JP2004079050A - Optical head and optical recording medium reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head for achieving both miniaturization and good tilt characteristics, and to provide an optical recording medium reproducing device. <P>SOLUTION: The optical head is provided with a condensing objective lens having a first surface formed into a projected aspheric shape, a second surface formed into a roughly planar shape and a numerical aperture smaller than 1, and a solid immersion lens having a third surface formed into a projected aspheric shape, a fourth surface formed into a roughly planar shape on which light passed through the condensing objective lens is condensed, and a refractive index larger than 1. Since the first and third surfaces are aspheric, condensing performance is hardly deteriorated even when the optical head is tilted with respect to the surface of an optical recording medium. Since the second and fourth surfaces are planar, the application of a microfabrication technology such as dry etching and wet etching is facilitated. Thus, by simultaneously improving tilt characteristics and facilitating microfabrication, the manufacturing of the lens compact and good in tilt characteristics is facilitated. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical head for reproducing information recorded on an optical recording medium, and an optical recording medium reproducing device.
[0002]
[Prior art]
2. Description of the Related Art An optical recording medium reproducing apparatus (optical disk system) for reading (reproducing) information recorded on an optical recording medium such as a CD (Compact Disk) and a DVD (Digital Versatile Disk) by an optical means is used. By irradiating the recording layer of the optical recording medium with light and detecting the intensity or the like of the light reflected from the recording layer, information is read from the optical recording medium (and writing is performed as necessary).
Here, in recent years, an optical head of a system for reading and writing information from an optical recording medium using near-field light has been receiving attention. In this type of optical head, a condensing optical system combining a light source side objective lens with a numerical aperture NA smaller than 1 and a solid immersion lens (SIL: Solid Immersion Lens) on an optical recording medium side with a numerical aperture NA larger than 1 is used. Is used.
By bringing the bottom surface of the solid immersion lens and the surface of the optical recording medium close to each other to about 100 nm or less and optically coupling (evanescent coupling) with evanescent light, it is possible to realize a numerical aperture NA exceeding 1 as the entire condensing optical system. . Since it has a high numerical aperture NA, the focus (spot size) of the light beam can be narrowed, and reading and writing at a high recording density can be realized.
[0003]
[Problems to be solved by the invention]
Here, as a method of keeping the bottom surface of the solid immersion lens and the surface of the optical recording medium close to each other, it is conceivable that the optical head is of a floating type used in a hard disk drive (HDD) or the like. For this purpose, it is necessary to reduce the size of the objective lens and the solid immersion lens. It is difficult to produce such a small lens by, for example, a general molding technique, and it is conceivable to use a fine processing technique such as dry etching or wet etching.
On the other hand, in order to apply dry etching or wet etching, it is preferable that one surface of the lens is a plano-convex lens having a flat surface.
However, if the lens has a plano-convex shape, if the incident light incident on the optical recording medium has a tilt (tilt), there is a fear that the light condensing property of the lens is greatly reduced.
The present invention has been made in order to solve such a problem, and an object of the present invention is to provide an optical head and an optical recording medium reproducing apparatus capable of achieving both miniaturization and good tilt characteristics.
[0004]
[Means for Solving the Problems]
A. In order to achieve the above object, an optical head according to the present invention has a convex aspheric first surface on which light from a light source is incident, and a substantially planar second surface, and has a numerical aperture. A converging objective lens smaller than 1, a convex aspherical third surface facing the second surface, and light from the light source facing the optical recording medium and passing through the converging objective lens. And a solid immersion lens having a substantially planar fourth surface for condensing light and having a refractive index of greater than 1.
Since one of the surfaces (first and third surfaces) of the converging objective lens and the solid immersion lens is aspherical, even if the optical head is tilted (there is a tilt) with respect to the surface of the optical recording medium, the light is collected. The light performance can be hardly reduced.
Further, since the other surfaces (second and fourth surfaces) of the condensing objective lens and the solid immersion lens are flat, it is easy to apply a fine processing technique such as dry etching or wet etching. That is, a lens can be formed by etching one surface of the optical material. At this time, the surface on the etched side is an aspherical surface, and the surface on the unetched side is a plane.
As described above, by making both the improvement of the tilt characteristics and the easiness of fine processing compatible, it is possible to easily produce a lens having a small size and excellent tilt characteristics.
The numerical aperture of the optical head as a whole is determined by the product of the numerical aperture of the focusing objective lens and the refractive index of the solid immersion lens. That is, the solid immersion lens aims to improve the numerical aperture of the entire optical head by its refractive index, and does not need to consider the numerical aperture of itself.
[0005]
(1) The optical head may be a floating optical head that floats from the optical recording medium by a flow of air generated by relative movement with the optical recording medium.
The use of the floating optical head enables stable reading even when the optical head scans the optical recording medium.
Note that “relative” means that one or both of the optical recording medium and the optical head may be moved.
[0006]
(2) The optical head may further include a focusing position adjusting unit that adjusts a focusing position of the focused light from the optical head in a thickness direction of the optical recording medium. It becomes easy to focus light to an appropriate depth (position) of the optical recording medium.
The focusing position adjusting means can be configured using, for example, any one of a variable refractive index element and a wedge prism.
[0007]
(3) The optical head may further include aberration correction means for correcting aberration of the optical head. The convergence of light to the optical recording medium is improved.
