Abstract:
An optical pickup apparatus for recording and/or reproducing information for an optical information recording medium, comprising a light converging optical system including a coupling lens and an objective lens, wherein a first diffraction structure is installed on the objective lens, and a second diffraction structure is arranged on the coupling lens, whereby spherical aberration caused by change of ambient temperature is reduced by giving a condition to reduce the number of areas formed in a ring-shaped of the diffractive structure.

Description:
This application is based on Japanese Patent Application No. 2006-61081 filed on Mar. 7, 2006, in Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to an optical pickup apparatus, particularly an optical pickup apparatus capable of information recording and/or reproduction using different types of optical information recording medium with compatibility maintained among these mediums. 
   BACKGROUND OF THE INVENTION 
   In recent years, there has been a rapid progress in the research and development of a high-density optical disk system capable of recording and/or reproduction (hereinafter referred to as “recording/reproduction) of information, using a blue-violet semiconductor laser having a wavelength of about 405 nm. For example, an optical disk for conducting information recording/reproduction based on NA of 0.85 and light source wavelength of 405 nm—a so-called Blu-ray Disc (hereinafter referred to as “BD”)—is capable of recording 23 through 27 GB information per layer, as compared to the optical disk having a diameter of 12 cm, the same diameter as that of the DVD (NA: 0.6; light source wavelength: 650 nm, memory size: 4.7 GB). Further, an optical disk to conduct information recording/reproduction based on NA of 0.65 and light source wavelength of 405 nm—so-called HD DVD (hereinafter referred to as “HD”)—is capable of recording 15 through 20 GB information per layer as compared to the optical disk having a diameter of 12 cm. When the BD is used, there is an increase in the coma aberration caused by the skew of the optical disk. In the present Specification, such an optical disk will be referred to as a “high-density optical disk”. 
   The value of the optical disk player/recorder as a commercial product such as optical disk player or recorder (hereinafter referred to as “optical disk player/recorder) cannot be said to be sufficient in some cases if information recording/reproduction can be conducted only when a high-density optical disk is used. At present, since the DVD and CD (compact disks) for recording a great variety of information are put on the market, the commercial value of the optical disk player/recorder for high density optical disk is enhanced if adequate information recording/reproduction capacity is ensured for the DVD and CD owned by the user, for example. Thus, the optical pickup apparatus mounted on the optical disk player/recorder for high-density optical disk is desired to ensure adequate information recording/reproduction for any of the high-density optical disk, DVD and CD. 
   A method has been proposed to permit adequate information recording and/or reproduction while maintaining compatibility with any of the high-density optical disk, DVD and CD. According to this method, selective switching is carried out between the optical system for high-density optical disk and the optical system for DVD and CD in response to the recording density of the optical disk for information recording and/or reproduction. However, this method requires a plurality of optical systems, and is disadvantageous from the viewpoint of downsizing and cost cutting. 
   To simplify the structure of the optical pickup apparatus and to reduce the cost, it is preferred in the optical pickup apparatus characterized by compatibility that standardization should be achieved between the optical system for high-density optical disk and the optical system for the DVD and CD wherever possible, thereby minimizing the number of the optical parts constituting the optical pickup apparatus. Further, standardization of the objective optical system laid opposite to the optical disk is the shortest way to simplify the structure of the optical pickup apparatus and to reduce the cost. 
   Inexpensive and lightweight resin can be used for the optical element to provide a low-cost and lightweight optical pickup apparatus. One of the problems when using such resin is that the refractive index of the resin material changes significantly when ambient temperature changes and that the change of ambient temperature causes notable spherical aberration deterioration. The Patent Document 1 discloses an optical pickup apparatus wherein a spherical aberration deterioration caused by the change of ambient temperature is reduced by generating under spherical aberration or over spherical aberration. Where the under or over spherical aberration is generated by introducing the light flux coming from a deformed collimate lens as a divergent light or as a converged light. 
   [Patent Document 1] Unexamined Japanese Patent Application Publication No. 2001-23228 
   However, the optical pickup apparatus of the Patent Document 1 does not take account of change of oscillation wavelength due to change of ambient temperature of a semiconductor laser. There is a semiconductor laser with a temperature character of about 0.05 (nm/° C.) among the semiconductor lasers employed in a optical pickup apparatus. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to solve the problems of the conventional art and to provide an optical pickup apparatus capable of information recording and/or reproduction compatibly, regardless of change of ambient temperature, using different types of optical information recording medium with compatibility maintained among these medium. 
   According to the first aspect of the present invention, there is provided an optical pickup apparatus comprising: a first light source for emitting a first light flux having a wavelength λ 1  (nm); 
   a second light source for emitting a second light flux having a wavelength λ 2  (nm) (λ 2 &gt;λ 1 ); and 
   a light converging optical system including a coupling lens and an objective lens, 
   wherein the light converging system converges a light flux from the first light source onto an information recording surface of a first optical information recording medium through a protective layer having a thickness t 1  so that recording and/or reproducing information is conducted for the first optical information recording medium, and converges a light flux from the second light source onto an information recording surface of a second optical information recording medium through a protective layer having a thickness t 2  (t 1 ≦t 2 ) so that recording and/or reproducing information is conducted for the second optical information recording medium, 
   wherein the first light source and the second light source are arranged so that a distance between the first light source and the coupling lens become optically equal to a distance between the second light source and the coupling lens, 
   wherein a first diffraction structure is provided on an optical surface of the objective lens through which both the light flux of the wavelength λ 1  and that of the wavelength λ 2  pass, 
   wherein the following equation (1) is satisfied when d 1  denotes an average depth of step structures in the optical axial direction of the first diffraction structure, 
   wherein the following equation (2) is satisfied by the chromatic aberration ΔfB at the time when the light flux of the wavelength λ 1  passes through an optical surface of the objective lens, 
   wherein a second diffraction structure is formed on an optical surface of the coupling lens through which both the light flux of the wavelength λ 1  and that of the wavelength λ 2  pass, and 
   wherein the following equation (3) is satisfied when d 2  denotes the average depth of step structures in the optical axial direction of the second diffraction structure:
 
