Radial graded-index objective lens system

Abstract: A rod lens (1) made of a transparent cylindrical medium having a refractive index distribution in the radial direction is used as a graded index lens. An object surface (2) and an image surface (3) are arranged on both sides of the rod lens (1). A first surface of the lens is a convex spherical surface (1a), and a second surface of the lens is a planar surface (1b), and the lens is arranged so that the convex spherical surface (1a) faces the object surface (2), and the planar surface (1b) faces an image surface (3). The equations 1.0 ≤ R / r 0 ≤ 1.6, and R / f ≤ 1.2 are satisfied, wherein r 0 is a radius of a portion of the rod lens (1) measured from an optical axis of the lens, where the RMS wavefront aberration on the optical axis (1c) is below 0.072, R is a curvature radius of the convex spherical surface (1a), and f is a focal length of the rod lens (1). Thus, a compact and light-weighted objective lens system is provided which has a large NA and which is particular useful in an optical head for reading/writing information on an optical disk.

Description: The present invention relates to an objective lens system using a single lens with a refractive index distribution (also referred to as "graded index lens" in the following). More specifically, the present invention relates to an objective lens system using a graded index lens having a convex spherical surface on one side and a planar surface on the other side. This optical system can be used, for example, for an object lens system of an optical head used for reading information stored on an optical disk and writing information onto the optical disk. In all kinds of optical disk systems, such as compact disk players or DVD players, optical heads irradiating laser light that is focused by an object lens onto a recording plane are used for reading information from recording media and writing information onto recording media. To increase the recording density of the recording medium, it is necessary to make the focus spot diameter of the object lens smaller, and therefore it is preferable that the image-side NA (numerical aperture) of the object lens is large. Furthermore, striving for a reduction of the reproduction and access times, it is important that the object lens is small and light. Conventionally, aspherical plastic lenses with NA ≈ 0.45 were used for the object lens in compact disk players, and aspherical molded glass lenses with NA ≈ 0.60 were used for the object lens in DVD players. In any case, conventional object lenses have an outer diameter of several mm (typically, about 4mm). In order to achieve further miniaturization of recording devices using for example optical disks, the object lens itself has to be made even smaller. However, since aspherical lenses are press-formed using a die, it is very difficult to make miniature aspherical lenses with a diameter of 1mm or less (see "Bishoukougaku Handobukku", p. 6, Optical Society of Japan, 1995). Rod lenses having a refractive index distribution in the radial direction can be used as optical imaging systems with an outer diameter of 1mm or less. The refractive index distribution for this kind of rod lenses can be expressed, for example, as (Eq. 1) n ( r ) 2 = n 2 0 · {1-( g · r ) 2 + h 4 ( g · r ) 4 + h 6 ( g · r ) 6 + h 8 ( g · r ) 8 + ...} wherein r is a distance in a radial direction from the optical axis of the rod lens; n(r) is the refractive index at a distance r in the radial direction from the optical axis of the rod lens; n 0 is a refractive index on the optical axis of the rod lens (central refractive index); and g, h 4 , h 6 , h 8 , etc. are refractive index distribution coefficients. Such a graded index lens can be manufactured, for example, from a rod-shaped glass by ion exchange (see for example "Hikarigijutsu Yougo Jiten" 2 nd Ed., by Shuuji KOYANAGI, Optronics, Tokyo 1998, p. 12). Using ion exchange, a miniature lens with an outer diameter of not more than 1mm can be manufactured at low cost. Furthermore, since the material itself has a positive refractive power, simple rod lenses having planar surfaces on both ends can be used as object lenses. The refractive power of a rod lens having the refractive index distribution of Eq. 1 can be expressed by n 0 · g · r 0 . The larger this value, the larger is the NA of the object lens. However, if a rod lens having a refractive index distribution in the radial direction is produced by ion exchange, there is a limit to the refractive index difference that can be attained, and a realizable range for n 0 · g · r 0 is n 0 · g · r 0 ≦ 0.71. However, to increase the recording density of recording media even further, there is a need for an object lens system with an even larger NA. Moreover, to