Patent Application
20070286054
Referring now to the drawings, a description will be given in detail of a preferred embodiment in accordance with the present invention.
In the following, an objective lens in accordance with an embodiment of the present invention will be described. The objective lens of this embodiment, which is installed in an optical information recording/reproducing device, has the compatibility with three types of optical discs according to different standards (protective layer thickness, data density, etc.).
In the following explanation, an optical disc of a type (one of the three types) having the highest data density (e.g. new-standard optical disc such as HD DVD, DVD or BD) will be referred to as a “first optical disc D1”, an optical disc of a type having a relatively low data density compared to the first optical disc D1 (DVD, DVD-R, etc.) will be referred to as a “second optical disc D2”, and an optical disc of a type having the lowest data density (CD, CD-R, etc.) will be referred to as a “third optical disc D3” for convenience of explanation.
The protective layer thicknesses t1-t3 of the first through third optical discs D1-D3 satisfy the following relationship:
t1≦t2<t3
In order to carry out the information read/rite on each of the optical discs D1-D3, the NA (Numerical Aperture) required for the information read/write has to be varied properly so that a beam spot suitable for the particular data density of each disc can be formed. The optimum design NAs required for the information read/write on the three types of optical discs D1, D2 and D3 (hereinafter described as “NA1”, “NA2” and “NA3”) satisfy the following relationships:
(NA1>NA3) and (NA2>NA3)
Specifically, for the information read/write on the first or second optical disc D1, D2 (having high data density), a relatively large NA is required since a relatively small spot has to be formed. On the other hand, for the information read/write on the third optical disc D3 (having the lowest data density), the required NA is relatively small. Incidentally, each optical disc is set on an unshown turntable and rotated at high speed when the information read/write is carried out.
In cases where three types of optical discs D1-D3 (having different data densities) are used as above, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device so that a beam spot suitable for each data density can be formed on the record surface. Specifically, for the information read/write on the first optical disc D1, a “first laser beam” having the shortest wavelength (first wavelength) is emitted from a light source so as to form the smallest beam spot on the record surface of the first optical disc D1. On the other hand, for the information read/write on the third optical disc D3, a “third laser beam” having the longest wavelength (third wavelength) is emitted from a light source so as to form the largest beam spot on the record surface of the third optical disc D3. For the information read/write on the second optical disc D2, a “second laser beam” having a wavelength longer than that of the first laser beam and shorter than that of the third laser beam (second wavelength) is emitted from a light source so as to form a relatively small beam spot on the record surface of the second optical disc D2.
As shown in
By letting each coupling lens 1B-3B transform each laser beam (to be incident upon the objective lens 10) into a collimated beam as above, off-axis aberration occurring during the tracking of the objective lens 10 (e.g. coma aberration) can be suppressed.
Incidentally, there are cases where each light beam emerging from each coupling lens 1B-3B is not necessarily a collimated beam in a strict sense, due to various factors such as individual differences and installation positions of the light sources 1A-3A, variations in the environment around the optical information recording/reproducing device 100, etc. However, the divergence angle of the light beam caused by the above factors is extremely small and the aberration occurring during the tracking shifts can also be regarded to be small, by which substantially no problem is caused in practical use.
As shown in
where X(h) denotes a SAG amount of a coordinate point on the aspherical surface whose height (distance) from the optical axis is h (SAG amount: distance measured from a tangential plane contacting the aspherical surface on the optical axis), “C” denotes the curvature (1/r) of the aspherical surface on the optical axis, “K” denotes a cone constant, and each “A2i” (i: integer larger than 1) denotes an aspherical coefficient of the 2i-th order (the summation in the expression includes aspherical coefficients A4, A6, A8, A10, A12, . . . of the fourth order, sixth order, eighth order, tenth order, twelfth order, and so forth).
