JP2010123214A - Objective lens, optical pickup and optical disc device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens made of plastics that compensates for the third-order coma aberration when environmental temperature and the thickness of a cover layer are changed, and an optical pickup and an optical disc device using the same. <P>SOLUTION: The objective lens 4 made of plastics and used for the optical pickup 1. The lens condenses light beams emitted from a light source 3 into one or more layers of the optical disc. The numerical aperture is 0.8 or more. The sign of the third-order coma aberration generated by a lens tilt of the objective lens is the same with respect to a tilt angle direction in a range of an incident magnification for correcting the third-order spherical aberration caused by change in temperature and the thickness of the cover layer of the optical disc. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical pickup for recording and / or reproducing information with respect to an optical recording medium such as an optical disc, an objective lens used in an optical disc apparatus, and the like, and an optical pickup and an optical disc apparatus using the same.

  Conventionally, as an information signal recording medium, a CD (Compact Disc) using a light beam with a wavelength of about 785 nm, a DVD (Digital Versatile Disc) using a light beam with a wavelength of about 660 nm, which realizes higher density recording than a CD, and the like. There is an optical disc. Furthermore, the optical disk is an optical disk capable of high-density recording (hereinafter referred to as “high-density recording optical disk”) in which a signal is recorded and reproduced using a light beam having a wavelength of about 405 nm by a blue-violet semiconductor laser that realizes higher-density recording than DVD. "). As this high-density recording optical disc, for example, a BD (Blu-ray Disc (registered trademark)) having a structure in which the thickness of a cover layer (protective layer) for protecting a recording layer for recording a signal is reduced is proposed. ing.

  An optical pickup is used to record an information signal on an optical disk such as the above-described CD, DVD, BD, or to reproduce an information signal recorded on the optical disk. In such an objective lens for an optical pickup, it is desired to replace the material with glass (synthetic resin), which has been widely used in the past, in order to improve mass productivity and weight reduction.

  However, when a plastic objective lens is used as compared with a glass objective lens, it is necessary to avoid degradation of optical characteristics in consideration of the characteristics as plastic. The most important factor when using a plastic lens is that plastic has a characteristic that its refractive index changes greatly due to a temperature change compared to glass.

For example, as shown in FIG. 15, L-BAL42 as a general glass material has a substantially constant refractive index regardless of temperature, as indicated by Lg. On the other hand, plastic materials such as Zeonex 340R, for example, have a characteristic that the refractive index changes greatly according to the temperature change as indicated by Lp, that is, the refractive index has a large temperature dependency. In addition, dn / dT which shows the change rate with respect to the temperature of the refractive index of L-BAL42 is dn / dT = -3.30 * 10 < ^-6 > [/ degreeC], and shows the change rate with respect to the temperature of the refractive index of Zeonex340R. dn / dT is dn / dT = −1.29 × 10 ⁻⁴ [/ ° C.]. When such a plastic material is used for an objective lens for an optical pickup, there is a concern that various optical characteristics deteriorate with respect to temperature changes.

  As one of the deteriorated optical characteristics, there is a problem of sensitivity deterioration due to a change in third-order coma aberration that occurs when the objective lens is tilted (hereinafter also referred to as “lens tilt”). Hereinafter, the third-order coma aberration amount generated per lens tilt of 1 degree is referred to as “lens tilt sensitivity”.

  Here, it will be described that deterioration of lens tilt sensitivity becomes a problem in a plastic objective lens. Optical pickups are used in various environments, and are required to operate normally in the range of 0 ° C. to 75 ° C., for example. Due to this temperature change, when a plastic objective lens is used, the lens tilt sensitivity will fluctuate. The lens tilt sensitivity also varies when the cover layer thickness changes.

  Further, in an optical pickup for a high-density recording optical disk such as a BD, there is a precondition that aberration management is severe due to factors such as an increase in the numerical aperture of the objective lens and a reduction in the wavelength of the used light beam. In this case, a method of changing the incident magnification to the objective lens is being adopted in order to reduce third-order spherical aberration caused by a cover layer error or the like. Furthermore, a technique of changing the incident magnification on the objective lens is also used in an optical pickup for a so-called two-layer optical disk or multilayer optical disk having two or three or more recording layers. The lens tilt sensitivity described above has a problem that it deteriorates even when the incident magnification to the objective lens fluctuates. In addition, when there is a temperature change as described above, a third-order spherical aberration occurs due to a change in the refractive index of the objective lens, and in this case, in order to reduce the third-order spherical aberration, It is necessary to change the incident magnification. Even when the incident magnification to the objective lens is changed by this temperature change, there is a problem that the lens tilt sensitivity is deteriorated.

When a plastic objective lens is formed with the same guidelines as a glass objective lens having a general lens tilt sensitivity, the lens tilt sensitivity deteriorates due to factors such as temperature change and cover layer thickness change. May be fatal. Specifically, there are the following problems when the objective lens is formed with a conventional pointer with a temperature of 35 ° C. and a cover layer thickness of 87.5 μm as the design center state. That is, such an objective lens has a problem that the lens tilt sensitivity becomes almost zero as shown in FIG. 16A, for example, under the setting conditions of a temperature of 75 ° C. and a cover layer thickness of 100 μm. FIG. 16A is a diagram for showing the amount of third-order coma aberration generated with respect to the change of the lens tilt angle in the state where the temperature is 75 ° C. and the cover layer thickness is 100 μm, that is, the lens tilt characteristic. is there. In FIG. 16A, L _JLT indicates the lens tilt sensitivity under the above-described setting conditions of 75 ° C. and 100 μm when the objective lens is formed with a conventional pointer.

  Further, the problem due to the deterioration of the lens tilt sensitivity will be described with reference to FIG. In the optical pickup for various optical disks as described above, a compensation method is generally used in which coma aberration caused by warping or tilting of the optical disk is canceled by tilting the objective lens.

That is, when the optical disc is tilted, the third-order coma aberration as shown in FIG. 16B, for example, occurs according to the disc tilt angle. In FIG. 16B, L _DT indicates the disc tilt characteristic, that is, the amount of third-order coma aberration generated with respect to the disc tilt angle, the horizontal axis indicates the disc tilt angle [degree], and the vertical axis indicates the third-order coma aberration. [Mλrms] is shown. In the example as shown in FIG. 16B, there is about 98.7 disc tilt sensitivity indicating the third-order coma aberration amount generated per disc tilt of 1 degree. In the conventional compensation method, the third-order coma aberration that cancels out the third-order coma aberration generated according to the disc tilt angle is canceled by causing the objective lens to tilt, thereby canceling it.

  Then, when the lens tilt sensitivity as described above is deteriorated, especially when it is close to 0, even if the objective lens is tilted, the third-order coma aberration generated due to the tilt of the optical disk cannot be canceled, and thus Various signal deteriorations due to residual coma are a problem. In other words, there is a problem in that coma generated due to the tilt of the optical disk cannot be reduced, leading to deterioration of recording / reproducing characteristics.

JP 2008-112575 A

  The object of the present invention is to improve the mass productivity and weight reduction by making the objective lens made of plastic, and when used in an optical pickup, even when there is a change in environmental temperature and a change in cover layer thickness, It is an object of the present invention to provide an objective lens that can realize compensation for the next coma aberration and realize good recording / reproducing characteristics, and an optical pickup and an optical disc apparatus using the objective lens.

  In order to achieve this object, the objective lens according to the present invention has a numerical aperture of 0.8 or more, and in the range of the incident magnification that corrects the third-order spherical aberration caused by the temperature and the thickness change of the cover layer of the optical disc. The objective lens made of plastic having the same sign of the third-order coma aberration generated by the lens tilt of the objective lens.

The objective lens according to the present invention has a numerical aperture of 0.8 or more, a temperature change amount is ΔT, a thickness change amount of the cover layer of the optical disk is ΔL, and a refractive index temperature coefficient of the objective lens is dn. / DT, where f is the focal length of the objective lens, the third-order coma aberration that occurs around 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × ( | Dn / dT |) + 2.0 × (ΔL) × (1 / f) [mλrms] or more.

  The optical pickup according to the present invention further includes an objective lens for condensing the light beam emitted from the light source on each of one or more layers of the optical disc. The objective lens includes a temperature and a thickness of the cover layer of the optical disc. This is an optical pickup in which a light beam is incident at an incident magnification that corrects third-order spherical aberration caused by a change in height. The objective lens used in this optical pickup is the one described above.

  Furthermore, the optical disc apparatus according to the present invention includes an optical pickup having an objective lens that condenses the light beam emitted from the light source on each of one or more layers of the optical disc. An optical disk device in which a light beam is incident at an incident magnification that corrects third-order spherical aberration caused by a change in the thickness of the cover layer. The objective lens used in the optical disk device is as described above. .

