JP3485456B2 - Optical recording device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium such as a magneto-optical disk or a DVD-RAM using laser light.
The present invention relates to an optical recording device such as an optical head capable of at least writing (recording) information. 2. Description of the Related Art In order to improve the surface recording density of an optical recording medium such as a magneto-optical disk or a DVD-RAM, it is necessary to make the beam spot diameter of a recording laser beam as small as possible. Generally, the beam spot diameter of laser light is determined by the wavelength λ of the light source and the NA (numerical aperture) of the objective lens, and is approximately 0.8 × λ / NA. For this reason, a blue laser using a gallium nitride (GaN) -based material has been developed as a semiconductor laser capable of further shortening the wavelength from the light wavelength of about 630 nm actually used in the optical disk device. However, even if such a wavelength of blue laser light is used, it is only about three times the current areal recording density. So, Solid Immersion
An optical system in which a hemispherical lens called a lens (hereinafter, also referred to as SIL) is disposed between the optical recording medium and the objective lens reduces the beam spot diameter transmitted through the objective lens by 1 /. n times (where n is SI
(Refractive index of L) (see, for example, US Pat. No. 5,125,750). [0004] Here, the laser beam transmitted through the SIL and emitted into the air tries to expand to the original beam diameter again, but the distance between the recording surface of the optical recording medium and the SIL bottom surface facing the SIL bottom surface. However, in an area within about ４ of the light wavelength (generally called a near-field area), the laser light
The light is emitted with the same property as the inside of L, and the beam spot diameter is reduced to 1 / n times the diffraction limit. [0005] Glass such as borosilicate glass is generally used for the SIL. The refractive index of glass is usually up to about 1.8. To obtain a higher refractive index than this, for example, La ₂ O ₃ , ThO ₂ , ZrO ₂ ,
Special glass containing a rare element oxide such as Ta ₂ O ₅ as a main component must be used. Further, even such a special glass has a refractive index of 2.0 or less, and it has been difficult to perform recording at a higher density. In particular, the near field area is 630
When a laser wavelength in the vicinity of nm is used as a light source, the laser spot diameter is about 300 nm or less, so the surface of the SIL facing the recording surface of the optical recording medium is required to have high flatness processing accuracy. There was a problem of too much. Even if a polycrystalline material having a high refractive index is used for the SIL, the transmittance of light becomes extremely small due to a large number of grain boundaries existing in the polycrystalline material (for example, a light wavelength of 500 nm).
(The transmittance of about 600 nm is about 50% or less), and an optical head having excellent performance cannot be provided. Therefore, the present invention solves the above-mentioned problems and has a very high refractive index and excellent transmittance.
It is an object of the present invention to provide an optical recording apparatus having a condensing lens made of a material which is easy to manufacture and has a very excellent workability. In order to achieve the above object, an optical recording apparatus according to the present invention comprises: an objective lens for narrowing a laser beam for recording information on an optical recording medium to a predetermined beam diameter; A hemispherical solid immersion lens (SIL) for condensing the laser light transmitted through the objective lens, wherein the SIL contains 3 to 7 mol% of magnesia, lithium niobate, lead molybdate and It is characterized by being composed of any single crystal of tellurium dioxide. Here, if the SIL is a lithium niobate single crystal containing 3 to 7 mol% of magnesia, not only the light damage resistance is large and the change in the refractive index is very small, but also
Since the grown single crystal also has few cracks and good crystallinity, it can be suitably used as an SIL. [0010] The optical recording apparatus is only required to be capable of recording at least on an optical recording medium, and may be capable of recording and reproducing. An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic perspective view illustrating a state in which optical recording is performed on an optical recording medium. FIG. 2 is a schematic partial cross-sectional view taken along the line AA of FIG. A dielectric layer made of, for example, silicon nitride, a magnetic layer made of a Gd-Fe-Co-based alloy, a Tb-Fe-Co-based alloy, or the like, made of silicon nitride, etc. on a substrate 1 made of a resin such as polycarbonate or glass. A laser beam L having a wavelength of 500 nm to 600 nm is applied to a magneto-optical disk D having a recording layer 2 including a dielectric layer, a metal layer made of aluminum or the like, and a protective layer made of UV resin or the like by an optical head 3 which is an optical recording device. The light is focused and irradiated with a beam spot having a predetermined diameter. As a result, information is recorded on the magnetic layer in the area of the recording layer 2 of the magneto-optical disk D irradiated with the beam spot. The optical head 3
Is supported by a movable arm 7 connected to a control device (not shown). The recording of information on the magneto-optical disk D by the optical head 3 is performed as follows. First, the laser beam L is converged by the objective lens 4 which is a biconvex lens, and further comprises a single crystal such as lithium niobate.
