Patent Publication Number: US-7596071-B2

Title: Objective lens, optical pickup, and optical information processing apparatus using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 10/875,304, filed on Jun. 25, 2004 now U.S. Pat No. 7,313,074, the subject matter of which is incorporated in its entirety by reference herein. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to optical information professing and more particularly to an objective lens and an optical pickup for conducting at least one of information recording, information playback and erasing of information to optical media designed for blue wavelength band, red wavelength band or infrared wavelength band, as well as the information processing apparatus that uses the same. 
   For the means for saving video information, audio information or data in a computer, the use of optical recording media such as a CD having a recording capacity of 0.65 GB, or a DVD having a recording capacity of 4.7 GB, is spreading. In these days, there is an acute demand for further improvement of recording density for more recording capacity. 
   In order to increase the recording density in such an optical recoding medium, it is necessary to increase the numerical aperture (NA) value of the objective lens or reduce the wavelength of the optical sources the optical pickup used for writing or reading information to and from the optical recording medium, such that the size of the beam spot formed on the optical recording medium by the objective lens is reduced. 
   In the case of the optical recording medium called CD, a numerical aperture of 0.40-0.50 is used for the objective lens and a wavelength of about 785 nm is used for the optical source. In the case of the recording medium called DVD, in which the recording density is increased over the CD, a numerical aperture of 0.60-0.65 is used together with the wavelength of the optical source of about 660 nm. 
   As noted before, there is a keen demand for improvement of recording density and increase of recording capacity in the optical recording medium in these days, and thus, there is a desire to increase the numerical aperture of the objective lens beyond the conventional value of 0.65 and/or to decrease the wavelength of the optical source to less than 660 nm. 
   Thus, there appears a situation in which a new specification of higher numerical aperture value or shorter wavelength appears every year for the optical pickup, while the users of the optical information processing apparatus maintain CDs and DVDs of conventional specification. Because of this, there is arising a demand that such conventional optical recording media can be processed also in the optical information processing apparatus designed for such novel optical recording media of novel specification. 
   While such compatibility between different specifications can be achieved by providing a conventional optical pickup in addition to the optical pickup designed according to the new specification, such a construction is contradictory to the requirement of downsizing and cost reduction. 
   It is preferable to provide a construction shown in  FIG. 1A  in which a blue optical source  100  and a DVD optical source  200  use a single objective lens  104  that focuses the optical beams of the respective optical sources to an optical recording medium  103 , such that the optical pickup, designed for a large-capacity (blue) optical recording medium by using the optical source  100  of the blue wavelength band, is still compatible with the conventional CDs or DVDs. Alternatively, it is preferable to provide a construction shown in  FIG. 1B  in which a blue optical source  100 , a DVD optical source  200  and a CD optical source  300  cooperate with the single objective lens  2  that focuses the exit beams of the respective optical sources to the optical recording medium  1 . 
   On the other hand, in order to focus the optical beams to the optical media designed for various different specifications such as blue color systems, DVDs or CDs, there arise various problems in relation to the difference in the wavelength/substrate thickness of the optical recording medium  103  as will be explained hereinafter. 
   Japanese Laid-Open Patent Publication 2002-107617 describes the problem of aberration (see  FIG. 3 ) arising in the case an objective lens  110  of  FIG. 2  designed of the optical source wavelength of 405 nm is used in the wavelength range of 400-800 nm. In  FIG. 2 , the reference numeral  111  shows an optical recording medium. 
   As will be explained below, it is generally required that the wavefront aberration should be 0.02 λrms or less. On the other hand,  FIG. 3  shows that there appears an aberration of 0.10 λms or more at the wavelength of 660 nm or 785 nm used with a DVD system or CD system. In view of this situation, the foregoing Japanese Laid-Open Patent Publication 2002-107617 proposes the use of a compound lens for the objective lens. However, the use of such a compound lens raises the problems in that it requires a bonding process at the time of manufacturing of the objective lens and there arises the problem of increase of weight associated with the construction that uses two lenses. 
   As an alternative, there is a proposal in ISOM2001 Abstract, “BLUE/DVD/CD COMPATIBLE OPTICAL HEAD WITH THREE WAVELENGTHS AND A WAVELENGTH SELECTIVE FILTER”, or in Japanese Laid-Open Patent Application 2003-67972 to construct the respective optical paths of DVD or CD by a finite optical system in view of the fact that there occurs a spherical aberration in the event an optical beam is focused to the optical recording medium of DVD or CD by the objective lens designed for a blue optical beam. In this proposal, there is further provided a phase compensation element such that the phase distribution is changed for the optical beam of DVD or CD while not causing any change of phase distribution for the blue optical beam of the blue wavelength band. By using a finite optical system, in which the incident optical beam to the objective lens is made divergent, an effect of reducing the spherical aberration is achieved. However, there still remains some spherical aberration, and compensation of this residual spherical aberration is achieved by inducing a spherical aberration of opposite phase by the phase compensating element. 
   For example, in the case of using a single objective lens, which is designed for minimizing the wavefront aberration when it is used with an infinite optical system (a parallel beam is incident to the objective lens) and with a blue optical recording medium (λ 1 =407 nm, NA(λ 1 )=0.67, substrate thickness t 1 =0.6 mm), for focusing a beam spot on a DVD recording medium (λ 2 =660 nm, NA(λ 2 )=0.65, substrate thickness t 2 =0.6 mm), there arises a spherical aberration shown in  FIG. 12B  due to the difference in the wavelength. 
   Such a problem was also noticed in the DVD/CD compatible optical pickup. Thus, in the case of using a single objective lens, which is designed for minimizing the wavefront aberration when it is used with an infinite optical system (a parallel beam is incident to the objective lens) and with a DVD recording medium (λ 2 =660 nm, NA(λ 2 )=0.65, substrate thickness t 2 =0.6 mm), for focusing a beam spot on a CD recording medium (λ 3 =780 nm, NA(λ 3 )=0.50, substrate thickness t 3 =1.3 mm), there arises a similar spherical aberration shown due to the difference in the wavelength. 
   In order to attend to this problem, Japanese Patent 2,725,653 or Japanese Laid-Open Patent Application 10-334504 teaches the construction in which there are provided two laser diodes of different wavelengths and a phase compensating element having wavelength selectivity, such that recording or playback is conducted for a DVD optical recording medium having the thickness of 0.6 mm by using a beam of 660 nm wavelength emitted from one of the laser diodes and carries out recording or playback for a CD optical recording medium having a thickness of 1.2 mm by using the beam of 780 nm wavelength emitted from the other laser diode. Thereby, the phase compensating element having the wavelength selectivity causes no change of phase distribution for the optical beam of the 660 nm wavelength, while the phase compensating element causes a phase distribution change for the optical beam of the 780 nm wavelength in such a manner that the spherical aberration caused by the difference in the thickness of the substrate is successfully compensated for. 
   As an alternative, there is generally known the means of compensating for the spherical aberration caused by difference in the thickness or wavelength between the DVD optical recording medium and the CD optical recording medium, by using an infinite optical system when the optical beam of the 660 nm wavelength for DVD is incident to the objective lens and by using a finite optical system (a state in which a divergent beam enters into the objective lens in the form of a divergent beam) for the case the optical beam for CD is incident to the objective lens. 
   Particularly, the ISOM2001 Abstract, op. cit., proposes the method of recording or playing back information to or from any of the three optical recording media of blue optical recording medium, DVD optical recording medium and CD optical recording medium while using a single objective lens. According to the teaching of this prior art, there is provided a system including three different laser diodes of the wavelengths of 405 nm, 650 nm and 780 nm and a wavelength-selective phase compensating element having wavelength selectivity, such that optical irradiation is made to a blue optical recording medium having a thickness of 0.1 mm with the wavelength of 405 nm by using an infinite optical system, and an optical irradiation is made to a DVD optical recording medium having a thickness of 0.6 mm with the wavelength of 660 nm by using a finite optical system, and optical irradiation is made to a CD optical recording medium having a thickness of 1.2 mm with the wavelength of 780 nm by using a finite optical system. Thereby, the wavelength-selective phase plate does not change the phase distribution for the optical beam of the wavelength of 405 nm while it changes the phase distribution for the optical beams of 660 nm and 780 nm. In this construction, the wavelength-selective phase compensating means and the finite optical system used for the case of the DVD/CD recording media constitute the wavefront compensating means compensating for the spherical aberration caused by the difference in the thickness of the substrate. 
   In addition to the foregoing problem of aberration, there arises another problem in that the numerical aperture value changes between the optical recording media of different specifications. 
   Generally, the numerical aperture values NA of 0.60 and 0.65 are used respectively for the playback system and recording system of a DVD optical recording medium, while the numerical aperture values NA of 0.45 and 0.50 are used respectively for the play back system and recording system of a CD optical recording medium. 
   Most of the optical information processing apparatuses currently sold are capable of recording for both of DVD and CD, and it is preferable to use the numerical aperture value NA of 0.65 for DVD and the numerical aperture value NA of 0.50 for CD. 
   On the other hand, with regard to the blue optical recording technology, there is proposed the use of the numerical aperture value NA of 0.65 in relation to the HD-DVD technology. 
   This means that there is a need of switching the numerical aperture value in response to the optical recording medium used for recording and playback. 
   Thus, in order to achieve two-generation compatibility between DVD and CD, there is proposed a technology that uses an aperture switching element as disclosed in the Japanese Patent 3,240,846, Japanese Patent 2,713,257, Japanese Patent 2,725,653, and Japanese Patent 3,036,314. Further, in order to achieve two-generation compatibility between a large-capacity optical recording medium and DVD, there is proposed a technology that uses an aperture switching element as disclosed in the Japanese Laid-Open Patent Application 2001-216676. 
   Further, with regard to the technology that achieves three-generation compatibility, there is proposed the use of a three-step aperture switching element in Japanese Laid-Open Patent Application 2000-187870 and Japanese Laid-Open Patent Application 2003-67972. 
   With regard to such a conventional technology, the phase compensating element disclosed in the foregoing ISOM Abstract, op. cit, or Japanese Laid-Open Patent Application has to be driven integrally with the objective lens in view of the possibility of comma aberration when there is caused a displacement between the objective lens and the optical axis. With regard to the actuator that drives the objective lens in the focusing direction and the track direction, there is a demand of reducing the number of the parts in order to achieve weight reduction and reduction of the number of assembling steps. In order to suppress formation of comma aberration, there is a need for the adjustment of alignment between the objective lens and the phase compensating element. 
   Further, according to the method described in the foregoing ISOM Abstract, op. cit., no satisfactory wave front performance is obtained when to achieve the compatibility between the DVD optical recording medium and the CD optical recording medium. 
   Generally, the reference value of 0.07 λrms, known as Marshal criterion, is used as the wavefront aberration for the case of the diffraction limit. In the case of an optical pickup, on the other hand, there exist various factors that may become the cause of the error such as the thickness error of the optical recording medium, the tilt error of the optical recording medium, the defocus error caused by displacement between the optical recording medium and the objective lens. Thus, it is preferred that the wavelength aberration (median value) is suppressed to 0.03 λrms or less in the state there is caused no error, in view of the fact that there may be caused deterioration of the wavefront aberration as a result of buildup of these errors. 
   Contrary to this, it should be noted that the wavefront aberration (median value) becomes about 0.05 λrms for a DVD optical recording medium in the technology described in the foregoing ISOM Abstract, op. cit. in view of the situation that it is not possible to minimize the wavefront aberration for both of DVD and CD by using a single element. Because of this, it was inevitable to make the design to target the intermediate point of phase compensation condition when to minimize the aberration in both of DVD and CD, while such an approach could not reduce the aberration sufficiently. 
   The same applies also to the numerical aperture switching elements. In the case of the numerical aperture switching element, it is preferable that it is driven integrally with the objective lens in view of the fact that the desired numerical aperture value is no longer attained when the optical axis of the objective lens is offset. 
   Generally, a numerical aperture switching element is mounted on an actuator such that the numerical aperture switching element is driven integrally with an objective lens so that the change of the numerical aperture value is avoided. On the other hand, an actuator is also required to include smaller number of parts for weight reduction and reduction of the number of the assembling steps. Thus, it is desired that such a numerical aperture switching element is removed from the actuator. Particularly, the numerical aperture switching element designed for three-generation optical recording and playback so as to operate in three steps has a complex structure, and design and manufacture thereof becomes difficult. Further, in spit of its complex construction, such a three-step numerical aperture switching element cannot provide sufficient compensation with regard to the wavefront characteristics and transmission characteristics. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a general object of the present invention to provide an objective lens, an optical pickup and an optical information processing apparatus that uses the same wherein the foregoing problems are eliminated. 
   Another and more specific object of the present invention is to achieve a two-generation compatibility between optical recording technology of different generations such as the blue optical recording and the DVD, without providing an aperture switching element and is simultaneously capable of simplifying the assembling process and achieving weight reduction of the actuator or reduction of the number of parts of the actuator. 
   Another object of the present invention is to achieve a three-generation compatibility between the optical recording technology of three different generations such as the blue optical recording, DVD and CD, while using a two-step numerical aperture switching element and the conventional art of the aperture switching element. 
   Another object of the present invention is to provide an optical component and an optical system capable of providing sufficient compensation of spherical aberration that arises at the time of achieving compatibility between different generations of optical recording technology. 
   Another object of the present invention is to provide an objective lens and an optical pickup sufficiently suppressing spherical aberration between two generations of optical recording technology such as the blue optical recording technology and DVD technology, or between three generations of optical recording technology such as the blue optical recording technology, DVD technology and CD technology, without the need of providing an aberration compensating element. 
   Another object of the present invention is to provide an objective lens and an optical pickup capable of reducing the cost and also the number of manufacturing steps by decreasing the number of the parts used therein, without the need of providing an aberration compensating element and an aperture switching element. 
   According to a first aspect of the present invention, there is provided an optical pickup carrying out at least one of recording, playback and erasing of information to and from an optical recording medium, comprising: 
   a first optical source producing a first optical beam having a wavelength λ 1 ; 
   a second optical source producing a second optical beam having a wavelength λ 2  (λ 1 &lt;λ 2 ); 
   a single objective lens focusing any of said first and second optical beams to an optical recording medium, 
   said first optical beam being produced in response to turning-on of said first optical source and being incident to said objective lens in the state of a parallel beam via an infinite optical system, 
   said second optical beam being produced in response to turning-on of said second optical source and being incident to said objective lens in the state of a divergent optical beam via a finite optical system, 
   an aperture being provided at a front side of said objective lens opposite to said optical recording medium with offset from a principal point of said objective lens at said front side by a distance t given by an equation
 
 t=L−NA 1 ·f /tan( a  sin( NA 2 obj )),
 
wherein f represents a focal distance of said objective lens, NA 1  represents a numerical aperture at a side of image surface in the state when said first optical source is turned on, NA 2 obj represents a numerical aperture at said front side in the state when said second optical source is turned on, and L represents an object distance at the time when said second optical source is turned on.
 
   According to the present invention, it becomes possible to achieve the two-generation compatibility between the optical recording technology of two, different generations, such as the blue optical recording technology and DVD. 
   According to a second aspect of the present invention, there is provided an optical pickup carrying out at least one of recording, playback and erasing of information with regard to an optical recording medium, comprising: 
   a first optical source producing a first optical beam having a wavelength λ 1 ; 
   a second optical source producing a second optical beam having a wavelength λ 2 ; 
   a third optical source producing a third optical beam having a wavelength λ 3  (λ 1 &lt;λ 2 &lt;λ 3 ); and 
   a single objective lens focusing any one of said first through third optical beams upon an optical recording medium, 
   said first optical beam being produced in response to activation of said first optical source and being incident to said objective lens in the form of a parallel beam via an infinite optical system, 
   said second and third optical beams being produced respectively in response to activation of said second and third optical sources, said second and third optical beams being incident to said objective lens in the state of a divergent beam via a finite optical system, 
   an aperture switching element providing an optical beam diameter φ 1  when any of said first and second optical beam is injected therethrough to said objective lens and an optical beam diameter φ 2  (φ 1 &gt;φ 2 ) when said third optical beam is injected therethrough to said objective lens, said aperture switching element being provided at a front side of said objective lens opposite to a side of said optical recording medium from a principal point of said objective lens at said front side with a distance offset t given by an equation
 
 t=L−NA 1 ·f /tan( a  sin( NA 2 obj )),
 
where f is a focal distance of said objective lens, NA 1  is a numerical aperture value at the side of an image surface in the state when said first optical source is turned on, NA 1 obj is a numerical aperture value at the side of said optical source in the state when said second optical source is turned on, and L is an object distance for the state in which said second optical source is turned on.
 
