SINGLE ASPHERICAL LENS

Abstract: A single lens (1) having first and second aspherical sur­ faces (2, 3) each having a positive power is usable for an objective lens of an optical pickup for an optical recording medium. The lens satisfies the following conditions: 0.1 < | m | < 0.5 0.9 < < 1.2 where m denotes magnification; d denotes thickness of the lens; and f denotes focal length of the lens.

Description: 1. Field of the Invention The present invention relates to a single lens usable as an objective lens which is used in an optical pickup of a recording and reproducing apparatus for an optical recording medium such as a video disk, an audio disk an optical memory disk, and the like, and more particularly to a single lens usable as an objective lens which is capable of converging a light from a light source without using a collimator lens. 2. Description of the Prior Art The objective lens used in an optical pickup of an optical disk recording and reproducing apparatus is required to precisely correct aberrations to its diffraction limit, to provide a wide angle of view, to be cheap, and to allow the optical pickup as compact as possible. The conventional objective lens, which is made by combining a plurality of spherical glass lenses to fully satisfy the desired optical performance, cannot satisfy those requirements mentioned above. On the other hand, optical pickups using a single aspherical lens have been proposed by Japanese Patent Laid-Open No. 57-76512 (1982), Japanese Patent Laid-Open No. 57-201210 (1982), Japanese Patent Laid-Open No. 59-26714 (1984), Japanese Patent Laid-Open No. 59-23313 (1984), and Japanese Patent Laid-Open No. 60-120310 (1985), respectively. However, each of these pickups disclosed by these prior arts converts a light from a light source into parallel beams by a collimator lens before converging the parallel beams onto the back surface of a disk by the objective lens. These pickups would be expensive due to use of a number of lenses, and cannot be made compact due to necessity of a space for the collimator lens. Japanese Patent Laid-Open No. 50-156945 (1975) discloses a finite-conjugate aspherical single lens. However, the magnification of this lens is only 1/20, which is very small, and yet, since there is a long distance between the light source and the light-condensing point, compact size cannot easily be realized for the optical pickup. Furthermore, when combining this lens with a laser diode, which is widely used for the light source, the numerical aperture of the lens toward the light-source end is too small to efficiently use the effective quantity of light from the light source. Japanese Patent Laid-Open No. 60-126616 (1985) proposes a single lens capable of converging a light from a light source. However, taking the manufacturing convenience into account, only one of the two surfaces of the single lens is made aspherical, and as a result, the lens cannot provide a wide field angle. Consequently, in order to effectively use this single lens in the optical pickup, the light source and the lens must be housed in a tubular housing which is movable for tracking. This structure does not allow the optical pickup compact, and in addition, due to its heavy weight, power-saving cannot be realized. Japanese Patent Laid-Open No. 61-56314 (1986) proposes a bi-aspheric single lens which is capable of satisfying optical performances to the extent of its diffraction limit even when moved independently without being integrally moved with the light source. However, in order to achieve a compact optical pickup, it is essentially required to insert a mirror and a prism in the optical path from the light source to the lens. As a result, in consideration of mounting errors of the mirror, prism, and light source, and also in consideration of the movement of the lens in the tracking direction, a still wider field angle is needed. The present invention provides a single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by having specified values of the following parameters, as specified in any one of claims 1 to 7: m: magnification of the lens, d: thickness of the lens, f: focal length of the lens, n: refractive index of the lens, K₁ : conic constant of the first surface, and K₂: conic