JP2008234814A - Objective optical system for optical information recording/reproducing device and optical information recording/reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective optical system for an optical information recording/reproducing device which has compatibility with multiple types of optical discs having specifications different from each other and which is capable of effectively correcting a longitudinal chromatic aberration caused when an optical disc having a high recording density is used, while achieving a simple configuration. <P>SOLUTION: The objective optical system for the optical information recording/reproducing device which has compatibility with multiple types of optical discs is provided, and the objective optical system includes a longitudinal chromatic aberration correction element having a negative power first lens and a positive power second lens made of materials different from each other and cemented together to correct a longitudinal chromatic aberration, at least one phase shift surface having a prescribed phase shift structure to give a predetermined optical path length difference of nearly three wavelengths to a first light beam, and an objective lens. The longitudinal chromatic aberration correction element is located along an optical path common to the first and second light beams on a light source side with respect to the objective lens and the longitudinal chromatic aberration correction element satisfies a prescribed condition in the relation with the phase shift structure. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical information recording / reproducing apparatus for recording / reproducing information on / from a plurality of types of optical disks having different recording densities and protective layer thicknesses, and an objective optical system mounted on the apparatus.

  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 particular, in recent years, a new standard optical disc having a higher recording density than that of a DVD has been 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. In view of the convenience of users who use a plurality of optical discs with different standards in this way, in recent years, optical information recording / reproducing apparatuses, more strictly, the objective optical system provided in the apparatus have been used for the above three types of optical disks. It is required to have compatibility.

  In the text, the term “optical information recording / reproducing apparatus” includes all of the information recording apparatus, the information reproduction apparatus, and the information recording / reproducing apparatus. “Compatible” means that recording or reproduction of information is guaranteed without changing parts even if the optical disk to be used is switched.

  In general, in order for an apparatus to be compatible with three types of optical discs with different standards, first, when replacing optical discs with different standards, while correcting spherical aberration that varies depending on the thickness of the protective layer, It is necessary to obtain a beam spot having a size corresponding to the difference in recording density by changing the numerical aperture (NA) of light used for reproduction. Generally, the spot diameter can be made smaller as the wavelength is shorter. Accordingly, it has been proposed that laser light having a plurality of wavelengths can be used properly in accordance with the recording density of the optical disk used. 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. In addition, when the optical disc of the new standard is used, light having a wavelength shorter than that used when recording or reproducing information on a DVD (for example, so-called blue laser light per 408 nm) is used because of its high recording density.

  When the optical disc of the new standard as described above is used, since the recording density is higher than that of the existing optical disc, it is necessary to converge the laser beam so that a spot having a smaller diameter is formed on the recording surface. For that purpose, it is necessary to correct various aberrations satisfactorily.

  One of the aberrations that need to be corrected is, for example, axial chromatic aberration. Axial chromatic aberration is caused by minute fluctuations in the wavelength of laser light emitted from a light source. The configurations for correcting axial chromatic aberration are exemplified in Patent Documents 1 to 3 below.

JP 2001-337269 A JP 2002-333575 A JP 2006-65902 A

  Patent Document 1 discloses an apparatus for recording or reproducing information on a new standard optical disk using so-called blue laser light. The apparatus described in Patent Document 1 corrects longitudinal chromatic aberration by disposing an optical element formed by joining two positive and negative lenses in front of the objective lens.

  However, the apparatus described in Patent Document 1 is configured so that information can be recorded or reproduced only on a new standard optical disc. For this reason, axial chromatic aberration may not be corrected appropriately for optical discs other than the new standard optical disc.

  Similarly to Patent Document 1, Patent Document 2 also discloses an apparatus for recording or reproducing information on a new standard optical disk using so-called blue laser light. This apparatus corrects axial chromatic aberration by providing a diffractive structure on any surface of a lens constituting the condensing optical system.

  However, in general, the diffractive structure is uniquely determined by the wavelength and diffraction order of light used for recording or reproducing information. Here, similarly to Patent Document 1, the apparatus described in Patent Document 2 is configured to be able to record or reproduce information only on a new standard optical disk. Therefore, the disclosed diffractive structure is set so as to satisfactorily correct axial chromatic aberration that occurs when so-called blue laser light is used. Therefore, even if an attempt is made to construct a compatible device for a plurality of types of optical disks based on the configuration described in Patent Document 2, when light having a wavelength other than the so-called blue laser light is used, The amount of upper chromatic aberration cannot be properly controlled.

  Patent Document 3 discloses an apparatus that is compatible with three types of optical disks: a new standard optical disk, a DVD, and a CD. In this apparatus, one in the common optical path of each laser beam used for recording or reproducing information on each optical disc, and one in the optical path of laser beam used for recording or reproducing information on the new standard optical disc. A chromatic aberration correction element is arranged. That is, chromatic aberration is corrected by two types of optical elements when a new standard optical disk is used, and chromatic aberration is corrected by one type of optical elements when a DVD or CD is used.

  However, in the configuration of Patent Document 3, a plurality of optical elements for correcting chromatic aberration must be provided. This increase in the number of parts is not preferable because it becomes a factor that hinders cost reduction and simplification of the entire optical system.

  In view of the above circumstances, the present invention is compatible with a plurality of types of optical discs having different standards, and has a simple configuration, but at least effectively reduces axial chromatic aberration that occurs when using an optical disc with a high recording density. It is an object of the present invention to provide an objective optical system for an optical information recording / reproducing apparatus that can be corrected, and an optical information recording / reproducing apparatus in which the objective optical system is suitably mounted.

を満たすことを特徴とする。 In order to solve the above-mentioned problem, the objective optical system for an optical information recording / reproducing apparatus according to claim 1 has the first wavelength in order from the short wavelength side to the first and second optical discs in order of increasing recording density. An objective optical system in an optical information recording / reproducing apparatus that records or reproduces information with respect to each optical disc by selectively using two types of light beams having a second wavelength, and has a negative power made of different materials. A chromatic aberration correcting element having an axial chromatic aberration correcting action and a plurality of concentric refracting surfaces, and a predetermined optical path for the incident light beam. A phase shift surface having a phase shift structure in which a step providing a length difference is formed between adjacent refractive surfaces, and an objective lens, and a chromatic aberration correcting element and a phase shift surface, The chromatic aberration correction element is disposed on the light source side of the objective lens, and the phase shift structure has an optical path length of approximately three wavelengths with respect to the first wavelength light beam. A first step which gives a difference, the number of annular zones within the effective radius of the phase shift structure is N, the refractive index of the first lens at the first wavelength is nB1, and the first refractive index at the first wavelength; The refractive index of the second lens is nB2, the refractive index of the first lens at the second wavelength is nR1, the refractive index of the second lens at the second wavelength is nR2, and the radius of curvature of the joint surface of the chromatic aberration correcting element. Is R, the combined focal length of the objective optical system at the first wavelength is f1, the refractive index of the objective lens at the first wavelength is nB3, and the refractive index of the objective lens at the second wavelength is nR3. Condition (1) shown in the following equation 1,

It is characterized by satisfying.

  According to the objective optical system for an optical information recording / reproducing apparatus according to claim 1, it is possible to have compatibility with at least two types of optical discs having different standards, and relative to each other when information is recorded or reproduced on each optical disc. Axial chromatic aberration can be corrected.

を満たすことが望ましい。 According to the objective optical system for an optical information recording / reproducing apparatus according to claim 2, the condition (2) shown in the following equation (2),

It is desirable to satisfy.

を満たすことが望ましい。 Furthermore, according to the objective optical system for an optical information recording / reproducing apparatus according to claim 3, the condition (3) shown in the following equation (3),

It is desirable to satisfy.

  According to the objective optical system for an optical information recording / reproducing apparatus described in claim 4, the phase shift surface may be disposed on at least one surface of the objective lens.

  According to the objective optical system for an optical information recording / reproducing apparatus described in claim 5, the phase shift surface may be disposed on an optical element independent of the objective lens.

