Patent Publication Number: US-2005122883-A1

Title: Optical pickup apparatus and optical information recording and/or reproducing apparatus

Description:
RELATED APPLICATION  
      This application is based on Japanese Patent Application No. 2003-404333, the entire content of which is hereby incorporated by reference.  
     TECHNICAL FIELD  
      The present invention relates to an optical pickup apparatus and an optical information recording and/or reproducing apparatus.  
     TECHNICAL BACKGROUND  
      There has been known an optical pickup apparatus capable of recording and/or reproducing for optical discs of plural types each having different recording density. For example, there is available the one wherein recording and/or reproducing are conducted for DVD (digital versatile disc) and CD (compact disc) by the same optical pickup apparatus. For obtaining compatibility for optical discs in plural types each having a different recording density, there is a method wherein a light beam having a different wavelength is used in the optical pickup apparatus,.and recording and/or reproducing are conducted for the optical disc having recording density corresponding to each wavelength. For example, there are given Japanese Un-examiner Patent Application Publication Nos. 2002-277732 and 2002-298422 respectively as Patent Document 1 and Patent Document 2.  
      The technology described in the Patent Document 1 is one wherein ring-shaped zone structures whose center is on the optical axis is provided on the surface of an objective lens, and plural stair-structures are formed in each of the ring-shaped zones, and it is a technology wherein a step of the stair-structure formed the in ring-shaped zone is set for either one wavelength (for example, λ 2 ) of recording/reproducing wavelength λ 2  for DVD and recording/reproducing wavelength λ 3  for CD so that an optical path difference is not added substantially at adjoining stair-structures and an optical path difference is given only to the wavelength (for example, λ 3 ) on the other side, and the incident light flux is diffracted wavelength-selectively, thus, spherical aberration caused by a difference of a protective layer thickness between DVD and CD is corrected.  
      In recent years, there has been a demand for an optical pickup apparatus having compatibility for a high density disc using a violet semiconductor laser and conventional DVD and CD, as optical discs each having different recording density. In an objective lens in the foregoing, it is possible to realize high transmittance even for any wavelength, while correcting spherical aberration caused by a difference of protective layer thickness between a high density disc and DVD or by a difference of recording/reproducing wavelength, by setting a step between adjoining stair-structures to the height calculated by d=2·λ 1 /(N 1 −1) for recording/reproducing wavelength λ 1  of the high density disc, and by dividing one ring-shaped zone into five parts or six parts. Incidentally, N 1  represents a refractive index of the objective lens for light with wavelength λ 1 .  
      However, since the recording/reproducing wavelength λ 3  for CD is substantially twice the recording/reproducing wavelength λ 1  for the high density disc, an optical path difference of 1×λ 3  is added to the light with wavelength λ 3 , in the stair-structure wherein the step is determined to satisfy the aforesaid expression. This corresponds to that an optical path difference is not added substantially to the light with wavelength λ 3 . Therefore, in the stair-structure of this kind, spherical aberration caused by a difference of protective layer thickness between the high density optical disc and CD or by a difference of recording/reproducing wavelength cannot be corrected.  
      Further, there has been known a technology to provide on the surface of an objective lens a diffractive structure of a blaze type wherein the order number of diffraction of the diffracted light generated when recording/reproducing wavelength λ 2  of DVD enters is lower than the order number of diffraction of the diffracted light generated when recording/reproducing wavelength λ 1  of the high density optical disc enters, as described in Patent Document 2.  
      For example, by making the order number of diffraction of the diffracted light generated when light with wavelength λ 1  enters to be 2 and by making the order number of diffraction of the diffracted light generated when light with wavelength λ 2  enters to be 1, it is possible to correct spherical aberration caused by a difference of protective layer thickness between a high density optical disc and DVD or by a difference of recording/reproducing wavelength, by utilizing a difference of diffracting actions (diffraction angles) for each wavelength, while securing high diffraction efficiency for each wavelength.  
      However, diffracting actions (diffraction angles) for wavelength λ 1  are substantially the same as diffracting actions (diffraction angles) for wavelength λ 3  because the wavelength λ 3  is almost twice the wavelength λ 1 . Therefore, spherical aberration caused by a difference of protective layer thickness between the high density disc and CD or by a difference of recording/reproducing wavelength cannot be corrected in the diffractive structure of this kind, although high diffraction efficiency can be secured for wavelength λ 3 .  
      From the foregoing, when utilizing the diffractive structure described in the Patent Documents 1 and 2, the diffracting actions can hardly be utilized, for correcting the spherical aberration caused by a difference of protective layer thickness between the high density disc and CD or by a difference of recording/reproducing wavelength. It is therefore necessary to correct the spherical aberration caused by a difference of protective layer thickness between the high density disc and CD or by a difference of recording/reproducing wavelength, by making an angle of divergence of incident light with wavelength λ 3  on an objective lens to be different from an angle of divergence of incident light of a high density optical disc.  
      However, for correcting spherical aberration caused by a difference of protective layer thickness between the high density disc and CD or by a difference of recording/reproducing wavelength, it is necessary to set a divergence degree for light with wavelength λ 3  to be great when light with wavelength λ 1  is parallel light, and thereby, a large coma aberration is caused when tracking of the objective lens takes place, and excellent tracking characteristics cannot be obtained, which is a problem.  
      Incidentally, as a standard for the high density optical disc that employs a violet semiconductor laser as a light source for recording/reproducing, there are proposed two-types including a Blu-ray disc (BD) and High-Density DVD (HD DVD).  
      BD is an optical disc on which recording/reproducing is conducted by an objective lens having a protective layer whose thickness is 0.1 mm and having a numerical aperture of 0.85, while, HD DVD is an optical disc on which recording/reproducing is conducted by an objective lens having a protective layer whose thickness is 0.6 mm which is the same as that of DVD and having a numerical aperture of 0.65.  
      In order to attain compatibility between the high density optical disc and DVD, it is necessary to correct the spherical aberration caused by a difference of protective layer thickness between the high density disc and DVD or by a difference of recording/reproducing wavelength. On the other hand, in the case of the HD DVD, it is necessary to correct the spherical aberration caused by a difference of recording/reproducing wavelength.  
      In view of the problems stated above, an object of the invention is to provide an optical pickup apparatus and an optical information recording and/or reproducing apparatus wherein recording/reproducing can be conducted for three different types of recording media and occurrence of coma aberration caused by tracking of an objective lens in conducting recording/reproducing for CD can be reduced.  
     SUMMARY  
      The aspect mentioned above can be achieved by the structures described below.  
      The first structure is an optical pickup apparatus comprising a first light source emitting a first light flux with wavelength λ 1  a second light source emitting a second light flux with wavelength λ 2  and a light-converging optical system including at least an objective lens. In the optical pickup apparatus of the first structure, a diffractive structure is formed on at least one optical surface of the aforesaid light-converging optical system, the first light flux is used to conduct recording and/or reproducing of information for the first optical disc having a protective layer having a thickness of t 1 , the second light flux is used to conduct recording and/or reproducing of information for the second optical disc having a protective layer having a thickness of t 2 , t 2  being same as or larger than t 1 , and having recording density that is lower than that of the first optical disc and the second light flux is used to conduct recording and/or reproducing of information for the third optical disc having a protective layer having a thickness of t 3 , t 3  being larger than t 2 , and having recording density that is lower than that of the second optical disc.  
      The second structure is an optical pickup apparatus having therein a first light source emitting a first light flux with wavelength λ 1 , a second light source emitting a second light flux with wavelength λ 2 , a third light source emitting a third light flux with wavelength λ 2 , and a light-converging optical system including at least an objective lens.  
      In the optical pickup apparatus of the second structure, a diffractive structure is formed on at least one optical surface of the aforesaid light-converging optical system, the first light flux is used to conduct recording and/or reproducing of information for the first optical disc having a protective layer having a thickness of t 1 , the second light flux is used to conduct recording and/or reproducing of information for the second optical disc having a protective layer having a thickness of t 2 , t 2  being same as or larger than t 1 , and having recording density that is lower than that of the first optical disc, and the third light flux is used to conduct recording and/or reproducing of information for the third optical disc having a protective layer having a thickness of t 3 , t 3  being larger than t 2 , and having recording density that is lower than that of the second optical disc.  
      In the first structure and the second structure stated above, by conducting recording/reproducing for CD by the diffractive structure with the second light flux that receives diffracting actions different from those of the first light source or with the third light flux having the same wavelength as that of the second light flux, the spherical aberration caused by a difference of protective layer thickness between the high density optical disc and CD can be corrected. As a result, occurrence of coma aberration caused by tracking of the objective lens can be reduced because it is not necessary to set the magnification of the objective lens to be small in the case of conducting recording/reproducing of CD, thus, excellent tracking characteristics can be obtained.  
      The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.  
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a plan view of primary parts showing the structure of an optical pickup apparatus.  
      FIGS.  2 ( a ) to  2 ( c ) are diagrams showing the structure of an objective optical system representing a part of a light-converging optical system.  
      FIGS.  3 ( a ) to  3 ( c ) are diagrams showing the structure of a diffraction lens.  
       FIG. 4  is a plan view of primary parts showing the structure of an optical pickup apparatus.  
       FIG. 5  is a plan view of primary parts showing the structure of an optical pickup apparatus.  
       FIG. 6  is a graph showing lens shift characteristics of objective optical system OBJ. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      As stated above, in the present specification, optical discs using a violet semiconductor laser and a violet SHG laser as a light source for recording/reproducing of information are called (high density optical disc” generically, and Blu-Ray Disc (BD) representing the optical disc that conducts recording/reproducing of information with an objective optical system with NA 0.85 and has a standard of a protective layer thickness of about 0.1 mm and also High-Density DVD (HD DVD) representing an optical disc that conducts recording/reproducing of information with an objective optical system with NA 0.65 and has a standard of about 0.6 mm for the protective layer thickness, are included. Further, in addition to the optical disc having on its information recording surface the protective layer of this kind, the optical disc having on its information recording surface a protective layer with a thickness of several—several tens +nm and the optical disc wherein a thickness of a protective layer or a protective film is 0 are included. Further, in the present specification, a magneto-optical disc employing a violet semiconductor laser or a violet SHG laser as a light source for recording/reproducing of information is also included in the high density optical disc.  
      Further, in the present specification, optical discs of a DVD series such as DVE-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD−R, DVD−RW, DVD+R and DVD+RW are called “DVD” generically and optical discs of a CD series such as CD-ROM, CD-Audio, CD-Video, CD-R and CD-RW are called “CD” generically.  
      Further, in the present specification, the order number of diffraction ni of the diffracted light generated when i th  light flux enters the blazed diffractive structure, the stepwise diffractive structure dr the superposition diffractive structure means the order number of diffraction of the diffracted light having the maximum diffraction efficiency among various diffracted light generated when i th  light flux enters the diffractive structure.  
      It is preferable that wavelength λ 1  and wavelength k2 satisfy the following relationship, in the aforesaid first structure and the second structure. 
 