λ1×2/( n 1−1)×1.0≦ d 1(nm)≦λ1×2/( n 1−1)×1.3  (1)
 
−0.5≦Δ fB (μm/nm)≦0.5  (2)
 
λ1×2/( n 2−1)×1.0≦ d 2(nm)≦λ1×2/( n 2−1)×1.3  (3)
 
−1.5×10 −4   ≦Δn (° C. −1 )≦−1.0×10 −4   (4)
 
0.04 ≦ΔW (nm/° C.)≦0.07  (5)
 
−0.08 &lt;m 2 −m 1≦0.01  (6)
 
where m 1  denotes the magnification of the objective lens at the time of recording or reproduction of information using the first optical information recording medium;
 
   f 1  indicates the focal distanced of the objective lens at the time of recording or reproduction of information using the first optical information recording medium; 
   m 2  shows the magnification of the objective lens at the time of recording or reproduction of information using the second optical information recording medium; 
   n 1  represents the refractive index of the material constituting the first diffraction structure with respect to the light of wavelength λ 1 ; 
   n 2  represents the refractive index of the material constituting the second diffraction structure with respect to the light of wavelength λ 1 ; 
   Δn represents the change amount of refractive index of the material constituting the coupling lens and the objective lens with respect to the light of wavelength λ 1  for a change of ambient temperature; and 
   ΔW represents the change amount of wavelength of the first light source. 
   The present invention is to correct spherical aberration caused by a change of refractive index of optical element by utilizing a change of oscillation wavelength due to change of ambient temperature of a semiconductor laser. In principle, it is possible to correct spherical aberration by changing a diffractive effect, which arises when a light flux passes through a diffractive structure formed on a surface of the optical element, according to a change of oscillating wavelength. But in the case of forming diffractive structure on both the optical surface of the coupling lens and the optical surface of the objective lens, when there is a large number of areas formed in a ring-shape, substantial change of spherical aberration arises due to pitch difference at the time of thermal deformation. Such phenomenon is notable especially when utilizing a light flux with short wavelength such as blue-violet light. With that, in the present invention, spherical aberration caused by change of ambient temperature is reduced by giving a condition to reduce the number of areas formed in a ring-shaped of the diffractive structure. 
   To put it more specifically, when the equation (2) is satisfied by the chromatic aberration ΔfB at the time when the light flux of the wavelength λ 1  passes through an optical surface of the objective lens, the chromatic aberration becomes equivalent to or less than that of common objective lens which comprises only refractive surface. Accordingly, the diffractive structure does not need to have function of reducing chromatic aberration. Further satisfying the equation (6), it is possible to reduce the number of areas formed in a ring-shaped of the diffractive structure. This makes appropriate correction of aberration. Accordingly, when forming an objective lens using a common resin material that satisfies the formula (4) and utilizing a light source with characteristics satisfying the formula (5), it is possible to effectively reduce spherical aberration deterioration caused by the change of ambient temperature. In addition, the chromatic aberration means a shift amount of a defocus position where a wave aberration of the objective lens becomes the best when the wavelength of the light source changes by 1 nm. 
   Further, if the first diffraction structure and second diffraction structure are designed so as to satisfy the equations (1) and (3), the intensity of the secondary diffracted light is maximized in the light flux of the wavelength λ 1  and the intensity of the primary diffracted light is maximized in the light flux of the wavelength λ 2 , regardless of which of the diffraction structures has been passed through by light. Thus, by using the diffracted light of lower order, it is possible to enhance the change of diffraction efficiency due to change of wavelength and change of ambient temperature. The average step depth d 1  is defined as the average value of the step depths of diffraction structures formed in the area through which both the wavelengths λ 1  and λ 2  pass, in the objective lens. Namely, it corresponds to the value obtained by dividing the total of the depth of the step formed in this area, by the number of steps. When a third light source of the wavelength λ 3  is provided, the average step depth d 1  is defined as the average value of the step depths diffraction structures formed in the area through which all of the wavelengths λ 1 , λ 2  and λ 3  pass, in the objective lens. The average step depth d 2  is defined as the average value of the step depths of diffraction structures formed in the area through which both the wavelengths λ 1  and λ 2  pass, in the coupling lens. Namely, it corresponds to the value obtained by dividing the total of the depth of the step formed in this area, by the number of steps. Even when the third light source of the wavelength λ 3  is provided, the average step depth d 2  is defined as the average value of the step depths of diffraction structures formed in the area through which both the wavelengths λ 1  and λ 2  pass, in the objective lens. Namely, it corresponds to the value obtained by dividing the total of the depth of the step formed in this area, by the number of steps. 
   According to the second aspect of the present invention, there is provided the optical pickup apparatus of the first aspect of the present invention that satisfies the following equation. Therefore it is possible to make the optical pickup apparatus thin-shaped by reducing the focal distance of the objective lens. To put it more specifically, by setting f 1  less than upper limit of the equation (7), it is possible to enhance the effect of correcting the spherical aberration caused by change of ambient temperature when using the first optical information recording medium. And by setting f 1  more than lower limit of the equation (7), it is possible to make the size of pitch of ring-shaped area of the diffractive structure suitable for manufacturing.
 
1.5&lt; f 1(mm)&lt;3.5  (7)
 
   According to the third aspect of the present invention, there is provided the optical pickup apparatus of the first aspect of the present invention, wherein the light flux reflected from the information recording surface of the first optical information recording medium and the light flux reflected from the information recording surface of the second optical information recording medium enter the light receiving surface of a common optical detector. Therefore, compact configuration of the optical pickup apparatus is achieved by use of the common optical detector. 
   According to the fourth aspect of the present invention, there is provided the optical pickup apparatus of the first aspect of the present invention, wherein the following equation is satisfied, and therefore, the effect of correcting the spherical aberration caused by ambient temperature change can be enhanced when the first optical information recording medium is used. And it is possible to control laser power of a semiconductor laser commercially offered now so that light amount reaching a sensor become an appropriate amount for recording and/or reproducing.
 
−⅙≦ mG≦−  1/10  (8)
 