In cases where multiple laser beams of different wavelengths are used for various optical discs D1-D3 as in the optical information recording/reproducing device 100 of this embodiment, spherical aberration occurs due to variations in the refractive index of the objective lens 10, the thickness of the protective layer 21, etc. (which vary depending on which optical disc is used).
Therefore, in order to correct the spherical aberration (occurring in different ways when the three types of optical discs D1-D3 are used) and achieve the compatibility with the optical discs D1-D3, at least the cementing surface 13 of the objective lens 10 of this embodiment is provided with a diffracting structure having diffracting effects on the three types of light beams. The diffracting structure formed at the cementing surface 13 includes a plurality of concentric refracting surfaces (annular zones) around the optical axis AX and a plurality of minute level differences each of which is formed between adjacent refracting surfaces.
The objective lens 10 of this embodiment has the function of converging the first through third laser beams on the record surfaces of the corresponding optical discs (D1, D2, D3) respectively while correcting the spherical aberration to approximately 0 by the diffracting effect and refracting effect of the cementing surface 13 and refracting effects of the first and second surfaces 11 and 12.
The configuration of the diffracting structure (cementing surface 13) of the objective lens 10 of this embodiment is specified by an optical path difference function which will be explained below. The optical path difference function represents the function of the objective lens 10 as a diffracting lens, in terms of an optical path length addition at each height h from the optical axis. The optical path difference function φ(h) can be expressed by the following expression:
In the above optical path difference function φ(h), each “P2i” (i: positive integer) denotes a coefficient of the 2i-th order (the summation in the expression includes coefficients P2, P4, P6, . . . of the second order, fourth order, sixth order, and so forth), “m” denotes the diffraction order maximizing the diffraction efficiency of the laser beam being used, and “λ” denotes the design wavelength of the laser beam being used. In this embodiment, the diffraction orders “m” of the first through third laser beams maximizing their diffraction efficiency satisfies the following condition:
m(λ1)>m(λ2)≧m(λ3)≧1
where m(λ1) denotes the diffraction order of the first laser beam (having the first wavelength) maximizing its diffraction efficiency, m(λ2) denotes the diffraction order of the second laser beam (having the second wavelength) maximizing its diffraction efficiency, and m(λ3) denotes the diffraction order of the third laser beam (having the third wavelength) maximizing its diffraction efficiency. By setting the diffraction orders of the first through third laser beams, high performance (light utilization efficiency, etc.) can be achieved while maintaining higher degree of freedom of material selection compared to cemented diffracting lenses which have been proposed.
Each optical member (10A, 10B) forming the objective lens 10 is made of material that is specified by a particular ratio among diffraction orders (of the first through third laser beams) maximizing diffraction efficiency of each of the laser beams employed (hereinafter simply referred to as “the ratio among the diffraction orders”) so that high “light utilization efficiency” can be achieved irrespective of which of the first through third laser beams is incident upon the objective lens 10.
In the following, the selection of the materials of the optical members 10A and 10B suitable for two configuration examples of the objective lens 10 (first configuration example having a ratio of 3:2:2, second configuration example having a ratio of 5:3:3) will be described in detail. In the first configuration example, a diffracting structure whose ratio among the diffraction orders is 3:2:2 (listed from the diffraction order of the first laser beam) is formed at the cementing surface 13 of the objective lens 10. In the second configuration example, a diffracting structure whose ratio among the diffraction orders is 5:3:3 is formed at the cementing surface 13 of the objective lens 10.
As the materials of the optical members 10A and 10B forming the objective lens 10 of the first configuration example, those satisfying the following conditions (1) and (2) are selected:
1.00≦Δn(λ2)/Δn(λ1)≦1.18 (1)
1.02≦Δn(λ3)/Δn(λ1)≦1.30 (2)
where:
Δn(λ1)=n2(λ1)−n1(λ1),
Δn(λ2)=n2(λ2)−n1(λ2),
Δn(λ3)=n2(λ3)−n1(λ3),
The conditions (1) and (2) are those for achieving relatively high light utilization efficiency in the use of the second or third optical disc (D2, D3) with reference to the light utilization efficiency in the use of the first optical disc D1. In the conditions (1) and (2), when the value (ratio) goes below the lower limit or over the upper limit, the light utilization efficiency in the use of each optical disc gets too low and stray light deriving from diffracted beams of unnecessary orders becomes a problem.
