JP2011086349A - Optical pickup and optical recording and reproducing device - Google Patents

Abstract

When recording / reproducing on a plurality of recording layers by a near-field method, large spherical aberration occurs when the convergence position is adjusted by a beam expander. Further, when this spherical aberration is corrected using a slight change in the lens transmission region, the tracking range is narrowed.
A wavefront conversion optical system that converts a wavefront of light emitted from a light source, and an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer, the divergence emitted from the light source Wavefront conversion is performed by the first lens 9a for converging light, the third lens 9c for condensing the diverged light after convergence, and the second lens 9b provided between the first lens and the third lens. Convergence position adjustment and spherical aberration correction by using the fact that the light transmission area on the lens surface changes greatly by configuring the optical system and moving the second lens 9b in the convergent divergent light in the optical axis direction. I do.
[Selection] Figure 11

Description

  The present invention relates to an optical pickup that records or reproduces information on an optical recording medium by irradiating the converged light onto the optical recording medium, and an optical recording / reproducing apparatus using the same.

  Conventionally, an optical disc such as a CD, a DVD, or a BD (Blu-ray disc) has been widely used as a medium for recording various information including video and audio. In such an optical recording / reproducing apparatus using an optical recording medium, information is recorded or reproduced by irradiating the optical recording medium with light, so the information recording density depends on the size of the light spot that converges on the optical recording medium. To do. Therefore, the capacity of the recording medium can be increased by reducing the light spot irradiated by the optical pickup. Since the size of this light spot is proportional to the numerical aperture of the objective lens and inversely proportional to the wavelength of the irradiating light, in order to obtain a smaller light spot, the wavelength of the light used can be further shortened, or the objective What is necessary is just to enlarge the numerical aperture of a lens further. However, in an optical recording / reproducing apparatus that has been put to practical use so far, the distance between the optical recording medium and the objective lens is sufficiently large compared to the wavelength, and when the light incident on the objective lens has a numerical aperture exceeding 1, the lens Since the light is totally reflected at the exit surface, the recording density cannot be increased.

  Therefore, a near-field optical recording / reproducing method using SIL (solid immersion lens) has been developed as an optical recording / reproducing method in which the numerical aperture of the objective lens exceeds 1. Usually, when the numerical aperture exceeds 1, the critical angle is exceeded, so that the light in this region is totally reflected at the exit end face of the objective lens. The totally reflected light oozes out from the emission end face. In the near-field optical recording / reproducing method, the evanescent light can be propagated. For this reason, the air gap between the exit end face of the objective lens and the surface of the optical recording medium is maintained shorter than the attenuation distance of the evanescent light, and light having a numerical aperture exceeding 1 is transmitted from the objective lens to the optical recording medium. Yes.

  In the near-field optical recording and reproducing method using SIL, information is conventionally recorded only on the recording layer provided on the surface of the optical recording medium. However, in order to further increase the recording capacity, information is recorded on a plurality of recording layers. Recording methods are being considered. However, in order to record on a plurality of recording layers by the near-field optical recording / reproducing method, in addition to keeping the air gap constant, the light convergence position is adjusted to each of the recording layers having different depths, and It is necessary to correct spherical aberration occurring at the convergence position.

  In order to form a plurality of recording layers, there has conventionally been a lens unit that adjusts the convergence position and corrects spherical aberration with liquid crystal (see, for example, Patent Document 1). FIG. 15 shows a conventional method described in Patent Document 1. In FIG. 15, the optical recording medium 50 has a plurality of recording layers 51 formed to a predetermined depth. The light beam 53 emitted from the semiconductor laser 52 becomes substantially parallel light at the condenser lens 54, passes through the liquid crystal element 55 for correcting spherical aberration and the lens unit 56 for convergence position control, and enters the objective lens optical system 57. . In the present specification, the convergence position means the beam waist position of the converged light. The objective lens optical system 57 includes a diaphragm lens 57a and a solid immersion lens SIL 57b, and an air gap existing between the exit end face of the SIL 57b and the surface of the optical recording medium 50 facing the SIL 57b is made shorter than the evanescent attenuation length. The light propagation by the evanescent light is performed.

  The lens unit 56 includes a lens 56a having a negative power and a lens 56b having a positive power, and is configured such that an interval between the lenses 56a and 56b is changed by an actuator (not shown). By changing the distance between the lenses 56a and 56b, the light convergence position inside the optical recording medium 50 is controlled, and the light is converged on a desired information recording layer. However, when the convergent light incident on the inside of the optical recording medium 50 is converged on the desired recording layer 51, spherical aberration occurs and the convergence state deteriorates. The above-mentioned liquid crystal element 55 is an optical element for correcting this spherical aberration, has a concentric circular electrode pattern, and the correction amount of the spherical aberration according to the voltage applied to these electrodes. A substantially equivalent wavefront is generated.

  The convergent light emitted from the objective lens optical system 57 is incident on the inside of the optical recording medium 50, and converges on a desired recording layer by the convergence position control by the lens unit 56 and the spherical aberration correction of the liquid crystal element 55. Information is recorded on or reproduced from the recording layer 51.

  As another conventional example, a beam expander using three lenses is used to adjust the light convergence position and correct spherical aberration generated at the convergence position (see, for example, Patent Document 2). FIG. 16 shows a conventional method described in Patent Document 2. In FIG. 16, an optical recording medium 60 has a plurality of recording layers 61 formed to a predetermined depth. Light emitted from the semiconductor laser 62 is converted into substantially parallel light 64 by the condenser lens 63, passes through the beam splitter 65, and enters the beam expander 66. The beam expander 66 converts the incident parallel light 64 into slightly diverging or convergent light, and includes three lenses 66a and 66b having negative power and a lens 66c having positive power. The lenses 66a and 66b are supported so as to be movable in a direction parallel to the optical axis. The light whose wavefront is converted by the beam expander 66 enters the objective lens optical system 67. The objective lens optical system 67 is a lens having a numerical aperture exceeding 1, and is composed of a diaphragm lens 67a and a solid immersion lens SIL 67b, as in Patent Document 1, and the incident light is converged to converge on an optical recording medium. A light spot is formed on the information recording layer 61 in the 60. The surface of the optical recording medium 60 and the SIL element 67 b have an air gap of about 20 nm, and the evanescent light is propagated to the optical recording medium 60.

