JP3980677B2 - Optical pickup device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device.
[0002]
[Prior art]
An optical pickup device is known as a device for recording / reproducing information on / from an optical information recording medium such as an optical disk.
[0003]
The optical pickup device guides the return light from the optical information recording medium (reflected light beam from the recording surface of the optical information recording medium) to the detection system for focusing control and tracking control. It is necessary to separate from the “optical path toward the medium” to the detection system side.
[0004]
In general, the return light is separated by combining a quarter-wave plate and a polarization beam splitter so that the polarization directions of the light beams incident on the polarization beam splitter are orthogonal to each other when the light is directed to the optical information recording medium and the return light, This is performed using transmission and reflection by a polarizing beam splitter. Recently, it is also intended to use a “polarization-dependent diffractive optical element” instead of a polarizing beam splitter.
[0005]
In order to perform “good recording / reproduction of information” with an optical pickup device, the light beam must be condensed as a good light spot on the recording surface of the optical information recording medium. For this reason, the “total amount of wavefront aberration (wavefront aberration generated by each optical element: a: a)” of an optical element (collimator lens, objective lens, polarizing beam splitter, ¼ wavelength plate, etc.) between the light source and the recording surface. The square root of the sum of the squares of _i , that is, √ {Σa _i ² }) ”is“ λ / 14 ”or less, that is,“ approximately 0.07λ ”or less with respect to the wavelength of light emitted from the light source: λ. It is said that it is necessary.
[0006]
The quarter-wave plate and the diffractive optical element have a “parallel plate” shape. However, if the surface does not have high-precision flatness, wavefront aberration occurs due to these. For this reason, the surface accuracy required for the quarter-wave plate and the diffractive optical element becomes strict, and the “cost of manufacturing them” is high, which hinders cost reduction of the optical pickup device.
[0007]
For example, in the case of a quarter-wave plate, the surface accuracy that can be realized at a relatively low cost is about λ / 10 with respect to an oscillation wavelength: λ (for example, 633 nm) of an LD (semiconductor laser) used as a light source, which is higher than this If it is attempted to satisfy the surface accuracy, the manufacturing cost becomes extremely high.
[0008]
When the surface accuracy of the quarter-wave plate is about λ / 10, the wavefront aberration generated by the quarter-wave plate is “approximately 0.05λ”. For this reason, when a quarter-wave plate having a surface accuracy of about λ / 10 is used in an optical pickup device, it is applied to other optical elements such as a collimating lens and an objective lens arranged on the optical path from the LD to the recording surface. The allowable amount of wavefront aberration is as small as about {square root} {(0.07λ) ² − (0.05λ) ² } ≈0.049λ, and the tolerance for wavefront aberration of these other optical systems is significantly limited. Will end up.
[0009]
[Problems to be solved by the invention]
In view of the circumstances described above, the present invention uses an optical pickup device that uses a “¼ wavelength plate or a diffractive optical element” whose surface accuracy is lower than about λ / 10 and has a high tolerance for wavefront aberration of other optical elements. Realization is the issue.
[0010]
[Means for Solving the Problems]
The optical pickup apparatus of claim 1, wherein the optical path leading to the recording surface of the optical information recording medium from the LD (semiconductor laser) is a "light source, has a portion which the light beam becomes a substantially parallel light beam, said light beam This is an “optical pickup device having a parallel-plate-shaped quarter-wave plate in a portion that becomes a substantially parallel light beam ”. Therefore, a “parallel beam” is incident on the quarter-wave plate.
[0011]
The optical pickup device according to claim 1 is characterized by the following points.
That is, the incident angle of the light flux from the LD to the quarter-wave plate is θ, the refractive index of the material of the quarter-wave plate is n, the surface accuracy of the quarter-wave plate is η, and the laser light emitted from the LD When the wavelength of λ is λ, the incident angle is θ:
(1) | (η / cos θ) −n · η / √ {1- (sin θ / n) ² } | <λ / 45
The quarter wavelength plate is inclined with respect to the incident light beam so as to satisfy the above.
