JP2004039010A - Optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To sufficiently reduce the spherical aberration caused by the variance of thickness between disks, even though a numerical aperture NA is enlarged. <P>SOLUTION: A beam magnifying optical system 33 of a magnification factor (m) of 1.2≤m≤1.5, is arranged in a parallel optical path up to an objective lens 26 from a light branching element 23. The spherical aberration caused by the thickness tolerance of a covering glass 27 of an optical recording medium 28 is compensated. The beam magnifying optical system 33 includes a biconcave lens 24 which is a 1st lens, and a plano-convex lens 25 which is a 2nd lens. The mounting of the lens is simplified by expanding the lens mounting tolerance, then stabilization of a recording/reproducing signal at the spherical aberration correction is also attainable. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
[Industrial applications]
The present invention is directed to irradiating a recording medium such as an optical disk with a light beam from a light source such as a semiconductor laser and writing and recording information on a recording surface of the recording medium, or information written on the recording surface of the recording medium. The present invention relates to an optical pickup device that reproduces an image.
[0002]
[Prior art]
2. Description of the Related Art In recent years, in the field of information recording, researches on optical information recording methods have been advanced in various places. This optical information recording system has many advantages such as non-contact recording / reproduction and compatibility with read-only, write-once, and rewritable memory types, and realizes inexpensive large-capacity media. A wide range of applications from industrial use to consumer use are considered as possible.
[0003]
In these optical information recording systems, an optical disk, which is a disk-shaped recording medium, is employed, and has an advantage that random access is possible without being limited to sequential access such as a tape-shaped recording medium. Recent trends in optical disk devices include increasing the information recording capacity per unit area in optical disks having a de facto standard shape, such as 120 mm diameter disks such as CDs (Compact Discs) and DVDs (Digital Versatile Discs). There are two coordinate axes, that is, an optical disk without reducing (rather increasing) the information recording amount and a direction of reducing the size of the optical disk reproducing device, and research has been actively conducted in recent years.
[0004]
Methods for increasing the information recording capacity per unit area include a method of shortening the wavelength of the light source and increasing the numerical aperture (NA) of the objective lens. Shortening the wavelength of the light source has made a great leap forward with the advent of blue lasers in recent years, but the further shortening of the wavelength has reached a plateau due to the problem of absorption of optical components. On the other hand, with respect to the increase in the lens NA, it is possible to design a lens with a high NA by designing the lens, but the influence of spherical aberration due to a thickness error of the light transmission layer covering the recording surface of the optical disk and coma due to the tilt of the disk increase. There are issues such as. Although the light transmitting layer is not necessarily made of glass, it is referred to as “cover glass” in this specification.
[0005]
The influence of the latter coma can be avoided by reducing the thickness of the cover glass. Regarding the former thickness error of the cover glass, if there is a thickness error of about 5 μm, it will not be possible to focus the spot only by the focus servo using the actuator that moves the objective lens in the optical axis direction. There is.
[0006]
FIG. 11 shows a conventional example in which such spherical aberration compensation is performed. This conventional example is based on the one disclosed in FIG. 1 in Japanese Patent Application Laid-Open No. H11-259893. Hereinafter, this conventional example is referred to as “conventional example 1”. After the light from the semiconductor laser 1 passes through the diffraction grating 2 and the light splitting element 3, it is converted into a parallel light beam by a collimating lens 4, and is converted into a circularly polarized light by a λ / 4 plate 5. The light is condensed at 6 and condensed on a recording medium 8 having a cover glass 7. The objective lens 6 is provided with a two-axis objective lens actuator 9 for adjusting focus and tracking. Thereafter, the signal light reflected from the recording medium 8 is split by the light splitting element 3, and the reflected light is condensed on the light receiving element 11 by a composite lens 10 having a cylindrical lens on the incident side and a concave lens on the output side, and the recording signal and The servo signal is read. Here, by using the collimating lens actuator 12 to move the collimating lens 4 in the optical axis direction according to the thickness of the cover glass 7, it is possible to suppress the spherical aberration caused by the thickness error of the cover glass 7 to be low. Become.