  With such a configuration, when the lenses 66a and 66b move in the optical axis direction while changing the distance between each other, the light emitted toward the objective lens optical system 67 is slightly diverged or converged. The light converging position changes at. The spherical aberration is also changed in accordance with the change in the distance between the lenses 66a and 66b, so that light can be applied to the desired recording layer 61 while the distance between the surface of the optical recording medium 60 and the SIL 66b is kept constant. Can be converged and spherical aberration can be corrected.

Japanese Patent Laid-Open No. 2003-263770 JP 2009-170076 A

  When recording or reproducing information on a plurality of recording layers in a conventional optical recording medium, the adjustment of the light convergence position can be achieved by changing the distance between the objective lens and the optical recording medium, that is, the working distance. . Further, the correction of the spherical aberration can be achieved by moving the condenser lens in the optical axis direction and slightly diverging or converging the light emitted from the condenser lens.

  On the other hand, in the near-field optical recording / reproducing method, since the distance between the objective lens and the optical recording medium, that is, the air gap needs to be kept constant, the convergence position can be adjusted by changing the working distance. However, another adjustment mechanism is required. Furthermore, since the numerical aperture of the objective lens is much larger than the conventional value, when the light is converged on each recording layer having a different depth from the surface of the optical recording medium, the spherical aberration generated at the convergence position becomes very large. Therefore, it is necessary to correct the spherical aberration according to the convergence position.

  In the above Patent Document 1, in the near-field optical recording / reproducing method, a beam expander for adjusting the convergence position and a liquid crystal element for correcting spherical aberration are provided for the light source and the objective lens. Recording or reproduction on a plurality of recording layers is realized. However, if, for example, the numerical aperture is 1.7 and the refractive index of the protective layer of the optical recording medium is 2.0, and the convergence position is adjusted to a recording layer 5 μm away by a beam expander, spherical aberration of 300 mλ or more occurs. . Generally, spherical aberration correction by a liquid crystal element is 100 mλ or less, and spherical aberrations exceeding 300 mλ have insufficient resolution and are difficult to practically correct. In addition, as a result of using the beam expander and the liquid crystal element in combination, the optical system becomes complicated.

  In the above-mentioned Patent Document 2, the light expander is adjusted with a beam expander using three lenses, and spherical aberration generated at the convergent position is corrected, and recording on a plurality of recording layers is performed in the near-field optical recording / reproducing method. Or play is realized. However, the spherical aberration of the light incident on the beam expander as parallel light is corrected by a slight change in the use area of the lens caused by a change in the distance between the lenses 66a and 66b. For this reason, when the objective lens optical system is moved, it deviates from the correction region and a large aberration occurs. For this reason, there is a problem that the tracking control range is limited by this spherical aberration correction. FIG. 17 shows the movement characteristics of the objective lens optical system 67 when the light is converged to a position of 7 μm from the surface of the optical recording medium 60 based on the lens data shown in this document. The horizontal axis is the ratio of the lens movement amount to the diameter of the light beam incident on the objective lens optical system, and is the normalized lens movement amount. As can be seen from this, if the region where the aberration is up to 50 mλ is the practical use range, the normalized lens movement amount is 10 μm or less, and the tracking control range when converted to a light beam diameter of 1 mm is very narrow. Further, when the convergence position of the recording layer separated by 6 μm is adjusted with this beam expander, there is a problem that spherical convergence of about 40 mλ occurs.

  The present invention has been made in view of the above problems, and is used in particular for recording or reproducing information on an optical recording medium having a plurality of recording layers in a near-field optical recording and reproducing method. An object is to provide an optical pickup having a configuration and an optical recording / reproducing apparatus using the optical pickup optical system.

  In order to achieve the above object, an optical pickup according to the present invention corrects light convergence positions and spherical aberrations in a plurality of recording layers by a wavefront conversion optical system having a convergent and diverging optical path.

  A desirable first configuration according to the present invention is an optical pickup that records or reproduces information by irradiating light emitted from a light source onto an optical recording medium having a plurality of recording layers. A wavefront conversion optical system for converting a wavefront of light, and an objective lens optical system for converging a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1. The optical system includes a condensing lens that converts divergent light emitted from the light source into parallel light, a first lens that converges the parallel light, and a third lens that condenses the divergent light after convergence. A second lens provided between the first lens and the third lens, wherein at least the second lens is configured to be movable in the direction of the optical axis, the surface of the optical recording medium, and the surface Closest lens of the objective lens optical system In the state where the distance between the first and second lenses is kept constant, at least the second lens is moved in the optical axis direction to correct the light convergence position and spherical aberration in the recording layer.

  A desirable second configuration according to the present invention is an optical pickup that records or reproduces information by irradiating light emitted from a light source onto an optical recording medium having a plurality of recording layers. A wavefront conversion optical system that converts a wavefront of light, an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1, and the optical recording medium A separation optical system that separates the reflected light from an optical path from the light source to the optical recording medium, and a detection unit that detects the light separated by the separation optical system, and the wavefront conversion optical system includes: A condensing lens that converts divergent light emitted from the light source into parallel light, a first lens that converges the parallel light, a third lens that condenses the divergent light after convergence, and a first lens And the third lens The second lens is provided, and at least the second lens is configured to be movable in the optical axis direction. The distance between the surface of the optical recording medium and the lens of the objective lens optical system closest to the surface is set. The light converging position and spherical aberration in the recording layer are corrected by moving at least the second lens in the optical axis direction while maintaining a constant value.

A desirable third configuration according to the present invention is an optical pickup that records or reproduces information by irradiating light emitted from a light source onto an optical recording medium having a plurality of recording layers. A wavefront conversion optical system that converts a wavefront of light, and an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1.
The wavefront conversion optical system includes a first lens for converging diverging light emitted from the light source, a third lens for converging diverging light after convergence, and a space between the first lens and the third lens. And at least the second lens is configured to be movable in the optical axis direction, and includes a surface of the optical recording medium and a lens element of the objective lens optical system closest to the surface. The light convergence position and spherical aberration in the recording layer are corrected by moving at least the second lens in the optical axis direction while maintaining a constant interval.