In the second configuration example, materials satisfying the following conditions (3) and (4) are selected for the first and second optical members 10A and 10B of the objective lens 10:
0.85≦Δn(λ2)/Δn(λ1)≦1.10 (3)
0.88≦Δn(λ3)/Δn(λ1)≦1.25 (4)
The upper and lower limits in the above conditions (3) and (4) are set for the objective lens 10 of the second configuration example to let it achieve effects similar to those of the objective lens 10 of the first configuration example satisfying the conditions (1) and (2).
Both in the first and second configuration examples, the materials of the first and second optical members 10A and 10B are selected to satisfy the following condition (5):
0.01≦|Δn(λ1)|≦0.15 (5)
The above condition (5) is related to whether the cementing surface 13 can be formed with ease or not. The depth At of the diffracting level difference in the optical axis direction is given by the following expression:
Δt=m·λ/Δn(λ) (6)
where “λ” and “m” denote the wavelength and diffraction order maximizing the diffraction efficiency and Δn(λ) denotes the difference between the refractive indexes of the first and second optical members 10A and 10B at the wavelength λ.
Specifically, the level difference becomes extremely deep when Δn(λ) is too small since the depth Δt of the level difference is inversely proportional to Δn(λ). Therefore, the first and second optical members 10A and 10B are required to have a refractive index difference Δn(λ) not less than the lower limit of the condition (5). On the other hand, if the refractive index difference Δn(λ) becomes too large, the amount of aberration occurring at the cementing surface 13 becomes excessive, by which permissible ranges of shape error and decentering of the cementing surface 13 become extremely narrow. Thus, the materials of the first and second optical members 10A and 10B are selected so as not to exceed the upper limit of the condition (5).
In the following, four specific examples of the objective lens 10 of this embodiment explained above will be described.
In the following examples, a first optical disc D1 having the highest data density and a protective layer thickness of 0.6 mm, a second optical disc D2 having a data density lower than that of the first optical disc D1 and a protective layer thickness of 0.6 mm, and a third optical disc D3 having the lowest data density and a protective layer thickness of 1.2 mm are assumed to be used.
The overall configuration of the objective lens 10 as a first example is shown in
According to Table 1, the values of the conditions (1), (2) and (5) of the objective lens 10 of the first example are 1.105, 1.127 and 0.015, respectively. Thus, the objective lens 10 of the first example satisfies all the conditions (1), (2) and (5). With this configuration, diffraction efficiency of 100% can be secured for the first and second laser beams as well as securing diffraction efficiency of 79% for the third laser beam. As above, the objective lens 10 of the first example is capable of securing high light utilization efficiency irrespective of which of the optical discs D1-D3 is used.
The overall configuration of the objective lens 10 as a second example is also shown in
According to Table 2, the values of the conditions (3), (4) and (5) of the objective lens 10 of the second example are 1.080, 1.095 and 0.024, respectively. Thus, the objective lens 10 of the second example satisfies all the conditions (3), (4) and (5). With this configuration, diffraction efficiency of 100% can be secured for the first laser beam, 71% can be secured for the second laser beam, and 88% can be secured for the third laser beam. As above, similarly to the first example, the objective lens 10 of the second example is capable of securing high light utilization efficiency irrespective of which of the optical discs D1-D3 is used.
