JP2010086621A - Optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head device for performing a recording and reproducing operation by achieving miniaturization and suppressing an astigmatic component amount with respect to an optical disk having a plurality of information recording surface. <P>SOLUTION: The satisfied recording and reproducing operation are attained with respect to each information recording surface of multi-layered optical disk and also the miniaturization of the optical head device is obtained by reducing the astigmatic component amount generated on a rising prism 15 with respect to the rising prism 15 having a beam shaping ratio m different from 1 and also by being provided with a variable focus section 14 for making the astigmatic component amount outgoing from the rising prism 15 to be a constant level or lower even though a focus component (Power) amount is changed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  As an optical system that handles optical storage, the present invention records information on optical recording media such as CDs, DVDs, magneto-optical disks, and high-density optical recording media such as “Blu-ray” (registered trademark: BD). The present invention relates to an optical head device that performs reproduction (hereinafter referred to as “recording / reproduction”) and an optical disk device that uses this optical head device. These optical recording media and high-density optical recording media are hereinafter referred to as “optical discs”, and in particular, an optical disc having a condensing surface (information recording surface) of a plurality of information recording layers is hereinafter referred to as a “multi-layer optical disc”.

  With downsizing of personal computers and the like, downsizing of optical disk devices used in these devices is required. In response to this requirement, the direction (space) in which light travels is changed by using a rising prism in the shape of a triangular prism in the optical head device mounted on the optical disk device, thereby reducing the thickness of the optical head device, and the optical disk device. The miniaturization is realized.

  This rising prism may have a beam shaping function that shapes the shape of the emitted light into a circular shape when light having an elliptical shape (plane perpendicular to the optical axis) is incident. In addition, in order to focus on each information recording surface of the multilayer optical disk, the amount of spherical aberration component generated by the difference in the cover thickness of each information recording surface is controlled by changing the parallelism of the light incident on the rising prism. An optical head device has been reported (Patent Document 1). However, when this rising prism is used, if non-parallel light is incident on the prism in order to control the spherical aberration component amount, the amount of astigmatism component when condensing on the information recording surface increases, and recording / recording There was a problem that the reproduction characteristics deteriorated.

  Further, as a solution to the problem in the optical head device described in Patent Document 1, in order to suppress the amount of astigmatism component generated due to the rising prism, the characteristic that the beam shaping magnification is brought close to 1.00. An optical head device has been reported that suppresses the generation of the amount of astigmatism components of light collected on each information recording surface having a different focal point by arranging the rising prism to have a circular shape. (Patent Document 2).

JP 2004-5903 A JP 2007-213755 A

  However, as described above, the optical head device described in Patent Document 1 uses a rising prism whose beam shaping magnification is as large as 1.1 to 1.3, so that an astigmatism component amount is generated. Is not considered. As a result, for an optical disc with a single information recording surface, the incident light can be adjusted to be parallel light so as to reduce the amount of astigmatism component, but it is condensed during recording / reproduction of a multilayer optical disc. When the information recording surface is switched, for example, when the control is performed so as to switch between the light incident while converging on the rising prism and the light incident while diverging, the beam shaping magnification of 1.1 to 1.3 is increased. In the prism, a large amount of astigmatism component is generated, and there is a problem in that good condensing characteristics cannot be obtained for all information recording surfaces to be condensed.

  Furthermore, since the rising prism of the optical head device described in Patent Document 2 has a beam shaping magnification of 0.97 to 1.03, a large beam shaping function cannot be generated by the rising prism. In other words, since the light incident on the rising prism in an elliptical shape does not have a function of emitting the light in a circular shape, the light incident on the rising prism must be circular in order to focus the light on the optical disk in a circular shape. There is a limitation. Furthermore, since the beam shaping magnification is set to 0.97 to 1.03, the shape of the rising prism becomes larger in the thickness direction of the optical head device, or the processing becomes difficult. There has been a problem that there is a limit to miniaturization and cost reduction of the head device.

  The present invention has been made to solve the above problem, and includes a light source, a beam shaping element that changes the shape of the light in a plane orthogonal to the optical axis of the light emitted from the light source, and the beam shaping element. In an optical head device comprising an objective lens for condensing emitted light on an optical disc having a plurality of information recording surfaces and a photodetector for receiving light reflected from the optical disc, the optical head device is provided between the light source and the objective lens. Provided is an optical head device having, in the optical path, the beam shaping element, and a variable focus section that varies a focus component amount and an astigmatism component amount to change a light divergence state.

  With this configuration, it is possible to reduce the size of an optical head device that records and reproduces a multi-layer optical disc, and to suppress astigmatism of light that converges on each of the information recording surfaces of the multi-layer optical disc. Therefore, good recording / reproduction can be realized.

The beam shaping element has a first direction in which the shape of the incident light A in a plane orthogonal to the optical axis of the incident light A traveling in parallel corresponds to the major axis or the minor axis is the diameter d ₁ , A plane orthogonal to the direction of 1 and having a diameter d ₂ in the second direction corresponding to the other of the major axis or the minor axis and a plane orthogonal to the optical axis of the outgoing light A that exits the beam shaping element When the shape of the emitted light A is a circle having the diameter d ₁ as a diameter, the variable focusing section has a beam shaping ratio m represented by d ₂ / d ₁ , and the variable focus section travels in parallel. A position where the shape of the incident light B in a plane perpendicular to the optical axis of the light B is a circle, and the phase of the emitted light B is the same on a plane perpendicular to the optical axis of the emitted light B emitted from the variable focus section The shape of the equiphase wavefront that is formed by connecting the long axis diameter is a and the short axis diameter is a. When the ellipse is b, the above optical head device is provided, wherein a × m × c = b or a = m × c × b, and 0.97 ≦ c ≦ 1.03 is satisfied. The beam shaping ratio m of the beam shaping element is m> 1.009 or m <0.991.

  With this configuration, a beam shaping element can be selected without any large limitation on the size of the beam shaping ratio m, so that the degree of freedom in design is increased and the amount of astigmatism components is suppressed in accordance with the corresponding beam shaping ratio. Since the variable focus portion can be designed as described above, astigmatism of light that converges on each of the information recording surfaces of a plurality of information recording surfaces of the multilayer optical disc can be suppressed. Reproduction can be realized.

  Further, the variable focus unit includes a liquid crystal lens element in which two transparent substrates are opposed to each other, and a focal length of light transmitted through the liquid crystal is changed according to a voltage applied to the liquid crystal sandwiched between the transparent substrates. The above-described optical head device is provided. One surface of the transparent substrate is made of a transparent electrode and a transparent material having a sawtooth shape or a cross-sectional shape approximating a sawtooth in a staircase shape, and has an ellipticity κ similarity around the optical axis on the incident plane. Provided is an optical head device having a plurality of concavo-convex portions formed in an elliptical shape.

  The ellipticity κ of the uneven portion is 0.97 × m ≦ κ ≦ 1.03 × m, or 0.97 / m ≦ κ ≦ 1.03 / m. provide.