   According to the present invention, there is provided an optical pickup capable of achieving compatibility for optical recording technology of three generations such as blue optical recording technology, DVD and CD, while using a two-step numerical aperture switching element, not a three-step numerical aperture element. 
   According to a third aspect, there is provided an optical pickup according to said second aspect of the present invention, wherein said aperture switching element carries out said switching of optical beam diameter between φ 1  and φ 2  by way of any of reflection, diffraction or absorption, depending on the wavelength band or polarization direction. According to the third aspect, it becomes possible to provide an optical pickup capable of achieving compatibility for optical recording of three generations such as blue optical recording technology, DVD and CD while using a two-step numerical aperture switching element, not a three-step numerical aperture element. 
   According to a fourth aspect of the present invention, there is provided an optical pickup according to any of the first through third aspects noted above, wherein there is provided a correction element that provides a predetermined spherical aberration to said second optical beam upon turning on of said second optical source, wherein said correction element induces said spherical aberration with opposite phase at the time when said second optical beam is incident to said objective lens. Thereby, the spherical aberration caused with the optical beam upon irradiation of said optical recording medium is successfully compensated for. 
   According to a fifth aspect of the present invention, there is provided an optical pickup according to said fourth aspect, wherein said correction element has a concentric pattern in a surface perpendicular to an optical axis, said correction element further having a cross-sectional from taken in the direction parallel to said optical axis such that said cross-sectional form is optimized for a particular wavelength. 
   According to a sixth aspect of the present invention, there is provided an optical pickup according to said fourth aspect, wherein said correction element has a phase pattern concentric in a surface perpendicular to an optical axis, said correction element further having a stepped phase pattern in a cross-section taken in the direction parallel to said optical axis. 
   According to a seventh aspect of the present invention, there is provided an optical pickup according to said fourth aspect, wherein said correction element comprises a liquid crystal element inducing a phase change in an optical beam passing therethrough in response to application of a voltage such that there is inducted a spherical aberration of opposite polarity in said optical beam upon incidence to said objective lens. 
   According to the present invention as set forth in any of the fifth through seventh aspects of the present invention, it becomes possible to compensate for the spherical aberration caused when the optical beam hits the optical recording medium. 
   According to an eighth aspect of the present invention, there is provided an optical pickup according to any of said first through third aspects, wherein said objective lens is formed of one group/two element construction in which a concave lens and a convex lens are bonded with each other. 
   According to a ninth aspect of the present invention, there is provided an optical pickup according to any of said first through third aspects, wherein said objective lens has a construction in which there is formed a diffraction pattern or in the form of a stepwise phase pattern on any of an incident surface or exit surface of said objective lens. 
   According to any of the eighth aspect and ninth aspect of the present invention, it becomes possible to provide an optical pickup that induces a spherical aberration of opposite phase to the incident optical beam when it is incident to the objective lens, without the need of using a correction element. 
   According to a tenth aspect of the present invention, there is provided an optical information processing apparatus capable of carrying out at least any one of recording, playback and erasing of information to and from any of an optical recording medium designed for blue optical recording technology that uses a blue optical beam and an optical recording medium designed for a red optical recording technology that uses a red optical beam while using an optical pickup, wherein said optical pickup according to any of said first aspect or said fourth through ninth aspects of the present invention is used for said optical pickup. 
   According to an eleventh aspect of the present invention, there is provided an optical information processing apparatus capable of carrying out at least one of recording, playback and erasing of information to and from any of an optical recording medium designed for a blue optical recording technology that uses a blue optical beam, an optical recording medium designed for a red optical recording technology that uses a red optical beam and an optical recording medium designed for an infrared recording technology that uses an infrared optical beam while using the same optical pickup, wherein an optical pickup according to any of the first through ninth aspects of the present invention is used for said optical pickup. 
   According to a twelfth aspect of the present invention, there is provided an objective lens focusing any of a first optical beam having a first wavelength λ 1  and a second optical beam having a second wavelength λ 2  (λ 1 &lt;λ 2 ) incident to an incident side of said objective lens, to an optical recording medium provided at an exit side of said objective lens, 
   said objective lens having a single-lens construction and defined by a non-spherical convex surface at said incident side and said exit side, 
   wherein there is satisfied a condition:
 
νd&gt;35  (1)
 
1.58&gt;nd  (2)
 
0.58 nd− 0.29 ≦R 1 /f ≦ 0.62 nd− 0.31  (3)
 
   where nd and νd represent respectively the refractive index and Abbe number for a D-line, R 1  represent the paraxial radius of curvature and f represents the focal distance. 
   According to a thirteenth aspect of the present invention, there is provided an objective lens according to said twelfth aspect, wherein said conditions (1), (2) and (3) are set for an optical beam incident from an infinite distance when said optical beam has said wavelength λ 1 , and wherein said conditions (1), (2) and (3) are set for an optical beam incident form a finite distance when said optical beam has said wavelength λ 2 . 
   According to a fourteenth aspect of the present invention, there is provided an objective lens according to any of said twelfth or thirteenth aspect, wherein said optical beam incident to said objective lens with said wavelength λ 1  is an optical beam of a blue wavelength band, and wherein said optical beam incident to said objective lens with said wavelength λ 2  is an optical beam of a red wavelength band, and wherein said conditions (1), (2) and (3) are set for said optical beams of said wavelengths λ 1 , λ 2  and λ 3 . 
   According to a fifteenth aspect of the present invention, there is provided an objective lens focusing any of a first optical beam having a wavelength λ 1 , a second optical beam having a wavelength λ 2  and a third optical beam having a wavelength λ 3  (λ 1 &lt;λ 2 &lt;λ 3 ) incident to an incident side of said objective lens to an optical recording medium provided at an exit side of said objective lens, 
   said objective lens having a single-lens construction and defined by a non-spherical convex surface at said incident side and said exit side, 
   wherein there is satisfied a condition:
 
νd&gt;35  (1)
 
1.58&gt;nd  (2)
 
0.58 nd− 0.29 ≦R 1 /f≦ 0.62 nd− 0.31  (3)
 
   where nd and νd represent respectively the refractive index and Abbe number for the D-line, R 1  represent the paraxial radius of curvature and f represents the focal distance. 
   The objective lens according to the twelfth or fifteenth aspect of the present invention is a positive lens focusing a parallel beam incident to said objective lens at said incident side. Because the objective lens for the optical pickup of the present invention has a single lens construction and is defined by a non-spherical surface at both sides, the objective lens can be configured in the form of a convex lens or meniscus lens, wherein the use of the convex lens is most preferable in view of the possibility of reducing the curvature at the incident side and in view of the easiness of manufacture. Thereby, it should be noted that the foregoing conditions (1), (2) and (3) are the conditions for focusing the optical beam on the optical recording medium with the best state of wavefront aberration for the case of conducting two-wavelength optical recording or three-wavelength optical recording. 
   In the case the condition (2) is not satisfied, for example, the refractive index of the objective lens becomes too small, and it becomes necessary to increase the curvature of the lens surface at the incident side for achieving the desired numerical aperture value. However, formation of such a lens surface with high precision is difficult and the cost of the objective lens is increased inevitably. 
   When the conditions (1) and (2) for the refractive index nd and the Abbe number νd is not satisfied, on the other hand, there arises the problem of excessively large chromatic aberration, and the compatibility between two wavelengths or three wavelengths becomes difficult. 
   In the case where the lens surface of the objective lens at the incident side is convex, there is caused an increase of positive refracting power at this surface with increase of the paraxial curvature radial R 1 . Because the objective lens for an optical pickup has a large numerical aperture NA of about 0.65, it is necessary to increase the positive refracting power at the lens. Thus, in the case of increasing the paraxial curvature radius R 1  as noted above while maintaining a large numerical value, it becomes necessary to increase the refractive index of the lens material. Thus, there exists a relation ship “the refractive index nd of the lens increases with increase of the paraxial curvature radius R 1 . 
   According to a sixteenth aspect of the present invention, there is provided an objective lens according to said fourteenth aspect, wherein said conditions (1), (2) and (3) are set for an optical beam from an infinite distance when said optical beam has said wavelength λ 1  and wherein said conditions (1), (2) and (3) are set for an optical beam from a finite distance when said optical beam has any of said wavelength λ 2  or λ 3 . 
   According to a seventeenth aspect, there is provided an objective lens according to any of said fifteenth or sixteenth aspect, wherein said optical beam incident to said objective lens with said wavelength λ 1  is an optical beam of a blue wavelength band, and wherein said optical beam incident to said objective lens with said wavelength λ 2  is an optical beam of a red wavelength band, and wherein said conditions (1), (2) and (3) are set for said optical beams of said wavelengths λ 1 , λ 2  and λ 3 . 
   Thus, in the objective lens of the present invention, an incident optical beam is incident to the objective lens in the form of an optical beam of a finite system in the case the optical beam is an optical beam of long wavelength. 
   In the case an objective lens designed for use in the blue wavelength band with an incident optical beam of the infinite system is used with the wavelength and substrate thickness of DVDs or CDs, there arises a spherical aberration because of the difference of substrate thickness (06 mm, 06 mm, 1.2 mm) and the difference of wavelength (405 nm, 660 nm, 785 nm). 
   This spherical aberration can be compensated for by using a divergent optical beam as the incident optical beam incident to the objective lend when conducting any of recording, playback or erasing to or from a DVD or CD. In other words, the. objective lens is used as the lens of a finite optical system when carrying out any of recording, playback or erasing to and from a DVD or a CD. 
   Hereinafter, these inventions will be examined closely. 
   In order to form an optical spot on a recording surface with a “desired spot size” with a predetermined wavelength through a substrate of predetermined thickness, it is necessary to set the “upper limit value of the wavefront aberration permitted for formation of satisfactory optical spot” to be 0.04 λms, where λ represents the wavelength. It should be noted that this value of 0.04 λms includes the effect of deviation of curvature radius at the front lens surface and the rear lens surface, deviation of the thickness, deviation of the non-spherical form, shifting of the front and rear lens surfaces, tilting of the front and rear lens surfaces, and the like, caused by the manufacturing error of the lens. 
   Thus, the appropriate median value set for the degradation of wavefront is thought to be about 0.02 λms. Hereinafter, explanation will be made by using the foregoing value of 0.2 λrms as the approximate medium value. 
   Because the present invention achieves at least one of recording, playback and erasing for the three generations of blue optical recording media, DVDs and CDs, there is a need of achieving the wavefront aberration of 0.2 λms or less for each of the wavelengths. 
     FIGS. 30A-30C  show the wavefront aberrations respectively for the case of using an infinite optical system together with the numerical aperture value NA of 0.65-0.75 for a blue incident optical beam having the wavelength of 405 nm together with the optical recording medium having a substrate thickness of 0.6 mm, the case of using a finite optical system of DVD together with the numerical aperture value NA of 0.65 for the incident optical beam having the wavelength of 660 nm with the optical recording medium having a substrate thickness of 0.6 mm, and the case of using a finite optical system of CD together with the numerical aperture NA of 0.50 for the optical beam having the wavelength of 785 nm with the optical recording medium having a substrate thickness of 1.2 mm, wherein the lens material is represented in terms of the refractive index nd and the Abbe number νd. 
   It should be noted that the condition that the wavefront aberration becomes 0.02 λrms or less for any of the blue system, DVD and CD is the range shown in  FIG. 31  by hatching, wherein this range corresponds to the foregoing conditions (1) and (2). Further,  FIG. 31  shows the compositional range of commercially available glasses by a broken line. 
   It should be noted from  FIG. 30B  that when these conditions (1) and (2) are not met, there appears an excessively large chromatic aberration due to the different wavelength band of DVD, and the required aberration of 0.02 λrms or less cannot be satisfied. 
     FIG. 32  shows the relationship between the paraxial curvature radius R 1  and the refractive index nd of the objective lens of an optical pickup that has a lens surface of larger curvature at the incident side, under the condition that the wavefront aberration of 0.02 λrms or less is achieved. In the relationship, it was assumed that the focal distance f is 3.05 mm and the numerical aperture value NA is 0.65. Here, it should be noted that the marks ●, ▪, Δ, ×, *, ◯ and + represent the Abbe number of 65, 60, 55, 50, 45, 40 and 35, respectively. 
   As can be seen, the Abbe numbers fall within the region defined by the lines a and b. Because the refractive index depends not only on the refractive index nd of a D-line but also on the Abbe number νd, the relationship between the paraxial curvature radius R 1  and the refractive index nd is not determined uniquely. Nevertheless, it is possible to achieve the wavefront aberration of 0.02 λrms or less, by satisfying the condition (3) with regard to the paraxial curvature radius R 1  and the refractive index nd determined by the range between the lines a and b and simultaneously satisfying the condition of the refractive index nd determined by the condition (1). 
   Thus, according to the objective lens of the twelfth through seventeenth aspect of the present invention, it becomes possible to achieve compatibility between two or three wavelengths used in the blue optical recording, DVDs and CDs, without using a conventional compound lens or adding phase compensation elements. 
   As noted before, the optimum numerical aperture value is different in constructing an optical pickup between the blue optical recording medium, DVD recording medium and CD recording medium. Thus, there is a need of changing the diameter of the incident beam to the objective lens according to the type of the optical recording medium used. 
   Thus, according to an eighteenth aspect of the present invention, there is provided an optical pickup comprising: 
   a first optical source producing a first optical beam having a first wavelength λ 1 ; 
   a second optical source producing a second optical beam having a second wavelength λ 2 ; and 
   an objective lens according to any of the twelfth, thirteenth or fourteenth aspect of the present invention, said objective lens focusing any of said first and second optical beams on an optical recording medium, 
   said objective lens having a first effective numerical aperture value NA(λ 1 ) for said first optical beam and a second effective numerical aperture value NA(λ 2 ) for said second optical beam, 
   said first optical beam incident to said objective lens having a first beam diameter φ 1 , 
   said second optical beam incident to said objective lens having a second beam diameter φ 2 , 
   said first and second effective numerical aperture values NA(λ 1 ) and NA(λ 2 ) and said first and second beam diameters satisfying a relationship
 
 NA (λ1)= NA (λ2)  (4)
 
φ2&gt;φ1.  (5)
 
   Generally, the effective numerical aperture value NA is represented by
 
 NA=φ/ 2 /f  
 
where φ represents the diameter of the optical beam incident to the objective lens, while f represents the focal distance.
 
   The focal distance increases with increasing wavelength. Thus, by satisfying the conditions (4) and (5) noted above, it becomes possible to achieve the compatibility between the blue optical recording technology known as HD-DVD that uses the numerical aperture value NA of 0.65 and the DVD optical recording technology that uses the numerical aperture value NA of 0.65. 
   According to a twentieth aspect of the present invention, there is provided an optical pickup comprising: 
   a first optical source producing a first optical beam having a first wavelength λ 1 ; 
   a second optical source producing a second optical beam having a second wavelength λ 2 ; and 
   an objective lens according to any of the twelfth, thirteenth or fourteenth aspect of the present invention, said objective lens focusing any of said first and second optical beams on an optical recording medium, 
   said objective lens having a first effective numerical aperture value NA(λ 1 ) for said first optical beam and a second effective numerical aperture value NA(λ 2 ) for said second optical beam, 
   said first optical beam incident to said objective lens having a first beam diameter φ 1 , 
   said second optical beam incident to said objective lens having a second beam diameter φ 2 , 
   said first and second effective numerical aperture values NA(λ 1 ), and NA(λ 2 ) and said first and second beam diameters satisfying a relationship
 
 NA (λ1)&gt; NA (λ2)  (6)
 
φ2=φ1.  (7)
 
   For example, by setting the numerical aperture value NA for the blue beam to 0.67, which is slightly larger than the numerical aperture value for the DVD technology, it becomes possible to use the same beam diameter (φ 2 =φ 1 ) for the incident optical beams, and the switching of the numerical aperture can be omitted. 
   Generally, the degradation of wavefront caused by the thickness error of the transparent substrate forming an optical recording medium increases with fourth power of NA, while the degradation of wavefront caused by the tilt error of the transparent substrate increases with third power of NA. Thus, increase of capacity cannot be achieved simply by increasing NA. 
   On the other hand, in the case the degree of degradation of wavefront is such that the use of the numerical aperture value NA of 0.65 for HD-DVD is allowed also in the technology that uses the numerical aperture value NA of about 0.67, the degradation of such a margin is sufficiently small and the use of the numerical aperture value NA of 0.65 is possible without problem. 
   According to a twenty second aspect of the present invention, there is provided an optical pickup comprising: 
   a first optical source producing a first optical beam having a first wavelength λ 1 ; 
   a second optical source producing a second optical beam having a second wavelength λ 2 ; 
   a third optical source producing a third optical beam having a third wavelength λ 3 ; and 
   an objective lens according to any of the fourteenth, fifteenth or sixteenth aspect of the present invention, said objective lens focusing any of said first, second and third optical beams on an optical recording medium, 
   said objective lens having a first effective numerical aperture value NA(λ 1 ) for said first optical beam, a second effective numerical aperture value NA(λ 2 ) for said second optical beam, and a third effective numerical aperture value NA(λ 3 ) for said third optical beam, 
   said first optical beam incident to said objective lens having a first beam diameter φ 1 , 
   said second optical beam incident to said objective lens having a second beam diameter φ 2 , 
   said third optical beam incident to said objective lens having a third beam diameter φ 3 , 
   said first, second and third effective numerical aperture values NA(λ 1 ), NA(λ 2 ), NA(λ 3 ) and said first, second and third beam diameters φ 2 , φ 1 , φ 3  satisfying a relationship
 
 NA (λ1)= NA (λ2)&gt; NA (λ3)  (8)
 
φ2&gt;φ1&gt;φ3.  (9)
 
   With this, it becomes possible to achieve the optimum numerical aperture values NA for each of the generation of the blue optical recording technology, DVD optical recording technology and CD optical recording technology. 
   According to a twenty third aspect of the present invention, there is provided an optical pickup comprising: 
   a first optical source producing a first optical beam having a first wavelength λ 1 ; 
   a second optical source producing a second optical beam having a second wavelength λ 2 ; 
   a third optical source producing a third optical beam having a third wavelength λ 3 ; and 
   an objective lens according to any of the fourteenth, fifteenth or sixteenth aspect of the present invention, said objective lens focusing any of said first, second and third optical beams on an optical recording medium, 
   said objective lens having a first effective numerical aperture value NA(λ 1 ) for said first optical beam, a second effective numerical aperture value NA(λ 2 ) for said second optical beam, and a third effective numerical aperture value NA(λ 3 ) for said third optical beam, 
   said first optical beam incident to said objective lens having a first beam diameter φ 1 , 
   said second optical beam incident to said objective lens having a second beam diameter φ 2 , 
   said third optical beam incident to said objective lens having a third beam diameter φ 3 , 
   said first, second and third effective numerical aperture values NA(λ 1 ), NA(λ 2 ), NA(λ 3 ) and said first, second and third beam diameters φ 1 , φ 2 , φ 3  satisfying a relationship
 
