Patent Application
20050249064
This application claims priority from Japanese Patent Application No. 2004-138635 filed on May 7, 2004, which is incorporated hereinto by reference.
The present invention relates to an optical element (herein after, there is a case where it is called wavelength selection element) adequate when it is used for an optical pick-up apparatus by which the recording and/or reproducing of the information can be compatibly conducted on different kinds of optical disks, an objective optical system, an optical pick-up apparatus, and a drive apparatus of optical disks.
Recently, in the optical pick-up apparatus, the wavelength-shortening of the laser light source used as the light source for reproducing of the information recorded in an optical disk or recording of the information in the optical disk is advanced, for example, a blue violet laser light source of wavelength 400-420 nm such as a blue violet semiconductor laser, or a blue violet SHG laser which conducts the wavelength conversion of the infrared semiconductor laser by using the second harmonic wave, is putting to a practical use. When these blue violet laser light sources are used, in the case where an objective optical system of the same numerical aperture (NA) as DVD (Digital Versatile Disk) is used, the information of 15-20 GB can be recorded in an optical disk of diameter 12 cm, and in the case where NA of the objective optical system is increased to 0.85, the information of 23-25 GB can be recorded in the optical disk of diameter 12 cm. Hereinafter, in the present specification, the optical disk and photo-magnetic disk for which the blue violet laser light source is used, are generally referred as “high density optical disk”.
Hereupon, only by saying that the information can be adequately recorded/reproduced for such a high density optical disk, it can not be said that a value as a product of the optical disk player/recorder is enough. In the present time, when the actuality that DVD or CD (Compact Disk) in which various information are recorded is put in a market, is based on, by only a case where the information can be recorded/reproduced for the high density optical disk, it is insufficient, and for example, a fact that the information can be adequately recorded/reproduced in the same manner also for a user-own DVD or CD, introduces to a fact that a commercial value as the optical disk player/recorder is increased. For such a background, it is desired that the optical pick-up apparatus mounted in the optical disk player/recorder for the high density optical disk has a performance by which the information can be adequately recorded/reproduced while the compatibility is being kept with also any one of optical disks of the high density optical disk, DVD and CD.
Herein, when the recording and/or reproducing of the information is conducted on, for example, the high density optical disk, DVD and CD, by using one optical pick-up apparatus, there is a problem that values of numerical apertures NA for each of optical disks are different in such a manner that the high density optical disk is 0.85, DVD is 0.6, CD is 0.45.
Accordingly, when, by using the same objective lens, the recording and/or reproducing of the information is conducted on all optical disks, the necessity that the numerical aperture of the objective optical system is switched during the exchange to the different optical disk is generated. It is general that the switching of the large and small numerical apertures is conducted by changing the light flux diameter, however, it is considered that, for example, the liquid crystal shutter is driven in timed relationship with the timing of switching to the different optical disk, and the diameter of the light flux passing this liquid crystal shutter is changed.
However, when the liquid crystal shutter is used for the purpose that the light flux diameter is changed, the electric control system for controlling the liquid crystal shutter becomes necessary, and the production cost is increased. Further, because the light which can pass the liquid crystal shutter, is only a polarized light having a predetermined oscillation direction, it is necessary that the lens design is conducted considering the oscillation direction for the purpose that the irradiation light is effectively passed, and there is also a problem that the degree of the freedom of the design work is limited.
In contrast to this, in Tokkaihei No. 11-194207 (hereinafter, Patent Document 1), a diffraction type filter in which a zone plate which has the first area structuring the central part of the filter, and the second area positioned outside the central part of the filter, and the aberration correction function to only any one hand light of the light of two wavelengths λ1, λ2 which are different from each other, is formed in the first area, and the diffraction lattice which practically has the light shielding function only to one hand light of the light of 2 wavelengths λ1, λ2, is formed in the second area, is disclosed. When such a diffraction type filter is used, the recording and/or reproducing of the information can be compatibly conducted on DVD and CD.
However, the diffraction type filter written in Patent Document 1 is a filter which corresponds to light fluxes of different 2 wavelengths, and is inadequate for the use for the optical pick-up apparatus by which the recording and/or reproducing of the information is conducted on optical disks more than 3 kinds of, for example, the high density optical disk, DVD, CD.
The present invention is attained in view of the conventional problem, and the object of the present invention is to provide an optical pick-up apparatus by which, although the recording and/or reproducing of the information can be adequately conducted on different 3 kinds of optical disks, the size reduction can be intended, and a drive apparatus of the optical disk, and an objective optical system and an optical element used for it.
The first aspect of the present invention is an optical pick-up apparatus.
The optical pick-up apparatus comprises: a first light source which emits a first light flux of a first wavelength λ1; a second light source which emits a second light flux of a second wavelength λ2 (λ1<λ2); a third light source which emits a third light flux of a third wavelength λ3 (λ2<λ3); and at least an optical element positioned so that the first light flux, the second light flux and the third light flux pass through the optical element.