  With this configuration, when the light is converged on each information recording surface among a plurality of information recording surfaces by controlling the voltage applied to the liquid crystal lens, the focus component amount and the astigmatism component amount are easily optimal values. Thus, good recording / reproduction can be realized in the optical head device.

  The variable focus unit includes a collimator lens that changes a divergence state of light emitted from the light source, a collimator lens position adjustment unit that has a mechanism for moving the collimator lens in parallel to the optical axis direction, and an astigmatism component. An optical head device as described above, comprising an astigmatism generation unit that can be generated. Further, the astigmatism generation unit includes a liquid crystal aberration in which a liquid crystal is sandwiched between two transparent substrates, and a transparent electrode divided into a specific pattern that generates an astigmatism component is formed on at least one of the transparent substrates. The above-described optical head device which is a correction element is provided.

  With this configuration, it is possible to easily change the focus component amount by moving the collimator lens. By generating the astigmatism component amount in proportion to the focus component amount, the focus component amount and the astigmatism component can be easily obtained. The amount can be controlled to an optimum value, and good recording / reproduction can be realized in the optical head device.

  The present invention provides an optical head device capable of recording and reproducing with good astigmatism component amount suppressed on each information recording surface of a multi-layer optical disc, realizing miniaturization, and having a high degree of design freedom. Can be realized.

(First embodiment)
FIG. 1 is a schematic diagram of an optical head device 10 according to a first embodiment of the present invention. The light emitted from the light source 11 becomes parallel light by the collimator lens 13, passes through the variable focus section 14, and enters the rising prism 15 that exhibits the beam shaping function. The light incident on the rising prism 15 is deflected toward the optical disc, and is focused on the information recording surface (for example, the information recording surface 18b) of the optical disc 18 by the objective lens 17. The light reflected by the optical disk 18 is deflected by the polarization beam splitter 12, passes through the lens 19, and is recorded on the information recording surfaces 18 a and 18 b of the optical disk 18 by the photodetector 20. Optical information such as a tracking error signal is detected. The optical path from the light source 11 to the optical disk 18 is referred to as “outward path”, and the optical path from the optical disk 18 to the photodetector 20 is referred to as “return path”.

  Further, a quarter wavelength plate 16 that provides a phase difference of λ / 4 with respect to light of wavelength λ may be disposed in the optical path between the polarizing beam splitter 12 and the objective lens 17. The polarization beam splitter 12 is not limited to be disposed in the optical path between the light source 11 and the collimator lens 13 but may be in the optical path between the collimator lens 13 and the rising prism 15. In FIG. 1, the dotted line schematically indicates the width of light, and the alternate long and short dash line indicates the center (optical axis) of light. The arrangement of the variable focus section 14 and the rising prism 15 in the optical path is not limited to the order shown in FIG. 1. For example, in the forward optical path, the variable focus section 14 is arranged after passing through the rising prism 15. It may be. For example, when the variable focus section 14 is arranged in front of the rising prism 15 as in the configuration of the optical head device 10 in FIG. 1, the width of the optical head device 10 in the Z direction (thickness direction) can be reduced. it can.

  The optical head device 10 includes a focus servo (not shown) that controls the movement of the objective lens 17 in the optical axis direction based on the focus error signal, and the objective lens 17 that is perpendicular to the optical axis direction based on the tracking error signal. And a tracking servo (not shown) for controlling in the direction to be.

  The light source 11 may be configured to emit two or three types of linearly polarized light having a single wavelength. As a light source having such a configuration, a so-called hybrid two-wavelength laser light source or three-wavelength laser light source in which two or three semiconductor laser chips are mounted on the same substrate, or two light sources emitting different wavelengths of light are used. Alternatively, a monolithic type two-wavelength laser light source or three-wavelength laser light source having three light emitting points may be used. For example, as three types of wavelengths, a semiconductor laser having a 405 nm wavelength band (385 to 420 nm) for BD, a 660 nm wavelength band (640 to 675 nm) for DVD, and a 785 nm wavelength band (770 to 800 nm) for CD is used. it can.

  The variable focus section 14 of the optical head device 10 according to the first embodiment is composed of a liquid crystal lens element as will be described later. The liquid crystal lens element includes a transparent electrode for applying a voltage to the liquid crystal, and can change the wavefront shape of transmitted light by an electric signal. For example, when parallel light traveling without changing the shape of the plane perpendicular to the optical axis of the light incident on the liquid crystal lens element is incident, if no voltage signal is applied, the voltage is transmitted without changing the wavefront, and the voltage is transmitted. Is applied to change the amount of focus component (Power) and the amount of astigmatism component of the transmitted light. Also, the focus component amount is 0, for example, in the case of parallel light, because the shape of the plane orthogonal to the optical axis of the light does not change, but is 0 when the size of the shape of the plane orthogonal to the optical axis of the light changes. It is a value different from, and represents a change in the size of this shape. The “shape of a plane perpendicular to the optical axis of light” is hereinafter referred to as “light shape”.

  In this way, for example, when parallel light is incident on the variable focus section 14 in which the amount of the focus component changes, the transmitted light proceeds while diverging or converging. As described above, "changing the divergence state" includes one or both of the functions of emitting light so as to be converged with respect to incident light or emitting light so as to diverge. Express. On the other hand, the amount of astigmatism component is 0 when the shape of the light is similar and does not change even when converged or diverged, but the shape changes, for example, such that the shape changes from a circle to an ellipse. In this case, the value is different from 0.

  The rising prism 15 has a triangular prism shape whose section is an isosceles triangle shape. Further, when the cross section is as shown in FIG. 1, the shape is an isosceles triangle in which the length of the side corresponding to the incident surface 15a and the length of the side corresponding to the reflecting surface 15c are equal. Light enters from the incident surface 15a of the rising prism 15, travels in the direction of the exit surface 15b, is reflected by the exit surface 15b, and travels in the direction of the reflective surface 15c. The light reflected by the reflecting surface 15 c and traveling again to the exit surface 15 b exits from the rising prism 15 and travels toward the optical disk 18. In this way, the traveling direction of the emitted light is approximately 90 ° with respect to the traveling direction of the light incident on the rising prism 15. Such a rising prism 15 has a function as an anamorphic prism that changes the shape of light. The exit surface 15b reflects or transmits light depending on the angle formed by the exit surface 15b and the traveling direction of light.

Next, the optical head device 10 using the rising prism 15 will be described. Here, the direction in which the light is emitted from the light source 11 and the traveling direction is the X direction, and the direction in which the light is traveling from the rising prism 15 is the Z direction, that is, the direction in which the light is incident on the rising prism 15. It is assumed that the direction of emitting light 15 is deflected in a perpendicular direction and proceeds. Further, as the beam shaping function of the rising prism 15, for example, consider that the shape of the emitted light is a circle with respect to the light having an elliptical shape. Of the light incident on the light incident surface 15a of the rising prism 15 (the optical axis direction is the X direction), the diameter in the Z direction is d ₂ , and the diameter in the Y direction perpendicular to the Z direction is d _1. (the optical axis direction Z direction) of the light emitted from the light emitting surface 15c is a circle of diameter d _1. That is, although not shown, the diameters in the Y direction are the same among the shapes of the light incident on the rising prism 15 and the light emitted. The beam diameter ratio d ₂ / d ₁ is expressed as a beam shaping ratio m. The present invention provides an optical head with a high degree of freedom in design that can adjust the variable focus portion 14 so as not to generate a large amount of astigmatism component even when a rising prism having a beam shaping ratio different from 1 (m ≠ 1) is arranged. A device can be realized.

