RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP2010/052726 filed Feb. 23, 2010. 
     
    
       [0002]    This application claims the priority of Japanese application No. 2009-082300 filed Mar. 30, 2009, the entire content of which is hereby incorporated by reference. 
       TECHNICAL FIELD 
       [0003]    The present invention relates to a lens with a NA of 0.7 or more in highly accurate shape. 
       BACKGROUND ART 
       [0004]    In recent years, research and development of a high density optical disc system capable of recording and/or reproducing (hereinafter, “record and/or reproduce” will be referred as “record/reproduce”) information by using a blue-violet semiconductor laser with wavelength of about 400 nm are proceeding swiftly. As an example, in the case of an optical disc on which information is recorded/reproduced under the specifications that NA is 0.85 and alight source wavelength is 405 an, namely, in the case of the so-called Blu-ray Disc (hereinafter, BD), it is possible to record information of 23 to 27 GB per layer for an optical disc with a diameter of 12 cm, which is same in size as DVD (NA is 0.6, wavelength of a light some is 650 nm, and memory capacity is 4.7 GB). This optical disc is referred as a high density optical disc in the present specification. 
         [0005]    As an objective lens for use in an optical pickup apparatus, there is disclosed an objective lens obtained by molding resin, for example, in Patent Literature 1. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP-A No. 2003-15031 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    As for the lens disclosed in Patent Literature 1, a connecting area is arranged on a portion on a lens surface which is outside how an intersection of the outermost ray within numerical aperture NA and the lens surface, in order to avoid defective molding of a lens. According to a result of study of the present inventor, it has been found that, even when such the connection area is provided, defective molding such as flow marks can be generated under the condition that a thickness deviation ratio (which will be described below) is large. On the other hand, excessively small thickness deviation ratio makes the flange thickness thick, and the effective aperture is hardly enlarged. Accordingly, in a high-NA lens with a large effective aperture, a lens design cannot be achieved in view of optics under the condition that the thickness deviation ratio is small. 
         [0008]    The present invention has been achieved in view of the above problem in the prior art, and is intended to provide a lens which allows to control defects caused in a molding process in a high-NA lens and which is easy to handle. 
       Solution to Problem 
       [0009]    A lens described in Item 1 is a lens which is molded out of a resin material, comprises a flange section on a periphery and has a numerical aperture of 0.7 or more, characterized by comprising:
       a first surface including an optical axis;   a second surface having a curvature which is smaller than a curvature of the first surface, formed on aside opposite to the first surface;   an intermediate end surface formed at an area between the first surface and the flange section along a direction perpendicular to the optical axis; and   a transition surface connecting the intermediate end surface and the first surface,   wherein an angle θ 1  (which is an acute angle) formed between a normal line of the first surface at a bonier of the transition surface and the first surface and the optical axis, is larger than an angle θ 2  formed between a normal line of the transition surface at a border of the first surface and the transition surface and the optical axis, and   the lens satisfies the following expression, where d (mm) is a distance between the first surface and the second surface on the optical axis and t (mm) is a minimum thickness along the optical axis on the intermediate end surface.       
 
         [0000]      4.1 &gt;d/t&gt; 2.8  (1)
 
         [0016]    According to the results of the study of the present inventor, it has been found that, though the defective molding can be controlled by arranging the transition surface, the defective molding such as flow marks can be generated even when the transition surface is provided under the condition that d/t (which is referred as a thickness deviation ratio) is 4.1 or more. On the other hand, under the condition that d/t is 2.8 or less, the thickness of a flange becomes excessively thin and the lens is hardly attached to a device with accuracy, for example, when it is an objective leas for an optical pickup apparatus. Therefore, by satisfying the expression (1), a lens with controlled defective molding and excellent property of attachment can be provided. 
         [0017]    A lens descried in Item 2 is characterized by satisfying the following accession, in the leas of Item 1. Therefore, a lens with NA of 0.7 or more can be molded while the defective molding is controlled more effectively. 
         [0000]      3.8 &gt;d/t&gt; 3.3  (1′)
 
         [0018]    A lens described in Item 3 is characterized by satisfying the following expression, in the lens of Item 1 or 2, where Δ (mm) is a thickness of the flange section along the optical axis. 
         [0000]        t&lt;Δ   (2)
 