The optical pick-up apparatus conducts recording and/or reproducing of information of a first optical disk having a protective layer of thickness t1 by using the first light flux, recording and/or reproducing of information of a second optical disk having a protective layer of thickness t2 (t2≧t1) by using the second light flux, and recording and/or reproducing of the information on a third optical disk having a protective layer of thickness t3 (t3>t2) by using the third light flux.
In the optical pick-up apparatus, the optical element has a first optical surface having a first area including an optical axis of the optical element, and a second area positioned outside the first area, the second area includes a light-shielding structure being capable of substantially shielding the third light flux and being capable of transmitting the first light flux and the second light flux. The third light flux which passed the second area does not effectively contribute to the recording and/or reproducing on the third-optical disk.
The second aspect of the present invention is an objective optical system.
The objective optical system is used for the optical pick-up apparatus for conducting recording and/or reproducing information of each of a first optical disk, a second optical disk and a third optical disk by using each of a first light flux emitted having first wavelength λ1, a second light flux having second wavelength λ2 (λ1<λ2) and a third light flux having third wavelength λ3 (λ2<λ3).
The objective optical system comprises a first optical surface having a first area including the optical axis of the objective optical system, and a second area positioned outside the first area. the second area includes a light-shielding structure being capable of substantially shielding the third light flux, and being capable of transmitting the first light flux and the second light flux. The third light flux, which passed the second area, does not effectively contribute to the recording and/or reproducing on the third optical disk.
The third aspect of the present invention is an optical element for the optical pick-up apparatus.
The optical element is used for the optical pick-up apparatus for conducting recording and/or reproducing information of each of a first optical disk, a second optical disk and a third optical disk by using each of a first light flux emitted having first wavelength λ1, a second light flux having second wavelength λ2 (λ1<λ2) and a third light flux having third wavelength λ3 (λ2<λ3).
The optical element comprises a first optical surface having a first area including the optical axis of the objective optical system, and a second area positioned outside the first area. The second area includes a light-shielding structure being capable of substantially shielding the third light flux, and being capable of transmitting the first light flux and the second light flux. The third light flux, which passed the second area, does not effectively contribute to the recording and/or reproducing on the third optical disk.
The fourth aspect of the present invention is a drive apparatus of the optical disk.
The drive apparatus of the optical disk comprises the optical pick-up apparatus of the first aspect and a moving device for moving the optical pick-up apparatus in a direction perpendicular to the radius direction of the optical disk, and the recording and/or reproducing of the information is conducted by the optical pick-up apparatus.
Referring to
Herein, a phrase “a light shielding structure being capable of substantially shielding a predetermined light flux” indicates that the predetermined light flux passing the light-shielding structure T is made not to effectively contribute to the recording and/or reproducing conducted on the optical disk. As the light-shielding structure T, for example, when the predetermined light flux passing the light-shielding structure T is diffracted, a structure in which the predetermined light flux is made not to contribute to the recording and/or reproducing on the optical disk, or by a method by which the phase is dislocated by adding the optical path difference to the predetermined light flux incident on the light-shielding structure T, when mutual light fluxes passed the convex part and the concave part of the light-shielding structure T are cancelled with each other, a structure by which they are made not so as to contribute to the recording and/or reproducing on the optical disk, are listed. The light-shielding structure T may be provided on the whole of the second area R2, or on a part of the second area R2.
In the present invention, a phrase that “a phase structure being capable of compensating aberration of a predetermined light flux” expresses that the phase structure has an action to reduce the aberration on the wave-front of the light conversing spot formed on the optical disk by the predetermined light flux, to the predetermined light flux passed the phase structure.
The shape of the phase structure or the light-shielding structure, used in the present invention, when they have the above action, particularly they are not limited, however, a structure, in which, for example, patterns whose sectional shape including the optical axis is rectangular, are arranged concentric circularly, is listed.
The step difference of this rectangular pattern can be adequately set by the action to be given to the light flux passing the phase structure or the light-shielding structure.
For example, when the depth of the step difference of the phase structure or the light-shielding structure is d, the wavelength of the light flux passing the phase structure D1 is λ, and the refractive index of the wavelength selection element WSE to the light flux of wavelength λ is n, the height of the step difference can be designed by using X expressed by the following expression.
X=(d×(nλ−1))/λ
When the light flux of the wavelength λ passing the phase structure or the light-shielding structure is transmitted, it is preferable that the following expression is satisfied.
0≦|INT(X)−1|≦0.1
Herein, INT (X) expresses a natural number closest to X. When set to such a step difference, the optical path difference generated by the step difference becomes about integer times of the wavelength λ, and the light flux of the wavelength λ passing the phase structure or light-shielding structure can be transmitted as it is without receiving any action.
Further, it can be considered, for example, that the step difference is structured so that X satisfies the following expression.