The focusing position adjusting means can be configured using, for example, a small liquid crystal element.
[0008]
B. An optical recording medium reproducing apparatus according to the present invention has a light source that emits light, a first surface having a convex aspheric surface on which light from the light source is incident, and a second surface having a substantially planar shape, A condenser objective lens having a numerical aperture smaller than 1, a convex aspherical third surface facing the second surface, and the light source facing an optical recording medium and passing through the condenser objective lens And a solid immersion lens having a refractive index greater than 1 and a solid immersion lens having a refractive index greater than 1. An optical member for converging return light returned from an optical recording medium, and a light receiving unit for receiving light converged by the optical member are provided.
[0009]
Since one surface (first and third surfaces) of the condensing objective lens and the solid immersion lens is aspherical and the other surface (second and fourth surfaces) is flat, light on two surfaces of each lens is used. It is not necessary to align the axes, and it is possible to achieve both improvement in tilt characteristics and ease of fine processing when viewed as a whole light collecting system. That is, it is possible to provide an optical recording medium reproducing apparatus using a small lens having a good tilt characteristic.
The “return light” may include any of light transmitted, reflected, or generated in the optical recording medium. That is, any of transmitted light, reflected light, and generated light from the recording layer can be used for reading information from the recording layer.
Here, the optical recording medium may be built in the optical recording medium reproducing apparatus itself, or may be detachably mounted on a stage provided in the optical recording medium reproducing apparatus.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(1st Embodiment)
In the present embodiment, an optical recording medium reproducing apparatus 100 using a floating optical head 80 for reading information from the optical recording medium 10 is shown.
A. First, the configuration of the optical recording medium 10 will be described.
The optical recording medium 10 may be any of read-only (read only), write-once (write-once: write-once), and rewritable (rewritable). That is, the optical recording medium reproducing device 100 may perform any of reading, writing, and rewriting from the optical recording medium 10 in some cases.
The optical recording medium 10 can have a plurality of variations depending on how tracking is performed. 1A and 1B are enlarged sectional views of optical recording media 10A and 10B according to a configuration example of the optical recording medium 10, respectively.
[0011]
The optical recording medium 10A has a plurality of recording layers 11A and a lowermost groove layer 12A. Information is recorded on the recording layer 11A of the optical recording medium 10A in an optically readable form. Reflection type recording pits (or recording marks) are recorded on the recording layer 11A. The recording pits may be of a transmission type depending on the case. When light enters the recording pits, reflection or transmission of light occurs, and information can be read from the intensity of the reflected light or transmitted light.
[0012]
It is also possible to generate light (for example, fluorescence) different from the wavelength of the incident light from the recording pit, and read information using the generated light. This generated light may be either coherent light having coherence or incoherent light having no coherence.
Since ordinary fluorescent light is incoherent light, it is possible to generate incoherent light in accordance with information recorded from the recording layer 11 by scattering a fluorescent substance on the recording layer 11 and using this as recording pits.
[0013]
The groove layer 12A is a layer in which a guide groove (groove) serving as a reference for performing tracking when reading information recorded on the recording layer 11 or a structure similar thereto is formed. Tracking can be performed using any of reflection, transmission, and generation of light from the groove.
[0014]
On the other hand, the optical recording medium 10B has a plurality of recording layers 11B, but does not have a groove layer unlike the optical recording medium 10A. Instead, a guide groove (groove) as a reference for tracking is formed in each of the recording layers 11B or a structure similar thereto.
As the optical recording medium 10, any of the optical recording media 10A and 10B may be used.
[0015]
B. Next, the floating type optical head 80 will be described.
FIG. 2 is a schematic diagram illustrating a configuration example of the flying optical head 80.
The floating optical head 80 performs recording and / or reproduction of a signal on the optical recording medium 10 by using near-field optics. As shown in the drawing, a slider member 81, a second group lens 82, a suspension 83 and a focus adjustment mechanism 84.
Although the aberration correction element 126 is provided in the optical path of the light incident on the floating optical head 80, it can be downsized and installed in the floating optical head 80.
The floating optical head 80 is a light collecting optical system that collects light on the optical recording medium 10 and is connected to the optical system of the optical recording medium reproducing device 100 through parallel light.
[0016]
The slider member 81 is supported by the suspension 83 so as to face the optical recording medium 10 similarly to a head slider used in a hard disk device or the like. Thus, when writing or reading a signal to or from the optical recording medium 10, the optical recording medium 10 receives an airflow generated between the optical recording medium 10 due to rotation of the optical recording medium 10 and the like, so that the optical recording medium 10 has a thickness of about 50 to 100 nm. The vehicle can travel while levitating at the flying height (distance d).
The slider member 81 is provided with a through hole 81a penetrating at a predetermined position in the thickness direction. The second group lens 82 is disposed in the through hole 81a.
[0017]
The second group lens 82 includes an objective lens 85 and a solid immersion lens (SIL) 86, and can realize one or more numerical apertures NA.
The objective lens 85 is a light source-side lens having a first surface 85a having one aspherical convex surface and a second surface 85b having the other surface substantially flat.
The solid immersion lens 86 has a third surface 86a whose surface facing the objective lens 85 is an aspherical convex surface, and a fourth surface 86b whose other surface is substantially flat. The lens on the side facing the medium 10.