   wherein mG denotes the magnification of the composite system made up of the objective lens and coupling lens at the time of information recording or reproduction using the first optical information recording medium. 
   According to the fifth aspect of the present invention, there is provided the optical pickup apparatus of the first aspect of the present invention, wherein the first and second light sources are accommodated in a common package, thereby permitting further downsizing of the optical pickup apparatus. 
   According to the sixth aspect of the present invention, there is provided the optical pickup apparatus of the first aspect of the present invention, wherein a third light source of wavelength λ 3  (λ 2 &lt;λ 3 ) is further provided, and the light converging optical system converges a light flux from the third light source onto an information recording surface of a third optical information recording medium through a protective layer having a thickness t 3  (t 2 &lt;t 3 ), whereby information recording and/or reproduction is enabled. 
   In the present specification, if there are a lens capable of light convergence arranged opposite the optical information recording medium at the position closest to this recording medium mounted on the optical pickup apparatus; and an optical element or lens which is mounted on an actuator for driving the lens and is driven integrally with the lens for light convergence, then the objective lens is defined as the optical element group including the optical element or lens. In other words, the objective lens can be made up of a plurality of lenses although it is preferably made up of a single lens. 
   The present invention provides an optical pickup apparatus capable of information recording and/or reproduction using different types of optical information recording medium with compatibility maintained among these mediums. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a drawing schematically representing the structure of an optical pickup apparatus PU 1  of the present embodiment capable of correct information recording/reproduction using an HD, DVD and CD as different types of optical information recording medium (also called the optical disks). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following describes the embodiment of the present invention with reference to drawings:  FIG. 1  is a drawing schematically representing the structure of an optical pickup apparatus PU 1  of the present embodiment capable of correct information recording/reproduction using an HD, DVD and CD as different types of optical information recording medium (also called the optical disks). This optical pickup apparatus PU 1  can be mounted on an optical information recording/reproduction apparatus. 
   The optical pickup apparatus PU 1  includes: 
   a 2-laser 1-package 2L1P wherein a common package located equidistant from the objective lens accommodates: 
   a first semiconductor laser for emitting the blue-violet laser light flux (the first light flux) of λ 1 =407 nm, wherein light is emitted at the time of information recording/reproduction using the HD as a high-density optical disk; and 
   a second semiconductor laser for emitting the red laser light flux (the second light flux) of λ 2 =655 nm, wherein light is emitted at the time of information recording/reproduction using the DVD; 
   a CD hologram laser LD 3  integrally containing a third semiconductor laser (third light source) for emitting infrared laser light flux (third light flux) of λ 3 =785 nm, wherein light is emitted at the time of information recording/reproduction using the CD; and a CD optical detector; 
   an optical detector PD shared by an HD and DVD; 
   a coupling lens (also called the outgoing angle conversion element, the same hereafter) CUL, wherein a diffraction structure is formed; 
   an objective lens OBJ capable of converging the incoming laser light flux on the information recording surface of the optical disk; 
   a polarized beam splitter (also called the separation unit, the same hereafter) PBS; 
   a dichroic prism DP (or a half-mirror); 
   a λ/4 wave plate QWP; and 
   a sensor lens SN for providing astigmatism to the light flux reflected by the optical disk. 
   The optical surfaces of the coupling lens CUL and objective lens OBJ are provided with the diffraction structures, wherein the amount of the secondary diffracted light is maximized when the light flux of wavelength λ 1  has passed by, and the amount of the primary diffracted light is maximized when the light flux of wavelengths λ 2  and λ 3  has passed by. In addition to the semiconductor laser LD 1 , a blue-violet SHG laser can be used as a light source for HD. 