  In the present invention, the objective lens is made of plastic, so that mass productivity and weight reduction are improved, and the lens temperature sensitivity change and the cover layer thickness change are made by considering the variation of the lens tilt sensitivity. Sometimes compensation of third-order coma aberration is realized. That is, the present invention achieves mass productivity and weight reduction, and realizes good recording / reproduction characteristics by realizing good aberration correction by maintaining good lens tilt sensitivity under the set conditions.

Hereinafter, the best mode for carrying out the invention will be described in the following order.
1. 1. Overall configuration of optical pickup and optical disc apparatus 2. Correction of third-order spherical aberration by optical pickup 3. Compensation of third-order coma aberration by objective lens Objective lens configuration examples and comparative examples

[1. Overall configuration of optical pickup and optical disc apparatus]
Hereinafter, an optical pickup 1 to which the present invention is applied and an optical disc apparatus using the same will be described with reference to the drawings.

  An optical pickup 1 to which the present invention is applied records and reproduces information with respect to an optical disc 2 as an optical recording medium. The optical pickup 1 constitutes an optical disk device together with a spindle motor serving as a drive unit for rotating the optical disk 2 and a feed motor for moving the optical pickup 1 in the radial direction of the optical disk. The optical pickup 1 records and reproduces information with respect to the optical disc 2 that is rotated by a spindle motor.

  The optical disk 2 used here is, for example, a high-density recording optical disk capable of high-density recording using a semiconductor laser having an emission wavelength of about 405 nm (blue purple). The present invention is applicable not only to the above-described optical disc but also to an optical pickup and an optical disc apparatus that perform recording and / or reproduction on an optical recording medium capable of optical recording and / or reproduction. The present invention is also applicable to an optical pickup that records and / or reproduces information on an optical disc in which one or a plurality of recording layers (also referred to as “layers”) are stacked in the incident direction of the light beam. The

  As shown in FIG. 1, an optical pickup 1 to which the present invention is applied collects a light source unit 3 that emits a light beam having a wavelength of about 405 nm and a light beam emitted from the light source unit 3 on a signal recording surface of an optical disc 2. And an objective lens 4 that emits light. The optical pickup 1 includes a collimator lens 5 provided between the light source unit 3 and the objective lens 4.

  The optical pickup 1 also includes a photodetector 6 that detects a returning light beam reflected by the optical disc 2 and a beam splitter 7 that guides the returning light beam reflected by the optical disc 2 to the photodetector 6.

  The light source unit 3 is made of, for example, a semiconductor laser and has a light emitting unit that emits a light beam having a predetermined wavelength with a design wavelength of about 405 nm. The wavelength of the light beam emitted from the light source unit 3 is not limited to this.

  The beam splitter 7 is disposed on the optical path between the light source unit 3 and the collimator lens 5, transmits the light beam from the light source unit 3 and guides it to the collimator lens 5, and returns the light beam reflected by the optical disc 2. Is reflected and guided to the photodetector 6. That is, the beam splitter 7 is an optical element that branches the optical path of the return light beam from the optical path of the forward light beam.

  The collimator lens 5 is disposed between the beam splitter 7 and the objective lens 4, and is a divergence angle conversion unit that converts the divergence angle of the light beam that passes through. The light emitted from the light source unit 3 and reflected by the beam splitter 7. The beam divergence angle is converted to a desired angle such as substantially parallel light.

  Further, the collimator lens 5 is moved in order to correct third-order spherical aberration caused by factors such as an error in the cover layer thickness of the optical disc 2 and a temperature change. The divergence angle of the light beam incident on the objective lens 4 is converted. That is, the collimator lens 5 is movable in the optical axis direction, and the optical pickup 1 is provided with a collimator lens driving unit 11 that drives the collimator lens 5 to move in the optical axis direction. The collimator lens driving unit 11 may be configured to rotate and move the lead screw by a feed motor, for example, or may be another configuration. That is, the collimator lens driving unit may be configured to move the collimator lens 5 by the action of the current flowing through the magnet and the coil, like the objective lens driving unit. Further, a linear motor or the like may be used. Then, the collimator lens 5 is moved so as to enter the objective lens 4 in a convergent light state that is slightly converged from, for example, parallel light or in a divergent light state that is slightly diverged. By doing so, the generated spherical aberration can be reduced.

  Further, when the optical pickup 1 is an optical pickup that records and / or reproduces information with respect to a multilayer type optical disc provided with a plurality of recording layers, the collimator lens 5 is provided for each recording layer. Is moved to an appropriate position. At this time, the collimator lens 5 is moved to a position corresponding to each recording layer (each recording layer), so that the thickness from each recording layer to the light incident side surface of the optical disc (also referred to as “cover layer thickness”). The spherical aberration due to the difference can be reduced. That is, the collimator lens 5 and the collimator lens driving unit 11 can appropriately form a beam spot of the light beam for each of the plurality of recording layers. As described above, the collimator lens 5 and the like can reduce third-order spherical aberration caused by a temperature change and a cover layer thickness change, and can form an appropriate beam spot.

  As described above, the collimator lens 5 and the collimator lens driving unit 11 function as an incident magnification variable unit that converts the incident magnification of the light beam to the objective lens 4. Here, the incident magnification variable unit constituting the optical pickup 1 to which the present invention is applied is not limited to this, and may be a so-called beam expander, a liquid crystal element, or the like.

  The objective lens 4 focuses the light beam whose divergence angle is converted by the collimator lens 5 on the recording surface of the optical disk 8. An aperture stop 8 is provided on the incident side of the objective lens 4, and the aperture stop 8 limits the aperture so that the numerical aperture of the light beam incident on the objective lens 4 becomes a desired numerical aperture.

  The objective lens 4 is movably held by an objective lens driving unit 12 provided in the optical pickup 1. The objective lens 4 is displaced by the objective lens driving unit 12 based on the tracking error signal and the focus error signal generated by the return light from the optical disc 2 detected by the photodetector 6. As a result, the objective lens 4 is displaced in two axial directions: a direction approaching and separating from the optical disc 2 shown in FIG. 2 (focus direction F) and a radial direction of the optical disc 2 (tracking direction T). The objective lens 4 focuses the light beam from the light source unit 3 so that the light beam is always focused on the recording surface of the optical disc 2 and forms the focused light beam on the recording surface of the optical disc 2. Follow the recorded track. The objective lens 4 can be tilted not only in the above-described biaxial direction but also in the tilt direction of the objective lens 4, and the objective lens driving unit 12 in the tilt direction based on the signal detected by the photodetector 6. Is moved by. Thus, the objective lens drive unit 12 drives the objective lens 4 in the focus direction, the tracking direction, and the tilt direction, and is a so-called triaxial actuator. The objective lens 4 can reduce coma aberration by being tilted in the tilt direction.

  Here, as shown in FIG. 2, the tilt direction means a so-called radial tilt direction Tir that is a direction around an axis with the tangential direction Tz orthogonal to the focus direction F and the tracking direction T described above as an axis. It is not limited to this. That is, the objective lens 4 may be configured to be drivable in a so-called tangential tilt direction that is a direction around an axis with the tracking direction as an axis. Further, it may be configured to be able to be driven in a four-axis direction that can be driven in the radial tilt direction and the tangential tilt direction. As described above, in a configuration that can also be driven in the tangential tilt direction, the coma aberration in the tangential tilt direction depends on the setting state of the temperature and the cover layer thickness due to the effect of the objective lens 4 described later. Always achieve a reduction.

  The objective lens driving unit 12 includes a fixed unit and a movable unit that holds the objective lens 4 and is movable with respect to the fixed unit, and includes a coil and a magnet that generate a driving force in each driving direction. Configured. The objective lens driving unit 12 corresponds to, for example, a focus coil that generates a driving force in the focus direction, a tracking coil that generates a driving force in the tracking direction, a tilt coil that generates a driving force in the tilt direction, and each coil. Has a magnet. Here, for the purpose of independent tilt driving, a difference is provided in the driving force generated in the focus coils arranged side by side in the tracking direction and the tangential direction, without providing the coils independently. May be. In such a case, the driving force can be generated in the tilt direction due to the difference in the driving force.

  The objective lens 4 is a single lens objective lens made of plastic having a numerical aperture (NA) of 0.8 or more, and both the first surface 4a on the incident side and the second surface 4b on the output side are aspheric. It is made into a shape.

  Specifically, the aspheric shapes of the first surface 4a and the second surface 4b of the objective lens 4 are given by the following equation (1). In Equation (1), r represents a distance (mm) from the optical axis, and f (r) is a distance from the tangent plane of the aspheric surface vertex at a position where the distance from the optical axis is r. The distance (mm) is shown. K represents a conic constant, and A to J represent fourth-order, sixth-order, eighth-order, tenth-order, twelfth-order, fourteenth-order, sixteenth-order, eighteenth-order, and twentieth-order aspheric coefficients, respectively. is there.

  Since the objective lens 4 is made of plastic, mass productivity and weight reduction are improved as compared with a conventional glass.