 NA (λ1)&gt; NA (λ2)&gt; NA (λ3)  (10)
 
φ2&gt;φ1&gt;φ3.  (11)
 
   With this, it becomes possible to achieve the optimum numerical aperture values NA for each of the generation of the blue optical recording technology, DVD optical recording technology and CD optical recording technology. 
   According to a twenty fourth aspect of the present invention, there is provided an optical pickup comprising: 
   a first optical source producing a first optical beam having a first wavelength λ 1 ; 
   a second optical source producing a second optical beam having a second wavelength λ 2 ; 
   a third optical source producing a third optical beam having a third wavelength λ 3 ; and 
   an objective lens according to any of the fourteenth, fifteenth or sixteenth aspect of the present invention, said objective lens focusing any of said first, second and third optical beams on an optical recording medium, 
   said objective lens having a first effective numerical aperture value NA(λ 1 ) for said first optical beam, a second effective numerical aperture value NA(λ 2 ) for said second optical beam, and a third effective numerical aperture value NA(λ 3 ) for said third optical beam, 
   said first optical beam incident to said objective lens having a first beam diameter φ 1 , 
   said second optical beam incident to said objective lens having a second beam diameter φ 2 , 
   said third optical beam incident to said objective lens having a third beam diameter φ 3 , 
   said first, second and third effective numerical aperture values NA(λ 1 ), NA(λ), NA(λ 3 ) and said first, second and third beam diameters φ 1 , φ 2 , φ 3  satisfying a relationship
 
 NA (λ1)&gt; NA (λ2)&gt; NA (λ3)  (12)
 
φ1=φ2&gt;φ3  (13)
 
   With this, it becomes possible to achieve the optimum numerical aperture values NA for each of the generation of the blue optical recording technology, DVD optical recording technology and CD optical recording technology. 
   It should be noted that beam diameter of the optical pickup according to any of the sixteenth through twenty sixth aspects of the present invention is determined by the aperture or a wavelength-selective aperture switching means moving integrally with the objective lens. 
   Thus, according to a twenty sixth aspect of the present invention, there is provided an optical pickup according to any of the eighteenth through twentieth aspect of the present invention, wherein there is provided an aperture having a diameter generally identical with a maximum diameter of the incident optical beam, said aperture being movable integrally with said objective lens. 
   According to a twenty-sixth aspect of the present invention, there is provided an optical pickup according to any of said eighteenth, twenty first, twenty second and twenty third aspects of the present invention, wherein there is provided an aperture having a diameter generally identical with a maximum diameter of the incident optical beam and a wavelength-selective aperture switching means, said aperture and said wavelength-selective aperture switching means being movable integrally with said objective lens. 
   According to a twenty seventh aspect of the present invention, there is provided an optical pickup according to any of the nineteenth or twenty third aspects of the present invention, wherein said first optical beam is incident to said objective lens via an infinite optical system when said first optical source is turned on, said second optical beam is incident to said objective lens via a finite optical system when said second optical source is turned on, 
   wherein there is provided an aperture having a diameter generally equal to said first diameter of said first optical beam incident to said objective lens at a predetermined distance t
 
 t≈L−NA 1 f /tan( a  sin( NA 2 obj ))
 
as measured from a principal point of said objective lens located at an incident-side lens surface in the direction opposite to an exist-side lens surface, wherein f represents a focal distance of said objective lens, NA 1  represents a numerical aperture at a side of image surface in the state when said first optical source is turned on, NA 2 obj represents a numerical aperture at said front side in the state when said second optical source is turned on, and L represents an object distance at the time when said second optical source is turned on.
 
   According to the present invention, it becomes possible to provide optimum numerical aperture values for two generations by using the aperture only (no aperture switching means). Further, it becomes possible to provide optimum numerical aperture values for three generations by merely using the aperture and one step switching (not two-step switching). 
   According to a twenty eighth aspect of the present invention, there is provided an optical pickup according to the twenty sixth aspect, wherein the aperture switching means comprises a wavelength-selective coating or a wavelength-selective diffraction grating and is formed on any of the incident-side surface or exit-side surface of said objective lens. 
   Thus, it becomes possible to eliminate a dedicated element for aperture switching and the weight of the actuator is reduced. Thereby, the number of steps for assembling the actuator is reduced. 
   According to a twenty-ninth aspect of the present invention, there is provided an optical information processing apparatus that carries out at least one of recording, playback and erasing of information to and from an optical recording medium while using the optical pickup according to any of the twelfth through seventeenth aspect or eighteenth through twenty eighth aspect of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a diagram showing the construction of an optical pickup capable of achieving compatibility between the large-capacity optical recording technology that uses a blue wavelength band and a conventional DVD recording technology, while  FIG. 1B  is a diagram showing the construction of an optical pickup capable of achieving compatibility between the large-capacity optical recording technology that uses a blue wavelength band and a conventional DVD or CD recording technology; 
       FIG. 2  is a diagram showing an exemplary construction of a conventional objective lens; 
       FIG. 3  is a diagram showing the relationship between the wavelength of a conventional optical source and the wave-front aberration characteristics; 
       FIGS. 4A-4C  are characteristic diagrams showing the distribution of wave-front aberration for various refractive index values and various the Abbe numbers for the case of a blue optical recording medium, DVD medium and CD medium; 
       FIG. 5  is a characteristic diagram showing the constraint imposed on the glass composition from the relationship between the Abbe number and the refractive index; 
       FIG. 6  is the characteristic diagram showing the relationship between the refractive index and a first surface curvature radius; 
       FIG. 7  is a diagram showing the outline construction of an optical pickup according to a first embodiment of the present invention; 
       FIG. 8  is a diagram showing the construction of a hologram unit in which a laser diode, a hologram and a photodetection device are integrated; 
       FIG. 9  is a diagram showing the relationship between the diameter of the incident optical beam and the distance between the aperture and the objective lens and further the relation ship between the diameter of the incident optical beam and the numerical aperture value; 
       FIG. 10  is a diagram showing the focusing action of an objective lens focusing an incident optical beam for the case of an infinite optical system and a finite optical system; 
       FIG. 11A  is a diagram showing focusing of an incident optical beam having a wavelength of 407 nm by an objective lens and a phase compensation element, while  FIG. 11B  shows the spherical aberration caused in the construction of  FIG. 11A  for the optical beam of the 407 nm wavelength; 
       FIG. 12A  is a diagram showing focusing of an incident optical beam having a wavelength of 660 nm by an objective lens and a phase compensation element; 
       FIG. 12B  is a diagram showing the spherical aberration caused in an optical beam of a 660 nm wavelength incident to the objective lens that is designed so as to minimize the aberration for the optical beam having the wavelength of 407 nm, via an infinite optical system; 
       FIG. 12C  is a diagram showing the spherical aberration for the case an optical beam of a 660 nm wavelength has been incident to the objective lens via a finite optical system; 
       FIG. 12D  is a diagram showing the spherical aberration caused in the optical beam of the 660 nm wavelength for the case the optical beam is incident via a finite optical system together with phase compensation; 
       FIG. 13  is a diagram showing the relationship between the object distance and the wavefront aberration for the case of a finite DVD optical system; 
       FIGS. 14A and 14B  are respectively a plan view and a cross-sectional view showing the construction of a phase compensation element according to Example 1 of the first embodiment of the present invention; 
       FIG. 15  is a diagram showing the general construction of an optical pickup according to Example 2 of the first embodiment of the present invention; 
       FIG. 16  is a cross-sectional diagram showing the construction of the phase compensation element according to Example 2 of the first embodiment of the present invention; 
       FIG. 17  is a diagram showing the general construction of an optical pickup according to Example 3 of the first embodiment of the present invention; 
       FIG. 18  is a front view diagram showing an electrode pattern used in a liquid crystal device used for the phase compensation element of Example 3; 
       FIG. 19  is a diagram showing the spherical aberration caused as a result of difference of the wavelength of the used optical beam; 
       FIG. 20A  is a diagram showing a wavefront of the spherical aberration (continuous line) and a phase shift pattern (broken line) in the form of a two-dimensional curve; 
       FIG. 20B  is a diagram showing the wavefront after the correction of the aberration has been made; 
       FIG. 21  is a diagram showing the general construction of an optical pickup according to Example 4 of the first embodiment of the present invention; 
       FIG. 22A  is a diagram showing the focusing action of an optical beam by an optical system including a phase compensation element for the wavelength of 407 nm and an objective lens having a one group/two element construction; 
       FIG. 22B  is a diagram showing the spherical aberration caused in the optical system of  FIG. 21A  at the wavelength of 407 nm; 
       FIG. 23A  is a diagram showing the focusing action of an optical beam by an optical system including a phase compensation element for the wavelength of 660 nm and an objective lens having a one group/two element construction; 
       FIG. 23B  is a diagram showing the spherical aberration caused in the optical system of  FIG. 23A  at the wavelength of 660 nm; 
       FIG. 24  is a diagram showing the general construction of an optical pickup according to Example 5 of the first embodiment of the present invention; 
       FIG. 25  is a diagram showing the construction of an objective lens carrying a phase pattern; 
       FIG. 26  is a diagram showing the general construction of an optical pickup according to a second embodiment of the present invention; 
       FIGS. 27A-27C  are diagrams showing the construction of an aperture switching element that changes the beam diameter of an incident optical beam in response the wavelength of the optical beam by causing reflection according to a second embodiment of the present invention; 
       FIGS. 28A-28C  are diagrams showing the construction of an aperture switching element that changes the beam diameter of an incident optical beam in response the wavelength of the optical beam by causing diffraction according to a second embodiment of the present invention; 
       FIGS. 29A-29C  are diagrams showing the construction of an aperture switching element that changes the beam diameter of an incident optical beam in response the wavelength of the optical beam by causing absorption according to a second embodiment of the present invention; 
       FIG. 30A  is a diagram showing the focusing action of an optical system including an aperture switching element for the wavelength of 780 nm and an objective lens, while  FIG. 30B  shows the aberration caused in the optical system of  FIG. 30A  at the wavelength of 780 nm; 
       FIG. 31A  is a diagram showing the focusing action of an objective lens having the one group/two element construction, while  FIG. 31B  shows the spherical aberration caused in the optical system of  FIG. 31A  at the wavelength of 780 nm; 
       FIG. 32  is an oblique view diagram showing the general construction of an optical recording and playback apparatus according to a third embodiment of the present invention; 
       FIGS. 33A-33C  are diagrams showing an objective lens according to a fourth embodiment of the present invention; 
       FIG. 34  is a characteristic diagram showing the wave-front aberration of the objective lens of the fourth embodiment; 
       FIGS. 35A-35C  are diagrams showing an objective lens according to a fifth embodiment of the present invention; 
       FIG. 36  is a characteristic diagram showing the wave-front aberration of the objective lens of the fifth embodiment; 
       FIGS. 37A-37C  are diagrams showing an objective lens according to a sixth embodiment of the present invention; 
       FIG. 38  is a characteristic diagram showing the wave-front aberration of the objective lens of the sixth embodiment; 
       FIG. 39  is a diagram showing an optical system constructing an optical pickup according to a seventh embodiment of the present invention; 
       FIG. 40  is a diagram showing the relationship between the wavelength and the effective diameter for achieving the numerical aperture of 0.65; 
       FIGS. 41A-41C  are diagrams showing the construction of switching the aperture of the optical pickup according to the seventh embodiment; 
       FIGS. 42A-42C  are diagrams showing the construction of switching the aperture of the optical pickup according to an eighth embodiment; 
       FIG. 43  is a diagram showing the relationship between the numerical aperture value for the blue wavelength band and the numerical aperture value for the red wavelength band; 
       FIGS. 44A-44C  are diagrams showing the schematic construction of numerical aperture switching of an optical pickup according to a ninth embodiment of the present invention; 
       FIG. 45  is a characteristic diagram showing the relationship between the diameter of the optical beam incident to an objective lens and the distance of the aperture from the objective lens; 
       FIG. 46  is a diagram showing the principle of the calculation for calculating the aperture position; 
       FIG. 47  is a characteristic diagram showing the relationship between the wavefront aberration and the object distance for the case of the objective lens of which wavelength is 660 nm; 
       FIGS. 48A-48C  are schematic diagram showing an example of the coating applied to an objective lens; and 
       FIG. 49  is a schematic oblique view diagram showing an optical information processing apparatus according to a tenth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
     FIG. 7  is a diagram showing the schematic construction of an optical pickup according to a first embodiment of the present invention capable of carrying out any of recording, playback and erasing of information to and from any of: a blue optical recording medium having a substrate thickness of 0.6 mm at the side where an optical irradiation is made and designed for use with an optical source having a wavelength of 407 nm and an optical system having a numerical aperture value NA of 0.67; and a DVD optical recording medium having a substrate thickness of 0.6 mm at the side where an optical irradiation is made and designed for use with an optical source having a wavelength of 660 nm and an optical system having a numerical aperture value NA of 0.65. 
   While the technology of the blue generation optical recording technology addressed in the first embodiment of the present invention is still in the process of consolidation, the present embodiment will be explained for an exemplar construction that uses the wavelength of 407 nm and the numerical aperture value NA of 0.67. By satisfying these conditions, a capacity of about 2.7 times as large as the capacity of 4.7 GB of the conventional DVD optical recording medium is achieved. 
   Referring to  FIG. 7 , the optical pickup according to the first embodiment of the present invention comprises: a blue optical system formed of a laser diode  101  producing an optical beam of a blue wavelength band, a collimator lens  102 , a polarization optical beam splitter  103 , a dichroic prism  204 , a deflection prism  104 , a quarter-wavelength plate  106 , a single aperture  107 , an objective lens  108 , a detection lens  110 , an optical beam splitter  111  and a photodetection device  112 ; and a DVD optical system formed of a hologram unit  201 , a coupling lens  202 , a phase compensation element  203   a , a dichroic prism  204 , the deflection prism  104 , the quarter-wavelength plate  106 , the single aperture  107  and the objective lens  108 , wherein the dichroic prism  204 , the deflection prism  104 , the quarter-wavelength plate  106 , the single aperture  107 , and the objective lens  108  are used commonly by the foregoing two optical system. 
   Here, it should be noted that the objective lens  108  is designed so as to minimize the spherical aberration when it is used with a blue optical recording medium having a substrate thickness of 0.6 mm at the side thereof where irradiation is made by a blue optical beam having the wavelength of 407 nm, provided that the blue optical beam is incident to the objective lens  108  in the form of a parallel beam by way of the optical system forming an infinite optical system. 
   Further, it should be noted that there are provided optical recording media  109   a  and  109   b  designed for different wavelengths, wherein the optical recording medium  109   a  represents the blue optical recording medium having the substrate thickness of 0.6 mm, while the optical recording medium  109   b  represents the DVD optical recording medium having the substrate thickness of 0.6 mm. At the time of recording or playback any one of the optical recording media is mounted to a rotating mechanism not illustrated that rotates the optical recording medium mounted thereon at a high speed. 
   It should be noted that the single aperture  107  restricting the incident optical beam may be provided on a bobbin, which in turn is provided on an actuator that holds the objective lens  108  in such a manner that the objective lens  108  is moved in the focusing direction or tracking direction, and thus, there is no need of providing a separate optical component for this purpose. 
   First, explanation will be made for the case of carrying out recording, playback and erasing of information to or from a blue optical recording medium having the substrate thickness of 0.6 mm at the side where the irradiation by the optical beam having the wavelength of 407 nm is made while using the optical system together with the numerical aperture value NA of 0.67. 
   The optical beam emitted from the laser diode  101  oscillating at the wavelength of 407 nm in the form of a divergent beam having a linear polarization is shaped by the collimator lens  102  in the form of a parallel optical beam, wherein the parallel optical beam thus formed is passed through the polarization beam splitter  103  and the dichroic prism  204  disposed along an optical path thereof consecutively, wherein the optical path of the optical beam thus passed through the polarization beam splitter  103  and the dichroic prism  204  is deflected by 90 degrees by the polarization prism  104 . Thereafter, the optical beam is passed through the quarter-wavelength plate  106  and is converted to a circular polarization beam upon passage therethrough. Further, the optical beam is restricted by the single aperture  107  to the numerical aperture value NA of 0.67 and is incident to the objective lens  108 , wherein the objective lens  108  focuses the optical beam on the optical recording medium  109   a  in the form of a minute beam spot. With this beam spot, the playback, recording or erasing of information is achieved. 
   The optical beam reflected back from the optical recording medium  109   a  forms a circular polarized beam opposite having an opposite rotating direction to the optical beam incident to the optical recording medium  109   a  and is shaped by the objective lens  108  in the form of a parallel optical beam. The parallel optical beam thus shaped travels in the opposite direction and passes through the foregoing quarter-wavelength plate  106  in the opposite direction. Thereby, the optical beam is converted to a linear polarized beam having a polarization plane perpendicular to the polarization plane of the optical beam traveling to the optical recording medium, wherein the optical beam is reflected by the polarization optical beam splitter to the detection lens  110 . Thereby, the optical beam is converged by the detection lens  110  and is divided into plural paths leading to the photodetection element  112 . Thereby, the photodetection element  112  detects an aberration signal, information signal and the servo signal from the optical beam thus, focused thereon. 
   Next, explanation will be made for the case of carrying out recording, playback and erasing of information to or from a DVD optical recording medium having the substrate thickness of 0.6 mm at the side where the irradiation by the optical beam having the wavelength of 660 nm is made while using the optical system together with the numerical aperture value NA of 0.65. 
   Today, a DVD pickup generally uses a hologram unit  201  having a construction in which a light emitting device and a photodetection device are accommodated in a single. Thereby, separation of the optical beam is made by using a hologram. 
   Thus, as shown in  FIG. 8 , the hologram unit  201  has a construction in which a laser diode chip  201   a , a hologram  201   b  and a photodetection device  201   c  are integrated. 
   The optical beam having the wavelength of 660 nm emitted from the laser diode  201   a  is caused to pass through the hologram  201   b  and is converted to a predetermined divergent beam of a finite optical system by the coupling lens  202 . 
   The optical beam thus processed is added with a predetermined spherical aberration by the phase compensation element  203   a  as will be described later and is reflected toward the deflection prism  104  by the dichroic prism  204 , wherein the dichroic prism  204  passes an optical beam of blue wavelength band therethrough while causes reflection in an optical beam of red wavelength band. 
   The optical beam thus incident to the deflection prism  104  is deflected by the angle of 90 degrees and is passed through the ¼ wavelength plate  106 , and the optical beam is converted to a circular polarized optical beam upon passage through the ¼ wavelength plate  106 . 
   Thereafter, the optical beam is restricted to the numerical aperture value NA of 0.65 by the single aperture  107  and is incident to the objective lens  108 . The objective lens  108 , in turn, focuses the optical beam upon the optical recording medium  109   b  in the form of a minute beam spot, and playback, recording or erasing of information is achieved with this beam spot. 
   The optical beam reflected from the optical recording medium  109   b  is then deflected by the deflection prism  104  and is reflected by the dichroic prism  204 . Further, the optical beam thus reflected is converted to a converging beam by the coupling lens  202  and is directed to the photodetection device  201   c  inside the can that also accommodates the laser diode  201   a  by way of diffraction caused by the hologram  201   b . Thereby, the photodetection device  201   c  detects the aberration signal, information signal and the servo signal. 
   Here, explanation will be made on the single aperture and the aberration compensating element of the present invention. 
   The optical pickup of the present embodiment is an optical pickup achieving so-called two-generation compatibility by providing two optical sources, one producing an optical beam in the wavelength band an the other producing an optical beam in the red wavelength band, wherein the optical pickup of the present embodiment does not require an aperture switching element for achieving the desired two-generation compatibility. 
     FIG. 9  is a diagram showing the relationship between the incident beam diameter (X axis) for the case an optical beam of a red wavelength band of 660 nm is passed through an objective lens and the distance between the single aperture  107  and the front side (incident side) principal point of the objective lens (Y axis at the left) for the case of using a glass material of BaCD5 (provided by HOYA) and the substrate thickness of the optical recording medium of 0.6 mm. Further, the Y axis at the right of  FIG. 9  represents the numerical aperture value NA for the incident optical beam diameter in the case an optical beam having a wavelength of 407 nm is caused to pass through the objective lens. 
   In the particular case of the present embodiment in which recording and playback is made for the blue optical recording media by using the numerical aperture value NA of 0.67, it can be seen from  FIG. 9  that the optimum diameter φ of the incident optical beam to the objective lens is 4.03 mm and the optimum distance t between the single aperture and the front side principal point of the objective lens is 1.4 mm. 
   In the present embodiment, there is provided an optical pickup carrying out at least one of recording, playback and erasing of information to and from an optical recording medium, comprising: a first optical source producing a first optical beam having a wavelength λ 1 ; a second optical source producing a second optical beam having a wavelength λ 2  (λ 2 &lt;λ 2 ); a single objective lens focusing any of said first and second optical beams to an optical recording medium, said first optical beam being produced in response to turning-on of said first optical source and being incident to said objective lens in the state of a parallel beam via an infinite optical system, said second optical beam being produced in response to turning-on of said second optical source and being incident to said objective lens in the state of a divergent optical beam via a finite optical system, an aperture being provided at a front side of said objective lens opposite to said optical recording medium with offset from a principal point of said objective lens at said front side by a distance t given by an equation
 