constant of the second surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A₂₁ (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and the values are as specified in any one of claims 1 to 7, all distances being in mm. A primary advantage of the present invention is the provision of a novel single lens which is capable of converging a light from a light source to a diffraction limit, and has an extremely wide field angle. The single lens of this invention provides the following particular advantages: (1) Since the single lens of the invention can directly converge the light from the light source to the diffraction limit, the collimator lens for converting the light from the light source into parallel beams can be removed from the pickup. (2) Since the single lens of the invention has an extremely wide field angle, it is possible to widen the allowable ranges of the movement of the lens itself in the tracking direction and of the mounting location errors of the laser diode and the mirror and prism which may in some cases be inserted into the optical path of the pickup, thus allowing the pickup to be easily assembled. (3) Since the working distance can be made long, the focal length can be shortened, so that the lens can be made compact and light in weight. (4) Mass-production at low cost of the single lens of the invention is possible by using a molding process. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention in which; FIG. 1 is a simplified schematic side view of an optical pickup using a single lens of the invention as its objective lens; FIG. 2 is a graphical chart showing variations of the off-axis wave-front aberration with respect to the thickness of the single lens of the invention; FIGs. 3 through 9 are respectively charts showing aberrations generated in the first through seventh embodiments of the single lens of the invention; and FIG. 10 is a simplified schematic side view of another optical pickup incorporating a single lens of the invention as its objective lens. DESCRIPTION OF THE EMBODIMENTS ACCORDING TO THE INVENTION FIG. 1 is a simplified schematic side view showing an optical pickup using a single lens of the invention as its objective lens. The optical pickup is provided with an objective lens 1 having a first surface 2 and a second surface 3, and a semiconductor laser diode 5. Beams emitted from the semiconductor laser 5 are refracted through the first and second surfaces 2 and 3 of the lens 1 to be focussed onto the back surface 6 of a recording disk 4. Lens 1 is substantially a single lens with the first and second surfaces 2, 3 each being aspherical and having a positive power, satisfying the following conditions: 0.1< | m | <0.5 (1) 0.9< d f <1.2 (2) where m denotes magnification of the lens; d denotes thickness at the center of the lens; and f denotes focal length of the lens. Condition (1) represents the effective range of magnification. If the magnification is below the lower limit of condition (1), the working distance becomes insufficient. Conversely, if the focal length is extended to maintain a sufficient working distance, then the entire size of the lens will be increased, thus making it difficult to realize a compact optical pickup. If the magnification exceeds the upper limit of condition (1), the lens cannot compensate for the coma aberrations against the off-axis ray, thus reducing the field angle. Condition (2) represents the thickness of the lens. If the lens thickness is below the lower limit of condition (2), then the low-order astigmatism will increase. Generally, when the both surfaces of a lens are aspherical , both of the spherical and coma aberrations can be corrected. However, in order to securely provide a wide field angle, the astigmatism should also be corrected. It is possible to properly balance the low-and high-order astigmatisms within the specific range defined by the above condition (2). If the lens thickness exceeds the upper limit of condition (2), the lens will become excessively thick, thus causing the lens itself become heavy and incur difficulty in maintaining the required working distance. FIG. 2 shows a relationship between the lens thickness and the off-axis optical performance of the single lens of the invention. The horizontal