0.2≦|Int(λ 2 /λ 1 )−λ 2 /λ 1 |≦0.5 
 
      In the expression above, Int (λ 2 /λ 1 ) indicates the integer that is closest to λ 2 /λ 1 .  
      When wavelength λ 1  and wavelength λ 2  satisfy the relationship above, effects of the invention are exhibited more splendidly.  
      It is further preferable that the following relationship is satisfied. 
 
λ 1 ≦450 nm 
 
630 nm≦λ 2 ≦680 nm 
 
      It is one of the preferable structures that the light-converging optical system has therein the objective lens and a diffraction lens having an optical surface on which the diffractive structure is formed, in the optical pickup apparatuses respectively of the first structure and the second structure. In this case, in the first structure, the objective lens converges the first light flux which has passed through the diffraction lens on the information recording surface of the first optical disc and converges the second light flux which has passed through the diffraction lens respectively on the information recording surface of the second optical disc and on the information recording surface of the third optical disc. In the second structure, the objective lens converges the first, second and third light fluxes which have passed through the diffraction lens respectively on the information recording surfaces of the first, second and third optical discs.  
      In this case, it is preferable that the objective lens and the diffraction lens are formed integrally, the optical pickup apparatus further has an actuator and the actuator drives the objective lens and the diffraction lens in the direction perpendicular to the optical axis of the light-converging optical system.  
      Since the foregoing structure makes it possible to control occurrence of coma caused by disagreement of optical axes between the diffraction lens and the objective lens in the case of tracking of the objective lens, it is possible to make the tracking properties of the objective lens in the case of conducting recording/reproducing for a high density optical disc, DVD and CD to be excellent.  
      Further, in the first structure and the second structure stated above, it is also one of the preferable embodiments that the diffractive structure is formed on at least one optical surface of the objective lens.  
      In the first structure, it is preferable that the diffractive structure has ring-shaped zones in a shape of plural concentric circles whose centers are on the optical axis of the light-converging optical system, the diffractive structure is of a superposition diffractive structure wherein a plurality of stepwise structures are formed on each ring-shaped zone and the superposition diffractive structure does not give substantially a phase difference to the first light flux but gives a phase difference to the second light. In the same way, in the second structure, it is preferable that the diffractive structure has ring-shaped zones in a shape of plural concentric circles whose centers are on the optical axis of the light-converging optical system, the diffractive structure is of a superposition diffractive structure wherein a plurality of stepwise structures are formed on each ring-shaped zone and the superposition diffractive structure does not give substantially a phase difference to the first light flux but gives a phase difference to the second light flux and the third light flux.  
      When the superposition diffractive structure of this kind is used, it is further preferable that thickness t 1  of a protective layer of the first optical disc and t 2  of a protective layer of the second optical disc satisfy the following relationship. 
 
t 1 &lt;t 2  
 
      Further, it is more preferable that t 1  and t 2  satisfy the following relational expression. 
 
t 1 ≦0.2 mm 
 
0.55 mm≦t 2 ≦0.65 mm 
 
      It is further preferable to satisfy the following relationship. 
 
λ 1 ≦450 nm 
 
630 nm≦λ 2 ≦680 nm 
 
3≦N≦10   (1) 
 
 d= 2 m ·λ 1 /( N   1 −1)   (2) 
 
0 &lt;|INT (φ 2 )−φ 2 |≦0.4   (3) 
 
φ 2   =d·N· ( N   2 −1)/λ 2    (4) 
 
      In the expressions above, N represents the number of the stepwise structures on a given ring-shaped zone of the superposition diffractive structure, d μm represents a height of the step between adjoining stepwise structures on the given ring-shaped zone, φ 2  represents an optical path added to the second light flux (and the third light flux) by the given ring-shaped zone, m represents a positive integer of not more than 5, N 1  represents the refractive index of the optical element having an optical surface having the diffractive structure for the light flux with wavelength λ 1 , N 2  represents the refractive index of the optical element having an optical surface having the diffractive structure for the light flux with wavelength λ 2 , and INT (φ 2 ) represents an integer closest to φ 2 .  
      The foregoing structure relates to the preferable form of the superposition diffractive structure for correcting properly the spherical aberration caused by a difference of protective layer thickness between the high density optical disc and DVD, while securing high transmittance for recording/reproducing wavelength for the high density optical disc and for recording/reproducing wavelengths for DVD and CD. When the number of division N of each ring-shaped zone and step d between adjoining stepwise structures are determined to satisfy expression (1) and expression (2), a width of each stepwise structure does not become too small and a height of each ring-shaped zone in the optical axis direction does not become too great, which makes it possible to process easily a metallic mold for the superimposed type of:diffractive structure, and to control a decline of transmittance caused by individual differences of oscillation wavelength of a semiconductor laser light source. When the number of division N of each ring-shaped zone and step d between adjoining stepwise structures are determined so that amount of addition φ 2  for each ring-shaped zone for the second light flux (and the third light flux) may satisfy the expression (3), it is possible to enhance the transmittance for the second light flux (and the third light flux).  
      In the optical pickup apparatuses respectively of the first structure and the second structure, it is also one of the preferable embodiments that the diffractive structure has ring-shaped zones in a shape of concentric circles having their centers on the optical axis of the light-converging optical system, and the diffractive structure is a blazed diffractive structure wherein a sectional view including the optical axis is in a saw tooth shape.  
      In the optical pickup apparatuses respectively of the first structure and the second structure, it is also one of the preferable embodiments that the diffractive structure is formed on an optical surface that is a concave surface or a convex surface macroscopically to have ring-shaped zones in a shape of plural concentric circles having their centers on the optical axis of the light-converging optical system, and is of the stepwise diffractive structure wherein a sectional view including the optical axis is in a stepwise shape.  
      In the first structure, it is further preferable that diffraction order number n 2  of the diffracted light generated when the second light flux enters satisfies the following relationship for diffraction order number n 1  (n 1 ≧2) of the diffracted light generated when the first light flux in the blazed diffractive structure and the stepwise diffractive structure. 
 