 t=L−NA 1 ·f /tan( a  sin( NA 2 obj )),  Eq. (1.1)
 
wherein f represents a focal distance of said objective lens, NA 1  represents a numerical aperture value at a side of image surface in the state when said first optical source is turned on, NA 2 obj represents a numerical aperture value at said front side in the state when said second optical source is turned on, and L represents an object distance at the time when said second optical source is turned on.
 
   With this, it becomes possible to achieve two-generation compatibility between the blue optical recording technology and the DVD technology, without providing an aperture switching element. 
     FIG. 10  shows the optical path of the optical beam of an infinite system, which is produced at an infinite point and incident to an objective lens with the wavelength λ 1  and the numerical aperture value NA 1  by a one-dotted line. 
   On the other hand, it is known that there exists the relationship
 
 NA 1=φ/2 /f   Eq. (1.2)
 
between the focal distance f of the objective lens, the diameter φ of the incident optical beam and the numerical aperture value NA 1 .
 
   On the other hand, in the case of the finite optical system in which the optical beam has the wavelength λ 2 , the numerical aperture value NA 2 , the object distance L and the numerical aperture value NA 2 obj at the object side is represented in  FIG. 10  by the continuous line. 
   In the case the first aperture is provided at the distance t offset from the principal point of the objective lens at the front side in the direction toward the optical source such that the beam diameter of the incident optical beam having the wavelength λ 2  becomes φ, the following relationship is obtained
 
φ/2=( L−t )×tan( a  sin( NA 2 obj )).  Eq. (1.3)
 
   The foregoing equation (1.1) is obtained by substituting the equation (1.2) into the equation (1.3). 
   In the present embodiment, the construction shown in  FIGS. 12A-12D  provides the relationship φ=4.03 mm and t=1.44 mm for the optical path condition of λ 1 =407 nm and NA 1 =0.67 for the: blue wavelength band and the optical path condition of λ 2 =660 nm, Na 2 =0.65, L=157 mm and NA 2 obj=0.013. 
   Further, it should be noted that the glass material of the objective lens is not limited to BaCD5 but other materials such as BaCD12, LaC1130, BaF41, NbF1, and the like, can be used. Thereby, it is preferable to choose the glass material allowing non-spherical molding process. The foregoing diameter φ of the incident optical beam or the distance t between the single aperture and the front side principal point of the objective lens is affected also by the glass material used for the objective lens. 
   EXAMPLE 1 
   Next, an optical pickup equipped with a phase compensation element that uses optical diffraction for the compensation element of the spherical aberration will be explained with reference to  FIG. 7  as Example 1 of the present invention. 
   In the case an optical spot is formed on a DVD recording medium  109  by using an optical beam having a wavelength of 660 nm together with the single objective lens  108 , which is optimized such that the wavefront aberration becomes minimum for the optical beam having the wavelength of 407 nm, by supplying the foregoing optical beam of the 660 nm wavelength to the objective lens  108  via an infinite optical system, there arises a spherical aberration shown in  FIG. 12B  due to the difference in the wavelength. It should be noted that  FIG. 12B  shows the wavefront aberration at the wavelength of 407 nm. 
   Thus, in Example 1, the optical pickup uses a finite optical system for DVD and further provides the phase compensation element  203   a  in such a manner that the phase compensation element  203   a  induces a spherical aberration of opposite phase for canceling out the spherical aberration caused in DVD shown in  FIG. 12B . 
   It should be noted that the use of the finite optical system means that the optical beam incident to the objective lens is a divergent beam or convergent beam. Because changing of the diverging state of the optical beam incident to the objective lens is equivalent of changing the spherical aberration, a diverging state appropriate for reducing the spherical aberration may be used. 
   As shown in  FIG. 13 , the wavefront aberration can be suppressed when the object distance of the DVD optical system (the distance between the optical source and the objective lens) is changed. According to  FIG. 13 , it can be seen that the wavefront aberration reduced when the object distance is set to the range of 110-160 mm. 
   While  FIG. 10  represents the case in which no component is interposed between the objective lens and the optical source, the actual construction of the optical pickup such as the one shown in  FIG. 7  disposes other component such as the coupling lens  202  therebetween, wherein the coupling lens  202  is provided for reducing the optical path length. 
   On the other hand, the use of the finite optical system alone is not sufficient for compensating for the wavefront aberration in the case the optical system for DVD is used. 
   In view of the foregoing, Example 1 proposes the use of the phase compensation element  203   a  in addition to the use of the finite optical system for suppressing the wavefront aberration caused in the DVD optical path. 
   More specifically, the phase compensation element  203   a  causes a +1th order diffraction in the optical beam of the 660 nm wavelength and induces thereby a spherical aberration capable of canceling out the sum of the spherical aberration in the exit optical beam of the objective lens caused as it passes through the DVD optical recording medium having the thickness of 0.6 mm and the spherical aberration and the spherical aberration of the objective lens  108 . With this, the +1th order diffraction beam of the phase compensation element  203   a  is focused upon the recording surface of the DVD recording medium  109   b  by way of the objective lens  108  without aberration. 
     FIG. 14A  shows the phase compensation element in a plan view while  FIG. 14B  shows a cross sectional views  FIG. 14A . 
   Referring to  FIG. 14A , the phase compensation element  203   a  has a concentric interference pattern and plays the role of the spherical aberration correction mentioned before with regard to the +1th diffraction beam. 
   Here, it should be noted that the phase compensation surface of Example 1 is optimized for a specific wavelength. As described in “INTRODUCTION TO DIFFRACTION OPTICAL ELEMENTS” Optical Design Study Group, Optical Society of Japan, (ed)., The Physical Society of Japan, not all the energy of the incident optical beam is converted to the exit optical beam in a diffraction element but only a part of the incident optical energy is converted to the exit optical energy with a factor called diffraction efficiency. In the case of so-called kinoform (sawtooth) pattern, which is thought ideal in the diffraction elements, a theoretical diffraction efficiency of 100% can be achieved at a specific wavelength when it is optimized (blazed) at that wavelength. Further, a diffraction efficiency of 90% or more can be achieved even in the case of using a diffraction grating that approximates the foregoing kinoform grating. In Example 1, such a blazing of the phase compensation element  203   b  can be achieved to the DVD optical system. 
   Hereinafter, the actual construction and corresponding optical performance of the optical pickup of Example 1 will be explained. 
   First, the optical performance of the objective lens will be explained for the blue optical beam of the 407 nm wavelength while using  FIG. 11A  and Table 1. 
   It should be noted that the objective lens  108  of Example 1 is the objective lens designed so that the spherical aberration is minimized at the wavelength of 407 nm. Here, it should be noted that the numerical aperture value NA is set to 0.67, the focal distance f is set to 3 mm, and the refractive index n at the wavelength of 407 nm is set to 1.6049 by using BaCD5 of HOYA for the glass of the objective lens. The non-spherical form of the lens surface is given by a commonly known equation (1.4) of non-spherical surface by specifying the parameters of: coordinate X in the direction of the optical axis; coordinate Y in the direction perpendicular to the optical axis; paraxial curvature radius R; conical constant K; and higher coefficients A, B, C, D, E, F, . . . as follows.
 
 X =( Y   2   /R )/[1+√{square root over ( )}{1−(1 +K ) Y/R   2   }+AY   4   +BY   6   +CY   8   +DY   10   +EY   12   +FY   14   +GY   16   +HY   18   +JY   20 +  Eq. (1.4)
 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
                 
                 
               Glass 
             
             
                 
                 
               RDY (curvature 
               THI 
               (refractive 
             
             
                 
               surface 
               radius) 
               (thickness) 
               index) 
             
             
                 
                 
             
           
          
             
                 
               OBJ 
               INFINITY 
               INFINITY 
                 
             
             
                 
               STO 
               INFINITY 
               1.44 
             
             
                 
               S2 
               2.01507 
               1.700000 
               BaCD5 (1.6049) 
             
          
         
         
             
             
          
             
                 
               K: −0.674258 
             
             
                 
               A: 0.364557E−02 B: 0.410494E−04; 
             
             
                 
               C: 0.815925E−04; D: −.444548E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −18.13584 
               1.656788 
                 
             
          
         
         
             
             
          
             
                 
               K: 69.056492 
             
             
                 
               A: 0.132534E−01; B: −.410601E−02; 
             
             
                 
               C: 0.595437E−03; D: −.200993E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.6202) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: entrance pupil 
                 
               4.03 
                 
             
             
                 
               diameter (mm) 
             
             
                 
               WL: wavelength (nm) 
                 
               407 
             
             
                 
                 
             
          
         
       
     
   
   In Table 1, “OBJ” represents the object point (laser diode used for optical source). Because the objective lens  108  forms the infinite optical system, “INFINITY” for the curvature radius RDY and the thickness THI means that the optical source is located at the infinite distance. Further, “STO” represents the single aperture, wherein it will be noted that the curvature radius thereof is designated as “INFINITY” in Table 1. The thickness of the single aperture is set to zero for the convenience of the design. Here, all the quantities having the dimension of length is represented by “mm”. 
   S 2  represents the lens surface of the objective lens  108  at the side of the optical source, while “S 3 ” represents the lens surface of the same objective lens  108  at the side of the optical recording medium. The objective lens  108  has a thickness of 1.7 mm, and the thickness value “1.656788 mm” at the right of “curvature” in the column for S 3  represents the “working distance”. Further, “S 4 ” represents the surface of the optical recording medium  109   a  irradiated with the optical beam and hence located at the side of the optical source, while IMG represents the surface coincident to the recording surface. It will be noted that the distance between the surfaces S 4  and IMG corresponds to the substrate thickness at the side where the optical radiation is made, wherein the substrate thickness is 0.6 mm and the refractive index n is 1.6202. 
   Further, EPD: entrance pupil diameter represents the beam diameter (4.03 mm) of the incident optical beam, while WL: wavelength represents the wavelength (407 nm) used for the recording. In the representation of the non-spherical coefficients, the representation such as D:−.200993E-04 means D=−0.200993×10 −4 . 
   Next, the numerals of the phase compensation element and the optical performance at the wavelength of 660 nm will be explained for the phase compensation element  203   a  of the DVD optical system while using  FIG. 12A , Table 2 and Table 3. 
   Referring to Tables 2 and 3, it can be seen that the shape of the objective lens  108  is identical with that of the objective lens  108  of  FIG. 1 , while the refractive index an the working distance are different. Further, the phase increment caused by the increment of optical path length by the interference pattern formed on the phase compensation element  203   a  at the side of the objective lens  108  is represented as
 
γ( h )=( C 1 h   2   +C 2 h   4   +C 3 h   6 + . . . ) ×m×λ,   Eq. (1.5)
 
wherein h represents the height from the optical axis, Ci represents the optical path difference function coefficient of the nth (even number) order, m represents the order of the diffraction, and λ represents the wavelength. Here, the phase increment is defined such that it takes a positive value in the case the optical path length increases with regard to the optical path length on the optical axis.
 
   Here, it should be noted that the concentric interference pattern of Example 1 formed on the phase compensation element  203   a  is designed so as to use the +1th order diffraction. It is, however, possible to use the diffraction of any order. For example, it is possible to use a second order diffraction. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
               Glass 
             
             
                 
                 
               (curvature 
               THI 
               (refractive 
             
             
                 
               surface 
               radius) 
               (thickness) 
               index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               146.5 
                 
             
             
                 
               S1 
               INFINITY 
               0.5 
               FCD1 (1.4951) 
             
             
                 
               S2 (※) 
               INFINITY 
               10 
             
             
                 
               STO 
               INFINITY 
               1.44 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               2.01507 
               1.700000 
               BaCD5 (1.6049) 
             
          
         
         
             
             
          
             
                 
               K: −0.674258 
             
             
                 
               A: 0.364557E−02 B: 0.410494E−04; 
             
             
                 
               C: 0.815925E−04; D: −.444548E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S5 
               −14.62096 
               1.800894 
                 
             
          
         
         
             
             
          
             
                 
               K: 69.056492 
             
             
                 
               A: 0.132534E−01; B: −.410601E−02; 
             
             
                 
               C: 0.595437E−03; D: −.200993E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S6 
               INFINITY 
               0.6 
               PC (1.5791) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.03 
                 
             
             
                 
               WL: (nm) 
                 
               660 
             
             
                 
                 
             
             
                 
               (※) With regard to the phase correction surface date, reference should be made to Table 3. 
             
          
         
       
     
   
   In Example 1, the interference fringe pattern of the phase compensation element  203  is designed such that the spherical aberration is minimized at the wavelength of 660 nm, and the optical path difference coefficients Ci are determined as represented in Table 3 below. 
   