  A desirable fourth configuration according to the present invention is an optical pickup that records or reproduces information by irradiating light emitted from a light source onto an optical recording medium having a plurality of recording layers. A wavefront conversion optical system that converts a wavefront of light, an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1, and the optical recording medium A separation unit that separates reflected light from an optical path from the light source to the optical recording medium; and a detection unit that detects light separated by the separation unit, and the wavefront conversion optical system includes: The first lens for converging the emitted divergent light, the third lens for condensing the divergent light after convergence, and the second lens provided between the first lens and the third lens. At least the second lens It is configured to be movable in the direction of the optical axis, and at least the second lens is irradiated with light while maintaining a constant distance between the surface of the optical recording medium and the lens element of the objective lens optical system closest to the surface. By moving in the axial direction, the light convergence position and spherical aberration are corrected in the recording layer.

  If the optical pickup is configured as described above, the spherical aberration is corrected simultaneously with the adjustment of the convergence position of the convergent light by controlling the distance between the lenses included in the wavefront conversion optical system.

  In this specification, “adjustment of convergence position and correction of spherical aberration” means that both the convergence position and spherical aberration are within an allowable range that can be used as an optical pickup, and both of them are not necessarily limited. Does not mean that the optical characteristics coincide with the best state.

  According to the present invention, it is possible to adjust the convergence position and correct the spherical aberration only by the beam expander of the wavefront conversion optical system. Further, since the spherical aberration is corrected in the convergent and divergent optical path, the tracking range can be expanded unlike the correction of the spherical aberration caused by a slight change in the lens use area as in the conventional example. Therefore, an optical pickup having a simple configuration and an optical recording / reproducing apparatus using the same can be realized.

1 is a schematic diagram showing a configuration of an optical pickup according to a first embodiment of the present invention. FIG. 1 is a schematic diagram showing details of a part of an objective lens optical system and an optical recording medium in a first embodiment of the present invention. The schematic diagram which shows the operation | movement of the beam expander in the 1st Embodiment of this invention. The graph which shows the relationship between the air gap and reflectance in the 1st Embodiment of this invention. The graph which shows the spherical aberration of the design example in the 1st Embodiment of this invention The graph which shows the depth and aberration of the recording layer of the design example in the 1st Embodiment of this invention The graph which shows the relationship between the recording depth of the example of a design in the 1st Embodiment of this invention, and a lens movement amount. The graph which shows the relationship between the recording depth of the example of a design in the 1st Embodiment of this invention, and a lens moving amount | distance. The graph which shows the relationship between the movement amount of the normalized objective lens optical system of the design example in the 1st Embodiment of this invention, and an aberration. FIG. 2 is a schematic diagram showing a chromatic aberration correction lens configuration in the first embodiment of the present invention. Schematic diagram showing the configuration of the optical pickup according to the second embodiment of the present invention. Schematic diagram showing an optical recording / reproducing apparatus according to a third embodiment of the present invention. Schematic diagram showing a computer apparatus according to a fourth embodiment of the present invention. Schematic diagram showing an optical disk recorder according to a fifth embodiment of the present invention. 1st schematic diagram which shows schematic structure of an example of the conventional optical pick-up 2nd schematic diagram which shows schematic structure of an example in the conventional optical pick-up The graph which shows the relationship between the movement amount and aberration of the normalized objective lens optical system in the second schematic diagram of the conventional optical pickup

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
FIG. 1 is a schematic diagram showing the configuration of an optical pickup according to the first embodiment of the present invention. FIG. 2 is a schematic diagram showing details of a part of the objective lens optical system and the optical recording medium shown in FIG. FIG. 3 is a schematic diagram showing details of the beam expander shown in FIG.

  1 and 2, 1 is a semiconductor laser that emits light having a wavelength of about 405 nm, 3 is a condenser lens, 5 is a beam expander, and 8 is an objective lens optical system. 21 is a semiconductor laser that emits light having a wavelength of about 655 nm, and 24 is a condenser lens. Reference numeral 30 denotes an optical recording medium, on which 31 recording layers are formed.

  A light beam 2 emitted as divergent light from the semiconductor laser 1 is condensed by a condensing lens 3 to become substantially parallel light and enters a beam splitter 4. The beam splitter 4 has a polarization separation characteristic in which the polarization plane reflects polarized light in a direction parallel to the incident surface (hereinafter referred to as S-polarized light) and transmits polarized light in a direction perpendicular thereto (hereinafter referred to as P-polarized light). The light beam 2 emitted from the semiconductor laser 1 is set to be incident on the beam splitter 4 as P-polarized light, passes through the beam splitter 4, and enters the beam expander 5. The beam expander 5 includes three lenses: a first lens 5a having a positive power, a second lens 5b that is a meniscus lens, and a third lens 5c having a positive power. Furthermore, in the present embodiment, among the three lenses, the second lens 5b and the third lens 5c are supported so as to be movable in the optical axis direction. The second lens 5a and the third lens 5c are configured to be moved by an actuator (not shown). Therefore, the light beam 2 incident on the beam expander 5 is once converged by the first lens 5a, then transmitted through the second lens 5b, and condensed again by the third lens 5c. The light beam 2 emitted from the beam expander becomes parallel light, slight divergent light, or convergent light, is converted from P-polarized light to circularly-polarized light by the wave plate 6, and enters the beam splitter 7. The beam splitter 7 has a wavelength selectivity for transmitting light having a wavelength of 405 nm and reflecting light having a wavelength of 655 nm. The light beam 2 having a wavelength of 405 nm is transmitted therethrough and enters the objective lens optical system 8. The objective lens optical system 8 is a lens having a numerical aperture exceeding 1, and converges incident light to form a light spot on the recording layer 31 inside the optical recording medium 30. More specifically, the objective lens optical system 8 has a lens 8a having a positive power for converging the incident light beam 2, and further converges the light converged by the lens 8a and emits evanescent light from its exit end face. It consists of and. The SIL 8b is a so-called solid immersion lens, and as shown in FIG. 2, the SIL 8b has a super hemispherical shape in which a part of a spherical lens is cut out and a part exceeding the size of the hemisphere is left. Is refracted and converges to the plane side. Since the SIL 8b is a spherical surface on the incident side and uses the refraction action, the numerical aperture of the lens 8a combined with the SIL 8b can be suppressed to about 0.45. The lens 8a and the SIL 8b are fixed to a lens barrel (not shown) and constitute an objective lens optical system 8.