  Here, a case where a BD that is a multilayer optical disk is recorded and reproduced as the optical disk 18 is considered. In the BD, the distance between the information recording surfaces in the thickness direction is standardized to 25 μm, and adjacent information recording surfaces, for example, the information recording surface 18a and the information recording surface 18b in FIG. Thus, the divergence state of the light emitted from the variable focus section 14 is changed so that the amount of the component to be focused (Power) is different on each information recording surface.

  Then, as shown in the optical head device 10 of FIG. 1, consider a case where the rising prism 15 which is a beam shaping element having a beam shaping ratio different from 1 (m ≠ 1) is arranged. At this time, if the variable focus section 14 generates the same divergence and convergence characteristics (same magnification) in the Y and Z directions on the YZ plane centered on the optical axis, it occurs in the rising prism 15. Due to the amount of astigmatism component, it is not effectively condensed on the information recording surface, and good recording / reproducing characteristics cannot be obtained.

  In this case, even if adjustment is made so that the light condensing characteristic with respect to one of the information recording surfaces (18a or 18b) is improved, the amount of astigmatism components increases with respect to the other information recording surface, which is favorable. Light condensing characteristics cannot be obtained. For example, even if collimated light is incident on the rising prism 15 and can be condensed so as not to cause large astigmatism on the information recording surface 18b, it is condensed on the information recording surface 18a located far away from the focal point. As described above, when the diverging light is incident on the rising prism 15 by controlling the variable focus unit 14, the astigmatism generated according to the amount of the focus component (Power) increases, so that a good condensing characteristic can be obtained. I can't.

  FIG. 2 shows an example in which collimated light having a focus component (Power) amount of 1λrms is incident on a beam shaping element such as a prism 15, and the amount of aberration component of the emitted light with respect to a change in the beam shaping ratio m. It is a graph which shows the characteristic when it divides | segments into a focus component (Power) amount and an astigmatism component (AST) amount. Thus, in the case of a rising prism that does not generate a beam shaping function (horizontal axis: beam shaping ratio m = 1), a focus component (Power) amount 1λrms of incident light appears as it is in the emitted light, and the amount of astigmatism component However, as the beam shaping ratio m becomes a value away from 1, a large astigmatism component amount is generated. In general, the amount of astigmatism component allowed when recording / reproducing with an optical head device is about 0.04 λrms. On the other hand, if the beam shaping ratio m is a value away from 1, the astigmatism value is increased accordingly. Since an aberration component amount is generated, it is necessary to devise a technique for suppressing this to a certain level or less.

  For the sake of simplicity, FIG. 2 shows the characteristics when the amount of focus component of light incident on the beam shaping element 15 is 1λrms. However, the amount of focus component of light to be actually applied to an optical disc having a plurality of information recording surfaces is shown. Is different. The amount of focus component of incident light is proportional to the amount of generated astigmatism component. For example, when light having a focus component amount of 10λ rms enters the beam shaping element 15, the amount of astigmatism component shown in FIG. Astigmatism component amount is 10 times as large as. Actually, in the case of a BD, the wavelength λ of the light used is 405 nm, the numerical aperture NA of the objective lens 16 is as large as 0.85, and as described above, between two adjacent information recording surfaces of the BD, as described above. The cover thickness differs by 25 μm. For this reason, when focusing on one information recording surface with respect to the other information recording surface, the amount of the focus component to be diversified or convergent light incident on the objective lens is actually about 3.36 λrms. For this reason, it is necessary to adjust the variable focus section 14 to give a focus component amount suitable for each information recording surface.

  Specifically, in consideration of the above, for example, when considering an optical head device that includes only the rising prism 15 and does not have the variable focus section 14, the astigmatism component amount can be allowed to be about 0.04λrms. The shaping ratio m is limited to those having a characteristic value of 0.991 ≦ m ≦ 1.009. On the other hand, in the case of an optical head device having the variable focus unit 14 in the optical head device, the astigmatism component amount is 0. Even if the beam shaping ratio m is m> 1.009 or m <0.991. It is possible to suppress to about 04λrms. Therefore, the degree of freedom in design for realizing good recording / reproduction in the optical head device is increased.

  Further, when light is incident on the beam shaping element (rising prism 15) in which the beam shaping ratio m is fixed at a constant value, as described above, the amount of incident light focus component (Power) [λrms] With respect to the increase, the focus component amount and the astigmatism component amount of the light emitted from the beam shaping element 15 increase in proportion. FIG. 3 shows, as an example, a focus component amount of light emitted from the beam shaping element 15 when the light shaping component amount is changed and incident on the beam shaping element 15 having a beam shaping ratio m = 0.8. It is a graph which shows the amount of astigmatism components. From this, it can be seen that these are in a proportional relationship.

  When the beam shaping element 15 having the beam shaping ratio m ≠ 1 is used in this way, in order to obtain a good light collection characteristic by suppressing the amount of astigmatism component condensed on each information recording surface of the multilayer optical disk, The variable focus unit 14 is required to have a function of controlling the amount of astigmatism component generated along with the control of the amount of focus component so that the information is recorded on each information recording surface of the multilayer optical disc. Therefore, if these two component amounts can be controlled, the range that the beam shaping ratio m required for the beam shaping element 15 (rise prism 15) can take can be widened.

  That is, the variable focus unit considers the astigmatism component amount generated in the beam shaping element 15 in accordance with the focus component amount, and generates a function for generating in advance the astigmatism component amount that cancels this astigmatism. . By doing so, the amount of astigmatism component of the light emitted from the beam shaping element 15 is canceled, and the shape of the light emitted from the beam shaping element 15 approaches a circle. The amount of astigmatism components can be reduced, and good light collection characteristics can be obtained.

  FIG. 4 schematically shows the shape of the light emitted from the beam shaping element when parallel light having a circular plane (XY plane) orthogonal to the optical axis is incident on the variable focus portion. In the variable focus portion, astigmatism component amount increases as the focus component amount increases as described above. Thus, when the astigmatism component amount increases, the light emitted from the variable focus section becomes elliptical. For example, as shown in FIG. 4, when the light emitted from the variable focus portion has an elliptical shape in which the Y direction is the major axis direction, the light enters the beam shaping element and is emitted as circular light. Adjust as follows. That is, when the beam shaping element has a characteristic that diverges more in the X direction than in the Y direction shown in FIG. 4, the light changes from an ellipse to a circle. Here, when the shape of the light emitted from the beam shaping element (the rising prism 15) (on the plane orthogonal to the optical axis) is the aspect ratio that is the ratio of the minor axis diameter to the major axis diameter, this aspect ratio is If it is 1, it becomes a circle. As described above, when the aspect ratio is 1, that is, a = b in FIG. 4, the astigmatism component amount does not increase even when the focus component amount is changed. As described above, the astigmatism component amount generated in the variable focus section may be adjusted so as to cancel the astigmatism component amount generated in the beam shaping element.