ただし、ｈは、光軸からの高さ、
Ｐ_２、Ｐ_４、Ｐ_６、…は、それぞれ第ｉの光路差関数（ｉは自然数）における二次、四次、六次、…の係数、
ｍは、入射光束の回折効率が最大となる回折次数、
λは、入射光束の使用波長、を表す、
により表される少なくとも第一の光路差関数によって規定され、該回折構造は、第一の波長の光束に対して回折効率が最大となる回折次数が三次であり、第一の波長での第一のレンズの屈折率をｎＢ１、第一の波長での第二のレンズの屈折率をｎＢ２、第二の波長での第一のレンズの屈折率をｎＲ１、第二の波長での第二のレンズの屈折率をｎＲ２、色収差補正素子の接合面の曲率半径をＲ、第一の波長での対物光学系の合成焦点距離をｆ１、第一の波長での対物レンズの屈折率をｎＢ３、第二の波長での対物レンズの屈折率をｎＲ３、とすると、以下の数５に示す条件（４）、

を満たすことを特徴とする。 The objective optical system for an optical information recording / reproducing apparatus according to claim 6 has two types, a first wavelength and a second wavelength, in order from the short wavelength side with respect to the first and second optical disks in order from the highest recording density. Is an objective optical system in an optical information recording / reproducing apparatus that records or reproduces information with respect to each optical disc by using different light beams, and has a positive power with a first lens made of different materials and having a negative power A chromatic aberration correcting element having an axial chromatic aberration correcting action; at least one diffractive surface having a diffractive structure; and an objective lens. The chromatic aberration correcting element is disposed on the light source side with respect to the objective lens, and the diffractive structure is represented by the following formula 4,

Where h is the height from the optical axis,
P ₂ , P ₄ , P ₆ ,... Are coefficients of second order, fourth order, sixth order,... In the i th optical path difference function (i is a natural number),
m is the diffraction order that maximizes the diffraction efficiency of the incident light beam,
λ represents the used wavelength of the incident light beam,
The diffraction structure is defined by at least a first optical path difference function represented by: the diffraction structure having a diffraction order that has the maximum diffraction efficiency with respect to a light beam of the first wavelength, and the first at the first wavelength. NB1, the refractive index of the second lens at the first wavelength is nB2, the refractive index of the first lens at the second wavelength is nR1, and the second lens at the second wavelength. NR2, the radius of curvature of the joint surface of the chromatic aberration correcting element is R, the synthetic focal length of the objective optical system at the first wavelength is f1, the refractive index of the objective lens at the first wavelength is nB3, and the second If the refractive index of the objective lens at a wavelength of nR3 is nR3, the condition (4) shown in the following formula 5

It is characterized by satisfying.

  According to the objective optical system for an optical information recording / reproducing apparatus according to claim 6, it is possible to have compatibility with two types of optical discs having different standards, and to generate relative information when recording or reproducing information on each optical disc. Axial chromatic aberration can be corrected.

を満たすことが望ましい。 According to the objective optical system for an optical information recording / reproducing apparatus according to claim 7, the condition (5) shown in the following formula 6

It is desirable to satisfy.

を満たすことが望ましい。 Furthermore, according to the objective optical system for an optical information recording / reproducing apparatus according to claim 8, the condition (6) shown in the following equation 7

It is desirable to satisfy.

  According to the objective optical system for an optical information recording / reproducing apparatus described in claim 9, the diffractive surface may be disposed on at least one surface of the objective lens.

  According to the objective optical system for an optical information recording / reproducing apparatus according to the tenth aspect, the diffractive surface may be disposed on an optical element independent of the objective lens.

  According to the objective optical system for an optical information recording / reproducing apparatus according to claim 11, the first lens is a plano-concave lens, the second lens is a plano-convex lens, and the chromatic aberration correcting element includes the first lens and the first lens. Each curved surface of the second lens is configured to be a cemented surface.

From another point of view, the optical information recording / reproducing apparatus according to claim 12 is one of three kinds of light beams having first to third wavelengths with respect to first to third optical disks having different recording densities. Is an optical information recording / reproducing apparatus for recording or reproducing information to / from each optical disc, wherein the first wavelength is λ1 (nm), the second wavelength is λ2 (nm), and the third wavelength is If λ3 (nm),
λ1 <λ2 <λ3
NA1 for the numerical aperture necessary for recording or reproducing information on the first optical disc, NA2 for the numerical aperture necessary for recording or reproducing information on the second optical disc, and recording or reproducing information on the third optical disc If the required numerical aperture is NA3,
NA1> NA3 and NA2> NA3
The thickness of the protective layer of the first optical disc on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is t1, which is longer than the first wavelength. The recording layer of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength is t2, and the information recording is performed using a light beam having the longest third wavelength among the first to third wavelengths. Or if the protective layer thickness of the third optical disk to be played is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
And having three light sources for irradiating three kinds of light fluxes and the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 1 to 11, at a first wavelength, The objective optical system's composite imaging magnification at M1, the second wavelength, the objective optical system's synthetic focal length and the synthetic imaging magnification at f2, M2, and the third wavelength, respectively. When the combined imaging magnification is f3 and M3, respectively, the following conditions (7) to (9)
−0.02 <f1 × M1 <0.02 (7)
−0.02 <f2 × M2 <0.02 (8)
−0.19 <f3 × M3 <−0.05 (9)
It is characterized by satisfying.

  According to the optical information recording / reproducing apparatus according to claim 12, when recording or reproducing information on each of the first and second optical discs having a relatively high recording density among the three types of optical discs having different standards, the substantially parallel light flux Is incident on the objective optical system, axial chromatic aberration and spherical aberration can be corrected. In addition, when recording or reproducing information on the third optical disk having the lowest recording density, axial chromatic aberration and spherical aberration can be corrected by making divergent light incident on the objective optical system.

From another point of view, the optical information recording / reproducing apparatus according to claim 13 is any one of three kinds of light beams having first to third wavelengths with respect to first to third optical disks having different recording densities. Is an optical information recording / reproducing apparatus for recording or reproducing information to / from each optical disc, wherein the first wavelength is λ1 (nm), the second wavelength is λ2 (nm), and the third wavelength is If λ3 (nm),
λ1 <λ2 <λ3
NA1 is the numerical aperture necessary for recording or reproducing information on the first optical disc, NA2 is the numerical aperture necessary for recording or reproducing information on the second optical disc, and information is recorded or reproduced on the third optical disc. If the numerical aperture required for NA is NA3,
NA1> NA3 and NA2> NA3
The thickness of the protective layer of the first optical disc on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is t1, which is longer than the first wavelength. The recording layer of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength is t2, and the information recording is performed using a light beam having the longest third wavelength among the first to third wavelengths. Or if the thickness of the protective layer of the third optical disk to be reproduced is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
And the three light sources for irradiating three kinds of light beams and the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 1 to 5, wherein the phase shift structure is a first At least a second step that gives a difference in optical path length different from the step of the first step, and the light beams having the first to third wavelengths are incident on the chromatic aberration correction element in a substantially parallel light beam state. .

  According to the optical information recording / reproducing apparatus of the fourteenth aspect, it is preferable that the second step provides an optical path length difference of substantially even wavelengths with respect to the light beam having the first wavelength.

  According to the optical information recording / reproducing apparatus of claim 13 or claim 14, a substantially parallel light beam is made incident on the objective optical system, and an optical path length difference of approximately three wavelengths is given to the light beam of the first wavelength. A phase shift structure having at least two types of steps, that is, a first step that is different from the first step and a second step that is different in optical path length from the first step is employed. As a result, it is possible to satisfactorily correct both the longitudinal chromatic aberration and the spherical aberration even when any of the three types of optical discs having different standards is used.