  As shown in FIG. 2, in the optical recording medium 30, a plurality of recording layers 31 are formed at a predetermined depth, and a protective layer 30a is provided on the surface. The recording layer 31 includes a first recording layer 31a, a second recording layer 31b, and a third recording layer 31c. Each recording layer transmits a part of light, absorbs a part, and reflects a part. have. The recording layers 31 may be laminated at equal intervals, or may be laminated at predetermined intervals that are not equal intervals. The protective layer 30a is provided so that information recorded on the recording layer 31 can be protected even when the objective lens optical system 8 and the optical recording medium 30 collide during use. An air gap of about 20 to 30 nm is formed between the exit end face of the SIL 8b and the protective layer 30a of the optical recording medium 30. Due to this air gap, the SIL 8b and the optical recording medium 30 are not in contact with each other, and evanescent light emitted from the SIL 8b is propagated to the optical recording medium 30. Accordingly, in the light beam 2, light having a critical angle or less is normally propagated to the optical recording medium 30 at the emission end face of the SIL 8b, and light having the critical angle or more is propagated to the optical recording medium 30 as evanescent light. The light beam 2 propagated to the optical recording medium 30 passes through the protective layer 30 a and converges on the desired recording layer 31. A part of the converged light is reflected by the recording layer 31 and enters the wave plate 6 through the objective lens optical system 8 and the beam splitter 7. The light beam 2 incident on the wave plate 6 is converted from circularly polarized light to S-polarized light orthogonal to the forward path, and then enters the beam splitter 4 through the beam expander 5. Since the beam splitter 4 reflects S-polarized light, the light beam 2 is reflected here. Further, the light beam 2 is given astigmatism by the detection lens 9 and enters the photodetector 10. By calculating the output of the photodetector 10, servo signals and information signals used for focus control and tracking control can be obtained. The focus signal can be detected by, for example, the astigmatism method, and the tracking signal can be detected by, for example, the push-pull method.

  On the other hand, various methods can be used as a control method for keeping the air gap constant. In this embodiment, reflected light of evanescent light emitted from the SIL 8b using a semiconductor laser is used. A light beam 22 emitted from the semiconductor laser 21 enters a beam splitter 23. The beam splitter 23 has a polarization separation characteristic that reflects S-polarized light and transmits P-polarized light. The light beam 22 emitted from the semiconductor laser 21 is set to be incident on the beam splitter 23 as P-polarized light. The light beam 22 is transmitted therethrough and becomes substantially parallel light by the condenser lens 24. The light beam 12 that has become parallel light passes through the wave plate 25, is converted from P-polarized light to circularly-polarized light, and enters the beam splitter 7. As described above, the beam splitter 7 has wavelength selectivity for reflecting light having a wavelength of 655 nm, and the light beam 22 having a wavelength of 655 nm is reflected from this and enters the objective lens optical system 8. The light beam 22 incident on the objective lens optical system 8 is converged by the lens 8a and emits evanescent light from the exit end face of the SIL 8b. The evanescent light is divided into light propagating to the optical recording medium 30 and light reflected by the exit end face of the SIL 8b. The light reflected by the exit end face of the SIL 8b passes through the lens 8a, is reflected by the beam splitter 7, and is reflected by the wave plate 25. Is incident on. The light beam 22 incident on the wave plate 25 is converted from circularly polarized light to S-polarized light orthogonal to the forward path, and enters the beam splitter 23. Since the beam splitter 23 reflects the S-polarized light, the light beam 22 is reflected here and enters the photodetector 26. Note that a part of the light beam 22 incident on the optical recording medium 30 is reflected by the recording layer 31 and returns in the direction of the photodetector 26, but is hardly incident on the photodetector because it diffuses.

  Of the evanescent light emitted from the exit end face of the SIL 8b, the ratio of the light propagating to the optical recording medium 30 and the light reflected from the exit end face of the SIL 8b varies depending on the size of the air gap. FIG. 4 shows an example of the relationship between the air gap and the reflected light, and the average reflectance is calculated when the objective lens optical system has a numerical aperture of 1.7, a wavelength of 405 nm, and circularly polarized light is incident. is there. As shown in FIG. 4, the reflectance of evanescent light increases as the air gap increases, and the reflectance of evanescent light decreases as the air gap decreases. Therefore, the size of the air gap can be obtained by detecting the light beam 22 reflected by the exit end face of the SIL 8b by the photodetector 16, and a servo signal for controlling the air gap can be obtained from this.

  Next, functions of the optical pickup according to the present embodiment will be described with reference to FIG. FIG. 3A shows a state in which the second lens 5b among the three lenses constituting the beam expander 5 is at an intermediate position between the first lens 5a and the third lens 5c. When the second lens 5 b is in this position, the light beam 2 emitted from the beam expander 5 becomes parallel light and converges on the second recording layer 31 b inside the optical recording medium 30. FIG. 3B shows a state in which the second lens 5b of the three lenses constituting the beam expander 5 has moved a certain amount in the direction of the third lens 5c. When the second lens 5b is in this position, the light beam 2 emitted from the beam expander 5 becomes slightly convergent light, and the convergence position of the light in the optical recording medium 30 changes, and the first recording layer 31a is changed. Converge. The spherical aberration occurring at this time is corrected mainly by making one lens surface 5b1 of the second lens 5b an aspherical surface. Next, FIG. 3C shows a state in which the second lens 5b of the three lenses constituting the beam expander 5 has moved by a certain amount in the direction of the first lens 5a. When the second lens 5b is in this position, the light beam 2 emitted from the beam expander 5 becomes slightly divergent light, and the convergence position of the light in the optical recording medium 30 changes, and the third recording layer 31c is changed. Converge. The spherical aberration generated at this time is corrected mainly by making the other lens surface 5b2 of the second lens 5b an aspherical surface.

