i in the optical path difference function for defining the phase shift structure to be formed on the first surface 11 of the objective lens 10 of Example 5. Table 39 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized.

As shown in Table 38, the value of the coefficient P ₂ in the optical path difference function defining each region is different. Therefore, as shown in Table 34 to Table 36, the first surface 11 of the objective lens 10 of Example 5 has a different radius of curvature r for each region.

  As shown in Table 39, the diffraction order m is set to a different value for each laser beam used and for each region where the phase shift structure is provided. Specifically, the first region has a phase shift structure that contributes to the convergence of the first to third laser beams, although the diffraction orders at which the diffraction efficiency is maximized are different. The second region has a phase shift structure that contributes to the convergence of the first laser beam and the second laser beam.

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 5 is specifically shown in Table 40. Table 40 is a table showing the ranges of the annular zones formed on the first surface 11 of the objective lens 10 of Example 5 and the optical path length variation.

  As described above, in the objective lens 10 according to the fifth embodiment, different phase shift structures are formed depending on the region of the first surface 11. Therefore, as shown in Table 40, the amount of change in the optical path length to which the first laser beam is applied due to the step between the annular zones is different for each region.

That is, as shown in Table 40, in the first region, it can be seen that the change amount of the optical path length to which the first laser light is applied by the step between the annular zones is −5, 2 or 8 wavelengths. That is, i _A = 5, i _B = 2 and i _C = 8, and Δ _A = Δ _B = Δ _C = 0.05. In the second region, it can be seen that the amount of change in the optical path length is ± 5 or −3 wavelengths. That is, i _A = 5, i _B = 3, and Δ _A = Δ _B = 0. Therefore, the objective lens 10 of Example 5 satisfies the above conditions (1) to (4).

In the first region, the steps formed between the ring zones with the ring numbers 20 and 21, between the ring zones 21 and 22, and between the ring zones 23 to 27 are i _A = 5 and i _C = 8 is a special step obtained as the sum of two types of optical path length variation amounts. Further, in the second region, the step formed between the ring zones of the ring numbers 33 and 34 and between the ring zones 45 and 46 is a special step obtained as the sum of two types of optical path length variation.

  12A to 12C 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 fifth embodiment. is there. FIG. 12A shows the spherical aberration generated when the first laser beam is used, FIG. 12B shows the spherical aberration generated when the second laser beam is used, and FIG. 12C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented.

  As shown in FIGS. 12A to 12C, when the objective lens 10 of Example 5 is used, a laser having a corresponding design wavelength is used at the time of recording or reproducing information on any of the optical disks D1 to D3. It can be seen that the light converges well on the recording surface 22 of each optical disk without causing spherical aberration. It can also be seen that the change in spherical aberration due to the laser beam deviating from the design wavelength is suppressed to the extent that there is no problem in practical use. In particular, according to the objective lens 10 of the fifth embodiment, as shown in FIG. 12C, when the third optical disk D3 is used, even if the third laser beam deviates from the design wavelength, the spherical surface is obtained. It can be seen that the change in aberration is very well suppressed and the spherical aberration is always corrected.

  The objective lens 10 of Example 6 has a phase shift structure on the first surface 11 that gives three types of optical path length variation. Specific specifications of the objective lens 10 of Example 6 are shown in Table 41.

  In Table 41, as indicated by the magnification value, also in Example 6, each laser beam when using any one of the optical discs D1 to D3 enters the objective lens 10 as a parallel light flux. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective lens 10 shown in Table 41 when the optical discs D1 to D3 are used are shown in Tables 42 to 44.

  Both surfaces 11 and 12 (surface numbers 1 and 2) of the objective lens 10 are aspherical surfaces as in the above embodiments. Table 45 shows conic coefficients and aspheric coefficients that define the shape of each aspheric surface of each surface 11, 12.