[0007]
In the paragraph [0066] of Conventional Example 1, it is described that the numerical aperture NA of the objective lens is set to be larger than about 0.65 required for DVD. It is described that the numerical aperture NA is set to 0.85 or more.
[0008]
FIG. 12 shows another conventional example different from the first conventional example. This conventional example is based on the one disclosed in FIG. 1 in JP-A-5-266511. Hereinafter, this conventional example is referred to as “conventional example 2”. Light collimated by a combination of a semiconductor laser and a collimating lens passes through a beam expanding optical system that expands a beam diameter by two lenses 14 and 15. After passing through the beam expanding optical system, the light is focused on a recording medium 8 having a cover glass 7 by an objective lens 16 including four groups of lenses. It is presumed that the light reflected from the recording medium 7 is branched by an optical branching element (not shown) and the recording signal and the servo signal are read by the light receiving element, as in the first conventional example.
[0009]
In Conventional Example 2, one of the lenses 14 and 15 of the beam expanding optical system is moved in the optical axis direction by the moving mechanism 17 according to the thickness of the cover glass 7, so that spherical aberration caused by a thickness error of the cover glass 7 is obtained. Can be kept low. The moving mechanism 17 is controlled by a conversion / drive unit 19 in response to the inter-lens correction interval data read from the surface of the recording medium 8 by the data reading unit 18. As shown in the figure, the distance between the lenses 14 and 15 of the beam expanding optical system is small, and the magnification of the beam diameter by the lenses 14 and 15 is also small.
[0010]
[Problems to be solved by the invention]
However, the optical system of Conventional Example 1 has the advantage of reducing the number of components in terms of using conventional components because spherical aberration compensation is performed using the collimating lens 4, while the collimating lens 4 has a size including a holder. In addition, the disadvantage is that the weight becomes very heavy, and it is not suitable for electromagnetic driving as shown in FIG. 11 in terms of power consumption.
[0011]
On the other hand, the optical system of Conventional Example 2 has a configuration in which spherical aberration compensation is performed using a beam expanding optical system having a small beam expanding rate as shown in FIG. 12, and lenses 14 and 15 of the beam diameter expanding optical system. While there is an advantage that the optical pickup device can be easily miniaturized because the interval between them is short, on the other hand, as described later in the first embodiment, in a beam expansion optical system having a low expansion rate, an optical element such as a lens used for wavefront aberration uses a wavefront aberration. Since it is easily affected by shift / tilt errors, there is a demerit that mounting accuracy is required and mass production at low cost is difficult.
[0012]
As described above, the optical disk serving as the recording medium 8 has the cover glass 7 as a light transmitting layer for protection of the recording surface or the like, and recording / reproduction is performed by irradiating light through the cover glass 7. Is Here, when the cover glass 7 has a thickness error (δt), the spherical aberration (Wt) represented by the following relational expression (1) is obtained. ₄₀ (Where n is the refractive index of the cover glass).
W ₄₀ = (N ² -1) / (8n ³ ) ・ Δt ・ NA ⁴ … (1)
[0013]
From equation (1), it can be seen that the spherical aberration caused by the thickness error of the cover glass 7 increases in proportion to the fourth power of the numerical aperture NA. That is, in the case of an optical system having a large numerical aperture NA, it is important to suppress a thickness error of the cover glass 7. However, it is very difficult to manage the thickness error of the cover glass 7, and although it is possible to manage the thickness variation within a single disc, it is not possible to control the thickness variation between the discs at the time of disc replacement. It is extremely difficult in terms of production technology, and it is not advisable to control the disk thickness accurately even when considering mass productivity and compatibility with other companies' products.
[0014]
An object of the present invention is to solve such a problem in high-density recording, and is an adjustment capable of sufficiently reducing spherical aberration due to thickness variation between disks even when the numerical aperture NA is increased. An object of the present invention is to provide an optical pickup having a mechanism.
[0015]
[Means for Solving the Problems]
According to the present invention, a light beam emitted from a light source is converted into a parallel light beam by a collimating lens, and condensed on a recording medium by an objective lens, and reflected light from the recording medium is passed through the objective lens to form a light beam between the objective lens and the collimating lens. In an optical pickup device that receives light by branching with an optical branching element disposed therebetween,
A beam expanding optical system between the light splitting element and the objective lens for expanding the diameter of a light beam transmitted through the light splitting element so that the expansion rate (m) becomes 1.2 or more. It is a pickup device.