   In the optical pickup apparatus PU 1 , light is emitted from the first semiconductor laser (also called the first light source, the same hereinafter) of the 2-laser 1-package 2L1P at the time of information recording/reproduction using the HD. The divergent light flux emitted from the first semiconductor laser passes through the polarized beam splitter PBS and the dichroic prism DP, and is converted into the finite convergent light flux of convergent angle θ 1  by the coupling lens CUL. Then this light passes through the λ/4 wave plate QWP. After the diameter of the light flux has been controlled by an aperture (not illustrated), the objective lens OBJ causes the light to form a spot on the information recording surface through the HD protective layer. The objective lens OBJ conducts focusing and tracking through the biaxial actuator (not illustrated) arranged on the periphery. 
   The reflected light flux having been modulated by the information pit on the information recording surface of the HD again passes through the objective lens OBJ and λ/4 wave plate QWP. After passing through the coupling lens CUL and the dichroic prism DP, the light is reflected by the polarized beam splitter PBS and is provided with astigmatism by the sensor lens SN. Then the light is converged on the light receiving surface of the optical detector PD. Then the information recorded on the HD can be read using the output signal of the optical detector PD. 
   In the optical pickup apparatus PU 1 , at the time of information recording/reproduction using the DVD, light is emitted from the second semiconductor laser (also called the second light source, the same hereafter) of the 2-laser 1-package 2L1P. The divergent light flux emitted from the second semiconductor laser passes through the polarized beam splitter PBS and the dichroic prism DP, and is converted into the finite light flux or infinite light flux of convergent angle θ 2  (θ 1 ≠θ 2 ) by the coupling lens CUL. The light flux then passes through the λ/4 wave plate QWP. After the diameter of the light flux has been controlled by an aperture (not illustrated), the objective lens OBJ causes the light to form a spot on the information recording surface through the protective layer of the DVD. The objective lens OBJ conducts focusing and tracking through the biaxial actuator (not illustrated) arranged on the periphery. 
   The reflected light flux having been modulated by the information pit on the information recording surface of the DVD again passes through the objective lens OBJ and λ/4 wave plate QWP. After passing through the coupling lens CUL and the dichroic prism DP, the light is reflected by the polarized beam splitter PBS and is provided with astigmatism by the sensor lens SN. Then the light is converged on the light receiving surface of the optical detector PD. Then the information recorded on the DVD can be read using the output signal of the optical detector PD. 
   In the optical pickup apparatus PU 1 , at the time of information recording/reproduction using the DVD, light is emitted from the hologram laser LD 3 . The divergent light flux from the hologram laser LD 3  is reflected by the dichroic prism DP, and is converted into the finite divergent light flux having an angle of divergence of θ 3  by the coupling lens CUL. The light flux then passes through the λ/4 wave plate QWP. After the diameter of the light flux has been controlled by an aperture (not illustrated), the objective lens OBJ causes the light to form a spot on the information recording surface through the protective layer of the DVD. The objective lens OBJ conducts focusing and tracking through the biaxial actuator (not illustrated) arranged on the periphery. 
   The reflected light flux having been modulated by the information pit on the information recording surface of the CD again passes through the objective lens OBJ, λ/4 wave plate QWP and the coupling lens CUL, and is reflected by the dichroic prism DP. Then the light is converged on the light receiving surface of the optical detector inside the hologram laser LD 3 . Then the information recorded on the CD can be read using the output signal of the optical detector. 
   EXAMPLE 
   The following describes the preferred example of the embodiment. In the following description (including the lens data), the power multiplier of 10 (e.g., 2.5×10 −3 ) will be expressed in terms of E (e.g., 2.5E−3). 
   The optical surface of the objective optical system is formed on the axially symmetric aspheric surface around the optical axis, defined by the mathematical expression obtained by substituting the coefficient of the Table into the mathematical formula 1.
 