Table 46 shows the coefficient P _2i in the first to third optical path difference functions for defining the phase shift structure to be formed on the first surface 11 of the objective lens 10 of Example 6. Table 47 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized. In Example 6, the diffraction order m that maximizes the diffraction efficiency in the third laser light is represented by k.

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 6 is specifically shown in Table 50. Table 48 shows the range of each annular zone formed on the first surface 11 of the objective lens 10 of Example 6, and the optical path length change amount given by the first laser beam passing through the steps between the annular zones. It is the table | surface which showed.

As shown in Table 48, it can be seen that the change amount of the optical path length to which the first laser light is applied by the step between the annular zones is 3, 2 or −10 wavelengths. That is, i _A = 3, i _B = 2 and i _C = 10, Δ _A = −0.10, and Δ _B = Δ _c = 0. Therefore, the objective lens 10 of Example 6 satisfies the above conditions (5) to (8). In addition, between each annular zone of the annular zone numbers 12 and 13, between each annular zone of 24 and 25, between each annular zone of 32 and 33, between each annular zone of 34 and 35, between each annular zone of 38 and 39, The steps formed between the annular zones 40 and 41, between the annular zones 46 and 47, between the annular zones 48 and 49, and between the annular zones 51 and 52 are i _B = 2 and i, respectively. _This is a special step obtained as the sum of two types of optical path length variation amounts of _A = 3.

Further, the steps formed between the ring zones of the ring numbers 25 and 26, between the ring zones of 35 and 36, between the ring zones of 39 and 40, and between the ring zones of 47 and 48 are i. _This is a special step obtained as the sum of two types of optical path length variation amounts of _B = 2 and i _C = 10.

Further, the steps formed between the annular zones of the annular zones 42 and 43 and between the annular zones 50 and 51 are three types of optical paths of i _B = 2, i _A = 3, and i _C = 10. This is a special step determined as the sum of long changes.

  FIGS. 13A to 13C 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 sixth embodiment. is there. FIG. 13A shows the spherical aberration generated when the first laser beam is used, FIG. 13B shows the spherical aberration generated when the second laser beam is used, and FIG. 13C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented.

  As shown in FIGS. 13A to 13C, when the objective lens 10 of Example 6 is used, a laser having a corresponding design wavelength is used at the time of recording or reproducing information on any of the optical disks D1 to D3. It can be seen that the light converges well on the recording surface 22 of each optical disk without causing spherical aberration. Furthermore, the objective lens 10 of Example 6 is formed with a phase shift structure having a level difference that gives three types of optical path length variation. That is, the phase shift structure formed in the objective lens 10 of Example 6 has both a three-wavelength compatibility effect and a wavelength shift compensation effect. Therefore, as shown in FIGS. 13A to 13C, even when the first to third laser beams deviate from the design wavelength, the change in spherical aberration can be suppressed very well, and the spherical surface is always spherical. It can be seen that the aberration is corrected.

  The light utilization efficiency when information is recorded or reproduced on the first to third optical disks D1 to D3 using the objective lens 10 of Example 4 is about 85% for the first laser beam, The laser light is about 73% and the third laser light is 56%. In addition, when the information is recorded or reproduced on the first to third optical disks D1 to D3 using the objective lens 10 of the fifth embodiment, the light utilization efficiency is about 98% for the first laser beam, The laser beam is about 75% and the third laser beam is 41%. The light utilization efficiency when information is recorded or reproduced on the first to third optical disks D1 to D3 using the objective lens 10 of Example 6 is about 97% for the first laser beam, The laser beam is about 69% and the third laser beam is 51%. As described above, according to the objective lens of the embodiment of the present invention, the optical disk having a high recording density is configured by satisfying each condition according to the diffraction order at which the diffraction efficiency becomes maximum in the third laser light. When D1 and D2 are used, a larger amount of light can be used, and when the third optical disc D3 is used, a sufficient amount of light can be ensured with no problem in practical use regarding information recording or reproduction.