  Next, a specific aspect of the variable focus unit 14 will be described. First, in order to cancel the astigmatism component amount generated by the rising prism (beam shaping ratio m ≠ 1) as described above, the wavefront emitted from the variable focus section 14 will be considered. FIG. 5 is a schematic diagram showing an equiphase wavefront shape in a plane orthogonal to the optical axis. The “equal phase wavefront shape” refers to a shape formed by connecting positions having the same phase on a plane orthogonal to the optical axis. FIG. 5A is a schematic diagram illustrating an example of a wavefront shape when the focus component amount and the astigmatism component amount are taken into consideration, and here corresponds to the wavefront shape of the light emitted from the variable focus section 14. However, the isotropic wavefront shape is an ellipse.

In this example, the radius in the major axis direction that is the first direction is a ₁ and the radius in the minor axis direction that is the second direction is b ₁ . For example, this wavefront-shaped light is adjusted so that the variable focus unit 14 is adjusted so that the first direction coincides with the Z direction in FIG. 1, and the beam shaping ratio m (= d ₂ / d ₁ )> 1 is established. When entering the prism 15, only the beam diameter in the Z direction is emitted at 1 / m times. Therefore, the wavefront shape after passing through the rising prism 15 is deformed as shown in FIG. 5B, and a ₂ = a ₁ / m and b ₂ = b ₁ are obtained. At this time, in order to sufficiently reduce the amount of astigmatism component, it is preferable to adjust the wavefront shape after passing through the beam shaping element 15 to be a circle (a ₂ = b ₂ ), that is, the aspect ratio is 1.

Here, in order to control the amount of astigmatism component of the light emitted from the beam shaping element 15,
a ₁ × m × c = b ₁ (1)
C is satisfied. At this time, when the formula (1) is satisfied in any of 0.97 ≦ c ≦ 1.03, the focus component amount is preferably 1λ rms and the astigmatism component amount is preferably about 0.04λ rms or less, preferably 0.98 When Expression (1) is satisfied at any of ≦ c ≦ 1.02, the astigmatism component amount when the focus component amount is 1λrms is about 0.027λrms or less, which is more preferable.

As described above, when the amount of the focus component is 3.36 λrms corresponding to a change in focal length of 25 μm between the BD information recording surfaces, 0.991 ≦ c ≦ 1 in equation (1). .009 is preferable because the astigmatism component amount is about 0.04 λrms or less. Although it has been described so far as m> 1, even if m <1, the wavefront emitted from the variable focus section 14 may be adjusted so that a ₁ <b _1. It is preferable to adjust the value of c so that 1) is satisfied. Even if m <1, the above preferable range and more preferable range that c can take are the same.

FIG. 6 shows the astigmatism of the light emitted from the beam shaping element 15 when the beam shaping ratio of the beam shaping element 15 is m and the focus component (Power) amount of the light emitted from the beam shaping element 15 is 1λrms. It is a graph which shows the conditions of the focus component amount and astigmatism component amount of the light which radiate | emits the variable focus part 14, in order to set an aberration component (AST) amount to 0 (lambda) rms. That is, when the value of the beam shaping ratio m of the beam shaping element 15 is determined, the variable focus section 14 is designed so as to emit light having a focus component amount and an astigmatism component amount as shown in FIG. Good light condensing characteristics can be obtained on any information recording surface of the multilayer optical disk. If the amount of focus (Power) component of light incident on the beam shaping element 15 is P [λrms] and the amount of astigmatism component is A [λrms],
T = | A / P− (0.66 × (m−1) ² −1.41 × (m−1)) | (2)
In this relational expression, it is preferable to control so that light of each component amount of P and A satisfying T ≦ 0.15 can be emitted. For the same reason, the amount of astigmatism component can be suppressed as T approaches “0”, preferably T ≦ 0.10, more preferably T ≦ 0.05, and T ≦ 0. More preferably, it is 0.02.

  Thus, the variable focus section 14 is set so that the ratio of P and A is substantially constant so that the value of m is determined and the relationship in which the value of T in Equation (2) approaches 0 is established. As a result of the adjustment, the light emitted from the beam shaping element 15 only needs to consider the focus component amount, and the astigmatism component amount can be set to a sufficiently low level. Therefore, it is preferable that recording / reproduction can be performed with high quality on each information recording surface of the multilayer optical disk.

  Next, a liquid crystal lens element that is specifically configured as the variable focus section 14 will be described. FIG. 7 shows a specific configuration of the liquid crystal lens element 30 used as the variable focus section 14 of the optical head device 10 of FIG. FIG. 7A is a schematic plan view of the liquid crystal lens element 30, and FIG. 7B is a schematic cross-sectional view showing a cross section along AA ′ of the schematic plan view.

As shown in the schematic cross-sectional view of FIG. 7B, the liquid crystal lens element 30 includes transparent substrates 31 and 32 and transparent electrodes 33 and 34, a seal 35, a liquid crystal layer (liquid crystal) 36, and an uneven portion 37. Terminal electrodes 41 and 42 electrically connected to the transparent electrode, and connected to a drive power supply circuit (not shown). The liquid crystal layer 36, using a nematic liquid crystal having a ordinary refractive index n _o and extraordinary refractive index _{n e (n o ≠ n e} ). Uneven portion 37 is formed by using a transparent material having a refractive index n _s, a cross section having a depth d is an uneven shape. The uneven portion 37 preferably has a sawtooth shape (blazed shape) or a shape approximating the sawtooth shape in a staircase shape (pseudo blaze shape). In addition, although the transparent electrode 33 has shown the example formed on the uneven | corrugated | grooved part 37, it is not restricted to this, You may be formed on the transparent substrate 31. Moreover, although the transparent electrode 34 is described as an electrode having a solid surface, it may be divided into annular regions so as to correspond to a blazed region as described later.

Further, in the schematic plan view of FIG. 7A, an elliptical line in which the ellipticity κ (ratio of the length of the major axis to the length of the minor axis: b ₁ / a ₁ ) is constant around the optical axis. 38 is shown. Note that, by determining the beam shaping ratio m of the beam shaping element 15 to be used, the ellipticity κ can be set so as to satisfy the above formula (2). That is, the Fresnel lens pattern is formed so that the beam shaping ratio m (or 1 / m) is equal to the ellipticity κ, and a ratio of the focus component (Power) amount and the astigmatism component amount at a specific ratio is obtained. It is good to adjust as follows. Each ellipse 38 corresponds to a line connecting the vertices of the convex portions of the concavo-convex portions shown in the schematic sectional view of FIG. 7B, and the phase of the light emitted from the liquid crystal lens element 30 is along this ellipse 38. It will be the same. It should be noted that the outermost ellipse 39 corresponds to a line connecting points where the outermost blaze-shaped step becomes zero in the cross-sectional schematic diagram of FIG. The elliptical region including the optical axis shown in FIG. 7A corresponds to the central region of the concavo-convex portion 37 in FIG. 7B, and the annular zone surrounded by the adjacent ellipses shown in FIG. Each corresponds to one blaze-shaped uneven region in FIG.