achieve both a smaller device and faster access speeds, the number of parts has to be kept to a minimum, while making the object lens system smaller and lighter. Nishi H.; Ichikawa H.; Toyama M.; Kitano I.: "Gradient-index objective lens for the compact disk system" in: Applied Optics, vol. 25, no. 19, 1 October 1986, pages 3340-3345, XP002106460 and US 4 647 159 A disclose an objective lens system as specified in the preamble of claim 1. It is an object of the present invention to provide an objective lens system with a large NA that is smaller and lighter, while the number of parts has been kept to a minimum. To achieve this object, an objective lens system in accordance with the present invention comprises the features of claim 1. With this configuration of an object lens system, the refractive power of the entire lens is increased, and the image-side NA is increased. As a result, if the object lens system is used for an optical head, the recording density of the recording medium can be increased. Moreover, since the object lens system consists of a single graded index lens with a certain structure, a smaller and lighter object lens system can be achieved, while keeping the number of parts to a minimum and providing for higher access speeds. It is preferable that the radius r 0 of the portion of the lens that operates as a lens, measured from the optical axis of the lens, is in a range of 0.05mm ≦ r 0 ≦ 0.20mm. This configuration facilitates the manufacture of the graded index lens (especially, when the graded index lens is a rod lens) and the processing of the convex spherical surface, while the entire object lens system can be made smaller. Moreover, it is more preferable that the radius r 0 of the portion of the lens that operates as a lens, measured from the optical axis of the lens, is in a range of 0.10mm ≦ r 0 ≦ 0.15mm, most preferably about 0.125mm. It is preferable that this object lens system is mounted in an optical head, in a manner that the convex spherical surface of the lens faces a light source, and the planar surface of the lens faces a recording medium. With this configuration, the NA on the side of the recording medium can be increased, and the recording density of the recording medium can be increased. The lens is a rod lens, whose refractive index distribution is expressed by Eq. 1, i.e. (Eq. 1) n ( r ) 2 = n 2 0 · {1-( g · r ) 2 + h 4 ( g · r ) 4 + h 6 ( g · r ) 6 + h 8 ( g · r ) 8 + ...} wherein r is a distance in a radial direction from the optical axis of the rod lens, n 0 is a refractive index on the optical axis of the rod lens, and g, h 4 , h 6 , h 8 , etc. are refractive index distribution coefficients. In this case, it is preferable that the refractive index n 0 on the optical axis of the rod lens is in a range of 1.40 ≦ n 0 ≦ 1.80. This range for the refractive index n 0 is governed by the range which can actually be achieved for example with ion exchange. It is also preferable that a product n 0 · g · r 0 is in a range of 0.45 ≦ n 0 · g · r 0 ≦ 0.71, where the refractive index on the optical axis of the rod lens 1 is n 0 , the radius r 0 is the radius of the portion of the rod lens that operates as a lens, measured from the optical axis of the rod lens, and the coefficient g is a coefficient for the refractive index distribution. If n 0 · g · r 0 is less than 0.45, the NA cannot be increased (the lens cannot be made brighter) and if n 0 · g · r 0 exceeds 0.71, it is difficult to manufacture the rod lens for example by ion exchange. It is also preferable that the refractive index distribution coefficient h 4 is in a range of -1.5 ≦ h 4 ≦ 2.5. With this configuration, the spherical aberration of the entire object lens system can be suitably corrected. Moreover, L 2 / r 0 is in the range L 2 / r 0 ≦ 0.37, wherein L 2 is a distance between the planar surface of the lens and the image plane. With this configuration, the spherical aberration of the entire object lens system can be corrected easily. Furthermore, it is also preferable that L 1 / r 0 is in the range 5 ≦ L 1 / r 0 ≦ 20, wherein L 1 is a distance between the object plane (light source in an optical disk system) and the convex spherical surface of the lens. Usually, an optical fiber with an NA of about 0.05 to 0.2 is used as a light source in an optical disk system, so that by using the object lens system for such a finite system the widening of the light from the light source can be matched with the diameter of the rod lens, and the loss of light energy can be kept low. Moreover, it is also preferable that the distance L 1 between the image plane and the convex spherical surface of the lens is infinite. Such an infinite system is useful when using a parallel light beam for the light source. Fig. 1 is a cross-sectional diagram