The beam spot diameter is reduced to a predetermined beam spot by the SIL 5, which is a hemispherical condenser lens having a flat portion for focusing on the lower surface 5a. Then, the recording layer 2 in the beam spot irradiated on the magneto-optical disk D is heated, and when the area is cooled, the magnetic layer is magnetized by the magnetic field applied by the magnetic field modulation coil 6 to perform recording. Do. The objective lens 4
May be a uniconvex lens. [0014] Here, SIL5 is lithium niobate (LiNbO _3: mp 1250 ° C., the ordinary refractive index _{n o} = 2.29,
Besides, lead molybdate of extraordinary refractive index _{n e} = 2.20) (P
bMoO _4: mp _{1070 ℃, n o = 2.39,} n e = 2.26) and tellurium dioxide (TeO _2: mp 733 ℃, _{n o} = 2.4
3, n e ₌ 2.27) one of a single crystal of. Each of these single crystals has a refractive index of 2.1 or more, and has an excellent transmittance of 70% or more at a used light wavelength of 500 to 600 nm. In particular, lithium niobate can be easily grown by the Czochralski method or the like, and is softer than glass (Mohs hardness of 5 to 6). Furthermore, the refractive index fluctuation is smaller than that of glass (1 × 10
^-4 or less). In particular, in the case of lithium niobate, if 3 to 7 mol% of magnesia (MgO ₂ ) is added, there is no change in refractive index (optical damage) due to short-wavelength laser light such as blue-green light, and the condensing lens It can be suitably used. Further, when the magnesia in the lithium niobate single crystal is 4 to 5 mol%, there is almost no change in the refractive index, which is very preferable. Here, if the magnesia in the lithium niobate is less than 3 mol%, the light damage resistance is reduced, and the refractive index fluctuation caused by light is undesirably increased. Also,
If the content of magnesia exceeds 7 mol%, cracks will increase in the grown single crystal, which is not preferable. As described above, the above-mentioned single crystal, which has a refractive index much larger than that of the conventional glass, has a smaller refractive index fluctuation than that of the glass, and is very excellent in workability, etc., is very preferably used as an SIL. And the beam spot diameter of the laser beam can be made very small. As a result, the current recording pit size can be significantly reduced, and high-density recording on an optical recording medium can be performed several times as high as the current state. As shown in FIGS. 1 and 2, an optical system for optical recording includes an objective lens 4 and a condenser lens 5 on an optical head 3.
Although an example in which information and the like are provided integrally to record information has been described, the configuration of the optical system is not limited to this, and an optical head including only a condenser lens without an objective lens may be used. . Also, for example, an optical head capable of recording and reproducing may be used. The optical head is not only a magneto-optical disk but also a DVD-R which is a phase-change type optical disk having an optical recording layer made of, for example, various Te-based alloys.
AM can also be used. Further, the shape of the condensing lens is not limited to a general hemisphere, and may be a shape such as a so-called super hemisphere having a large curved surface area as long as a desired spot diameter can be obtained. The shape and the like can be appropriately changed and implemented without departing from the gist of the present invention. Embodiments of the present invention will be described below. A lithium niobate single crystal having a diameter of about 3 inches was obtained by the Czochralski method using a resistance heating type growth furnace. At this time, magnesia was added to the single crystal growing raw material so that the amount of magnesia after the growth was 5 mol%. The change in the photo-induced refractive index of this lithium niobate single crystal was measured by the Senarmont method.
It was 2.0 × 10 ⁻⁵ or less, which was much smaller than the above glass. After the lithium niobate single crystal is cut into a 0.7 mm square wafer, the optical axis (Z axis (C axis)) of the lithium niobate single crystal becomes perpendicular to the recording surface of the magneto-optical disk. The hemisphere having a radius of about 0.5 mm was polished as described above (the refractive index is 2.29 because the ordinary light refractive index and the extraordinary light refractive index match). This polishing was performed using a grinding wheel for optical glass as in Example 1. Next, a laser beam having a wavelength of 529 nm is condensed by an objective lens, and further condensed by the SIL made of the above-mentioned single crystal, so that the distance between the recording surface of the magneto-optical disk and the plane of the hemisphere SIL is reduced. Was brought close to about 100 nm, and the recording spot diameter was measured.
A spot diameter smaller than the above glass of about nm was obtained. As described above, according to the optical recording apparatus of the present invention, any one of lithium niobate, lead molybdate and tellurium dioxide containing 3 to 7 mol% of magnesia can be used. Since these single crystals were used as solid immersion lenses, these single crystals were excellent in transmittance for the light wavelength used, and a very excellent optical recording device having a solid immersion lens that was easy to manufacture and had good workability. Can be provided. Further, since the refractive index of the single crystal is 2.0 or more, the beam spot diameter of the laser beam can be made very small, and the current recording pit size can be greatly reduced. It is possible to provide an excellent optical recording apparatus capable of recording on a medium at several times or more the density of the current state. Further, magnesia is used as a solid immersion lens in an amount of 3 to 7 mol%, more preferably 4 to 5 mol%.
By employing a lithium niobate single crystal containing mol%, an optical recording apparatus having an excellent solid immersion lens having high light damage resistance and a very small change in refractive index can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view illustrating an embodiment of an optical system according to the present invention. FIG. 2 is a schematic partial cross-sectional view taken along line AA in FIG. [Description of Signs] 1: substrate 2: recording layer 3: optical head (optical recording device) 4: objective lens 5: SIL (solid immersion lens) 6: coil for applying a magnetic field D: magneto-optical disk

Claims (1)

(57) and the objective lens to narrow the Patent Claims 1 An optical recording medium in the information recording laser beam rows as Utame a predetermined beam diameter, the pair Monore
Hemispherical solid that collects laser light transmitted through the lens
An optical recording device comprising an immersion lens , wherein the solid immersion lens is
Lithium niobate containing 3 to 7 mol% of Nesia, moly
An optical recording device comprising a single crystal of either lead butyrate or tellurium dioxide .