Due to their shapes, the objective lens 85 and the solid immersion lens 86 can be miniaturized using microfabrication technology, and can suppress an increase in aberration when the lens is inclined (tilt) with respect to the optical axis. It is. The details will be described later.
[0018]
The solid immersion lens 86 utilizes the oozing of evanescent light due to near-field optics between the fourth surface 86b and the optical recording medium 10 by bringing the fourth surface 86b and the optical recording medium 10 close to each other. By setting the distance d between the fourth surface 86b of the solid immersion lens 86 and the optical disk to about 100 nm or less, a numerical aperture NA of 1 or more is realized, and reading of the optical recording medium 10 with high resolution or the like is possible. And
[0019]
When the fourth surface 86b and the optical recording medium 10 are brought close to each other, the inflow and outflow of light between the fourth surface 86b and the optical recording medium 10 are performed by seepage of evanescent light. . As a result, the refractive index of the air layer between the fourth surface 86b and the optical recording medium 10 does not affect the light passing through the air layer, and the medium that transmits the light is substantially solid immersion. Only the lens 86 is provided.
Since the numerical aperture NA is proportional to the refractive index n of the medium, and the refractive index n of the solid immersion lens 86 is larger than the refractive index (n0 = 1) of the air layer, the numerical aperture NA can be increased.
[0020]
The suspension 83 can maintain an appropriate floating amount of the slider member 81 by providing a suspension spring having elasticity between itself and the slider member 81.
As the focus adjustment mechanism 84, for example, a variable refractive index element whose refractive index changes by application of a voltage or the like can be used. The variable refractive index element can be configured using a liquid crystal element or the like. By changing the refractive index, the optical path length changes, and the position of the focal point can be moved to each recording layer 11. As a result, it is possible to appropriately select the recording layer 11 on which light is focused.
Further, by controlling the position of the focal point based on the focus error signal, it becomes possible to focus on the optimal position for each of the recording layers 11.
[0021]
(Details of the objective lens 85 and the solid immersion lens 86)
FIG. 3 is a side view showing an objective lens OL and a solid immersion lens SIL which are configuration examples of the objective lens 85 and the solid immersion lens 86 according to the present invention.
As described above, the objective lens OL has an aspheric surface on the upper surface (S1) and a convex flat shape with a lower surface (S2) being flat. On the other hand, the solid immersion lens SIL has a combination of an aspherical surface (region near the center: S3) and a flat surface (peripheral region of the aspherical region) on the upper surface, and has a flat and convex lower surface. However, since light is not expected to pass through a planar region on the upper surface of the solid immersion lens SIL, there is no substantial effect on optical characteristics.
[0022]
Specifically, the objective lens OL has a convex flat shape in which the constituent material is GaP, the numerical aperture NA is 0.8, the diameter D1 is 0.5 mm, and the center thickness T1 is 0.3 mm. The solid immersion lens SIL is made of GaP (refractive index n = 3.29 (when wavelength λ: 650 nm)), diameter D2 is 0.5 mm, and center thickness T2 (the thickness of the aspherical portion) is 0. 1 mm.
The objective lens OL and the solid immersion lens SIL are arranged such that the distance L between the upper surface of the objective lens OL and the lower surface of the solid immersion lens SIL is 0.447 mm, and the effective numerical aperture NA as a whole is 2.6. A light focusing system with an effective diameter D0 of 0.376 mm (at a wavelength of 650 ± 10 nm) is realized.
[0023]
FIG. 4 is a side view showing an objective lens OL0 and a solid immersion lens SIL0, which are comparative examples of the objective lens 85 and the solid immersion lens 86.
The objective lens OL0 has a biconvex shape in which both the upper surface (S1) and the lower surface (S2) are aspherical. The solid immersion lens SIL0 has a convex flat shape in which the upper surface (S3) is spherical and the lower surface (S4) is flat.
[0024]
Specifically, the objective lens OL0 has a convex and flat shape in which the constituent material is NbFD82 glass, the numerical aperture NA is 0.6, the diameter D1 is 1.4 mm, and the center thickness T1 is 0.75 mm. The solid immersion lens SIL is made of glass of NbFD82 (refractive index n = 1.8 (when wavelength λ: 650 nm)), numerical aperture NA is 1.1, diameter D2 is 0.5 mm, and center thickness T2 is 0.25 mm.
The objective lens OL0 and the solid immersion lens SIL0 are arranged such that the distance L between the upper surface of the objective lens OL0 and the lower surface of the solid immersion lens SIL0 is 1.065 mm, and the effective numerical aperture NA is 1.1 as a whole. A light focusing system having an effective diameter D0 of 0.9 mm (at a wavelength of 650 ± 10 nm) is realized.
[0025]
As described above, the embodiment of FIG. 3 and the comparative example of FIG. 4 are different from each other in that the objective lenses OL1 and OL0 are convex / planar and biconvex, respectively, and that the top surfaces of the solid immersion lenses SIL and SIL0 are aspherical and spherical, respectively. Is different. The size of the comparative example of FIG. 4 is larger than that of the case of FIG. 3 in consideration of manufacturing by a mold.
[0026]
FIG. 5 is a graph showing the tilt characteristics of the embodiment of FIG. 3 and Comparative Examples 1 and 2 in comparison.
The horizontal axis of the graph in FIG. 5 represents the incident angle (tilt angle) of light with respect to the axes of the objective lens OL and the solid immersion lens SIL, and the vertical axis represents the amount of aberration.