From another point of view, the optical information recording / reproducing apparatus according to claim 15 is any of three types of light beams having first to third wavelengths with respect to first to third optical discs having different recording densities. Is an optical information recording / reproducing apparatus for recording or reproducing information to / from each optical disc, wherein the first wavelength is λ1 (nm), the second wavelength is λ2 (nm), and the third wavelength is If λ3 (nm),
λ1 <λ2 <λ3
NA1 is the numerical aperture necessary for recording or reproducing information on the first optical disc, NA2 is the numerical aperture necessary for recording or reproducing information on the second optical disc, and information is recorded or reproduced on the third optical disc. If the numerical aperture required for NA is NA3,
NA1> NA3 and NA2> NA3
The thickness of the protective layer of the first optical disc on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is t1, which is longer than the first wavelength. The recording layer of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength is t2, and the information recording is performed using a light beam having the longest third wavelength among the first to third wavelengths. Or if the thickness of the protective layer of the third optical disk to be reproduced is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
And the three light sources for irradiating three kinds of light beams and the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 6 to 10, wherein the diffractive structure is the first The optical path difference function is defined by at least two types of optical path difference functions, ie, a second optical path difference function having a diffraction order that has the maximum diffraction efficiency with respect to the light beam having the first wavelength, and is not a third order. The third wavelength light beam is incident on the chromatic aberration correction element in a substantially parallel light beam state.

  According to the optical information recording / reproducing apparatus of the sixteenth aspect, it is preferable that the second optical path difference function has an even-order diffraction order at which the diffraction efficiency becomes maximum with respect to the light beam having the first wavelength.

  According to the optical information recording / reproducing apparatus of claim 15 or 16, the substantially parallel light beam is made incident on the objective optical system, and the diffraction order that maximizes the diffraction efficiency for the light beam of the first wavelength is the third order. A diffraction structure defined by at least two types of optical path difference functions, that is, a certain first optical path difference function and a second optical path difference function having a diffraction order different from that of the first step is employed. As a result, it is possible to satisfactorily correct both the longitudinal chromatic aberration and the spherical aberration even when any of the three types of optical discs having different standards is used.

を満たすことが好ましい。 Furthermore, according to the optical information recording / reproducing apparatus according to claim 17, the chromatic aberration correcting element has a condition (10) represented by the following equation 8:

It is preferable to satisfy.

  As described above, according to the objective optical system for an optical information recording / reproducing apparatus according to the present invention, two types of optical disks can be obtained by configuring each optical member constituting the objective optical system so as to satisfy a predetermined condition. It is possible to satisfactorily correct the longitudinal chromatic aberration that occurs when each of these is used.

  Further, an optical information recording / reproducing apparatus according to the present invention uses the above-described objective optical system, and an optical disc to be recorded or reproduced with a phase shift structure, a diffraction structure, or information included in the objective optical system. The divergence of the light beam emitted from the light source is controlled according to the above. As a result, even when any of the three types of optical discs having different standards is used, it is possible to satisfactorily correct axial chromatic aberration and spherical aberration, and to realize highly accurate information recording or reproduction.

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

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

  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 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm

Further, when recording or reproducing information on each of the optical discs D1 to D3, it is necessary to change the required NA value so that a beam spot having a size corresponding to the difference in recording density can be obtained. There is. Here, assuming that the optimum numerical apertures required for recording or reproducing information on each of the optical discs D1 to D3 are NA1, NA2, and NA3, respectively, in each embodiment, the following relationships are associated with NA1 to NA3. There is.
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. Note that any optical disk rotates while being placed on a turntable (not shown) 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 so as to obtain a beam spot having a size corresponding to each recording density. Hereinafter, a wavelength suitable for obtaining a spot diameter corresponding to each recording density is referred to as a design wavelength. Specifically, when information is recorded on or reproduced from the first optical disc D1, the beam spot having the smallest diameter is formed on the recording surface of the first optical disc D1. Therefore, when the first optical disc D1 is used, a laser beam having the shortest wavelength (first wavelength) (hereinafter referred to as the first laser beam) is irradiated from the light source. Further, when information is recorded on or reproduced from the third optical disc D3, a beam spot having the largest diameter is formed on the recording surface of the third optical disc D3. Therefore, when the third optical disc D3 is used, a laser beam having the longest wavelength (third wavelength) (hereinafter referred to as a third laser beam) is emitted from the light source. When information is recorded on or reproduced from the second optical disc D2, a spot having a relatively small diameter is formed on the recording surface of the second optical disc D2. Therefore, when the second optical disk D2 is used, a laser beam (hereinafter referred to as a second laser beam) having a longer wavelength than the first laser beam and a shorter wavelength (second wavelength) than the third laser beam. ) Is emitted from the light source.

  FIG. 1 is a schematic diagram showing a schematic configuration of an optical information recording / reproducing apparatus 100 having an objective optical system 30 of the first embodiment. The optical information recording / reproducing apparatus 100 includes a light source 1A for irradiating a first laser beam, a light source 1B for irradiating a second laser beam, a light source 1C for irradiating a third laser beam, diffraction gratings 2A, 2B, 2C, a cup. Ring lenses 3A, 3B, and 3C, beam splitters 41 and 42, half mirrors 5A, 5B, and 5C, light receiving units 6A, 6B, and 6C, and an objective optical system 30 are included.

  FIGS. 2A to 2C are diagrams showing the optical path including the objective optical system 30 and each optical disk when using each optical disk D1 to D3 separately for each optical disk to be used. 2A shows the optical path when using the first optical disk D1, FIG. 2B shows the optical path when using the second optical disk d2, and FIG. 2C shows the optical path when using the third optical disk. Represent each. 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. 2A to 2C, the optical axis of the objective optical system 30 coincides with the reference axis AX. However, the optical axis of the objective optical system 30 or the objective lens 10 is a reference by a tracking operation or the like. There is also a state in which it is off the axis AX.

  As shown in FIGS. 1 and 2A to 2C, the objective optical system 30 includes a chromatic aberration correction element 20 and an objective lens 10 in order from the light source side on the optical path.

  In the optical information recording / reproducing apparatus 100, it is necessary to cope with different NAs required when using each optical disk as described above. Therefore, the optical information recording / reproducing apparatus 100 may be provided with an aperture limiting element that defines the beam diameter of the third laser light as shown in FIGS. 3 (A) and 3 (B).

  FIG. 3A is an enlarged view showing the vicinity of the objective optical system 30 in a state where the aperture limiting element 60 is disposed as an example. As shown in FIG. 3, the aperture limiting element 60 that can be disposed has a first surface 61 and a second surface 62 in order from the light source side. FIG. 3B is a schematic view of the aperture limiting element 60 viewed from the first surface 62 side. As shown in FIG. 3B, the first surface 61 of the aperture limiting element 60 has two light transmission regions 63a and 63b divided concentrically. The first light transmission region 63a has a property of transmitting all the first to third laser beams. The second light transmission region 63b has a property of transmitting only the first and second laser beams and shielding the third laser beam. In the present embodiment, the same state as in FIG. 3B is observed even when the aperture limiting element 60 is faced from the second surface 62 side. By disposing such an aperture limiting element 60 between the chromatic aberration correcting element 20 and the objective lens 10, the third laser beam can be defined (squeezed) to a predetermined diameter. That is, the required spot can be formed on the third optical disc D3.

  As shown in FIGS. 2A to 2C, each of the optical disks D1 to D3 has a protective layer 51 and a recording surface 52, respectively. In the actual optical disks D1 to D3, the recording surface 52 is sandwiched between the protective layer 51 and a substrate layer or label layer (not shown).

  As shown in FIG. 1 and FIGS. 2A to 2C, the first to third laser beams irradiated from the light sources 1A to 1C are the diffraction gratings 2A to 2C and the coupling lenses 3A to 3C, respectively. 3C is guided to a common optical path via the beam splitters 41 and 42 and enters the objective optical system 30. Each light beam that has passed through the objective optical system 30 converges in the vicinity of the recording surface 52 of each of the optical disks D1 to D3 to be recorded or reproduced. Each laser beam reflected by the recording surface 52 returns the same optical path as that at the time of incidence, passes through the half mirrors 5A to 5C, and is detected by the light receiving units 6A to 6C.