n 2 &lt;n 1  
 
      In the same way, even in the first structure, it is further preferable that diffraction order number n 2  of the diffracted light generated when the second light flux and the third light flux enter satisfies the following relationship for diffraction order number n 1  (n 1 ≧n 2 ) of the diffracted light generated when the first light flux in the blazed diffractive structure and the stepwise diffractive structure. 
 
n 2 &lt;n 1  
 
      By determining the step between the adjoining ring-shaped zones in the blazed diffractive structure or in the stepwise diffractive structure so that diffraction order number n 2  of the second light flux (and the third light flux) may be lower in terms of the order number than the diffraction order number n 1  of the first light flux, it is possible to correct the spherical aberration caused by a difference of protective layer thickness between the high density optical disc and DVD or by a difference of recording/reproducing wavelength, while securing high diffraction efficiency for all wavelengths.  
      When the blazed diffractive structure or the stepwise diffractive structure stated above is used, it is preferable that thickness t 1  of the protective layer of the first optical disc and thickness t 2  of the protective layer of the second optical disc satisfy the following structure. 
 
t 1 =t 2  
 
      In particular, 0.55 mm≦t 1  and t 2 ≦0.65 mm are preferable.  
      Further, when the blazed diffractive structure or the stepwise diffractive structure stated above is used, it is preferable that the combination of the n 1  and n 2  is any one of 
 
(n 1 , n 2 )=(2, 1), (3, 2), (5, 3), (8, 5) and (10, 6) 
 
 and the following relational expression is satisfied. 
 
λ 1 ≦450 nm 
 
630 nm≦λ 2 ≦680 nm 
 
      Compatibility between the high density optical disc and DVD can be attained by utilizing diffracting functions by means of the diffractive structure. However, when designing a compatible optical system, it is preferable to select a type of the diffractive structure in accordance with the type of the high density optical disc.  
      When using BD as a high density optical disc, it is necessary, for achievement of compatibility, to correct spherical aberration caused by a difference of protective layer thickness between BD and DVD as stated above. In such a case, when a blazed diffractive structure or a stepwise diffractive structure, which diffracts any wavelength, the wavelength-dependency of the spherical aberration caused by the diffractive structure grows greater, and an influence by individual differences of oscillation wavelength of the laser light source grows greater. Therefore, the superposition diffractive structure that diffracts the wavelength on one side selectively is preferably used. On the other hand, when HD DVD is used as a high density optical disc, spherical aberration caused by a difference of recording/reproducing wavelength has only to be corrected. Therefore, compared with an occasion where BD is used as a high density optical disc, the spherical aberration to be corrected becomes smaller. In this case, the blazed diffractive structure or the stepwise diffractive structure which can be manufactured easily is preferably used, rather than the superposition diffractive structure wherein the structure is complicated.  
      In the first structure and the second structure stated above, when NA 2  represents a numerical aperture of the objective lens in the case of conducting recording and/or reproducing of information for the second optical disc with the second light flux, it is preferable that the diffractive structure is formed in an area corresponding to the inside of the NA 2 , and the following relational expression is satisfied. 
 
t 1 ≦0.2 mm 
 
0.55 mm≦t 2 ≦0.65 mm 
 
      In the structure of this kind, the spherical aberration caused by a difference of protective layer thickness between a high density optical disc and DVD and by a difference of recording/reproducing wavelength can be corrected only within numerical aperture NA 2  of the objective lens in the case of conducting recording/reproducing for DVD. Therefore, a spot is not stopped down more than necessary and an amount of coma caused by a tilt of DVD does not grow too large. Further, the second light flux that passes through an area that is outside the numerical aperture NA 2  has spherical aberration caused by a difference of protective layer thickness between the high density optical disc and DVD and by a difference of recording/reproducing wavelength, and it becomes flare component that does not contribute to formation of a spot on an information recording surface of DVD. Since this is equivalent to the diffractive structure that has an aperture switching function for DVD, it is not necessary to provide separately a diaphragm corresponding to NA 2 , and its structure can be made simple.  
      In the optical pickup apparatus of the first structure, it is preferable that the light-converging optical system has the objective lens and a diffraction lens having the optical surface on which the diffractive structure is formed, and the objective lens converges the first light flux having passed through the diffraction lens on the information recording surface of the first optical disc, and converges the second light-flux having passed through the diffraction lens on information recording surfaces respectively of the second optical disc and the third optical disc, and further satisfies the following relational expressions; 
 
1.15 mm≦t 3 ≦1.25 mm 
 
−0.01≦m 3 &lt;0   (5) 
 
 wherein, m 3  represents the magnification of a composite optical system including the diffraction lens and the objective lens, in the case of conducting recording and/or reproducing of information for the third optical disc by the second light flux. 
 
      Equally, in the optical pickup apparatus with the second structure, it is preferable that the light-converging optical system has the objective lens and the diffraction lens having an optical surface on which the diffractive structure is formed, and the objective lens converges the first light flux, the second light flux and the third light flux which have passed through the diffraction lens respectively on information recording surfaces respectively of the first optical disc, the second optical disc and the third optical disc, and satisfies the following relational expressions.; 
 
1.15 mm≦t 3 ≦1.25 mm 
 
−0.01≦m 3 &lt;0   (5) 
 
 wherein, m 3  represents the magnification of a composite optical system including the diffraction lens&#39; and the objective lens, in the case of conducting recording and/or reproducing of information for the third optical disc by the third light flux. 
 
      These structures mentioned above relates to a range of magnification of the composite system including the diffraction lens and the objective lens, in the case of conducting recording/reproducing for CD when using the second light flux identical to DVD (or the third light flux that is the same as the second light flux in terms of a wavelength) as recording/reproducing wavelength for CD. When optimizing a width of each ring-shaped zone of the diffractive structure so that spherical aberration caused by a difference of protective layer thickness and a difference of recording/reproducing wavelength between a high density optical disc and DVD may be corrected, there still is residual spherical aberration corresponding to a difference of protective layer thickness between DVD and CD. For correcting this residual spherical aberration, it is preferable to establish magnification m 3  of a composite optical system including the diffraction lens and the objective lens in the case of conducting recording/reproducing for CD in a way that the magnification satisfies the expression (5) above. When using a light flux whose wavelength is close to conventional 785 nm as a recording/reproducing wavelength for CD, magnification m 3  needs to be established to be in a range that is smaller than that of the expression (5) above, which increases occurrence of coma aberration caused by tracking of the objective lens. However, by using the second light flux identical to DVD as a recording/reproducing wavelength for CD, a range for the magnification m 3  can be made to be the same as the range in the expression (5), thereby, occurrence of coma aberration caused by tracking of the objective lens can be reduced.  
      In the optical pickup apparatus with the first structure, it is preferable that a spherical aberration correcting means to correct spherical aberration that is caused when conducting recording and/or reproducing of information for the third optical disc by the third light flux is provided, and the following relational expression is satisfied. 
 
1.15 mm≦t 3 ≦1.25 
 
      Equally, in the optical pickup apparatus with the second structure, it is preferable that a spherical aberration correcting means to correct spherical aberration that is caused when conducting recording and/or reproducing of information for the third optical disc by the second light flux is provided, and the following relational expression is satisfied.  
      In place of setting magnification m 3  of the composite optical system including the diffraction lens and the objective lens to satisfy the expression (5), it is also possible to provide a spherical aberration correcting means to correct spherical aberration that is caused when conducting recording and/or reproducing of information for the third optical disc by the second light flux (or the third light flux). Or, it is also possible to establish m 3  to be greater and thereby to make the structure wherein the residual spherical aberration is corrected by the spherical aberration correcting means, which makes it possible to improve tracking characteristics in the case of recording/reproducing for CD.  
      In the optical pickup apparatus with the first structure, it is preferable that numerical aperture NA 3  of the objective lens in the case of conducting recording and/or reproducing of information for the third optical disc by the second light flux satisfies the following relational expression. 
 
0.36&lt;NA 3 &lt;0.43 
 
      Equally, in the optical pickup apparatus with the second structure, it is preferable that numerical aperture NA 3  of the objective lens in the case of conducting recording and/or reproducing of information for the third optical disc by the third light flux satisfies the following relational expression. 
 