The solid line is the embodiment of FIG. 3 (the objective lens is convex and flat, the top surface of the solid immersion lens is aspherical, the effective numerical aperture NA: 2.6), and the dotted line is Comparative Example 1 (the objective lens is convex and flat, the solid immersion lens). Is an example of a spherical upper surface, which corresponds to an effective numerical aperture NA: 2.6). A dashed line corresponds to Comparative Example 2 (an example in which the objective lens shown in FIG. 4 is biconvex and the top surface of the solid immersion lens is spherical, and the effective numerical aperture NA is 1.1).
[0027]
Comparative example 1 is different from the example in that the upper surface of the solid immersion lens is spherical. This is to clarify the effect of the difference between the upper surface of the SIL and the spherical surface on the tilt characteristics.
In Comparative Example 2, as described above, the upper surface of the solid immersion lens is spherical, while both surfaces of the objective lens are aspherical. This is to compare the example with an example having good tilt characteristics (Comparative Example 2).
[0028]
In the case of the embodiment of FIG. 3, even if the incident angle changes from 0 ° (when light is incident parallel to the axes of the objective lens and the solid immersion lens) to about 3 °, the amount of spherical aberration is 0.04λ (λ Is smaller than the wavelength), whereas in the case of Comparative Example 1, it is larger than 0.3λ at an incident angle of 3 °.
It can be seen that the tilt characteristics of the example are as good as the tilt characteristics of the comparative example 2 when the tilt characteristics are good and as good as the tilt characteristics.
[0029]
A lens having a small size and good tilt characteristics as in the embodiment can be manufactured by a fine processing technique. Here, in the lens of the embodiment, since one surface of the objective lens is flat, it is easy to apply the fine processing technology.
In this method, a lens can be manufactured by subjecting the upper surface of an optical material (for example, GaP) to dry etching or wet etching by photolithography. For example, a desired shape (aspherical surface) can be obtained by partially adjusting the amount of exposure by partially changing the amount of exposure using a gray scale mask. At this time, the lower surface of the optical material that has not been etched is generally flat.
It should be noted that the specific contents of this technique are described in literatures (for example, W. Dashner, P. Long, R. Stein, C. Wu, and SH Lee: "Cost-effective mass fabricating abstraction derivative illustrations." In a single optical exposure with a gray scale mask on high-energy beam-sensitive glass ", Appl. Opt. Vol. 36 (1997) No. 20, p.
[0030]
The characteristics of the objective lens OL and the solid immersion lens SIL are summarized as follows.
(1) Since the objective lens OL and the solid immersion lens SIL can be formed in a plano-convex shape, a small lens is manufactured using a fine processing technique such as dry etching or wet etching (one-time etching is also possible). can do.
[0031]
(2) Since both the objective lens OL and the solid immersion lens SIL can be miniaturized, a floating optical head can be configured using a small slider (nano slider). This optical head can stably maintain the lower surface of the solid immersion lens SIL close to the optical recording medium 10 (for example, at an interval of about 100 nm or less). As a result, the optical head can function as a light-collecting system for near-field light recording / reproduction using near-field light (evanescent light).
[0032]
Here, the numerical aperture NA of the entire floating optical head 80 is determined by the product of the numerical aperture NA of the objective lens 85 and the refractive index n of the solid immersion lens 86. As a result, one or more numerical apertures NA can be realized by combining the high numerical aperture objective lens 85 and the high refractive index solid immersion lens 86. Since the solid immersion lens 86 is optically coupled to the optical recording medium 10 by evanescent light (so-called light leaching), its own numerical aperture NA does not matter.
[0033]
(3) By combining the shapes of the upper surfaces of both the objective lens OL and the solid immersion lens SIL with appropriate aspherical coefficients, it is equivalent to using a biconvex aspherical objective lens (for example, Comparative Example 2). , Or higher tilt characteristics. That is, it is possible to increase the margin of the incident angle as an optical system in which the variation of the aberration with respect to the incident angle (tilt angle) is small.
[0034]
(Operation of the floating optical head 80)
The incident light from the objective lens 85 side is optically recorded via the objective lens 85, the focus adjustment mechanism 84, the solid immersion lens 86, and the air layer (distance d) between the floating optical head 80 and the optical recording medium 10. The light enters the recording layer 11 of the medium 10. The light reflected by the recording layer 11 exits the objective lens 85 as return light through the reverse path.
As will be described later, aberrations (mainly spherical aberrations) caused by the movement of the recording layer 11 are appropriately corrected by the aberration correction element 126.
[0035]
By adjusting the focal position and correcting the aberration as described above, it is possible to appropriately condense light on a desired recording layer 11, that is, read and write information.
In addition, by using near-field optics, a numerical aperture NA of 1 or more can be realized, and high-resolution reading and writing on the optical recording medium 10, that is, an increase in the capacity of the optical recording medium 10 can be achieved.
[0036]
(Modification of the flying optical head 80)
The focus adjustment mechanism 84 can also be realized by means other than the variable refractive index element.
FIG. 6 is a schematic diagram illustrating a configuration of a floating optical head 80A in which a focus adjustment mechanism is realized by a pair of wedge prisms 87a and 87b.