  As in the optical information recording / reproducing apparatus 100, when using laser beams having different wavelengths when using the optical discs D1 to D3, changes in the refractive index of the objective lens 10 and differences in the thickness of the protective layer 21 of the optical discs D1 to D3. Due to this, the spherical aberration changes. In order to make the optical information recording / reproducing apparatus 100 compatible with each of the optical discs D1 to D3, it is necessary to satisfactorily correct spherical aberration that occurs when any of the optical discs is used. Also, the laser light emitted from each light source does not always remain at the design wavelength, and may fluctuate slightly due to external factors such as temperature changes. Wavelength variations cause axial chromatic aberration. When axial chromatic aberration occurs, a spot having a diameter suitable for recording or reproducing information is not formed on the recording surface of each optical disc when a wavelength change such as mode hop occurs. Therefore, in the optical information recording / reproducing apparatus 100, it is necessary to satisfactorily correct axial chromatic aberration as well as spherical aberration.

  However, in an objective optical system for an optical information recording / reproducing apparatus constituted by a small number of lenses, if the axial chromatic aberration generated when the first optical disk D1 is used is suppressed to substantially zero, the second optical disk D2 is used. The axial chromatic aberration that sometimes occurs is overcorrected. Further, in the same type of objective optical system, if the axial chromatic aberration generated when the second optical disk D2 is used is suppressed to substantially zero, the correction for the axial chromatic aberration generated when the first optical disk D1 is used is insufficient. . Therefore, in this embodiment, the objective optical system 30 is configured as follows in order to satisfactorily correct the longitudinal chromatic aberration generated when using each optical disk while having compatibility with a plurality of optical disks.

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 single lens in which each of the surfaces 11 and 12 is an aspheric surface. 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 curvature (1 / r) above is C, the conic coefficient is K, the fourth, sixth, eighth, tenth, twelfth, etc. aspheric coefficients are A ₄ , A ₆ , A ₈ , A ₁₀ , A ₁₂ is expressed by the following equation (9).

  On at least one surface (first surface 11 in the present embodiment) of the objective lens 10, a plurality of concentric divided refracting surfaces with a reference axis AX as a center and a plurality of refracting surface boundaries are formed. A phase shift structure composed of minute steps is provided.

  The phase shift structure can be represented by the following optical path difference function φi (h). The optical path difference function φi (h) expresses the function of the objective lens 10 as a diffractive lens in the form of an optical path length addition amount at a height h from the optical axis. In other words, in the diffractive structure. It defines the position and height at which the step is provided. The optical path difference function φi (h) is expressed by the equation shown in Equation 4.

In the i-th optical path difference function φi (h) shown in Equation 4, P _i2 , P _i4 , P _i6 ,... _Are coefficients of the second order, fourth order, sixth order _,. m _i represents the diffraction order at which the diffraction efficiency of the laser beam to be used is maximized, and λ represents the design wavelength of the laser beam to be used (incident).

The phase shift structure provided in the objective lens 10 of the present embodiment is configured to give an optical path length difference of approximately three wavelengths to the first laser light (more precisely, light of the first wavelength). The That is, in the present embodiment, in the above formula 4, λ is set to the first wavelength and m ₁ is set to 3. By setting in this way, it is possible to effectively suppress the occurrence of spherical aberration when the first laser beam or the second laser beam is incident on the objective lens 10 as a parallel light beam.

  The chromatic aberration correcting element 20 is an optical element for correcting chromatic aberration, which is disposed mainly for appropriately controlling the amount of axial chromatic aberration when using the first optical disc D1 and the second optical disc D2. The chromatic aberration correction element 20 is configured by bonding a positive lens and a negative lens, which are made of different materials. The chromatic aberration correction element 20 of the present embodiment is configured by joining a curved surface of a plano-concave lens 20a and a plano-convex lens 20b in order from the light source. 2A to 2C, reference numeral 21 is attached to the joint surfaces of the lenses 20a and 20b.

Here, since the chromatic aberration correcting element 20 has an action of appropriately controlling the amount of axial chromatic aberration when using the first optical disc D1 and the second optical disc D2, the following conditions (1), (4) and Is configured to satisfy the conditions (2) and (5).

ただし、Ｎは、有効半径内での輪帯数を、
ｎＢ１は、第一の設計波長での平凹レンズ２０ａの屈折率を、
ｎＢ２は、第一の設計波長での平凸レンズ２０ｂの屈折率を、
ｎＲ１は、第二の設計波長での平凹レンズの屈折率を、
ｎＲ２は、第二の設計波長での平凸レンズの屈折率を、
ｎＢ３は、第一の設計波長での対物レンズの屈折率を、
ｎＲ３は、第二の設計波長での対物レンズの屈折率を、
Ｒは、色収差補正素子２０の接合面２１の曲率半径を、
ｆ１は、第一の設計波長での対物光学系３０の合成焦点距離を、それぞれ表す。

Where N is the number of zones within the effective radius,
nB1 is the refractive index of the plano-concave lens 20a at the first design wavelength,
nB2 is the refractive index of the plano-convex lens 20b at the first design wavelength,
nR1 is the refractive index of the plano-concave lens at the second design wavelength,
nR2 is the refractive index of the plano-convex lens at the second design wavelength,
nB3 is the refractive index of the objective lens at the first design wavelength,
nR3 is the refractive index of the objective lens at the second design wavelength,
R represents the radius of curvature of the joint surface 21 of the chromatic aberration correction element 20,
f1 represents the combined focal length of the objective optical system 30 at the first design wavelength.

  Conditions (1), (2), (4), and (5) are related to the paraxial power component of the first surface 11 of the objective lens 10 and the materials of the lenses 20a and 20b constituting the chromatic aberration correction element 20. This is a condition for selecting and defining the radius of curvature of the joining surface 21. By satisfying the conditions (1), (4), and also the conditions (2) and (5), the axial chromatic aberration generated when either the first optical disk D1 or the second optical disk D2 is used can be corrected well. It becomes possible to do.

  If the values of the conditions (1) and (4) are below the lower limit, correction of axial chromatic aberration that occurs particularly when the first optical disk D1 is used is not preferable. Further, if the values of the conditions (1) and (4) are equal to or higher than the upper limit, the correction of the longitudinal chromatic aberration that occurs particularly when the second optical disc D2 is used is not preferable.

  In general, it is known that the higher the recording density of the optical disc used, the narrower the tolerance for aberration. Therefore, in order to further reduce the longitudinal chromatic aberration that occurs when the first optical disc D1 is used, the objective optical system 30 is configured to satisfy the following conditions (3) and (6).

  If the values of the conditions (3) and (6) are below the lower limit, the correction of the longitudinal chromatic aberration that occurs when the first optical disc D1 is used is insufficient. On the other hand, if the values of the conditions (3) and (6) exceed the upper limit, the axial chromatic aberration generated when the first optical disc D1 is used becomes excessive, which is not preferable.

  The materials of the plano-concave lens 20a and the plano-convex lens 20b are selected so as to satisfy the following condition (10).

  Condition (10) is a condition relating to the ratio of the refractive index difference of each lens 20a, 20b to the first design wavelength and the refractive index difference of each lens 20a, 20b to the second design wavelength. By setting the ratio appropriately, the absolute value of the imaging magnification of the objective lens 10 can be kept small. If the value of the condition (10) is below the lower limit, the absolute value of the imaging magnification of the objective lens 10 when the first optical disc D1 is used is not preferable. On the other hand, if the value of the condition (10) exceeds the upper limit, the absolute value of the imaging magnification of the objective lens 10 when the second optical disk D2 is used is not preferable.

  As described above, by using the objective optical system 30, the two types of optical disks D1 and D2 having a relatively high recording density achieve compatibility while correcting axial chromatic aberration satisfactorily. The optical information recording / reproducing apparatus 100 of this embodiment uses such an objective optical system 30 and makes laser light having different divergence incident according to the optical disk to be used, so that three types of optical disks D1 having different standards are used. ~ D3 compatibility is achieved. Hereinafter, this point will be described in detail.

  Usually, the objective optical system 30 is disposed on the reference axis AX of the optical information recording / reproducing apparatus 100. However, the position of the objective optical system 30 or the objective lens 10 may deviate from the reference axis AX due to a tracking shift executed in the process of recording or reproducing information. In this case, if a parallel light beam is incident on the objective optical system 30 or the objective lens 10, no aberration is generated. However, if non-parallel light such as divergent light or convergent light is incident, coma aberration or non-magnification is not generated. An off-axis aberration such as point aberration occurs. As described above, generally, an optical disc that requires a high NA for recording or reproducing information has a narrower tolerance for aberration. Therefore, when using an optical disc that requires a high NA for recording or reproducing information, even if the objective optical system 30 is subjected to tracking shift or the like, the objective optical system is used to suppress the occurrence of various aberrations due to off-axis light. It is desired that a substantially parallel light beam is incident on 30.