0.36&lt;NA 3 &lt;0.43   (6) 
 
      The foregoing structure relates to the range of the numerical aperture of the objective lens in the case of conducting recording/reproducing for CD. When using a light flux whose wavelength is close to conventional 785 nm as a recording/reproducing wavelength for CD, numerical aperture NA 3  of the objective lens is 0.45 or 0.50, and a spot diameter obtained by this wavelength and numerical aperture is 1.43 μm or 1.29 μm. When using the second light flux whose wavelength is the same as the recording/reproducing wavelength for DVD or the third light flux whose wavelength is the same as that of the second light flux as a recording/reproducing wavelength for CD, as in the optical pickup apparatus of the invention, the numerical aperture NA 3  that is needed for obtaining the aforesaid spot diameter is 0.375 or 0.417 (provided that the wavelength of the second light flux is 655 nm). Therefore, in the optical pickup apparatus of the invention, a desired spot diameter necessary for recording/reproducing for CD can be obtained by setting the numerical aperture NA 3  so that it satisfies the expression (6). Though the coma aberration caused in the objective lens increases in proportion to NA 3 /λ (NA represents a numerical aperture of the objective lens, and λ represents a wave of incidence to the objective lens), occurrence of coma aberration caused by tracking of the objective lens can be controlled more, compared with an occasion where a light flux whose wavelength is close to 785 nm is used as a recording/reproducing wavelength, because numerical aperture NA 3  of the objective lens in the case of conducting recording/reproducing for CD is set within a range of the expression (6) in the optical pickup apparatus of the invention.  
      In the optical pickup apparatus with the first structure, when NA 2  represents a numerical aperture of the objective lens in the case of conducting recording and/or reproducing of information for the second optical disc by the second light flux and NA 3  represents a numerical aperture of the objective lens in the case of conducting recording and/or reproducing of information for the third optical disc by the second light flux, it is preferable that the optical pickup apparatus further has an aperture element that switches an aperture of the objective lens in accordance with a difference between the NA 2  and the NA 3 .  
      Equally, even in the optical pickup apparatus with the second structure, when NA 2  represents a numerical aperture of the objective lens in the case of conducting recording and/or reproducing of information for the second optical disc by the second light flux and NA 3  represents a numerical aperture of the objective lens in the case of conducting recording and/or reproducing of information for the third optical disc by the third light flux, it is preferable that the optical pickup apparatus further has an aperture element that switches an aperture of the objective lens in accordance with a difference between the NA 2  and the NA 3 .  
      In the foregoing structure, the spot is not stopped down more than necessary in the case of conducting recording/reproducing for CD, an amount of coma aberration caused by a tilt of CD does not grow greater too much and occurrence of coma aberration caused by tracking of the objective lens can be reduced. The aperture element of this kind may include a variable diaphragm that can change a size of its aperture depending on recording density of the optical disc that requires recording/reproducing, the one wherein a diaphragm corresponding to a numerical aperture is prepared for each type of an optical disc, and a diaphragm to be inserted in an optical path between the light source and an optical disc is switched mechanically depending on recording density of the optical disc that require recording/reproducing, a polarization filter having polarization dependency of the transmittance, a liquid crystal shutter and a polarization hologram. In addition, the aperture element may either be arranged in an optical path between the light source and the diffraction lens, in an optical path between the diffraction lens and the objective lens, or in an optical path between the objective lens and an optical disc.  
      Further, it is preferable that the aperture element is provided with a plane of polarization converting means that converts, based on control signals from the outside, a plane of polarization of the second light flux (and the third light flux) entering the objective lens by a prescribed amount. It is a preferred embodiment that the plane of polarization converting means is composed of a liquid crystal element.  
      The aperture element switching an aperture of the objective lens in accordance with a difference between the NA 2  and the NA 3  which is of the structure to have the plane of polarization converting means that converts the plane of polarization of the second light flux entering the objective lens by a prescribed amount can make the structure of the aperture element to be simple, which is preferable. In particular, when the plane of polarization converting means is composed of a liquid crystal element and is used in a form of combination with the polarization filter, an aperture element in a simple structure having no mechanically movable portions can be obtained.  
      As a third structure of the invention, there is given an optical information recording and/or reproducing apparatus having the optical pickup apparatus and a holding member that holds the first optical disc, the second optical disk or the third optical disc in the case of conducting recording and/or reproducing of information. The optical information recording and/or reproducing apparatus of this kind naturally makes it possible to obtain the same effect as in the foregoing optical pickup apparatus.  
     PREFERRED EMBODIMENTS OF THE INVENTION  
      Preferred embodiments to practice the invention will be explained in detail as follows, referring to the drawings.  
     First Embodiment  
       FIG. 1  is a diagram showing schematically the structure of first optical pickup apparatus PU 1  capable of conducting recording/reproducing of information properly for high density optical disc HD (first optical disc), DVD (second optical disc) and CD (third optical disc). Optical specifications of the high density optical disc HD include wavelength λ 1 =408 nm, thickness t 1  of protective layer PL 1 =0.0875 mm and numerical aperture NA 1 =0.85, optical specifications of DVD include wavelength λ 2 =658 nm, thickness t 2  of protective layer PL 2  =0.6 mm and numerical aperture NA 2 =0.65, and optical specifications of CD include wavelength λ 2 =658 nm, thickness t 3  of protective layer PL 3 =1.2 mm and numerical aperture NA 3 =0.386, wherein a combination of the wavelength, the thickness of the protective layer and the numerical aperture is not limited to the foregoing combination.  
      The first optical pickup apparatus PU 1  shown in  FIG. 1  is an optical pickup apparatus capable of conducting recording/reproducing of information properly for high density optical disk HD (first optical disc), DVD (second optical disc) and CD (third optical disc). The is composed of violet semiconductor laser LD 1  (first light source) that emits light when conducting recording/reproducing of information for high density optical disc HD and emits the first light flux with wavelength 408 nm, red semiconductor laser LD 2  (second light source) that emits light when conducting recording/reproducing of information for DVD and CD and emits the second light flux with wavelength 658 nm, photodetector PD that is used commonly for the first light source and the second light source, beam shaping element BSH for changing a sectional form of a laser beam emitted from violet semiconductor laser LD 1  from an oval form to a circular form, objective optical system OBJ axis actuator AC having a function to converge each light flux on information recording surfaces RL 1  and RL 2 , first beam splitter BS 1 , second beam splitter BS 2 , first collimating optical system COL 1 , second collimating optical system COL 2 , diaphragm STO, sensor lens SEN and liquid crystal element LCD (aperture element).  
      When conducting recording/reproducing of information for high density optical disc HD in the optical pickup apparatus PU 1 , the violet semiconductor laser LD 1  is first made to emit light as its path of a ray of light is drawn with solid lines in  FIG. 1 . A divergent light flux emitted from violet semiconductor laser LD 1  is changed in terms of its sectional form from an oval form to a circular form while it is passing through beam shaping element BSH, then, it is converted into a parallel light flux while it is transmitted through first collimating optical system COL 1 , and it passes successively through first beam splitter BS 1 , second beam splitter BS 2  and liquid crystal element LCD to become a spot formed by objective optical system OBJ on information recording surface RL 1  through protective layer PL 1  of high density optical disc HD.  