  Further, the objective lens 4 always corrects, that is, reduces, third-order spherical aberration by changing the incident magnification to the objective lens 4 even when there is a change in environmental temperature or a change in cover layer thickness. Realize that. A “method for correcting third-order spherical aberration” by the objective lens 4 will be described later.

  Further, the objective lens 4 has a tilt angle in which the sign of the third-order coma aberration generated by the lens tilt of the objective lens 4 at the incident magnification for correcting the third-order spherical aberration generated by the change in temperature and the thickness of the cover layer of the optical disc. It is comprised so that it may become the same with respect to.

  The objective lens 4 having such a configuration always realizes third-order coma aberration compensation even when there is a change in environmental temperature or a change in cover layer thickness. This “compensation of third-order coma aberration” will be described later.

  The photodetector 6 has a photodetector for receiving each of the light beams reflected by the signal recording surface of the optical disc 1, and detects various signals such as a tracking error signal and a focus error signal together with the RF signal.

  The optical pickup 1 configured as described above drives the objective lens 4 on the basis of the focus servo signal and tracking servo signal generated by the return light detected by the photodetector 6, and performs focus servo and tracking. Servo is performed. When the objective lens 4 is driven, the objective lens 4 is moved to a focus position where the objective lens 4 is focused on the signal recording surface of the optical disc 2, and the light beam is focused on the recording surface of the optical disc 2. Records or reproduces information. Further, the optical pickup 1 can reduce the coma generated by the warp of the optical disk and the like by generating a coma that cancels out the tilt of the objective lens 4 in the tilt direction by the objective lens driving unit 12. As a result, the optical pickup 1 and the optical disk apparatus using the optical pickup 1 have good recording / reproducing characteristics.

  The optical pickup 1 and the optical disc apparatus to which the present invention is applied have an incident magnification variable unit such as the collimator lens 5 and the collimator lens driving unit 11 described above, so that even when there is a temperature change or a cover layer thickness change, Spherical aberration can be reduced. In addition, the optical pickup 1 and the optical disc apparatus include the objective lens 4 described above, so that coma aberration caused by warpage of the optical disc can be reduced even when there is a temperature change or a cover layer thickness change. it can. This point will be described in detail below.

[2. Correction of third-order spherical aberration by optical pickup]
Next, a “method for correcting third-order spherical aberration” by the objective lens 4 of the optical pickup to which the present invention is applied will be described in detail. That is, in the following, a method for correcting spherical aberration that occurs when there is a change in environmental temperature or a change in cover layer thickness will be described. When the numerical aperture of the lens is set high as in the BD objective lens, the amount of spherical aberration caused by the change in the thickness of the cover layer (recording cover layer) is large. Furthermore, when the material is changed from glass to plastic, as shown in the example of FIG. 15 described above, the amount of spherical aberration caused by temperature change is large due to the high temperature dependence of the refractive index.

  In the objective lens 4 to which the present invention is applied and the optical pickup 1 using the same, the incident magnification of the light beam on the objective lens 4 is changed in order to correct the spherical aberration caused by the change in the cover layer thickness and the environmental temperature. I am letting. That is, the objective lens 4 and the optical pickup 1 correct the spherical aberration by driving the collimator lens 5 in the optical axis direction and changing the incident magnification of the light beam to the objective lens 4 as described above.

  This is because the driving amount of the collimator lens 5 is set so that the spherical aberration generated by the change in the incident magnification of the objective lens 4 has the opposite polarity to the spherical aberration generated by the cover layer thickness change or the spherical aberration generated by the temperature change. This is a technique for canceling aberrations by adjusting. Here, the spherical aberration caused by the temperature change refers to the spherical aberration caused by the lens shape expansion / contraction change due to the temperature change, the lens material refractive index change due to the temperature change, and the wavelength change of incident light due to the temperature change. The driving amount of the collimator lens 5, that is, the position to be moved may be determined according to a signal detected by the detector, for example. Alternatively, it may be determined according to the environmental temperature or the cover layer thickness by detecting the environmental temperature and detecting the cover layer thickness.

The arrangement of the optical system in the spherical aberration correction state will be described below. Prior to the description, the incident magnification to the objective lens 4 will be described with reference to FIG. As shown in FIG. 3, the incident magnification β of the light beam to the objective lens 4 is defined by β = S ′ / S. Here, S indicates the distance in the optical axis direction from the object surface S _O1 including the object point O ₁ to the object side main surface S _M1 of the objective lens 4, and S ′ is the image side main surface S of the objective lens 4. _M2 shows the distance in the direction of the optical axis to the image plane _{S I2} including image point _{I 2} from. S and S ′ are positive values that are directed in the traveling direction of the light beam. The state shown in FIG. 3 is a diagram when diverging light is incident on the objective lens 4, and the incident magnification in this state is a positive incident magnification. That is, when the convergent light to the objective lens 4 is incident becomes a possible object surface S _O1 is positioned in the light beam traveling direction from the object side principal plane S _M1, since S is a negative value, the incident magnification The indicated β is a negative value. In other words, the incident magnification when the convergent light is incident on the objective lens 4 is a negative incident magnification.

Specifically, FIGS. 4 and 5 are conceptual diagrams of the arrangement of the optical systems in the spherical aberration correction state when the temperature changes and the cover layer thickness changes. FIGS. 4A and 5A are schematic diagrams conceptually showing the arrangement relationship between the collimator lens 5 and the objective lens 4 when the temperature and the cover layer thickness are in the design center state. In the following description, the temperature of the designed center state called T _c, the cover layer thickness of the designed center condition that L _c. In general, in a design center state, a light beam that has been collimated by a collimator lens is often incident on an objective lens. Therefore, also in the optical pickup 1, when the temperature is T _c and the cover layer thickness L _c , as shown in FIGS. 4A and 5A, the light is incident on the objective lens 4 in the state of parallel light. It demonstrates as what is comprised in this way. That is, in the case shown in FIGS. 4A and 5A, the magnification of the light beam incident on the objective lens 4 is zero. However, the present invention is not limited to this.

FIG. 4 (b), when the cover layer thickness is L _c, is a schematic view showing a collimator lens 5 and the positional relationship of the objective lens 4 when the maximum temperature T _max. In the case of FIG. 4B, the collimator lens 5 is positioned at the light source side position P (T _max , L _c ) compared to the position P (T _c , L _c ) in the case of FIG. The light is incident on the objective lens 4 in a slightly divergent light state rather than a parallel light state. That is, in the case shown in FIG. 4B, the magnification of the light beam incident on the objective lens 4 is in a positive state.

FIG. 5 (b), when the temperature is T _c, is a schematic view showing a collimator lens 5 and the positional relationship of the objective lens 4 at the maximum cover layer thickness L _max. In the case of FIG. 5B, the collimator lens 5 is located at the position (T _c , L _max ) on the light source side and parallel to the position P (T _c , L _c ) in the case of FIG. The light is incident on the objective lens 4 in a slightly divergent light state from the light state. That is, in the case shown in FIG. 5B, the magnification of the light beam incident on the objective lens 4 is in a positive state. Incidentally, FIG. 4, in FIG. 5, the cover layer 2a is that the cover layer L _c in the design center state, the cover layer 2b show that the maximum cover layer thickness L _max.

  Next, the relationship between the incident magnification of the objective lens 4 and the third-order spherical aberration is shown in FIGS. Here, the relationship regarding a configuration example (design example) of the objective lens 4 of the present invention described later is shown in FIGS. 6A and 7A, and a comparative example for comparison with the configuration example of the present invention is shown. The relationship is shown in FIG. 6 (b) and FIG. 7 (b). 6 and 7, the horizontal axis indicates the incident magnification, and the vertical axis indicates the third-order spherical aberration [mλrms].

6A and 6B show the relationship between the incident magnification and the third-order spherical aberration when the cover layer thickness is fixed at 87.5 μm and the temperatures are 0 ° C., 35 ° C., and 75 ° C. It is a figure shown about. In FIG. _6A , L _1PTL shows the relationship at 0 ° C., L _1PTC shows the relationship at 35 ° C., and L _1PTH shows the relationship at 75 ° C. FIG 6 (b) Medium _{L 1CTL} shows the relationship when the 0 _{℃, L 1CTC} represents the relationship when the 35 _{℃, L 1CTH} shows the relationship in the case of 75 ° C..

7A and 7B show the relationship between the incident magnification and the third-order spherical aberration when the temperature is fixed at 35 ° C. and the cover layer thickness is 75 μm, 87.5 μm, and 100 μm. FIG. FIGS. 7 (a) Medium _{L 2PLL} shows the relationship when the _{75μm, L 2PLC} represents the relationship when the _{87.5μm, L 2PLH} shows the relationship when the 100 [mu] m. Figure 7 (b) Medium _{L 2CLL} shows the relationship when the _{75μm, L 2CLC} represents the relationship when the _{87.5μm, L 2CLH} shows the relationship when the 100 [mu] m.

