JP2012119042A - Objective lens and optical head with the same - Google Patents

Abstract

【Task】
It is an object of the present invention to provide means for realizing a three-wavelength compatible objective lens corresponding to BD / DVD / CD with a single objective lens with high efficiency.
[Solution]
When a light beam of the first wavelength is incident, a first phase is imparted. When a light beam of the second wavelength is incident, a second phase is imparted, and a light beam of the third wavelength is incident. In this case, the objective lens imparts a third phase, and the objective lens is composed of at least a material A and a material B, and the material A and the material B are different in refractive index with respect to the light beam of the first wavelength. The difference between the refractive indices of the material A and the material B with respect to the light beam with the second wavelength is Δ2, and the difference between the refractive indexes with respect to the light beam with the third wavelength is Δ3. Δ2 and Δd3 are increased, and a phase step is provided in a predetermined region at the boundary between the material A and the material B.
[Selection] Figure 1

Description

The present invention relates to a lens having three wavelength selectivity. In particular, the present invention relates to an objective lens and an optical head compatible with BD, DVD, and CD.

  As a background art in this technical field, there is JP-A-2007-280600 (Patent Document 1). This publication states that “there is chromatic aberration that can be recorded / reproduced without any problem even when there is an instantaneous wavelength jump that the actuator cannot follow, and that the adverse effect that occurs on the spot diameter of the focused spot and the sensor read signal can be suppressed. A diffractive optical element and an optical pickup apparatus including the diffractive optical element are disclosed. Further, an objective lens using a phase step is disclosed.

JP2007-280600A

  As optical discs, BD (Blu-ray Disc), DVD (Digital Versatile Disc), CD (Compact Disc) and the like are standardized. In an optical head for recording or reproducing an optical disc, a light beam is emitted from a light source, the light beam is condensed on an information layer of the optical disc by an objective lens, and a reflected light beam from the optical disc is detected by a photodetector. Information on the optical disk is reproduced from the signal.

  Now, optical heads compatible with BD, DVD, and CD differ in the wavelength of the light beam emitted from the light source, the NA (Numerical Aperture) of the objective lens, and the distance from the surface of the optical disk to the information surface (hereinafter referred to as “between surfaces”). The point is a feature. For BD / DVD / CD, use a wavelength of 405 nm band / 660 nm band / 785 nm band, NA of 0.85 / 0.6 / 0.45, and a space of 0.1 mm / 0.6 mm / 1.2 mm. It is common. In such a BD / DVD / CD compatible optical head that records or reproduces three optical disks with a single optical head, an objective lens dedicated to BD and a dual wavelength compatible objective lens for DVD / CD are provided. It is common to use two in combination. In a two-wavelength compatible objective lens, a phase step is provided on the lens surface, and the phase generated at the step is set to an integral multiple of the wavelength on one side, so that the phase step functions at one wavelength and the other wavelength. The wavelength selectivity that prevents the phase step from functioning is used.

  In recent years, a three-wavelength compatible objective lens that supports reproduction or recording of the BD / DVD / CD with a single objective lens, and an optical head using the objective lens have been studied. Patent Document 1 describes a three-wavelength compatible objective lens using a phase step that utilizes the difference in wavelength as in a conventional two-wavelength compatible objective lens. However, in order to achieve the three wavelength selectivity in the phase step, the step becomes large in order to achieve matching, and the light is greatly diffracted, resulting in a decrease in light efficiency. For example, in Table 3 of Patent Document 1, the diffraction efficiency of CD is described as 46.1%. That is, it can be said that a three-wavelength compatible objective lens using a phase step generally has a disadvantage in that it has low light efficiency. An optical head with low light efficiency requires an increase in the output of the light source, so there are problems in terms of cost and reliability (lifetime of the light source), and the light quantity of the reflected light beam to the photodetector is also small. Another problem is that N is deteriorated and the reproduction margin is reduced.

  Accordingly, an object of the present invention is to provide a means for realizing a three-wavelength compatible objective lens corresponding to BD / DVD / CD with a single objective lens with high efficiency.

  The above object can be achieved by, for example, the configuration described in the claims.

  According to the present invention, it is possible to realize a high-efficiency three-wavelength compatible objective lens and a BD / DVD / CD compatible optical head that is inexpensive, highly reliable, and has a high reproduction margin.