         [0019]    For example, in an objective lens for use in a general optical pickup apparatus, a flange section which connects to a connecting area has a uniform thickness. Such the form can control the defective molding with relatively ease, by providing a connecting mea. On the other hand, in recent years, a lens achieving both of a thin axial thickness and a thick flange section has been desired. In such the lens, the intermediate end surface is needed to be located closer to the second surface than a surface of the flange section at the first-surface side. Under such the situation, the flow direction of resin which goes to a cavity changes rapidly, which causes defective molding such as flow marks easily. According to that, when the lens is configured such that the thickness deviation ratio d/t satisfies the expression (1), the defective molding can be controlled effectively. In other words, it is especially preferable that the thickness deviation ratio satisfies the expression (1) in a lens satisfying the expression (2). 
         [0020]    A lens described in Item 4 is characterized by satisfying the following expression, in the lens of any one of Items 1 to 3, where D 1  is a diameter of an area from the optical axis to an inner circumference of the transition surface and D 2  is a diameter of an area from the optical axis to an inner circumference of the intermediate and surface. 
         [0000]      0.88 &lt;D 1 /D 2&lt;0.98  (3)
 
         [0021]    When the value of D 1 /D 2  is the upper limit of the expression (3) or more, D 1 ≈D 2  holds, which makes a forming process of a mold for molding the above lens difficult. On the other hand, the value of D 1 /D 2  is the lower limit of the expression (3) or less, the transition surface becomes excessively large, which brings decrease of the thickness deviation ratio d/t and is not preferable. Therefore, the condition satisfying the expression (3) is preferable. 
         [0022]    A lens described in Item 5 is characterized by satisfying the following expression, in the lens of Item 4. 
         [0000]      0.94&lt; D 1 /D 2&lt;0.975  (3′).
 