[0016]
According to the present invention, when a tilt occurs in which the optical axis of the optical element constituting the beam expansion optical system is inclined with respect to the direction of the light beam, if the maximum amount of the tilt is ± 1 deg or less, the expansion ratio ( By setting m) to 1.2 or more, even if the numerical aperture NA increases, the wavefront aberration can be suppressed to 0.05λ or less, so that the condensed spot in the recording track of the recording medium has a predetermined size. Can be focused.
[0017]
Therefore, the mounting accuracy when attaching the beam expanding optical system to the optical pickup device can be simplified.
[0018]
Further, in the present invention, a magnification (m) of a light beam diameter by the beam expanding optical system is 1.5 or less.
[0019]
According to the present invention, when a thickness error occurs in the cover glass of the recording medium and the thickness error is ± 30 μm or less, the magnification (m) of the beam expanding optical system is set to 1.5 or less. As a result, the amount of generated aberration can be suppressed to 0.05λ or less, so that the condensed spot can be condensed with a predetermined size in the recording track of the recording medium.
[0020]
In the present invention, the beam expanding optical system includes:
A first lens having an optical axis in a direction of the light beam to be converted into a parallel light beam and disposed on a light branching element side;
It is characterized by including a second lens which shares the optical axis with the first lens and is disposed on the objective lens side.
[0021]
According to the present invention, by changing the distance between the first lens and the second lens, the magnification of the beam expanding optical system that expands the light beam can be easily adjusted, and appropriate aberration correction can be performed.
[0022]
Further, according to the present invention, a light beam emitted from a light source is converted into a parallel light beam by a collimating lens, and condensed on a recording medium by an objective lens, and reflected light from the recording medium passes through the objective lens. In an optical pickup device that receives light by branching with an optical branching element disposed between
Including a beam expanding optical system for expanding the diameter of a light beam transmitted through the light branching element, between the light branching element and the objective lens,
The beam expanding optical system includes:
A first lens having an optical axis in a direction of the light beam to be converted into a parallel light beam and disposed on the light branching element side;
An optical pickup device comprising: a second lens that shares an optical axis with a first lens and is disposed on an objective lens side; and a moving unit that moves the second lens along the optical axis. is there.
[0023]
According to the present invention, of the lenses of the beam expanding optical system composed of a combination of two lenses, the second lens which is disposed on the objective lens side which is the recording medium side and has a small increase in wavefront aberration with respect to tilt is used. Since the lens is moved by the moving means, it is possible to reduce the influence on the tilt generated by the so-called "play" between the lens holder and the guide rail, which occurs when the lens is moved in the optical axis direction.
[0024]
Therefore, even when tilt occurs due to movement of the lens constituting the beam expanding optical system for aberration correction when the numerical aperture NA is increased, the amount of generated aberration can be further reduced. A condensed spot can be condensed in a predetermined size in a recording track of the medium.
[0025]
By setting the magnification (m) of the light beam diameter to 1.2 or more, if the lens of the beam expanding optical system tilts, if the maximum amount of the tilt is ± 1 deg or less, the wavefront aberration Can be suppressed to 0.05λ or less, so that the condensed spot can be condensed in a predetermined size in the recording track of the recording medium.
[0026]
Further, by defining the magnification (m) of the luminous flux diameter to 1.5 or less, if a thickness error occurs in the cover glass of the recording medium and the thickness error is ± 30 μm or less, the amount of generated aberration is Can be suppressed to 0.05λ or less, so that the condensed spot can be condensed in a predetermined size in the recording track of the recording medium.
[0027]
In the present invention, the first lens is a biconcave lens,
The second lens is a plano-convex lens.
[0028]
According to the present invention, the focal length of the biconcave lens which is the first lens is -f. ₁ , The focal length of the plano-convex lens which is the second lens is f ₂ Then, the magnification (m) of the beam diameter is m = f ₂ / F ₁ Is led by For a plano-convex lens, the focal length f ₂ Is determined by the lens radius on the convex surface side, so that the magnification can be changed by this lens radius.