For example, the optical information recording / reproducing apparatus 100 including the objective optical system 30 designed to satisfy the above conditions is designed to satisfy the following conditions (7) and (8).
−0.02 <f1 × M1 <0.02 (7)
−0.02 <f2 × M2 <0.02 (8)
However, M1 is the composite imaging magnification of the objective optical system 30 at the first design wavelength,
f2 and M2 respectively represent the combined focal length and the combined imaging magnification of the objective optical system 30 at the second design wavelength.

In the optical information recording / reproducing apparatus 100, the objective optical system 30 is designed so as to satisfy the conditions (7) and (8), so that information can be recorded or reproduced on the first optical disc D1 or the second optical disc D2. The light used is a substantially parallel light beam. Therefore, the amount of occurrence of coma and astigmatism during tracking shift can be reduced while effectively correcting the longitudinal chromatic aberration and spherical aberration while effectively exhibiting the performance of the objective optical system 30 as described above. It can be satisfactorily reduced to a negligible level.

  In the present embodiment, the light source 1A and the light source 1B are arranged at positions where the laser beams emitted from the light sources 1A and 1B are converted into parallel light beams by the coupling lenses 3A and 3B. As a result, the combined image forming magnifications M1 and M2 of the objective optical system 30 are set to zero. That is, each coupling lens 3A, 3B of this embodiment functions as a collimating lens for the first laser light and the second laser light.

As described above, when the objective lens 10 is provided with a phase shift structure that effectively suppresses various aberrations when the optical disks D1 and D2 having a narrow tolerance for aberration are used, information is recorded or reproduced on the third optical disk D3. Sometimes using a parallel light beam leaves spherical aberration. Therefore, the spherical aberration generated when the third optical disk D3 is used is corrected by making the light beam incident on the objective optical system 30 into divergent light as shown in FIG. Specifically, assuming that the combined focal length of the objective optical system 30 at the third design wavelength is f3 and the combined imaging magnification is M3, the objective optical system 30 has the following conditions in the optical information recording / reproducing apparatus 100: Designed to satisfy (9).
−0.19 <f3 × M3 <−0.05 (9)

  If the value of the condition (9) exceeds the upper limit, excessive spherical aberration remains when the optical disc D3 is used, which is not preferable. On the other hand, when the value of the condition (9) is less than the lower limit, an under spherical aberration occurs when the optical disc D3 is used, which is not preferable. By designing the objective optical system 30 so as to satisfy the condition (9), it is possible to satisfactorily suppress the occurrence of spherical aberration when the third optical disc D3 is used.

  Note that the optical information recording / reproducing apparatus according to the present invention can achieve the same effects as described above even if it is not necessarily configured to satisfy the condition (9). For example, there is a modification in which the imaging magnification of the objective lens 10 is made substantially zero by disposing the light source 1C at a position where the laser light emitted from the light source 1C is converted into a parallel light beam by the coupling lens 3C. Can be mentioned. That is, in this modification, like the other coupling lenses 3A and 3B, the coupling lens 3C is also arranged and configured to have a function as a collimating lens for the third laser light.

  In the case of adopting the configuration of the above modified example, the refractive index change of the objective lens 10 due to the difference in wavelength between the first to third laser beams and the spherical aberration due to the difference in the protective layer thickness of each of the optical discs D1 to D3 are incident. It is necessary to correct well by a method other than adjusting the divergence of the luminous flux. Therefore, in this modification, the phase shift structure disposed in the objective optical system 30 is designed so as to suppress the refractive index change and the spherical aberration to approximately 0 by controlling the refractive index change and the spherical aberration, respectively. Specifically, the phase shift structure of this modification has at least two types of steps that differ in the optical path length difference imparted to the incident light flux. Such a phase shift structure is defined by at least two types of optical path difference functions having different ratios of diffraction orders at which the diffraction efficiencies of the first to third laser light beams are maximum.

Specifically, at least one of the at least two types of steps gives an optical path length difference of approximately three wavelengths to the first laser beam. That is, m ₁ in the optical path difference function that defines the one type of step is 3. Thus, when at least one of the at least two types of steps is configured to give an optical path length difference corresponding to a substantially odd multiple of the wavelength of the first laser light, In particular, the light utilization efficiency at the time of recording or reproducing information on the third optical disk D3 is lowered. Therefore, the other steps are designed to increase the light use efficiency particularly when the third optical disc D3 is used. Specifically, the other steps are configured so as to give an optical path length difference corresponding to a substantially even multiple of wavelengths to the first laser light. Accordingly, high light utilization efficiency can be ensured in recording or reproducing information on any of the optical disks D1 to D3.

  Four specific examples of the objective optical system 30 of the present embodiment described above and the optical information recording / reproducing apparatus 100 including the optical system 30 and a conventional configuration as a comparative example are shown below.

  In each example, when the third optical disc D3 is used, an aperture limiting element 60 as shown in FIGS. 3A and 3B is used to obtain a numerical aperture suitable for recording or reproducing information. Defines the beam diameter. 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.

  The optical disks used in the examples and comparative examples are the first optical disk D1 having the highest recording density with the protective layer thickness t1 = 0.6 mm, and the first optical disk D1 with the protective layer thickness t2 = 0.6 mm. Further, a second optical disk D2 having a lower recording density and a third optical disk D3 having the lowest recording density with a protective layer thickness t3 = 1.2 mm are assumed.

  The optical information recording / reproducing apparatus 100 of Example 1 is shown by FIG. 1 and FIG. 2 (A)-(C). The same applies to the optical information recording / reproducing apparatus 100 of the second and third embodiments. Specific specifications of the objective optical system 30 mounted on the optical information recording / reproducing apparatus 100 of Example 1 are shown in Table 1.

  As shown by the value of magnification M in Table 1, in Example 1, the laser light is incident on the objective optical system 30 as a parallel light beam when the optical disks D1 to D2 are used, and the laser light as a divergent light beam when the optical disk D3 is used. To do. Tables 2 to 4 show specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective optical system 30 shown in Table 1 when the optical discs D1 to D3 are used.

  As shown in the remarks in Tables 2 to 4, the surface number 0 is the light sources 1A to 1C, the surface numbers 1 and 2 are the diffraction gratings 2A to 2C, the surface numbers 3 and 4 are the coupling lenses 3A to 3C, Surface numbers 5 and 6 in Tables 2 to 3 are beam splitters 41, surface numbers 7 and 8 in Tables 2 to 3, and surface numbers 5 and 6 in Table 4 are beam splitters 42, and surface numbers 9 to 11 in Tables 2 to 3. And surface numbers 7 to 9 in Table 4 are chromatic aberration correction elements 20, surface numbers 12 and 13 in Tables 2 to 3, and surface numbers 10 and 11 in Table 4 are objective lenses 10, and surface numbers 14 and 15 in Tables 2 to 3. The surface numbers 12 and 13 in Table 4 indicate the protective layer 51 and the recording surface 52 of each of the optical disks D1 to D3, which are media. In Tables 2 to 4, r is a radius of curvature (unit: mm) of each lens surface, d is a lens thickness or lens interval (unit: mm) at the time of recording or reproducing information, and n (Xnm) is a wavelength Xnm. Refractive index. The same applies to each table in Example 2 and Example 3 to be described later.

  Further, the second surfaces of the coupling lenses 3A to 3C and the both surfaces 11 and 12 of the objective lens 10 are aspherical surfaces. Tables 5 to 7 show conical coefficients and aspheric coefficients that define the shape of each aspheric surface when information is recorded or reproduced on the first optical disc D1, the second optical disc D2, and the third optical disc D3. 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.