FIG. 2 is a schematic configuration diagram of an objective lens 001 for compatibility with three wavelengths according to the first embodiment. FIG. 2 is a diagram illustrating a BD optical system in the objective lens 001 in Example 1. FIG. 3 is a diagram illustrating a DVD optical system in the objective lens 001 in Example 1. FIG. 2 is a diagram illustrating a CD optical system in the objective lens 001 in Example 1. FIG. 3 is a schematic configuration diagram of an objective lens for 050-wavelength compatibility in Example 2. FIG. FIG. 4 is a schematic configuration diagram of an optical head 100 in Embodiment 3. FIG. 6 shows a schematic configuration diagram of an optical head 200 in Embodiment 4.

  Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings, but the present invention is not limited thereby.

  Embodiment 1 of the present invention will be described with reference to the drawings. Here, an objective lens that can be designed to have a predetermined performance with respect to three different wavelengths will be specifically described by taking a BD, DVD, and CD three-wavelength compatible objective lens as an example.

  FIG. 1 is a schematic configuration diagram illustrating a three-wavelength compatible objective lens 001 in the first embodiment. The objective lens 001 in the first embodiment is an optical element including two components, a phase plate 002 and a lens 003. The phase plate 002 is a transparent flat plate and is composed of two materials A004 and B005. The center wavelengths of BD / DVD / CD are λ1 = 405 nm / λ2 = 660 nm / λ3 = 785 nm, respectively. As the material A004 and the material B005, materials having different dispersion characteristics are used. The dispersion characteristic is intended to be a characteristic in which the change sensitivity of the refractive index with respect to the wavelength is different. Specifically, the difference in refractive index with respect to λ1 of the material A004 and the material B005 is Δ1, the difference in refractive index with respect to λ2 of the material A004 and the material B005 is Δ2, and the difference in refractive index with respect to λ3 of the material A004 and the material B005 is Δ3. It is assumed that Δ2 and Δ3 are larger than Δ1. In particular, Δ1 is assumed to be as small as approximately the same level.

In addition, a phase step 006 is provided at the boundary between the material A004 and the material B005. This phase step is set so that the refractive index difference ΔN between the material A004 and the material B005 and the step Δd with respect to λ2 is a multiple of an integer M of λ2 as shown in Equation 1.

ΔN × Δd = M × λ2 (M = 1.2.3....) [Equation 1]

Note that M is preferably 1. This is because the actual step Δd can be minimized.

  Now, it is assumed that the lens 003 is an objective lens dedicated to a BD having an NA of 0.85. In the figure, the light beam diameter φ1 indicates the diameter of the incident light beam (diameter corresponding to NA 0.85). The light beam diameter φ2 is the same diameter as that of the material A004 (diameter corresponding to NA 0.6). The light beam diameter φ3 is the same diameter as that of the region having the phase step 006 (diameter corresponding to NA 0.45). As will be described later, this is provided to make the NA of the objective lens 001 different for the light beams of λ1, λ2, and λ3.

  Next, the function of the objective lens 001 when the light beam of λ1 enters the objective lens 001 will be described with reference to FIG. In the figure, an optical disc 011 is a BD, and is composed of a surface 012 and an information surface 013. The space between the surface 012 and the information surface 013 is a transparent substrate, and the space between the surfaces t1 is 0.1 mm of BD. A light beam of λ1 (light beam 010 in the figure) incident with a light beam diameter φ1 first enters the phase plate 002. Since the refractive index of the material A004 and the material B005 of the phase plate 002 is very small with respect to λ1, the phase plate 002 is merely a transparent flat plate and gives any phase to the light beam of λ1 passing through the phase plate 002. Not. That is, the light beam of λ1 passes through the phase plate 002 and reaches the lens 003. Since the lens 003 is a BD-only lens with NA of 0.85, the light beam of λ1 that has passed through the lens 003 is condensed on the information surface 013 with minimum aberration. That is, when the light beam of λ1 enters the objective lens 001, it can be said that the phase is given only by the outer shape of the lens 003. Hereinafter, the phase imparted by the outer shape will be referred to as a phase B. From the above, the objective lens 001 functions as a BD-dedicated objective lens with NA of 0.85 for the light beam of λ1. Further, since the phase plate 002 passes through, it can be said that there is no decrease in light efficiency and high light efficiency is realized.