The wedge-shaped portions of the wedge (wedge-shaped) prisms 87a and 87b are vertically overlapped to constitute a plate-like member 87 as a whole. By adjusting the relative positions of the wedge prisms 87a and 87b, the thickness of the plate member 87 can be changed. This change in thickness causes a change in the optical path length of light passing through the plate-like member 87. As a result, the focal position can be adjusted by moving one or both of the wedge prisms 87a and 87b.
[0037]
The focus adjusting mechanism 84 can be provided separately from the floating optical head 80.
FIG. 7 is a schematic diagram illustrating an example in which the focal position is adjusted using an afocal optical system 93 including a pair of lenses 91 and 92. The floating optical head 80B is not provided with a focus adjustment mechanism. Instead, the focal position on the optical recording medium 10 can be adjusted by incorporating the afocal optical system 93 into the optical system of the optical recording medium reproducing apparatus 100. By varying the distance between the pair of lenses 91 and 92, the angle of the incident light incident on the objective lens 85 can be delicately changed, and the focal position can be adjusted.
[0038]
It is also possible to add another optical element such as a mirror to the floating optical head 80. FIG. 8 is a configuration example of a floating optical head 80C having a mirror 88. A mirror 88 is provided above the objective lens 85, and the objective lens 85 enters and emits light to and from the optical system of the optical recording medium reproducing device 100 via reflection by the mirror 88.
The suspension 83A is connected to one side of the slider member 81 by a suspension spring.
[0039]
FIG. 9 shows a configuration example of a floating optical head 80D having a condenser lens 89 and an optical fiber 95. The light emitted from the objective lens 85 is sent to one end of an optical fiber 95 via a mirror 88 and a condenser lens 89. The light emitted from one end of the optical fiber 95 enters the objective lens 85 via the condenser lens 89 and the mirror 88. That is, the floating optical head 80D inputs and outputs light to and from the optical system of the optical recording medium reproducing apparatus 100 via the optical fiber 95.
[0040]
C. The aberration correction element 126 is an optical element for correcting aberration (mainly spherical aberration) caused by movement of the recording layer 11 to improve the efficiency of focusing light on the recording layer 11 of the optical recording medium 10. It can be configured using, for example, a liquid crystal element.
The aberration correction element 126 can change the correction amount of the spherical aberration according to the applied voltage. Therefore, it is possible to appropriately correct the aberration in accordance with the position of the focal point of the flying optical head 80.
[0041]
FIG. 10 is a graph showing the calculation results of the aberration. The calculation is performed on the assumption that the constituent material of the optical recording medium 10 is SiO _{2 and} that the center layer (the center recording layer 11) is located about 500 μm below the center (the top of the curved surface) of the upper surface 86a of the solid immersion lens 86. .
10 (A) is the case of FIGS. 2 and 6 (when the focus position adjusting means is incorporated in the floating optical head 80), and FIG. 10 (B) is the case of FIG. 7 (where the focus position adjusting means is the floating optical head 80). When installed outside). Here, the solid line and the broken line of the graph are the calculation results when the numerical aperture NA is 0.88 and 1.047, respectively.
For example, when the numerical aperture NA in FIG. 10A is 0.88, a change in aberration of about 0.2λ (rms value) occurs with a movement amount of 50 μm. This 0.2λ aberration is in a range that can be corrected using a commercially available spherical aberration correcting liquid crystal element.
[0042]
The distance of 50 μm corresponds to 10 layers when the interval (layer interval) of the recording layers 11 is 5 μm. That is, by using a commercially available gradation type spherical aberration correcting liquid crystal element, reading from the optical recording medium 10 having about 10 recording layers 11 becomes possible. As a result, it is found that the storage capacity of the optical recording medium 10 can be increased to ten times or more as compared with the case of a single layer (when the numerical aperture NA is 1.047).
The setting of the layer spacing of 5 μm is smaller than that of a conventional DVD disk having a two-layer structure, but is a value sufficient to avoid Rayleigh scattering. This is effective when fluorescence detection is selected as a reproduction method.
[0043]
Since general fluorescent light is incoherent, it is possible to read information using the generation of incoherent light by dispersing fluorescent substances in the recording layer 11 so as to correspond to the pits.
Further, by using a light source that emits incoherent light (for example, a light source that does not emit laser light) as a light source (light source 121 described later) that emits incident light on the recording layer 11, the light from the recording layer 11 due to the incoherent light can be used. Reading of information can be realized.
As described above, by reading information from the optical recording medium 10 using the incoherent light, light interference between the recording layers 11 is suppressed, and the optical recording medium 10 with a narrower layer interval can be read. It becomes.
[0044]
D. Overall Configuration of Optical Recording Medium Reproducing Apparatus 100 FIG. 11 is a schematic diagram showing the overall configuration of the optical recording medium reproducing apparatus 100. As shown in the figure, the optical recording medium reproducing apparatus 100 includes an optical recording medium 10, a light source (Light Source) 121, a collimator lens (Collimator Lens) 122, an anamorphic prism (Anamorphic Prism) 123 and 124, a non-polarizing beam splitter. (NPBS: Non-Polarization Beam Splitter) 125, Aberration Correction Element (Spherical Aberration Compensator) 126, Polarization Beam Splitter (PBS: Polarization Beam Splitter) 127, Quarter-Wave Optical, QWP: Quarter-Wave 128 129, Dichroic Combine r) 131, a floating optical head 80, and a tilt servo unit 160.