         [0023]    A lens described in Item 6 is characterized in that, in the lens of any one of Items 1 to 5, a width of the intermediate end surface along the direction perpendicular to the optical axis is 50 μm or more. Therefore, it has an advantage that a mold for molding the lens can be costly formed. 
         [0024]    A lens described in Item 7 is characterized in that, in the lens of any one of Items 1 to 6, a diffractive structure is formed on at least apart of the fast surface. As for a lens baying a diffractive structure formed on at least a part of the first surface, its molding is especially difficult. Therefore, in the case that the lens is a lens which has compatibility for three media including a BD and which includes diffractive structures on all the surfaces, its molding process is further more difficult and the effect of the present invention is hoped. 
         [0025]    That is no limitation for resin, as far as it is a transparent thermoplastic resin which is generally used as an optical material. However, in view of workability as an optical element, the followings are preferable: actyl resin, cyclic olefin resin, polycarbonate resin, polyester resin, polyether resin, polyimide resin and polyimide resin. As an applicable thermoplastic resin, for example, there can be cited compositions described in JP-A No. 2003-73559. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0026]    According to the present invention, there can be provided a lens such that defects caused in a molding process can be controlled in a high-NA lens and an excellent attachment property is exhibited. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]    Each of  FIGS. 1   a  to  1   d  are diagrams showing steps of molding a lens relating to the present embodiment by using a mold. 
           [0028]      FIG. 2  is an enlarged view of the mold. 
           [0029]      FIG. 3  shows lens OE of the present embodiment, molded with a mold. 
           [0030]      FIG. 4   a  is an enlarged view of a section  11  pointed by an avow in lens OE in  FIG. 3 , and  FIG. 4   b  is the similar view of lens OE′ of a comparative example. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]    Embodiments of the present invention will be described below with referring to the drawings.  FIGS. 1   a  to  1   d  showing steps of molding a lens with NA of 0.7 or more for use in an optical pickup apparatus for a BD, by using a mold.  FIG. 2  is an enlarged view of the mold. The mold includes first mold  10 , second mold  20  and insert  30  which form a cavity under the condition that they are closed. Each of  FIGS. 1   a  to  1   d  shows an outline form of the cavity. 
         [0032]    As shown in  FIG. 2 , fast mold  10  arranged to face second mold  20  includes opening  11   a  in cylindrical shape, and insert  30  in cylindrical shape is fitted with the inside of opening  11   a . On the end surface of insert  30  which protrudes toward second mold  20 , there are formed first optical surface transferring surface  31   a  for transferring and molding the first optical surface of a lens; transition surface transferring surface  31   b  for forming a transition surface, arranged next to the first optical surface transferring surface  31   a ; intermediate end surface transferring surface  31   c  for forming a intermediate end surface, arranged next to transition surface transferring surface  31   b ; and taper surface  31   d  arranged next to intermediate end surface transferring surface  31   c . Intermediate end surface transferring surface  31   c  is located at a position closest to second mold  20 , and is formed so as to be perpendicular to the axis X. On first optical surface transferring surface  31   a , there may be formed a fine structure such as ring-shaped zones corresponding to a diffractive structure to be formed on a lens. 
         [0033]    On first mold  10 , there are formed first flange end transferring surface  11   b  to be connected to taper surface  31   d  of insert  30  which is fitted into first mold  10 , for transferring and molding the first end surface of a flange section of a lens; and first flange circumference transferring surface  11   c  which is perpendicular to first flange end transferring surface  11   b , for transferring and molding the circumference surface of the flange section. First flange end transferring surface  11   b  is perpendicular to axis X of first optical surface transferring surface  31   a  (an optical axis of the lens). It is preferable that a chamfer section is arranged on an intersectional position of first flange end transferring surface  11   b  and fast flange circumference transferring surface  11   c.    
         [0034]    Second mold  20  includes, around optical axis X as the center, second optical surface transferring surface  21   a  for transferring and molding the second optical surface of the lens; taper surface  21   b  arranged next to second optical surface transferring surface  21   a ; second flange end transferring surface  21   c  arranged next to taper surface  21   b , for transferring and molding the second end surface of the flange section; and second flange circumference transferring surface  21   d  which is perpendicular to second flange end transferring surface  21   c , for transferring and molding the circumference of flange section. Second flange end transferring surface  21   c  is perpendicular to central axis X of second optical surface transferring surface  21   a  (the optical axis of the lens). It is preferable that a chamfer section is arranged on an intersectional position of second flange end transferring surface  21   c  and second flange circumference transferring surface  21   d , too. 
         [0035]    As shown in As shown in  FIGS. 1   a  to  1   c , gate GT (entrance channel) is formed on first mold  10 . 
         [0036]    Next, a method of molding a lens will be described. At first, insert  30  is inserted into opening  11   a  of first mold  10  as shown in  FIG. 1   a , and first mold  10  is set to face second mold  20 . After that, as shown in  FIG. 1   b , first mold  10  and insert  30  are located to approach and come in contact with second mold  20  relatively, and mold closing is carried out with a predetermined pressure kept. At that time, positions of butting end sections of first flange circumference transferring surface  11   c  and opposing second flange circumference transferring surface  21   d  coincide with each other. 
         [0037]    Optical surface transferring surfaces  31   a  and  21   a  art heated to a predetermined temperature at the time of mold closing by heating first mold  10  and second mold  20  by a heater which is not illustrated. After that, resin which is pressurized at an arbitral pressure is supplied through runner  22  and gate GT from a nozzle which is not illustrated (see  FIG. 1   c ). 
         [0038]    Next, after molten resin is solidified with shapes of transferring surfaces  31   a  to  31   d ,  11   b  to  11   c  and  21   a  to  21   d  being transferred thereon, the mold temperature is decreased to cool and solidify the resin. 
         [0039]    After that, when first mold  10  and insert  30 , and second mold  20  are relatively moved to carry out mold opening, a molded body including lens OE is exposed with sticking to fast mold  10 . When lens OE is separated from such the molded body, lens OE as a single body is formed. 
         [0040]    According to the present embodiment, transition surface transferring surface  31   b  for forming a transition surface is formed between the first optical surface transferring surface  31   a  and intermediate end surface transferring surface  31   c , which can prevent a rapid change of the direction of resin moving toward a cavity in a molding process and allows highly accurate molding with controlling generation of flow marks even when lens OE with large thickness deviation ratio is molded. 