A third example described below is a specific example of the optical information recording/reproducing device 100 equipped with the objective lens 10 of the embodiment. The overall configuration of the optical information recording/reproducing device 100 of the third example is shown in
As indicated by the “Maginfication M” in Table 3, the laser beam is incident upon the objective lens 10 as a collimated beam in the optical information recording/reproducing device 100 of the third example irrespective of which of the optical discs D1-D3 is used (i.e. irrespective of which of the first through third laser beams is used). The following Tables 4-6 show specific numerical configurations of the optical information recording/reproducing device 100 (equipped with the objective lens 10 having the specifications shown in Table 3) when each of the optical discs D1-D3 is used.
In Tables 4-6, “r” denotes the curvature radius [mm] of each optical surface, “d” denotes the distance [mm] from each optical surface to the next optical surface during the information read/write, “n (X nm)” denotes the refractive index of a medium between each optical surface and the next optical surface for a wavelength of X nm (ditto for Tables 10, 11 and 12 explained later in the fourth example).
As shown in the “REMARKS” in Tables 4-6, the surface No. 0 represents the light source (1A-3A), the surface No. 1 represents the first surface 11 of the objective lens 10, the surface No. 2 represents the cementing surface 13 of the objective lens 10, the surface No. 3 represents the second surface 12 of the objective lens 10, the surface No. 4 represents the surface of the protective layer 21 of the optical disc (D1-D3) as the medium, and the surface No. 5 represents the record surface 22 of the optical disc (D1-D3). Incidentally, numerical configurations of optical members (elements) placed between each light source (1A-3A) and the objective lens 10 are omitted in Tables 4-6 for convenience of explanation.
The surfaces 11, 13 and 12 of the objective lens 10 (surfaces Nos. 1, 2 and 3) are aspherical surfaces. The following Table 7 shows the cone constant and aspherical coefficients specifying the shape of each aspherical surface (11, 13, 12). Incidentally, the notation “E” in Table 7, etc. means the power of 10 with an exponent specified by the number to the right of E (e.g. “E-04” means “×10−4”).
The following Table 8 shows the coefficients P2i (i: positive integer) of the optical path difference function specifying the diffracting structure formed at the cementing surface 13 of the objective lens 10 installed in the optical information recording/reproducing device 100 of the third example. The ratio among the diffraction orders of the first through third laser beams (incident upon the diffracting structure) maximizing their diffraction efficiency is 3:2:2 (according to the aforementioned first configuration example). Specifically, for the first and second optical members 10A and 10B forming the objective lens 10 of the optical information recording/reproducing device 100 of the third example, materials having refractive indexes shown in the above Tables 4-6 have been selected as those suitable for the diffracting structure achieving the above ratio 3:2:2.
According to Tables 4-6, the values of the conditions (1), (2) and (5) of the objective lens 10 of the optical information recording/reproducing device 100 of the third example are 1.057, 1.036 and 0.028, respectively. Thus, the objective lens 10 in the third example satisfies all the conditions (1), (2) and (5). With this configuration, diffraction efficiency of 100% can be secured for the first laser beam, 99% can be secured for the second laser beam, and 56% can be secured for the third laser beam. As above, the optical information recording/reproducing device 100 of the third example is capable of securing high light utilization efficiency irrespective of which of the optical discs D1-D3 is used.
A fourth example described below is another specific example of the optical information recording/reproducing device 100 equipped with the objective lens 10 of the embodiment. The overall configuration of the optical information recording/reproducing device 100 of the fourth example is also shown in
As indicated by the “MAGNIFICATION M” in Table 9, the laser beam is incident upon the objective lens 10 as a collimated beam in the optical information recording/reproducing device 100 of the fourth example irrespective of which of the optical discs D1-D3 is used (i.e. irrespective of which of the first through third laser beams is used), similarly to the third example. The following Tables 10-12 show specific numerical configurations of the optical information recording/reproducing device 100 (equipped with the objective lens 10 having the specifications shown in Table 9) when each of the optical discs D1-D3 is used.
The surfaces 11, 13 and 12 of the objective lens 10 (surfaces Nos. 1, 2 and 3) are aspherical surfaces. The following Table 13 shows the cone constant and aspherical coefficients specifying the shape of each aspherical surface (11, 13, 12).