0.4≦|INT(X)−1|≦0.6
When the step difference is set to such a height, because the height of the step difference becomes about a half integer times of the wavelength λ, the phase of the light flux passing the concave convex part of the rectangular pattern is dislocated by π to the light flux of λ passing the phase structure. Accordingly, almost the light amount of the light flux incident on the phase structure D1 is distributed into 1-order diffraction light and −1-orer diffraction light, and the light flux is bended.
The height of the step difference in the phase structure or light-shielding structure is not limited to these ones, but can appropriately be set corresponding to the action to the passing light flux.
When the phase structure or the light-shielding structure is formed at a position through which a plurality of light fluxes pass, by combining the relationships described above to respective light fluxes, when the light flux of a part is practically passed, and the other light fluxes are diffracted, the aberration compensation action can be given, or the light shielding action can be practically given.
Referring to
Herein, N1 is a refractive index of the wavelength selection element WSE in the first wavelength λ1, N2 is a refractive index of the wavelength selection element WSE in the second wavelength λ2, and N3 is a refractive index of the wavelength selection element WSE in the third wavelength λ3.
That is, the step difference of the phase structure D1 is set to a height satisfying the following expressions.
When
x1=(d1×(N1λ1−1))/λ1,
x2=(d1×(N2λ2−1))/λ2,
x3=(d1×(N3λ3−1))/λ3,
then, it is made as follows,
0≦|INT(x1)−1≦0.1,
0≦|INT(x2)−1|≦0.1
and
0.4≦|INT(x3)−x3|≦0.6.
Where, INT (x1), INT (x2) and INT (x3) respectively express a natural number closest to x1, a natural number closest to x2 and a natural number closest to x3, and in the example described above, INT (x1)=5, INT (x2)=3, INT (x3)=2.
Accordingly, because the optical path difference generated by this step difference d1 is 5 times of the first wavelength λ1, and 3 time of the second wavelength λ2, the first light flux and the second light flux transmit as they are, without receiving any action by the phase structure D1.
Because the optical path difference generated by this step difference d1 is a half integer times of the third wavelength λ3, and the phase of the third light flux passing the concave convex part of the rectangular pattern is dislocated by π, almost light amount of the third light flux incident on the phase structure D1 is distributed into the 1-order diffraction light and −1-order diffraction light. The phase structure D1 is designed in such a manner that the 1-order diffraction light of them is light converged on the information recording surface (RL3) of the third optical disk (for example, CD). Herein, the paraxial diffraction power of the phase structure D1 is negative, and the third light flux incident on the phase structure D1 is converted into the divergent light flux and incident on the light converging element CE. In this case, the spherical aberration in the under compensation direction generated by the magnification change of the light converging element CE, and the spherical aberration in the over compensation direction generated due to the difference of thickness between the protective layer of the high density optical disk and that of CD, are cancelled out, and the 1-order diffraction light of the third light flux is light converged on the information recording surface (RL3) under the condition that the spherical aberration is compensated. Herein, the diffraction power PD is defined by PD=−2 dor·B2 by the 2-order diffraction surface coefficient B2 of the optical path difference function φ which will be described later, and the diffraction order dor of the diffraction light used for the recording/reproducing of the information on the optical disk. In the present specification, a phrase “diffracts and converts into the divergent light flux” means that the diffraction power defined by the above expression is negative”.
Further, also the light-shielding structure T is, as the same as the phase structure D1, a structure in which patterns whose sectional shape including the optical axis is rectangular, are arranged on the concentric circle, and the step difference d2 of the rectangular pattern is set to a height satisfying d2=5·λ1/(N1−1)≈3λ2/(N2−1)≈2.5λ3/(N3−1).
That is, the step difference of the light-shielding structure T is set to a height satisfying the following expressions.
When
x1=(d2×(N1λ1−1)/λ1
x2=(d2×(N2λ2−1)/λ2
x3=(d2×(N3×3−1)/λ3,
then,
0≦|INT(x1)−1|≦0.1,
0≦|INT(x2)−1|≦0.1,
and
0.4≦|INT(x3)−x3|≦0.6.
Where, INT(x1), INT (x2), and INT (x3) respectively express a natural number closest to x1, a natural number closest to x2, and a natural number closest to x3, and in the example described above, INT(x1)=5, INT(x2)=3, INT(x3)=2.
By the same principle as the phase structure D1, the first light flux and the second light flux transmit as they are, without receiving any action by the light-shielding structure T.
On the one hand, in the light-shielding structure T, when the concave convex part is one period, because the rectangular patterns are arranged so that the width of this 1 period is periodic, (in
Hereupon, in the wavelength selection element WSE, although rectangular patterns are arranged so that the width of one period of the concave convex part of the light-shielding structure T is constant, this arrangement may be allowed when the third light flux receives the diffraction action, further, when the diffraction light of the third light flux passing the light-shielding structure T has the periodicity so that the diffraction light spreads on the position sufficiently separated from the spot, and other than the arrangement in which the width of one period becomes constant, an arrangement in which the width of one period becomes small as facing the outer periphery, or the width of one period becomes large as facing the outer periphery, may also be allowed.