of an object lens system in accordance with the present invention. Fig. 2 is a perspective view of a rod lens used for an object lens system in accordance with the present invention. Fig. 3 is a graph of the refractive index distribution in a rod lens used for an object lens system in accordance with the present invention. Fig. 4(A) is a cross-sectional diagram of Example 1 of an object lens system in accordance with the present invention. Fig. 4(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 4(A). Fig. 5(A) is a cross-sectional diagram of Example 2 of an object lens system in accordance with the present invention. Fig. 5(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 5(A). Fig. 6(A) is a cross-sectional diagram of Example 3 of an object lens system in accordance with the present invention. Fig. 6(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 6(A). Fig. 7(A) is a cross-sectional diagram of Example 4 of an object lens system in accordance with the present invention. Fig. 7(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 7(A). Fig. 8(A) is a cross-sectional diagram of Example 5 of an object lens system in accordance with the present invention. Fig. 8(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 8(A). Fig. 9(A) is a cross-sectional diagram of Example 6 of an object lens system in accordance with the present invention. Fig. 9(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 9(A). Fig. 10(A) is a cross-sectional diagram of Example 7 of an object lens system in accordance with the present invention. Fig. 10(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 10(A). Fig. 11(A) is a cross-sectional diagram of Example 8 of an object lens system in accordance with the present invention. Fig. 11(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 11(A). Fig. 12(A) is a cross-sectional diagram of the Comparative Example of an object lens system in accordance with the present invention. Fig. 12(B) is a graph of the lateral spherical aberration of the object lens system in Fig. 12(A). The following is a detailed description of the preferred embodiments of the present invention, with reference to the accompanying drawings. In accordance with the present invention, an object lens system comprises (a) a rod lens 1 having a cylindrical transparent medium as shown in Figs. 1 and 2, which has a refractive index distribution in the radial direction and is used as a graded index lens, and (b) an object plane 2 and an image plane 3 which are arranged on respective sides of the rod lens 1. The surface on one side of the rod lens 1 is a convex spherical surface 1a, and the surface on the other side is a planar surface 1b. The convex spherical surface 1a faces the object plane 2, and the planar surface 1b faces the image plane 3 of the rod lens 1. In Fig. 1, an aperture stop 4 is further provided at the vertex of the convex spherical surface 1a of the rod lens 1. The refractive index n of the rod lens 1 has a distribution in the radial direction as shown in Fig. 3, which can be expressed by (Eq. 1) n ( r ) 2 = n 2 0 · {1-( g · r ) 2 + h 4 ( g · r ) 4 + h 6 ( g · r ) 6 + h 8 ( g · r ) 8 + ...} wherein r is the distance in the radial direction from the optical axis 1c of the rod lens 1, n(r) is the refractive index at the distance r in the radial direction from the optical axis 1c of the rod lens 1, n 0 is the refractive index on the optical axis 1c of the rod lens 1 (central refractive index), and g, h 4 , h 6 , h 8 etc. are the coefficients for the refractive index distribution. An object lens system in accordance with this embodiment satisfies the equations (Eq. 2) 1.0 ≤ R / r 0 ≤ 1.6 (Eq. 3) R / f ≤ 1.2 wherein r 0 is the radius of the portion that functions as a lens measured from the optical axis 1c of the rod lens 1, that is, the radius of the effective lens portion of the rod lens 1, R is the curvature radius of the convex spherical surface 1a, and f is the focal length of the rod lens 1. The radius r 0 of the effective portion of the rod lens 1 can be defined, for example, as "the range where the RMS wave front aberration on the optical axis 1c is below 0.07 λ". The radius r 0 of the effective portion of the rod lens 1 has an influence on the refractive power of the rod lens 1, and is one of the design parameters for the rod lens 1. In practical rod lenses 1, the refractive index distribution at a peripheral portion of the lens often deviates from the design value, so that this portion of the rod lens does not function well as a lens. For example, even when the actual outer diameter is 0.5mm, if the effective lens diameter is actually 0.4mm, then r 0 is 0.2mm. It is preferable that the radius r 0 of the effective lens portion of the rod lens 1 is 0.05mm ≦ r 0 ≦ 0.20mm. If the radius