  Next, the function of the objective lens 001 when the light beam of λ2 is incident on the objective lens 001 will be described with reference to FIG. In the figure, an optical disk 021 is a DVD, and is composed of a surface 022 and an information surface 023. The space between the surface 022 and the information surface 023 is a transparent substrate, and the space between the surfaces t2 is 0.6 mm of DVD. A light beam of λ2 (light beam 010 in the figure) incident with a light beam diameter φ1 enters the phase plate 002 first. Since the material A004 and the material B005 of the phase plate 002 have different refractive indexes with respect to λ2, the phase is given to the light beam of λ2 that passes through the phase plate 002 by the ratio difference of the distance that has passed through the material A004 and the material B005. Is done. In addition, the phase given by the ratio difference of the distance which passed the material A004 and the material B005 in this way is hereafter described as the phase D. Further, the phase D to be applied is only when the material A004 and the material B005 are both passed through, so that λ2 is provided with the phase only in the range of the light beam diameter φ2. This corresponds to that the effective diameter of the light beam of λ2 can be changed with respect to λ1. That is, it can be said that it can be equivalent to 0.6, which is the NA of DVD different from λ1. Since the phase step 006 is an integral multiple of λ2, the phase step 006 cannot be seen with respect to the light beam of λ2.

  Now, the light beam of λ2 that has passed through the phase plate 002 and to which the phase D is given enters the lens 003. Since the lens 003 is a BD-dedicated lens similarly to λ1 with respect to the light beam of λ2, the phase B that is condensed on the information surface of the BD is given to the light beam 020 of λ2 that has passed through the lens 003. Of course, since the material of the lens 003 also has dispersion characteristics, it is not exactly the same phase as λ1, but for the sake of easy explanation, the same phase B is described. The phase D applied when λ2 passes through the phase plate 002 and the phase B applied when passed through the lens 003 are combined so that the light beam of λ2 is focused on the information surface 023 with minimum aberration. The boundary between the material A004 and the material B006 of the plate 002 is designed. Here, the phase D is a free parameter. That is, when the light beam of λ2 enters the objective lens 001, a phase in which the phase B and the phase D are combined is given by the boundary shape of the material A and the material B of the phase plate 002 and the outer shape of the lens 003. The light can be condensed on the surface 023 with minimum aberration. The path of the light beam λ2 collected at this time is indicated by a light beam 020 in the figure. As described above, the objective lens 001 functions as a DVD-only objective lens having a NA of 0.6 with respect to the light beam of λ2. Further, the phase plate 002 has a phase step 006, but since the step is small, there is no decrease in light efficiency as in the conventional objective lens for DVD and CD compatibility, and high light efficiency is realized. I can say that.

  Next, the function of the objective lens 001 when the light beam of λ3 is incident on the objective lens 001 will be described with reference to FIG. In the figure, an optical disk 031 is a CD, and is composed of a surface 032 and an information surface 033. The space between the surface 032 and the information surface 033 is a transparent substrate, and the space between the surfaces t3 is 1.2 mm of CD. A light beam of λ3 incident at a light beam diameter φ1 first enters the phase plate 002. Since the material A004 and the material B005 of the phase plate 002 have different refractive indexes with respect to λ3, the phase D of the light beam of λ3 that passes through the phase plate 002 is a ratio difference between the distances that have passed through the material A004 and the material B005. Is granted. Since the phase step 006 is not an integral multiple of λ3, the phase step 006 gives a phase to the light beam of λ3. Hereinafter, the phase imparted by the phase step 006 will be referred to as a phase C. Further, since the phase to be applied is only when the phase has passed through the region of the phase step 006, the phase is applied to λ3 within the range of the light beam diameter φ3. This corresponds to that the effective diameter of the light beam of λ3 can be changed with respect to λ1 and λ2. That is, it can be said that it can be equivalent to 0.45 which is the NA of CD different from λ1 and λ2.

  Now, the light beam of λ3 having passed through the phase plate 002 and provided with the phase D and the phase C enters the lens 003. Since the lens 003 is a BD-dedicated lens similarly to λ1 with respect to the light beam of λ3, the light beam of λ3 that has passed through the lens 003 is given a phase B that is condensed on the information surface of the BD. When the light beam of λ3 is incident on the objective lens 001, the phase difference 006 is designed such that the phase B, the phase D, and the phase C are combined and the light beam of λ3 is condensed on the information surface 033 with minimum aberration. Here, the phase C is a free parameter. That is, when the light beam of λ3 enters the objective lens 001, phase B, phase D, and phase C are given by the boundary shape of the material A and material B of the phase plate 002, the phase step 006, and the outer shape of the lens 003, and CD The information surface 033 can be condensed with minimum aberration. The path of the light beam of λ3 condensed at this time is indicated by a light beam 030 in the drawing. As described above, the objective lens 001 functions as a CD-only objective lens with NA of 0.45 with respect to the light beam of λ3. Further, the phase plate 002 has a phase step 006, but since the step is small, it can be said that a high light efficiency is realized without a decrease in light efficiency like DVD.