 z =( y   2 /γ)/[1+√{square root over ( )}{1−( K+ 1)( y /γ) 2   }]+A   4   y   4 + A   6   y   6   +A   8   y   8   +A   10   y   10   +A   12   y   12   +A   14   y   14   +A   16   y   16   +A   18   y   18   +A   20   y   20   [Mathematical Formula 1]
 
   z: Shape of aspheric surface (distance along the optical axis from the plane surface bordering on the vertex of surface of a aspheric surface) 
   y: Distance from optical axis 
   γ: Curvature radius 
   K: Cornic coefficient 
   A 4 , A 6 , A 8 , A 10 , A 12 , A 14 , A 16 , A 18 , A 20 : Aspheric surface coefficient 
   The difference in optical path applied to the light flux of each wavelength by the diffraction structure (phase structure) is defined by the mathematical expression obtained by substituting the coefficient of the Table into the function of optical path difference of the mathematical formula 2.
 
φ=dor×λ/λ B ×( C   2   y   2   +C   4   y   4   +C   6   y   6   +C   8   y   8   +C   10   y   10 )  [Mathematical Formula 2]
 
   where 
   φ: Function of optical path difference 
   λ: Wavelength of light flux entering the diffracted structure 
   λ B : Blaze wavelength 
   dor: Order of diffraction of the diffracted light for recording/reproduction using the optical disk 
   y: Distance from optical axis 
   C 2 , C 4 , C 6 , C 8 , C 10 : Optical path difference function coefficient 
   Example 1 
   Table 1 shows the lens data of Example 1, wherein the optical magnification m 1  in the light flux of wavelength λ 1  is 0.035, and the optical magnification m 2  in the light flux of wavelength λ 2  is 0.035, and therefore, the difference is zero. In this case, the spherical aberration ΔT in the composite system made up of a coupling lens and an objective lens, when ambient temperature changes 30° C., is 0.021. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Example 1 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               i-th 
                 
                 
                 
                 
                 
                 
                 
             
             
               surface 
               ri 
               di(407 nm) 
               ni(407 nm) 
               di(655 nm) 
               ni(655 nm) 
               di(785 nm) 
               ni(785 nm) 
             
             
                 
             
             
               0 
                 
               0.00 
                 
               0.00 
                 
               0.00 
             
             
               1 
               ∞ 
               29.21 
                 
               29.21 
                 
               18.21 
             
             
               2 
               35.7364 
               1.70 
               1.5598 
               1.70 
               1.5407 
               1.70 
               1.5372 
             