     
       
         
             
             
           
             
               TABLE 3 
             
             
                 
             
           
          
             
               S2 
               C1: −6.3005E−04; C2: 1.9631E−03; C3: −2.4763E−03; 
             
             
               (phase 
               C4: 1.4279E−03; C5: −3.7237E−03; C3: 3.5567E−03; 
             
             
               compensation 
             
             
               plane) 
             
             
                 
             
          
         
       
     
   
     FIG. 12D  shows the wavefront aberration at the wavelength of 660 nm for the case the phase compensation element is inserted, wherein the horizontal axis represents the height from the optical axis while the vertical axis represents the spherical aberration. The RMS value of the spherical aberration is 0.001 λrms in this case, while this value satisfies the requested value of 0.030 λrms and excellent spot formation becomes possible. 
   EXAMPLE 2 
     FIG. 15  is a diagram showing the schematic construction of the optical pickup according to Example 2 of the present invention. 
   Referring to  FIG. 15 , Example 2 shows an optical pickup capable of carrying out recording, playback and erasing of information to and from any of the blue optical recording medium by using the wavelength band of 407 nm, numerical aperture value NA of 0.67 and the substrate thickness of 0.6 mm at the side to which the optical irradiation is made and the red optical recording medium by using the wavelength band of 660 nm, numerical aperture value NA of 0.65 and the thickness of 0.6 mm at the side to which the optical irradiation is made. 
   The difference of the optical pickup of  FIG. 15  over the optical pickup of  FIG. 7  is in the point that a phase compensation element of stepped shape shown in  FIG. 16  is used in place of the phase compensation element  203   a  having a blazed pitch pattern shown in  FIG. 14B . Associated with this, the present example achieves the aberration compensation by changing the phase of the transmission light (0 th  order light) itself, contrary to the case of Example 1 that uses the +1 st  diffraction light. 
   In the case the phase compensation element  203   b  is not used, there remains a spherical aberration at the time the exit beam of the 660 nm wavelength passes through the substrate of the 0.6 mm thickness, similar to the case explained before. In Example 2, the stepped phase compensation element  203   b  is used for the means of suppressing the spherical aberration, wherein the stepped form is formed in conformity with the residual aberration as shown in  FIG. 12C . 
     FIG. 16  is a cross-sectional diagram of the phase compensation element  203   b  according to Example 2. As shown in  FIG. 16 , the phase compensation element  203   b  is formed by forming a concentric pattern on a glass substrate. 
   EXAMPLE 3 
     FIG. 17  is a diagram showing the schematic construction of an optical pickup according to Example 3 of the present embodiment. 
   Referring to  FIG. 17 , the optical pickup of Example 3 is an optical pickup capable of carrying out recording, playback or erasing of information to and from any of the blue optical recording medium by using the wavelength of 407 nm wavelength, numerical aperture value NA of 0.67 and the thickness of 0.6 nm for the substrate at the side where the optical irradiation is made and the DVD optical recording medium by using the wavelength of 660 nm wavelength, numerical aperture value NA of 0.65 and the thickness of 0.6 nm for the substrate at the side where the optical irradiation is made. 
   The difference of the optical pickup of Example 3 over the optical pickup of Example 1 or 2 is that the optical pickup of Example 2 uses a dynamic phase compensation element  302   c  for the compensation means of the spherical aberration. 
   For such a dynamic phase compensation element  302   c , it is possible to use an electro-optic element such as a liquid crystal device. 
   As shown in  FIG. 18 , at least one of the transparent electrodes of the liquid crystal device is patterned to form a concentric pattern, and each of the concentric electrode patterns is driven by applying thereto a voltage independently from other concentric electrode patterns. Thereby, the refractive index n of the liquid crystal can be changed arbitrarily from n 1  to n 2  in the part corresponding to the concentric electrode. 
   By changing the refractive index n, there is induced a phase difference of Δn·d(2π/λ) for the optical beam passing through each part of the liquid crystal cell, wherein Δn represents the refractive index change for that part of the liquid crystal cell constituting the liquid crystal device while d represents the cell thickness. 
   Consider now the case in which there is induced a spherical aberration shown in  FIG. 19  as a result of the difference of wavelength of the optical beam. 
     FIG. 20A  shows the wavefront of the spherical aberration in the form of a two-dimensional curve by a continuous line. When a voltage is applied to the concentric electrodes of the liquid crystal cell such that there is provided a phase difference shown in  FIG. 20A  by a broken line in the optical beam incident to the objective lens from the optical source, it becomes possible to cancel out the foregoing aberration of wavefront as a result of the delay caused in the wavefront of the optical beam passing through the liquid crystal device. 
     FIG. 20B  shows the sum of the wavefront aberration shown by the continuous line in  FIG. 20A  and the delay of the wavefront caused by the liquid crystal device shown in  FIG. 20A  by the broken line. In other words,  FIG. 20B  shows the wavefront after the foregoing phase correction by the liquid crystal device. As can be seen in  FIG. 20B , the wavefront aberration is reduced significantly as compared with the original wavefront aberration shown by the continuous line in  FIG. 20A . 
   In the phase compensation element  203   c , it is further possible to compensate for the spherical aberration caused by a layer gap in the case of using a two-layer optical recording medium or the assembling error of the optical components in the optical pickup by adjusting the voltage applied to the liquid crystal device. 
   EXAMPLE 4 
     FIG. 21  is a diagram showing the schematic construction of an optical pickup according to Example 4 of the present embodiment. 
   Referring to  FIG. 21 , the optical pickup of Example 4 is an optical pickup capable of carrying out recording, playback or erasing of information to and from any of the blue optical recording medium by using the wavelength of 407 nm wavelength, numerical aperture value NA of 0.67 and the thickness of 0.6 nm for the substrate at the side where the optical irradiation is made and the DVD optical recording medium by using the red wavelength of 660 nm wavelength, numerical aperture value NA of 0.65 and the thickness of 0.6 nm for the substrate at the side where the optical irradiation is made. 
   The difference of the optical pickup of Example 4 over the optical pickup of Example 1, 2 or 3 s that the optical pickup of Example 4 suppresses the spherical aberration at the time of recording and playback of the DVD optical recording medium not by using the phase compensation element but by way of optimizing the objective lens. More specifically, Example 4 uses an objective lens  108   b  of one group/two element construction for the objective lens. 
   Generally, it is known that chromatic aberration can be reduced by bonding a lens of positive refraction power and a lens of a negative refraction power. In Example 4, a lens capable of compensating for the chromatic aberration in the wavelength band from the blue wavelength band to the red wavelength band is used. 
   Hereinafter, the actual numeric construction of the one group/two element objective lens used Example 4 with reference to  FIGS. 22A ,  22 B and  FIGS. 23A and 23B . 
   Referring to the drawings, the incident optical beam from the optical source (left side of  FIGS. 22A and 23A ) is passed through the aperture (beam diameter radius φ=4.03 mm) and is incident to the objective lens  108   b , wherein the optical beam is focused upon a recording surface of the optical recording medium  109  after passing through the substrate to which the optical irradiation is made. It should be noted that the recording surface is provided coincident to the right surface of the optical recording medium  109 . 
     FIG. 4  shows the actual data for the case of using the blue wavelength band. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
               Glass 
             
             
                 
                 
               (curvature 
               THI 
               (refractive 
             
             
                 
               surface 
               radius) 
               (thickness) 
               index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               146.5 
                 
             
             
                 
               STO 
               INFINITY 
               1.44 
             
             
                 
               S2 
               2.31274 
               1.847986 
               LAC8 (1.7364) 
             
          
         
         
             
             
          
             
                 
               K: −0.661400 
             
             
                 
               A: 0.197340E−02 B: 0.756070E−04; 
             
             
                 
               C: 0.392551E−04; D: −.986201E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −36.95971 
               0.615995 
               EFD8 (1.7317) 
             
             
                 
               S5 
               −17.05615 
               1.232228 
             
          
         
         
             
             
          
             
                 
               K: −176.103564 
             
             
                 
               A: 0.859781E−02; B: −1.87435E−02; 
             
             
                 
               C: −.171848E−03; D: 0.783833E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S5 
               INFINITY 
               0.6 
               PC (1.6202) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.03 
                 
             
             
                 
               WL: (nm) 
                 
               407 
             
             
                 
                 
             
          
         
       
     
   
   The non-spherical surface of the lens is represented as
 
 X =( Y   2   /R )/[1+√{square root over ( )}{1−(1 +K ) Y/R   2   }+AY   4   +BY   6   +CY   8   +DY   10   +EY   12   +FY   14   +GY   16   +HY   18   +JY   20   +   Eq. (1.6)
 
   In Table 4, “OBJ” represents the object point (laser diode used for optical source). Because the objective lens forms the infinite optical system, “INFINITY” in the columns for the curvature radius RDY and the thickness THI means that the optical source is located at the infinite distance. Further, “STO” represents the single aperture, wherein it will be noted that the curvature radius thereof is designated as “INFINITY” in Table 4. The thickness of the single aperture is set to zero for the convenience of the design. Here, all the quantities having the dimension of length is represented by “mm”. 
   S 2  represents the lens surface of the objective lens at the side of the optical source, while “S 3 ” represents the surface at which the two lenses are bonded and “S 4 ” represents the surface of the same objective lens at the side of the optical recording medium. The separation between the surfaces S 2  and S 4  corresponds to the thickness of the lens having the value of 2.463981 mm, while the thickness 1.23228 mm at the right of the curvature radius of the column for S 4  represents the “working distance”. 
   Further, “S 5  represents the surface of the optical recording medium  109  irradiated with the optical beam and hence located at the side of the optical source, while IMG represents the surface coincident to the recording surface. It will be noted that the distance between the surfaces S 5  and IMG corresponds to the substrate thickness at the side where the optical radiation is made, wherein the substrate thickness is 0.6 mm and the refractive index n is 1.6202. Further, EPD: entrance pupil diameter represents the beam diameter (4.03 mm) of the incident optical beam, while WL: wavelength represents the wavelength (407 nm) used for the recording. 
   Table 5 represents the actual data for the case of using the red wavelength band. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
               Glass 
             
             
                 
                 
               (curvature 
               THI 
               (refractive 
             
             
                 
               surface 
               radius) 
               (thickness) 
               index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               156 
                 
             
             
                 
               STO 
               INFINITY 
               1.44 
             
             
                 
               S2 
               2.31274 
               1.847986 
               LAC8 (1.7088) 
             
          
         
         
             
             
          
             
                 
               K: −0.661400 
             
             
                 
               A: 0.197340E−02 B: 0.756070E−04; 
             
             
                 
               C: 0.392551E−04; D: −.986201E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −36.95971 
               0.615995 
               EFD8 (1.6822) 
             
             
                 
               S5 
               −17.05615 
               1.368239 
             
          
         
         
             
             
          
             
                 
               K: −176.103564 
             
             
                 
               A: 0.859781E−02; B: −1.87435E−02; 
             
             
                 
               C: −.171848E−03; D: 0.783833E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S5 
               INFINITY 
               0.6 
               PC (1.5791) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.03 
                 
             
             
                 
               WL: (nm) 
                 
               660 
             
             
                 
                 
             
          
         
       
     
   
   It should be noted that the shape of the objective lens  108   b  is identical with the case of Table 4. On the other hand, because a finite optical system is used, the THI of the OBJ is not infinity but is set to a finite distance of 156 mm. Further, the refractive index and working distance are different between the elements. 
   EXAMPLE 5 
     FIG. 21  shows the construction of an optical pickup according to Example 5 of the present embodiment. 
   It should be noted that the optical pickup according to Example 5 is used for carrying out recording, playback and erasing of information to and from any of the blue optical recording medium having the thickness of 0.6 mm at the side where irradiation of the optical beam is made by using a blue optical source of 407 nm wavelength with the numerical aperture value NA of 0.67 and a DVD optical recording medium having the thickness of 0.6 mm at the side where irradiation of the optical beam is made by using a blue optical source of 660 nm wavelength with the numerical aperture value NA of 0.65, similarly to the case of Example 1, wherein the optical pickup of Example 5 is different from the optical pickup of Examples 1-4 in the point that the function of the phase compensation element explained with Examples 1 and 2 is integrated with the objective lens. 
   Thus, in Example 5, the spherical aberration caused when the optical beam of the 660 nm wavelength is passed through the substrate of the DVD recording medium having the thickness of 0.6 mm at the side where the optical irradiation is made, is suppressed by the phase pattern formed on an objective lens  108   c  as shown in  FIG. 25 , in place of using the phase compensation element  203   a  or  203   b  in combination with the objective lens  108  as shown in  FIG. 7  or  FIG. 15 . 
   In the example of  FIG. 25 , a concentric pattern is formed about the optical axis on the lens surface of the objective lens at the side of the optical source, while the surface on which the foregoing pattern is formed is not limited to the lens surface at the side of the optical source. Thus, it is also possible to form such a concentric pattern on the lens surface at the side of the optical recording medium. Further, such a pattern may be formed on both of the lens surface at the side of the optical source and the optical recording medium. 
   Second Embodiment 
     FIG. 26  is a diagram showing the general construction of an optical pickup capable of carrying out recording, playback and erasing of information to and from any of: a blue optical recording medium having a substrate thickness of 0.6 mm at the side where the irradiation of the optical beam of the 407 mm wavelength is made with the numerical aperture value of 0.67; a DVD optical recording medium having a substrate thickness of 0.6 mm at the side where the irradiation of the optical beam of the 660 mm wavelength is made with the numerical aperture value of 0.65; and a CD optical recording medium having a substrate thickness of 1.2 mm at the side where the irradiation of the optical beam of the 780 mm wavelength is made with the numerical aperture value of 0.50. 
   Referring to  FIG. 26 , the optical pickup includes a blue optical system similar to the one shown in  FIG. 7  that includes the laser diode  101  oscillating at the wavelength of 407 nm, the collimator lens  102 , the polarization beam splitter  103 , the dichroic prism  204 , the polarization prism  104 , the quarter-wavelength plate  106 , the single aperture  107 , the objective lens  108 , the detection lens  110 , the beam splitter  111  and the photodetector  112 , wherein the optical pickup of the present embodiment further includes a dichroic prism  304  and an aperture switching element  107   b.    
   Further, the optical pickup includes a DVD optical system for the 660 nm wavelength similar to the one explained in  FIG. 7  including the hologram unit  201 , the coupling lens  202 , the phase compensation element  203   a  (or  203   b  or  203   c ), the dichroic prisms  204  and  304 , the quarter-wavelength plate  106 , the single aperture  107 , the aperture switching element  107   b , and the objective lens  108 . 
   Further, the optical pickup includes a CD optical system for the 780 nm wavelength including a hologram unit  301 , a coupling lens  302 , the dichroic prisms  304 , the quarter-wavelength plate  106 , the single aperture  107 , the aperture switching element  107   b , and the objective lens  108 . 
   Thus, it will be noted that the dichroic prisms  204 ,  304 , the polarization prism  104 , the quarter wavelength plate  106 , the single aperture  107 , the aperture switching element  107   b  and the objective lens  108  are used commonly by the two or three optical systems. 
   Here, it should be noted that the objective lens  108  is designed so that eh spherical aberration is minimized when it is used for the blue optical recording medium having the substrate thickness of 0.6 mm at the side irradiated with the optical beam of the 407 nm for the case the optical beam is incident to the objective lens via an infinite optical system having a numerical aperture value NA of 0.65. 
   Further, it should be noted that the optical pickup cooperates with any of an optical recording medium  109   a , an optical recording medium  109   b  and an optical recording medium  109   c  designed for different wavelengths, wherein the optical recording medium  109   a  is the blue optical recording medium having the substrate thickness of 0.6 mm, the optical recording medium  109   b  is the DVD optical recording medium having the substrate thickness of 0.6 mm, and the optical recording medium  109   c  is the CD optical recording medium having the substrate thickness of 1.2 mm. As the time of recording or playback, only one of the optical recording media  109   a - 109   c  is mounted on the rotating mechanism and rotated at high speed. 
   Hereinafter, explanation will be made for the case of carrying out recording, playback and erasing of information to or from a blue optical recording medium having the substrate thickness of 0.6 mm at the side where the irradiation by the optical beam having the wavelength of 407 nm is made while using the the numerical aperture value NA of 0.67 with reference to  FIG. 26 . 
   The optical beam emitted from the laser diode  101  oscillating at the wavelength of 407 nm in the form of a divergent beam having a linear polarization is shaped by the collimator lens  102  in the form of a parallel optical beam, wherein the parallel optical beam thus formed is passed through the polarization beam splitter  103  and the dichroic prisms  204  and  304  disposed along an optical path thereof consecutively, wherein the optical path of the optical beam thus passed through the polarization beam splitter  103  and the dichroic prisms  204  and  304  is deflected by 90 degrees by the polarization prism  104 . Thereafter, the optical beam is passed through, the quarter-wavelength plate  106  and is converted to a circular polarization beam upon passage therethrough. Further, the optical beam is restricted by the single aperture  107  to the numerical aperture value NA of 0.67 and is incident to the objective lens  108 , wherein the objective lens  108  focuses the optical beam on the optical recording medium  109   a  in the form of a minute beam spot. With this beam spot, the playback, recording or erasing of information is achieved. 
   The optical beam reflected back from the optical recording medium  109   a  forms a circular polarized beam opposite having an opposite rotating direction to the optical beam incident to the optical recording medium  109   a  and is shaped by the objective lens  108  in the form of a parallel optical beam. The parallel optical beam thus shaped travels in the opposite direction and passes through the foregoing quarter-wavelength plate  106  in the opposite direction. Thereby, the optical beam is converted to a linear polarized beam having a polarization plane perpendicular to the polarization plane of the optical beam traveling to the optical recording medium, wherein the optical beam is reflected by the polarization optical beam splitter to the detection lens  110 . Thereby, the optical beam is converged by the detection lens  110  and is divided into plural paths leading to the photodetection element  112 . Thereby, the photodetection element  112  detects an aberration signal, information signal and the servo signal from the optical beam thus focused thereon. 
   Next, explanation will be made for the case of carrying out recording, playback and erasing of information to or from a DVD optical recording medium having the substrate thickness of 0.6 mm at the side where the irradiation by the optical beam having the wavelength of 660 nm is made while using the numerical aperture value NA of 0.65. 
   Today, a DVD pickup generally uses a hologram unit  201  having a construction in which a light emitting device and a photodetection device are accommodated in a single. Thereby, separation of the optical beam is made by using a hologram. 
   Thus, as explained with reference to the first embodiment, the hologram unit  201  has a construction in which a laser diode chip  201   a , a hologram  201   b  and a photodetection device  201   c  are integrated. 
   The optical beam having the wavelength of 660 nm emitted from the laser diode  201   a  is caused to pass through the hologram  201   b  and is converted to a predetermined divergent beam of a finite optical system by the coupling lens  202 . 
   The optical beam thus processed is added with a predetermined spherical aberration by the phase compensation element  203   a  as will be described later and is reflected toward the deflection prism  104  by the dichroic prism  204 , wherein the dichroic prism  204  passes an optical beam of blue wavelength band therethrough while causes reflection in an optical beam of red wavelength band. 
   The optical beam thus incident to the deflection prism  104  is deflected by the angle of 90 degrees and is passed through the ¼ wavelength. plate  106 , and the optical beam is converted to a circular polarized optical beam upon passage through the ¼ wavelength plate  106 . 
   Thereafter, the optical beam is restricted to the numerical aperture value NA of 0.65 by the single aperture  107  and is incident to the objective lens  108 . The objective lens  108 , in turn, focuses the optical beam upon the optical recording medium  109   b  in the form of a minute beam spot, and playback, recording or erasing of information is achieved with this beam spot. 
   The optical beam reflected from the optical recording medium  109   b  is then deflected by the deflection prism  104  and is reflected by the dichroic prism  204 . Further, the optical beam thus reflected is converted to a converging beam by the coupling lens  202  and is directed to the photodetection device  201   c  inside the can that also accommodates the laser diode  201   a  by way of diffraction caused by the hologram  201   b . Thereby, the photodetection device  201   c  detects the aberration signal, information signal and the servo signal. 
   It should be noted that the phase compensation elements  203   a ,  203   b  and  203   c  explained with reference to Examples 1-3 of the first embodiment of the present invention is applicable also to the present embodiment. Further, it is possible to carry out the compensation of the aberration by the objective lens  108   b  or  108   c  in place of using the phase compensation element  203   a  or  203   b  similarly to Examples 4 and 5 of the first embodiment explained with reference to  FIG. 15  or  18 . 
   Further, explanation will be made for the case of carrying out recording, playback and erasing of information to or from a CD optical recording medium having the substrate thickness of 1.2 mm at the side where the irradiation by the optical beam having the wavelength of 780 nm is made while using the numerical aperture value NA of 0.50. 
   Similarly to the case of DVD, a CD pickup generally uses a hologram unit  201  having a construction in which a light emitting device and a photodetection device are accommodated in a single. Thereby, separation of the optical beam is made by using a hologram. 
   Thus, similarly to the hologram unit  201  explained above, there is constructed a hologram unit  301  in which a laser diode chip  301   a , a hologram  301   b  and a photodetection device  301   c  are integrated. 
   The optical beam having the wavelength of 780 nm emitted from the laser diode  301   a  of the hologram unit  301  is caused to pass through the hologram  301   b  and is converted to a predetermined divergent beam of a finite optical system by the coupling lens  302 . 
   The optical beam thus processed is reflected toward the deflection prism  104  by the dichroic prism  304 , wherein the dichroic prism  304  passes an optical beam of blue and red wavelength band therethrough while causes reflection in an optical beam of infrared wavelength band. 
   The optical beam thus incident to the deflection prism  104  is deflected by the angle of 90 degrees and is passed through the ¼ wavelength plate  106 , and the optical beam is converted to an elliptical a circular polarized optical beam upon passage through the ¼ wavelength plate  106 . 
   Thereafter, the optical beam is restricted to the numerical aperture value NA of 0.50 by the single aperture  107  and is incident to the objective lens  108 . The objective lens  108 , in turn, focuses the optical beam upon the optical recording medium  109   c  in the form of a minute beam spot, and playback, recording or erasing of information is achieved with this beam spot. 
   The optical beam reflected from the optical recording medium  109   b  is then deflected by the deflection prism  104  and is reflected by the dichroic prism  304 . Further, the optical beam thus reflected is converted to a converging beam by the coupling lens  302  and is directed to the photodetection device  301   c . Thereby, the photodetection device  301   c  detects the aberration signal, information signal and the servo signal. 
   Thus, the optical pickup according to the second embodiment of the present invention is a so-called three-generation optical pickup having three optical sources, one for the blue wavelength band, one for the DVD wavelength band and one for the infrared wavelength band. 
   In the optical pickup of the present embodiment, the desired three-generation compatibility is achieved by providing the aperture switching element  107   b . On the other hand, it should be noted that the present embodiment uses the construction that uses the same aperture for the blue optical recording technology and the DVD technology when achieving the foregoing three-generation compatibility. Thereby, it becomes possible with the present embodiment to use the conventional three-step aperture switching technology used in the conventional DVD/CD compatible optical system also in the three-generation compatible optical system. It should be noted that the aperture switching element  107   b  may be the one that uses any of reflection, diffraction or absorption when carrying out the switching of the optical beam diameter in response to the wavelength band or polarization direction. 
   Further, according to the present embodiment, the three-generation compatibility is achieved by an optical pickup comprising: 
   a first optical source producing a first optical beam with a wavelength λ 1 ; 
   a second optical source producing a second optical beam with a wavelength λ 2 ; 
   a third optical source producing a third optical beam with a wavelength λ 3  (λ 1 &lt;λ 2 &lt;λ 3 ); and 
   a single objective lens focusing any of said first through third optical beams to an optical recording medium, 
   said first optical beam is incident to said objective lens in the form of a parallel optical beam when said first optical source is activated, 
   said second optical beam and said third optical beam is incident to said objective lens in the form of a divergent optical beam when any of said second and third optical sources is activated, 
   wherein there is provided an aperture switching element providing a first beam diameter φ 1  when any of said first and second optical beams is passed therethrough and a second beam diameter φ 2  (φ 1 &gt;φ 2 ) when said third optical beam is passed therethrough, such that said aperture switching element is disposed at a distance t from a principal point of said objective lens at a side away from said optical recording medium given as
 