[0012]
The optical pickup device according to claim 2 has a portion in which the light beam becomes a substantially parallel light beam in the optical path from the LD as the light source to the recording surface of the optical information recording medium, and the light beam is substantially parallel. An optical pickup device having a parallel plate-like and polarization-dependent diffractive optical element in a portion that becomes a light beam. Accordingly, a parallel light beam enters the diffractive optical element.
[0013]
The phrase “diffractive optical element is polarization-dependent” means that the diffractive action differs depending on whether the incident parallel light beam to the diffractive optical element is P-polarized light or S-polarized light. It means that the diffractive action acts only on one of the polarized light.
[0014]
The optical pickup device according to claim 2 is characterized by the following points.
That is, the incident angle of the light beam from the LD to the diffractive optical element is α, the refractive index of the material of the diffractive optical element is N, the surface accuracy of the diffractive optical element is ξ, and the wavelength of the laser light emitted from the LD is λ. When the above incident angle: α is the condition:
(2) | (ξ / cos α) −N · ξ / √ {1- (sin α / N) ² } | <λ / 45
The diffractive optical element is inclined with respect to the incident light beam so as to satisfy the above.
[0015]
The optical pickup apparatus of claim 3, wherein the optical path leading to the recording surface of the optical information recording medium from the LD is "light source has a portion which the light beam becomes a substantially parallel light beam, the light beam is substantially parallel a parallel flat plate-shaped optical pickup device having a diffractive optical element of the polarization dependency with a quarter wavelength plate and the parallel plate "in the portion serving as the light beam. Accordingly, a parallel light beam enters both the quarter-wave plate and the diffractive optical element.
[0016]
The optical pickup device according to claim 3 is characterized by the following points.
That is, the aforementioned incident angles: θ, α, refractive indexes: n, N, surface accuracy: η, ξ, wavelength: λ, and incident angles: θ and α are:
(3) √ {| (η / cos θ) −n · η / √ {1- (sin θ / n) ² } | ² +
| (Ξ / cos α) −N · ξ / √ {1- (sin α / N) ² } | ² } <λ / 45
The quarter-wave plate and / or the diffractive optical element are inclined with respect to the incident light beam so as to satisfy the above. In the optical pickup device according to the third aspect, the quarter-wave plate and the diffractive optical element can be integrated with each other (claim 4).
[0017]
In the optical pickup device according to claim 1, 3, or 4, the incident angle to the quarter-wave plate: θ is a condition:
(4) | (1 / cos θ) −n / √ {1- (sin θ / n) ² } | ≈0
(Claim 5), and in the optical pickup device according to claim 2, 3 or 4, the incident angle to the diffractive optical element: α is a condition:
(5) | (1 / cos α) −N / √ {1- (sin α / N) ² } | ≈0
Can be satisfied (claim 6).
[0018]
In the optical pickup device according to claim 3, the incident angle to the quarter-wave plate: θ is a condition:
(4) | (1 / cos θ) −n / √ {1- (sin θ / n) ² } | ≈0
And the incident angle to the diffractive optical element: α is the condition:
(5) | (1 / cos α) −N / √ {1- (sin α / N) ² } | ≈0
Can be satisfied (claim 7).
[0019]
In addition, as a quarter wavelength plate, in addition to the well-known “quartz”, for example, a material using lithium niobate (LiNbO ₃ ) or the like can be used.
[0020]
As described above, the pickup device of the present invention is characterized in that a parallel plate (a quarter wavelength plate or a diffractive optical element) provided in the optical path of the parallel light beam is inclined with respect to the optical path of the parallel light beam. The meaning of tilting the parallel plate in this way will be described.
[0021]
In FIG. 4, the code | symbol 1 has shown the transparent parallel plate. There is a step on one side of the parallel plate 1, and the size of this step is “Λ”. When a parallel light beam is incident on the parallel plate 1 at an incident angle “A” and is transmitted, a “step difference due to Λ” is generated in the transmitted light beam.
[0022]
Assuming that the refractive index of the material of the parallel plate 1 is n1, the refractive index of the region outside the parallel plate 1 is n2, the refraction angle by the parallel plate 1 is B, and the wavelength of the parallel light beam is λ, the above “step difference due to Λ” Is
{N1 · Λ / (λ · cos A)} − {n2 · Λ / (λ · cos B)} (a)
Given in.