[0029]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows a schematic configuration of an optical system of an optical pickup device 20 according to the first embodiment of the present invention. Light from the semiconductor laser 21 is converted into a parallel light beam by the collimator lens 22. The collimated light beam is expanded through a light splitting element 23 by a biconcave lens 24 as a first lens and a plano-convex lens 25 as a second lens. After that, the light is focused on the recording surface of the optical recording medium 28 via the cover glass 27. The reflected light from the optical recording medium 28 follows an optical path opposite to that of the incident light, is reflected by the light branching element 23, is condensed by the spot lens 29, passes through the cylindrical lens 30, and is converged on the same plane. The light is irradiated on the light receiving element 31 having the divided light receiving portion, where the recording signal and the servo signal are detected. Here, the objective lens 26 is not limited to a doublet lens, but may be an optical system using a single lens.
[0030]
The diameter of the light beam incident on the objective lens 26 needs to be larger than the opening diameter of the aperture 32. For this reason, in the optical pickup device 20, it is necessary to enlarge the beam diameter of the light from the semiconductor laser 21. In the present embodiment, the beam expanding optical system 33 is configured by disposing the biconcave lens 24 and the plano-convex lens 25, which are optical elements, after the light splitting element 23, thereby expanding the light beam diameter. The beam expanding optical system 33 is arranged at this position because the beam diameter at the light branching element 23 increases when passing through the light branching element 23 after passing through the beam expanding optical system 33, This is because it is necessary to increase the part shape. Therefore, in order to reduce the size of the optical pickup device 20, the position of the beam expanding optical system 33 is limited to the position after the light beam to be expanded has passed through the light splitting element 23.
[0031]
Here, the beam expanding optical system 33 is provided for compensating for spherical aberration caused by a thickness error of the cover glass 27, and as shown in FIG. 1, a lens having a concave surface on at least one side on the incident side (hereinafter referred to as L1). ), And a combination of lenses (hereinafter, referred to as L2) having a convex surface on at least one surface on the emission side. Note that L1 and L2 may be spherical lenses, or may be aspheric lenses to improve off-axis aberration characteristics.
[0032]
Without the beam expanding optical system 33, when the parallel light emitted from the collimating lens 22 is condensed on the optical recording medium 28 by the objective lens 26, a spherical aberration occurs due to a thickness error of the cover glass 27. A biconcave lens 24 and a plano-convex lens 25, which are optical elements of a beam magnifying optical system 33, are arranged between the lens 22 and the objective lens 26. By adjusting the distance between the lenses, spherical aberration caused by a thickness error of the cover glass 27 is reduced. Generates spherical aberration of opposite phase, and can perform spherical aberration compensation.
[0033]
FIG. 2 shows an example of a specific optical arrangement for performing spherical aberration compensation using the beam expanding optical system 33. When the light beam diameter D1 on the incident side is 2.27 mm and the light beam diameter D2 on the output side is 3.4 mm, the beam expansion ratio required is D2 / D1 = 3.4 / 2.27 = 1.498. . The light beam diameter D2 on the emission side needs to be larger than the opening diameter D3 of the aperture 32, for example, 3 mm. The aperture diameter D3 of the aperture 32 is set according to the effective diameter of the objective lens 26. The lens radii R11 and R12 of the biconcave lens 24 are 32.65 mm and 17.39 mm, respectively. The thickness t1 of the central portion of the biconcave lens 24 which is the thinnest is 1 mm. The thickness t2 of the central portion where the plano-convex lens 25 is the thickest is 1.42 mm. The inter-lens distance S1 between the biconcave lens 24 and the plano-convex lens 25 is 5.914 mm.
[0034]
The light that has been converted into a parallel light beam by the collimating lens 22 is passed through a light splitting element 23 and then expanded in diameter by a beam expanding optical system 33 composed of a biconcave lens 24 and a plano-convex lens 25. The light is guided to an optical recording medium 28 via a cover glass 27. The thickness t0 of the cover glass 27 on the optical recording medium 28 is 0.1 mm.