  In this way, when the second lens 5b is moved in the optical axis direction, the light emitted toward the objective lens optical system 8 is slightly diverged or converged. For this reason, the convergence position in the optical recording medium 30 can be changed, and spherical aberration can be corrected by utilizing the change in the region of the lens surface of the light passing through the second lens 5b. Therefore, the light can be converged on the desired recording layer 31 with the air gap between the surface of the optical recording medium 30 and the SIL 8b being constant.

  The beam expander 5 according to the present embodiment can be designed so that spherical aberration can be sufficiently corrected since the transmission region of the lens surface changes greatly when the light beam 2 passes through the second lens 5b. Therefore, even if the objective lens optical system moves in the tracking direction, it does not deviate from the correction region, and the tracking control range is not limited in accordance with the spherical aberration correction.

  In the optical pickup according to this embodiment, when recording or reproducing on the optical recording medium 30, the second lens 5 b is set at a predetermined position so that the light beam 2 converges on the desired recording layer 31. Alternatively, the second lens 5 b may be controlled using the focus signal so that the light beam 2 always converges on the desired recording layer 31.

  In the description so far, the second lens 5b of the three lenses constituting the beam expander 5 is moved in the optical axis direction to adjust the convergence position and correct the spherical aberration. 5b is moved in the optical axis direction, and the third lens 5c is moved slightly in the optical axis direction, thereby further reducing spherical aberration at positions other than the recording layer 31 and reducing spherical aberration with respect to wavelength changes. Can be achieved. Thus, when the second lenses 5b and 5c are moved in the optical axis direction to adjust the convergence position and correct the spherical aberration, they may be performed simultaneously or sequentially or stepwise.

  Next, a specific design example will be described. It is assumed that the first to third recording layers 31a, 31b, and 31c are provided at positions having a depth of 1 μm, 4 μm, and 7 μm from the surface of the optical recording medium, respectively. In the design example, the optical pickup was designed with the light source wavelength of 405 nm, the numerical aperture of the objective lens optical system 8 being 1.7, and the focal length of the objective lens optical system being 0.708. The first lens 5a and the third lens 5c are lenses having positive power, the second lens 5b is a meniscus lens, and the refractive index is 1.6239.

  The focal lengths of the first lens 5a and the third lens 5c are each 6 mm, the lens surface 5b2 of the second lens 5b has a radius of curvature of 2.912 mm and a negative power, and the lens surface 5b1 has a radius of curvature of 3. Assume positive power at 5 mm. The lens thickness of the second lens 5b was 2.8 mm. As shown in FIG. 3A, the aspheric coefficients of the first lenses 5a and 5c are such that when the second lens 5b is in an intermediate position and the light beam 2 converges on the second recording layer 31b. It is set to be smaller. The aspherical coefficient of the lens surface 5b1 of the second lens 5b is such that the lens surface 5b2 of the second lens 5b is in the vicinity of the focal position of the first lens 5a as shown in FIG. The aberration is set to be small when converging on the first recording layer 31a. As shown in FIG. 3C, the aspheric coefficient of the lens surface 5b2 of the second lens 5b is such that the lens surface 5b1 of the second lens 5b is in the vicinity of the focal position of the third lens 5c, and the luminous flux 2 is The aberration is set to be small when converging on the third recording layer 31c.

  5 and 6 are graphs showing the spherical aberration of the optical pickup according to the present embodiment. 5A corresponds to the state of convergence on the first recording layer 31a having a depth of 1 μm, and FIG. 5B corresponds to the state of convergence on the second recording layer 31b having a depth of 4 μm. c) corresponds to the state of convergence on the third recording layer 31c having a depth of 7 μm. As can be seen from this graph, spherical aberration is well corrected in the three recording layers having depths of 1 μm, 4 μm and 7 μm from the optical recording medium 30. On the other hand, FIG. 6 shows the spherical aberration in a state of being converged to an arbitrary depth. 6A shows spherical aberration when only the second lens 5b is moved in the optical axis direction and the convergence position is changed. (B) represents spherical aberration when the second lens 5b and the third lens 5c are moved in the optical axis direction to change the convergence position. From this graph, it can be seen that the spherical aberration can be further reduced at an arbitrary depth by moving the third lens 5c. However, even when only the second lens 5b is moved in the optical axis direction, the maximum value of the spherical aberration is about 30 mλ, which is sufficiently within the practical range at any position. When the second lens 5b and the third lens 5c are moved in the optical axis direction, the aberration due to the change in the wavelength of the semiconductor laser 1 can be corrected well.

  FIG. 7 shows the correspondence between the depth of the recording layer and the amount of lens movement when the second lens 5b constituting the beam expander 5 is moved in the optical axis direction. In FIG. 7, in order to change ± 3 μm around the depth of 4 μm of the recording layer, the second lens 5b is moved from −0.87 mm to 0.85 mm, and the optical path length changes by about ± 1.4 mm. become. Therefore, the lens surface 5b2 of the second lens 5b is in the vicinity of the focal position of the first lens 5a, and the lens surface 5b1 is in the vicinity of the focal position of the third lens 5c.

  FIG. 8 shows the correspondence between the depth of the recording layer and the amount of lens movement when the second lens 5b and the third lens 5c constituting the beam expander 5 are moved in the optical axis direction. 8A shows the relationship between the movement amount of the second lens 5b and FIG. 8B shows the relationship with the movement amount of the third lens 5c. In FIG. 8, the light spot in the optical recording medium can be formed at an arbitrary position by moving the second lens 5b greatly and moving the third lens 5c slightly.

  FIG. 9 shows the movement characteristics of the objective lens optical system when the light beam 2 is converged on the third recording layer 31c. The horizontal axis is the ratio of the lens movement amount to the diameter of the light beam incident on the objective lens optical system, and is the normalized movement amount of the objective lens optical system. As can be seen, the aberration relative to the amount of movement of the normalized objective lens optical system is improved to about half that of the conventional example. For this reason, the tracking control range is expanded, and the control performance can be further improved.