[0023]
Between the incident angle: A and the refraction angle: B, Snell's law:
n1 · sinA = n2 · sinB (b)
Therefore, the condition for the phase difference to be 0 is
(n1 / cosA) −n2 / √ {1- (n1 · sinA / n2) ² } = 0 (c)
Is true. Considering that the quarter-wave plate and the diffractive optical element are placed in the air, if n1 = 1.0, the formula (c) is
(1 / cosA) -n2 / √ {1- (sinA / n2) ² } = 0 (d)
It becomes. For example, when n2 = 1.5, Λ = 0.01 μm, and λ = 0.633 μm, the incident angle is A≈56 degrees and the phase difference becomes zero. That is, no wavefront aberration occurs at this time. That is, for parallel monochromatic light with a wavelength of 0.633 μm, when a parallel plate with a refractive index of 1.5 is tilted by 56 degrees, the wavefront aberration is not dependent on the size of the surface step: Λ. Does not occur. The present invention takes advantage of this fact.
[0024]
“Surface accuracy” is generally expressed as “difference between maximum value and minimum value (PV value)” of surface irregularities, and “surface accuracy of λ / 10” is the difference between the maximum value and the minimum value. It means that the maximum is 1/10 of the wavelength: λ. For example, if the wavelength is 0.6 μm, the maximum value of the difference is 0.06 μm.
[0025]
When the incident angle: A is arbitrary, the generated wavefront aberration: Y depends on the surface accuracy: Λ, and assuming the model of FIG.
Y = (Λ / cosA) −n2 · Λ / √ {1- (sinA / n2) ² } (e)
It is represented by FIG. 5 shows how the right side of the equation (e) changes together with the incident angle: A when the surface accuracy is λ / 10 and λ / 5 (λ = 0.633 μm). . A “solid line” curve 51 in FIG. 5 is for the surface accuracy: λ / 10, and a “dashed line” curve 52 is for the surface accuracy: λ / 5.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of an optical pickup device according to claim 1.
A light beam emitted from the LD 10 that is a light source is converted into a parallel light beam by the collimator lens 12, passes through the polarization beam splitter 14 and the quarter-wave plate 16, is converged by the objective lens 18, and is recorded on the recording surface of the optical information recording medium 20. 21 is condensed as a light spot. That is, in this embodiment, the optical pickup device has a “part where the light beam becomes a substantially parallel light beam” in the optical path from the LD 10 as the light source to the recording surface 21 of the optical information recording medium 20. Have a quarter-wave plate 16.
[0027]
The reflected light from the recording surface 21 becomes “returned light” that passes through the objective lens 18 and the quarter-wave plate 16 and enters the polarizing beam splitter 14. At this time, the return light is transmitted through the quarter-wave plate 16 twice, so that its polarization plane is rotated 90 degrees from the state when it was originally emitted from the LD 10. Accordingly, the return light is separated from the “optical path from the LD 10 toward the recording surface 21” by being reflected by the polarization beam splitter 14.
[0028]
The separated return light is condensed by the condensing lens 22, partly reflected by the knife edge prism 24 in the course of condensing, and incident on the tracking control light receiving element 28, and is not reflected by the knife edge prism 24. The portion is incident on the light receiving element 26 for focusing control. In this embodiment, focusing control is performed by a known “knife edge method” and tracking control is performed by a “push / pull method”. The focusing and tracking control is performed by any other known appropriate method. Also good.
[0029]
The quarter-wave plate 16 is made of a material having a refractive index: n, and the incident angle: θ of the light beam from the LD 10 depends on the wavelength: λ of the laser light emitted from the LD 10 and the surface accuracy: η:
(1) | (η / cos θ) −n · η / √ {1- (sin θ / n) ² } | <λ / 45
So as to satisfy the following conditions:
[0030]
The left side of Expression (1) is “wavefront aberration” generated by the quarter-wave plate 16.