[0045]
Also, a light source (Light Source) 141, a collimator lens (Collimator Lens) 142, an anamorphic prism (Anamorphic Prism) 143, 1644, a condenser lens (Condenser Lens) 151 to 153, a light receiving unit 30, a monitor PD (Front monitor PhotoD) ) 171 and 172.
[0046]
The optical recording medium reproducing apparatus 100 has a scanning unit (not shown) for scanning the optical recording medium 10 with a light beam emitted from the light source 121 in order to read information from the entire surface of the optical recording medium 10. For example, by rotating the optical recording medium 10 and moving the floating optical head 80 in the radial direction with respect to the center of rotation of the optical recording medium 10, optical recording is performed by light emitted from the floating optical head 80. It is possible to scan almost the entire surface of the medium 10.
[0047]
The light source 121 is a light source used for generating an RF signal and a focus error signal, and has a wavelength λ of, for example, 400 nm. Reading from the optical recording medium 10 can be performed with higher resolution by shortening the wavelength.
The collimator lens 122 converts the light emitted from the light source 121 into substantially parallel light.
The anamorphic prisms 123 and 124 form an anamorphic optical system, shape the light emitted from the collimator lens 122 into an appropriate shape, and make the light enter the non-polarizing beam splitter 125.
The non-polarizing beam splitter 125 reflects a part of the incident light and transmits the other, and the ratio of the amount of reflection to the amount of transmission is set to, for example, 1: 9.
The aberration correction element 126 is an optical element for correcting the spherical aberration of the floating optical head 80 and the like as described above to improve the efficiency of converging light to the optical recording medium 10, and is configured using, for example, a liquid crystal element. it can.
[0048]
The polarization beam splitter 127 is an optical element that transmits light having a predetermined polarization component and reflects light having another polarization component.
The quarter-wave plate 128 is an optical element that imparts a quarter-wave phase difference to orthogonally polarized components.
The afocal optical system 129 is composed of a pair of lenses as described above, and by changing the distance between the lenses, the condensing position at which the light emitted from the light source 121 is condensed on the optical recording medium 10 is adjusted. In addition, it is possible to select the recording layer 11 on which reading or the like is performed.
The dichroic combiner 131 is an optical element that combines light of a plurality of wavelengths, and in this case, combines light emitted from the light sources 121 and 141.
[0049]
The light source 141 is a light source used for generating a tracking error signal.
The collimator lens 142 converts light emitted from the light source 141 into substantially parallel light.
The anamorphic prisms 143 and 144 constitute an anamorphic optical system, shape the light emitted from the collimator lens 142 into an appropriate shape, and make the light enter the dichroic combiner 131.
The condenser lenses 151 to 153 focus light on the light receiving unit 30 and the monitor PDs 171 and 172.
The monitor PDs 171 and 172 monitor the outputs of the light sources 121 and 141, respectively.
[0050]
The light source 141 and the anamorphic prisms 143 and 144 are useful when the optical recording medium 10 has a tracking groove layer 12A. On the other hand, when each recording layer 11B has a groove for tracking like the optical recording medium 10B, the light from the light source 121 is used for information reading and tracking, excluding the light source 141 and the anamorphic prisms 143 and 144. It can be used for both applications.
[0051]
E. FIG. The light receiving unit 30 includes a hologram element 31, a pinhole 32, and light receiving elements 33 to 35.
The hologram element 31 is an optical element that diffracts the light incident from the polarization beam splitter 127 by the hologram and causes the light to enter each of the light receiving elements 33 to 35. It is inserted for the purpose of generating a beam pattern suitable for both focus servo signals.
[0052]
The pinhole 32 has a hole formed corresponding to the focal point of the light incident on the light receiving element 33 from the hologram element 31, passes light passing near the focal point of the incident light, and receives light other than that. To block incident light.
The focused light on the optical recording medium 10 and the focused light of the light receiving element 33 have focal points corresponding to each other. For this reason, by passing the light focused on the light receiving element 33 through the pinhole 32, the focus on the optical recording medium 10, that is, stray light from other than the recording layer 11 from which reading is being performed (especially, recording that is not targeted for reading) (Reflected light from the layer) can be excluded, and the S / N ratio of the signal can be improved.
[0053]
It is considered effective to adopt, for example, a value about the diameter of an Airy disk (1.22λ / NA) as the diameter φ of the hole. The Airy disk of the focused light on the light receiving element 33 side corresponds to the Airy disk of the focused light on the recording layer 11 of the optical recording medium 10. Therefore, it is considered that stray light can be removed from the light incident on the light receiving element 33 by passing only the light of the focused light on the light receiving element 33 side within the Airy disk. If the diameter φ of the hole is too large than the diameter of the Airy disk, stray light increases in the light incident on the light receiving element 33. On the other hand, if the diameter φ of the hole is too small than the diameter of the Airy disk, the intensity of light incident on the light receiving element 33 decreases.
Assuming that the numerical aperture NA (numerical aperture of the condenser lens 151) of light incident on the light receiving element 33 (return light from the optical recording medium 10) is about 0.1 and the wavelength λ of the light source 121 is 400 nm, the diameter φ of the hole is Is about 5 μm.
[0054]
The light receiving element 33 is an element for receiving the return light from the optical recording medium 10 and generating information recorded on the optical recording medium 10 as an RF signal.
