Abstract:
Disclosed are a lens efficiently manufactured into an accurate shape at a low cost, a manufacturing method thereof, and an optical pickup using the lens. The lens includes a geometrical optics portion for converging light having been incident thereon from its light incoming plane, and two diffraction optics portions provided on the light incoming plane and a light outgoing plane of the geometrical optics portion, wherein the diffraction optics portions are made from a material different from that of the geometrical optics portion.

Description:
RELATED APPLICATION DATA 
     This application is a divisional application of U.S. application Ser. No. 09/352,753 filed Jul. 14, 1999 now U.S. Pat. No. 6,215,591. The present application and the parent application claim priority to Japanese application No. P10-203829 filed Jul. 17, 1998. All of the foreign applications are incorporated herein by reference to the extent permitted by law. 
    
    
     BACKGOUND OF THE INVENTION 
     The present invention relates to a lens, a manufacturing method thereof, and an improved optical pickup, and particularly to a lens capable of being accurately and efficiently manufactured, a manufacturing method thereof, and an optical pickup using the lens. 
     At present, the recording density of an optical disk has been increased as exemplified by a DVD (Digital Video Disk), and to realize the high density of an optical disk, it has been required to shorten the wavelength of a laser beam and to make a beam spot size small. In order to make a beam spot size small, it is necessary to increase the numerical aperture (hereinafter, referred to as “NA”) of a lens. The numerical aperture of a lens can be increased by making the diameter of the lens large, however, the size of the lens is restricted if the lens is assembled into a small-sized mechanical deck such as an optical pickup. In particular, it is difficult to manufacture a lens which is small in diameter having large radius of curvature. For this reason, to obtain a lens having a high NA, there has been used a so-called hologram integrally formed lens in which diffraction optics portions configured as blazed holograms are formed on a light incoming plane and a light outgoing plane of a geometrical optics portion configured as an aspherical lens. In addition, the hologram integrally formed lens is also used as an objective lens adapted to obtain two focal points for one lens. 
     A prior art lens manufacturing apparatus for manufacturing a hologram integrally formed lens is shown in FIGS. 1A and 1B. A lens manufacturing apparatus  1  shown in FIG. 1A includes an upper die  2 , a lower die  3 , and a core die  4 . A hollow portion having a shape equivalent to that of a lens is formed between the upper die  2  and the lower die  3 , and glass as a material for forming the lens is supplied to the hollow portion. The upper die  2  is movable in the direction shown by an arrow Y 2  along the core die  4 , to apply a pressure to glass. A hologram integrally formed lens is manufactured by executing the steps of; supplying glass into the hollow portion as shown in FIG. 1A, heating the upper die  2  and the lower die  3  to a glass formable temperature, moving the upper die  2  in the direction Y 2  to press-form the glass, thereby transferring the shape of the die onto the glass. After cooling glass down, a hologram integrally formed lens is obtained. 
     Here, each of the upper and lower dies  2  and  3  must be accurately formed into a shape equivalent to that of a lens to be formed. FIGS. 2A and 2B show a state in which the die is shaped by machining. In addition, since the process of shaping the upper die  2  is the same as the process of shaping the lower die  3 , only the latter process will be described with reference to FIGS. 2A and 2B, omitting the description of the upper die  2 . Referring to FIG. 2A, the lower die  3  is composed of a base member  3   b  and a layer  3   a  to be machined. The layer  3   a  is formed of, for example, a film made from a noble metal such as platinum (Pt) or iridium (Ir). The reason for this is that since the melting point of glass is high, the layer  3   a  is required to be made from such a material which is not to be fusion-bonded to glass upon formation of the lens. The layer  3   a  is machined into a shape equivalent to that of a diffraction optics portion of a lens using a bite  5  made from diamond or the like. Then, as shown in FIG. 2B, after formation of the layer  3   a  into a specific shape, a protective layer  6  is formed on the surface of the layer  3   a  for preventing the diffraction optics portion from losing its shape. 