In the optical pickup device configured as shown in FIG. 1, a collimating lens 12, a polarizing beam splitter 14, a quarter wavelength plate 16, and an objective lens 18 are provided between the LD 10 and the recording surface 21, and these optical elements are used. Since the total amount of wavefront aberration should be λ / 14 (= 0.07λ) or less, if the incident angle: θ of the quarter-wave plate 16 satisfies the formula (1), a quarter-wave plate is used. The generated wavefront aberration is 0.02λ or less, and is 1/3 or less of the total amount of wavefront aberration by the optical element. At this time, the wavefront aberration allowed for the other optical elements (the collimating lens 12, the objective lens 18, and the polarization beam splitter 14) is approximately {square root} {(0.07λ) ² − (0.02λ) ² } = 0.067λ. Therefore, it is easy to manufacture these other optical systems, and the cost of the optical pickup device can be effectively reduced.
[0031]
If the wavelength of the LD: λ is 0.633 μm and the refractive index n of the material of the quarter-wave plate 16 is 1.5, the range of the incident angle: θ that satisfies the above formula (1) is As can be seen from FIG. 5, the surface accuracy is 49 to 61 degrees with respect to λ / 10, and the surface accuracy is 53 to 58 degrees with respect to λ / 5. In particular, when the incident angle: θ with respect to the quarter-wave plate 16 is substantially 56 degrees, the above-mentioned formula (d) is satisfied, and the quarter-wave plate 16 is independent of the surface accuracy of the quarter-wave plate 16. Therefore, the tolerance for the wavefront aberration of other optical systems becomes extremely large.
[0032]
FIG. 2 shows an embodiment of the optical pickup device according to claims 2 and 3. In order to avoid complications, the same reference numerals as those in FIG.
[0033]
The light beam emitted from the LD 10 as the light source is converted into a parallel light beam by the collimator lens 12, the optical path is deflected by the deflecting mirror 13, passes through the diffractive optical element 15 and the quarter wavelength plate 16, and is converged by the objective lens 18. The light is condensed as a light spot on the recording surface 21 of the optical information recording medium 20.
[0034]
The diffractive optical element 15 is “polarization-dependent” and has a parallel plate shape. That is, in this embodiment, the optical pickup device has a “part where the light beam becomes a substantially parallel light beam” in the optical path from the LD 10 as the light source to the recording surface 21 of the optical information recording medium 20. The diffractive optical element 15 of “parallel plate shape and polarization dependency” and the quarter wavelength plate 16 are included.
[0035]
The diffractive optical element 15 is made of a material having a refractive index: N and is formed in a parallel plate shape. The incident angle of the light beam from the LD 10 to the diffractive optical element 15 is α, the surface accuracy of the diffractive optical element 15 is ξ, and from the LD 10. When the wavelength of the emitted laser light is λ, the incident angle: α is the condition:
(2) | (ξ / cos α) −N · ξ / √ {1- (sin α / N) ² } | <λ / 45
Inclined with respect to the incident light flux so as to satisfy
[0036]
The reflected light from the recording surface 21 is “returned light” that is transmitted through the objective lens 18 and the quarter-wave plate 16, and enters the diffractive optical element 15 in a state in which the polarization surface is rotated 90 degrees from the original direction. . The diffractive optical element 15 does not exert a diffractive effect on a light beam incident from the LD 10 side, but a “hologram” is formed so as to exert a diffractive action on return light. The return light separated from the “optical path from to the recording surface 21” enters a detection system (an optical system that detects the return light to generate a reproduction signal, a focusing control signal, and a tracking control signal). At this time, the diffractive action of the diffractive optical element 15 can also be determined such that “the separated return light is separately incident on the light receiving elements for focusing control and tracking control by diffraction”.
[0037]
By arranging the diffractive optical element 15 so that the above expression (2) is satisfied, the wavefront aberration generated in the diffractive optical element 15 becomes 0.02λ or less, and the wavefront aberration in the other optical elements is the same as in the above case. As a permissible amount, about 0.067 out of the limit value: 0.07λ is left, and manufacturing of other optical elements becomes easy.
[0038]
In the embodiment of FIG. 2 described above, the quarter wavelength plate 16 is one of the other optical elements, and in this case, the wavefront aberration of the other optical elements including the quarter wavelength plate is reduced. The sum must be suppressed to about 0.067λ.