             
               3 
               −16.1301 
               10.00 
               1.0 
               10.00 
               1.0 
               10.00 
               1.0 
             
             
               4 
               ∞ 
               0.00 
                 
               −0.17 
                 
               0.08 
             
             
               5 
               ∞ 
               0.00 
                 
               0.00 
                 
               0.00 
             
             
               (Aperture 
             
             
               diameter) 
             
             
               6 
               2.1231 
               1.76 
               1.5598 
               1.76 
               1.5407 
               1.76 
               1.5372 
             
             
               7 
               −26.7426 
               1.54 
               1.0 
               1.71 
               1.0 
               1.46 
               1.0 
             
             
               8 
               ∞ 
               0.60 
               1.6187 
               0.60 
               1.5775 
               1.20 
               1.6187 
             
             
               9 
               ∞ 
               0.00 
               1.0 
               0.00 
               1.0 
               0.00 
               1.0 
             
             
                 
             
           
        
         
             
               di indicates the displacement from the i-th surface to the i + 1-th surface 
             
           
        
         
             
               3rd surface 
             
             
               Aspheric surface coefficient 
             
             
               Optical path difference function (HD DVD secondary, 
             
             
               DVD: primary, CD: primary) 
             
           
        
         
             
                 
               κ 
               −1.0002E+00 
               λB 
               407 nm 
             
             
                 
               AsphericA4 
                 1.9416E−05 
               C2 
               −1.4955E−03 
             
             
                 
               A6 
                 0.0000E+00 
             
           
        
         
             
               6th surface 
             
             
               Aspheric surface coefficient 
             
             
               Optical path difference function (HD DVD secondary, 
             
             
               DVD: primary, CD: primary) 
             
           
        
         
             
                 
               κ 
               −4.9315E−01 
               λB 
               395 nm 
             
             
                 
               A4 
                 1.6515E−03 
               C2 
               −1.0985E−02 
             
             
                 
               A6 
                 5.5701E−04 
               C4 
               −3.4938E−04 
             
             
                 
               A8 
               −9.3443E−06 
               C6 
                 8.3919E−05 
             
             
                 
               A10 
               −3.0976E−05 
               C8 
               −3.9176E−05 
             
             
                 
               A12 
                 1.1487E−05 
               C10 
                 4.3351E−06 
             
             
                 
               A14 
               −1.3823E−06 
             
           
        
         
             
               7th surface 
             
             
               Aspheric surface coefficient 
             
           
        
         
             
                 
               κ 
               −1.9917E+01 
             
             
                 
               A4 
                 1.0542E−02 
             
             
                 
               A6 
               −1.4002E−03 
             
             
                 
               A8 
               −1.6501E−04 
             
             
                 
               A10 
                 7.2173E−05 
             
             
                 
               A12 
               −1.0340E−05 
             
             
                 
               A14 
                 6.3473E−07 
             
             
                 
                 
             
           
        
       
     
   
   Example 2 
   Table 2 shows the lens data of Example 2, wherein the optical magnification m 1  in the light flux of wavelength λ 1  is 0.035, and the optical magnification m 2  in the light flux of wavelength λ 2  is 0.000, and therefore, the difference is −0.035. In this case, the spherical aberration ΔT in the composite system made up of a coupling lens and an objective lens, when ambient temperature changes 30° C., is 0.023. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Example 2 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               i-th 
                 
                 
                 
                 
                 
                 
                 
             
             
               surface 
               ri 
               di(407 nm) 
               ni(407 nm) 
               di(655 nm) 
               ni(655 nm) 
               di(785 nm) 
               ni(785 nm) 
             
             
                 
             
             
               0 
                 
               0.00 
                 
               0.00 
                 
               114.60 
             
             
               1 
               ∞ 
               28.42 
                 
               28.42 
                 
               0.00 
             
             
               2 
               −29.0384 
               1.70 
               1.5598 
               1.70 
               1.5407 
               0.00 
             
             
               3 
               −94.4348 
               10.00 
               1.0 
               10.00 
               1.0 
               10.00 
               1.0 
             
             
               4 
               ∞ 
               0.00 
                 
               −0.22 
                 
               0.00 
             
             
               5 
               ∞ 
               0.00 
                 
               0.00 
                 
               0.00 
             
             
               (Aperture 
             
             
               diameter) 
             