[0035]
FIG. 3 shows a wavefront aberration characteristic on a light-converging spot surface when spherical aberration correction is performed for the thickness error of the cover glass 27 by the biconcave lens 24 and the plano-convex lens 25, which are optical elements of the beam expanding optical system 33, and The beam expansion optical system 33 shows the inter-lens distance characteristics. The ideal value of the thickness of the cover glass 27 is set to 0.1 mm according to FIG.
[0036]
If the cover glass 27 has no thickness error, that is, 0.1 mm, the wavefront aberration is about 0.003λ, but if the cover glass 27 has a thickness error, the biconcave lens 24 of the beam expanding optical system 33 and In the case where correction for changing the distance between the plano-convex lenses 25 is not performed, or without correction, the wavefront aberration becomes very large. On the other hand, if the inter-lens distance between the biconcave lens 24 and the plano-convex lens 25, which are the optical elements of the beam expanding optical system 33, is changed in accordance with the relationship shown in the drawing according to the thickness of the cover glass 27, the wavefront aberration is greatly reduced. It is possible to do.
[0037]
By the way, the thickness error of the cover glass 27 is ± 2 to 3 μm or less in the disk surface in the current production process, ± 5 to 7 μm or less between disks, and it is sufficient to consider a total ± 10 μm thickness error in a normal disk. However, when the same optical pickup device is used for a disk having a double-layered recording surface, such as DVD double-layer recording, it is necessary to perform recording and reproduction with an interlayer thickness of about 20 μm. Therefore, it is necessary to cope with a cover glass thickness error of about ± 30 μm as a whole. The setting parameters of each optical component are as follows, and the numerical aperture NA of the objective lens 26 is 0.85, which is larger than 0.65 required for DVD.
Light source wavelength: 405 nm
First lens glass material: SF4
Second lens glass material: BK7
Objective lens numerical aperture: 0.85
Objective lens effective diameter: 3.0mm
Cover glass thickness: 0.1mm
[0038]
FIG. 4 shows a specific optical arrangement for reducing the beam diameter by exchanging the biconcave lens 24 and the plano-convex lens 25, which are optical elements of the beam expanding optical system 33. In this drawing, the plano-convex lens 25 and the biconcave lens 24 and the incident and outgoing directions thereof are exchanged for the light emitted from the collimating lens 22 and thereafter. If the light beam diameter D11 incident on the plano-convex lens 25 is 5.1 mm and the light beam diameter D2 emitted from the biconcave lens 24 is 3.4 mm, the enlargement ratio becomes 0.667 times (= 3.4 / 5.1). , The luminous flux diameter is reduced. In this optical system, the light beam diameter is reduced by changing the arrangement order of the optical elements constituting the beam expanding optical system 33. Therefore, in order to secure a light beam diameter larger than the effective diameter of the objective lens 26, the collimated emission light is At this stage, the beam diameter is increased by a factor of 1.5.
[0039]
FIG. 5 shows an example of a specific optical arrangement at each magnification of the biconcave lens 24 and the plano-convex lens 25, which are optical elements of the beam expanding optical system 33. The magnification (m) of the light beam by an optical system that combines a concave lens and a convex lens such as the biconcave lens 24 and the plano-convex lens 25 is determined by the focal length of the concave lens (−f). ₁ ) And the focal length of the convex lens (f ₂ ), The relational expression m = (f ₂ / F ₁ ). Focal length f of convex lens ₂ Is related to the reciprocal of the lens radius. In the plano-convex lens 25, the lens radius on the plane side is infinite and its reciprocal is 0, so only the lens radius on the convex side is related to the focal length. Therefore, in the beam expanding optical system 33 that combines the biconcave lens 24 and the plano-convex lens 25, the biconcave lens 24 is used in common, and by changing the lens radius R on the convex surface side of the plano-convex lens 25 and the distance S between the lenses. , The design change to change the enlargement ratio (m) can be made relatively easily.
[0040]
In FIG. 5A, the distance S between lenses is set to 3.841 mm and the radius R of the convex surface side lens of the plano-convex lens 25 is set to 10.04 mm in order to set the enlargement ratio m to 1.35. In (b), in order to make the magnification ratio m = 1.20, the distance S between the lenses is set to 1.777 mm, and the radius R of the plano-convex lens 25 on the convex surface side is set to 8.96 mm. In (c), the distance S between lenses is set to 0.411 mm, and the radius R of the convex-side lens of the plano-convex lens 25 is set to 8.25 mm in order to set the magnification ratio m to 1.10. In (d), the distance S between lenses is set to 5.916 mm, and the radius R of the convex surface of the plano-convex lens 25 is set to 11.13 mm in order to set the enlargement ratio m to 0.667.