      Incidentally, objective optical system OBJ will be explained in detail later.  
      The objective optical system OBJ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OB. A reflected light flux modulated by information pits on information recording surface RL 1  passes again through objective optical system OBJ and liquid crystal element LCD, then, is branched by second beam splitter BS 2 , and is given astigmatism when it passes through sensor lens SEN to be converged on light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on high density optical disc HD by using output signals of photodetector PD.  
      When conducting recording/reproducing of information for DVD, the red semiconductor laser LD 2  is first made to emit light as its path of a ray of light is drawn with dotted lines in  FIG. 1 . A divergent light flux emitted from red semiconductor laser LD 2  is converted into a parallel light flux while it is transmitted through second collimator optical-system COL 2 , and then, is reflected on the first beam splitter BS 1 , and passes successively through the second beam splitter BS 2  and liquid crystal element LCD to become a spot formed by objective optical system OBJ on information recording surface RL 2  through protective layer PL 2  of DVD.  
      Then, the objective optical system OBJ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OB. A reflected light flux modulated by information pits on information recording surface RL 2  passes again through objective optical system OBJ and liquid crystal element LCD, then, is branched by second beam splitter BS 2 , and is given astigmatism when it passes through sensor lens SEN to be converged on light-receiving surface of photodetector PD. Then, it is possible to read information recorded on DVD by using output signals of photodetector PD.  
      When conducting recording/reproducing of information for CD, the red semiconductor laser LD 2  is first made to emit light as its path of a ray of light is drawn with two-dot chain lines in  FIG. 1 . A divergent light flux emitted from red semiconductor laser LD 2  is converted into a parallel light flux while it is transmitted through second collimator optical system CLD 2 , and then, is reflected on the first beam splitter BS 1 , and passes successively through the second beam splitter BS 2  and liquid crystal element LCD to become a spot formed by objective optical system OBJ on information recording surface RL 3  through protective layer PL 3  of CD.  
      Then, the objective optical system OBJ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OBJ. A reflected light flux modulated by information pits on information recording surface RL 3  passes again through objective optical system OBJ and liquid crystal element LCD, then, is branched by second beam splitter BS 2 , and is given astigmatism when it passes through sensor lens SEN to be converged on light-receiving surface of photodetector PD. Then, it is possible to read information recorded on CD by using output signals of photodetector PD.  
      Next, the structure of the objective optical system OBJ will be explained.  
      The objective optical system OBJ is composed of diffraction lens L 1  and the objective lens L 2  that has a function to converge a laser light flux transmitted through the diffraction lens L 1  on an information recording surface of an optical disc and has an aspheric surface on each of its both sides, as shown in  FIG. 2 . Each of the diffraction lens L 1  and the objective lens L 2  is a plastic lens, and on circumferences of the respective optical functional sections of them, there are formed respectively flange portions FL 1  and FL 2  each being united solidly with the optical functional section, and both of them are united solidly when a part of the flange portion FL 1  and a part of the flange portion FL 2  are fitted together. Though the diffraction lens equipped with the diffractive structure is provided separately in the present embodiment, the diffractive structure may also be provided on the optical surface of the objective lens, making it unnecessary to provide the diffraction lens separately in that case.  
      With respect to optical specifications for high density optical disc HD, DVD and CD which are assumed in the case of designing objective optical system OBJ, the specifications including wavelength λ 1 =408 nm, thickness t 1 =0.0875 mm for protective layer PL 1  and numerical aperture NA 1 =0.85 are for the high density optical disc HD, the specifications including wavelength λ 2 =658 nm, thickness t 2 =0.6 mm for protective layer PL 2  and numerical aperture NA 2 =0.65 are for DVD, and the specifications including wavelength λ 2 =658 nm, thickness t 3 =1.2 mm for protective layer PL 3  and numerical aperture NA 3 =0.386 are for CD. However, the combination of the wavelength, the thickness of the protective layer and the numerical aperture is not limited to the foregoing.  
      Optical surface S 1  closer to the light source on the diffraction lens L 1  is divided into first area AREA  1  corresponding to an area within NA 2  and second area AREA  2  corresponding to an area from NA 1  to NA 2  as shown in  FIG. 3 ( a ), and the superposition diffractive structure HOE representing the structure wherein plural ring-shaped zones having therein stair-structure are arranged with their centers positioned on the optical axis is formed in the first area AREA  1 , as shown in  FIG. 2 ( a ).  
      In the superposition diffractive structure HOE formed in the first area AREA  1 , step d μm between stair-structures formed in each ring-shaped zone is established to the value calculated by d=2·λ 1 /(N 1 −1) (μm), and the number of division for each ring-shaped zone N is established to 5.  
      In the expression above, λ 1  represents a wavelength shown in a unit of micron of a laser light flux emitted from the violet semiconductor laser (in this case, λ 1 =0.408 μm), and N 1  represents a refractive index for wavelength λ 1  of the diffractive lens L 1  (in this case, N 1 =1.5242).  
      When the laser light flux having wavelength λ 1  enters the superposition diffractive structure HOE, an optical path difference of 2×λ 1  (μm) is generated between the adjoining stairs, and the laser light flux with wavelength λ 1  is hardly given a phase difference to be transmitted as it is without being diffracted. Incidentally, in the following explanation, a light flux that is hardly given a phase difference by the superposition diffractive structure and is transmitted while being left intact is called a zero-order diffracted light.  
      On the other hand, when the laser light flux having wavelength λ 2  (in this case, λ 2 =0.658 μm) emitted from the red semiconductor laser enters the superposition diffractive structure HOE, an optical path difference of d×(N 2 −1−λ 2 =0.13 μm is generated, and an optical path difference between 0.13×5=0.65 μm and one wavelength of wavelength λ 2  is generated for one fifth of one ring-shaped zone which is divided into 5 parts, thus, wave fronts which have been transmitted through adjoining ring-shaped zones is shifted by one wavelength to be overlapped. Namely, the light flux with wavelength λ 2  is diffracted by the superposition diffractive structure HOE in the one-order direction to become the diffracted light. Incidentally, N 2  represents a refractive index of the diffraction lens L 2  for wavelength λ 2  (in this case, N 2 =1.5064). The diffraction efficiency of the first-order diffracted light of the laser light flux with wavelength λ 2  in this case is 87.5% which represents a sufficient amount of light for recording/reproducing of information for DVD and CD.  
      Since width Λ I  of each ring-shaped zone of the superposition diffractive structure HOE is optimized so that spherical aberration caused by a difference of protective layer thickness between a high density optical disc and DVD may be corrected, spherical aberration corresponding to a difference of protective layer thickness between DVD and CD still remains. In order to correct this remaining spherical aberration, magnification m 3  of objective optical system OBJ in the case of conducting recording/reproducing for CD is set to −0.0725 in the second optical pickup apparatus PU 2 .  
      Further, optical surface S 2  closer to an optical disc on the diffraction lens L 1  is divided into third area AREA 3  including the optical axis corresponding to an area in NA 2  and fourth area AREA 4  corresponding to an area from NA 2  to NA 1  as shown in  FIG. 3 ( c ), and blaze type diffractive structures DOE 1  and DOE 2  are formed respectively on the third area AREA 3  and the fourth area AREA 4 . Blaze type diffractive structures DOE 1  and DOE 2  are structures for correcting chromatic aberration of objective optical system OBJ in a violet area.  
      In the invention, division number N in optional ring-shaped zone of a stair-type diffractive structure, step d μm between adjoining stair-structures and optical path difference φ 2  to be added to the second light flux by optional ring-shaped zone satisfy the following expressions (1)-(4). 
 