  For example, in the case where the cover layer thickness is 87.5 μm and the temperature is 75 ° C. shown in FIG. 6A, a third-order spherical aberration of about 200 mλrms occurs at an incident magnification of 0. Therefore, as described above, the incident magnification is changed to 0.0072 and the third-order spherical aberration is corrected to be the minimum value.

  Further, in the case where the cover layer thickness is 100 μm at a temperature of 35 ° C. shown in FIG. 7A, a third-order spherical aberration of 103.7 mλ rms occurs at an incident magnification of 0. Therefore, as described above, the incident magnification is changed to 0.0040 and the third-order spherical aberration is corrected to be the minimum value.

  Tables 1 and 2 show a list of incident magnifications on the objective lens 4 that minimizes the third-order spherical aberration in each setting state as described above. Table 1 is a list of configuration examples of the objective lens 4 of the present invention, and Table 2 is a list of comparative examples. Tables 1 and 2 correspond to FIGS. 6 and 7 and FIGS.

[3. Compensation of third-order coma aberration by objective lens]
Next, the lens tilt sensitivity of the objective lens 4 will be described. The lens tilt sensitivity will be described with reference to FIGS. 8 and 9 in consideration of the relationship with the incident magnification. The lens tilt sensitivity means the amount of third-order coma aberration that occurs per 1 degree of lens tilt.

  That is, the relationship between the lens tilt sensitivity of the objective lens 4 and the incident magnification is shown in FIGS. Here, the relationship of the configuration example of the objective lens 4 of the present invention is shown in FIGS. 8A and 9A, and the relationship of the comparative example for comparison with the configuration example of the present invention is shown in FIG. It is shown in b) and FIG. 9 (b). 8 and 9, the horizontal axis represents the incident magnification, and the vertical axis represents the lens tilt sensitivity [mλrms / degree].

8A and 8B show the relationship between the incident magnification and the third-order spherical aberration when the cover layer thickness is fixed at 87.5 μm and the temperatures are 0 ° C., 35 ° C., and 75 ° C. It is a figure shown about. Figure 8 (a) Medium _{L 3PTL} shows the relationship when the 0 _{℃, L 3PTC} represents the relationship when the 35 _{℃, L 3PTH} shows the relationship in the case of 75 ° C.. Figure 8 (b) Medium _{L 3CTL} shows the relationship when the 0 _{℃, L 3CTC} represents the relationship when the 35 _{℃, L 3CTH} shows the relationship in the case of 75 ° C..

9A and 9B show the relationship between the incident magnification and the third-order spherical aberration when the temperature is fixed at 35 ° C. and the cover layer thickness is 75 μm, 87.5 μm, and 100 μm. FIG. Figure 9 (a) Medium _{L 4PLL} shows the relationship when the _{75μm, L 4PLC} represents the relationship when the _{87.5μm, L 4PLH} shows the relationship when the 100 [mu] m. Figure 9 (b) Medium _{L 4CLL} shows the relationship when the _{75μm, L 4CLC} represents the relationship when the _{87.5μm, L 4CLH} shows the relationship when the 100 [mu] m.

  As shown in FIGS. 8 and 9, the value of the lens tilt sensitivity becomes the smallest when the magnification needs to be maximized in the positive direction, that is, “temperature 75 ° C./cover layer thickness”. It is shown that this is the case of “100 μm”.

  In order to sort out the matters described in FIG. 8 and FIG. 9, FIG. 10 shows a graph of the relationship between the incident magnification and the third-order spherical aberration in the typical six states assumed as actual use states, and the use magnification range. Shown with. Here, FIG. 10A shows a configuration example of the objective lens 4 of the present invention, and FIG. 10B shows a comparative example. Here, the typical six states are a maximum temperature of 75 ° C., a design temperature of 35 ° C., and a minimum temperature of 0 for cover layer thicknesses of 75 μm and 100 μm up to the recording layers L1 and L0 used in a so-called double-layer optical disc. It means the state of ° C.

In FIG. 10, the horizontal axis indicates the incident magnification, and the vertical axis indicates the lens tilt sensitivity [mλrms / degree]. In FIG. 10A and later-described FIG. 11A, L _5PL1TL indicates a state where the cover layer thickness is 75 μm up to the so-called L1 recording layer and the temperature is 0 ° C. _L5PL0TL indicates a state where the cover layer thickness up to a so-called L0 recording layer is 100 μm and the temperature is 0 ° C. L _5PL1TC shows a state where the cover layer thickness is 75 μm and the temperature is 35 ° C. _L5PL0TC shows a state where the cover layer thickness is 100 μm and the temperature is 35 ° C. L _5PL1TH indicates a state where the cover layer thickness is 75 μm and the temperature is 75 ° C. L _5PL0TH indicates a state where the cover layer thickness is 100 μm and the temperature is 75 ° C.

In FIG. 10B and later-described FIG. _11B , L _5CL1TL indicates a state where the cover layer thickness is 75 μm and the temperature is 0 ° C. L _5CL0TL indicates a state where the cover layer thickness is 100 μm and the temperature is 0 ° C. _L5CL1TC indicates a state where the cover layer thickness is 75 μm and the temperature is 35 ° C. _L5CL0TC shows a state where the cover layer thickness is 100 μm and the temperature is 35 ° C. L _5CL1TH indicates a state where the cover layer thickness is 75 μm and the temperature is 75 ° C. L _5CL0TH indicates a state where the cover layer thickness is 100 μm and the temperature is 75 ° C.

Also, the medium _{R PTL} 10 (a) shows the range of use of incident magnification in the case of temperature 0 ° _{C., R PTC} represents the range of use of incident magnification when the temperature 35 ° _{C., R PTH,} the temperature 75 The range of use of the incident magnification in the case of ° C is shown. In FIG. 10B, R _CTL indicates the use range of the incident magnification when the temperature is 0 ° C., R _CTC indicates the use range of the incident magnification when the temperature is 35 ° C., and R _CTH indicates that the temperature is 75 The range of use of the incident magnification in the case of ° C is shown. 10 (a) and 10 (b) show that the lens tilt sensitivity value is the smallest when “temperature is 75 ° C. and cover layer thickness is 100 μm” as described above.

  As shown in FIG. 10B, the objective lens of the conventional example includes a value at which the minimum value of the lens tilt sensitivity is substantially 0 within the use range. On the other hand, as shown in FIG. 10A, in the configuration example of the objective lens 4 of the present invention, the entire lens tilt sensitivity is raised and the minimum value is 48.3. Thus, a large lens tilt sensitivity can be ensured within the use range. Thus, the configuration example of the objective lens 4 of the present invention is a lens that has lens tilt sensitivity in the entire use setting range and solves the above-described problems. In other words, the objective lens 4 to which the present invention is applied ensures appropriate lens tilt sensitivity in the entire use setting range, and corrects third-order coma aberration well, that is, compensates for third-order coma aberration. .

10 shows the range of use of the incident magnification at temperatures of 0 ° C., 35 ° C., and 75 ° C. Similarly, FIG. 11 shows the range of use of the incident magnification for each cover layer thickness. Figure 11 (a) in _RPL1 indicates the use range of the incident magnification when the cover layer thickness of 75 [mu] _{m, R PL0} shows the use range of the incident magnification when the cover layer thickness of 100 [mu] m. In FIG. 11A, R _CL1 indicates the use range of the incident magnification when the cover layer thickness is 75 μm, and R _CL0 indicates the use range of the incident magnification when the cover layer thickness is 100 μm. 11 (a) and 11 (b) clearly show that the lens tilt sensitivity value is the smallest when “temperature is 75 ° C. and cover layer thickness is 100 μm” as described above. ing.

  As shown in FIG. 11B, in the objective lens of the conventional example, the minimum value of the lens tilt sensitivity is included within the use range. On the other hand, as shown in FIG. 11A, in the configuration example of the objective lens 4 of the present invention, the entire lens tilt sensitivity is raised, and the lens tilt sensitivity is within the use range, thus solving the above-described problem. Lens. In other words, the objective lens 4 to which the present invention is applied ensures appropriate lens tilt sensitivity in the entire use setting range, and corrects third-order coma aberration well, that is, compensates for third-order coma aberration. .

  Next, an appropriate value will be described as the minimum value of the lens tilt sensitivity in the objective lens 4 described above. FIG. 16A and FIG. 14 described later show the relationship between the lens tilt sensitivity and the generated third-order coma aberration in the state of “temperature 75 ° C./cover layer thickness 100 μm” where the lens tilt sensitivity takes the minimum value. Is. FIG. 16B shows the relationship between the disc tilt sensitivity and the generated third order coma aberration (disc tilt characteristic). The characteristics shown in FIG. 16B are values that depend only on the numerical aperture of the objective lens 4 and the characteristics of the optical disk, and do not depend on the lens design.