     The above method of machining the layer formed of the film made from a noble metal poor in machinability such as platinum (Pt) or iridium (Ir), however, has a problem in occurrence of heavy wear of the bite. To solve such a problem, it may be considered to replace the material of the layer to be machined, from a noble metal to a material exhibiting good machinability, for example, electroless-plated nickel, however, in this case, the electroless nickel plating has another problem that since the melting point of glass is high, the electroless-plated nickel layer on the surface of the die may be fusion-bonded to glass or severely consumed upon formation of the lens. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a lens having a high numerical aperture which is capable of being accurately, efficiently manufactured at a low cost, a manufacturing method thereof, and an optical pickup using the lens. 
     To achieve the above object, according to a first aspect of the present invention, there is provided a lens including; a geometrical optics portion for converging light having been incident thereon from its light incoming plane, and two diffraction optics portions provided on the light incoming plane and a light outgoing plane of the geometrical optics portion, wherein the diffraction optics portions are made from a material different from that of the geometrical optics portion and joined each other. With this configuration, the diffraction optics portions, which are made from the material different from that of the geometrical optics portion, for example, a resin, are joined to the light incoming plane and the light outgoing plane of the geometrical optics portion, and accordingly it is possible to easily form the diffraction optics portions. 
     To achieve the above object, according to a second aspect of the present invention, there is provided a method of manufacturing a lens including a geometrical optics portion and two diffraction optics portions, including the steps of; inserting a material for forming the geometrical optics portion in a first die having a shape equivalent to that of the geometrical optics portion, press-forming the material by applying a pressure to the first die to form the geometrical optics portion, pouring a material for forming the diffraction optics portions in a second die having shapes equivalent to those of the diffraction optics portions, and inserting the geometrical optics portion having been formed by press-forming in the second die to join the geometrical optics portion to the diffraction optics portions. With this configuration, since a material for forming the diffraction optics portions, for example, a resin is joined to the light incoming plane and the light outgoing plane of the previously formed geometrical optics portion to form the diffraction optics portions, and accordingly, it is possible to easily form the diffraction optics portions on the geometrical optics portion. 
     To achieve the above object, according to a third aspect of the present invention, there is provided an optical pickup including; a light source for outputting a laser beam, a light splitting means for splitting the laser beam emitted from the light source into pieces, an objective lens for converging the laser beam from the light splitting means on a signal recording plane of an optical recording medium, and an optical detector for detecting a return laser beam reflected from the signal recording plane, wherein the objective lens includes a geometrical optics portion and two diffraction optics portions formed on the geometrical optics portion, the diffraction optics portions being made from a material different from that of the geometrical optical portion. With this configuration, the objective lens is configured such that the diffraction optics portions, which are made from the material different from that of the geometrical optics portion, are joined to the light incoming plane and the light outgoing plane of the geometrical optics portion, and accordingly it is possible to easily form the diffraction optics portions on the geometrical optics portion, and hence to provide a high performance optical pickup at a low cost by using a lens which has a high numerical aperture and a small spherical aberration and which can be efficiently manufactured at a low cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are configuration views showing one example of a manufacturing apparatus of a hologram integrally formed lens in a related art; 
     FIGS. 2A and 2B are configuration views showing a state in which a die shown in FIGS. 1A and 1B is shaped by machining; 
     FIG. 3 is a system diagram showing a preferred embodiment of an optical pickup of the present invention; 
     FIG. 4 is a sectional view showing a preferred embodiment of a lens of the present invention; 
     FIGS. 5A to  5 C are configuration views showing a state in which a geometrical optics portion of the lens shown in FIG. 4 is manufactured; and 
     FIGS. 6A to  6 E are configuration views showing a state in which a diffraction optics portion of the lens shown in FIG. 4 is manufactured. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 3 is a configuration diagram showing a preferred embodiment of an optical pickup of the present invention. Now, an optical pickup  10  will be described in detail with reference to FIG.  3 . 
     The optical pickup  10  shown in FIG. 3 includes a light source  11 , a beam splitter  12  as a light splitting means, a collimator lens  13 , an objective lens  14 , and an optical detector  15 . 
     The light source  11  is adapted to emit a laser beam-having a wavelength λ of 650 nm for recording and/or reproducing information on and/or from an optical disk D and is configured as a semiconductor laser element. A laser beam emitted from the light source  11  is incident on the beam splitter  12 . The beam splitter  12  has a beam splitter film  12   a  tilted approximately 45 with respect to the optical axis. The laser beam having been incident on the beam splitter  12  is emerged from the beam splitter  12  and is incident on the collimator lens  13 . 