[0041]
In FIG. 5, the optical component parameters other than the lens radius R of the plano-convex lens 25 are the same as in the case of m = 1.5 shown in FIG. 3, but the luminous flux diameter (φ) of the collimated incident light is φ = ( 3.4 / m), the luminous flux diameter at the stage of emission from the beam expanding optical system 33 is 3.4 mm.
[0042]
FIG. 6 shows a change characteristic of a distance between lenses of the beam expanding optical system when an error is given to the cover glass and spherical aberration compensation is performed at each magnification of the beam expanding optical system 33 by the biconcave lens 24 and the plano-convex lens 25. An example is shown below. m = 1.5 corresponds to the optical configuration of FIG. 2, and m = 1.35, m = 1.2 and m = 1.2 correspond to the optical configurations of FIGS. 5 (a), (b) and (c). Each corresponds to an arrangement. As the magnification m of the beam expanding optical system 33 decreases, the distance between the lenses decreases, and as the cover glass 27 becomes thicker, the distance between the lenses in the beam expanding optical system 33 decreases.
[0043]
FIG. 7 shows an example of the coupling efficiency of the objective lens 26 when a thickness error is given to the cover glass 27 and spherical aberration compensation is performed at each of the above-described magnifications. For the magnifying optical system with m> 1, the change rate of the coupling efficiency is stable at ± 2% with a cover glass thickness change of ± 10 μm. In the reduction optical system with m = 0.667, the variation rate of the coupling efficiency becomes ± 5 to 6%, and the difference in the coupling efficiency becomes larger as the cover glass thickness becomes thicker. This means that the amount of light emitted from the objective lens fluctuates by compensating for spherical aberration, and the light reflected from the optical recording medium 28 also fluctuates. Therefore, in order to stabilize the signal light, in an optical system that performs spherical aberration compensation, it is necessary that the enlargement factor m satisfies at least m> 1 so as to be an enlargement optical system.
[0044]
FIG. 8 shows the wavefront aberration on the converging spot surface when the incident side lens (L1) and the exit side lens (L2) are tilted with respect to the optical axis at each magnification ratio m of the beam expanding optical system 33. Show. The main component of the aberration that appears at this time is coma, but the smaller the magnification of the beam expanding optical system 33, the smaller the amount of aberration with respect to the tilt. It turns out that it is big.
[0045]
When it is assumed that a normal optical pickup device is mounted on a general-purpose mounting machine, it is assumed that a tilt of ± 1 deg at the maximum occurs during mounting. Further, in order to narrow the focused spot in the recording track of the optical recording medium 28, it is necessary to reduce the amount of generated aberration to 0.05λ or less. If there is a tilt of 1 deg on the L1 side at m = 1.1, the wavefront aberration becomes larger than 0.05λ. Therefore, in order to satisfy these conditions, the magnification of the beam expanding optical system 33 needs to be m ≧ 1.2.
[0046]
As shown in FIG. 6, when the magnification ratio m is 1.2 or less, the distance between lenses in the beam expanding optical system 33 becomes 0 or less at a cover glass thickness of ± 30 μm, and the distance between lenses must be ensured. Can not be done. Therefore, from the point of view of the mechanism, it is necessary to satisfy m ≧ 1.2.
[0047]
FIG. 9 shows an example of the wavefront aberration on the condensing spot surface when a thickness error is given to the cover glass 27 and the spherical aberration is compensated for the beam expansion optical system 33 at each magnification ratio m. As can be seen, the spherical aberration increases as the magnification m of the beam expanding optical system 33 increases. In order to reduce the amount of generated aberration to 0.05 λ or less when the cover glass thickness error is ± 30 μm, the wavefront aberration increases when the magnification ratio is m = 1.6, and the magnification ratio must be set to m ≦ 1.5. You can also see that there is.