  The above is a specific example of the objective lens designed by the designing method according to the present invention. In addition, each said Example is an example of the objective lens based on this invention to the last. That is, the objective lens according to the present invention is not limited to the specific numerical configuration of each embodiment. For example, the number of optical elements such as lenses constituting the objective optical system of the optical information recording / reproducing apparatus may be plural. When the objective optical system is composed of a plurality of optical elements, the optical element designed by the designing method according to the present invention can be provided with phase shift structures on both sides as well as on one side.

  Further, the value of the spherical aberration adjusted by the three-wavelength compatibility action may not be 0, and can be arbitrarily set by the designer. Similarly, it is not always necessary to cancel the spherical aberration that changes due to the wavelength shift in the wavelength shift compensation function, and the degree of change of the spherical aberration can be arbitrarily set by the designer.

Furthermore, the combination of i of each step presented in each of the above embodiments is also an exemplification, and is not necessarily limited to the combination of each embodiment. For example, when there are three types of optical path length change amounts provided by the phase shift structure provided on the first surface of the objective lens, the combinations of the steps are i _A = 5, i _B = 2 and i _C = 10. There may be.

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 phase shift structure provided in the 1st surface of the objective lens of embodiment of this invention. It is a graph shown about the light utilization efficiency of each 1st-3rd laser beam when i is set to 2 of the objective lens of the embodiment of the present invention. It is a graph which shows about the light utilization efficiency of each 1st to 3rd laser beam when i is set to 3 of the objective lens of the embodiment of the present invention. It is a graph which shows about the light utilization efficiency of each 1st-3rd laser beam when i is set to 5 of the objective lens of embodiment of this invention. It is a graph shown about the light utilization efficiency of each 1st-3rd laser beam when i is set to 10 of the objective lens of the embodiment of the present invention. FIG. 6 is an aberration diagram showing spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus of Example 1. FIG. 7 is an aberration diagram showing spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus in Example 2. FIG. 10 is an aberration diagram showing spherical aberration generated when the first to third laser beams are used in the optical information recording / reproducing apparatus in Example 3. FIG. 10 is an aberration diagram showing spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus in Example 4. FIG. 11 is an aberration diagram showing spherical aberration that occurs when the first to third laser lights are used in the optical information recording / reproducing apparatus in Example 5. FIG. 12 is an aberration diagram showing spherical aberration that occurs when the first to third laser lights are used in the optical information recording / reproducing apparatus in Example 6.

Explanation of symbols

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

Claims (19)