     The laser beam having been incident on the collimator lens  13  is collimated and is made incident on the objective lens  14 . The laser beam is then converged on a signal recording plane D 1  of the optical disk D through the objective lens  14 . A return laser beam reflected from the signal recording plane of the optical disk D is made sequentially incident on the objective lens  14 , collimator lens  13 , and beam splitter  12 , and is reflected from the beam splitter film  12   a  to enter the optical detector  15 . The optical detector  15  is adapted to convert the return laser beam into an electric signal and output a readout signal or the like. 
     FIG. 4 is a sectional view showing a preferred embodiment of the objective lens  14  of the present invention. The objective lens  14  shown in FIG. 4 is composed of a geometrical optics portion  20  and a diffraction optics portion  21 . The geometrical optics portion  20  is formed of an aspherical lens made from, for example, glass. To be more specific, the geometrical optics portion  20  is of a convex lens for converging a laser beam having been incident thereon from a light incoming plane  20   a . A light incoming plane side diffraction optics portion  21   a  and a light outgoing plane side diffraction optics portion  21   b  are formed on the light incoming plane  20   a  and the light outgoing plane  20   b  of the geometrical optics portion  20 , respectively. The diffraction optics portion  21  composed of the light incoming plane side diffraction optics portion  21   a  and the light outgoing plane side diffraction optics portion  21   b  is made from, for example, a CR39 (thermosetting resin) or a photo-setting resin such as an ultraviolet curing resin. The diffraction optics portion  21  is formed into a blazed shape or serrated shape. 
     The laser beam having been incident on the objective lens  14  from the light incoming plane  20   a  is nearly all diffracted and converged by the light incoming plane side diffraction optics portion  21   a  to be made incident on the geometrical optics portion  20 . The geometrical optics portion  20  converges the incident laser beam by the function of the convex lens, and makes the laser beam incident on the light outgoing plane side diffraction optics portion  21   b . The light outgoing plane side diffraction optics portion  21   b  diffracts nearly all of the quantity of the incident laser beam, to thereby further converge the laser beam. Accordingly, the objective lens  14  attains a high numeral aperture NA. For example, the objective lens  14  shown in FIG. 4, which has a width L of 3.035 mm and a diameter φ of 5.76 mm, attains the numerical aperture NA of 0.85. 
     In this embodiment, to diffract nearly all the quantity of the incident laser beam, the diffraction optics portion  21  is configured to have a depth ranging from 600 nm to 700 nm and a pitch ranging from 0.020 mm to 0.340 mm, wherein the depth and the pitch are changed in the radial direction in such a manner that the pitch becomes smaller and the depth becomes narrower in the direction from the center to the outer periphery of the objective lens  14 . 
     FIGS. 5A to  5 C and FIGS. 6A to  6 E are configuration views showing a state of manufacturing the objective lens  14 . A process of manufacturing the objective lens  14 , called a replica process, will be described in detail with reference to FIGS. 5A to  5 C and FIGS. 6A to  6 E. The process of manufacturing the objective lens  14  is generally divided into a process of manufacturing the geometrical optics portion  20  and a process of manufacturing the diffraction optics portion  21 . First, the process of manufacturing the geometrical optics portion  20  will be described with reference to FIGS. 5A to  5 C. 
     FIG. 5A shows a die  30  composed of an upper die  31  and a lower die  32  between which a space having a shape being substantially the same as that of the geometrical optics portion  20  is formed for inserting glass therein. First, glass is inserted in the space between the upper and lower dies  31  and  32 . 
     In this case, a predetermined amount of glass is inserted to be formed into a spherical shape. Then, to avoid fusion-bonding between the die  30  and the glass, an inert gas is injected in the die  30 , and the glass is heated up to a glass formable temperature. After the glass is heated until it exhibits a specific viscosity, the upper die  31  is moved, for example, in the direction shown by an arrow Y 2  as shown in FIG. 5B to press-form the glass by applying a pressure thereto. The glass is gradually cooled and then rapidly cooled, to obtain the geometrical optics portion  20  shown in FIG.  5 C. 