Next, materials constituting the liquid crystal lens element 30 will be described. As long as the transparent substrates 31 and 32 are transparent to incident light, various materials such as a resin plate and a resin film can be used. However, when an optically isotropic material such as glass or quartz glass is used, This is preferable because the transmitted light is not affected by birefringence. As the material of the concavo-convex portion 37, various inorganic materials and organic materials such as a photosensitive resin and a thermosetting resin can be used. As the inorganic material, a SiO _x N _y film (x and y are atomic ratios of O and N to Si), a SiO ₂ film, a Si ₃ N ₄ film, an Al ₂ O ₃ film, etc. can be used. _{The xN y} film is preferably used because it can be adjusted to a desired refractive index by changing x and y depending on the film forming conditions, and is excellent in transparency and durability. Further, the uneven portion 37 may be formed of an optically isotropic material or a material exhibiting birefringence, and the uneven portion 37 causes a change in the refractive index of the liquid crystal layer 36 in accordance with an applied voltage. to the polarization direction of the incident light, it is sufficient to form a transparent material having a refractive index n _s between the refractive index n ₁ and the refractive index n ₂ of the liquid crystal can take (n ₁ <n _2). As the transparent electrodes 33 and 34, an oxide transparent conductive film such as an ITO (tin oxide doped indium oxide) film, an AZO (aluminum doped zinc oxide) film, or a GZO (gallium doped zinc oxide) film has high transparency and conductivity. Is preferably used.

  The liquid crystal layer 36 is composed of nematic phase liquid crystal, and the alignment direction of the liquid crystal molecules of the nematic phase liquid crystal will be described below. Here, considering the liquid crystal having the characteristic that the dielectric anisotropy Δε> 0, this liquid crystal alignment method will be described as an example. In order to align the liquid crystal, an alignment film (not shown) is formed on the transparent electrodes 33 and 34. In the homogeneous alignment using a liquid crystal material satisfying Δε> 0 for the liquid crystal layer 36, an alignment film is formed so as to have the same alignment direction. Specifically, as the alignment film, a film formed by applying a polyimide film and rubbed in a certain direction, an oblique deposition film formed by depositing SiO in an oblique direction with respect to the substrate surface, or the like is used. . The liquid crystal satisfying the dielectric anisotropy Δε <0 is not limited to the liquid crystal satisfying the dielectric anisotropy Δε> 0.

Next, the cross-sectional shape of the concavo-convex portion 37 having a blaze shape or a pseudo blaze shape will be described. In the transmission wavefront of the plane wave incident on the liquid crystal lens element 30 of the present invention, the optical path length difference OPD (r) of the light beam passing through the position separated by the length of the radius r with respect to the light beam of the optical axis (x = y = 0). ) Is the ellipticity κ of the liquid crystal lens element as b ₁ / a ₁ from FIG.
OPD (r) = α ₁ r ² + α ₂ r ⁴ + α ₃ r ⁶ + α ₄ r ⁸ +... (3)
However, r ² = {(b ₁ / a ₁ ) · x} ² + y ² ,
α ₁ , α ₂ , α ₃ , α ₄ ...; satisfy a constant power series formula. With this configuration, when the liquid crystal lens element 30 is disposed in the optical head device, it is possible to generate a transmitted wavefront that corrects the amount of spherical aberration component generated due to the difference in the cover thickness of the multilayer optical disk. At the same time, it acts to cancel the astigmatism component amount generated by the rising prism. In particular, by adjusting the focus component amount and the astigmatism component amount so as to satisfy the expression (2), the focus (Power) component while suppressing the astigmatism component amount of the light emitted from the rising prism to a low level. The amount can be changed and condensed on each information recording surface.

  Here, the specific shape of the curve of the expression (3) is indicated by the symbol α in FIG. The horizontal axis represents the radius r, and the vertical axis represents the optical path length difference OPD in units of the wavelength λ of the incident light. Then, the transmitted wavefront having an optical path length difference that is an integral multiple of λ with respect to the incident light having the wavelength λ can be regarded as equivalent. Therefore, the graph β showing the optical path length difference obtained by dividing the graph (optical path length difference) indicated by α in FIG. 8 at the wavelength λ interval and projecting the optical path length difference onto the plane of zero is substantially equivalent to the graph α. . The optical path length differences shown in the graph β are all within λ (in the figure, in the range of −λ to zero), and the cross-section has a sawtooth shape reflecting this.

Next, the depth d of the uneven portion 37 is as follows. In this case, the convex portion of the concavo-convex portion 37 (isotropic material portion having a refractive index n _s) and distance g (hereinafter, referred to as "cell gap") between the transparent electrode 34 substantially identical voltage is applied to the liquid crystal in The For this reason, the change in the refractive index of the liquid crystal in the cell gap only shifts the wavefront of the transmitted light uniformly, so that the wavefront change can be ignored.

On the other hand, when a voltage V is applied between the transparent electrodes 33 and 34, the substantial refractive index of the liquid crystal layer 36 (liquid crystal) with respect to light having a polarization direction indicating the extraordinary light refractive index (hereinafter referred to as “abnormal light polarization”). When the a n (V), the refractive index difference of the liquid crystal layer 36 and the uneven portion 37 [Delta] n (V) becomes Δn (V) = n (V ) -n s. For example, assuming that the applied voltage is V ₊₁ and that the optical path length difference of the transmitted wavefront corresponding to the graph β in FIG. 8 is generated at this time, the depth d of the concavo-convex portion 37 shown in FIG.
d = λ / | Δn (V ₊₁ ) | (4)
Where λ is the wavelength of the incident light,
Δn (V ₊₁ ) = n (V ₊₁ ) −n _s = n ₁ −n _s ,
n ₁ , n _s ; refractive index,
It is good to set it to a value.

Here, when the applied voltage V is changed, the refractive index difference Δn (V) also changes. For example, at an applied voltage V _{0 where} Δn (V ₀ ) = 0, the transmitted wavefront of the liquid crystal lens element 30 does not change. In addition, at an applied voltage V _{−1 where} Δn (V ₋₁ ) = − Δn (V ₊₁ ), a transmission wavefront having an optical path length difference shown in the graph γ of FIG. This corresponds to a transmitted wavefront having an optical path length difference which is plane-symmetric with respect to the graph β in FIG. 8 with respect to a plane having an optical path length difference of zero. As described above, three types of wavefront states can be generated by changing the magnitude of the applied voltage, that is, the state without the transmitted wavefront change and the wavefront states of the graphs β and γ in FIG. Further, when the refractive index n _s substantially equal to n ₁ or n _2, transmission wavefront is able to produce either of the two wavefronts states of the state of the no transmission wavefront change state, beta or gamma. Further, although the depth d is set based on 1λ in the equation (4), it may be set based on 2λ, for example.