Further, on the optical surface S1 on the light source side of the wavelength selection element WSE, a second phase structure D2 by which the first light flux and the third light flux are not diffracted but the second light flux is diffracted, is formed. The second phase structure D2 is a structure in which patterns whose sectional shape including the optical axis is step-like, are arranged on the concentric circle, and in which the step is shifted by the height for the number of steps corresponding to the number of level surfaces (in
Hereupon, there are 2 examples in the structure in which patterns whose sectional shape including the optical axis is step-like, are arranged on the concentric circle. One is the saw-toothed type as described above, and the other one is a structure in which the step difference of the adjoining level surfaces is always one step, and this is called a continuous type. Hereupon, in
The saw-toothed type shown in
Herein, the step difference d3 of the step-like pattern is set to a height satisfying d3=2·λ1/(N1−1)≈1.2·λ2/(N2−1)≈1·λ3/(N3−1). Herein, N1 is a refractive index of the wavelength selection element WSE in the first wavelength λ1, N2 is a refractive index of the wavelength selection element WSE in the second wavelength λ2, and N3 is a refractive index of the wavelength selection element WSE in the third wavelength λ3.
That is, the depth d3 of one step difference of the step-like patterns is set so as to satisfy the following relationships.
When x1=(d3×(N1λ1−1))/λ1, x3=(d3×(N3λ3−1))/λ3, then, 0≦|INT (x1)−1|≦0.1, 0≦|INT (x3)−1|≦0.1. Where, INT(x1), INT(x3) respectively express a natural number closest to x1, a natural number closest to x3.
Because the optical path difference generated by this step difference d3 is 2 times of the first wavelength λ1, and 1 time of the third wavelength λ3, the first light flux and the third light flux transmit as they are, without receiving any action by the second phase structure D2.
On the one hand, because the optical path difference generated by this step difference d3 is 1.2 times of the second wavelength λ2, the phase of the second light flux passing the level surface before or after the step difference is deviated by 2π/5. Because one saw-tooth is divided into 5, the deviation of the phase of the second light flux is just 5×2π/5=2π for one saw-tooth, and the 1-order diffraction light is generated. Herein, the paraxial diffraction power of the phase structure D1 is negative, the third light flux incident on the phase structure D1 is converted into the divergent light flux and is incident on the light converging element CE. In this case, the spherical aberration in the under compensation direction generated by the magnification change of the light converging element CE and the spherical aberration in the over compensation direction generated by the differences of the thickness between the protective layer of the high density optical disk and that of DVD are cancelled with each other, and the 1-order diffraction light of the second light flux is light-converged on the information recording surface (RL2) of the second optical disk (for example, DVD), under the condition that the spherical aberration is compensated.
Hereupon, the second phase structure D2 is formed only in the area corresponding to inside of the numerical aperture NA2, and because the spherical aberration due to the difference between the thickness of the protective layer of the high density optical disk and that of DVD is not compensated in the area of the outside of the numerical aperture NA2, the second light flux passed the area of the outside of the numerical aperture NA2 becomes a flare component spreading in the position sufficiently separated from the spot formed on the information recording surface (RL2) of the second optical disk (for example, DVD). Accordingly, because the wavelength selection element WSE used for the optical pick-up apparatus PU of the present embodiment has not only the aperture limit function corresponding to the numerical aperture NA3 of the third optical disk (for example, CD), but also the aperture limit function corresponding to the numerical aperture NA2 of the second optical disk (for example, DVD), the simplification of the structure of the optical pick-up apparatus or the reduction of the number of parts can be realized.
It is preferable that the light-shielding structure T formed in the second area in the present embodiment, is a structure in which patterns whose sectional shape including the optical axis is rectangular, are arranged about concentric circularly.
Further, when the step difference depth of the light-shielding structure is d2, the refractive index to the first light flux of the optical element is n1, and the refractive index to the second light flux of the optical element is n2, x1 and x2 expressed by the following expressions,
x1=(d2×(n1λ1−1))/λ1
x2=(d2×(n2λ2−1))/λ2,
respectively satisfy the following relational expressions.
0≦|INT (x1)−1|≦0.1
0≦|INT (x2)−1|≦0.1
Where, INT (x1), INT (x2) respectively express a natural number closest to x1, a natural number closest to x2.
In the optical pick-up apparatus according to the present embodiment, when, further, the refractive index to the third light flux of the optical element is n3, it is preferable that x3 expressed by the following expression satisfies the following relational expression.
x3=(d2×(n3λ3−1))/λ3
0.4≦|INT(x3)−x3|≦0.6
Where, INT (x3) expresses a natural number closest to x3. Further, it is preferable that the first wavelength λ1 and the second wavelength λ2 satisfy the following relationship, and in that case, it is further preferable mode that the INT(x1) and INT(x2) satisfy the following relationship.