  Here, the characteristic of the optical disk means the refractive index of the disk constituent material and the cover layer thickness. The maximum value of the warp of the optical disc is defined by the BD-ROM standard and is a maximum of 0.4 degrees. Therefore, from the disc tilt characteristic shown in FIG. 16B, the third-order coma aberration of 39.5 mλrms maximum occurs due to the warp of the optical disc.

  As described above, in the optical pickup system, in general, the third-order coma aberration generated by such disc tilt is corrected by tilting the objective lens itself. Therefore, in principle, this correction is possible if the objective lens has a lens tilt sensitivity greater than zero.

  That is, in the objective lens 4 described above, the third-order coma aberration generated by the lens tilt includes 0 in the range of the incident magnification that corrects the third-order spherical aberration generated by the change in the setting state such as the temperature and the cover layer thickness. The third-order coma aberration compensation is realized with a configuration that does not.

In other words, in the objective lens 4, the sign of the third-order coma aberration generated by the lens tilt of the objective lens 4 in the range of the incident magnification that corrects the third-order spherical aberration generated by the temperature and the cover layer thickness change of the optical disc is tilted. What is necessary is just to set it as the same structure with respect to an angle direction. Here, the sign of the third-order coma aberration generated by the lens tilt of the objective lens 4 being the same with respect to the tilt angle direction means that the sign of the lens tilt sensitivity in the tilt angle direction is the same. In other words, the sign of the inclination of a line segment (for example, L _7βTCLC in FIG. 13 described later) indicating the third-order coma aberration generated with respect to the lens tilt angle is the same in each incident magnification range. Here, the lens tilt angle means an angle at which the objective lens 4 is inclined in the tilt direction with respect to the reference state, with the objective lens 4 facing the optical disc (a parallel state) as a reference state. . The tilt direction here means a so-called radial tilt direction. The objective lens 4 to which the present invention is applied realizes third-order coma aberration compensation with this configuration, and thus realizes good aberration correction even when formed of plastic, regardless of the refractive index characteristics of the plastic. .

In consideration of the configuration of the actuator as the objective lens driving unit 12 that drives the objective lens 4 and the working distance such as BD, the maximum angle of the lens tilt of the objective lens is determined as follows. That is, the maximum lens tilt angle θmax uses a horizontal distance L from the rotation center of the objective lens to the protruding end of the lens holder, and a working distance WD that is a vertical distance between the protruding end of the lens holder and the optical disc. Can be expressed. That is, θmax is determined by a relational expression (θmax = tan ⁻¹ (WD / L)). Here, L is generally set to about 5.90 mm. Further, WD is a value determined by the thickness design value of the protruding end portions of the optical disc and the lens holder, and is calculated in consideration of the manufacturing tolerance of each component.

  Here, as the tolerance value to be considered, there is a tolerance (± 34 μm) due to factors on the optical disc side such as a tolerance of the cover layer thickness of the optical disc. Further, the tolerance values to be considered include tolerances due to factors on the objective lens side such as the tolerance of the working distance of the objective lens itself (± 40 μm) and the thickness tolerance of the lens protector (± 20 μm). WD, θmax, and the like should be determined by accumulating the tolerance due to the factors on the optical disk side and the tolerance due to the factors on the objective lens side. When considering these multiple tolerances, depending on how each factor value has a distribution within the tolerance, the sum of squared sums of squares of sums of squares can be simply taken. Logic types exist, and the following values are determined for each.

  That is, in the case of the design center value, WD = 0.270 [mm]. In addition, when the sum of the optical disc side simple sum tolerance and the objective lens side square sum square root tolerance is subtracted from the design center value, WD = 0.168 [mm] (= design center value− (disc side simple sum tolerance) + Lens side square sum square tolerance)). Further, when the sum of the square root tolerance on the optical disc side and the square root tolerance on the objective lens side is subtracted from the design center value, WD = 0.085 [mm] (= design center value− (disc side 2 Multiplier square root tolerance + Lens side square sum square tolerance)).

Accordingly, in each of these three types of WD, θmax is determined as follows. That is, when WD = 0.270, θmax is θmax = tan ⁻¹ (0.270 / 5.90) = 2.62 degrees. When WD = 0.168, θmax is θmax = tan ⁻¹ (0.168 / 5.90) = 1.63 degrees. When WD = 0.085, θmax is θmax = tan ⁻¹ (0.085 / 5.90) = 0.83 degrees.

  Therefore, for each of these values, the required lens tilt sensitivity is determined as follows by dividing the third-order coma generated by the disc tilt to be corrected by θmax. That is, when θmax = 2.62, the lens tilt sensitivity is 39.5 [mλrms] /2.62 [degree] = 15.1 [mλrms / degree]. When θmax = 1.63, the lens tilt sensitivity is 39.5 [mλrms] /1.63 [degree] = 24.2 [mλrms / degree]. In the case of θmax = 0.83, the lens tilt sensitivity is 39.5 [mλrms] /0.83 [degree] = 47.9 [mλrms / degree].

  Therefore, the objective lens 4 having a numerical aperture of about 0.85 for an optical disk having a used wavelength of about 405 nm has a lens tilt sensitivity of 15.1 mλ rms / degree or more. The maximum aberration of about 39.5 mλrms can be compensated. If the lens tilt sensitivity is 24.2 mλrms / degree or more, the third-order coma aberration can be compensated more reliably and with a smaller tilt angle. Furthermore, if the lens tilt sensitivity is 47.9 mλrms / degree or more, the third-order coma aberration can be compensated more reliably and with a smaller tilt angle. Such an objective lens 4 is made of plastic, thereby improving mass productivity and weight reduction and realizing good aberration correction in any range of use conditions, which is favorable for high-density recording optical disks such as BD. Realize recording and playback characteristics.

  The general use conditions of the high-density recording optical disk such as BD having a use wavelength of about 405 nm and a numerical aperture of about 0.85 have been described above. Specifically, the range of the appropriate lens tilt sensitivity in the case of a two-layer optical disk in which the recording layer is provided at positions of 75 μm and 100 μm from the light beam incident surface at a temperature of 0 ° C. to 75 ° C. has been described. The invention is not limited to this. That is, the use temperature may be changed, and further, it may be used for an objective lens for an optical disc having three or more recording layers. The range of lens tilt sensitivity suitable for various design conditions will be described below.

  In the following, the conditions under which the variation of the lens tilt sensitivity can be considered under the assumption that the numerical aperture is fixed in the case of the configuration conditions of the objective lens when the degree of freedom of various conditions is increased will be described with reference to FIGS. .

  FIG. 12 shows the relationship between the third-order spherical aberration and the incident magnification, and FIG. 13 shows the relationship between the third-order coma aberration and the lens tilt angle. In FIG. 12, the horizontal axis indicates the incident magnification to the objective lens 4, and the vertical axis indicates the third-order spherical aberration [a. u. ] Is shown. In FIG. 13, the horizontal axis represents the lens tilt angle [degree] of the objective lens 4, and the vertical axis represents the third-order coma aberration [a. u. ] Is shown.

L _{6 TCLC in} FIG. 12 indicates a relationship in the state of the design center temperature T _c and the design center cover layer thickness L _c . _{Furthermore, L6 TmaxLcc shows} the relationship in the state of the design maximum temperature _{T max} in and design center cover layer thickness _{L c.} L _6TmaxLmax indicates the relationship in the state of the design maximum temperature T _max and the design maximum cover layer thickness L _max .

  In FIG. 12, the incident magnification at which the third-order spherical aberration is minimized in each state is expressed by indicating the temperature and the cover layer thickness in parentheses on the symbol β. Similarly, the third-order spherical aberration is represented by displaying the temperature, the cover layer thickness, and the magnification β defined above in parentheses on the symbol SA.

For example, the incident magnification that minimizes the third-order spherical aberration at the temperature T _max and the cover layer thickness L _c is β (T _max , L _c ). Further, the third-order spherical aberration that occurs when the cover layer thickness is changed to L _max at the incident magnification is SA (T _max , L _max , β (T _max , L _c )). Furthermore, the slopes of the graphs in these three setting states are expressed by using symbols representing the respective setting states, g (T _c , L _c ), g (T _max , L _c ), g (T _max , L _max). ).

Here, for the sake of generalization, description will be made using the symbols as described above. In this configuration example, T _c = 35 ° C., T _max = 75 ° C., L _c = 87.5 μm, L Each represents _max = 100 μm.