 t=L−NA 1 ·f /tan( a  sin( NA 2 obj ))  Eq. (1.7)
 
where f represents the focal distance of the objective lens, NA 1  represents the numerical aperture value at the side of the image plane when the first optical source is turned on, NA 2 obj represents the numerical aperture value at the side away from the optical recording medium when the second optical source is turned on, and L represents the object distance for the case the second optical source is turned on.
 
   With this, a three-generation compatibility between the blue optical recording technology, DVD technology and CD technology can be achieved by using a two-step aperture switching element, not a three-step aperture switching element. 
   For the aperture switching element of the present embodiment, it is possible to use the means for switching the beam diameter depending on the wavelength of the optical beam emitted from the optical source by reflection as represented in  FIGS. 27A-27C . 
   More specifically, it is possible to use a dielectric optical multilayer film  107   bf  having wavelength selectivity for this purpose. 
   Referring to  FIGS. 27A-27C , the aperture switching element  107   b  shows high transmissivity for any of the blue wavelength band, red wavelength band and infrared wavelength band at the central region φ 2  where the dielectric optical multilayer film  107   bf  is not provided. On the other hand, the circumference region (outside of the central region φ 2  shows high transmissivity for only the optical beam of the blue wavelength band and the red optical wavelength band while low transmissivity for the optical beam of the infrared wavelength band. 
   Similarly to the first embodiment, the optical beam of the blue wavelength band or red wavelength band is restricted to have the beam diameter of φ 1  or less by the aperture (single aperture) formed on the actuator of the objective lens. Further, it is possible to restrict the optical beam of the blue wavelength band or red wavelength band by providing an opaque means such as rough surface on the surface of the aperture switching element. 
   As the aperture switching element of the present embodiment, it is also possible to provide the beam diameter by diffraction as shown in  FIGS. 28A-28C . More specifically, it is possible to provide a diffraction grating  107   bd  having a wavelength selectivity for this purpose. 
   Referring to  FIGS. 28A-28C , the aperture switching element  107  has a high transmissivity for any of the blue wavelength band, red wavelength band and infrared wavelength band at the central region having the diameter φ 2  where there is formed no diffraction grating  107   bd . In the region outside the central part φ 2 , the diffraction grating  107   bd  does not work on the blue and red wavelength beam and only the red wavelength beam experiences diffraction. 
   Similarly to the first embodiment, the optical beam of the blue wavelength band and the red wavelength band is restricted to have the beam diameter φ 1  by the aperture (single aperture) formed on the objective lens actuator. 
   Further, it is possible to switch the optical beam diameter according to the wavelength of the optical beam emitted from the optical source by absorption as shown in  FIGS. 29A-29C , wherein it will be noted that there is provided a selective absorption element  107   ba  on the aperture switching element  107   b  in the construction of  FIGS. 29A-29C . 
   Heretofore, explanation has been made on the aperture switching element for switching the beam diameter depending on the wavelength, while the present invention is not limited to such a specific embodiment but other construction may be used. For example, it is possible to use the polarization of the optical beam. In this case, the optical source for the red wavelength and the optical source for the infrared wavelength are disposed so that the polarization directions cross with each other. Thereby, the switching of the aperture is achieved according to the mutually perpendicular polarization directions. 
   Further, it is possible to provide the means for restricting the beam diameter to φ 2  in the infrared wavelength band as provided to the aperture switching element also on the objective lens surface. In this case, it is possible to eliminate the aperture switching element similarly to the first embodiment of the present invention. As the means for restricting the beam diameter, it is possible to provide a coating of thin film causing reflection or absorption as explained before. Alternatively, it is possible to provide a diffraction pattern. 
   Next, explanation will be made on the finite construction of the CD optical system with reference to  FIG. 30A  and Table 6, wherein it should be noted that Table 6 shows the numerical parameters of the CD optical system and the optical performance at the wavelength of 780 nm. 
   Referring to Table 6, the objective lens has the same shape as that of the objective lens of Table 1, while it will be noted that the refractive index, working distance and the aperture diameter are changed. Further, a finite optical system is used for the CD optical system for minimizing the spherical aberration. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 6 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
               glass 
             
             
                 
                 
               (curvature 
               THI 
               refractive 
             
             
                 
               surface 
               radius) 
               (thickness) 
               index 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               52.26 
                 
             
             
                 
               STO 
               INFINITY 
               1.44 
             
             
                 
               S2 
               2.01507 
               1.700000 
               BaCD5 (1.5825) 
             
          
         
         
             
             
          
             
                 
               K: −0.674258 
             
             
                 
               A: 0.364557E−02 B: 0.410494E−04; 
             
             
                 
               C: 0.915925E−04; D: −.4445481E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −14.62096 
               1.546207 
                 
             
          
         
         
             
             
          
             
                 
               K: 69.056492 
             
             
                 
               A: 0.132534E−02; B: −.410601E−02; 
             
             
                 
               C: 0.595437E−03; D: −.200993E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               1.2 
               PC (1.5728) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               3.14 
                 
             
             
                 
               WL: (nm) 
                 
               780 
             
             
                 
                 
             
          
         
       
     
   
   As represented in Table 6, an optimum wavefront is achieved by setting the object distance (the distance from OBJ to S 3 ) to 53.7 mm. In the actual construction, reduction of the optical path length is achieved by interposing a coupling lens  302  to the optical path between the optical source and the objective lens as shown in the construction of  FIG. 23 . 
     FIG. 30B  shows the wavefront aberration at the wavelength of 780 nm, wherein the horizontal axis represents the height from the optical axis while the vertical axis represents the wavefront aberration. In the state of  FIG. 30B , the RMS value of the wavefront is 0.006 λrms, while this value satisfies the condition of 0.030 λrms imposed from the viewpoint of actual use. Thus, excellent spot formation is achieved. 
   In the present embodiment, the optimum wavefront is achieved for the optical system of CD by using the finite construction, while the optimum wavefront is achieved also for the DVD optical path by using the aberration compensation element shown in Examples 1-5 of the first embodiment. 
   It should be noted that the one group/two element construction used in Example 4 of the first embodiment is applicable also to the CD optical path as showing in  FIG. 31A  and Table 7. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 7 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
               glass 
             
             
                 
                 
               (curvature 
               THI 
               (refractive 
             
             
                 
               surface 
               radius) 
               (thickness) 
               index) 
             
             
                 
                 
             
           
          
             
                 
               OBJ 
               INFINITY 
               INFINITY 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               2.31274 
               1.847986 
               LAC8 (1.7040) 
             
          
         
         
             
             
          
             
                 
               K: −0.661400 
             
             
                 
               A: 0.197340E−02; B: 0.756070E−04; 
             
             
                 
               C: 0.392551E−04; D: −.986201E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −36.95971 
               0.615995 
               EFD8 (1.6750) 
             
             
                 
               S5 
               −17.05615 
               1.204793 
             
          
         
         
             
             
          
             
                 
               K: −176.103564 
             
             
                 
               A: 0.859781E−02; B: −1.87435E−02; 
             
             
                 
               C: −.171848E−03; D: 0.783833E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S5 
               INFINITY 
               1.2 
               PC (1.5728) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               3.14 
                 
             
             
                 
               WL: (nm) 
                 
               780 
             
             
                 
                 
             
          
         
       
     
   
   Third Embodiment 
     FIG. 32  is a diagram showing an information recording apparatus  30  realized by the optical information processing apparatus according to a third embodiment of the present invention. 
   Referring to  FIG. 32 , the information recording apparatus  30  is an apparatus carrying out at least one of recording, playback and erasing of information to and from an optical recording medium by using an optical pickup  31 . 
   In the present embodiment, it should be noted that the optical recording medium  40  is implemented in the form of a disk accommodated in a cartridge  41  or protective case. Thus, the optical recording medium  40  is loaded to the information recording apparatus  30  by inserting the disk  40  into an insertion opening  32  of the information recording apparatus  30  in the direction of an arrow together with the cartridge  41 . The disk  40  thus loaded is rotated by a spindle motor  33 , and recording, playback or erasing of information is achieved by the optical pickup  31 . 
   For such the optical pickup  31 , it is possible to use the optical pickup described in first or second embodiment of the present invention. Further, any of the blue optical recording medium red optical recording medium or infrared optical recording medium can be used for the optical disk  40  when carrying out any of recording, playback or erasing of the information. 
   According to the present embodiment, it becomes possible to achieve the two-generation compatibility between blue/DVD technologies without using an aperture switching element. Further, it becomes possible to achieve the three-generation compatibility between blue/DVD/CD technologies while using the two-step aperture switching element, not the three-step aperture switching element. With this, it becomes possible to achieve multiple-generation compatibility with simple construction. Further, by using a static or dynamic phase compensation element or by optimizing the construction of the objective lens, it becomes possible to secure a satisfactory beam spot performance when carrying out recording or playback of DVD or CD. Thereby, a high S/N optical pickup compatible with plural generations of optical recording technologies and an optical information processing apparatus using such an optical pickup are obtained. 
   Fourth Embodiment 
   Hereinafter, an objective lens according to a fourth embodiment of the present invention will be described with reference to  FIGS. 33A-33C  and  FIG. 34 . While the description hereinafter will be made for the case of achieving three-generation compatibility between the blue/DVD/CD technologies, it is also applicable for achieving two-generation compatibility between blue/DVD technologies or blue/CD technologies. 
   It should be noted that the present embodiment relates to an objective lens used in an optical pickup carrying out any of recording, playback and erasing to and from any of: a blue optical recording medium having a substrate thickness of 0.6 mm and designed for use with the wavelength λ 1  of 405 nm and the numerical aperture value NA of 0.65; a DVD recording medium having a substrate thickness of 0.6 mm and designed for use with the wavelength λ 2  of 660 nm and the numerical aperture value NA of 0.65; and a CD recording medium having a substrate thickness of 1.2 mm and designed for use with the wavelength λ 3  of 785 nm and the numerical aperture value NA of 0.50. 
   First, the optical performance of an objective lens  2 A will be explained for the case when it is used with the blue recording medium  1   a  designed for the wavelength λ 1  of 405 nm with reference to  FIG. 33A  and Table 8A. The objective lens  2 A of the present embodiment has the numerical aperture value NA of 0.65, the focal distance f of 3.05 mm, the refractive index nd of a D-line of 1.50 and the Abbe number νd of 60 and is formed of a glass (500,000.600000). 
   Further, the objective lens  2 A of the present embodiment has its non-spherical surface described in terms of the coordinate X in the optical axis direction, the coordinate Y in the direction perpendicular to the optical axis, the paraxial curvature radius R, conical constant K, and higher order coefficients A, B, C, D, E, F, . . . according to the known relationship of
 
 X =( Y   2   /R )·[1+√{square root over ( )}{1−(1 +K ) Y/R   2   }+AY   4   +BY   6   +CY   8   +DY   10   +EY   12   +FY   14   +GY   16   +HY   18   +JY   20 + . . .
 