3≦N≦10   (1) 
 
 d= 2 m ·λ 1 /( N   1 −1)   (2) 
 
0 &lt;|INT (φ 2 )−φ 2 |≦0.4   (3) 
 
φ 2   =d·N· ( N   2 −1)/λ 2    (4) 
 
      In the above expressions, m represents positive ingeter of 5 and less and INT (φ 2 ) represents an integer closest to φ 2 .  
      Objective optical system OBJ is united solidly with liquid crystal element LCD through connecting member HM (see  FIG. 1 ), and it conducts tracking and focusing together with the liquid crystal element LCD.  
      When conducting recording/reproducing of information for CD, an aperture is switched by the liquid crystal element LCD. The technology to switch an aperture by the liquid crystal element LCD is described, for example, in TOKKAIHEI No. 10-20263, and it is a known technology. Therefore, the detailed explanation for that technology will be omitted here. Due to the foregoing technology, a spot is not stopped down more than necessary and an amount of coma caused by a tilt of CD does not grow too large.  
      Further, since the superposition diffractive structure HOE is formed only on the first area AREA 1  of the diffraction lens L 1 , the second light flux passing through the second area AREA 2  becomes flare components which do not contribute to formation of a spot, on information recording surface RL 2  of DVD. Therefore, when conducting recording/reproducing of information for DVD, an aperture is switched by the diffraction lens L 1 .  
      In addition to switching of an aperture, the liquid crystal element LCD may also conduct correcting of spherical aberration for a spot formed on information recording surface RL 1  of a high density optical disc HD and for a spot formed on information recording surface RL 2  of DVD, and thereby, recording/reproducing characteristics for the high density optical disc HD and DVD can be improved.  
      It is further possible to correct spherical aberration resulting from a difference of protective layer thickness between DVD and CD with the liquid crystal element LCD, and owing to this, the magnification m 3  of objective optical system OBJ in the case of conducting recording/reproducing of information for CD can be set to be greater, which makes it possible to improve tracking characteristics.  
      The technology to correct spherical aberration by the liquid crystal element LCD is described, for example, in TOKKAIHEI No. 10-20263, and it is a known technology. Therefore, the detailed explanation for that technology will be omitted here.  
     Second Embodiment  
      Next, the Second Embodiment of the invention will be explained as follows, and the same structures therein as those in the First Embodiment are given the same reference symbols, and explanations for them will be omitted here.  
      As shown in  FIG. 4 , optical pickup apparatus PU 2  is composed of violet semiconductor laser LD 1  emitting the first light flux, red semiconductor laser LD 2  emitting the second light flux used for DVD, red semiconductor laser LD 2 ′ emitting the second light flux used for CD, photodetector PD to be used commonly for the first and second light fluxes, beam expander optical system EXP including first lens EXP 1  and second lens EXP 2 , uniaxial actuator AC 1 , objective optical system OBJ, biaxial actuator AC 2 , first beam splitter BS 1 , second beam splitter BS 2 , third beam splitter BS 3 , first collimating optical system COL 1 , second collimating optical system CLD 2 , third collimating optical system COL 3  and polarizing filter PF (aperture element).  
      The first light flux for a high density optical disc HD and the second light flux for DVD are emitted respectively from violet semiconductor laser LD 1  and red semiconductor laser LD 2  under the condition that the light flux is polarized in the prescribed direction (for example, the direction which is perpendicular to the page), and the second light flux to be used for CD is emitted from the red semiconductor laser LD 2  in the direction perpendicular to the prescribed direction (for example, the direction that is in parallel with the page).  
      The polarizing filter PF is united solidly with objective optical system OBJ through connecting member HM (see  FIG. 4 ), and it conducts tracking and focusing together with objective optical system OBJ.  
      Polarizing filter PF has characteristics to transmit the first light flux for high density optical disc HD polarized in the prescribed direction and the second light flux for DVD and to intercept or reflect the second light flux for CD polarized in the direction perpendicular to the prescribed direction in the area corresponding to a range from NA 3  to NA 1  (a hatched section in the front view of polarizing filter PF in  FIG. 4 ), and has characteristics to transmit the first light flux for high density optical disc HD, the second light flux for DVD and the second light flux for CD in the area corresponding to the inside of NA 3  (a section surrounded by the hatched section in the front view of polarizing filter PF in  FIG. 4 ). Therefore, it is possible to conduct switching of an aperture in the case of conducting recording/reproducing of information for CD by the polarizing filter PF.  
      Incidentally, since the structure of the objective optical system OBJ in the present embodiment is the same as that of the objective optical system OBJ in the First Embodiment, an explanation of the structure of the objective optical system OBJ in the present embodiment will be omitted here.  
      When conducting recording/reproducing of information for high density optical disc HD in the optical pickup apparatus PU 2 , the violet semiconductor laser LD 1  is first made to emit light as its path of a ray of light is drawn with solid lines in  FIG. 4 . A divergent light flux emitted from violet semiconductor laser LD 1  is changed in terms of its sectional form from an oval form to a circular form while it is passing through beam shaping element BSH, then, it is converted into a parallel light flux while it is transmitted through first collimating optical system COL 1 , and it passes successively through first beam splitter BS 1 , second beam splitter BS 2 , third-beam splitter BS 3 , first lens EXP 1  and second lens EXP 2  to become a spot formed by objective optical system OBJ on information recording surface RL 1  through protective layer PL 1  of high density optical disc HD.  
      The objective optical system OBJ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OB. A reflected light flux modulated by information pits on information recording surface RL 1  passes again through objective optical system OBJ, second lens EXP 2  and first lens EXP 1 , then, is branched by third beam splitter BS 3 , and is converged on light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on high density optical disc HD by using output signals of photodetector PD.  
      When conducting recording/reproducing of information for DVD, the red semiconductor laser LD 2  is first made to emit light as its path of a ray of light is drawn with dotted lines in  FIG. 4 . A divergent light flux emitted from red semiconductor laser LD 2  is converted into a parallel light flux while it is transmitted through second collimator optical system COL 2 , and then, is reflected on the first beam splitter BS 1 , and passes successively through the second beam splitter BS 2 , the third beam splitter BS 3 , first lens EXP 1  and second lens EXP 2  to become a spot formed by objective optical system OBJ on information recording surface RL 2  through protective layer PL 2  of DVD.  
      Then, the objective optical system OBJ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OBJ. A reflected light flux modulated by information pits on information recording surface RL 2  passes again through objective optical system OBJ, second lens EXP 2  and first lens EXP 1 , then, is branched by third beam splitter BS 3  to be converged on a light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on high density optical disc HD by using output signals of photodetector PD.  
      When conducting recording/reproducing of information for CD, the first lens EXP 1  is moved by uniaxial actuator UAC in a way that magnification m 3  of objective optical system OBJ for a light flux with wavelength λ 2  satisfies expression (5) so that a light flux emitted from infrared semiconductor laser LD 2 ′ may enter the objective optical system OBJ as a divergent light.  
      Then, the infrared semiconductor laser LD 2 ′ is first made to emit light as its path of a ray of light is drawn with two-dot chain lines in  FIG. 4 . A divergent light flux emitted from the infrared semiconductor laser LD 2 ′ passes through the third collimating optical system COL 3  to be converted into a parallel light flux, then, it is reflected by the third beam splitter BS 3 , and passes through the first lens EXP 1  and the second lens EXP 2  to arrive at polarizing filter PF as a divergent light.  
      As stated above, the polarizing filter PF has characteristics to intercept or reflect the second light flux for CD polarized in the direction perpendicular to the prescribed direction, in the area corresponding to an area from NA 3  to NA 1 , and has characteristics to transmit the second light flux for CD in the area corresponding to the inside of NA 3 . Therefore, only the light flux having arrived at the area corresponding to the inside of NA 3  of the polarizing filter PF is transmitted through the polarizing filter PF to become a spot that is formed by the objective optical system OBJ on information recording surface RL 3  through protective layer PL 3  of CD.  
      Then, the objective optical system OBJ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OBJ. A reflected light flux modulated by information pits on information recording surface RL 3  passes again through objective optical system OBJ, second lens EXP 2  and first lens EXP 1 , then, is branched by third beam splitter BS 3  to be converged on a light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on high density optical disc HD by using output signals of photodetector PD.  
      Incidentally, in the present embodiment, the first light flux for high density optical disc HD and the second light flux for DVD are emitted respectively from violet semiconductor laser LD 1  and red semiconductor laser LD 2  under the condition of polarization in the prescribed direction, and the second light flux to be used for CD is emitted from red semiconductor laser LD 2 ′ under the condition of polarization in the prescribed perpendicular to the prescribed direction. However, it is also possible to employ, without being limited to the foregoing, the structure wherein there is arranged a polarization direction changing means (illustration omitted) that can change the polarization direction of the light flux emitted from the red semiconductor laser LD 2 , depending on a type of an optical disc for which the recording/reproducing of information is conducted. Specifically, when conducting recording/reproducing of information for DVD by the polarization direction changing means of this kind, the second light flux is made to enter the objective optical system OBJ under the condition where it is polarized in the prescribed direction (for example, the direction perpendicular to the page), while, when conducting recording/reproducing of information for CD, the second light flux is made to enter the objective optical system OBJ under the condition where the second light flux is polarized in the direction perpendicular to the prescribed direction (for example, the direction that is in parallel with the page).  
      As a polarization direction changing means of this kind, the liquid crystal element LCD stated above may be used, or the structure wherein a wavelength plate such as a ¼ wavelength plate or a ½ wavelength plate is rotated mechanically can be used.  
      Further, spherical aberration of the spot formed on information recording surface RL 1  of high density optical disc HD by beam expander optical system EXP, or spherical aberration of the spot formed on information recording surface RL 2  of DVD may be corrected, and thereby, recording/reproducing characteristics for the high density optical disc HD and for DVD can be improved.  
     Third Embodiment  
      Next, the Second Embodiment will be explained as follows, and the same structures therein as those in the First Embodiment are given the same reference symbols, and explanations for them will be omitted here.  