  As described above, λ1, λ2, and λ3 each have phase B, phase D, and phase C as free design parameters. That is, the light beams of λ1, λ2, and λ3 can be focused on the information surfaces of BD, DVD, and CD with minimum aberration by setting these parameters to optimum values. In addition, when light beams having wavelengths of λ1, λ2, and λ3 are incident, the light beam diameters that are collected with minimum aberration on the information surfaces of BD, DVD, and CD are φ1, φ2, and φ3, which are predetermined effective diameters. Can be designed. In other words, this corresponds to that a predetermined NA can be realized for each of the light beams having the wavelengths λ1, λ2, and λ3. In addition, the objective lens 001 compatible with three wavelengths does not require a large phase step, which required the conventional wavelength selectivity of three wavelengths, so that high light efficiency can be realized.

  Here, an example in which the material A004 and the material B005 have the smallest refractive index difference with respect to λ1 has been described, but the material with the material A004 and the material B005 having the smallest refractive index difference with respect to λ3 should be selected. Thus, it is of course possible to add a phase to each of λ1 and λ2.

  That is, as described above, by using this mechanism, a lens capable of imparting arbitrary phases to the three wavelengths can be realized with high efficiency. Since the phase design itself is a known technique, the details are omitted.

  A second embodiment of the present invention will be described with reference to the drawings. Here again, an objective lens that can be designed to have a predetermined performance for each of three different wavelengths will be described by taking a BD, DVD, and CD three-wavelength compatible objective lens as an example. In the first embodiment, the objective lens 001 including the phase plate 002 and the lens 003 has been described. In Example 2, an objective lens 050 in which a phase difference 002 and a lens 003 are integrated will be described.

  FIG. 5 is a schematic configuration diagram illustrating an objective lens 050 for compatibility with three wavelengths according to the second embodiment. The objective lens 050 in the second embodiment is one optical element. It is assumed that the objective lens 050 has the same external shape of a BD dedicated lens as the lens 003 of the first embodiment. The objective lens 050 is composed of two materials A051 and B052. The center wavelengths of BD / DVD / CD are λ1 = 405 nm / λ2 = 660 nm / λ3 = 785 nm, respectively. Similar to the materials A004 and B005 described in the first embodiment, materials A051 and B052 having different dispersion characteristics are used. Further, a phase step 053 is provided at the boundary between the material A051 and the material B052. This phase step is set so that the refractive index difference ΔN and the step Δd of the material A051 and the material B053 with respect to λ2 are multiples of an integer M of λ2 as shown in the above equation 1. Note that M is preferably 1. This is because the actual step Δd can be minimized.

  In the figure, the light beam diameter φ1 indicates the diameter of the incident light beam (diameter corresponding to NA 0.85). The light beam diameter φ2 is the same diameter as that of the material A051 (diameter corresponding to NA 0.6). The light beam diameter φ3 is set to the same diameter as that of the region having the phase step 053 (diameter corresponding to NA 0.45). This is provided to make the NA of the objective lens 050 different from the light beams of λ1, λ2, and λ3 as described above.

  The light beam of λ1 incident with the light beam diameter φ1 has a small difference in refractive index between the material A051 and the material B052 of the objective lens 050 with respect to λ1, and therefore the light beam of λ1 that passes through the objective lens 050 has an outer shape. Phase B is given only at. The outer shape of the objective lens 050 is a BD-dedicated objective lens with an NA of 0.85. Therefore, the light beam of λ1 that has passed through the objective lens 050 is focused on the information surface of the BD with minimum aberration. From the above, the objective lens 050 functions as a BD-dedicated objective lens with NA of 0.85 for the light beam of λ1. Further, since the difference in refractive index between the material A051 and the material B052 is very small, it can be said that the objective lens 050 does not decrease in light efficiency and achieves high light efficiency as in the case of a normal objective lens dedicated to BD.