   Table 8A shows exemplary data, wherein “OBJ” represents the object point (laser diode used for optical source). Because the objective lens  108  forms the infinite optical system, “INFINITY” for the curvature radius RDY and the thickness THI means that the optical source is located at the infinite distance. Further, “STO” represents the incident pupil surface of which curvature radius is designated as “INFINITY” in Table 8A and the thickness thereof is set to zero for the convenience of the design. Here, all the quantities having the dimension of length is represented by “mm”. 
   S 2  represents the lens surface of the objective lens  108  at the side of the optical source, while “S 3 ” represents the lens surface of the same objective lens  108  at the side of the optical recording medium. The objective lens  108  has a thickness of 1.85 mm, and the thickness value “1.651193 mm” at the right of “curvature radius” in the column for S 3  represents the “working distance”. Further, “S 4 ” represents the surface of the optical recording medium  109   a  irradiated with the optical beam and hence located at the side of the optical source, while IMG represents the surface coincident to the recording surface. It will be noted that the distance between the surfaces S 4  and IMG corresponds to the substrate thickness at the side where the optical radiation is made, wherein the substrate thickness is 0.6 mm and the refractive index n is 1.62. 
   Further, EPD: entrance pupil diameter represents the beam diameter (3.965 mm) of the incident optical beam, while WL: wavelength represents the wavelength (405 nm) used for the recording. In the representation of the non-spherical coefficients, the representation such as D:0.306790E-05 means D=0.306790×10 −5 . 
   Further, it should be noted that the objective lens  2 A of the present invention satisfies the conditions
 
νd&gt;35  (2.1)
 
1.58&gt;nd  (2.2)
 
0.58 nd− 0.29 ≦R 1 /f ≦0.62 nd− 0.31  (2.3)
 
   Next, explanation will be made for the case of using the objective lens with a DVD medium  1   b  designed for the wavelength of 660 nm will be explained with reference to  FIG. 33B  and Table 8B. 
   Referring to  FIG. 33B  and Table 8B, the shape of the objective lens  2 A is the same as that of Table 8A, while the refractive index and the working distance are different. Further, the DVD medium  1   b  is used with a finite optical system in which the optical beam is incident to the objective lens in the form of a divergent beam, and thus, the distance between the object point OBJ (laser diode used for the optical source) and the single aperture of the first surface STO is set to 137 mm, while it should be noted that this value is chosen for minimizing the wavelength aberration. 
   Next, the case of using the objective lens with the CD medium  1   c  designed for the wavelength of 785 nm will be explained with reference to  FIG. 33C  and Table 8C. 
   Referring to  FIG. 33C  and Table 8C, the shape of the objective lens  2 A is identical with the case of  FIG. 33A  and Table 8A, while it will be noted that the refractive index, working distance and the substrate thickness at the side where the optical irradiation is made are different. Further, the CD medium  1   c  is used with a finite optical system in which the optical beam is incident to the objective lens in the form of a divergent beam, and the distance from the object point OBJ (laser diode used for the optical source) to the single aperture in the first surface STO is set to 45.0 mm. It should be noted that this value is chosen for minimizing the wavefront aberration similarly to the case of DVD. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 8A 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               Surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
               OBJ 
               INFINITY 
               INFINITY 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.85765 
               1.85 
               500000.600000 (1.514) 
             
          
         
         
             
             
          
             
                 
               K: −0.677487 
             
             
                 
               A: 0.273561E−02 B: 0.386515E−03; C: 0.536182E−04; 
             
             
                 
               D: 0.306790E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −6.68963 
               1.651193 
                 
             
          
         
         
             
             
          
             
                 
               K: −10.002091 
             
             
                 
               A: 0.138585E−01; B: −.255540E−02; C: 0.277769E−03; 
             
             
                 
               D: −.140136E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.621) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               3.965 
                 
             
             
                 
               WL: (nm) 
                 
               405 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 8B 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               Glass 
             
             
                 
               Surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               137 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.85765 
               1.85 
               500000.600000 (1.497) 
             
          
         
         
             
             
          
             
                 
               K: −0.677487 
             
             
                 
               A: 0.273561E−02 B: 0.386515E−03; 
             
             
                 
               C: 0.536182E−04; D: 0.306790E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −6.68963 
               1.801922 
                 
             
          
         
         
             
             
          
             
                 
               K: −10.002091 
             
             
                 
               A: 0.138585E−01; B: −.255540E−02; 
             
             
                 
               C: 0.277769E−03; D: −.140136E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.579) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.095 
                 
             
             
                 
               WL: (nm) 
                 
               660 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 8C 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               45.0 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.85765 
               1.85 
               500000.600000 (1.494) 
             
          
         
         
             
             
          
             
                 
               K: −0.677487 
             
             
                 
               A: 0.273561E−02 B: 0.386515E−03; 
             
             
                 
               C: 0.536182E−04; D: 0.306790E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −6.68963 
               1.651193 
                 
             
          
         
         
             
             
          
             
                 
               K: −10.002091 
             
             
                 
               A: 0.138585E−01; B: −.255540E−02; 
             
             
                 
               C: 0.277769E−03; D: −.140136E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               1.2 
               PC (1.573) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               3.28 
                 
             
             
                 
               WL: (nm) 
                 
               785 
             
             
                 
                 
             
          
         
       
     
   
     FIG. 34  is a characteristic diagram showing the wavefront aberration caused when the objective lens  2 A of the present invention is used with the blue recording medium  1   a , DVD medium  1   b  and the CD medium with respective, predetermined wavelengths, wherein the horizontal axis represents the optical recording media  1   a ,  1   b  and  1   c , while the vertical axis represents the wavefront aberration on the optical axis for the best image point. 
   It will be noted from  FIG. 34  that there is achieved excellent wavefront aberration of 0.02 λrms for each of the optical recording media  1   a ,  1   b  and  1   c.    
   Fifth Embodiment 
   Next, an objective lens  2 B according to a fifth embodiment of the present invention will be explained with reference to  FIGS. 35A-35C  and  FIG. 36 , wherein the present embodiment uses the objective lens  2 B with: the blue recording medium la having the substrate thickness of 0.6 mm and designed for use with the wavelength λ 1  of 405 nm and the numerical aperture value NA of 0.70; the DVD medium having the substrate thickness of 0.6 mm and designed for use with the wavelength λ 2  of 660 nm and the numerical aperture value NA of 0.65; and the CD medium having the substrate thickness of 1.2 mm and designed for use with the wavelength λ 3  of 785 nm and the numerical aperture value NA of 0.50, for carrying out recording, playback and erasing to and from any of these three different optical recording media. 
   It should be noted that the objective lens  2 B of the present embodiment has the focal distance f of 3.05 mm, the refractive index nd and the Abbe number νd of respectively 1.55 and 60 for the D-line, and the glass type of (550000.600000). Similarly to the fourth embodiment, the objective lens  2 B of the present embodiment is used with the infinite optical system when used with the blue optical recording medium  1   a , while it is used with a finite optical system in the case of using the objective lens  2 B with the DVD or CD medium. 
     FIG. 35A  and Table 9A show the construction for the case of using the objective lens  2 B with the blue optical recording medium  1   a , while  FIG. 35B  and Table 9B show the construction for the case of using the objective lens  2 B with the DVD medium  1   b . Further,  FIG. 35C  and Table 9C show the construction for the case of using the objective lens  2 B with the CD medium  1   c . Here, it should be noted that the non-spherical shape of the objective lens  2 B and the items shown in Tables 9A-9C are identical with the case of the fourth embodiment explained before. 
   Further, it should be noted that the objective lens  2 B of the present embodiment also satisfies the foregoing conditions of
 
νd&gt;35  (2.1)
 
1.58&gt;nd  (2.2)
 
0.58 nd− 0.29 ≦R 1 /f≦ 0.62 nd− 0.31.  (2.3)
 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 9A 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
               OBJ 
               INFINITY 
               INFINITY 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.9547 
               1.85 
               550000.600000 (1.565) 
             
          
         
         
             
             
          
             
                 
               K: −0.679252 
             
             
                 
               A: 0.311827E−02 B: 0.382688E−03; 
             
             
                 
               C: 0.272528E−04; D: 0.200561E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −9.67786 
               1.662831 
                 
             
          
         
         
             
             
          
             
                 
               K: −15.331354 
             
             
                 
               A: 0.128339E−01; B: −.272936E−02; 
             
             
                 
               C: 0.365079E−03; D: −.233260E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.621) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.271 
                 
             
             
                 
               WL: (nm) 
                 
               405 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 9B 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               153 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.9547 
               1.85 
               550000.600000 (1.547) 
             
          
         
         
             
             
          
             
                 
               K: −0.679252 
             
             
                 
               A: 0.311827E−02 B: 0.382688E−03; 
             
             
                 
               C: 0.272528E−04; D: 0.200561E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −9.67786 
               1.781112 
                 
             
          
         
         
             
             
          
             
                 
               K: −15.331354 
             
             
                 
               A: 0.128339E−01; B: −.272936E−02; 
             
             
                 
               C: 0.365079E−03; D: −.233260E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.5791) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.095 
                 
             
             
                 
               WL: (nm) 
                 
               660 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 9C 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               47 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.9547 
               1.85 
               550000.600000 (1.544) 
             
          
         
         
             
             
          
             
                 
               K: −0.679252 
             
             
                 
               A: 0.311827E−02 B: 0.382688E−03; 
             
             
                 
               C: 0.272528E−04; D: 0.200561E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −9.67786 
               1.577408 
                 
             
          
         
         
             
             
          
             
                 
               K: −15.331354 
             
             
                 
               A: 0.128339E−01; B: −.272936E−02; 
             
             
                 
               C: 0.365079E−03; D: −.233260E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               1.2 
               PC (1.573) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               3.275 
                 
             
             
                 
               WL: (nm) 
                 
               785 
             
             
                 
                 
             
          
         
       
     
   
     FIG. 36  is a characteristic diagram showing the wavefront aberration caused when the objective lens  2 B of the present invention is used with the blue recording medium  1   a , DVD medium  1   b  and the CD medium with respective, predetermined wavelengths, wherein the horizontal axis represents the optical recording media  1   a ,  1   b  and  1   c , while the vertical axis represents the wavefront aberration on the optical axis for the best image point. 
   It will be noted from  FIG. 34  that there is achieved excellent wavefront aberration of 0.02 λrms for each of the optical recording media  1   a ,  1   b  and  1   c.    
   Sixth Embodiment 
   Next, an objective lens  2 C according to a sixth embodiment of the present invention will be explained with reference to  FIGS. 37A-37C  and  FIG. 38 , wherein the present embodiment uses the objective lens  2 C with: the blue recording medium  1   a  having the substrate thickness of 0.6 mm and designed for use with the wavelength λ 1  of 405 nm and the numerical aperture value NA of 0.70; the DVD medium having the substrate thickness of 0.6 mm and designed for use with the wavelength λ 2  of 660 nm and the numerical aperture value NA of 0.65; and the CD medium having the substrate thickness of 1.2 mm and designed for use with the wavelength λ 3  of 785 nm and the numerical aperture value NA of 0.50, for carrying out recording, playback and erasing to and from any of these three different optical recording media. 
   It should be noted that the objective lens  2 C of the present embodiment has the focal distance f of 3.05 mm, the refractive index nd and the Abbe number νd of respectively 1.55 and 55 for the D-line, and the glass type of (550000.600000). Similarly to the fourth embodiment, the objective lens  2 B of the present embodiment is used with the infinite optical system when used with the blue optical recording medium  1   a , while it is used with a finite optical system in the case of using the objective lens  2 B with the DVD or CD medium. 
     FIG. 37A  and Table 10A show the construction for the case of using the objective lens  2 C with the blue optical recording medium  1   a , while  FIG. 37B  and Table 10B show the construction for the case of using the objective lens  2 C with the DVD medium  1   b . Further,  FIG. 37C  and Table 10C show the construction for the case of using the objective lens  2 C with the CD medium  1   c . Here, it should be noted that the non-spherical shape of the objective lens  2 B and the items shown in Tables 10A-10C are identical with the case of the fourth embodiment explained before. 
   Further, it should be noted that the objective lens  2 C of the present embodiment also satisfies the foregoing conditions of
 
νd&gt;35  (2.1)
 
1.58&gt;nd  (2.2)
 
0.58 nd− 0.29 ≦R 1 /f ≦0.62 nd− 0.31.  (2.3)
 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 10A 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
               OBJ 
               INFINITY 
               INFINITY 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.95795 
               1.85 
               550000.600000 (1.567) 
             
          
         
         
             
             
          
             
                 
               K: −0.677791 
             
             
                 
               A: 0.313034E−02 B: 0.374056E−03; 
             
             
                 
               C: 0.278874E−04; D: 0.796110E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −9.78389 
               1.627965 
                 
             
          
         
         
             
             
          
             
                 
               K: −12.939440 
             
             
                 
               A: 0.133065E−01; B: −.290188E−02; 
             
             
                 
               C: 0.395558E−03; D: −.254794E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.621) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.11 
                 
             
             
                 
               WL: (mm) 
                 
               405 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 10B 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               142 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.95795 
               1.85 
               550000.600000 (1.547) 
             
          
         
         
             
             
          
             
                 
               K: −0.677791 
             
             
                 
               A: 0.313034E−02 B: 0.374056E−03; 
             
             
                 
               C: 0.278874E−04; D: 0.796110E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −9.78389 
               1.795218 
                 
             
          
         
         
             
             
          
             
                 
               K: −12.939440 
             
             
                 
               A: 0.133065E−01; B: −.290188E−02; 
             
             
                 
               C: 0.395558E−03; D: −.254794E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               0.6 
               PC (1.5791) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               4.11 
                 
             
             
                 
               WL: (nm) 
                 
               660 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 10C 
             
             
                 
                 
             
             
                 
                 
               RDY 
                 
                 
             
             
                 
                 
               (curvature 
               THI 
               glass 
             
             
                 
               surface 
               radius) 
               (thickness) 
               (refractive index) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               OBJ 
               INFINITY 
               48 
                 
             
             
                 
               STO 
               INFINITY 
               0.0 
             
             
                 
               S2 
               1.95795 
               1.85 
               550000.600000 (1.543) 
             
          
         
         
             
             
          
             
                 
               K: −0.677791 
             
             
                 
               A: 0.313034E−02 B: 0.374056E−03; 
             
             
                 
               C: 0.278874E−04; D: 0.796110E−05 
             
          
         
         
             
             
             
             
             
          
             
                 
               S3 
               −9.78389 
               1.584558 
                 
             
          
         
         
             
             
          
             
                 
               K: −12.939440 
             
             
                 
               A: 0.133065E−01; B: −.290188E−02; 
             
             
                 
               C: 0.395558E−03; D: −.254794E−04 
             
          
         
         
             
             
             
             
             
          
             
                 
               S4 
               INFINITY 
               1.2 
               PC (1.573) 
             
             
                 
               IMG 
               INFINITY 
               0.0 
             
          
         
         
             
             
             
             
             
          
             
                 
               EPD: (mm) 
                 
               3.285 
                 
             
             
                 
               WL: (nm) 
                 
               785 
             
             
                 
                 
             
          
         
       
     