The light receiving elements 34 and 35 are elements for receiving return light from the optical recording medium 10 and generating a tracking error signal for performing tracking on the optical recording medium 10.
The light receiving element 33 and the light receiving elements 34 and 35 are light receiving elements corresponding to the light sources 121 and 141, respectively, and preferably have a sensitive wavelength range corresponding to the emission wavelength of each of the light sources 121 and 141.
[0055]
The knife edge method can be used to generate the focus error signal. The knife edge method is a method of disposing a knife edge that asymmetrically shields a part of return light reflected by the optical recording medium 10. By taking the difference between the outputs from the light receiving elements 34 and 35, a focus error signal can be obtained.
[0056]
The output of the tracking error signal can be performed by performing arithmetic processing on outputs from the light receiving elements 34 and 35 based on the push-pull method. A tracking error signal is output using reflected light from the groove of the recording layer 11, and tracking adjustment (tracking servo) to the optical recording medium 10 can be performed based on the tracking error signal.
[0057]
It is also conceivable to use a DPD (Differential Phase Detection) method to obtain a tracking error signal. The DPD method requires that the pit depth of the groove be a certain depth of about λ / 4, which means that it is difficult to increase the number of recording layers 11B. For this reason, the present embodiment employs the push-pull method. However, this is based only on the ease of manufacturing the recording layer 11, and does not absolutely exclude the application of the DPD method.
In addition, when burying of pits is used for tilt detection described later, it is conceivable to use a sample servo method for generating a tracking error signal from the viewpoint of the manufacturing efficiency of the optical recording medium 10.
[0058]
F. Tilt servo unit 160
The tilt servo (skew servo) unit 160 performs optical recording based on a detection result of tilt (tilt (tilt, skew) of the optical recording medium 10 with respect to the optical axis of light incident on the optical recording medium 10 from the floating optical head 80). This is an adjustment mechanism for adjusting the incident light so as to be perpendicularly incident on the surface of the medium 10.
[0059]
Since the optical recording medium 10 has a multilayer structure, if the optical recording medium 10 is inclined with respect to the optical axis A of the incident light, the information reading position shifts for each recording layer 11. This is illustrated in FIG. Here, FIG. 12A shows a case where the surface of the optical recording medium 10 is perpendicular to the optical axis A of the incident light (there is no inclination), and FIG. 3 shows a case where the surface of the optical recording medium 10 is inclined by the angle θ.
[0060]
In FIG. 12A, the optical axis of the recording / reproducing light incident on each recording layer 11B and the light incident on the tracking layer 12 coincide with each other with the incident light of the optical axis A, and the tracking servo is appropriately applied. While the signal can be recorded and reproduced, it can be seen from FIG. 12B that an offset occurs in the track direction according to the inclination θ of the optical recording medium 10. In this case, generation of an appropriate recording / reproducing signal cannot be expected.
[0061]
The tilt servo unit 160 is used to correct the tilt of the optical recording medium 10, and the tilt of the floating optical head 80 with respect to the optical recording medium 10 is adjusted (including the tilt of the optical recording medium 10 itself). I do.
To detect the tilt (tilt, skew) of the optical recording medium 10, for example, a method of embedding a pit serving as a tilt reference at the same position on each recording layer 11 can be considered. The optical axis of the light incident on the optical recording medium 10B is adjusted so that the reference pits are aligned on the same optical axis (see FIG. 12A).
In addition to the tilt detection, a method using a change in the position of the reflected light according to the inclination of the optical recording medium 10 (a method of differentially detecting the movement of the position of the reflected light by the tilt sensor), a DVD -A method used in a RAM, a method using coma, and the like are also conceivable.
[0062]
(Operation of Optical Recording Medium Reproducing Apparatus 100)
The linearly polarized light emitted from the light source 121 is converted into substantially parallel light by a collimator lens 122, shaped by anamorphic prisms 123 and 124, and enters a non-polarizing beam splitter 125. Part of the light that has entered the non-polarizing beam splitter 125 enters the monitor PD 171 via the condenser lens 152, and the output is monitored.
[0063]
Most of the light that has entered the non-polarizing beam splitter 125 enters the aberration correction element 126, and thereafter enters the quarter-wave plate 128 via the polarizing beam splitter 127. The linearly polarized light that has entered the quarter-wave plate 128 becomes circularly polarized light, and enters the optical recording medium 10 via the afocal optical system 129, the dichroic combiner 131, and the floating optical head 80.
Light incident on the optical recording medium 10 is reflected by the recording layer 11 and becomes return light. The return light passes through the floating optical head 80, the dichroic combiner 131, and the afocal optical system 129, is converted into linearly polarized light orthogonal to the outward path by the quarter wavelength plate 128, and is reflected by the polarization beam splitter 127. The light is focused on the light receiving unit 30 by the condenser lens 151, and an RF signal and a focus error signal are generated.
[0064]
The light emitted from the light source 141 is converted into substantially parallel light by the collimator lens 142, shaped by the anamorphic prisms 143 and 144, and incident on the dichroic combiner 131. Part of the light incident on the dichroic combiner 131 is incident on the monitor PD 172 via the condenser lens 153, and the output of the light source 141 is monitored.