  In the optical pickup according to the first embodiment, when the light beam 2 is converged on the first recording layer 31a, the lens surface 5b2 is in the vicinity of the focal position of the first lens 5a and converges on the third recording layer 31c. When it is set, the lens surface 5b1 is set in the vicinity of the focal position of the third lens 5c. However, the lens surfaces 5b1 and 5b2 of the second lens 5b are not limited to this position. The second lens 5b may be at any position as long as it is between the first and third lenses 5a and 5c, and may be designed so that the spherical aberration is reduced at that position. However, when the lens surfaces 5b1 and 5b2 are far away from the focal positions of the third and first lenses, the lens surfaces 5b1 and 5b2 can be corrected to correct the spherical aberration of the first recording layer 31a and the third recording layer 3c. It is necessary to design the aspheric coefficient together.

  Further, by forming a hologram for correcting chromatic aberration on the surface of at least one lens constituting the beam expander 5, spherical aberration that occurs when the wavelength of the semiconductor laser 1 changes can be suppressed. FIG. 10 is a schematic view in which a hologram 5a1 for correcting chromatic aberration is formed on the first lens 5a. In FIG. 10, the hologram 5a1 is formed in a concentric circle shape, and the pitch is configured so as to gradually decrease from the center toward the outer periphery. In order to further improve the diffraction efficiency, the cross-sectional shape is serrated. When the wavelength is shortened, the refractive index is increased and the refraction angle is increased. However, when the wavelength is shortened, the diffraction angle is decreased when the wavelength is shortened, and the spherical aberration due to the wavelength change can be canceled.

  As described above, the beam expander 5 according to this embodiment can correct spherical aberration simultaneously with the adjustment of the convergence position in the optical recording medium 30. Therefore, according to the present embodiment, there is no need to provide a separate optical element such as a liquid crystal element in order to correct spherical aberration due to the difference in the depth of the recording layer and spherical aberration of the objective lens system. An optical pickup can be realized with a simple configuration. Further, even if the objective lens optical system moves in the tracking direction, it does not deviate from the correction area. Such an optical pickup is particularly useful when the working distance of the objective lens optical system 8 needs to be strictly maintained within a certain range as in the near-field optical recording / reproducing system.

(Embodiment 2)
FIG. 11 is a schematic diagram showing a configuration of an optical pickup according to the second embodiment of the present invention. This embodiment is different from the first embodiment in that the beam expander 9 and the condensing lens are integrated, and other than that is the same as in the first embodiment. Therefore, in the present embodiment, the structural members given the same reference numerals as those in the first embodiment have the same functions as those in the first embodiment unless otherwise specified. In FIG. 11, reference numeral 9 denotes a beam expander, which is composed of three lenses: a first lens 9a having a positive power, a third lens 9c, and a second lens 9b which is a meniscus lens.

  A light beam 2 emitted from the semiconductor laser 1 as divergent light is incident on a beam splitter 12 having polarization separation characteristics as P-polarized light, passes therethrough, and enters a beam expander 9. Of the three lenses of the beam expander 9, the second lens 5b and the third lens 5c are supported so as to be movable in the optical axis direction, and are configured to be moved by an actuator (not shown). The light beam 2 incident on the beam expander 9 is converted from divergent light into convergent light by the first lens 9a, passes through the second lens 9b, and becomes divergent light again and is collected by the third lens 9c. The light beam 2 emitted from the beam expander 9 becomes parallel light, slight divergent light or convergent light, passes through the wave plate 6 and the beam splitter 7 and enters the objective lens optical system 8. By propagating the evanescent light, the objective lens optical system 8 having a numerical aperture exceeding 1 converges the incident light to form a light spot on the recording layer 31 inside the optical recording medium 30. A part of the light is reflected by the recording layer 31 and enters the beam splitter 12 through the objective lens optical system 8, the beam splitter 7, the wave plate 6, and the beam expander 5. The light beam 2 converted into S-polarized light by the action of the wave plate 6 is reflected by the beam splitter 12 and enters the photodetector 10 through the detection lens 13 that gives astigmatism. Servo signals and information signals used for focus control and tracking control can be obtained from the photodetector 10.

  The light beam 22 emitted from the semiconductor laser 21 passes through the beam splitter 23, the condenser lens 24, and the wave plate 25, is reflected by the beam splitter 7, and enters the objective lens optical system 8. A part of the light beam 22 incident on the objective lens optical system 8 is reflected by the exit end face of the SIL 8b, passes through the lens 8a, the beam splitter 7, the wave plate 25, and the beam splitter 23, and enters the photodetector 26. Similar to the first embodiment, a control signal for keeping the surface of the optical recording medium and the air gap of the SIL 8b constant can be obtained from the signal detected here.

  In the optical pickup according to the present embodiment, of the three lenses constituting the beam expander 9, the first lens 9a serves as both the condenser lens 3 and the first lens 5a in the first embodiment, and the optical system Can be configured more simply. The beam splitter 12 needs to separate the light emitted from the semiconductor laser 1 and directed to the optical recording medium 30 and the light reflected by the optical recording medium 30 and directed to the photodetector 11 in the divergent light path, and is angle-dependent. Less polarization separation characteristics are required. The operation of the beam expander 9 is the same as that of the first embodiment. When the second lens 9b is at an intermediate position between the first lens 9a and the third lens 9c, the light beam emitted from the beam expander 9 2 becomes parallel light and converges on the second recording layer 31 b inside the optical recording medium 30. When the second lens 9b moves by a certain amount in the direction of the third lens 9c, the light beam 2 emitted from the beam expander 9 becomes slightly convergent light, and the convergence position of the light in the optical recording medium 30 changes, and the first lens 9b changes. Converges to one recording layer 31a. The spherical aberration occurring at this time is corrected mainly by making one lens surface 9b1 of the second lens 9b an aspherical surface. When the second lens 9b moves a certain amount in the direction of the first lens 9a, the light beam 2 emitted from the beam expander 9 becomes slightly divergent light, and the light convergence position in the optical recording medium 30 changes, and the first lens 9b changes. 3 converges on the recording layer 31c. The spherical aberration generated at this time is corrected mainly by making the other lens surface 9b2 of the second lens 9b an aspherical surface.