Table 8 shows coefficients P ₂ in the optical path difference function for defining the phase shift structure formed on the first surface 11 of the objective lens 10 of Example 1. Table 9 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized. As shown in Table 9, the diffraction order m is set to a different value depending on the laser beam used.

  From each table, the values of the conditions (4) and (5) are 0.631. The value of condition (6) is 0.631. Therefore, the objective optical system 30 of Example 1 satisfies all the conditions (4) to (6).

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 1 is specifically shown in Table 10. Table 10 is a table showing ranges of each annular zone formed on the first surface 11 of the objective lens 10 of Example 1. The numbers of the annular zones are assigned in order from the optical axis side, and the range of each annular zone is represented by the heights hmin to hmax from the optical axis. The same applies to the same type of table presented in the examples described below.

  As shown in Table 10, the number N of ring zones within the effective radius of the first surface 11 of the objective lens 10 of Example 1 is 17. Therefore, the values of the conditions (1) and (2) are 0.830. The value of condition (3) is 0.978. Therefore, the objective optical system 30 of Example 1 satisfies all the conditions (1) to (3).

  Further, as can be seen from Table 1, in the optical information recording / reproducing apparatus 100 of Example 1, f1 × M1 is 0.000, f2 × M2 is 0.000, and f3 × M3 is −0.077. The conditions (9) are also satisfied from 7).

  FIGS. 4A to 4C are aberration diagrams showing spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 of the first embodiment. 4A shows the spherical aberration generated when the first laser beam is used, FIG. 4B shows the spherical aberration generated when the second laser beam is used, and FIG. 4C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. In each aberration diagram, the solid line represents spherical aberration at the design wavelength, and the broken line represents spherical aberration at a wavelength changed by +10 nm from the design wavelength. The definitions of the spherical aberration diagrams and line types shown in each of the drawings (A) to (C) are the same for the comparative examples described below and the spherical aberration diagrams presented in each of the examples described later.

  5A to 5C are aberration diagrams showing spherical aberrations that occur when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 of the comparative example. The comparative example assumes an optical information recording / reproducing apparatus 100 configured substantially the same as that of the first embodiment except that only one objective lens is used as an alternative to the objective optical system 30 of the first embodiment.

  When comparing FIGS. 4A to 4C and FIGS. 5A to 5C, the optical information recording / reproducing apparatus 100 equipped with the objective optical system 30 of Example 1 is different from the configuration of the comparative example. It can be seen that the axial chromatic aberration that occurs during recording or reproduction of information on either of the optical disks D1 and D2 is also well corrected. Similarly, in the optical information recording / reproducing apparatus 100 of Example 1, it can be seen that the spherical aberration is satisfactorily corrected even when information is recorded or reproduced on any of the optical disks D1 to D3. That is, the optical information recording / reproducing apparatus 100 according to the first embodiment can form a spot suitable for recording or reproducing information on the recording surface regardless of which optical disk is used.

  An optical information recording / reproducing apparatus 100 of Example 2 is shown in FIG. Specific specifications of the objective optical system 30 mounted on the optical information recording / reproducing apparatus 100 of Example 2 are shown in Table 11.

  As shown by the value of magnification M in Table 11, in Example 2, when the optical disks D1 to D2 are used, the laser light is incident on the objective optical system 30 as a parallel light beam, and when the optical disk D3 is used, the laser light is incident as a divergent light beam. To do. Tables 12 to 14 show specific numerical configurations when the optical discs D1 to D3 of the optical information recording / reproducing apparatus 100 including the objective optical system 30 shown in Table 11 are used.

  Further, the second surfaces of the coupling lenses 3A to 3C and the both surfaces 11 and 12 of the objective lens 10 are aspherical surfaces. Tables 15 to 17 show the conical coefficient and the aspheric coefficient that define the shape of each aspheric surface at the time of recording or reproducing information on the first optical disc D1, the second optical disc D2, and the third optical disc D3 in order.

Table 18 shows coefficients P ₂ in the optical path difference function for defining the phase shift structure formed on the first surface 11 of the objective lens 10 of Example 2. Table 19 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized. As shown in Table 19, the diffraction order m is set to a different value depending on the laser beam used.

  From each table, the values of conditions (4) and (5) are 0.833. Therefore, the objective optical system 30 of Example 2 satisfies the conditions (4) and (5).

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 2 is specifically shown in Table 20. Table 20 is a table showing the range of each annular zone formed on the first surface 11 of the objective lens 10 of Example 2.

  As shown in Table 20, the number N of ring zones within the effective radius of the first surface 11 of the objective lens 10 of Example 2 is 23. Therefore, the values of the conditions (1) and (2) are 1.017. Therefore, the objective optical system 30 of Example 2 satisfies the conditions (1) and (2).

  Further, as can be seen from Table 11, in the optical information recording / reproducing apparatus 100 of Example 2, f1 × M1 is 0.000, f2 × M2 is 0.000, f3 × M3 is −0.063, and the condition ( The conditions (9) are also satisfied from 7). As can be seen from Tables 12 to 13, (nB1-nB2) / (nR1-nR2) is -1.067, which also satisfies the condition (10).

  FIGS. 6A to 6C are aberration diagrams showing spherical aberrations that occur when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 of the second embodiment. 6A shows the spherical aberration that occurs when the first laser beam is used, FIG. 6B shows the spherical aberration that occurs when the second laser beam is used, and FIG. 6C shows the third laser beam that is used. Each of the spherical aberrations that sometimes occurs is represented. In each aberration diagram, the solid line represents spherical aberration at the design wavelength, and the broken line represents spherical aberration at a wavelength changed by +10 nm from the design wavelength.

  As shown in FIGS. 6A to 6C, the optical information recording / reproducing apparatus 100 equipped with the objective optical system 30 according to the second embodiment has an on-axis generated when information is recorded or reproduced on any one of the optical disks D1 and D2. It can be seen that the chromatic aberration is also corrected well. Similarly, it can be seen that the optical information recording / reproducing apparatus 100 of Example 2 has the spherical aberration corrected satisfactorily at the time of recording or reproducing information on any of the optical disks D1 to D3. That is, the optical information recording / reproducing apparatus 100 according to the second embodiment can form a spot suitable for recording or reproducing information on the recording surface regardless of which optical disk is used.

  An optical information recording / reproducing apparatus 100 of Example 3 is shown in FIG. Table 21 shows specific specifications of the objective optical system 30 mounted on the optical information recording / reproducing apparatus 100 of Example 3.

  As shown by the value of magnification M in Table 21, in Example 3, when the optical discs D1 to D2 are used, the laser light is incident on the objective optical system 30 as a parallel light beam, and when the optical disc D3 is used, the laser light is incident as a divergent light beam. To do. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective optical system 30 shown in Table 21 when the optical discs D1 to D3 are used are shown in Tables 22 to 24.

  Further, the second surfaces of the coupling lenses 3A to 3C and the both surfaces 11 and 12 of the objective lens 10 are aspherical surfaces. Tables 25 to 27 show the conical coefficient and the aspheric coefficient that define the shape of each aspheric surface when information is recorded or reproduced on the first optical disc D1, the second optical disc D2, and the third optical disc D3, respectively.

Table 28 shows coefficients P ₂ in the optical path difference function for defining the phase shift structure formed on the first surface 11 of the objective lens 10 of Example 3. Table 29 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized. As shown in Table 29, the diffraction order m is set to a different value depending on the laser beam used.

  From each table, the values of the conditions (4) and (5) are 0.792. The value of condition (6) is 0.840. Therefore, the objective optical system 30 of Example 3 satisfies all the conditions (4) to (6).

  The phase shift structure formed on the first surface 11 of the objective lens 10 of Example 3 is specifically shown in Table 30. Table 30 is a table showing ranges of each annular zone formed on the first surface 11 of the objective lens 10 of Example 3.

  As shown in Table 30, the number N of ring zones within the effective radius of the first surface 11 of the objective lens 10 of Example 3 is thirteen. Therefore, the values of the conditions (1) and (2) are 0.810. The value of condition (3) is 0.861. Therefore, the objective optical system 30 of Example 3 satisfies all the conditions (1) to (3).