An objective lens used in an optical information recording / reproducing apparatus for recording or reproducing information with respect to each optical disc by selectively using three types of substantially parallel light beams having first to third wavelengths for a plurality of optical discs having different recording densities. Because
When the first to third wavelengths are λ1, λ2, and λ3, respectively,
λ1 <λ2 <λ3
1.9 <λ3 / λ1 <2.1
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, and the information recording or reproduction is performed on the light beam of the second wavelength. When the protective layer thickness of the optical disk is t2, and the protective layer thickness of the third optical disk on which information is recorded or reproduced using the light beam of the third wavelength is t3,
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
At least one surface has a phase shift structure composed of a plurality of concentric refracting surfaces,
The phase shift structure has a first region that converges the light beam of the third wavelength on the recording surface of the third optical disc,
The first region has a step for providing at least two kinds of optical path length variation amounts having different absolute values with respect to a light beam having a first wavelength at a boundary between adjacent refractive surfaces,
The absolute values of the at least two types of optical path length variation amounts are (i _A + Δ _A ) times and (i _B + Δ _B ) times (where i _A , i _B ) times the wavelength of the first wavelength. Is a natural number and i _A ≠ i _B , −0.5 <Δ _A <0.5, −0.5 <Δ _B <0.5),
Further, i _A is represented by i _A = 2k + 1 (where k is a natural number), and the diffraction order at which the diffraction efficiency is maximum in the light beam of the third wavelength is the (k + 1) th order, and Δ _A is The following conditions (1),
0.000 ≦ Δ _A ≦ 0.384 (1)
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 Δ _A is the following condition (2):
0.020 ≦ Δ _A ≦ 0.324 (2)
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 Δ _A is the following condition (3):
0.020 ≦ Δ _A ≦ 0.258 (3)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: In the objective lens for optical information recording / reproducing apparatus according to claim 1 or 3,
The Δ _A is the following condition (4):
0.020 ≦ Δ _A ≦ 0.178 (4)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: An objective lens used in an optical information recording / reproducing apparatus for recording or reproducing information with respect to each optical disc by selectively using three types of substantially parallel light beams having first to third wavelengths for a plurality of optical discs having different recording densities. Because
When the first to third wavelengths are λ1, λ2, and λ3, respectively,
λ1 <λ2 <λ3
1.9 <λ3 / λ1 <2.1
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, and the information recording or reproduction is performed on the light beam of the second wavelength. When the protective layer thickness of the optical disk is t2, and the protective layer thickness of the third optical disk on which information is recorded or reproduced using the light beam of the third wavelength is t3,
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
At least one surface has a phase shift structure composed of a plurality of concentric refracting surfaces,
The phase shift structure has a first region that converges the light beam of the third wavelength on the recording surface of the third optical disc,
The first region has a step for providing at least two kinds of optical path length change amounts having different absolute values with respect to a light beam having a first wavelength at a boundary between adjacent refractive surfaces,
Wherein the absolute values of at least two kinds of optical path length variation, the first light flux one wavelength of the wavelength _{(i A} + Δ _A) times, and _{(i B} + Δ _B) times _(where, i A, _{i B} Is a natural number and i _A ≠ i _B , −0.5 <Δ _A <0.5, −0.5 <Δ _B <0.5),
Further, the i _A is i _{A =} 2k + 1 (where, k is a natural number) is represented by, and is said third diffraction order k-th order diffraction efficiency is maximized in the light flux of the wavelength, the delta _A following Condition (5),
−0.384 ≦ Δ _A ≦ −0.070 (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 5,
The Δ _A is the following condition (6):
−0.324 ≦ Δ _A ≦ −0.070 (6)
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 5 or 6,
The Δ _A is the following condition (7):
−0.258 ≦ Δ _A ≦ −0.070 (7)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: The objective lens for an optical information recording / reproducing device according to any one of claims 5 to 7,
The Δ _A is the following condition (8):
−0.178 ≦ Δ _A ≦ −0.070 (8)
An objective lens for an optical information recording / reproducing apparatus, characterized in that: The objective lens for an optical information recording / reproducing device according to any one of claims 1 to 8,
There are two types of the optical path length change amount provided by the step of the first region,
I _A and i _B of each step are
i _A = 3,
i _B = 2
An objective lens for an optical information recording / reproducing apparatus. The objective lens for an optical information recording / reproducing device according to any one of claims 1 to 8,
There are two types of the optical path length change amount provided by the step of the first region,
I _A and i _B of each step are
i _A = 5,
i _B = 2
An objective lens for an optical information recording / reproducing apparatus. The objective lens for an optical information recording / reproducing device according to any one of claims 1 to 8,
There are three types of the optical path length change amount provided by the step in the first region,