[0039]
However, as shown in the figure, the quarter wave plate 16 is also tilted to effectively reduce the occurrence of wavefront aberration in the quarter wave plate 16 and tolerate the wavefront aberration in the remaining optical elements. It can be enlarged.
[0040]
That is, the incident angle of the light flux from the LD 10 on the quarter-wave plate 16: θ, the refractive index of the material of the quarter-wave plate: n, the surface accuracy of the quarter-wave plate: η, the incidence on the diffractive optical element Angle: α, refractive index of material of diffractive optical element: N, surface accuracy of diffractive optical element: ξ, wavelength of laser light emitted from LD: λ, θ and α are:
(3) √ {| (η / cos θ) −n · η / √ {1- (sin θ / n) ² } | ² +
| (Ξ / cos α) −N · ξ / √ {1- (sin α / N) ² } | ² } <λ / 45
If the ¼ wavelength plate 16 and / or the diffractive optical element 15 are arranged so as to be inclined with respect to the incident light flux so as to satisfy the above, the square of the wavefront aberration generated by the diffractive optical element 15 and the ¼ wavelength plate 16 Since the square root of the sum is 0.02λ or less, the tolerance for wavefront aberration in the remaining optical elements can be increased.
[0041]
In particular, the incident angle to the quarter-wave plate 16 is θ:
(4) | (1 / cos θ) −n / √ {1- (sin θ / n) ² } | ≈0
And the angle of incidence on the diffractive optical element 15 is α:
(5) | (1 / cos α) −N / √ {1- (sin α / N) ² } | ≈0
(Claim 7), the diffractive optical element 15 and the quarter-wave plate 16 do not substantially generate wavefront aberrations, so that the tolerance for wavefront aberrations in the remaining optical elements can be greatly increased. .
[0042]
FIG. 3 shows an embodiment of the optical pickup device according to the fourth aspect. Those that are not likely to be confused are given the same reference numerals as in FIG.
In this embodiment, “the quarter-wave plate 16 and the diffractive optical element 15 are integrated”, and the incident angle to these is determined so that the expression (3) is satisfied. Even in this case, since the sum of wavefront aberrations generated by the diffractive optical element 15 and the quarter-wave plate 16 is 0.02λ or less, the tolerance for wavefront aberration in the remaining optical elements can be increased.
[0043]
If the diffractive optical element 15 and the quarter-wave plate 16 are made of the same material, the refractive indexes n and N are equal to each other, so that the expression (3) is satisfied with respect to the incident angle: θ = α. In particular, the expressions (4) and (5) can be satisfied together with the condition: θ = α. Therefore, in the state in which the diffractive optical element 15 and the quarter-wave plate 16 are integrated, the wavefront by them can be obtained. The aberration can be 0.02λ or less, or substantially zero.
[0044]
If λ is 0.633 μm and the refractive index of the material of the quarter-wave plate 16 and the diffractive optical element 15: n = N is 1.5, the integrated diffractive optical element 15 and the quarter-wave plate When the incident angle: θ = α is substantially 56 degrees with respect to 15, wavefront aberration due to the quarter-wave plate 16 and the diffractive optical element 15 does not occur regardless of the surface accuracy of these optical elements. The tolerance for wavefront aberration of other optical systems becomes extremely large.
[0045]
2 and 3, a part of the light from the LD 10 passes through the deflection mirror 13 and is detected by the front detector 30. The output of the front detector 30 is used for output control of the LD.
[0046]
【The invention's effect】
As described above, according to the present invention, a novel optical pickup device can be provided. According to the optical pickup device of the present invention, as described above, even when the surface accuracy of the quarter-wave plate or the diffractive optical element is about λ / 10 or less, the wavefront aberration generated by them can be suppressed to λ / 45 or less, The tolerance for wavefront aberration in the remaining optical elements can be effectively relaxed, and the quarter-wave plate, diffractive optical element, and other optical elements can be easily manufactured, and the cost of the optical pickup device can be reduced.