         [0041]      FIG. 3  shows a sectional view of lens OE molded with a mold shown in  FIGS. 1   a  to  1   d  and  2 , relating to the present embodiment.  FIG. 4   a  is an enlarged view of the section II which is pointed by an arrow in lens OE in  FIG. 3 , and  FIG. 4   b  is a similar view of lens OE′ of a comparative example. Hereinafter, a lens of the present embodiment will be described, being compared with the comparative example. 
         [0042]    In  FIG. 3 , lens OE of the present embodiment includes, around optical axis X as the center, optical surface (first surface) OE 11   a  formed in spherical shape and including optical axis X; transition surface OE 11   b  formed in ringed shape and arranged next to the first optical surface; intermediate end surface OE 11   c  formed in ringed shape and arranged next to the transition surface; taper surface OE 11   d  arranged next to the intermediate end surface; first end surface OE 11   e  of flange section OEF formed in ringed shape and arranged next to the taper surface; and circumference surface OE 11   f  of flange section OEF arranged to be perpendicular to the first end surface. A diffractive structure may be arranged on fast optical surface OE 11   a . Lens OE includes second optical surface (second surface) OE 21   a  formed in spherical shape and concentrically facing first optical surface OE 11   a ; taper surface OE 21   b  formed in ringed shape and arranged next to the second optical surface; and second end surface OE 21   c  of the flange section arranged next to the taper surface. When the lens is attached to an optical pickup apparatus which is not illustrated, first optical surface OE 11   a  faces a light source and second optical surface OE 21   a  faces an optical disc. Herein, a distance between first optical surface OE 11   a  and second optical surface OE 21   a  along optical axis X is represented as d (axial thickness). The diameter of an area up to the inner circumference of transition surface OE 11   b  is represented as D 1 , and the diameter of an area up to the inner circumference of the intermediate end surface OE 11   c  is represented as D 2 . 
         [0043]    In lens OE′ of a comparative example shown in  FIG. 4   b , a transition surface does not exist and first optical surface OE 11   a  directly connects to intermediate end surface OE 11   c . On the other hand, in lens OE of the present embodiment shown in  FIG. 4   a , first optical surface OE 11   a  connects to intermediate end surface OE 11   c  through transition surface OE 11   b . Herein, angle θ 1  (which is an acute angle) formed between normal line wand optical axis X is larger than angle θ 2  formed between normal line β and optical axis X, where normal line α is a normal line of first surface  11   a  at border B of transition surface OE 11   b  and first surface  11   a , and normal line β is a normal line of transition surface OE 11   b  at border B of first surface OE 11   a  and transition surface OE 11   b . In other words, infest optical surface OE 11   a , the angle formed between the normal line of the first optical surface and optical axis X comes closer to 90 degrees gradually, at a position which goes from optical axis X to the radial direction, then, the angle of the normal line becomes shallow when the position goes into transition surface OE 11   b  across border B. Therefore, border B corresponds to an inflection point. Since transition surface OE 11   b  is arranged outside effective aperture E, it does not affect optical properties of lens OE. 
         [0044]    It is generally said that width W of intermediate end surface OE 11   c  in the direction perpendicular to the optical axis is required to be 50 μm or more for reasons of manufacturing insert  30 . As for an objective lens for an optical pickup apparatus, it is torn about 50 μm to about 150 μm generally. Accordingly, in lens OE relating to the present embodiment, its minimum thickness t along the optical axis at intermediate end surface OE 11   c  is larger than the minimum thickness t′ of lens OE′ of the comparative example along the optical axis at intermediate end surface OE 11   c , when transition surface OE 11   b  is formed in lens OE, under the assumption that the maximum outer circumferences are uniform. Herein, the minimum thickness t along the optical axis at intermediate end surface OE 11   c  is smaller than thickness Δ of flange section OEF along the optical axis. When thickness Δ of flange section OEF along the optical axis is secured to be large, it has advantages such that, under the condition that lens OE is put on a flat surface, the lens is hardly damaged because of a space formed between second optical surface OE 21   a  and the flat surface. In order to realize a compact lens OE, it is preferable that border B is arranged at a place that is closer to second optical surface OE 21   a  than first end surface OE 11   e  of flange section OEF and that is close to first optical surface OE 11   a  than intermediate end surface OE 11   c.    
       (Injection Molding of Lens) 
       [0045]    Next, results of experiments which have been conducted by the present inventor will be described. The present inventor designed objective lenses with a numerical aperture of 0.85 for an optical pickup apparatus exclusively for a Blu-ray Disc, where the objective lenses have various thickness deviation ratios and various sizes of transition surfaces. The present inventor prepared molds (see  FIG. 2 ) corresponding to them. After king resin material at 70° C. to 120° C. for five hours with hot-air dehumidifying and drying machine to remove water hum the material, the inventor conducted injection molding for the material with an injection molding machine “s-2000i 50A” (which is a product name, produced by FANUC CORPORATION) on which a selected mold is attached, at a cylinder temperature of 200° C. to 270° C. and holding pressure of 70 MPa to 100 MPa. At that time, the mold temperature was controlled by oil medium. Concretely, a mold cavity temperature in a process of an injection and filling process for resin was set to 120° C. to 150° C., and the mold was cooled at the same time when the filling process was completed. After the mold was opened and a lens was taken out from the mold with an automatic taking machine, its degree of appearance defect was evaluated. 
         [0046]    Tables 1 and 2 show the evaluation results. In Tables 1 and 2, the thickness deviation ratio is given by d/t. In Table 2, a splitting ratio is given by D 1 /D 2 ×100. In the tables, as for the evaluation of appearance defects, the condition that molding defects such as flow marks were caused in 2% or less of samples is represented by the symbol A, the condition that the mold defects were caused in 2% to 5% of samples is represented by the symbol B, the condition that the mold defects were caused in 5% to 10% of samples is represented by the symbol C, and the condition that the mold defects were caused in 10% or more of samples is represented by the symbol D. As for the evaluation of configuration, the condition that the thickness deviation ratio was 33 or more is represented by the symbol A, the condition that the ratio was 33 to 2.8 is represented by the symbol B, the condition that the ratio was 2.8 to 2.5 is represented by the symbol C, and the condition that the ratio was less than 2.5 is represented by the symbol D. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Thickness deviation ratio 
                 Appearance defects 
                 Configuration 
               