[0048]
Therefore, in order to secure the distance between the lenses in the beam expanding optical system 33, to secure a margin for error in assembling the lens, and to sufficiently narrow the focused spot, the magnification of the beam expanding optical system 33 is 1.2 ≦ m ≦ 1. .5 must be satisfied.
[0049]
FIG. 10 shows a schematic configuration of an optical system of an optical pickup device 40 according to the second embodiment of the present invention. In the present embodiment, portions corresponding to the embodiment of FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted. In the present embodiment, a beam expanding optical system 43 basically similar to the beam expanding optical system 33 of FIG. 1 is disposed between the light splitting element 23 and the objective lens 26. However, the plano-convex lens 25 serving as the second lens L2 can be moved in the optical axis direction, and the distance between the lenses can be changed. When the recording medium 28 having an arbitrary thickness error is replaced, or when recording / reproducing is performed on a disk having a two-layered recording surface, if the distance between the lenses of the beam expanding optical system 43 is changed, the spherical surface generated due to the thickness error will occur. Aberration can be compensated.
[0050]
In the present embodiment, the second lens L2 on the emission side is movable. This is because, when the first lens L1 and the second lens L2 on the incident side are compared, as shown in FIG. 8, the increase in the wavefront aberration with respect to the tilt is smaller in the second lens L2. . When the lens can be moved in the optical axis direction, it is inevitable that so-called “play” occurs between the lens holder 44 and the guide rail 45. The second lens L2 is moved because the margin of the second lens L2 is wider than that of the first lens L1 with respect to the tilt generated by such “play”.
[0051]
On the other hand, in Conventional Example 2 shown in FIG. 12, the lens on the incident side is movable. It is presumed that this is because the light beam diameter of the lens on the incident side is small, so that the lens outer shape and the lens weight can be reduced, so that high-speed driving by the actuator is facilitated. Since it is necessary to compensate for spherical aberration only when a disc is replaced, it is not always necessary to drive at high speed, and it is considered that the difference in lens weight does not have a great effect.
[0052]
In this embodiment, as a moving means of the second lens L2, in addition to the method using the drive motor 47 as shown in the figure, a method using an electromagnetic actuator as in the first conventional example shown in FIG. Can be used, but the means are not limited to these.
[0053]
【The invention's effect】
As described above, according to the present invention, when a tilt occurs in the optical element constituting the beam expansion optical system, if the maximum amount of the tilt is ± 1 deg or less, the magnification (m) is 1.2 or more. By doing so, the wavefront aberration can be suppressed to 0.05λ or less even when the numerical aperture NA is large, so that the condensed spot can be condensed in a predetermined size in the recording track of the recording medium. Therefore, it is possible to increase the lens mounting tolerance when attaching the beam expanding optical system to the optical pickup device, thereby providing an effect that the lens mounting at the time of mounting the optical pickup device can be simplified.
[0054]
Further, according to the present invention, since the magnification (m) of the light beam diameter in the beam expanding optical system is set to 1.5 or less, when a thickness error occurs in the cover glass of the recording medium, the thickness error amount is ± 10%. When the thickness is 30 μm or less, the amount of generated aberration can be suppressed to 0.05λ or less, and there is an effect that a focused spot can be focused in a predetermined size in a recording track of a recording medium.
[0055]
Further, according to the present invention, by changing the distance between the first lens and the second lens in the beam expanding optical system, it is possible to easily adjust the enlargement ratio of the light beam diameter and perform appropriate aberration correction.
[0056]
Furthermore, according to the present invention, of the lenses of the beam expanding optical system composed of a combination of two lenses, the lens on the emission side can be moved in the optical axis direction, so that the lens holder and the guide can be moved during spherical aberration correction. Even if the numerical aperture NA is large, the amount of aberration generated due to tilt due to "play" with the rail can be reduced. As a result, there is an effect that the beam expansion optical system can compensate for the spherical aberration generated when the disc of the optical pickup device is replaced or the like without deteriorating the light-collecting characteristics of the spot on the recording medium.
[0057]
Further, according to the present invention, the magnification (m) of the light beam diameter can be changed by the lens radius on the convex surface side of the plano-convex lens.