  The light beam of λ2 incident with the light beam diameter φ1 is given a phase B in the outer shape to the light beam of λ2 passing through the objective lens 050. Further, since the material A051 and the material B052 have different refractive indices with respect to λ2, the phase D is given to the light beam of λ2 that passes through the objective lens 050 by the ratio difference of the distance that passed through the material A051 and the material B052. . The light beam of λ2 is given a phase D only in a region that has passed both the material A051 and the material B052. This corresponds to that the effective diameter of the light beam of λ2 can be changed with respect to λ1. That is, it can be said that it can be equivalent to 0.6, which is the NA of DVD different from λ1. Since the phase step 053 is an integral multiple of λ2, the phase step 053 cannot be seen with respect to the light beam of λ2.

  Now, the boundary between the material A051 and the material B052 is designed so that the phase B and the phase D given when λ2 passes are focused on the information surface of the DVD with minimum aberration. That is, when the light beam of λ2 enters the objective lens 050, a phase is given by the boundary shape and the outer shape of the material A051 and the material B052 of the objective lens 050, and the light can be condensed on the information surface of the DVD with minimum aberration. Here, the phase D is a free parameter.

  From the above, the objective lens 050 functions as a DVD-only objective lens with a NA of 0.6 with respect to the light beam of λ2. Further, although there is a phase step 053, it can be said that high light efficiency is realized without a decrease in light efficiency as in the case of a conventional objective lens compatible with DVD and CD.

  The light beam of λ3 incident with the light beam diameter φ1 is given a phase B in the outer shape to the light beam of λ2 passing through the objective lens 050. Further, since the material A051 and the material B052 have different refractive indexes with respect to λ3, the phase D is given to the light beam of λ3 that passes through the objective lens 050 by the ratio difference of the distance that passed through the material A051 and the material B052. . Further, since the phase step 053 is not an integral multiple of λ3, the phase C is given to the light beam of λ3. Further, since the phase C given by the phase step 053 is only in the region having the phase step 053, the phase C is given in the range of λ3 within the light beam diameter φ3. This is because the light beam of λ3 can be set to an arbitrary effective diameter with respect to λ1 and λ2. That is, it can be said that it can be equivalent to 0.45 which is the NA of CD different from λ1 and λ2.

  Now, the boundary of the phase step 053 is designed so that the phase B, phase D, and phase C added when λ3 passes are focused on the information surface of the CD with minimum aberration. Here, the phase C is a free parameter. That is, when the light beam of λ3 enters the objective lens 050, phase B, phase D, and phase C are given by the boundary shape, outer shape, and phase step 053 of the material A051 and material B052 of the objective lens 050, and the information surface of the CD Can be focused with minimum aberration. As described above, the objective lens 050 functions as a CD-only objective lens with NA of 0.45 with respect to the light beam of λ3. Further, although there is a phase step 053, since the step is small, it can be said that there is no decrease in light efficiency like DVD, and high light efficiency is realized.

  As described above, similarly to the objective lens 001, the objective lens 050 collects light with the minimum aberration on the information surfaces of the BD, DVD, and CD at a predetermined NA when a light beam having wavelengths of λ1, λ2, and λ3 is incident. It becomes possible to make it light. Moreover, the objective lens 050 can be realized by a single optical element. As described above, the three-wavelength compatible objective lens 050 does not require the phase step that required the conventional wavelength selectivity of the three wavelengths. For this reason, the step of the phase step can be reduced, and the light efficiency can be greatly improved similarly to the objective lens 001.

  Embodiment 3 of the present invention will be described in detail with reference to the drawings. Here, description will be made assuming an optical head compatible with BD, DVD, and CD. FIG. 6 is a schematic diagram illustrating the configuration of the optical head 100 according to the third embodiment. Again, the center wavelengths of BD / DVD / CD are λ1 = 405 nm / λ2 = 660 nm / λ3 = 785 nm, respectively. In the figure, a broken line 103 indicates the optical path of the light beam.

  First, the BD optical system will be described. The light source 101 emits a light beam of λ1 as divergent light. The light beam emitted from the light source 101 is reflected by the beam splitters 104 and 105 and converted into a parallel light beam by the collimator lens 106. The collimating lens 106 is mounted on the spherical aberration correction mechanism and can be driven in the direction of the arrow 107. A spherical aberration correction mechanism for driving the collimator lens 106 in the optical axis direction (the direction of the arrow 107) can be realized by using an electromagnetic motor, a piezoelectric element, or the like. In a BD optical disc, spherical aberration occurs due to an error between the surfaces. For this reason, the optical head needs a spherical aberration correction function for correcting the spherical aberration. In general, the spherical aberration adjustment function can be realized by driving the collimating lens 106 in the optical axis direction. For this reason, it is assumed that the collimating lens 106 is equipped with a spherical aberration correction mechanism that can be moved in the optical axis direction. The spherical aberration adjustment function can also be realized by a liquid crystal element having a concentric pattern. For this reason, a liquid crystal element may be disposed in the vicinity of the collimating lens 106 instead of the spherical aberration correction mechanism.