INT(x1)=5
INT(x2)=3
Further, the rectangular pattern may be arranged periodically, or aperiodically.
In the present embodiment, it is also one of preferable modes that the second area further has the second (a) area adjoining the outside of the first area and the second (b) area positioned at the outside of the second (a) area, and the light-shielding structure is formed in only either one area of the second (a) area or the second (b) area, and the other area is a continuous surface on which the light-shielding structure is not formed.
Referring to
In the first area R1, the phase structure D1 which does not diffract the first light flux and the third light flux, and diffracts the second light flux, is formed, and on the optical surface S1 on the light source side, the second phase structure D2 which does not diffract the first light flux and the second light flux, and diffracts the third light flux, is formed. Because the respective functions or structures of the phase structure D1 and the second phase structure D2 in the wavelength selection element WSE in
In the wavelength selection element WSE in
Then, this rectangular pattern is, when the concave convex part is made one period, arranged aperiodically so that the diffraction efficiency of the ±1-order diffraction light of the third light flux is enough reduced. When the concave convex part is aperiodically arranged in this manner, the third light flux incident on the light-shielding structure T goes straight on as it is, without receiving the diffraction action, however, because the height of the step difference d1 is a half integer times of the third wavelength λ3, the dislocation of the phase of the wave-front passing the adjoining concave convex part becomes π, and the phase of the wave-front passing the adjoining concave convex part is cancelled each other. As the result, the intensity of the light in the light converging spot by the 0-order diffraction light (non-diffraction light) of the third light flux incident on the light-shielding structure is extremely lowered, and the aperture limit function corresponding to the numerical aperture NA3 can be given to the light-shielding structure T.
Further, when there is a shape error in the rectangular pattern, the transmission of the incident light flux is lowered by the influence of the scattering of the error part, and as larger the area forming the light-shielding structure T increases, this transmission lowering becomes large. On the one hand, in order to give the aperture limit function corresponding to the numerical aperture NA3 to the light-shielding structure T, it is necessary that, in the ray of light passing the area of the outside of the numerical aperture NA3, the ray of light converged in the vicinity of the spot formed on the information recording surface (RL3) of the third optical disk (for example, CD), is at least light-shielded.
In the wavelength selection element WSE in
Hereupon, in contrast with the wavelength selection element WSE in
In the present embodiment, it is preferable that the light flux whose aberration is compensated by the phase structure formed in the first area, is any one light flux of the second light flux and the third light flux.
It is preferable that the phase structure compensates the aberration by diffracting at least one light flux of the second light flux and the third light flux and increasing the divergent angle.
Further, it is one of further preferable modes that the wavelength selection element has the third area structuring the central area including the optical axis of the optical element on the optical surface S2 different from the optical surface S1 on the light source side, and the third area has the second phase structure being capable of compensating the aberration of the light flux whose aberration is not compensated by the phase structure, in the second or third light flux.
It is one of the preferable modes that the second phase structure of the wavelength selection element compensates the aberration by diffracting the light flux whose aberration is not compensated by the phase structure and by increasing the divergent angle.
As the wavelength selection element in the present embodiment, it is one of preferable modes that the phase structure is capable of compensating the aberration of the third light flux, and the phase structure is a structure in which patterns whose sectional shape including the optical axis is rectangular are arranged concentric circularly, and when the step difference depth of the phase structure is d1, the refractive index to the first light flux of the optical element is n1, and the refractive index to the second light flux of the optical element is n2, x1 and x2 expressed by the following expressions respectively satisfy the following relational expressions.
x1=(d1×(n1λ1−1))/λ1
x2=(d1'(n2λ2−1))/λ2,
0≦|INT(x1)−1|≦0.1
0≦|INT(x2)−1|≦0.1
Where, INT(x1), INT(x2) respectively express, a natural number closest to x1, a natural number closest to x2.
In the above embodiment, it is further preferable that the second phase structure provided on the optical surface S2 different from the optical surface S1 on the light source side is capable compensating the aberration of the second light flux, and the second phase structure is a structure in which patterns whose sectional shape including the optical axis is stepwise shape are arranged concentric circularly, and when the step difference depth of one of step shaped patterns of the second phase structure is d3, the refractive index to the first light flux of the optical element is n1, and the refractive index to the third light flux of the optical element is n3, x1 and x3 expressed by the following expressions respectively satisfy the following relational expressions.
x1=(d3×(n1λ1−1))/λ1
x2=(d3×(n3λ3−1))/λ3,
0≦|INT (x1)−1|≦0.1
0≦|INT (x3)−1|≦0.1
Where, INT(x1), INT(x3) respectively express, a natural number closest to x1, a natural number closest to x3.
In the wavelength selection element, it is preferable that each of the patterns of the second phase structure has predetermined number of level surfaces, and each of the steps of the pattern is shifted in accordance with the number of the level surfaces.