[Brief description of the drawings]
FIG. 1 is a simplified cross-sectional view showing a schematic configuration of an optical system of an optical pickup device 20 according to a first embodiment of the present invention.
FIG. 2 is a sectional view showing a configuration of a beam expanding optical system 33 of FIG.
3 is a graph showing the effect of spherical aberration correction by a beam expanding optical system 33 with respect to a change in cover glass thickness in the optical pickup device 20 of FIG. 1, and a distance between lenses required for correction.
FIG. 4 is a cross-sectional view showing a specific optical arrangement in a case where the arrangement of a first lens and a second lens is reversed in the beam expanding optical system 33 of FIG. 1 to reduce the light beam diameter.
FIG. 5 is a cross-sectional view showing an example of an optical arrangement in which a distance S between lenses and a lens radius R of a plano-convex lens 25 are changed to different magnifications m in the beam expanding optical system 33 of FIG.
FIG. 6 is a graph showing an inter-lens distance when spherical aberration compensation is performed by giving an error to a cover glass thickness at each magnification in the beam expansion optical system 33 of FIG. 1;
FIG. 7 is a graph showing the coupling efficiency of the objective lens when the thickness error of the cover glass 27 is given and spherical aberration compensation is performed in the embodiment of FIG.
FIG. 8 is a diagram illustrating a light-converging spot surface when giving a tilt with respect to an optical axis to the entrance lens (L1) and the exit lens (L2) at each magnification of the beam expanding optical system 33 in the embodiment of FIG. 6 is a graph showing wavefront aberration in the graph of FIG.
9 is a graph showing the wavefront aberration on the converging spot surface when performing the spherical aberration correction by the beam expanding optical system 33 in the embodiment of FIG. 1 with the magnification ratio m as a parameter.
FIG. 10 is a simplified cross-sectional view showing a schematic configuration of an optical system of an optical pickup device 40 according to a second embodiment of the present invention.
FIG. 11 is a simplified cross-sectional view showing a schematic configuration of an optical system according to Conventional Example 1.
FIG. 12 is a simplified cross-sectional view showing a schematic configuration of an optical system according to Conventional Example 2.
[Explanation of symbols]
20,40 Optical pickup device
21 Semiconductor laser
22 Collimating lens
23 Optical splitter
24 biconcave lens
25 Plano-convex lens
26 Objective lens
27 Cover glass
28 Optical recording media
31 Light receiving element
32 aperture
33, 43 Beam expansion optical system
45 Lens holder
46 Guide Rail
47 Drive motor

Claims (5)

The light beam emitted from the light source is converted into a parallel light beam by a collimating lens and condensed on a recording medium by an objective lens, and reflected light from the recording medium passes through the objective lens, and is disposed between the objective lens and the collimating lens. In an optical pickup device that receives light after being branched by an optical branching element,
A beam expanding optical system between the light splitting element and the objective lens for expanding the diameter of a light beam transmitted through the light splitting element so that the expansion rate (m) becomes 1.2 or more. Pickup device. 2. The optical pickup device according to claim 1, wherein an expansion rate (m) of a beam diameter by the beam expansion optical system is 1.5 or less. The beam expanding optical system includes:
A first lens having an optical axis in a direction of the light beam to be converted into a parallel light beam and disposed on a light branching element side;
3. The optical pickup device according to claim 1, further comprising a second lens that shares an optical axis with the first lens and is disposed on the objective lens side. The light beam emitted from the light source is converted into a parallel light beam by a collimating lens and condensed on a recording medium by an objective lens, and reflected light from the recording medium passes through the objective lens, and is disposed between the objective lens and the collimating lens. In an optical pickup device that receives light after being branched by an optical branching element,
Including a beam expanding optical system for expanding the diameter of a light beam transmitted through the light branching element, between the light branching element and the objective lens,
The beam expanding optical system includes:
A first lens having an optical axis in a direction of the light beam to be converted into a parallel light beam and disposed on the light branching element side;
An optical pickup device comprising: a second lens that shares an optical axis with a first lens and is disposed on an objective lens side; and a moving unit that moves the second lens along the optical axis. The first lens is a biconcave lens,
The optical pickup device according to claim 3, wherein the second lens is a plano-convex lens.

Images