Further, FIG. 13 shows magnifications β (T _c , L _c ), β (T _max , L _c ), β (T (T) that minimize the third-order spherical aberration determined in FIGS. _{The graph} shows the lens tilt characteristics when set to _max , L _max . That is, L _7βTcLc in FIG. 13 indicates the lens tilt characteristic in the case of the magnification β (T _c , L _c ) that minimizes the third-order spherical aberration at the design center temperature and cover layer thickness. L _7βTmaxLc indicates the lens tilt characteristic in the case of the magnification β (T _max , L _c ) that minimizes the third-order spherical aberration at the temperature T _max and the cover layer thickness at the design center. L _7βTmaxLmax indicates the lens tilt characteristic in the case of the magnification β (T _max , L _max ) that minimizes the third-order spherical aberration at the temperature T _max and the cover layer thickness L _max . LTA indicates the lens tilt sensitivity at the design center temperature and cover layer thickness, LTB indicates the amount of decrease in lens tilt sensitivity due to temperature change, and LTC indicates the decrease in lens tilt sensitivity due to cover layer thickness change. Indicates the amount. In other words, the lens tilt sensitivities LTA, LTB, and LTC represent the third-order coma aberration amount at the lens tilt angle of 1.0 _degree of the _respective lines L _7βTcLc , L _7βTmaxLc , and L _7βTmaxLmax shown in FIG.

  Here, if the LTA, LTB, and LTC symbols shown in FIG. 13 are used, the minimum value of the lens tilt sensitivity can be expressed as LTA-LTB-LTC. In order to keep the minimum value of the lens tilt sensitivity large, it is necessary to set LTA large and LTB and LTC small.

  Here, first, a method for reducing LTB and LTC will be described. As described above, these lens tilt sensitivity reduction amounts LTB and LTC are used with the third-order spherical aberration being minimized in each state, and therefore a large part depends on changing the incident magnification β to the objective. I found. Therefore, in order to reduce these LTB and LTC, it is necessary to design to reduce the amount of change in magnification.

That is, in order to reduce LTB, it is necessary to reduce β (T _max , L _c ) −β (T _c , L _c ), and in order to reduce LTC, β (T _max , L _max ) It is necessary to reduce −β (T _max , L _c ).

First, β (T _max , L _c ) −β (T _c , L _c ) relating to LTB, which is a portion indicated by β _LTB in FIG. In order to reduce the β _LTB , it is necessary to reduce SA (T _max , L _c , β (T _c , L _c )) or increase the slope g (T _max , L _c ). Means. Here, SA (T _max , L _c , β (T _c , L _c )) is the temperature change amount ΔT (= T _max −T _c ) and the absolute value of the refractive index temperature dependency of the material (| dn / dT |) and proportional to the focal length f of the objective lens 4. Further, g (T _max , L _c ) is substantially proportional to the focal length f. Therefore, LTB and _βLTB are values proportional to ΔT and | dn / dT |, and have no dependency on f.

In the above configuration example, when ΔT = 40, | dn / dT | = −1.29 × 10 ⁻⁴ , LTB = 70.4 mλrms, β _LTB = β (T _max , L _c ) −β (T _c , It was L _c) = 0.0072. From this relationship, the value of LTB can derive a relationship of LTB = 1.4 × 10 ⁻⁴ × ΔT × (| dn / dT |) as a function of ΔT and dn / dT. The value of beta _{LTB is, β LTB = β (T max} , L c) -β (T c, L c) = 1.4 × (ΔT) × be derived relationship (| | dn / dT) it can. Note that ΔT described above indicates the amount of temperature change with respect to the temperature in the design center state. Further, dn / dT is a so-called refractive index temperature coefficient of the objective lens 4, that is, a ratio indicating a change in refractive index with respect to a temperature change of the material constituting the objective lens 4.

Next, in order to examine β (T _max , L _max ) −β (T _max , L _c ), this is a portion indicated by β _LTC in FIG. In order to reduce the β _LTC , it is necessary to reduce SA (T _max , L _max , β (T _max , L _c )) or increase the slope g (T _max , L _max ). Means. Here, SA (T _max , L _max , β (T _max , L _c )) is a value that depends only on the numerical aperture of the objective lens 4, the refractive index that is a characteristic of the optical disc, and the cover layer thickness. That is, SA (T _max , L _max , β (T _max , L _c )) is a value proportional to the change amount ΔL = (L _max −L _c ) of the cover layer thickness, and is dependent on the focal length f. There is no. Further, g (T _max , L _max ) is substantially proportional to the focal length f, similarly to g (T _max , L _c ) described above. From this, LTC and _βLTC are values proportional to ΔL and inversely proportional to f. ΔL described above indicates the amount of change with respect to the cover layer thickness in the design center state.

In the above configuration example, ΔL = 12.5, f = 1.41, LTC = 17.3 mλrms, β _LTC = β (T _max , L _max ) −β (T _max , L _c ) = 0.039 there were. From this relationship, the LTC value can be derived as a function of ΔL and f as follows: LTC = 2.0 × (ΔL) × (1 / f). Moreover, beta _LTC values derives the relation _{β LTC = β (T max,} L max) -β (T max, L c) = 8.1 × 10 -4 × (ΔL) × (1 / f) be able to.

  Next, a method for increasing the LTA will be considered. LTA does not depend on the focal length f and is a value that can be set in any way by designing the lens surface shape, and has the highest design flexibility among LTA, LTB, and LTC.

From the above, LTA−LTB−LTC = LTA−1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) −2.0 × (ΔL) × (1 / f).

Therefore, in order to make the minimum value of (LTA−LTB−LTC) larger than 0, LTA−1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) −2.0 × (ΔL) It is necessary to satisfy x (1 / f)> 0. In other words, the lens tilt sensitivity LTA in the design center state (temperature Tc, cover layer thickness Lc) is LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL ) × (1 / f) [mλrms].

In order to set the minimum value of (LTA-LTB-LTC) to 15.1 [mλrms] or more as described above, LTA−1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) − It is necessary to satisfy 2.0 × (ΔL) × (1 / f)> 15.1. In other words, the lens tilt sensitivity LTA in the design center state (temperature Tc, cover layer thickness Lc) is LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL ) × (1 / f) +15.1 [mλrms].

Similarly, in order to make the minimum value of (LTA−LTB−LTC) equal to or greater than the above 24.2 [mλrms], the LTA is LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL) × (1 / f) +24.2 [mλrms] must be satisfied.

In order to set the minimum value of (LTA-LTB-LTC) to 47.9 [mλrms] or more, the LTA is LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL) × (1 / f) +47.9 [mλrms].

As described above, in the objective lens 4, the LTA indicating the third-order coma aberration generated around the lens tilt of 1 degree in the design center state is LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT | ) + 2.0 × (ΔL) × (1 / f) satisfies the following advantages. The objective lens 4 realizes compensation for third-order coma aberration even when there is a change in environmental temperature or a change in cover layer thickness. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

Further, the objective lens 4 has an LTA indicating third-order coma aberration generated per lens tilt of 1 degree in the design center state, LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) +2. By satisfying the relationship of 0 × (ΔL) × (1 / f) +15.1, the following advantages are obtained. The objective lens 4 maintains higher lens tilt sensitivity even when there is a change in environmental temperature or a change in cover layer thickness, and realizes third-order coma aberration compensation. Further, since the objective lens 4 takes into account the manufacturing tolerance of the actuator, the structure of the actuator and the like can be simplified to ease the manufacturing tolerance and the like. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

Further, the objective lens 4 has an LTA indicating third-order coma aberration generated per lens tilt of 1 degree in the design center state, LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) +2. By satisfying the relationship of 0 × (ΔL) × (1 / f) +24.2, the following advantages are obtained. The objective lens 4 maintains higher lens tilt sensitivity even when there is a change in environmental temperature or a change in cover layer thickness, and realizes third-order coma aberration compensation. Further, since the objective lens 4 takes into account the manufacturing tolerance of the actuator and the like, the structure of the actuator and the like can be simplified to ease the manufacturing tolerance and the like. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

Further, the objective lens 4 has an LTA indicating third-order coma aberration generated per lens tilt of 1 degree in the design center state, LTA> 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) +2. By satisfying the relationship of 0 × (ΔL) × (1 / f) +47.9, the following advantages are obtained. The objective lens 4 maintains higher lens tilt sensitivity even when there is a change in environmental temperature or a change in cover layer thickness, and realizes third-order coma aberration compensation. Further, since the objective lens 4 takes into account the manufacturing tolerance of the actuator and the like, the structure of the actuator and the like can be simplified to ease the manufacturing tolerance and the like. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

In addition, β _LTB and β _LTC shown in FIG. 12 obtained in the course of the above description are used to obtain the above-described “third order generated by a change in the setting state such as temperature and cover layer thickness”. It is possible to define “incidence magnification for correcting spherical aberration” as a general expression. In other words, (β _LTB + β _LTC ) represents a magnification difference between the incident magnifications in the set state (T _max , L _max ) with respect to the incident magnification in the design center state (T _c , L _c ). Similarly to the above description, the magnification difference of the incident magnification in the set state (T _min , L _min ) with respect to the design center state (Tc, Lc) is − (β _LTB + β _LTC ). Generally, in the set center state, the incident magnification is set to 0, that is, the incident light beam to the objective lens 4 is set as parallel light. In view of this, if the incident magnification in the set center state is approximately 0, the above-described “third order generated by lens tilt” is in the range of − (β _LTB + β _LTC ) to + (β _LTB + β _LTC ). The objective lens 4 may be configured so that the coma does not include 0. With such a configuration, even when the degree of freedom of various conditions is increased, the above-described effects can be obtained.