axis denotes d/f, whereas the vertical axis denotes root mean square value of the wavefront aberration which represents the optical performance of the single lens. The curve in FIG. 2 was obtained by plotting actually designed values under the condition that the angle of view of the lens was 1.3°, the refractive index n = 1.65, the magnification m = -0.23, and the on-axis wavefront aberration was almost zero. As shown in FIG. 2, the single lens of the invention exhibits the most satisfactory off-axis optical performance in the region close to d/f = 1.1. The single lens provides satisfactory performance even when the thickness is thinner or thicker than the region close to d/f = 1.1. However, it is needless to say that the single lens having the thinner thickness saves its own weight and achieves a longer working distance. In addition to the above conditions (1) and (2), it is preferable to further satisfy the following conditions: 1.6< n (3) 0 < K ₁ < 1 (4) where n denotes refractive index of the lens; and K₁ denotes conic constant of the first surface. If the lens refractive index is below the limit of condition (3), even after correcting the spherical aberration, coma aberration, and the astigmatism, some high-order astigmatism still remains, thus making it difficult to maintain sufficient field angle. Condition (4) denotes the range of the conic constant. If the conic constant exceeds the upper limit or is below the lower limit, it is difficult to simultaneously correct the spherical and coma aberrations. Furthermore, a large aberration is generated when the first and second surfaces horizontally deviate from each other. Embodiments No. 1 through No. 7 of the single lens according to the invention are shown below. In each of the embodiments, the aspherical surface is expressed as follows: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a height h from the optical axis h: height from the optical axis C: surface curvature at the vertex of the aspherical surface (= 1/R) Kj: conic constant of j-th surface A 2i (j) : 2i-th order aspherical surface coefficient of j-th surface (i denotes an integer of at least 2) The embodiments Nos. 1 through 7 each use the following reference characters: Rj: radius of curvature of j-th surface d: thickness of the lens m: magnification n: refractive index of the lens with respect to wavelength of 780 nm f: focal length of the lens W.D.: working distance The recording disk used has a thickness t of 1.2 mm and a refractive index of 1.57221 with respect to wavelength of 780 nm. FIGs. 3 through 9 respectively show the aberrations observed in the embodiments Nos. 1 through 7. In each of FIGs. 3 through 9, (a) shows spherical aberration and (b) astigmatism. Charts (c) through (e) show coma aberrations with respect to angles of view of 1.0, 0.5 and 0.0, respectively. In each chart (a) showing spherical aberration, the solid line represents the spherical aberration, whereas the broken line represents sine-wave condition. In each chart (b) showing astigmatism, the solid line represents the sagittal curvature of field, whereas the broken line represents the meridional curvature of field. In each of the charts (c) through (e) showing coma aberration, the solid line represents the meridional coma aberration, whereas the broken line represents the sagittal coma aberration. Each aberration is expressed in terms of millimeters. As is clear from respective aberration charts, each aberration is significantly small in any one of the embodiments Nos. 1 through 7. FIG. 10 is a simplified schematic side view of another optical pickup incorporating the single lens of the invention. The pickup is made compact by inserting a mirror 7 and a prism 8 between the semiconductor laser diode 5 and the single objective lens 1. If the positions of the semiconductor laser diode 5, mirror 7 and prism 8 deviate from each other, the relative positions of the lens 1 and the semiconductor laser diode 5 will eventually deviate. However, since the single lens embodied by the invention has an extremely wide field angle, such positioning errors of the semiconductor laser diode 5, mirror 7 and prism 8 are allowable.