r 0 of the effective lens portion of the rod lens 1 is less than 0.05, then manufacturing of the rod lens 1 and the polishing of the convex spherical surface 1a becomes difficult. If the radius r 0 is more than 0.20, the object lens system cannot be miniaturized. Moreover, it is even more preferable that the radius r 0 of the effective lens portion of the rod lens 1 is 0.10mm ≦ r 0 ≦ 0.15mm, most preferably about 0.125mm. If the surface of this rod lens 1 on the side of the object plane 2 is a convex spherical surface 1a and the equations (Eq. 2) and (Eq. 3) are satisfied, the refractive power of the entire lens is increased, so that the NA on the image side can be increased. As a result, this rod lens 1 can increase the recording density of a recording medium, if the convex spherical surface 1a faces the light source and the planar surface 1b faces the recording medium in the optical head. Moreover, this object lens system is made of a single graded index lens (rod lens 1), keeping the number of parts to a minimum, which allows a smaller and lighter device, so that faster access can be accomplished. The realizable value for the refractive index n 0 on the optical axis 1c of the rod lens (central refractive index) depends on the rod lens material (glass or plastic), and is in the range of 1.40 ≦ n 0 ≦ 1.80. The brightness of the rod lens 1 is governed by the aperture angle, which indicates the range over which the lens accepts light. The aperture angle  (°) can be expressed by (Eq. 4)  = ( n 0 · g · r 0 )/( π /180) It is preferable that the product n 0 · g · r 0 is in the range of 0.45 ≦ n 0 · g · r 0 ≦ 0.71. If n 0 · g · r 0 is less than 0.45, the NA cannot be increased (the lens cannot be made brighter) and if n 0 · g · r 0 exceeds 0.71, it is difficult to manufacture the rod lens for example by ion exchange. Using a rod lens 1 with a refractive index distribution expressed by Eq. 1, spherical aberration of the entire object lens system can be corrected through the refractive index distribution coefficients h 4 , h 6 , h 8 , etc. If, for reasons of simplicity, h 6 and higher terms are set to zero, and the refractive index distribution of the rod lens 1 is approximately (Eq. 5) n ( r ) 2 = n 2 0 · {1 - ( g · r ) 2 + h 4 ( g · r ) 4 } then it is preferable that the refractive index distribution coefficient h 4 is in the range of -1.5 ≦ h 4 ≦ 2.5, because it is very difficult to manufacture a rod lens 1 having a refractive index distribution outside this range. Moreover, even when h 6 and higher terms are included, spherical aberration can be corrected if h 4 is within this range. If L 2 / r 0 , wherein L 2 is the distance between the planar surface 1b of the rod lens 1 and the image plane 3, satisfies (Eq. 6) L 2 / r 0 ≤ 0.37 the spherical aberration of the entire object lens system can be corrected easily. L 2 is also referred to as the "working distance". An object lens system having this configuration is mounted in an optical head so that, for example, the convex spherical surface 1a of the rod lens 1 faces the light source, and the planar surface 1b faces the recording medium. Because the diameter of the rod lens 1 is extremely small, and the distance L 2 between planar surface 1b and the image plane 3 is very small, the object lens system can be mounted in the optical head in a position where it is very close to the recording medium. It is assumed that a monochromatic light source such as a laser is used for the light source. It is preferable that the wavelength λ of this light source is 0.3 µ m - 2.0 µ m. For wavelengths λ of less than 0.3 µ m, the transmittivity of the rod lens 1 typically reduces, and if the wavelength λ is more than 2.0 µ m, the spot diameter at the focus becomes large and the recording density of the recording medium decreases. Moreover, assuming that an optical fiber with an NA of about 0.05 - 0.2 is used for the light source, it is preferable that the object lens system is used as a finite system, where the distance L 1 between the object plane 2 and the convex spherical surface 1a of the rod lens 1 satisfies the condition 5 ≦ L 1 / r 0 ≦ 20. If a single mode fiber is used for the light source, it is more preferable that the condition 8 ≦ L 1 / r 0 ≦ 12 is satisfied, because then the NA is about 0.1. By using the rod lens 1 in such a finite system satisfying these conditions, the widening of the light beam from the light source can be matched to the diameter of the rod lens 1, and the loss of light energy can be kept low. However, if parallel light is used for the light source, the system can also be designed as an infinite system wherein the distance L 1 between the object plane 2 and the convex spherical surface 1a of the rod lens 1 is infinite. Table 1 lists specific examples. Figs. 