Therefore, the objective lens 4 has the following advantages because the third-order coma aberration generated by the lens tilt is the same with respect to the tilt angle direction in the range where the incident magnification is ± (β _LTB + β _LTC ). Here, the range where the incident magnification is ± (β _LTB + β _LTC ) is − (1.4 × (ΔT) × (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)) to + (1.4 × (ΔT) × (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)).

  The objective lens 4 realizes compensation for third-order coma aberration even when there is a change in environmental temperature or a change in cover layer thickness. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

Similarly, the objective lens 4 has the following effects by having a lens tilt sensitivity of 0.0151 λ rms / degree or more in the range where the incident magnification is ± (β _LTB + β _LTC ). In the above-mentioned range, having a lens tilt sensitivity of 0.0151 λrms / degree or more has the same meaning as that the third-order coma aberration generated per lens tilt of 1degree is 0.0151 λrms or more.

  The objective lens 4 realizes third-order coma aberration compensation with higher lens tilt sensitivity even when there is an environmental temperature change or a cover layer thickness change. Further, since the objective lens 4 takes into account the manufacturing tolerance of the actuator, the structure of the actuator and the like can be simplified to ease the manufacturing tolerance and the like. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

The objective lens 4 is the range of incident magnification _{± (β LTB + β LTC)} , by a lens tilt sensitivity than 0.0242λrms / degree, it has the following advantages. The objective lens 4 realizes third-order coma aberration compensation with higher lens tilt sensitivity even when there is an environmental temperature change or a cover layer thickness change. Further, since the objective lens 4 takes into account the manufacturing tolerance of the actuator, the structure of the actuator and the like can be simplified to ease the manufacturing tolerance and the like. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

Further, the objective lens 4 has the following effects by having a lens tilt sensitivity of 0.0479λ rms / degree or more in the range where the incident magnification is ± (β _LTB + β _LTC ). The objective lens 4 realizes third-order coma aberration compensation with higher lens tilt sensitivity even when there is an environmental temperature change or a cover layer thickness change. Further, since the objective lens 4 takes into account the manufacturing tolerance of the actuator, the structure of the actuator and the like can be simplified to ease the manufacturing tolerance and the like. As a result, the objective lens 4 and the optical pickup 1 using the objective lens 4 can maintain a good lens tilt sensitivity under the set conditions, that is, realize a good aberration correction and a good recording / reproduction characteristic. To do.

[4. Objective lens configuration examples and comparative examples)
The design data of the objective lens of the comparative example for comparison with the design data as a configuration example (design example) of the objective lens 4 according to the present invention will be shown below. In any of the examples, the focal length f = 1.41 [mm], the numerical aperture NA = 0.85, the lens center thickness = 1.80 [mm], and the refractive index of the constituent material = 1.5246. It is.

"Comparative example"
Below, R, K, AJ in the above-mentioned formula (1) which shows the shape of light source side lens surface S1 of a comparative example are as follows.

<Light source side lens surface S1>
R = 0.92557
K = -0.60684
A = 0.02141
B = -0.00050
C = 0.00808
D = 0.00647
E = -0.00382
F = -0.00705
G = 0.00371
H = 0.00852
J = -0.00580

  In the following, R, K, and A to J in the above formula (1) showing the shape of the optical disk side lens surface S2 of the comparative example are as follows.

<Optical disc side lens surface S2>
R = -1.21100
K = -27.13926
A = 0.23133
B = -0.18987
C = -0.26925
D = 0.37981
E = -0.00964
F = -0.10854
G = -0.00587
H = -0.03997
J = 0.04787

"Configuration Example"
Hereinafter, R, K, A to J in the above-described formula (1) indicating the shape of the light source side lens surface S1 of the configuration example of the objective lens 4 according to the present invention, that is, the first surface 4a, are as follows. is there. S1 indicates the first surface 4a in FIG.

<Light source side lens surface S1>
R = 0.92889
K = -0.60519
A = 0.01771
B = 0.00694
C = 0.12831
D = -0.67249
E = 1.61889
F = -2.12102
G = 1.56419
H = -0.60521
J = 0.09398

  In the following, R, K, A to J in the above-described formula (1) indicating the shape of the optical disk side lens surface S2 of the configuration example of the objective lens 4 according to the present invention, that is, the second surface 4b are as follows: Street. S2 shows the 2nd surface 4b in FIG.

<Optical disc side lens surface S2>
R = -1.23347
K = -30.3168
A = 0.22086
B = -0.21581
C = -0.23610
D = 0.37238
E = 0.00720
F = -0.17558
G = 0.04437
H = 0.00434
J = 0.00000

  According to the objective lens having the above configuration example, FIGS. 6B, 7B, 8B, 9B, 10B, and 11B described above. Compared to the objective lens of the comparative example as described above, the above-described effects are exhibited. That is, according to the objective lens of the configuration example of the present invention, as shown in FIGS. 6 (a), 7 (a), 8 (a), 9 (a), 10 (a), and 11 (a). As shown in the figure, it is possible to adopt a configuration that takes into account fluctuations in lens tilt sensitivity. In other words, the objective lens having such a configuration example realizes compensation for third-order coma aberration regardless of a change in temperature or a change in cover layer thickness.

Specifically, FIG. 14 shows the lens tilt characteristics of the objective lenses of the above-described configuration example and comparative example, as in FIG. In FIG. 14, the horizontal axis indicates the lens tilt angle [degree], and the vertical axis indicates the third-order coma aberration [mλrms]. L _8P indicates the amount of change in third-order coma aberration with respect to the lens tilt angle when the temperature is 75 ° C. and the cover layer thickness is 100 μm, which is the smallest lens tilt characteristic of the objective lens having the above-described configuration example. L _8C indicates the amount of change in the third-order coma aberration with respect to the lens tilt angle when the temperature is 75 ° C. and the cover layer thickness is 100 μm, which is the smallest lens tilt characteristic of the objective lens of the comparative example described above.

  As described above, the objective lens 4 to which the present invention is applied can ensure the lens tilt sensitivity even under the strictest conditions of use setting. Therefore, the objective lens 4 to which the present invention is applied, and the optical pickup 1 and the optical disk apparatus using the objective lens 4 always have lens tilt sensitivity within the range of the use setting state of the optical disk system, and coma aberration generated by the warp of the optical disk. Can be corrected by the lens tilt of the objective lens 4.