     Next, the process of manufacturing the diffraction optics portion  21  will be described with reference to FIGS. 6A to  6 E. In addition, although only the process of forming the diffraction optics portion  21  on the light incoming plane  20   a  of the geometrical optics portion  20  shown in FIG. 4 will be described with reference to FIGS. 6A to  6 E, the diffraction optics portion  21  is also actually formed on the light outgoing plane  20   b  by an upper die (not shown). First, as shown in FIG. 6A, there is prepared a lower die  40  having a surface formed into a shape equivalent to that of the diffraction optics portion  21 . The lower die  40  is formed of a main body made from a stainless steel based material or a sintered hard alloy such as tungsten carbide, wherein electroless nickel plating is applied to the main body. The electroless nickel plating layer of the lower die  40  is machined by single point turning using a bite made from diamond, to form a shape equivalent to that of the diffraction optics portion  21 . By provision of the electroless nickel plating layer on the surface of the lower die  40 , the machinability of the lower die  40  is improved, to thereby easily form the shape equivalent to that of the diffraction optics portion  21  on the surface of the lower die  40 . In this case, since the diffraction optics portion  21  is made from a resin formable at a temperature relatively lower than the glass formable temperature, there is no fear of fusion-bonding of the electroless nickel plating layer formed on the lower die  40  upon formation of the diffraction optics portion  21  by the electroless nickel plating layer formed on the lower die  40 . 
     A resin for forming the diffraction optics portion  21  is supplied to the lower die  40 . Then, the resin is subjected to vacuum-defoaming, and the lower die  40  is rotated at a high speed in the direction shown by an arrow R 11  around an axis CL of the lower die  40 . Thus, as shown in FIG. 6B, there remains the resin in an amount necessary for the diffraction optics portion  21  to be formed on the lower die  40 . 
     Then, as shown in FIG. 6C, the geometrical optics portion  20  manufactured in the process shown in FIGS. 5A to  5 C is mounted on the resin coated on the lower die  40 , whereby the diffraction optics portion  21   a  is joined to the light incoming plane  20   a  of the geometrical optics portion  20 . Next, as shown in FIG. 6D, in the case of using the ultraviolet curing resin, the resin is irradiated with ultraviolet rays, and in the case of using the thermosetting resin, the resin is cured by heating the die. In this way, as shown in FIG. 6E, the diffraction optics portion  21  is formed on the light incoming plane  20   a  of the geometrical optics portion  20 . 
     According to the above-described embodiment, the lens  14  having a high numerical aperture and a good transmission wave front aberration can be manufactured. Also, in manufacturing the lens  14  having the geometrical optics portion  20  and the diffraction optics portion  21 , since the geometrical optics portion  20  and the diffraction optics portion  21  are separately formed, it is possible to accurately obtain the shape of the diffraction optics portion  21  by transfer from the die  40  and also to manufacture the lens  14  at a low cost. Further, since the die  40  for forming the diffraction optics portion  21  can be easily machined, the lens  14  can be efficiently manufactured. In addition, the total cost of the optical pickup  10  can be reduced by using the lens  14  efficiently manufactured at a low cost as the objective lens. 
     The present invention is not limited to the above embodiment. While the objective lens  14  is manufactured in accordance with the so-called replica process shown in FIGS. 5A to  5 C and FIGS. 6A to  6 E, it may be manufactured by a so-called insert molding process of inserting the geometrical optics portion  20  made from glass in an injection mold for molding the diffraction optics portion  21 , and injecting molding a resin around the geometrical optics portion  20 . Although the lens  14  shown in FIGS. 5A to  5 C and FIGS. 6A to  6 E is configured as the objective lens  14  having a high numerical aperture, it may be configured as a bifocal lens in which the geometrical optics portion  20  is combined with the diffraction optics portion  21 . 
     Further, in the above embodiment, the lens of the present invention is used as the objective lens  14  of the optical pickup  10  shown in FIG. 3, it can be applied to a lens for converging light, for example, an image pickup lens. In addition, the objective lens  14  shown in FIG. 4 is manufactured in accordance with the two processes, however, the two process can be carried out using the same apparatus only by changing the die. 
     While the preferred embodiments of the present invention have been described using the specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.