  The light beam that has traveled through the collimating lens 106 is incident on the objective lens 001, and is condensed and irradiated onto a predetermined information surface of the BD (the optical disk 109 is described in the figure). The objective lens 001 is assumed to be the objective lens of Example 1, and is composed of a lens 003 and a phase plate 002. The holder 108 is a housing that supports the lens 003 and the phase plate 002. In order to remove the outside beam diameter larger than the NA of λ1, the holder 108 may have an opening of a predetermined size and restrict the opening of the BD. That is, the light beam of λ1 is focused on the predetermined information surface of the BD with the minimum aberration by the objective lens 001. The objective lens 001 is mounted on an actuator (not shown), and can be driven in a normal direction and a radial direction of a predetermined information surface of the optical disc. The radial direction is used for light beam track control and driving during lens shift, and the normal direction is used for focus control.

  The light beam reflected by the predetermined information surface proceeds in the order of the objective lens 001, the collimating lens 106, the beam splitter 105, and the detection lens 110, and is detected by the photodetector 111. The detection lens 110 includes a cylindrical lens and a spherical lens. When the light beam passes through the detection lens 110, a predetermined astigmatism is given in about 45 directions, which is used for detecting an FES by an astigmatism method. The The detection lens 110 functions to determine the diameter of the light spot on the photodetector 111 at the same time as rotating the direction of astigmatism in an arbitrary direction.

    Next, DVD and CD optical systems will be described. The light source 102 is a two-wavelength multi-laser light source that emits light beams of λ2 and λ3 as divergent light. The divergence angle of the light beam emitted from the light source 102 is corrected by the correction lens 112. Next, the light passes through the beam splitter 104, reflects off the beam splitter 105, and is converted into a parallel light beam by the collimator lens 106. In general, the magnification (the focal length ratio of the objective lens and the collimating lens) differs between the BD optical system and the DVD / CD optical system. For this reason, in the DVD and CD optical systems, it is assumed that the correction lens 112 is arranged and the magnification is virtually changed by the combination of the collimating lens 106 and the correction lens 112. The beam splitter 104 has a function of reflecting the light beam of λ1 and transmitting λ2,3. Such a beam splitter can be realized by a dichroic mirror. The beam splitter 105 has a function of reflecting the light beam coming from the beam splitter 104 side regardless of the wavelength and transmitting the light beam traveling to the photodetector 111. Such a beam splitter can generally be realized by a PBS prism using polarization. As described in the first embodiment, the light beams having the wavelengths λ2 and λ3 are collected by the objective lens 001 on a predetermined information surface of the DVD or CD (indicated by the optical disk 109 in the figure) with minimum aberration. The light beam reflected on the predetermined information surface proceeds in the order of the objective lens 001, the collimator lens 106, the beam splitter 105, and the detection lens 110, and is detected by the photodetector 111 in the same manner as the BD.

  As described above, by using the objective lens 001, the optical head 100 having a simple configuration and compatible with BD, DVD, and CD can be realized. The optical head 100 uses a highly efficient objective lens 001, so that the output of the light source can be as high as when a BD-dedicated objective lens and a DVD / CD compatible objective lens are used, and the cost and reliability of the light source are sufficient. Is good. Further, the S / N of the same level as the case where a BD-dedicated objective lens and a DVD / CD compatible objective lens are used can be secured for the reflected light beam to the photodetector 111.

  Embodiment 3 of the present invention will be described in detail with reference to the drawings. Here, description will be made assuming an optical head compatible with BD, DVD, and CD.