      As shown in  FIG. 5 , optical pickup apparatus PU 3  is composed of violet semiconductor laser LD 1  emitting the first light flux, red semiconductor laser LD 2  emitting the second light flux used for DVD, red semiconductor laser LD 2 ′ emitting the second light flux used for CD, photodetector PD to be used commonly for the first and second light fluxes, beam expander optical system EXP including first lens EXP 1  and second lens EXP 2 , uniaxial actuator AC 1 , objective optical system OBJ′, biaxial actuator AC 2 , first beam splitter BS 1 , second beam splitter BS 2 , third beam splitter BS 3 , first collimating optical system COL 1 , second collimating optical system CLD 2 , and polarizing filter PF (aperture element).  
      The first light flux for a high density optical disc HD and the second light flux for DVD are emitted respectively from violet semiconductor laser LD 1  and red semiconductor laser LD 2  under the condition that the light flux is polarized in the prescribed direction (for example, the direction which is perpendicular to the page), and the second light flux to be used for CD is emitted from the red semiconductor laser LD 2  in the direction perpendicular to the prescribed direction (for example, the direction that is in parallel with the page).  
      Polarizing filter PF has characteristics to transmit the first light flux for high density optical disc HD polarized in the prescribed direction and the second light flux for DVD and to intercept or reflect the second light flux for CD polarized in the direction perpendicular to the prescribed direction in the area corresponding to a range from NA 3  to NA 1  (a hatched section in the front view of polarizing filter PF in  FIG. 5 ), and has characteristics to transmit the first light flux for high density optical disc HD, the second light flux for DVD and the second light flux for CD in the area corresponding to the inside of NA 3  (a section surrounded by the hatched section in the front view of polarizing filter PF in  FIG. 5 ). Therefore, it is possible to conduct switching of an aperture in the case of conducting recording/reproducing of information for CD by the polarizing filter PF.  
      Incidentally, the objective optical system OBJ will be explained in detail later.  
      When conducting recording/reproducing of information for high density optical disc HD in the optical pickup apparatus PU 3 , the violet semiconductor laser LD 1  is first made to emit light as its path of a ray of light is drawn with solid lines in  FIG. 5 . A divergent light flux emitted from violet semiconductor laser LD 1  is changed in terms of its sectional form from an oval form to a circular form while it is passing through beam shaping element BSH, then, it is converted into a parallel light flux while it is transmitted through first collimating optical system COL 1 , and it passes successively through first beam splitter BS 1 , third beam splitter BS 3 , first lens EXP 1 , second lens EXP 2  and second beam splitter BS 2 , to become a spot formed by objective optical system OBJ on information recording surface RL 1  through protective layer PL 1  of high density optical disc HD.  
      The objective optical system OBJ′ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OBJ′. A reflected light flux modulated by information pits on information recording surface RL 1  passes again through objective optical system OBJ′, second beam splitter BS 2 , second lens EXP 2  and first lens EXP 1 , then, is branched by third beam splitter BS 3 , and is converged on light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on high density optical disc HD by using output signals of photodetector PD.  
      When conducting recording/reproducing of information for DVD, the red semiconductor laser LD 2  is first made to emit light as its path of a ray of light is drawn with dotted lines in  FIG. 5 . A divergent light flux emitted from red semiconductor laser LD 2  is converted into a parallel light flux while it is transmitted through second collimator optical system CLD 2 , and then, is reflected on the first beam splitter BS 1 , and passes successively through the third beam splitter BS 3 , first lens EXP 1  and second lens EXP 2  and the second beam splitter BS 2 , to become a spot formed by objective optical system OBJ′ on information recording surface RL 2  through protective layer PL 2  of DVD.  
      Then, the objective optical system OBJ′ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OBJ′. A reflected light flux modulated by information pits on information recording surface RL 2  passes again through objective optical system OBJ′, second beam splitter BS 2 , second lens EXP 2  and first lens EXP 1 , then, is branched by third beam splitter BS 3  to be converged on a light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on DVD by using output signals of photodetector PD.  
      When conducting recording/reproducing of information for CD, the infrared semiconductor laser LD 2 ′ is first made to emit light as its path of a ray of light is drawn with two-dot chain lines in  FIG. 5 . The divergent light flux emitted from the infrared semiconductor laser LD 2 ′ is reflected on the second beam splitter BS 2  and arrives at polarizing filter PF as a divergent light.  
      As stated above, the polarizing filter PF has characteristics to intercept or reflect the second light flux for CD polarized in the direction perpendicular to the prescribed direction, in the area corresponding to an area from NA 3  to NA 1 , and has characteristics to transmit the second light flux for CD in the area corresponding to the inside of NA 3 . Therefore, only the light flux having arrived at the area corresponding to the inside of NA 3  of the polarizing filter PF is transmitted through the polarizing filter PF to become a spot that is formed by the objective optical system OBJ′ on information recording surface RL 3  through protective layer PL 3  of CD.  
      Then, the objective optical system OBJ′ conducts focusing and tracking with biaxial actuator AC arranged on the circumference of the objective optical system OBJ′. A reflected light flux modulated by information pits on information recording surface RL 3  passes through objective optical system OBJ′, second beam splitter BS 2 , second lens EXP 2  and first lens. EXP 1 , then, is branched by third beam splitter BS 3  to be converged on a light-receiving surface of photodetector PD. Thus, it is possible to read information recorded on CD by using output signals of photodetector PD.  
      Next, the structure of objective optical system OBJ′ will be explained as follows.  
      The objective optical system OBJ′ is composed of diffraction lens L 1  and the objective lens L 2  that has a function to converge a laser light flux transmitted through the diffraction lens L 1  on an information recording surface of an optical disc and has an aspheric surface on each of its both sides, in the same way as in objective optical system OBJ. Each of the diffraction lens L 1  and the objective lens L 2  is a plastic lens, and on circumferences of the respective optical functional sections of them, there are formed respectively flange portions FL 1  and FL 2  each being united solidly with the optical functional section, and both of them are united solidly when a part of the flange portion FL 1  and a part of the flange portion FL 2  are fitted together.  
      Since the optical specifications for high density optical disc HD, bVD and CD which are assumed in the case of designing objective optical system OBJ′ are the same as those of the objective optical system OBJ, detailed explanation for them is omitted here.  
      In a similar way as in the objective optical system OBJ, optical surface S 1  closer to the light source on diffraction lens L 1  is divided into first area AREA 1  corresponding to an area inside NA 2  and second area AREA 2  corresponding to an area from NA 2  to NA 1 , and a blazed diffractive structure DOE 3  (illustration omitted) wherein ring-shaped zones in a shape of plural concentric circles whose sectional view including the optical axis is in a serrated form are formed on the first area AREA 1 .  
      In the blazed diffractive structure DOE 3  formed on the first area AREA 1 , a depth of a step in the optical axial direction of each ring-shaped zone is determined in a way that diffraction order number n 1  of the diffracted light having the maximum diffraction efficiency among diffracted light generated when the first light flux enters is 2, and diffraction order number n 2  of the diffracted light having the maximum diffraction efficiency among diffracted light generated when the second light flux enters is 1. Incidentally, manufacture wavelength λB of the blazed diffractive structure DOE 3  is set to 390 nm, and a high diffraction efficiency is secured for all wavelengths in a way that the diffraction efficiency of the second order diffracted light of the laser light flux with wavelength λ 1  is about 97% and the diffraction efficiency of the first order diffracted light of the laser light flux with wavelength λ 2  is about 94%.  
      Owing to the functions of the blazed diffractive structure DOE 3 , spherical aberration caused by a difference of protective layer thickness between high density optical disc HD and DVD is corrected by utilizing a difference between diffracting actions (diffraction angles) for wavelength λ 1  and wavelength λ 2 , while securing the high diffraction efficiency for each wavelength.  
      Further, for correcting spherical aberration corresponding to the difference of protective layer thickness between DVD and CD, magnification m 3  of objective optical system OBJ′ in the case of conducting recording/reproducing of information for Cb is set to −0.0725, in the third optical pickup apparatus PU 3 .  
      Further, the second light flux passing through the second area AREA 2  results in a flare component that does not contribute to spot formation on information recording surface RL 2  of DVD, because the blazed diffractive structure DOE 3  is formed only on the first area AREA 1  of diffraction lens L 1 . When conducting recording/reproducing of information for DVD, therefore, the diffraction lens L 1  switches an aperture.  
      Incidentally, beam expander optical system EXP is a spherical aberration correcting means for correcting spherical aberration for a spot formed on information recording surface RL 1  of high density optical disc HD or a spot formed on information recording surface RL 2  of DVD, which can improve recording/reproducing characteristics for the high density optical disc HD and for DVD.  
     EXAMPLE 1  
      Next, Example of objective optical system OBJ that is suitable for the second optical pickup apparatus PU 2  will be explained.  
      Table 1 shows lens data of optical elements.  
               TABLE 1-1                          Lens Data Table                 (Optical specification)       HD = NA1 = 0.85, f1 = 2.200 mm, λ1 = 408 nm, m1 = 0, t1 = 0.0875 mm       DVD = NA2 = 0.65, f2 = 2.309 mm, λ2 DVD  = 658 nm, m2 = 0, t2 = 0.6 mm       CD = NA3 = 0.386, f3 = 2.309 mm, λ2 CD  = 658 nm, m3 = −0.0725, t3 = 1.2 mm                 (Paraxial data)                                                 Surface   r   d1   d2   d3                       No.   (mm)   (mm)   (mm)   (mm)   Nλ1   Nλ2 DVD     Nλ2 CD     νd               OBJ       ∞   ∞   33.000       STO       0.050   0.050   0.050       1   *1   0.900   0.900   0.900   1.52424   1.50643   1.50497   56.5       2   *1   0.050   0.050   0.050       3     1.445   2.510   2.510   2.510   1.55965   1.54062   1.53724   56.3       4   −4.540   0.679   0.477   0.267       5   ∞   0.0875   0.600   1.200   1.62110   1.57975   1.57326   30.0       6   ∞                         *1 (Following Table)                 (Paraxial radius of curvature, aspheric surface coefficient,       diffraction order number, manufacture wavelength, optical       path function coefficient for the first and second surfaces)                             First surface   Second surface                                     AREA1   AREA2   AREA3   AREA4           (0 ≦ h ≦ 1.49)   (1.49 ≦ h)   (0 ≦ h ≦ 1.49)   (1.49 ≦ h)               r   ∞   231.761   −117.433   −167.005       κ   0.0000E+00   0.0000E+00   0.0000E+00   9.6672E+01       A4    0.0000E+00   −1.2634E−04   −2.3039E−03   1.0847E−03       A6    0.0000E+00   −1.4443E−03   3.1515E−03   −2.2698E−04       A8    0.0000E+00   6.3328E−04   −2.1791E−04   4.0064E−04       A10   0.0000E+00   −6.8934E−05   −5.9061E−05   −1.3815E−05       n1/n2/n3   0/+1/+1   —   +2/+1/+1   +2/+1/+1       λB   658 nm   —   390 nm   408 nm       B2    4.7000E−03   0.0000E+00   −5.3000E−03   −5.2595E−03       B4    −5.5308E−04   0.0000E+00   5.6232E−04   −3.8500E−04       B6    −2.5919E−04   0.0000E+00   −7.7644E−04   −2.8980E−04       B8    −2.0155E−05   0.0000E+00   5.1093E−05   5.6214E−05       B10   2.0712E−07   0.0000E+00   1.4877E−05   −1.4307E−05                  
 