As an another mode of the wavelength selection element, it is one of preferable modes that the phase structure is capable of compensating the aberration of the second light flux, and the first phase structure is a structure in which patterns whose sectional shape including the optical axis is a stepwise shape are arranged concentric circularly, and when the step difference depth of one of the stepwise patterns of the first phase structure is d1, the refractive index to the first light flux of the optical element is n1, and the refractive index to the third light flux of the optical element is n3, x1 and x3 expressed by the following expressions respectively satisfy the following relational expressions.
x1=(d1×(n1λ1−1))/λ1
x3=(d1×(n3λ3−1))/λ3,
0≦|INT (x1)−1|≦0.1
0<|INT (x3)−1|≦0.1
Where, INT(x1), INT(x3) respectively express a natural number closest to x1, a natural number closest to x3.
In the wavelength selection element, it is preferable that each of the patterns of the phase structure has predetermined number of level surfaces, and each of the steps of the pattern is shifted in accordance with the number of the level surfaces.
Further, in the above embodiment, it is preferable that the second phase structure is capable of compensating the aberration of the third light flux, and the second phase structure is a structure in which patterns whose sectional shape including the optical axis is rectangular are arranged concentric circularly, and when the step difference depth of the second phase structure is d3, the refractive index to the first light flux of the optical element is n1, and the refractive index to the second light flux of the optical element is n2, x1 and x3 expressed by the following expressions respectively satisfy the following relational expressions.
x1=(d3×(n1λ1−1))/λ1
x2=(d3×(n2λ2−1))/λ2,
0≦|INT (x1)−1|≦0.1
0≦|INT (x2)−1|≦0.1
Where, INT(x1), INT(x2) respectively express a natural number closest to x1, a natural number closest to x2.
The wavelength selection element may be an objective lens which is a light converging lens, or a lens separately provided from the objective lens, and when the wavelength selection element is a lens separately provided from the objective lens, it is further preferable that the wavelength selection element and the objective lens are held so that the mutual relative positional relationship is constant. Further, it is one of preferable modes that the wavelength selection element is made of resin.
In the wavelength selection element, it is one of preferable modes that the resin layer in which the phase structure and light-shielding structure are formed, is formed on the glass substrate.
As the wavelength selection element WSE, all optical resins or optical glass are applicable, however, in order to form the light-shielding structure T which is a minute structure, phase structure D1 or second phase structure D2, a material whose viscosity under the fused condition is small, that is, the resin used for the optical purpose (called optical resin) is adequate. Further, when the optical resin is used, as compared to the case where the optical glass is used, an element of stable performance can be mass-produced at low cost. Further, because it is light weight, it is enough even when the drive force of the actuator AC1 (refer to
In the present specification, the optical disk for which a blue violet semiconductor laser or blue violet SHG laser is used as the light source for the recording/reproducing of the information, is generally called “high density optical disk”, and other than the optical disk (for example, blue ray disk, abbreviated as BD) of a standard in which the recording/reproducing of the information is conducted by the objective optical system of NA 0.85, and the thickness of the protective layer is about 0.1 mm, the optical disk (for example, HD DVD, abbreviated as HD) of a standard in which the recording/reproducing of the information is conducted by the objective optical system of NA 0.65 to 0.67, and the thickness of the protective layer is about 0.6 mm, is also included. Further, other than the optical disk having such a protective layer on its information recording surface, the optical disk having the protective layer whose thickness is about several—several tens nm on the information recording surface, or the optical disk whose thickness of the protective layer or thickness of the protective film is 0, is also included. Further, in the present specification, in the high density optical disk, the photo-magnetic disk which uses the blue violet semiconductor laser or blue violet SHG laser as the light source for the recording/reproducing of the information, is also included.
Further, in the present specification, DVD is a general name of a DVD series optical disk such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD+R, DVD+RW, and CD is a general name of a CD series optical disk such as CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW. In the recording density, the high density optical disk is highest, and subsequently, it becomes low in order of DVD, CD.
Further, in the present specification, the “objective optical system” indicates, in the optical pick-up apparatus, the optical system which is arranged at a position facing the optical disk, and at least includes the light converging element having the function by which the light fluxes which are projected from the light source and whose wavelengths are different from each other, are light converged on respective information recording surfaces of the optical disks whose recording densities are different from each other.
Further, when there is the optical element which is integrated with the light converging element and which conducts tracking and focusing by the actuator, the optical system which is structured by these optical element and light converging element, is the objective optical system. When the objective optical system is structured by a plurality of optical elements in this manner, the phase structure may also be formed on the optical surface of the light converging element, however, in order to reduce the influence of the eclipse of the light flux by the step difference part of the phase structure, it is preferable that the phase structure is formed on the optical surface of the optical element other than the light converging element.