  FIG. 7 is a schematic diagram showing the configuration of the optical head 200 in the fourth embodiment. In the figure, a broken line 203 indicates the optical path of the light beam. The light source 201 is a three-wavelength laser light source that emits light beams of λ1, λ2, and λ3 as divergent light. Each light beam emitted from the light source 201 passes through the beam splitter 205, passes through the correction phase plate 213, and is converted into a parallel light beam by the collimator lens 206. The beam splitter 205 has a function of transmitting the light beam coming from the light source 201 side regardless of the wavelength and reflecting the light beam traveling to the photodetector 211. Such a beam splitter can generally be realized by a PBS prism using polarization. The correction phase plate 213 has a function of converting the magnification by combination with the collimating lens 206. That is, the correction phase plate 213 corresponds to the phase plate 002 of the objective lens 001, and the collimating lens 206 corresponds to the lens 003. For example, λ1 is incident by the combination of the correction phase plate 213 and the collimating lens 206. It is possible to realize an optical system in which a lens having an optimum magnification at BD, a lens having an optimum magnification at DVD when λ2 is incident, and a lens having an optimum magnification at CD when λ3 is incident can be realized. By using the correction phase plate in this way, it is possible to design the CD and DVD magnifications independently of the optical head 100 of the third embodiment, and the CD is more efficient than the optical head 100 of the third embodiment. Can improve.

  Now, the collimating lens 206 is mounted on the spherical aberration correction mechanism and can be driven in the direction of the arrow 207. A spherical aberration correction mechanism for driving the collimating lens 206 in the optical axis direction (the direction of the arrow 107) can be realized by using an electromagnetic motor, a piezoelectric element, or the like. Since the objective lens 050 is assumed to be the objective lens according to the second embodiment, the light beam having each wavelength traveling through the collimator lens 206 is incident on the objective lens 050, and each of the predetermined information surfaces (see FIG. The medium optical disk 209 is described) and focused and irradiated with minimum aberration. The objective lens 050 is mounted on an actuator (not shown), and can be driven in a normal direction and a radial direction of a predetermined information surface of the optical disc. The radial direction is used for light beam track control and driving during lens shift, and the normal direction is used for focus control.

  The light beam reflected by the predetermined information surface proceeds in the order of the objective lens 050, the collimating lens 206, the correction phase plate 213, the beam splitter 205, and the detection lens 210, and is detected by the photodetector 211. The detection lens 210 is composed of a cylindrical lens and a spherical lens. When the light beam passes through the detection lens 210, a predetermined astigmatism is given in about 45 directions, which is used for detecting FES by the astigmatism method. The The detection lens 210 functions to determine the diameter of the light spot on the photodetector 211 at the same time as rotating the direction of astigmatism in an arbitrary direction.

  As described above, a very simple optical head 200 compatible with BD, DVD, and CD can be realized. The optical head 200 uses a highly efficient objective lens 050, so that the output of the light source is about the same as when using an objective lens dedicated to BD and an objective lens compatible with DVD / CD. It can be said that the property is good. Further, the S / N of the same level as the case where a BD-dedicated objective lens and a DVD / CD compatible objective lens are used can be secured for the reflected light beam to the photodetector 111. Furthermore, by using the mechanism that can arbitrarily design the phase of the three wavelengths according to the present invention, BD, DVD, and CD can be set to a predetermined magnification, and the efficiency of CD can be improved over the conventional DVD / CD compatible optical head. become. Note that the conventional DVD / CD compatible optical system refers to an optical system using a two-wavelength multi-laser light source and a collimating lens, which are currently common. In the first to fourth embodiments, the NA is described as a predetermined value. However, the NA can be arbitrarily designed by changing the diameter of the material A and the diameter of the phase step.

  Further, it has been described that in the objective lenses 001 and 050 in the present embodiment, the diameter of the phase step and the diameter of the material A are determined so that a predetermined NA is obtained. However, a structure in which a phase different from that of the region smaller than the NA is given to the region larger than the predetermined NA may be provided. It is possible to design so that the light beam entering the region larger than the NA does not become an unnecessary light beam in the detection system with the phase provided in the region larger than the predetermined NA. In this embodiment, an astigmatism optical head is described as an example, but an optical system using a knife edge or a spot size may be used. Further, it is possible to add a mirror that is not included in the optical system of this embodiment to bend the optical path.

  As described above, when the light beam 1 having the wavelength 1, that is, λ1, is incident, the predetermined phase 1, that is, the phase B is given, and when the light beam 2, that is, the wavelength 2, that is, λ2, is incident, the predetermined phase 2, that is, the phase B + An objective lens that imparts a phase difference D and a predetermined phase difference 3, that is, phase B + phase D + phase C, when the light beam 3 of wavelength 3, that is, λ3, is incident. The material A and the material B have the same refractive index difference Δ1 with respect to the light beam 1 and are very small, and the material A and the material B have a refractive index difference Δ2 with respect to the light beam 2 and the light beam 3. Is larger than Δ1, and there is a phase step in a predetermined region at the boundary between material A and material B. The phase generated at the phase step at the boundary between material A and material B is an integer multiple of wavelength 2.