  The transparent electrode 34 has been described as a solid electrode, but is not limited thereto. FIG. 9 is a schematic plan view when the transparent electrode 34 shown in FIG. 7B is divided, and coincides with the ring zone region having one blaze shape in FIG. 7B (as viewed from the optical axis direction). It may be divided like this. That is, the electrode segment 34a including the optical axis coincides with the central region of the concavo-convex portion 37 in FIG. 7B, and similarly, one blazed concavo-convex region in FIG. Design to match 34h.

  Further, a liquid crystal lens element having a structure in which a diffractive liquid crystal lens element 30 and a refractive liquid crystal lens element (not shown) are laminated may be used. In this case, the diffractive liquid crystal lens element 30 controls switching of giving a large focus component amount and giving or not giving the astigmatism component amount, while the refractive liquid crystal lens element 30 controls the focus component amount and astigmatism. More preferably, the component amount can be finely adjusted according to the cover thickness (each information recording surface) of the multilayer optical disc.

In the above, when the applied voltage _{V 0} to be Δn _(V 0) = 0, and [Delta] n _{(V -1)} = - transmitted by the two voltage when the applied voltage _{V -1} as the [Delta] n _{(V +1)} The wavefront is switched. That is, one of the light emitted from the liquid crystal lens element 30 remains parallel light that does not change the wavefront of the incident light, and the other is light that converges or diverges (generates a focus component amount). is there. However, since the light is condensed on each information recording surface of the multi-layer optical disc in this way, the state of the emitted light generated by the liquid crystal lens is not limited to this. For example, one is converged light and the other is diverged light. May be switched. Furthermore, even if the state of the emitted light is the same converged light, the degree of convergence (the magnitude of the focus component amount) may be set at two or more points and condensed on each information recording surface by the change. Further, for example, in the case of a multi-layer optical disc having a three-layer information recording surface, the state of light emitted from the liquid crystal lens 30 is as follows: convergent light, parallel light that does not change the transmitted wavefront, and divergent light. It is also possible to switch the position where light is condensed on each information recording surface.

(Second Embodiment)
FIG. 10 is a schematic diagram of an optical head device 50 according to the second embodiment of the present invention. In the optical head device 10 according to the first embodiment, the liquid crystal lens element 30 has been described as the variable focus unit 14, but the variable focus unit 51 of the optical head device 50 according to the second embodiment includes the collimator lens 53, It comprises a collimator lens position adjustment unit 52 that adjusts the position of the collimator lens 53 in the optical axis direction, and an astigmatism generation unit 60 that corrects astigmatism. In addition, in the optical head apparatus 50, the same thing as the optical head apparatus 10 attaches | subjects the same number, and avoids duplication of description.

  The collimator lens position adjustment unit 52 can move the collimator lens 53 in parallel with the optical axis direction. The collimator lens position adjustment unit 52 converges or diverges the light emitted by this position adjustment, and a focus component (Power). The amount is adjusted and condensed on each information recording surface 18a, 18b of the multilayer optical disc 18. As described above, since the astigmatism component amount changes when the focus component (Power) amount changes, the astigmatism generation unit 60 is arranged to correct astigmatism.

  Next, FIG. 11 shows a specific configuration of a liquid crystal aberration correction element 60 using liquid crystal as the astigmatism generation unit 60. FIG. 11A is a schematic plan view of the liquid crystal aberration correcting element 60, and FIG. 11B is a schematic cross-sectional view showing a cross section taken along the line BB ′ of the schematic plan view. As shown in FIG. 11 (a), the liquid crystal aberration correction element 60 has one transparent substrate (for example, transparent electrodes 61, 62a, 62b, 63a, 63b formed on the transparent substrate 65a, and each transparent electrode is independently provided. Electrical wiring (not shown) is applied so that a voltage can be applied, and is connected to a voltage control device, and the transparent electrodes 61, 62a, 62b, 63a, and 63b are patterns that generate astigmatism by the applied voltage. The center of the effective area 64 of the incident light is adjusted so as to coincide with the center of the transparent electrode 61. The transparent electrodes 61, 62a, 62b, 63a, 63b are combined. Also, this electrode pattern is an example, and is not limited to this as long as it is a pattern electrode that can correct astigmatism.

  As shown in the schematic cross-sectional view of FIG. 11B, the liquid crystal aberration correction element 60 has a transparent substrate 65a on which a pattern electrode is formed, and a transparent electrode 67 formed on one side on the transparent substrate 65b side facing the pattern electrode. The liquid crystal layer 68 is sandwiched with a seal 69 around the periphery. The transparent substrates 65a and 65b are each provided with an alignment film (not shown) and are aligned in the same direction, for example, the X direction. The liquid crystal layer 68 is composed of a nematic liquid crystal, and here, a liquid crystal satisfying a dielectric anisotropy Δε> 0 is used.

When no voltage is applied to the liquid crystal layer 68 (hereinafter referred to as no voltage application), the major axis direction of the liquid crystal molecules is aligned in the X direction within the liquid crystal layer 68, but when a voltage is applied (hereinafter referred to as voltage). When applied, the major axis direction of the liquid crystal is oriented toward the thickness direction of the liquid crystal layer 68. At this time, the alignment of the liquid crystal changes for each electrode pattern by adjusting the voltage value with each pattern electrode. That is, since the liquid crystal is a birefringent material having a ordinary refractive index n _o and extraordinary refractive index _{n e (n o <n e} ), the phase of the light emitting respective pattern electrodes, the respective pattern electrodes It can be changed by controlling the applied voltage. For example, when oriented in the X direction (when no voltage is applied) and linearly polarized light in the X direction is incident, the refractive index of the incident light becomes an extraordinary light refractive index. Since the axial direction is aligned in the thickness direction of the liquid crystal layer, the ordinary refractive index is obtained. By utilizing this, the wavefront of the emitted light can be changed with respect to the wavefront of the incident light. The liquid crystal is not limited to having a dielectric anisotropy Δε> 0, but may be a dielectric anisotropy Δε <0.

Next, in order to give astigmatism, the voltage given to a pattern electrode is demonstrated concretely. For example, V _{a is} applied to the transparent electrode 61, the voltage V _b is applied to the transparent electrodes 62a and 62b, and the voltage V _c is applied to the transparent electrodes 63a and 63b, so that V _b > V _a and V _c <V _a are set. At this time, out of the light incident on the liquid crystal aberration correction element 60 from the Z direction with a plane wave (same phase), the phase of the light emitted from the transparent electrodes 62a and 62b advances with respect to the light emitted from the transparent electrode 61. The phase of the light emitted from 63a and 63b is delayed. By adjusting the voltage in this way to make the phase in the X direction and the phase in the Y direction asymmetric, astigmatism is given, and astigmatism generated in the rising prism 15 which is a beam shaping element is offset. Can be controlled. Then, even if the focus component amount is changed by adjusting the ratio of the focus (Power) component amount and the astigmatism component amount generated by the variable focus unit 51 to satisfy the expression (2), the rising prism is changed. Astigmatism component of the light emitted from 15 can be suppressed to a low level.