             
               6 
               1.9617 
               1.76 
               1.5598 
               1.76 
               1.5407 
               1.76 
               1.5372 
             
             
               7 
               −15.9922 
               1.57 
               1.0 
               1.79 
               1.0 
               1.49 
               1.0 
             
             
               8 
               ∞ 
               0.60 
               1.6187 
               0.60 
               1.5775 
               1.20 
               1.6187 
             
             
               9 
               ∞ 
               0.00 
               1.0 
               0.00 
               1.0 
               0.00 
               1.0 
             
             
                 
             
           
        
         
             
               * di indicates the displacement from the i-th surface to the i + 1-th surface 
             
           
        
         
             
               3rd surface 
             
             
               Aspheric surface coefficient 
             
             
               Optical path difference function (HD DVD secondary, 
             
             
               DVD: primary) 
             
           
        
         
             
                 
               κ 
               −1.1944E+00 
               λB 
               407 nm 
             
             
                 
               A4 
                 5.1490E−06 
               C2 
               1.4001E−02 
             
             
                 
               A6 
                 0.0000E+00 
             
           
        
         
             
               6th surface 
             
             
               Aspheric surface coefficient 
             
             
               Optical path difference function (HD DVD secondary, 
             
             
               DVD: primary, CD: primary) 
             
           
        
         
             
                 
               κ 
               −5.3934E−01 
               λB 
               395 nm 
             
             
                 
               A4 
               −5.6051E−04 
               C2 
               −3.4043E−03 
             
             
                 
               A6 
                 1.1888E−03 
               C4 
               −8.203E−04 
             
             
                 
               A8 
               −1.8756E−04 
               C6 
                 3.2078E−04 
             
             
                 
               A10 
               −6.2959E−05 
               C8 
               −1.2892E−04 
             
             
                 
               A12 
                 2.6777E−05 
               C10 
                 1.4692E−05 
             
             
                 
               A14 
               −3.4352E−06 
             
           
        
         
             
               7th surface 
             
             
               Aspheric surface coefficient 
             
           
        
         
             
                 
               κ 
               −1.0000E+02 
             
             
                 
               A4 
                 9.2238E−03 
             
             
                 
               A6 
               −7.6413E−04 
             
             
                 
               A8 
               −4.2891E−04 
             
             
                 
               A10 
                 9.0108E−05 
             
             
                 
               A12 
               −6.5760E−06 
             
             
                 
               A14 
                 5.7974E−08 
             
             
                 
                 
             
           
        
       
     
   
   Example 3 
   Table 3 shows the lens data of Example 3, wherein the optical magnification m 1  in the light flux of wavelength λ 1  is 0.045, and the optical magnification m 2  in the light flux of wavelength λ 2  is 0.026, and therefore, the difference is −0.019. In this case, the spherical aberration ΔT in the composite system made up of a coupling lens and an objective lens, when ambient temperature changes 30° C., is −0.17. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               Example 3 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               i-th 
                 
                 
                 
                 
                 
                 
                 
             
             
               surface 
               ri 
               di(408 nm) 
               ni(408 nm) 
               di(660 nm) 
               ni(660 nm) 
               di(784 nm) 
               ni(784 nm) 
             
             
                 
             
             
               0 
                 
               0.00 
                 
               0.00 
                 
               0.00 
             
             
               1 
               ∞ 
               18.08 
                 
               18.08 
                 
               11.47 
             
             
               2 
               46.2814 
               1.00 
               1.5583 
               1.00 
               1.5392 
               1.00 
               1.5359 
             
             
               3 
               −18.4386 
               10.00 
               1.0 
               10.00 
               1.0 
               10.00 
               1.0 
             
             
               4 
               ∞ 
               0.00 
                 
               −0.10 
                 
               0.13 
             
             
               5 
               ∞ 
               0.00 
                 
               0.00 
                 
               0.00 
             
             
               (Aperture 
             
             
               diameter) 
             
             
               6 
               1.4188 
               1.37 
               1.5583 
               1.37 
               1.5392 
               1.37 
               1.5359 
             
             
               7 
               −8.0337 
               1.04 
               1.0 
               1.14 
               1.0 
               0.91 
               1.0 
             
             
               8 
               ∞ 
               0.60 
               1.6183 
               0.60 
               1.5772 
               1.20 
               1.5707 
             