               
                   
               
             
             
               
                 4.1 
                 C 
                 A 
               
               
                 3.8 
                 B 
                 A 
               
               
                 3.5 
                 A 
                 A 
               
               
                 3.3 
                 A 
                 A 
               
               
                 2.8 
                 A 
                 C 
               
               
                 2.5 
                 A 
                 C 
               
               
                 Less than 2.5 
                 A 
                 D 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Thickness deviation ratio 
                 Split ratio (%) 
                 Configuration 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4.3 
                 98.5 
                 C 
               
               
                 4.06 
                 97.5 
                 A 
               
               
                 3.8 
                 96 
                 A 
               
               
                 3.3 
                 94.5 
                 A 
               
               
                 2.7 
                 90 
                 B 
               
               
                 2.5 
                 88 
                 C 
               
               
                   
               
             
          
         
       
     
         [0047]    The evaluation results in Table 1 show that, when the thickness deviation ratio this 4.1 or less and is 2.8 or more, a molding process can be conducted without problems in actual use. Further, the results show that the thickness deviation ratio d/t in the range of 3.8 or less and 3.3 or more is preferable, for achieving both of proper appearance defects and proper configuration which generally have a trade-off relationship. 
         [0048]    The evaluation results in Table 2 show that, when the split ratio (D 1 /D 2 ×100) is 88% or more and is 98% or less, a molding process can be conducted without problems. Further, the results show that the split ratio of 94% or more and 97.5% or less is preferable for making the configuration into an excellent condition. 
       INDUSTRIAL APPLICABILITY 
       [0049]    As described above, the present invention has been descried with reference to embodiments. However, it is to be understood that the present invention should not be interpreted under limitations of the above embodiments and various changes and modifications will be apparent to those skilled in the art. For example, the present invention can be applied for various lenses not only for an objective lens for use in an optical pickup apparatus. 
       CITATION LIST 
       [0000]    
       
           10  First mold 
           11   a  Opening 
           11   b  Flange end transferring surface 
           11   c  Flange circumference transferring surface 
           20  Second mold 
           21   a  Optical transferring surface 
           21   b  Taper surface 
           21   c  Flange end transferring surface 
           21   d  Flange circumference transferring surface 
           22  Runner 
           30  Insert 
           31   a  Optical surface transferring surface 
           31   b  Transition surface transferring surface 
           31   c  Intermediate end surface transferring surface 
           31   d  Taper surface 
         α Normal line 
         β Normal line 
         θ 1  Angle 
         θ 2  Angle 
         B Border 
         GT Gate 
         OE Lens 
         OE 11   a  First optical surface 
         OE 11   b  Transition surface 
         OE 11   c  Intermediate end surface 
         OE 11   d  Taper surface 
         OE 11   e  First end surface of the flange section 
         OE 11   f  Circumference surface of the flange section 
         OE 21   a  Second optical surface 
         OE 21   b  Taper surface 
         OE 21   c  Second end surface of the flange section 
         OEF Flange section 
         X Optical axis or Axis
Provided is a lens having a high NA, wherein defects caused when the lens is molded can be eliminated and the laps exhibits a high mountability. A transition surface transferring surface ( 31 b) is formed between a first optical surface transferring surface ( 31 a) and an intermediate end surface transferring surface ( 31 c) to form a transition surface. Consequently, if a lens (OE) has a large thickness deviation ratio, the occurrence of a flow mark, etc., is prevented in order to achieve a high precision molding of the lens.