Example 1
First, a specific method for manufacturing the liquid crystal lens element 30 as the variable focus section 14 according to the first embodiment will be described with reference to FIGS. As the transparent substrates 31 and 32, SiO _x N _y (where x and y indicate the element ratio of O and N) is formed on the quartz glass substrate surface by sputtering. In this case, a transparent uniform refractive index film SiO _x N _y having a refractive index n _s (= 1.64) is formed using, for example, a Si target and a discharge gas in which oxygen and nitrogen are mixed into Ar gas. The film thickness is d (= 3.5 μm).

Further, after the resist is patterned by photolithography using a photomask so as to correspond to the shape of the graph β in FIG. 8, the SiO _x N _y film is processed by a reactive ion etching method. As a result, the cross-sectional shape in which the sawtooth shape is approximated by an eight-step staircase shape in the region of an ellipse 39 (major axis: 6.25 mm, minor axis: 5.00 mm) in the schematic plan view of FIG. The uneven part 37 which shows the cross section in b) is processed. At this time, the ellipticity of each of the ellipse 38 and the ellipse 39 is 0.8. Next, an ITO film is formed as the transparent electrode 33 on the surface of the uneven portion 37. Further, a polyimide film (not shown) is applied on the transparent electrode 33 so as to have a film thickness of about 50 nm and then baked, and the polyimide film surface is rubbed in the X-axis direction to form an alignment film.

Further, an ITO film as a transparent electrode 34 is formed on a quartz glass substrate as the opposing transparent substrate 32, and a polyimide film (not shown) is applied thereon to a film thickness of about 50 nm, followed by baking, and the polyimide film surface is X An alignment film is formed by rubbing alignment treatment in the axial direction. Further, an adhesive mixed with a gap control material having a diameter of 7 μm is printed and patterned to form a seal 35. The seal 35 is overlapped with the transparent substrate 31 and pressed, and the distance between the transparent electrodes is 7 μm at the maximum and 3.5 μm at the minimum. (Gap g = 3.5 μm) and an empty cell with d = g is produced. Thereafter, nematic liquid crystal is injected from an injection port (not shown) of the empty cell, and the injection port is sealed to obtain the liquid crystal lens element 30 shown in FIG. The liquid crystal is used a nematic liquid crystal having a positive dielectric anisotropy of the ordinary refractive index n _o (= 1.50) and an extraordinary refractive index n _e (= 1.75). The liquid crystal has a homogeneous orientation in which the orientation of liquid crystal molecules is aligned in parallel to the surfaces of the transparent electrodes 33 and 34 and in the X-axis direction, and is filled in the concave portion of the concave and convex portion 37.

A voltage is applied to the liquid crystal layer 36 by connecting a drive power supply circuit (not shown) between the transparent electrodes 33 and 34 of the liquid crystal lens element 30 thus obtained. Increasing the voltage applied to the liquid crystal layer 36 from 0V, the substantial refractive index of the X-axis direction of the liquid crystal layer 36 is, n ₁ = n from _{e (= 1.75) n 2 =} n o (= 1. 50). As a result, the difference in refractive index between the liquid crystal layer 36 and the concavo-convex portion 37 with respect to linearly polarized incident light having an X-axis polarization plane,
Δn _max (= n ₁ −n _s ) = 0.11.
To Δn _min (= n ₂ −n _s ) = − 0.14,
And the transmitted wavefront changes according to the thickness distribution of the liquid crystal layer 36 filled in the concave portion of the concave and convex portion 37.

Here, for example, when recording / reproducing BD, the objective lens 17 with NA of 0.85 is designed so that the aberration is zero with respect to the optical disk 18 having a cover wavelength of 87.5 μm at the operating wavelength λ (= 405 nm). Is focused on two information recording surfaces having cover thicknesses of 100 μm and 75 μm. In the optical head device 10 shown in FIG. 1, a rising prism 15 having a beam shaping ratio m of 0.8 (the beam diameter in the Y direction is unchanged before and after transmission) is disposed, and the liquid crystal lens element 30 is disposed on the variable focus section 14. The ellipses 38 and 39 are arranged so that the major axis is in the Y direction and the minor axis is in the Z direction. By adjusting the depth d of the liquid crystal lens element 30, an information recording surface (for example, 18b) having a cover thickness of 75 μm when the voltage is V _{+ 1} , and an information recording surface (for example, 18a) having a cover thickness of 100 μm when the voltage is V− _1. ) To adjust the amount of focus component (Power). At this time, when parallel light having a circular light shape is incident from the light source 11 and is emitted from the liquid crystal lens element 30, the amount of focus component can be adjusted by the applied voltage, and the astigmatism generated in the liquid crystal lens element 30. Since the aberration component amount acts so as to cancel out the astigmatism component amount generated by the rising prism 15, the astigmatism component amount on each information recording surface can be suppressed and condensed.

(Example 2)
A specific method of manufacturing the liquid crystal aberration correction element 60 as the variable focus unit 51 according to the second embodiment will be described with reference to FIGS. An ITO film is formed as a transparent electrode on a quartz glass substrate surface as the transparent substrate 65a. Then, the ITO film on the surface of the transparent substrate 65a is etched so as to have a pattern as shown in FIG. 11A to form transparent electrodes 61, 62a, 62b, 63a and 63b. Further, a polyimide film (not shown) is applied on the ITO film so as to have a film thickness of about 50 nm and then baked, and the polyimide film surface is subjected to a rubbing alignment treatment to obtain an alignment film.

Further, an ITO film is formed as a transparent electrode 67 on a quartz glass substrate as the opposing transparent substrate 65b, and a polyimide film (not shown) is applied thereon to a thickness of about 50 nm, followed by baking and rubbing alignment treatment. An alignment film is used. Furthermore, an adhesive material mixed with a gap control material having a diameter of 7 μm is printed and patterned to form a seal 67, which is pressed against the transparent substrate 65a so that the rubbing direction coincides, and the distance between the transparent electrodes is about A 7 μm empty cell is produced. Thereafter, nematic liquid crystal is injected from an empty cell inlet (not shown), and the inlet is sealed to obtain the liquid crystal aberration correcting element 60 shown in FIG. The liquid crystal is used a nematic liquid crystal having a positive dielectric anisotropy of the ordinary refractive index n _o (= 1.50) and an extraordinary refractive index n _e (= 1.75). The liquid crystal has a homogeneous alignment in which the alignment of the liquid crystal molecules is parallel to the surfaces of the transparent electrodes 65 a and 65 b and aligned in the X-axis direction, and is filled in the liquid crystal layer 68.