[Brief description of the drawings]
FIG. 1 is a view for explaining one embodiment of the first aspect of the present invention;
FIG. 2 is a view for explaining one embodiment of the invention as set forth in claims 2 and 3;
FIG. 3 is a view for explaining one embodiment of the present invention as set forth in claim 4;
FIG. 4 is a diagram for explaining the principle of the present invention.
FIGS. 5A and 5B are diagrams showing two examples of the relationship between the incident angle of a parallel light beam on a parallel plate and the generated wavefront aberration, using the surface accuracy as a parameter. FIGS.
[Explanation of symbols]
10 LD
14 Polarizing beam splitter 16 1/4 wavelength plate 20 Optical information recording medium 21 Recording surface

Claims (7)

In the optical path from the LD as the light source to the recording surface of the optical information recording medium, there is a portion where the light beam becomes a substantially parallel light beam. In an optical pickup device having a / 4 wavelength plate,
The incident angle of the luminous flux from the LD to the quarter-wave plate is θ, the refractive index of the material of the quarter-wave plate is n, the surface accuracy of the quarter-wave plate is η, and the wavelength of the laser light emitted from the LD Is the above-mentioned incident angle: θ is a condition:
(1) | (η / cos θ) −n · η / √ {1- (sin θ / n) ² } | <λ / 45
An optical pickup device characterized in that the ¼ wavelength plate is disposed so as to be inclined with respect to the incident light flux so as to satisfy the above. In the optical path from the LD as the light source to the recording surface of the optical information recording medium, there is a portion where the light beam becomes a substantially parallel light beam, and the parallel light beam is polarized in the portion where the light beam becomes a substantially parallel light beam. In an optical pickup device having a diffractive optical element having dependency,
When the incident angle of the light beam from the LD to the diffractive optical element is α, the refractive index of the material of the diffractive optical element is N, the surface accuracy of the diffractive optical element is ξ, and the wavelength of the laser light emitted from the LD is λ, The above incident angle: α is the condition:
(2) | (ξ / cos α) −N · ξ / √ {1- (sin α / N) ² } | <λ / 45
An optical pickup device, wherein the diffractive optical element is disposed so as to be inclined with respect to an incident light beam so as to satisfy the above. In the optical path from the LD as the light source to the recording surface of the optical information recording medium, there is a portion where the light beam becomes a substantially parallel light beam. In an optical pickup device having a / 4 wavelength plate and a parallel plate-like and polarization-dependent diffractive optical element,
The incident angle of the light flux from the LD to the quarter wavelength plate is θ, the refractive index of the material of the quarter wavelength plate is n, the surface accuracy of the quarter wavelength plate is η,
When the incident angle of the light beam from the LD to the diffractive optical element is α, the refractive index of the material of the diffractive optical element is N, the surface accuracy of the diffractive optical element is ξ, and the wavelength of the laser light emitted from the LD is λ, The above incident angles: θ and α are the conditions:
(3) √ {| (η / cos θ) −n · η / √ {1- (sin θ / n) ² } | ² +
| (Ξ / cos α) −N · ξ / √ {1- (sin α / N) ² } | ² } <λ / 45
An optical pickup device characterized in that the quarter-wave plate and / or the diffractive optical element are arranged so as to be inclined with respect to an incident light beam so as to satisfy the above. The optical pickup device according to claim 3, wherein
An optical pickup device in which a quarter-wave plate and a diffractive optical element are integrated with each other. The optical pickup device according to claim 1, 3 or 4,
Incident angle to quarter wave plate: θ is a condition:
(4) | (1 / cos θ) −n / √ {1- (sin θ / n) ² } | ≈0
An optical pickup device satisfying the requirements. The optical pickup device according to claim 2, 3 or 4,
Incident angle to the diffractive optical element: α is the condition:
(5) | (1 / cos α) −N / √ {1- (sin α / N) ² } | ≈0
An optical pickup device satisfying the requirements. The optical pickup device according to claim 3, wherein
Incident angle to quarter wave plate: θ is a condition:
(4) | (1 / cos θ) −n / √ {1- (sin θ / n) ² } | ≈0
And satisfy
Incident angle to the diffractive optical element: α is the condition:
(5) | (1 / cos α) −N / √ {1- (sin α / N) ² } | ≈0
An optical pickup device satisfying the requirements.

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