  Thus, when the lens 9b is moved in the optical axis direction, the light emitted toward the objective lens optical system 8 is slightly diverged or converged. Thereby, the convergence position of the light beam in the optical recording medium 30 can be changed, and when the light beam 2 passes through the second lens 9b, the transmission area of the lens surface changes greatly, so that the spherical aberration can be sufficiently corrected. Further, even if the objective lens optical system moves in the tracking direction, it does not deviate from the correction area, and the tracking control range is not limited. Therefore, the light can be converged on the desired recording layer 31 in a state where the air gap between the surface of the optical recording medium 30 and the SIL 8b is constant.

  When recording or reproducing on the optical recording medium 30 with the optical pickup according to the present embodiment, the second lens 9b is set at a predetermined position so that the light beam 2 converges on each desired recording layer 31. Alternatively, the second lens 5 b may be controlled using the focus signal so that the light beam 2 always converges on the desired recording layer 31.

  In the description so far, the second lens 9b among the three lenses constituting the beam expander 9 is moved in the optical axis direction to adjust the convergence position and correct the spherical aberration. 9b is moved in the optical axis direction, and the third lens 9c is moved slightly in the optical axis direction, thereby further reducing spherical aberration at positions other than the recording layer 31 and reducing spherical aberration with respect to wavelength changes. Can be achieved. In this way, when the second lens 9b and the third lens 9c are moved in the optical axis direction to adjust the convergence position and correct the spherical aberration, they may be performed simultaneously or sequentially or stepwise. Also good.

  Similarly to the first embodiment, when the light beam 2 is converged on the first and third recording layers 31a and 31c, the lens surfaces 9b2 and 9b1 are the focal positions of the first and third lenses 9a and 9c, respectively. It is not limited to the vicinity. In the case where the wavelength of the semiconductor laser 1 is changed by forming a hologram for correcting chromatic aberration on the surface of at least one lens constituting the beam expander 9. The generated spherical aberration can be suppressed.

  In the optical pickup according to the present embodiment, it is not necessary to provide a separate optical element such as a liquid crystal element in order to correct spherical aberration, and a simple optical pickup can be realized. Further, even if the objective lens optical system moves in the tracking direction, it does not deviate from the correction area. Such an optical pickup optical system is particularly useful when the working distance of the objective lens optical system 8 needs to be strictly maintained within a certain range as in the near-field optical recording / reproducing system.

  In the optical pickups according to the first and second embodiments, the SIL 8b of the objective lens optical system 8 has a super hemispherical shape in which a part of the spherical lens is cut out and a part larger than the hemisphere is left. The light beam 2 incident from the incident side surface was refracted and converged on the exit surface. However, the SIL 8b may be formed in a hemispherical shape so that the light beam incident from the incident side surface is condensed on the output surface without being refracted. The super hemispherical shape is sometimes referred to as a super solid immersion lens and the hemispherical shape is sometimes referred to as a solid immersion lens. However, in this specification, the two are referred to as a solid immersion lens. Further, when the SIL 8b is hemispherical, the numerical aperture required for the lens 8a is larger than that when the super hemispherical shape is used to increase the numerical aperture of the objective lens optical system.

  In the first and second embodiments, the optical recording medium having three recording layers has been described as an example, but the recording layer is not limited to three layers. Further, the optical recording medium used here may be a rewritable phase change optical disc, a read-only optical disc, a magneto-optical disc, etc., as long as it is irradiated with light in at least one of information recording / reproduction and erasure, Various standard media can be used.

(Embodiment 3)
An embodiment of an optical recording / reproducing apparatus using the optical pickup of the present invention is shown in FIG. In FIG. 12, an optical recording medium 100 is mounted on a turntable 105 and rotated by a motor 104. The optical pickup 102 shown in the first and second embodiments is transported by the driving device 101 to the track on the optical recording medium 100 where desired information exists.

  The optical pickup 102 sends a focus error signal and a tracking error signal to the electric circuit 103 in accordance with the positional relationship with the optical recording medium 100. In response to this signal, the electric circuit 103 sends a signal for driving the objective lens to the optical head 102. In response to this signal, the optical pickup 102 performs focus control and tracking control on the optical recording medium 100 to read, write, or erase information.

  In the above description, the mounted optical recording medium 100 is an optical recording medium having a plurality of recording layers for recording / reproducing by near-field light. By using the optical pickup of the present invention, the optical recording / reproducing apparatus 107 of the present embodiment can record or reproduce on a plurality of recording layers with one optical pickup.

(Embodiment 4)
The present embodiment is an embodiment of a computer apparatus provided with the optical recording / reproducing apparatus 107 according to the third embodiment. FIG. 13 is a perspective view of a computer according to the present embodiment. A computer 109 shown in FIG. 13 includes an optical recording / reproducing device 107 according to the third embodiment, input devices such as a keyboard 111 and a mouse 112 for inputting information, information input from the input device, and optical recording An arithmetic device 108 such as a CPU that performs an operation based on information read from the playback device 107, and an output device 110 such as a cathode ray tube or a liquid crystal display device that displays information of a result calculated by the arithmetic device 108 are provided. Yes.

  The computer apparatus according to the present embodiment includes the optical recording / reproducing apparatus 107 according to the third embodiment, and stably records or records an optical recording medium having a plurality of recording layers for recording / reproducing by near-field light. Since it can be reproduced, it can be used for a wide range of purposes.

(Embodiment 5)
The present embodiment is an embodiment of an optical disc recorder provided with the optical recording / reproducing apparatus 107 according to the third embodiment. FIG. 14 is a perspective view of the optical disc recorder according to the present embodiment. The optical disk recorder 115 shown in FIG. 14 includes an optical recording / reproducing device 107 according to the third embodiment and a recording signal processing circuit 113 that converts an image signal into an information signal to be recorded on an optical recording medium by the optical recording / reproducing device 107. ing.

  It is also desirable to have a reproduction signal processing circuit 114 that converts an information signal obtained from the optical recording / reproducing apparatus 107 into an image signal. According to this configuration, it is possible to reproduce the already recorded portion. Further, an output device 110 such as a cathode ray tube or a liquid crystal display device for displaying information may be provided.

  The optical disc recorder according to the present embodiment includes the optical recording / reproducing apparatus 107 according to the third embodiment, and can stably stabilize an optical recording medium having a plurality of recording layers for recording / reproducing by near-field light. Since it can be recorded or reproduced, it can be used for a wide range of purposes.