Claims: A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.2 f = 4.0 d = 4.2 n = 1.61469 K₁ = 0.150002 K₂ = -11.4616 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A₂₁ (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 3.2 R₂ = -5.3111 A₄ (1) = -3.68053x10⁻³ A₄ (2) = 1.14305x10⁻³ A₆ (1) = -3.01191x10⁻⁴ A₆ (2) = -2.08737x10⁻⁴ A₈ (1) = 1.73907x10⁻⁶ A₈ (2) = 9.19832x10⁻⁶ A₁₀ (1) = -5.05962x10⁻⁶, all distances being in mm. A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.2 f = 4.0 d = 4.0 n = 1.78569 K₁ = 0.062211 K₂ = 16.4482 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A₂₁ (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 3.8 R₂ = -9.755 A₄ (1) = -1.99885x10⁻³ A₄ (2) = 6.46581x10⁻³ A₆ (1) = -1.04115x10⁻⁴ A₆ (2) = -4.46152x10⁻⁴ A₈ (1) = -5.53717x10⁻⁶ A₈ (2) = 1.06107x10⁻⁶ A₁₀ (1) = -2.10252x10⁻⁷ A₁₂ (1) = -1.20303x10⁻⁸ A₁₄ (1) = -4.68164x10⁻¹⁰ A₁₆ (1) = -9.88946x10⁻¹¹ A₁₈ (1) = 6.99966x10⁻¹² A₂₀ (1) = -8.75783x10⁻¹³ , all distances being in mm. A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.125 f = 4.0 d = 4.6 n = 1.78569 K₁ = 0.487522 K₂ = 13.0263 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A₂₁ (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 3.8 R₂ = -8.4926 A₄ (1) = -2.77029x10⁻³ A₄ (2) = 8.99115x10⁻³ A₆ (1) = -2.06423x10⁻⁴ A₆ (2) = -1.21495x10⁻³ A₈ (1) = -7.0466x10⁻⁴ A₈ (2) = 2.37092x10⁻⁴ A₁₀ (1) = -2.30016x10⁻⁶, all distances being in mm. A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.125 f = 4.0 d = 3.8 n = 1.684429 K₁ = 0.90252 K₂ = 15.8112 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A 2i (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 3.4 R₂ = -7.6721 A₄ (1) = -5.27421x10⁻³ A₄ (2) = 1.03076x10⁻² A₆ (1) = -5.17669x10⁻⁴ A₆ (2) = -5.50378x10⁻⁴ A₈ (1) = -8.11181x10⁻⁵ A₈ (2) = 1.57230x10⁻⁴ A₁₀ (1) = 2.32456x10⁻⁵ A₁₀ (2) = 8.29788x10⁻⁵ A₁₂ (1) = -2.15725x10⁻⁵ A₁₄ (1) = 8.92890x10⁻⁶ A₁₆ (1) = -2.35518x10⁻⁶ A₁₈ (1) = 3.36978x10⁻⁷ A₂₀ (1) = -2.16639x10⁻⁸, all distances being in mm. A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.125 f = 4.0 d = 3.8 n = 1.61469 K₁ = 0.18994 K₂ = -7.38611 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A 2i (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 3.1 R₂ = -6.34 A₄ (1) = -3.7531x10⁻³ A₄ (2) = 4.6524x10⁻³ A₆ (1) = -3.77662x10⁻⁴ A₆ (2) = -1.14388x10⁻³ A₈ (1) = -2.59548x10⁻⁶ A₈ (2) = 1.05207x10⁻⁴ A₁₀ (1) = -7.74807x10⁻⁶ A₁₀ (2) = 1.36623x10⁻⁶, all distances being in mm. A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.35 f = 4.0 d = 4.6 n = 1.785691 K₁ = 0.58024 K₂ = 2.13568 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R j ) surface curvature at the vertex of the j-th aspherical surface; A 2i (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 4.1 R₂ = -6.816 A₄ (1) = -3.11733x10⁻³ A₄ (2) = 5.83291x10⁻³ A₆ (1) = -1.68631x10⁻⁴ A₆ (2) = -5.19808x10⁻⁴ A₈ (1) = -1.8999x10⁻⁶ A₈ (2) = 2.84781x10⁻⁵ A₁₀ (1) = -1.81188x10⁻⁶ A₁₀ (2) = 1.46632, all distances being in mm. A single lens having first and second aspherical surfaces each having a positive power for converging light from a light source to form a spot on a recording surface of an information recording medium, characterized by satisfying the following conditions: m = -0.35 f = 4.0 d = 4.7 n = 1.68443 K₁ = 0.32002 K₂ = 0.463027 where m: magnification of the lens d: thickness of the lens f: focal length of the lens n: refractive index of the lens K j : conic constant of the j-th surface, and where the first and second aspherical surfaces are expressed by the following equation: where X: distance from a tangential plane at the vertex of the aspherical surface to a point on the aspherical surface at a distance h from the optical axis; h: the distance from the optical axis; C j : (=1/R ) surface curvature at the vertex of the j-th aspherical surface; A₂₁ (j) : 2i-th order aspherical surface coefficient of the j-th surface, in which i represents an integer greater than or equal to 2, and in which: R₁ = 3.7 R₂ = -5.0933 A₄ (1) = -3.71287x10⁻³ A₄ (2) = 8.17174x10⁻³ A₆ (1) = -1.73500x10⁻⁴ A₆ (2) = -9.20368x10⁻⁴ A₈ (1) = -1.34883x10⁻⁵ A₈ (2) = 9.60691x10⁻⁵ A₁₀ (1) = -3.42637x10⁻⁷ A₁₀ (2) = -4.54728x10⁻⁶ A₁₂ (1) = -1.37009x10⁻⁷ A₁₄ (1) = 2.71986x10⁻⁸, all distances being in mm.

Inventors: NAGAOKA, YOSHITOMI, NAKAJIMA, YASUO, TANAKA, YASUHIRO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: G02B3
IPC codes: G02B13, G02B3