  A voltage is applied to the liquid crystal layer 68 by placing the manufactured liquid crystal aberration correction element 60 in the optical head device 50 and connecting a driving power supply circuit (not shown) to each transparent electrode. When the collimator lens 53 is fixed at a position (point A) adjusted by the collimator lens position adjustment unit 52 so that the light emitted from the collimator lens 53 becomes parallel light, no voltage is applied to each transparent electrode, and liquid crystal aberration correction is performed. The light emitted from the element 60 is focused on the information recording surface 18b without giving a phase distribution.

Next, the collimator lens position adjusting unit 52 moves the collimator lens 53 to a position different from the point A so as to be parallel to the optical axis, and fixes the light emitted from the collimator lens 53 to a position (referred to as a point B). Then, V _{a is} applied to the transparent electrode 61, the voltage V _b is applied to the transparent electrodes 62a and 62b, and the voltage V _c is applied to the transparent electrodes 63a and 63b, so that V _b > V _a and V _c <V _a are set. At this time, the phase of the light emitted from the transparent electrodes 62a and 62b is advanced, the phase of the light emitted from the transparent electrodes 63a and 63b is delayed, and a certain amount of astigmatism component is generated. As a result, the rising prism 15 functions to cancel the astigmatism component amount corresponding to the focus component (Power) amount, and the astigmatism component amount is sufficiently suppressed on the other information recording surface 18a. Can be lighted.

  As described above, by using the variable focus unit of the present invention, the focus component amount can be varied even in an optical head device that realizes miniaturization by using a beam shaping element having a beam shaping ratio m different from 1. However, it is possible to maintain a low level of the astigmatism component amount. In particular, an optical head device with improved recording / reproduction quality of a multi-layer optical disk having a plurality of information recording surfaces can be realized and is useful.

1 is a schematic diagram showing an optical head device according to a first embodiment of the present invention. The graph which shows the characteristic of each component amount of the light radiate | emitted with respect to the light of the focus component amount of 1 (lambda) rms which injects into the beam shaping element of the beam shaping ratio m. The graph which shows the characteristic of each component amount of the light which changes the focus component amount of the light which injects into the beam shaping element whose beam shaping ratio m is 0.8, and radiate | emits a beam shaping element. The schematic diagram which shows the mode of the shape of the light which enters / emits an optical axis and a beam shaping element in a variable focus part. The schematic diagram which shows the present phase wavefront shape of the light which radiate | emits a variable focus part and a beam shaping element. The graph which shows the characteristic of each component amount of incident light, when the focus component amount of the light which radiate | emits the beam shaping element of the beam shaping ratio m is 1 (lambda) rms. 2A and 2B are a schematic plan view and a schematic cross-sectional view illustrating a configuration of a liquid crystal lens that serves as a variable focus portion of the optical head device according to the first embodiment. The graph of an example which shows the optical path length difference of the transmitted wave front produced | generated by a liquid crystal lens. The plane schematic diagram which shows the electrode pattern of a liquid crystal lens. FIG. 5 is a schematic diagram showing an optical head device according to a second embodiment of the present invention. FIG. 9 is a schematic plan view and a schematic cross-sectional view showing a configuration of a liquid crystal aberration correction element used in a variable focus section of an optical head device according to a second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 50 Optical head apparatus 11 Light source 12 Beam splitter 13, 52 Collimate lens 14, 51 Variable focus part 15 Rising prism (beam shaping element)
15a Rising prism light incident surface 15b Rising prism light exit surface 15c Rising prism light reflecting surface 16 1/4 wavelength plate 17 Objective lens 18 (Multi-layer) Optical disk 18a, 18b Information recording surface 19 Cylindrical lens 20 Light Detector 30 Liquid crystal lens 31, 32, 65a, 65b Transparent substrates 33, 34, 34a, 34b, 34c, 34d, 34e, 34f, 34g, 34h, 61, 62a, 62b, 63a, 63b, 67 Transparent electrodes 35, 69 Seals 36, 68 Liquid crystal layer 37 Concavity and convexity 38, 39 Ellipse 41, 42 Terminal electrode 52 Collimator lens position adjustment unit 60 Liquid crystal aberration correction element 64 Effective region (incident light)

Claims (8)

A light source;
A beam shaping element that changes the shape of the light in a plane orthogonal to the optical axis of the light emitted from the light source;
An objective lens for condensing the light emitted from the beam shaping element onto an optical disc having a plurality of information recording surfaces;
In an optical head device comprising a photodetector for receiving light reflected from the optical disc,
Light having the beam shaping element and a variable focus unit that varies a focus component amount and an astigmatism component amount to change a light divergence state in an optical path between the light source and the objective lens. Head device. The beam shaping element has a first direction in which the shape of the incident light A in a plane orthogonal to the optical axis of the incident light A traveling in parallel corresponds to the major axis or the minor axis is the diameter d ₁ , and the first An ellipse having a diameter d2 in the _second direction corresponding to the other of the major axis and the minor axis, and in a plane perpendicular to the optical axis of the outgoing light A that exits the beam shaping element. When the shape of the emitted light A is a circle having the diameter d ₁ as a diameter, it has a beam shaping ratio m expressed by d ₂ / d ₁ ,
In the variable focus section, the shape of the incident light B in a plane orthogonal to the optical axis of the incident light B traveling in parallel is a circle, and the plane orthogonal to the optical axis of the output light B emitted from the variable focus section When the shape of the equiphase wavefront formed by connecting the positions having the same phase of the outgoing light B is an ellipse having a major axis diameter a and a minor axis diameter b,
2. The optical head device according to claim 1, wherein a × m × c = b or a = m × c × b, and 0.97 ≦ c ≦ 1.03 is satisfied.   3. The optical head device according to claim 1, wherein the beam shaping ratio m of the beam shaping element is m> 1.009 or m <0.991.   The variable focus section is composed of a liquid crystal lens element in which two transparent substrates are arranged opposite to each other and a focal length of light transmitted through the liquid crystal is changed according to a voltage applied to the liquid crystal sandwiched between the transparent substrates. The optical head device according to claim 1.   One surface of the transparent substrate is made of a transparent electrode and a transparent material having a sawtooth shape or a cross-sectional shape approximating a sawtooth in a staircase shape, and has a plurality of similarities of ellipticity κ around the optical axis on the incident plane. The optical head device according to claim 4, comprising an uneven portion formed in an elliptical shape.   The ellipticity κ of the uneven portion is 0.97 × m ≦ κ ≦ 1.03 × m, or 0.97 / m ≦ κ ≦ 1.03 / m. The optical head device described. The variable focus unit includes a collimator lens that changes a divergence state of light emitted from a light source,
A collimator lens position adjusting unit having a mechanism for moving the collimator lens in parallel to the optical axis direction;
The optical head device according to any one of claims 1 to 3, further comprising an astigmatism generation unit capable of generating an astigmatism component.   The astigmatism generation unit includes a liquid crystal aberration correction element in which a liquid crystal is sandwiched between two transparent substrates, and a transparent electrode divided into a specific pattern that generates an astigmatism component is formed on at least one of the transparent substrates. The optical head device according to claim 7.

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