4 to 11 correspond to the Examples 1 to 8 in Table 1, whereas Fig. 12 corresponds to the Comparative Example in Table 1. Each of those diagrams shows (A) the configuration of the object lens system, and (B) its lateral spherical aberration. In the diagrams (A) of the object lens system, numeral 1 denotes the rod lens, numeral 2 denotes the object plane, numeral 3 denotes the image plane, and numeral 4 denotes the aperture stop. In the diagrams (B) of the lateral spherical aberration, the end of the vertical axis indicates the inner edge of the aperture stop 4. As can be seen from Table 1 and the Figs. 4 to 12, the NA on the image side is 0.556 in the case of a simple rod lens wherein both lens surfaces are planar surfaces (Comparative Example), whereas with the Examples 1 to 8, a large value of at least 0.6 can be attained for the NA on the image side, and the spherical aberration can be corrected. Especially with lens design parameters as in the Examples 1 to 3, an especially preferable result of at least 0.7 NA on the image side can be attained. However, when the curvature radius R of the convex plane 1a becomes very small as in Example 8, a correction of the spherical aberration by adjusting only the refractive index distribution coefficient h 4 is difficult, and it is necessary to set the refractive index distribution coefficient h 6 to an appropriate value as well. In the present embodiment, an aperture stop 4 is provided at the vertex of the convex spherical surface 1a of the rod lens 1. However, since in the present invention only the paraxial image is used, the aperture stop 4 in an actual product can be provided by an integrated frame portion such as a lens holder. Furthermore, in the present embodiment, a rod lens 1 has been given as an example for a graded index lens, however, the present invention is not limited to this rod lens 1, and can be any transparent medium having a refractive index distribution in the radial direction, which has a convex spherical surface on one side and a planar surface on the other side.

Claims: Objective lens system comprising a lens made of a transparent medium with a refractive index distribution in a radial direction, comprising the following features: a first surface of the lens is a convex spherical surface, and a second surface of the lens is a planar surface; the lens is arranged so that the convex spherical surface faces an object surface, and the planar surface faces an image surface; the equations 1.0 ≤ R / r 0 ≤ 1.6, and R / f ≤ 1.2 are satisfied, wherein r 0 is a radius of a range where the aberration is below a predetermined level, measured from an optical axis of the lens, R is a curvature radius of the convex spherical surface, and f is a focal length of the lens; the lens is a rod lens, whose refractive index distribution is expressed by n(r) 2 = n 2 0 · {1-(g·r) 2 + h 4 (g·r) 4 + h 6 (g·r) 6 + h 8 (g·r) 8 + ....} wherein r is a distance in a radial direction from the optical axis of the rod lens, n 0 is a refractive index on the optical axis of the rod lens, and g, h 4 , h 6 , h 8 , etc. are refractive index distribution coefficients, L 2 / r 0 is in a range of L 2 / r 0 ≦ 0.37, wherein L 2 is a distance between the planar surface of the lens and the image surface. The objective lens system according to claim 1, further characterized in that L 1 / r 0 is in a range of 5 ≦ L 1 / r 0 ≦ 20, wherein L 1 is a distance between the objective surface and the convex spherical surface of the lens. The objective lens system of claim 1, further characterized in that the radius r 0 is in a range of 0.05mm ≦ r 0 ≦ 0.20mm. The objective lens system of claim 1, further characterized in that the convex spherical surface of the lens faces a light source, and the planar surface of the lens faces a recording medium. The objective lens system of claim 1, further characterized in that the refractive index n 0 on the optical axis of the rod lens is in a range of 1.40 ≦ n 0 ≦ 1.80. The objective lens system of claim 1, further characterized in that a product n 0 · g · r 0 is in a range of 0.45 ≦ n 0 · g · r 0 ≦ 0.71. The objective lens system according to claim 1, further characterized in that the refractive index distribution coefficient h 4 is in a range of -1.5 ≦ h 4 ≦ 2.5. The objective lens system according to claim 1, further characterized in that a distance L 1 between the object surface and the convex spherical surface of the lens is infinite. Optical head, comprising an objective lens system defined in the preceeding claims 1 to 8.

Inventors: KAKIMOTO KOJI, KITTAKA SHIGEO, KOSHI HIROSHI, KAKIMOTO, KOJI, KITTAKA, SHIGEO, KOSHI, HIROSHI
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO., LTD.
Application year: 1999
Main classification: G02B3
IPC codes: G02B3, G02B13