     
       
         
           
               
             
               
                 TABLE 1-2 
               
             
            
               
                   
               
               
                   
               
               
                 (Aspheric surface coefficients for the third and fourth 
               
               
                 surfaces) 
               
            
           
           
               
               
               
            
               
                   
                 Third surface 
                 Fourth surface 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 k 
                 −6.6105E−01 
                 −1.5745E+02 
               
               
                   
                 A4 
                 1.1439E−02 
                 1.0519E−01 
               
               
                   
                 A6 
                 2.5153E−03 
                 −1.1661E−01 
               
               
                   
                 A8 
                 8.3248E−06 
                 1.0617E−01 
               
               
                   
                 A10 
                 2.9389E−04 
                 −7.0962E−02 
               
               
                   
                 A12 
                 6.6343E−05 
                 2.7343E−02 
               
               
                   
                 A14 
                 −4.2105E−05 
                 −4.3966E−03 
               
               
                   
                 A16 
                 −3.6643E−06 
                 0.0000E+00 
               
               
                   
                 A18 
                 7.9754E−06 
                 0.0000E+00 
               
               
                   
                 A20 
                 −1.2239E−06 
                 0.0000E+00 
               
               
                   
                   
               
            
           
         
       
     
      Optical specifications in the case of using the high density optical disc HD include wavelength λ 1 =408 nm, thickness t 1  of protective layer PL 1 =0.0875 mm, numerical aperture NA 1 =0.85, focal length f 1 =2.200 mm and magnification m 1 =−0, optical specifications in the case of using DVD include wavelength λ 2 =658 nm, protective layer PL 2  thickness t 2 =0.6 mm, numerical aperture NA 2 =0.65, focal length f 2 =2.309 mm and magnification m 2 =0 and optical specifications in the case of using CD include wavelength λ 2 =658 nm, protective layer PL 3  thickness t 3 =1.2 mm, numerical aperture NA 3 =0.386, focal length f 3 =2.309 mm and magnification m 3 =−0.0725.  
      An aspheric surface on the optical surface is expressed by the expression wherein coefficients in Table 1 are substituted in the following Numeral 1, when X (mm) represents an amount of transformation from a plane that is tangent to the aspheric surface at its vertex, h (mm) represents a height in the direction perpendicular to the optical axis and r (mm) represents a radius of curvature, wherein κ represents a conic constant and A 2i  represents an aspheric surface coefficient.  
             x   =           h   2     /   r       1   +       1   -       (     1   +   κ     )     ⁢       (     h   /   r     )     2               +       ∑     i   =   2       ⁢       A     2   ⁢   i       ⁢     h     2   ⁢   i                     Numeral   ⁢           ⁢     (   1   )               
 
      In Table 1, NA 1 , NA 2  and NA 3  represent numerical apertures respectively of high density optical disc HD, DVD and CD, while, f 1 , f 2  and f 3  represent focal lengths (mm) respectively of high density optical disc HD, DVD and CD, and λ 1 , λ 2   DVD  and λ 2   CD  represent design wavelength (nm) respectively of high density optical disc HD, DVD and CD, then, m 1 , m 2  and m 3  represent magnifications respectively for high density optical disc HD, DVD and CD, and t 1 , t 2  and t 3  represent protective layer thicknesses (mm) respectively of high density optical disc HD, DVD and CD, while, OBJ represents an object point (a luminous point of a semiconductor laser light source), STO represents a diaphragm, r represents a radius of curvature (mm), d 1 , d 2  and d 3  represent respectively surface distances (mm) for high density optical disc HD, DVD and CD, Nλ 1 , Nλ 2   DVD  and Nλ 2   CD  represent respectively refractive indexes for design wavelengths respectively of high density optical disc HD, DVD and CD, υd represents Abbe&#39;s number for d line (587.6 nm), n 1 , n 2  and n 3  represent respectively diffraction order numbers of beams for recording/reproducing respectively for high density optical disc HD, DVD and CD, and λB represents respectively manufacture wavelengths (nm) for the superposition diffractive structure HOE and for blazed diffractive structures DOE 1  and DOE 2 .  
      The diffractive structure in each example is expressed by an optical path difference that is added by the diffractive structure to a transmission wave front. Such optical path difference is expressed by optical path difference function φb (mm) defined by the following Numeral 2, when λ represents a wavelength of an incident light flux, λ B  represents a manufacture wavelength of the diffractive structure, h (mm) represents a height in the direction perpendicular to the optical axis, B 2i  represents an optical path difference function coefficient and n represents the diffraction order number.  
               ϕ   b     =       λ   /     λ   B       ×   n   ×       ∑     j   =   1       ⁢       B     2   ⁢   j       ⁢     h     2   ⁢   j                     Numeral   ⁢           ⁢   2             
 
      When a laser light flux with wavelength λ 3 =785 nm enters the superposition diffractive structure HOE provided on diffraction lens L 1 , an optical path of 1×λ 3  (μm) is generated between adjoining steps because of λ 3 ≈2×λ 1 . Therefore, the laser light flux with wavelength λ 3  also is not given a phase difference substantially similarly to the laser light flux with wavelength λ 1 , and is transmitted as it is (zero-order diffracted light). Accordingly, when a light flux with a wavelength of 785 nm as a wavelength for recording/reproducing for CD is used (NA 3 =0.45), it is necessary to establish the magnification m 3  of objective optical system OBJ to be smaller than in the occasion to use a light flux with a wavelength of 658 nm, because an effect of correcting spherical aberration by the superposition diffractive structure HOE is not obtained.  
      Specifically, the magnification m 3  is established to be −0.130.  
       FIG. 6  shows lens shift characteristics for the objective optical system OBJ, for the occasion where 658 nm (NA 3 =0.386, m 3 =−0.0725) is used as a wavelength for recording/reproducing of CD and for the occasion where 785 nm (NA 3 =0.45, m 3 =−0.130) is used.  
      As is apparent from the foregoing, the magnification m 3  can be made greater by using the light flux with a wavelength of 658 nm which is the same as that for DVD as a wavelength for recording/reproducing of CD, and the numerical aperture NA 3  can further be set to be smaller, which makes it possible to improve tracking characteristics of the objective optical system OBJ.  
     EFFECTS OF THE INVENTION  
      The invention makes it possible to obtain an optical pickup apparatus and an optical information recording and/or reproducing apparatus wherein compatibility for a high density optical disc, DVD and CD is attained, and occurrence of coma aberration caused by tracking of an objective lens in the case of conducting recording/reproducing for CD can be reduced.