The following Table 14 shows the coefficients P2i (i: positive integer) of the optical path difference function specifying the diffracting structure formed at the cementing surface 13 of the objective lens 10 installed in the optical information recording/reproducing device 100 of the fourth example. The ratio among the diffraction orders of the first through third laser beams (incident upon the diffracting structure) maximizing their diffraction efficiency is 5:3:3 (according to the aforementioned second configuration example). Specifically, for the first and second optical members 10A and 10B forming the objective lens 10 of the optical information recording/reproducing device 100 of the fourth example, materials having refractive indexes shown in the above Tables 10-12 have been selected as those suitable for the diffracting structure achieving the above ratio 5:3:3.
According to Tables 10-12, the values of the conditions (3), (4) and (5) of the objective lens 10 of the optical information recording/reproducing device 100 of the fourth example are 1.057, 1.036 and 0.028, respectively. Thus, the objective lens 10 in the fourth example satisfies all the conditions (3), (4) and (5). With this configuration, diffraction efficiency of 100% can be secured for the first laser beam, 82% can be secured for the second laser beam, and 67% can be secured for the third laser beam. As above, the optical information recording/reproducing device 100 of the fourth example is capable of securing high light utilization efficiency irrespective of which of the optical discs D1-D3 is used.
As described above, the objective lens for an optical information recording/reproducing device in accordance with the embodiment of the present invention is configured by cementing two optical members (differing in optical performance) together to face each other at a cementing surface 13 provided with a prescribed diffracting structure. With this configuration, it becomes possible to secure high diffraction efficiency of the light beams and achieve information read/write with higher accuracy irrespective of which of the optical discs is used (especially, even when an optical disc of low data density (CD, etc.) is used). Further, a substantially collimated light beam can be used irrespective of which of the optical discs is used, by which not only the spherical aberration but also the aberration (e.g. coma aberration) occurring during the tracking shifts can be reduced excellently whether an existing optical disc or a new-standard optical disc is used.
By the selection of a proper ratio among the diffraction orders of the first through third light beams maximizing their diffraction efficiency (with each of the diffraction orders set at the first order or higher and the diffraction order of the first light beam (of the shortest wavelength) set higher than those of the other light beams), a wider range of material selection, a higher degree of freedom of the design, and a cemented lens with the first and second optical members made of resin (facilitating the formation of the diffracting structure) are made possible. Consequently, an objective lens for an optical information recording/reproducing device, capable of forming a desired beam spot (with a sufficient light amount) on the record surface of each of the three types of optical discs having different data densities, is provided.
While a description has been given above of a preferred embodiment in accordance with the present invention, the present invention is not to be restricted by the particular illustrative embodiment and a variety of modifications, design changes, etc. are possible without departing from the scope and spirit of the present invention described in the appended claims.
For example, the objective lens described in the above embodiment (including the specific examples) is just an illustration of an objective lens in accordance with the present invention. Therefore, the objective lens in accordance with the present invention is not to be restricted to the specific numerical configurations described in the above embodiment.
While the objective lens in the above embodiment has the diffracting structure at the cementing surface 13 only, it is also possible to provide another surface (e.g. first surface 11) with a diffracting structure having a different diffracting effect. The different diffracting effect can be an effect of suppressing variation in the spherical aberration caused by slight deviation of the wavelength of the laser beam emitted by the light source from the design wavelength, an effect of suppressing variation in the spherical aberration caused by temperature variation, an effect of diverging the third laser beam incident upon an area of the objective lens 10 outside the area corresponding to the numerical aperture NA3, etc. Incidentally, the above “design wavelength” means the wavelength of each laser beam which is regarded to be optimum for the information read/write on each optical disc.
This application claims priority of Japanese Patent Application No. P2006-163367, filed on Jun. 13, 2006. The entire subject matter of the application is incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-163367 | Jun 2006 | JP | national |