Embodiments of the present invention will be described by using the drawings below. Initially, by using
The optical pick-up apparatus PU is structured by: a blue violet semiconductor laser LD1 (the first light source) which projects the laser light flux (the first light flux) of 405 nm emitted when the recording/reproducing of the information is conducted on BD; a DVD/CD-use laser light source unit LU in which the first light emitting point (the second light source) EP1 which projects the laser light flux (the second light flux) of 655 nm emitted when the recording/reproducing of the information is conducted on DVD, and the second light emitting point (the third light source) EP2 which projects the laser light flux (the third light flux) of 785 nm emitted when the recording/reproducing of the information is conducted on CD, are formed on one chip; a photo detector PD commonly used for BD/DVD/CD; an objective optical element OBJ structured by the wavelength selection element WSE and the light converging element CE, both surfaces of which are aspheric, having a function by which the laser light fluxes transmitted this wavelength selection element WSE are converged on the information recording surfaces RL1, RL2, RL3; a 2-axis actuator AC1; a 1-axis actuator AC2; an expander lens EXP structured by the first lens EXP1 whose paraxial refractive index is negative, and the second lens EXP2 whose paraxial refractive index is positive; the first polarizing beam splitter BS1; the second polarizing beam splitter BS2; the first collimator lens COL1; the second collimator lens COL2; the third collimator lens COL3; and a sensor lens SEN for adding the astigmatism to the reflection light fluxes from the information recording surfaces RL1, RL2, and RL3. Hereupon, as the light source for BD, other than the blue violet semiconductor laser LD1, the blue violet SHG laser can also be used.
In the optical pick-up apparatus PU, when the recording/reproducing of the information is conducted on BD, as its light ray path is drawn by a solid line in
The reflection light flux modulated by the information pit on the information recording surface RL1 becomes, after transmits again the objective optical system OBJ, second lens EXP2, first lens EXP1, second polarizing beam splitter BS2, first polarizing beam splitter BS1, the converging light flux when it passes the third collimator lens COL3, and the astigmatism is added to it by the sensor lens SEN, and it is converged on the light receiving surface of the photo detector PD. Then, by using the output signal of the photo detector PD, the information recorded in BD can be read.
Further, in the optical pick-up apparatus PU, when the recording/reproducing of the information is conducted on DVD, the light emitting point EP1 is light emitted. The divergent light flux projected from the light emitting point EP1 is, as its light ray path is drawn by a dotted line in
The reflection light flux modulated by the information pit on the information recording surface RL2 becomes, after transmits again the objective optical system OBJ, second lens EXP2, first lens EXP1, second polarizing beam splitter BS2, first polarizing beam splitter BS1, the converging light flux when it passes the third collimator lens COL3, and the astigmatism is added to it by the sensor lens SEN, and it is converged on the light receiving surface of the photo detector PD. Then, by using the output signal of the photo detector PD, the information recorded in DVD can be read.
Further, in the optical pick-up apparatus PU, when the recording/reproducing of the information is conducted on CD, the light emitting point EP2 is light emitted. The divergent light flux projected from the light emitting point EP2 is, as its light ray path is drawn by a one-dotted chain line in
The reflection light flux modulated by the information pit on the information recording surface RL2 becomes, after transmits again the objective optical system OBJ, second lens EXP2, first lens EXP1, second polarizing beam splitter BS2, first polarizing beam splitter BS1, the converging light flux when it passes the third collimator lens COL3, and the astigmatism is added to it by the sensor lens SEN, and it is converged on the light receiving surface of the photo detector PD. Then, by using the output signal of the photo detector PD, the information recorded in CD can be read.
In the optical pick-up apparatus of the present embodiment, the wavelength selection element WSE and the light converging element CE as the exclusive use lens for BD, are integrated coaxially through a jointing member B. Specifically, they are structured in such a manner that the wavelength selection element WSE is engaged and fixed on one end of the cylindrical jointing member B, and on the other end, the light converging element CE is engaged and fixed, and they are integrated coaxially along the optical axis X. Hereupon, when the wavelength selection element WSE and the light converging element CE are integrated, it is suitable when the mutual relative positional relationship of the wavelength selection element WSE and the light converging element CE is held so that it is constant, and other than the method through the jointing member B, a method by which respective mutual flange parts of the wavelength selection element WSE and the light converging element CE are engaged and fixed, may also be suitable.
In this manner, when the mutual relative positional relationship of the wavelength selection element WSE and the light converging element CE is held so that it is constant, the generation of the aberration at the time of the focusing or tracking can be suppressed, and the good focusing characteristic or tracking characteristic can be obtained.
Further, when the first lens EXP1 of the expander lens EXP is driven in the optical axis direction by 1-axis actuator AC2, the spherical aberration of the spot formed on the information recording surface RL1 of BD can be compensated. The cause of generation of the spherical aberration to be compensated by the position adjustment of the first lens EXP1 is, for example, the wavelength dispersion by the production error of the blue violet semiconductor laser LD1, the refractive index change or refractive index distribution of the objective lens system following the temperature change, the focus jump between the information recording layers of multi-layer disk such as 2-layer disk or 4-layer disk, or the thickness dispersion or thickness distribution by the production error of the protective layer of BD. Hereupon, instead of the first lens. EXP1, even when a structure by which the second lens EXP2 or the first collimator lens COL1 is driven in the optical axis direction, is applied, the spherical aberration of the spot formed on the information recording surface RL1 of BD can be compensated.