             
               9 
               ∞ 
               0.00 
               1.0 
               0.00 
               1.0 
               0.00 
               1.0 
             
             
                 
             
           
        
         
             
               * di indicates the displacement from the i-th surface to the i + 1-th surface 
             
           
        
         
             
               3rd surface 
             
             
               Aspheric surface coefficient 
             
             
               Optical path difference function (HD DVD secondary, 
             
             
               DVD: primary, CD: primary) 
             
           
        
         
             
                 
               κ 
               −9.9948E−01 
               λB 
               407 nm 
             
             
                 
               A4 
                 3.2017E−05 
               C2 
               7.1665E−03 
             
             
                 
               A6 
                 0.0000E+00 
             
           
        
         
             
               6th surface 
             
             
               Aspheric surface coefficient 
             
             
               Optical path difference function (HD DVD secondary, 
             
             
               DVD: primary, CD: primary) 
             
           
        
         
             
                 
               κ 
               −5.3111E−01 
               λB 
               395 nm 
             
             
                 
               A4 
                 1.8302E−03 
               C2 
                 1.0763E−03 
             
             
                 
               A6 
                 1.9628E−03 
               C4 
               −4.9838E−04 
             
             
                 
               A8 
                 7.2388E−04 
               C6 
                 6.5400E−04 
             
             
                 
               A10 
               −1.7748E−03 
               C8 
               −5.0621E−04 
             
             
                 
               A12 
                 1.0782E−03 
               C10 
                 1.2263E−04 
             
             
                 
               A14 
               −2.6515E−04 
             
           
        
         
             
               7th surface 
             
             
               Aspheric surface coefficient 
             
           
        
         
             
                 
               κ 
               −3.0573E+01 
             
             
                 
               A4 
                 2.7016E−02 
             
             
                 
               A6 
               −7.8590E−03 
             
             
                 
               A8 
               −2.2641E−05 
             
             
                 
               A10 
               −8.4844E−04 
             
             
                 
               A12 
                 5.6544E−04 
             
             
                 
               A14 
               −9.8702E−05 
             
             
                 
                 
             
           
        
       
     
   
   Table 4 summarizes the numerical values given in the examples. 
   
     
       
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
               Example 1 
               Example 2 
               Example 3 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               HD 
               Composite 
               Magnification 
               −0.112 
               −0.111 
               −0.143 
             
             
                 
               system 
               (mG) 
             
             
                 
                 
               ΔT = 30° C. 
               −0.021 
               0.023 
               0.017 
             
             
                 
                 
               RMS [λrms] 
             
             
                 
               Coupling 
               Focal distance 
               22.8 
               22.8 
               14.2 
             
             
                 
                 
               [mm] 
             
             
                 
               Objective 
               Focal distance 
               3.10 
               3.10 
               2.30 
             
             
                 
                 
               [mm] 
             
             
                 
                 
               NA 
               0.65 
               0.65 
               0.65 
             
             
                 
                 
               Magnification 
               0.035 
               0.0035 
               0.045 
             
             
                 
                 
               (m1) 
             
             
                 
                 
               Chromatic 
               −0.42 
               0.17 
               0.48 
             
             
                 
                 
               aberration ΔfB 
             
             
                 
                 
               [μm/nm] 
             
             
               DVD 
               Composite 
               Magnification 
               −0.118 
               −0.107 
               −0.138 
             
             
                 
               system 
               (mG) 
             
             
                 
               Coupling 
               Focal distance 
               23.1 
               30.1 
               15.7 
             
             
                 
                 
               [mm] 
             
             
                 
               Objective 
               Focal distance 
               3.28 
               3.23 
               2.37 
             
             
                 
                 
               [mm] 
             
             
                 
                 
               NA 
               0.65 
               0.65 
               0.65 
             
             
                 
                 
               Magnification 
               0.035 
               0.000 
               0.026 
             
             
                 
                 
               (m2) 
             
           
        
         
             
               m2 − m1 
               0.000 
               −0.035 
               −0.019 
             
             
               d1 [mm] 
               0.00149 
               0.00150 
               0.00152 
             
             
               (Objective lens) 
             
             
               d2 [mm] 
               0.00145 
               0.00145 
               0.00141 
             
             
               (Coupling lens) 
             
             
               Δn 
               −1.05 × 
               −1.05 × 
               −1.05 × 
             
             
                 
               10 −4   
               10 −4   
               10 −4   
             
             
               ΔW 
               0.06 
               0.06 
               0.06