The absolute value of the change in optical path length that is different from (i _A + Δ _A ) times and (i _B + Δ _B ) times as much as one wavelength of the light beam of the first wavelength is (i _C + Δ _C ) times, where i _C is Natural number, i _C ≠ i _A , i _C ≠ i _B , −0.5 <Δ _C <0.5
Represented by
I _A , i _B and i _C of each step are
i _A = 3,
i _B = 2
i _C = 8,
An objective lens for an optical information recording / reproducing apparatus. The objective lens for an optical information recording / reproducing device according to any one of claims 1 to 8,
There are three types of the optical path length change amount provided by the step in the first region,
The absolute value of the change in optical path length, which is different from (i _A + Δ _A ) times and (i _B + Δ _B ) times the light flux of the first wavelength, is (i _C + Δ _C ) times, where i _C is Natural number, i _C ≠ i _A , i _C ≠ i _B , −0.5 <Δ _C <0.5
Represented by
I _A , i _B and i _C of each step are
i _A = 3,
i _B = 2
i _C = 10,
An objective lens for an optical information recording / reproducing apparatus. The objective lens for an optical information recording / reproducing device according to any one of claims 1 to 8,
There are three types of the optical path length change amount provided by the step in the first region,
The absolute value of the change in optical path length, which is different from (i _A + Δ _A ) times and (i _B + Δ _B ) times the light flux of the first wavelength, is (i _C + Δ _C ) times, where i _C is Natural number, i _C ≠ i _A , i _C ≠ i _B , −0.5 <Δ _C <0.5
Represented by
I _A , i _B and i _C of each step are
i _A = 5,
i _B = 2
i _C = 8,
An objective lens for an optical information recording / reproducing apparatus. The objective lens for an optical information recording / reproducing device according to any one of claims 1 to 8,
There are three types of the optical path length change amount provided by the step in the first region,
The absolute value of the change in optical path length that is different from (i _A + Δ _A ) times and (i _B + Δ _B ) times as much as one wavelength of the light beam of the first wavelength is (i _C + Δ _C ) times, where i _C is Natural number, i _C ≠ i _A , i _C ≠ i _B , −0.5 <Δ _C <0.5
Represented by
I _A , i _B and i _C of each step are
i _A = 5,
i _B = 2
i _C = 10,
An objective lens for an optical information recording / reproducing apparatus.   15. The objective lens for an optical information reproducing apparatus according to claim 1, wherein the objective lens is a single lens. The phase shift structure, on the outside of the first region, 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. And having a second region that does not contribute to the convergence of the light flux of the third wavelength,
The second region has a level difference that gives at least one type of optical path length variation to the first light flux at a boundary between adjacent refracting surfaces,
The absolute value of the at least one kind of optical path length variation is different from the absolute value of the at least one kind of optical path length variation in the first region. Objective lens for optical information recording / reproducing apparatus. The objective lens for an optical information recording / reproducing apparatus according to claim 16,
Assuming that the focal length at the time of recording or reproducing information on the first optical disc is f1, and the focal length at the time of recording or reproducing information on the second optical disc is f2, the following condition (9):
f1 × NA1 <f2 × NA2 (9)
The filling,
The phase shift structure has a third region outside the second region that converges only the light flux of the second wavelength and does not contribute to the convergence of the light flux of the first and third wavelengths,
The third region has a step for providing at least one type of optical path length change amount to the second light flux at a boundary between adjacent refractive surfaces,
An objective lens for an optical information recording / reproducing apparatus, wherein an absolute value of the at least one kind of optical path length change amount is different from an absolute value of the optical path length change amount in the second region. The objective lens for an optical information recording / reproducing apparatus according to claim 16,
When the focal length at the time of recording or reproducing information on the first optical disc is f1, and the focal length at the time of recording or reproducing information on the second optical disc is f2, the following condition (10):
f1 × NA1> f2 × NA2 (10)
The filling,
The phase shift structure has a third region outside the second region that converges only the light flux of the first wavelength and does not contribute to the convergence of the light flux of the second and third wavelengths,
The third region has a step for providing at least one type of optical path length variation to the first light flux at a boundary between adjacent refractive surfaces,
An objective lens for an optical information recording / reproducing apparatus, wherein an absolute value of the at least one kind of optical path length change amount is different from an absolute value of the optical path length change amount in the second region. First to third light sources that respectively irradiate three types of light beams having first to third wavelengths corresponding to the optical disc used;
An objective lens for an optical information recording / reproducing device according to any one of claims 1 to 18,
Deflecting means for guiding light emitted from the first to third light sources to the optical disc through the objective lens;
An optical information recording / reproducing apparatus comprising: a sensor for receiving return light from the optical disc.

Images