Further, in the above description, a structure by which, when the first lens EXP1 is driven in the optical axis direction, the spherical aberration of the spot formed on the information recording surface RL1 of BD is compensated, is applied, however, a structure by which the spherical aberration of the spot formed on the information recording surface RL2 of DVD, furthermore, the spherical aberration of the spot formed on the information recording surface RL3 of CD is compensated, may also be applied.
Further, in the present embodiment, the DVD/CD use laser light source unit in which the first light emitting point EP1 and the second light emitting point EP2 are formed on one chip is used, however, it is not limited to this, a BD/DVD/CD use one chip laser light source unit LU in which further the light emitting point projecting the laser light flux of wavelength 405 nm of BD is also formed on the same chip, may also be used. Alternatively, the BD/DVD/CD use 1-can laser light source unit in which 3 laser light sources of the blue violet semiconductor laser, red semiconductor laser, and infrared semiconductor laser are housed in one casing may also be used.
Further, in the present embodiment, a structure in which the light source and the photo detector PD are separately arranged, is applied, however, it is not limited to this, a laser light source module in which the light source and the photo detector PD are integrated may also be used.
Hereupon, in the present embodiment, a structure in which the wavelength selection element WSE and the light converging element CE are arranged as separated optical elements is applied, however, it is not limited to this, when, on the optical surface of the light converging element CE, the phase structure D1, light-shielding structure T, and the second phase structure D2 are formed, a system in which a function of the wavelength selection element WSE is given to the light converging element CE may also be used as the objective optical system OBJ.
(A Numeric Value Example)
Next, a specific numeric value example of the objective optical system OBJ which is composed of the wavelength selection element WSE and the light converging element CE as shown in
In the present numeric value example, an optical path difference added to the incident light flux by the phase structure D1 and the second phase structure D2 is expressed by the following optical path difference function φ (mm).
(Optical Path Difference Function) φ=λ/λB×dor×(B2y2+B4y4+B4y6+B8y8+B10y10),
Where, φ: optical path difference function, λ: wavelength of the light flux incident on the diffraction structure, λB: production wavelength, dor: diffraction order of the diffraction light used for the recording/reproducing on the optical disk, y: distance from the optical axis, B2, B4, B6, B8, B10: diffraction surface coefficients.
Further, the optical surface of the objective lens system is formed into an aspheric surface which is axially symmetric around the optical axis, which is regulated by an equation in which coefficients shown in Table 1 are respectively substituted into the following aspheric surface expression.
(Aspheric Surface Expression)
z=(y2/R)/[1{square root}{square root over ( )}+{1−(K+1)(y/R)2}]+A4y4+A6y6+A8y8+A10y10+A12y12+A14y14+A16y16+A18y18+A20y20.
Where, z: aspheric surface shape (distance in the direction along the optical axis from the surface top of the aspheric surface), y: distance from the optical axis, R: radius of curvature, K: conic coefficient, A4, A6, A8, A10, A12, A14, A16, A18, A20: aspheric surface coefficients.
The phase structure D1 formed in the first area R1 (an area corresponding to within NA3) on the optical disk side of the wavelength selection element WSE is a structure in which rectangular patterns are arranged, and its height Δ1 is set to 3.89 μm. Further, in the second area which is an area corresponding to the outside of the NA3, the light-shielding structure T which is a structure in which rectangular patterns whose height is the same as that of the phase structure, are arranged, is formed, and the width of one period of the concave convex part is set to 10 μm, and its width is constant in the second area R2. Hereupon, on the optical surface S2 on the optical disk side of the wavelength selection element WSE, the diameter of the first area R1 is 2.22 mm.
Further, in the present numeric value example, because the ray of light passing the peripheral part of the second area R2 is collected in the vicinity of the spot, and the ray of light passing the inside (a side near the first area R1) of the second area R2 is spread in the position far separated from the spot, as shown in
Hereupon, in the objective optical system OBJ of the present numeric value example, because the stop STO of the diameter of 3.74 mm corresponding to NA1=0.85 is arranged at the surface top position of the optical surface S3 on the light source side of the light converging element CE, the diameters of the first light flux to the third light flux passed the wavelength selection element WSE are regulated by this stop. Further, the numerical aperture and the entrance pupil diameter when each of BD, DVD, CD is used, are shown in Table 2.
According to the present invention, although the recording and/or reproducing of the information can be adequately conducted on the three kinds of optical disks which are different from each other, an optical pick-up apparatus by which the size reduction can be intended, a drive apparatus of the optical disks, an objective optical system and a wavelength selection element, used for it, can be provided.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2004-138635 | May 2004 | JP | national |