1 is an optical path diagram showing an optical system of an optical pickup to which the present invention is applied. It is a figure for demonstrating about the triaxial direction by which the objective lens which comprises the optical pick-up to which this invention is applied is driven, and is a schematic diagram which shows the relationship between an objective lens and an optical disk. It is a figure for demonstrating the incident magnification to an objective lens. It is a figure for demonstrating correct | amending the spherical aberration which generate | occur | produces when there exists a temperature change by changing the incident magnification to an objective lens. (A) is a figure for demonstrating the arrangement | positioning relationship of a collimator lens and an objective lens when temperature and cover layer thickness are design center states. (B) is a figure for demonstrating the arrangement | positioning relationship of a collimator lens and objective lens when temperature changes to the maximum temperature with respect to the state of (a). It is a figure for demonstrating correcting the spherical aberration which generate | occur | produces when a cover layer thickness changes by changing the incident magnification to an objective lens. (A) is a figure for demonstrating the arrangement | positioning relationship of a collimator lens and an objective lens when temperature and cover layer thickness are design center states. (B) is a figure for demonstrating the arrangement | positioning relationship of a collimator lens and an objective lens when the cover layer thickness changes to the thickest state with respect to the state of (a). It is a figure for demonstrating that the relationship between the incident magnification to an objective lens and the change of a tertiary spherical aberration changes for every temperature. (A) is a figure which shows the relationship between the incident magnification and the third-order spherical aberration when the temperatures in the configuration example of the objective lens of the present invention are 75 ° C., 35 ° C., and 0 ° C., respectively. (B) is a diagram showing the relationship between the incident magnification and the third-order spherical aberration when the temperatures in the comparative example of the objective lens for comparison with the above-described configuration example are 75 ° C., 35 ° C., and 0 ° C., respectively. is there. It is a figure for demonstrating that the relationship between the incident magnification to an objective lens and the change of a 3rd-order spherical aberration changes for every cover layer thickness. (A) is a figure which shows the relationship between the incident magnification and the third-order spherical aberration when the cover layer thickness is 100 μm, 87.5 μm, and 75 μm in the configuration example of the objective lens of the present invention. (B) is a diagram showing the relationship between the incident magnification and the third-order spherical aberration when the cover layer thickness in the comparative example of the objective lens is 100 μm, 87.5 μm, and 75 μm. It is a figure for demonstrating that the relationship of the change of the incident magnification to an objective lens and a lens tilt sensitivity changes for every temperature. (A) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in the case where the temperature in the structural example of the objective lens of this invention is 75 degreeC, 35 degreeC, and 0 degreeC, respectively. (B) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in the case where the temperature in the comparative example of an objective lens is 75 degreeC, 35 degreeC, and 0 degreeC, respectively. It is a figure for demonstrating that the relationship of the change of the incident magnification to an objective lens and a lens tilt sensitivity changes for every temperature. (A) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in the case where the cover layer thickness in the structural example of the objective lens of this invention is 100 micrometers, 87.5 micrometers, and 75 micrometers, respectively. (B) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in the case where the cover layer thickness in the comparative example of the objective lens is 100 μm, 87.5 μm and 75 μm, respectively. It is a figure which shows the use range of the incident magnification in each temperature while showing the relationship between the incident magnification of an objective lens and lens tilt sensitivity about each case of six states which changed temperature and cover layer thickness. (A) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in each case in the structural example of the objective lens of this invention. (B) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in each case in the comparative example of an objective lens. It is a figure which shows the use range of the incident magnification in each cover layer thickness while showing the relationship between the incident magnification of an objective lens and a lens tilt sensitivity in each case of six states which changed temperature and cover layer thickness. (A) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in each case in the structural example of the objective lens of this invention. (B) is a figure which shows the relationship between the incident magnification and lens tilt sensitivity in each case in the comparative example of an objective lens. It is a figure which shows the relationship between the 3rd-order spherical aberration and incident magnification in the case of three states which changed temperature and cover layer thickness. It is a figure which shows the relationship between the lens tilt angle and the 3rd-order coma aberration in the case of the three states which changed temperature and cover layer thickness. It is a figure which shows the lens tilt characteristic in the structural example of the objective lens of this invention, and the comparative example of an objective lens. It is a figure for demonstrating that the dependence with respect to the temperature of a refractive index is large compared with a glass material, and shows the change of the refractive index with respect to the temperature of Zeonex340R as a plastic material, and L-BAL42 as a glass material. FIG. It is a figure for demonstrating a lens tilt characteristic and a disk tilt characteristic, (a) is a figure which shows a lens tilt characteristic when a lens tilt characteristic falls in setting conditions, when forming a lens with the conventional method. is there. (B) is a figure which shows the change of the 3rd-order coma aberration amount generate | occur | produced by the disc tilt of a general optical disc.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical pick-up, 2 Optical disk, 3 Light source part, 4 Objective lens, 5 Collimator lens, 6 Optical detector, 7 Beam splitter, 8 Aperture stop, 11 Collimator lens drive part, 12 Objective lens drive part

Claims (16)

  The third order coma aberration generated by the lens tilt of the objective lens in the range of the incident magnification in which the numerical aperture is 0.8 or more and the third order spherical aberration generated by the temperature and the thickness change of the cover layer of the optical disc is corrected. Plastic objective lens with the same sign. When the temperature variation is ΔT, the thickness variation of the optical disk cover layer is ΔL, the refractive index temperature coefficient of the objective lens is dn / dT, and the focal length of the objective lens is f,
The incident magnification is − (1.4 × (ΔT) × (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)) to + (1.4 × (ΔT) × The sign of the third-order coma aberration generated by the lens tilt of the objective lens is the same in the range of (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)). The objective lens according to 1. When the temperature variation is ΔT, the thickness variation of the optical disk cover layer is ΔL, the refractive index temperature coefficient of the objective lens is dn / dT, and the focal length of the objective lens is f,
The incident magnification is − (1.4 × (ΔT) × (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)) to + (1.4 × (ΔT) × In the range of (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)), the third-order coma aberration generated per 1 degree lens tilt of the objective lens is 0.0151 λrms or more. The objective lens according to claim 2. When the temperature variation is ΔT, the thickness variation of the optical disk cover layer is ΔL, the refractive index temperature coefficient of the objective lens is dn / dT, and the focal length of the objective lens is f,
The incident magnification is − (1.4 × (ΔT) × (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)) to + (1.4 × (ΔT) × In the range of (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)), the third-order coma aberration generated per lens tilt of 1 degree of the objective lens is 0.0242λrms or more. The objective lens according to claim 2. When the temperature variation is ΔT, the thickness variation of the optical disk cover layer is ΔL, the refractive index temperature coefficient of the objective lens is dn / dT, and the focal length of the objective lens is f,
The incident magnification is − (1.4 × (ΔT) × (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)) to + (1.4 × (ΔT) × In the range of (| dn / dT |) + 8.1 × 10 ⁻⁴ × (ΔL) × (1 / f)), the third-order coma aberration generated per 1 degree of lens tilt of the objective lens is 0.0479 λrms or more. The objective lens according to claim 2.   The third-order coma aberration generated per lens tilt of 1 degree of the objective lens in the range of the incident magnification for correcting the third-order spherical aberration generated by the temperature and the thickness change of the cover layer of the optical disc is 0.0151 λrms or more. Item 1. The objective lens according to Item 1.   The third-order coma aberration generated per lens tilt of 1 degree of the objective lens in the range of the incident magnification for correcting the third-order spherical aberration generated by the temperature and the change in the thickness of the cover layer of the optical disc is 0.0242λrms or more. Item 1. The objective lens according to Item 1.   The third-order coma aberration generated per lens tilt of 1 degree of the objective lens in the range of the incident magnification for correcting the third-order spherical aberration generated by the temperature and the thickness change of the cover layer of the optical disk is 0.0479 λrms or more. Item 1. The objective lens according to Item 1. The numerical aperture is 0.8 or more,
When the temperature variation is ΔT, the thickness variation of the optical disk cover layer is ΔL, the refractive index temperature coefficient of the objective lens is dn / dT, and the focal length of the objective lens is f,
The third-order coma aberration that occurs around 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL) × (1 / F) A plastic objective lens that is equal to or greater than [mλrms]. The third-order coma aberration that occurs around 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL) × ( The plastic objective lens according to claim 9, wherein 1 / f) +15.1 [mλrms] or more. The third-order coma aberration that occurs around 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL) × ( The plastic objective lens according to claim 9, wherein 1 / f) +24.2 [mλrms] or more. The third-order coma aberration that occurs around 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT |) + 2.0 × (ΔL) × ( The plastic objective lens according to claim 9, wherein 1 / f) +47.9 [mλrms] or more. An objective lens for condensing the light beam emitted from the light source on each of one or more layers of the optical disc;
A light beam is incident on the objective lens at an incident magnification that corrects third-order spherical aberration caused by temperature and thickness change of the cover layer of the optical disc.
The objective lens has a numerical aperture of 0.8 or more, and is generated by a lens tilt of the objective lens in a range of incident magnification for correcting third-order spherical aberration caused by temperature and thickness change of the cover layer of the optical disc. An optical pickup which is a plastic objective lens having the same third-order coma aberration sign. An objective lens for condensing the light beam emitted from the light source on each of one or more layers of the optical disc;
A light beam is incident on the objective lens at an incident magnification that corrects third-order spherical aberration caused by temperature and thickness change of the cover layer of the optical disc.
The objective lens has a numerical aperture of 0.8 or more, a temperature change amount is ΔT, a thickness change amount of the cover layer of the optical disc is ΔL, a refractive index temperature coefficient of the objective lens is dn / dT, Assuming that the focal length of the objective lens is f, the third-order coma aberration generated per 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT | ) + 2.0 × (ΔL) × (1 / f) [mλrms] or more, an optical pickup that is a plastic objective lens. An optical pickup having an objective lens for condensing the light beam emitted from the light source on each of one or more layers of the optical disc;
A light beam is incident on the objective lens at an incident magnification that corrects third-order spherical aberration caused by temperature and thickness change of the cover layer of the optical disc.
The objective lens has a numerical aperture of 0.8 or more, and is generated by a lens tilt of the objective lens in a range of incident magnification for correcting third-order spherical aberration caused by temperature and thickness change of the cover layer of the optical disc. An optical disc apparatus which is a plastic objective lens having the same third-order coma aberration sign. An optical pickup having an objective lens for condensing the light beam emitted from the light source on each of one or more layers of the optical disc;
A light beam is incident on the objective lens at an incident magnification that corrects third-order spherical aberration caused by temperature and thickness change of the cover layer of the optical disc.
The objective lens has a numerical aperture of 0.8 or more, a temperature change amount is ΔT, a thickness change amount of the cover layer of the optical disc is ΔL, a refractive index temperature coefficient of the objective lens is dn / dT, Assuming that the focal length of the objective lens is f, the third-order coma aberration generated per 1 degree of lens tilt of the objective lens in the design center state is 1.4 × 10 ⁻⁴ × (ΔT) × (| dn / dT | ) + 2.0 × (ΔL) × (1 / f) [mλrms] or more, an optical disc apparatus that is a plastic objective lens.

Images