  Further, the phase B imparted to the light beam 1 incident on the objective lens is generated by the outer shape, and the phase B + phase D imparted to the light beam 2 incident on the objective lens is determined by the outer shape and the material A. The phase B, the phase D, and the phase C that are generated by the refractive index difference between the material A and the material B and are applied to the light beam 3 incident on the objective lens are determined by the outer shape, the refractive index difference between the material A and the material B, and the phase step. generate.

  Further, by making Δ1 substantially the same, a theoretically complete three-wavelength compatible objective lens can be realized. For this reason, it is preferable to set Δ1 to be substantially zero. The objective lens can also be realized by a combination lens separated into a lens having a lens shape, that is, a lens 003, and a flat plate having materials A and B, that is, a phase plate 002.

  In Examples 1 and 2, the phase B generated by the outer shape, the phase D generated by the difference in refractive index of the material, and the phase C generated by the phase step are phase B with respect to the wavelength λ1, phase D with respect to λ2, The example in which the phase C is optimally designed independently with respect to λ3 has been described. However, as a matter of course, it does not matter if the phase B, the phase D, and the phase C are combined and optimized for λ1, λ2, and λ3. For example, using the fact that the phase B, the phase D, and the phase C are different depending on the wavelength, the phase B and the phase D are optimally combined for λ1 and λ2, and the phase C is used only for λ3. May be set optimally.

001 ... Objective lens 002 ... Phase plate 003 ... Lens 004 ... Material A
005 ... Material B
006 ... Phase step 010 ... Light beam diameter 011 ... Optical disc 012 ... Surface 013 ... Information surface 020 ... Light beam diameter 021 ... Optical disc 022 ... Surface 023 ... Information surface 030 ... Light beam diameter 031 ... Optical disc 032 ... Surface 033 ... Information surface 050 ... Objective lens 051 ... Material A
052 ... Material B
053 ... Phase difference 100 ... Optical head 101 ... Light source 102 ... Light source 103 ... Optical axis 104 ... Beam splitter 105 ... Beam splitter 106 ... Collimating lens 107 ... Drive shaft 108 ... Holder 109 ... Optical disc 110 ... Detection lens 111 ... Photo detector 112 ... Correction lens 200 ... Optical head 201 ... Light source 203 ... Optical axis 205 ..Beam splitter 206 ... Collimator lens 207 ... Drive shaft 209 ... Optical disc 210 ... Detection lens 211 ... Photo detector 213 ... Correction phase plate

Claims (7)

When the light beam of the first wavelength is incident, the first phase is given. When the light beam of the second wavelength is incident, the second phase is given.
An objective lens that imparts a third phase when a light beam of a third wavelength is incident;
The objective lens is composed of at least material A and material B,
The difference between the refractive index of the material A and the material B with respect to the light beam of the first wavelength is Δ1,
The difference between the refractive index of the material A and the material B with respect to the light beam of the second wavelength is Δ2,
The material A and the material B have a difference in refractive index of Δ3 with respect to the light beam of the third wavelength,
Δ2 and Δ3 are larger than Δ1,
An objective lens having a phase step in a predetermined region at a boundary between the material A and the material B. The objective lens according to claim 1,
2. The objective lens according to claim 1, wherein the phase generated at the phase step at the boundary between the material A and the material B is an integer multiple of the second wavelength. The objective lens according to claim 2,
The phase imparted to the light beam of the first wavelength incident on the objective lens is generated by the outer shape,
The phase imparted to the light beam of the second wavelength incident on the objective lens is generated by the outer shape and the refractive index difference between the material A and the material B,
The objective lens is characterized in that the phase imparted to the light beam of the third wavelength incident on the objective lens is generated by the outer shape, the material A, the refractive index difference of the material B, and the phase step. The objective lens according to claim 3,
Δ1 is substantially zero, the objective lens The objective lens according to claim 4,
The objective lens according to claim 1, wherein the objective lens is a combination lens separated into a lens having an outer shape and a flat plate having the materials A and B. An optical head equipped with the objective lens according to claim 3, 4 or 5,
The optical head is
A light source that emits a light beam having the first to third wavelengths;
An optical head comprising at least one photodetector for detecting reflected light from the optical disc of the light beams having the first to third wavelengths. The optical head according to claim 6, wherein
2. An optical head according to claim 1, wherein the light source for emitting the light beams having the first to third wavelengths is a three-wavelength light source in one housing.

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