  The optical pickup and optical recording / reproducing apparatus according to the present invention can stably record or reproduce information in the near-field optical recording / reproducing method, and can perform high-density recording on an optical recording medium. Therefore, it can be used for a large-capacity optical disk recorder, a computer memory device, and the like, which are applied devices.

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Light beam 3 Condensing lens 5 Beam expander
5a 1st lens 5b 2nd lens 5c 3rd lens 8 Objective lens optical system 8a Lens 8b SIL (Solid Immersion Lens)
9 Beam expander
9a first lens 9b second lens 9c third lens 10 detection lens 11 photodetector 13 photodetector 21 semiconductor laser 30 optical recording medium 30a protective layer 31 recording layer 31a first recording layer 31a second recording Layer 31a Third recording layer 107 Optical recording / reproducing apparatus 109 Computer apparatus 115 Optical disc recorder

Claims (13)

An optical pickup that records or reproduces information by irradiating an optical recording medium having a plurality of recording layers with light emitted from a light source,
A wavefront conversion optical system that converts a wavefront of light emitted from the light source, and an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1. With
The wavefront conversion optical system includes a condenser lens that converts divergent light emitted from the light source into parallel light, a first lens that converges the parallel light, and a third lens that condenses the divergent light after convergence. And a second lens provided between the first lens and the third lens, and at least the second lens is configured to be movable in the optical axis direction,
The recording layer is moved by moving at least the second lens in the optical axis direction while maintaining a constant distance between the surface of the optical recording medium and the lens of the objective lens optical system closest to the surface. An optical pickup which corrects the light convergence position and spherical aberration in the lens. An optical pickup that records or reproduces information by irradiating an optical recording medium having a plurality of recording layers with light emitted from a light source,
A wavefront conversion optical system that converts a wavefront of light emitted from the light source, and an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1. A separation optical system for separating the light reflected by the optical recording medium from an optical path from the light source to the optical recording medium, and a detection unit for detecting the light separated by the separation optical system,
The wavefront conversion optical system includes a condenser lens that converts divergent light emitted from the light source into parallel light, a first lens that converges the parallel light, and a third lens that condenses the divergent light after convergence. And a second lens provided between the first lens and the third lens, and at least the second lens is configured to be movable in the optical axis direction,
The recording layer is moved by moving at least the second lens in the optical axis direction while maintaining a constant distance between the surface of the optical recording medium and the lens of the objective lens optical system closest to the surface. An optical pickup which corrects the light convergence position and spherical aberration in the lens. One of the second lens and the first or third lens of the wavefront conversion optical system is movable in the optical axis direction, and the two movable lenses move in the optical axis direction while changing the distance between them. The optical pickup according to claim 1, wherein the focal position and spherical aberration of the spot on the recording layer are corrected thereby. 4. The hologram according to claim 1, wherein a hologram for correcting an aberration caused by a wavelength change of the light source is formed on at least one of the condenser lens and the first to third lenses of the wavefront conversion optical system. Optical pickup. An optical pickup that records or reproduces information by irradiating an optical recording medium having a plurality of recording layers with light emitted from a light source,
A wavefront conversion optical system that converts a wavefront of light emitted from the light source, and an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1. With
The wavefront conversion optical system includes a first lens for converging diverging light emitted from the light source, a third lens for converging diverging light after convergence, and a space between the first lens and the third lens. And at least the second lens is configured to be movable in the direction of the optical axis,
By moving at least the second lens in the optical axis direction while maintaining a constant distance between the surface of the optical recording medium and the lens element of the objective lens optical system closest to the surface, the recording An optical pickup which corrects a light convergence position and spherical aberration in a layer. An optical pickup that records or reproduces information by irradiating an optical recording medium having a plurality of recording layers with light emitted from a light source,
A wavefront conversion optical system that converts a wavefront of light emitted from the light source, and an objective lens optical system that converges a light beam emitted from the wavefront conversion optical system on a recording layer of the optical recording medium with a numerical aperture exceeding 1. A separation unit for separating the light reflected by the optical recording medium from an optical path from the light source to the optical recording medium, and a detection unit for detecting the light separated by the separation unit,
The wavefront conversion optical system includes a first lens for converging diverging light emitted from the light source, a third lens for converging diverging light after convergence, and a space between the first lens and the third lens. Comprising at least a second lens, and at least the second lens is configured to be movable in the optical axis direction.
By moving at least the second lens in the optical axis direction while maintaining a constant distance between the surface of the optical recording medium and the lens element of the objective lens optical system closest to the surface, the recording An optical pickup which corrects a light convergence position and spherical aberration in a layer. The second lens and the third lens of the wavefront conversion optical system are movable in the optical axis direction, and the second and third lenses are moved in the optical axis direction while changing the distance between them. The optical pickup according to claim 5, wherein the focal position of the spot on the recording layer and the spherical aberration are corrected. 8. The optical pickup according to claim 5, wherein a hologram for correcting an aberration caused by a wavelength change of the light source is formed on at least one of the first to third lenses of the wavefront conversion optical system. 9. The optical pickup according to claim 1, wherein the second lens of the wavefront conversion optical system is a positive or negative meniscus lens. The optical pickup according to claim 1, wherein the objective lens optical system includes a solid immersion lens and irradiates the optical recording medium with a light beam including evanescent light. An optical pickup according to any one of claims 1 to 10, a motor for rotating an optical recording medium, a motor used for the optical pickup, a lens used for the optical pickup, and a light source based on a signal obtained from the optical pickup. An optical recording / reproducing apparatus comprising: an electric circuit that controls and drives at least one of them. 12. An optical recording / reproducing apparatus according to claim 11, comprising: an arithmetic unit that performs an operation based on at least one of input information and information reproduced from the optical recording / reproducing apparatus; the input information; A computer apparatus comprising: an output device that outputs at least one of information reproduced from an optical recording / reproducing device and a result computed by the computing device. 12. The optical recording / reproducing apparatus according to claim 11, a recording signal processing circuit for converting image information into information to be recorded in the optical recording / reproducing apparatus, and a reproducing signal for converting a signal obtained from the optical recording / reproducing apparatus into an image. An optical disc recorder comprising a signal processing circuit.

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