  Further, as can be seen from Table 21, in the optical information recording / reproducing apparatus 100 of Example 3, f1 × M1 is 0.000, f2 × M2 is 0.000, and f3 × M3 is −0.179. The conditions (9) are also satisfied from 7). Further, as can be seen from Tables 22 to 23, (nB1-nB2) / (nR1-nR2) is -0.852, which satisfies the condition (10).

  FIGS. 7A to 7C are aberration diagrams showing spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 of the third embodiment. 7A shows the spherical aberration generated when the first laser beam is used, FIG. 7B shows the spherical aberration generated when the second laser beam is used, and FIG. 7C shows the third laser beam used. Each of the spherical aberrations that sometimes occurs is represented. In each aberration diagram, the solid line represents spherical aberration at the design wavelength, and the broken line represents spherical aberration at a wavelength changed by +10 nm from the design wavelength.

  As shown in FIGS. 7A to 7C, the optical information recording / reproducing apparatus 100 on which the objective optical system 30 of the third embodiment is mounted is on an axis generated when information is recorded or reproduced on any one of the optical disks D1 and D2. It can be seen that the chromatic aberration is also corrected well. Similarly, it can be seen that the optical information recording / reproducing apparatus 100 of Example 3 has the spherical aberration corrected satisfactorily at the time of recording or reproducing information on any of the optical disks D1 to D3. That is, the optical information recording / reproducing apparatus 100 according to the third embodiment can form a spot suitable for recording or reproducing information on the recording surface regardless of which optical disc is used.

  The optical information recording / reproducing apparatus 100 of Example 4 is shown by FIG. 1 and FIG. 8 (A)-(C). 8A to 8C correspond to FIGS. 2A to 2C. Specific specifications of the objective optical system 30 mounted on the optical information recording / reproducing apparatus 100 of Example 4 are shown in Table 31.

  As shown by the value of magnification M in Table 31, in Example 4, a parallel light beam is used when any of the optical disks D1 to D3 is used. Specific numerical configurations of the optical information recording / reproducing apparatus 100 including the objective optical system 30 shown in Table 31 when the optical discs D1 to D3 are used are shown in Tables 32 to 34.

  Similarly to the other embodiments, the second surfaces of the coupling lenses 3A to 3C and both surfaces 11 and 12 of the objective lens 10 are aspherical surfaces. Tables 35 to 37 show conical coefficients and aspheric coefficients in order to define the shape of each aspheric surface when recording or reproducing information on the first optical disc D1, the second optical disc D2, and the third optical disc D3.

As described above, in the fourth embodiment, a parallel light beam is used when any of the optical disks D1 to D3 is used. Accordingly, the phase shift structure formed on the first surface 11 of the objective lens 10 of Example 4 is defined by two types of optical path difference functions (a first optical path difference function and a second optical path difference function). Table 38 shows coefficients P _i2 ... In each of the first optical path difference function and the second optical path difference function. Table 39 shows the diffraction order m at which the diffraction efficiency of each laser beam is maximized in each optical path difference function. As shown in Table 39, the diffraction order m _i is set a different value by the laser beam to be used.

From each table, the values of conditions (4) and (5) are 0.67. The value of condition (6) is 0.67. Therefore, the objective optical system 30 of Example 4 satisfies all the conditions (4) to (6).

  FIGS. 9A to 9C are aberration diagrams illustrating spherical aberration that occurs when the first to third laser beams are used in the optical information recording / reproducing apparatus 100 of the fourth embodiment. 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. In each aberration diagram, the solid line represents spherical aberration at the design wavelength, and the broken line represents spherical aberration at a wavelength changed by +10 nm from the design wavelength.

  As shown in FIGS. 9A to 9C, the optical information recording / reproducing apparatus 100 on which the objective optical system 30 of the fourth embodiment is mounted is on the axis generated when information is recorded or reproduced on any one of the optical disks D1 and D2. It can be seen that the chromatic aberration is also corrected well. Similarly, it can be seen that the optical information recording / reproducing apparatus 100 of Example 4 has the spherical aberration corrected satisfactorily at the time of recording or reproducing information on any of the optical disks D1 to D3. That is, the optical information recording / reproducing apparatus 100 according to the fourth embodiment can form a spot suitable for recording or reproducing information on the recording surface regardless of which optical disc is used.

  The above is the embodiment of the present invention. The present invention is not limited to these embodiments, and can be modified in various ranges as exemplified below.

  For example, in the above embodiment, a phase shift structure, in other words, a diffractive structure is arranged on one surface of the objective lens 10. The objective optical system according to the present invention is not limited to such a configuration. For example, an optical element having a phase shift structure can be provided independently of the objective lens 10. In this case, the optical element is disposed closer to the light source than the objective lens 10.

  In the above embodiment, the objective optical system and the optical information recording / reproducing apparatus having compatibility with the three types of optical disks from the first to the third have been described. The present invention is not necessarily limited to an objective optical system and an optical information recording / reproducing apparatus compatible with three types of optical disks. For example, even an objective optical system and an optical information recording / reproducing apparatus having compatibility only with two types of optical disks D1 and D2 having a high recording density are preferably implemented.

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. FIG. 3A is an enlarged view showing the vicinity of the disposed objective optical system according to the embodiment of the present invention. FIG. 3B is a schematic view of the opening limiting element according to the embodiment as viewed from the first surface side. 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 of the comparative example. 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 2. 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 3. It is a figure which shows separately the optical information recording / reproducing apparatus of Example 4 of this invention for every optical path at the time of each optical disk use. 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 4.

Explanation of symbols

1A, 1B, 1C Light source 2A, 2B, 2C Diffraction grating 3A, 3B, 3C Coupling lens 41, 42 Beam splitter 5A, 5B, 5C Half mirror 6A, 6B, 6C Light receiving unit 10 Objective lens D1-D3 Optical disc 100 Optical information Recording / playback device

Claims (17)

Information is recorded or reproduced on each optical disc by selectively using at least the first and second optical fluxes in order from the short wavelength side to at least the first and second optical discs in descending order of recording density. An objective optical system in an optical information recording / reproducing apparatus for performing
A chromatic aberration correcting element having a longitudinal chromatic aberration correcting action, which is formed by joining a first lens having a negative power and a second lens having a positive power made of different materials;
At least one phase shift surface having a phase shift structure that is divided into a plurality of concentric refracting surfaces, and a step providing a predetermined optical path length difference with respect to an incident light beam is formed between adjacent refracting surfaces;
An objective lens,
The chromatic aberration correcting element and the phase shift surface are in an optical path common to the two types of light fluxes,
The chromatic aberration correction element is disposed closer to the light source than the objective lens,
The phase shift structure has a first step that gives an optical path length difference of approximately three wavelengths to the light flux of the first wavelength,
The number of annular zones within the effective radius of the phase shift structure is N, the refractive index of the first lens at the first wavelength is nB1, and the refractive index of the second lens at the first wavelength. nB2, the refractive index of the first lens at the second wavelength is nR1, the refractive index of the second lens at the second wavelength is nR2, and the radius of curvature of the joint surface of the chromatic aberration correcting element is R. F1 is the combined focal length of the objective optical system at the first wavelength, nB3 is the refractive index of the objective lens at the first wavelength, and nR3 is the refractive index of the objective lens at the second wavelength. Then, condition (1) shown in the following equation 1

An objective optical system for an optical information recording / reproducing apparatus, characterized in that: The objective optical system for an optical information recording / reproducing apparatus according to claim 1,
Condition (2) shown in the following formula 2,

An objective optical system for an optical information recording / reproducing apparatus, characterized in that: In the objective optical system for an optical information recording / reproducing apparatus according to claim 1 or 2,
Furthermore, the condition (3) shown in the following formula 3,