[0065]
Most of the light that has entered the dichroic combiner 131 enters the optical recording medium 10 via the floating optical head 80. The light incident on the optical recording medium 10 is reflected by a layer on which a tracking pattern is formed (the groove layer 12A in FIG. 1A and the recording layer 11B in FIG. 1B), and becomes return light. The return light passes through the floating optical head 80, the dichroic combiner 131, and the afocal optical system 129, is converted into linearly polarized light by the quarter-wave plate 128, is reflected by the polarization beam splitter 127, and is received by the condenser lens 151. Focusing to 30 produces a tracking error signal.
[0066]
(Other embodiments)
The embodiment of the present invention is not limited to the above embodiment, and can be extended and changed. Extended and modified embodiments are also included in the technical scope of the present invention.
(1) For example, the optical recording medium reproducing apparatus may be either a fixed type in which the optical recording medium is steadily installed or a removable (removable) type in which the optical recording medium is detachably installed. In the case of the removable system, the optical recording medium reproducing device is provided with an optical recording medium holding means such as a stage (table), and the optical recording medium holding means holds the optical recording medium.
[0067]
(2) The optical recording medium may have a disc shape or any other appropriate shape.
Further, information can be recorded on the optical recording medium by various means. That is, the optical recording medium referred to here may be any device as long as information can be read by optical means, and the writing means is not limited.
[0068]
(3) In the above embodiment, the case where the optical recording medium reproducing apparatus is used for reproducing (reading) the information recorded on the optical recording medium is mainly described. Writing, erasing, etc. may be performed. For example, means for optically writing information, such as a magneto-optical disk, can be provided to an optical recording medium reproducing apparatus.
[0069]
【The invention's effect】
As described above, according to the present invention, it is possible to provide an optical head and an optical recording medium reproducing apparatus capable of achieving both miniaturization and good tilt characteristics.
[Brief description of the drawings]
FIG. 1 is an enlarged cross-sectional view of a configuration example of an optical recording medium.
FIG. 2 is a perspective view illustrating a configuration example of a flying optical head.
FIG. 3 is a side view illustrating an embodiment of an objective lens and a solid immersion lens.
FIG. 4 is a side view illustrating a comparative example of an objective lens and a solid immersion lens.
FIG. 5 is a graph showing tilt characteristics of an example and a comparative example in comparison.
FIG. 6 is a perspective view illustrating a configuration example of a floating optical head.
FIG. 7 is a schematic diagram illustrating an example in which a focal position is adjusted using an afocal optical system.
FIG. 8 is a perspective view illustrating a configuration example of a flying optical head.
FIG. 9 is a perspective view illustrating a configuration example of a floating optical head.
FIG. 10 is a graph showing spherical aberration.
FIG. 11 is a schematic diagram showing the overall configuration of the optical recording medium reproducing device according to the first embodiment of the present invention.
FIG. 12 is a diagram for explaining that, when the optical recording medium is inclined with respect to the optical axis of incident light, the information reading position shifts for each recording layer.
[Explanation of symbols]
Reference Signs List 10 optical recording medium 30 light receiving unit 31 hologram element 32 pinholes 33 to 35 light receiving element 80 floating optical head 81 slider member 82 second group lens 83 suspension 84 focus adjustment mechanism 85 objective lens 86 solid immersion lens 87 plate members 87a, 87b Wedge prism 88 Mirror 89 Condensing lens 91, 92 Lens 93 Afocal optical system 95 Optical fiber 100 Optical recording medium reproducing device 121 Light source 122 Collimator lens 123, 124 Anamorphic prism 125 Non-polarizing beam splitter 126 Aberration correcting element 127 Polarized beam Splitter 128 Wave plate 129 Afocal optical system 131 Dichroic combiner 141 Light source 142 Collimator lens 143, 144 Anamorphic prism 151-153 Nsarenzu 160 tilt servo unit 171 monitor PD

Claims (7)

A condensing objective lens having a convex aspheric first surface on which light from the light source is incident and a substantially planar second surface, and having a numerical aperture smaller than 1;
A third surface having a convex aspherical shape facing the second surface, and a third surface having a substantially planar shape facing the optical recording medium and collecting light from the light source passing through the focusing objective lens. 4, a solid immersion lens having a refractive index greater than 1;
An optical head comprising: 2. The optical head according to claim 1, wherein the optical head is a floating optical head that floats from the optical recording medium by a flow of air generated by a relative movement with the optical recording medium. 2. The optical head according to claim 1, further comprising a focusing position adjusting unit that adjusts a focusing position of the focused light from the optical head in a thickness direction of the optical recording medium. The optical head according to claim 3, wherein the focusing position adjusting unit includes a variable refractive index element. The optical head according to claim 3, wherein the focusing position adjusting means includes a wedge-shaped prism. 2. The optical head according to claim 1, wherein the optical head further comprises an aberration correcting unit for correcting an aberration of the optical head. A light source that emits light,
A converging objective lens having a convex aspheric first surface on which light from the light source is incident and a substantially planar second surface, the numerical aperture of which is smaller than 1, and the second surface A third surface of a convex aspherical shape facing the optical recording medium, and a fourth surface of a substantially planar shape facing the optical recording medium and condensing light from the light source passing through the converging objective lens. An optical head comprising: a solid immersion lens having a refractive index greater than 1;
An optical member for focusing return light returned from the optical recording medium in response to the light focused by the optical head;
A light receiving unit that receives the light focused by the optical member,
An optical recording medium reproducing apparatus, comprising:

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