An objective optical system for an optical information recording / reproducing apparatus, characterized in that: In the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 1 to 3,
The objective optical system for an optical information recording / reproducing apparatus, wherein the phase shift surface is disposed on at least one surface of the objective lens. In the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 1 to 3,
The objective optical system for an optical information recording / reproducing apparatus, wherein the phase shift surface is disposed in an optical element independent of the objective lens. Information is recorded or reproduced on each optical disc by selectively using at least the first and second optical fluxes in order from the short wavelength side to at least the first and second optical discs in descending order of recording density. An objective optical system in an optical information recording / reproducing apparatus for performing
A chromatic aberration correcting element having an axial chromatic aberration correcting action, which is formed by joining a first lens having a negative power and a second lens having a positive power made of different materials;
At least one diffractive surface having a diffractive structure;
An objective lens,
The chromatic aberration correcting element and the diffractive surface are in an optical path common to the two kinds of light beams,
The chromatic aberration correction element is disposed closer to the light source than the objective lens,
The diffractive structure has the following formula 4,

Where h is the height from the optical axis,
P _i2 , P _i4 , P _i6 ,... _Are coefficients of second order, fourth order, sixth order,... In the i th optical path difference function (i is a natural number), respectively.
m is the diffraction order that maximizes the diffraction efficiency of the incident light beam,
λ represents a wavelength used for the incident light beam,
Defined by at least a first optical path difference function represented by:
The diffractive structure has a third diffraction order in which the diffraction efficiency is maximized with respect to the light flux of the first wavelength,
The refractive index of the first lens at the first wavelength is nB1, the refractive index of the second lens at the first wavelength is nB2, and the refraction of the first lens at the second wavelength. NR1, the refractive index of the second lens at the second wavelength is nR2, the radius of curvature of the joint surface of the chromatic aberration correction element is R, and the combined focal length of the objective optical system at the first wavelength Is f1, the refractive index of the objective lens at the first wavelength is nB3, and the refractive index of the objective lens at the second wavelength is nR3, the condition (4) shown in the following equation (5):

An objective optical system for an optical information recording / reproducing apparatus, characterized in that: The objective optical system for an optical information recording / reproducing apparatus according to claim 6,
Condition (5) shown in Equation 6 below,

An objective optical system for an optical information recording / reproducing apparatus, characterized in that: In the objective optical system for an optical information recording / reproducing apparatus according to claim 6 or 7,
Furthermore, the condition (6) shown in the following formula 7,

An objective optical system for an optical information recording / reproducing apparatus, characterized in that: In the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 6 to 8,
The objective optical system for an optical information recording / reproducing apparatus, wherein the diffraction surface is disposed on at least one surface of the objective lens. In the objective optical system for an optical information recording / reproducing apparatus according to any one of claims 6 to 8,
The objective optical system for an optical information recording / reproducing apparatus, wherein the diffractive surface is disposed in an optical element independent of the objective lens. The objective optical system for an optical information recording / reproducing apparatus according to any one of claims 1 to 10,
The first lens is a plano-concave lens;
The second lens is a plano-convex lens;
The objective optical system for an optical information recording / reproducing apparatus, wherein the chromatic aberration correcting element is configured such that a curved surface of each of the first and second lenses becomes the cemented surface. Optical information recording / reproduction for recording / reproducing information on / from each optical disc by using one of three kinds of light beams having first to third wavelengths for the first to third optical discs having different recording densities A device,
When the first wavelength is λ1 (nm), the second wavelength is λ2 (nm), and the third wavelength is λ3 (nm),
λ1 <λ2 <λ3
And
The numerical aperture required for recording or reproducing information on the first optical disc is NA1, the numerical aperture required for recording or reproducing information on the second optical disc is NA2, and the information is recorded or reproduced on the third optical disc. If the required numerical aperture is NA3,
NA1> NA3 and NA2> NA3
And
The protective layer thickness of the first optical disk on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is set to t1, which is longer than the first wavelength. The thickness of the protective layer of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength is t2, and information is obtained using a light beam having the longest third wavelength among the first to third wavelengths. When the protective layer thickness of the third optical disc on which recording or reproduction is performed is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
And
Three light sources for irradiating the three kinds of light fluxes;
An optical information recording / reproducing apparatus objective optical system according to any one of claims 1 to 11,
The synthetic imaging magnification of the objective optical system at the first wavelength is M1, and the synthetic focal length and synthetic imaging magnification of the objective optical system at the second wavelength are f2, M2, and the third, respectively. When the synthetic focal length and the synthetic imaging magnification of the objective optical system at the wavelength of f3 and f3 are respectively f3 and M3, the following conditions (7) to (9)
−0.02 <f1 × M1 <0.02 (7)
−0.02 <f2 × M2 <0.02 (8)
−0.19 <f3 × M3 <−0.05 (9)
An optical information recording / reproducing apparatus characterized by satisfying the above. Optical information recording / reproduction for recording / reproducing information on / from each optical disc by using one of three kinds of light beams having first to third wavelengths for the first to third optical discs having different recording densities A device,
When the first wavelength is λ1 (nm), the second wavelength is λ2 (nm), and the third wavelength is λ3 (nm),
λ1 <λ2 <λ3
And
The numerical aperture required for recording or reproducing information on the first optical disc is NA1, the numerical aperture required for recording or reproducing information on the second optical disc is NA2, and the information is recorded or reproduced on the third optical disc. If the required numerical aperture is NA3,
NA1> NA3 and NA2> NA3
And
The protective layer thickness of the first optical disk on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is set to t1, which is longer than the first wavelength. The thickness of the protective layer of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength is t2, and information is obtained using a light beam having the longest third wavelength among the first to third wavelengths. When the protective layer thickness of the third optical disc on which recording or reproduction is performed is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
And
Three light sources for irradiating the three kinds of light fluxes;
An objective optical system for an optical information recording / reproducing apparatus according to any one of claims 1 to 5,
The phase shift structure has the first step and at least a second step that provides an optical path length difference different from the first step,
The optical information recording / reproducing apparatus, wherein the light beams having the first to third wavelengths are incident on the chromatic aberration correction element in a substantially parallel light beam state. The optical information recording / reproducing apparatus according to claim 13,
The optical information recording / reproducing apparatus characterized in that the second step provides an optical path length difference of substantially even wavelengths to the light flux having the first wavelength. Optical information recording / reproduction for recording / reproducing information on / from each optical disc by using one of three kinds of light beams having first to third wavelengths for the first to third optical discs having different recording densities A device,
When the first wavelength is λ1 (nm), the second wavelength is λ2 (nm), and the third wavelength is λ3 (nm),
λ1 <λ2 <λ3
And
The numerical aperture required for recording or reproducing information on the first optical disc is NA1, the numerical aperture required for recording or reproducing information on the second optical disc is NA2, and the information is recorded or reproduced on the third optical disc. If the required numerical aperture is NA3,
NA1> NA3 and NA2> NA3
And
The protective layer thickness of the first optical disk on which information is recorded or reproduced using the light beam having the shortest first wavelength among the first to third wavelengths is set to t1, which is longer than the first wavelength. The thickness of the protective layer of the second optical disc on which information is recorded or reproduced using a light beam having a second wavelength is t2, and information is obtained using a light beam having the longest third wavelength among the first to third wavelengths. When the protective layer thickness of the third optical disc on which recording or reproduction is performed is t3,
t1 ≒ 0.6mm
t2 ≒ 0.6mm
t3 ≒ 1.2mm
And
Three light sources for irradiating the three kinds of light fluxes;
An objective optical system for an optical information recording / reproducing apparatus according to any one of claims 6 to 10,
The diffractive structure includes at least two types of the first optical path difference function and a second optical path difference function whose diffraction order that maximizes the diffraction efficiency for the light beam having the first wavelength is not a third order. It is defined by the optical path difference function,
The optical information recording / reproducing apparatus, wherein the light beams having the first to third wavelengths are incident on the chromatic aberration correction element in a substantially parallel light beam state. The optical information recording / reproducing apparatus according to claim 15,
2. The optical information recording / reproducing apparatus according to claim 1, wherein the second optical path difference function has a diffraction order at which the diffraction efficiency is maximized with respect to the light beam having the first wavelength, which is substantially an even order. The optical information recording / reproducing apparatus according to any one of claims 12 to 16,
Condition (10) where the chromatic aberration correcting element is shown in the following equation (8)

An optical information recording / reproducing apparatus characterized by satisfying the above.

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