   
     FIG. 38  is a characteristic diagram showing the wavefront aberration caused when the objective lens  2 C of the present invention is used with the blue recording medium  1   a , DVD medium  1   b  and the CD medium with respective, predetermined wavelengths, wherein the horizontal axis represents the optical recording media  1   a ,  1   b  and  1   c , while the vertical axis represents the wavefront aberration on the optical axis for the best image point. 
   It will be noted from  FIG. 38  that there is achieved excellent wavefront aberration of 0.02 λrms for each of the optical recording media  1   a ,  1   b  and  1   c.    
   Seventh Embodiment 
   Next, the application of an optical pickup  11  to the NA0.65 blue/DVD/CD technologies will be described as a seventh embodiment of the present invention. 
   First, the overall construction of the optical system that uses the optical pickup  11  of the present embodiment will be described with reference to  FIG. 39 . 
   It should be noted that the optical pickup  11  of the present embodiment is an optical pickup having an optical source  12  of the wavelength λ 1  of 405 nm for the blue wavelength band, an optical source  13  of the wavelength λ 2  of 660 nm for the red wavelength band, and an optical source  14  of the wavelength λ 3  of 785 nm for the infrared wavelength band, wherein the optical pickup  11  is capable of carrying out recording, playback and erasing to and from any of: the blue optical recording medium  1   a  having the substrate thickness of 0.6 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.65; the DVD medium having the substrate thickness of 0.6 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.65; and the CD medium having the substrate thickness of 1.2 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.50. 
   It should be noted that the optical pickup  11  of the present embodiment comprises: a blue optical system  26  for the blue wavelength band including a laser (optical source)  12  of the blue wavelength band, a collimator lens  15 , a polarization beam splitter  16 , dichroic prisms  17  and  18 , a deflection prism  19 , a quarter wavelength plate  20 , an aperture  21 , an aperture switching means  22 , an objective lens  2  (the lens  2 A in the present embodiment), a detection lens  23 , a beam splitter  24  and a photodetector  25 ; a red optical system  29  for DVD including a hologram unit  27 , a coupling lens  28 , the dichroic mirrors  17  and  18 , the deflection prism  19 , the quarter-wavelength plate  20 , the aperture  21 , the aperture switching means  22  and the objective lens  2 A; and an infrared optical system  32  for CD including a hologram unit  30 , a coupling lens, the dichroic prism  18 , the deflection prism  19 , the quarter-wavelength plate  20 , the aperture  21 , the aperture switching means  22  and the objective lens  32 . Thus, the dichroic prisms  17  and  18 , the deflection prism  19 , the quarter-wavelength plate  20 , the aperture  20 , the aperture switching means  22  and the objective lens  2 A are used commonly between the two or three optical systems. 
   In the hologram unit  27 , it should be noted that the chip of the laser diode  13 , hologram  33  and the photodetection device  34  are integrated. Further, the hologram unit  30  integrates the chip of the laser diode  14 , the hologram  35  an the photodetection device  36 . 
   Further, as noted before, the optical recording medium  1   a  is a blue optical recording medium having a substrate thickness of 0.6 mm, the optical recording medium  1   b  is a DVD medium having a substrate thickness of 0.6 mm, while the optical recording medium  1   c  is a CD optical recording medium having the substrate thickness of 1.2 mm. Any one of these optical recording media  1   a ,  1   b  or  1   c  is mounted on a rotating mechanism not illustrated and rotated at high speed at the time of recording, playback or erasing. 
   Here, it should be noted that the aperture  21  can be formed on a bobbin that holds the objective lens  2  on the actuator that moves the objective lens  2  in the focusing direction or tracking direction, and it is not necessary to provide a separate optical component for this purpose. 
   Hereinafter, operation of the optical system will be explained for each of the wavelength bands. 
   First, explanation will be made for the case of carrying out recording, playback or erasing of information to and from the optical recording medium  1   a  having the substrate thickness of 0.6 mm at the side where the optical irradiation is made while using optical source  12  that produces the blue wavelength λ 1  of 405 nm together with the numerical aperture value NA of 0.65. 
   The divergent optical beam of the 405 nm wavelength emitted from the laser diode  12  with linear polarization is converted to a generally parallel beam by the collimator lens  15  and is deflected by the deflection prism  19  by the angle of 90 degrees after passing through the polarization bam splitter  16  and the dichroic prisms  17  and  18 . 
   The optical beam is then converted to a circular polarization beam upon passage through the quarter wavelength plate  20  and is passed through the aperture  21 . Thereby, the numerical aperture is restricted to the numerical aperture value NA of 0.65 by the aperture switching means  22  and is incident to the objective lens  2 A. Thereby, the objective lens  2 A focuses the optical beam on the optical recording medium  1   a  in the form of a minute spot. With this spot, playback, recording or erasing of information is performed. 
   The optical beam thus focused on the optical recording medium  1   a  is reflected by the optical recording medium  1   a , and there is formed a reflection optical beam of generally parallel beam having a circular polarization of opposite rotating direction, and the reflection optical beam thus formed is converted to a linear polarization beam having a polarization plane perpendicularly to the polarization plane incident to the optical recording medium upon passage through the quarter wavelength plate  20  in the reverse direction. 
   The linear polarized reflection optical beam thus formed is then passed through the deflection prism  19  and the dichroic prisms  18  and  17  consecutively and is reflected to the detection lens  23  by the polarization beam splitter  16 . Thereby, the reflected optical beam is divided into plural optical paths by the beam splitter  24 , and detection of information and servo signal is achieved by the photodetector  25 . 
   Next, recording, playback or erasing of information to and from the DVD optical recording medium  1   b  having the thickness of 0.6 mm at the side where the optical irradiation is made by using the red optical beam of the 660 nm wavelength from the optical source  13  with the numerical aperture value NA of 0.65 will be explained. 
   The optical beam of the 660 nm wavelength emitted from the laser diode chip  13  in the hologram unit is passed through the hologram  33  and is converted to a predetermined divergent beam by the coupling lens  28 . 
   Thereafter, the optical beam is reflected toward the polarization prism  19  by the dichroic prism  17  that passes through the blue wavelength beam but reflects the red optical beam selectively, wherein the optical beam is deflected by the deflection prism  19  by 90 degrees after passing through the dichroic prism  14 . Thereafter, the optical beam is converted to a circular polarization beam upon passage through the quarter wavelength plate  20 , and the numerical aperture value NA is restricted to 0.65 at the aperture  21 . The optical beam is then incident to the objective lens  2 A and is focused upon the optical recording medium  1   b  in the form of minute spot. Playback, recording or erasing of information is achieved by using this optical spot. 
   Further, the optical beam reflected by the optical recording medium  1   b  is deflected by the deflection prism and is reflected by the dichroic prism  17  after passing through the dichroic prism  18 . The optical beam is then converted to a converging beam by the coupling lens  28  and is diffracted by the hologram  22  toward the photodetection device  34  provided in the same can in which the laser diode  13  is provided. Thereby, the photodetection device  34  detects the information signal and the servo signal. 
   Next, recording, playback or erasing of information to and from the CD optical recording medium  1   c  having the thickness of 1.2 mm at the side where the optical irradiation is made by using the red optical beam of the 785 nm wavelength from the optical source  14  with the numerical aperture value NA of 0.50 will be explained. 
   The optical beam of the 785 nm wavelength emitted from the laser diode chip  14  in the hologram unit is passed through the hologram  35  and is converted to a predetermined divergent beam by the coupling lens  31 . 
   Thereafter, the optical beam is reflected toward the polarization prism  19  by the dichroic prism  18  that passes through the blue wavelength beam but reflects the infrared optical beam selectively, wherein the optical beam is deflected by the deflection prism  19  by 90 degrees after passing through the dichroic prism  18 . Thereafter, the optical beam is converted to a circular polarization beam upon passage through the quarter wavelength plate  20 , and the numerical aperture value NA is restricted to 0.65 at the aperture  21 . The optical beam is then incident to the objective lens  2 A and is focused upon the optical recording medium  1   c  in the form of minute spot. Playback, recording or erasing of information is achieved by using this optical spot. 
   Further, the optical beam reflected by the optical recording medium  1   b  is deflected by the deflection prism and is reflected by the dichroic prism  17  after passing through the dichroic prism  18 . The optical beam is then converted to a converging beam by the coupling lens  28  and is diffracted by the hologram  22  toward the photodetection device  34  provided in the same can in which the laser diode  13  is provided. Thereby, the photodetection device  34  detects the information signal and the servo signal. 
   Further, the optical beam reflected by the optical recording medium  1   c  is deflected by the deflection prism and is reflected by the dichroic prism  18 . The optical beam is then converted to a converging beam by the coupling lens  31  and is diffracted by the hologram  35  toward the photodetection device  36  provided in the same can in which the laser diode  14  is provided. Thereby, the photodetection device  36  detects the information signal and the servo signal. 
   Hereinafter, the aperture switching part in the vicinity of the objective lens  2 A will be explained. 
   First, the beam diameter of the incident optical beam for the case of DVD will be explained. 
   When a red optical beam is incident to an objective lens optimized to provide an optimum wavefront in the blue wavelength band with the same beam diameter, the refraction power of the lens is decreased and there occurs decrease of the numerical aperture value NA. 
   In view of this, the present embodiment uses a beam diameter φ 2  in the red wavelength band larger than the beam diameter φ 1  used in the blue wavelength band. 
     FIG. 40  shows the relationship between the wavelength and the effective diameter for the case of obtaining the numerical aperture value NA of 0.65 in the objective lens having the following characteristics in the blue wavelength band
     φ: 3.965 mm   NA: 0.65   F: 3.05 mm   glass type : 500000.600000,
 
for the case a different wavelength is used.
   
   From  FIG. 40 , it can be seen that it is necessary to set the beam diameter φ 2  to about 4.095 mm in the case of carrying out recording, playback or erasing of information for the DVD recording medium  1   b  by using the wavelength of 660 nm. Further, the wavefront aberration is optimized in the case of using the wavelength other than 405 nm by changing the object distance. 
   Next, explanation will be made on the incident optical beam diameter used for the case of CD. 
   In the case of carrying out recording, playback or erasing to or from the CD optical recording medium  1   c , the optimum aperture number NA is about 0.5. On the other hand, the optimum effective beam diameter φ 3  for such a case of CD is estimated to be about 3.28 mm by the similar process explained with reference to  FIG. 40 . 
   Thus, it is necessary to carry out three-step switching for the aperture for achieving the beam diameter of any of φ 1 , φ 2  or φ 3  for the optical beam incident to the objective lens  2 A. In the present embodiment, the incident optical beam diameter φ 2  is restricted by the aperture  21  provided to the actuator part, and a wavelength selective diffraction element formed with a wavelength-selective diffraction pattern is used as the aperture switching element  22  that switches the optical beam diameters φ 1  and φ 3  in response to the wavelength of the optical beam emitted from the optical source as shown in  FIGS. 41A-41C . 
   The aperture switching means  22  has a central region having a diameter φ 3  where there is provided no wavelength-selective diffraction element while this central region is surrounded by a wavelength-selective diffraction element  41 . Reference should be made to  FIGS. 41A-41C . 
   Thus, the optical beam of any of the blue wavelength and the red wavelength having the beam diameter φ 1  or φ 2  can pass through the aperture switching element  22  freely as shown in  FIGS. 41A and 41B , while the infrared optical beam is reflected back by the wavelength-selective diffraction element  41  surrounding the central region, and the beam size of the infrared optical beam is restricted to the beam diameter of φ 3 . 
   Of course, the aperture switching means  22  is not limited to the one having the wavelength-selective diffraction element  41  but may be a device having a wavelength-selective transmission/reflection coating. 
   Eight Embodiment 
   Next, the application of an optical pickup  11  to the NA0.70blue/DVD/CD technologies will be described as an eighth embodiment of the present invention. 
   It should be noted that the optical pickup of the present embodiment is an optical pickup having an optical source  12  of the wavelength λ 1  of 405 nm for the blue wavelength band, an optical source  13  of the wavelength λ 2  of 660 nm for the red wavelength band, and an optical source  14  of the wavelength λ 3  of 785 nm for the infrared wavelength band, wherein the optical pickup is capable of carrying out recording, playback and erasing to and from any of: the blue optical recording medium  1   a  having the substrate thickness of 0.6 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.70; the DVD medium having the substrate thickness of 0.6 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.65; and the CD medium having the substrate thickness of 1.2 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.50. 
   The difference of the present embodiment over the seventh embodiment is that the numerical aperture value of 0.70 is used for the blue optical system in place of the numerical aperture value of 0.65, and thus, the beam diameter of the blue optical beam is increased as compared with the case of the red optical beam used for DVD. 
   Thus, although the overall construction is identical to that of  FIG. 39 , the present embodiment uses the objective lens  1 B in place of the objective lens  1 A. It should be noted that, with the numerical aperture value NA of about 0.70, the degradation of tolerance caused by various variations is not significant and it is possible to increase the storage capacity of the optical recording medium. By using the numerical aperture value of 0.70, the storage capacity can be increased by about 15% as compared with the case of using the numerical aperture value of 0.65. 
   In the present embodiment, the incident optical beam diameter φ 1  is restricted by the aperture provided on the actuator part, while the aperture switching means  22  carries a wavelength-selective diffraction element  42  having a wavelength-selective diffraction pattern for changing the beam diameter of the optical beam emitted from the optical source between the beam diameter φ 2  and the beam diameter φ 3  as shown in  FIGS. 42A-42C . 
   Referring to  FIG. 42A , the blue optical beam having the beam diameter φ 1  passes freely through the wavelength-switching means  22  in view of the fact that the wavelength-selective diffraction element  42  does not cause diffraction in the blue optical beam. 
   In the case of the red optical beam shown in  FIG. 42B , on the other hand, the incident optical beam is reflected by the wavelength-selective diffraction element  42  in the part thereof outside the region having the diameter φ 2 , and the beam diameter of the red optical beam is restricted to the diameter of φ 2 . 
   In the case of the infrared optical beam showing in  FIG. 42C , the incident optical beam is reflected by the wavelength-selective diffraction element  42  in the part hereof outside the region having the diameter φ 3 , and the beam diameter of the infrared optical beam is restricted to the diameter of φ 3 . 
   Ninth Embodiment 
   Next, the application of an optical pickup  11  to the NA0.67blue/DVD/CD technologies will be described as an eighth embodiment of the present invention. 
   It should be noted that the optical pickup of the present embodiment is an optical pickup having an optical source  12  of the wavelength λ 1  of 405 nm for the blue wavelength band, an optical source  13  of the wavelength λ 2  of 660 nm for the red wavelength band, and an optical source  14  of the wavelength λ 3  of 785 nm for the infrared wavelength band, wherein the optical pickup is capable of carrying out recording, playback and erasing to and from any of: the blue optical recording medium  1   a  having the substrate thickness of 0.6 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.67; the DVD medium having the substrate thickness of 0.6 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.65; and the CD medium having the substrate thickness of 1.2 mm at the side irradiated with the optical beam and designed for use with the numerical aperture value NA of 0.50. 
   The difference of the present embodiment over the seventh embodiment is that the numerical aperture value of 0.67 is used for the blue optical system in place of the numerical aperture value of 0.65, and thus, the beam diameter of the blue optical beam is increased as compared with the case of the red optical beam used for DVD. 
   Thus, although the overall construction is identical to that of  FIG. 39 , the present embodiment uses the objective lens  1 C in place of the objective lens  1 A. 
   When a red optical beam is incident to an objective lens optimized to provide an optimum wavefront in the blue wavelength band with the same beam diameter, the refraction power of the lens is decreased and there occurs decrease of the numerical aperture value NA. This means that there occurs increase of the numerical aperture value NA when the optical beam of the blue wavelength is used as compared with the case the optical beam of the red wavelength band is used, and a large capacity optical pickup is realized together with the effect of reduction of the wavelength. On the other hand, the numerical aperture value NA for the DVD generation is determined to be about 0.65. 
     FIG. 43  shows the relationship between the numerical aperture values NA for the case a blue optical beam having the 405 nm wavelength and a red optical beam of the 660 nm wavelength are incident to the objective lens having the following characteristics:
     focal distance F at the blue wavelength: 3.05 mm   glass type: 550000.550000,   optical recording medium substrate thickness: 0.6 mm.   
   From  FIG. 43 , it can be seen that, in the case the numerical aperture value NA of 0.65 is achieved for the red optical beam for DVD, the numerical aperture value NA of about 0.67 can be achieved for the optical beam of the blue wavelength and having the same beam diameter. 
   Thus, by using such a relationship, it is possible to use the two-step aperture switching similar to the one used for the compatibility between the DVD generation and the CD generation, not the three-step aperture switching as in the case of the fourth or fifth embodiment of the present invention. Thereby simplification of the device is achieved. 
   For the aperture switching means  22  of the present embodiment, it is possible to use a wavelength-selective diffraction element  43  including a wavelength-selective diffraction pattern for the switching of the optical beam diameter as shown in  FIGS. 44A-44C  in response to the wavelength of the optical beam emitted from the optical source. 
   Referring to  FIGS. 44A and 44B , the blue optical beam having the diameter φ 1  or the red optical beam having the diameter φ 2  can pass freely through the aperture switching element  22  while the infrared optical beam is reflected by the wavelength-selective diffraction element  43  at the region outside the central part having the diameter φ 3 , and thus, the diameter of the infrared optical is restricted to the foregoing diameter φ 3 . 
     FIG. 45  shows the relationship between the incident beam diameter for the case the red optical beam of the 660 nm wavelength is passed through the objective lens  2 C of the present embodiment. (X-axis) and the distance between the aperture switching means  22  and the front-side principal point of the objective lens  2 C (first Y-axis at the left) for the case of achieving the numerical aperture value NA of 0.65 with regard to the incident optical beam radius. Further, the second Y axis at the right of  FIG. 45  shows the numerical aperture value NA for the case the blue optical beam of the 405 nm wavelength is passed through the same lens as the function of the incident optical beam diameter. 
   In view of the use of the numerical aperture value NA of 0.67 for the case of using the blue optical recording medium  1   a  in the present embodiment, the optimum beam diameter φ for the incident optical beam and the optimum distance t between the aperture switching means  22  and the front side principal point of the objective lens  2 C is given as φ=4.1 mm and t=0 mm. 
   Hereinafter, the relationship of  FIG. 45  will be explained. 
   In the present embodiment, the optical pickup includes the first optical source  12  producing the first optical beam with the first wavelength λ 1 , the second optical source  13  producing the second optical beam with the second wavelength λ 2 , and the single objective lens  2 C that focuses any of the first and second optical beams to the optical recording medium  1 , wherein the first optical beam of the 405 nm wavelength is used with an infinite optical system upon activation of the first optical source  12  and the second optical beam of the 660 nm wavelength is used with the finite optical system upon activation of the second optical source  13 , wherein the predetermined aperture is provided with an offset from the principal point at the front side (incident side) of the objective lens with the distance t in the direction toward the optical source given as
 
 t≈L −NA1 ·f /tan( a  sin( NA 2 obj ))
 
where f represents the focal distance of the objective lens, NA 1  represents the numerical aperture value at the side of the image surface at the time the optical source  12  of the wavelength λ 1  (=405 nm) is turned on, NA 2 obj represents the numerical aperture value at the side of the optical source at the time the optical source  13  of the wavelength λ 2  (=660 nm) is turned on, and the L represents the object distance at the time the optical source  13  of the wavelength λ 2  (=660 nm) is turned on. As a result, switching of the aperture between the case of using the wavelength of 405 nm and the case of using the wavelength of 660 nm is not necessary.
 
   Here, the foregoing relationship
 
 t≈L−NA 1 ·f /tan( a  sin( NA 2 obj ))
 
will be explained with reference to  FIG. 46 .
 
   Referring to  FIG. 46 , the optical path of the first optical beam of the first wavelength λ 1  incident to the objective lens  1 C via the infinite optical system and focused upon the optical recording medium with the numerical aperture value NA 1  is represented by the one-dotted chain line. 
   Generally, it is known that there exists a relationship
 
 NA 1=2/ f 
 
between the focal distance f of the objective lens  1 C, the numerical aperture value NA 1  and the incident beam diameter φ.
 
   On the other hand, the optical path of the second optical beam of the second wavelength λ 2  incident to the objective lens  1 C via the finite optical system and focused upon the optical recording medium with the numerical aperture value NA 2  is represented by the one-dotted chain line. 
   Thus, in the case the aperture is disposed at the location offset from the front side principal point of the objective lens such that the second optical beam of the wavelength λ 2  is φ, the relationship
 
φ/2=( L−t )×tan( a  sin( NA 2 obj ))
 
is obtained. By substituting this to the equation for NA 1 , the foregoing equation for t is obtained.
 
   Incidentally, the present embodiment uses the construction of λ 1 =405 nm, NA 1 =0.67 for the blue optical beam and the construction of λ 2 =660 nm, NA 2 =0.65, L=142 mm, NA 2 obj=0.015 for the red optical beam and φ=4.1 mm, t=0.0 mm. 
   Next, the example of using the object distance determined so as to minimize the wavefront aberration in the case of using the finite optical system for the DVD technology and the CD technology will be explained. 
   For example, in the case of the sixth embodiment for the objective lens, the relationship between the object distance and the wavefront aberration is represented as shown in  FIG. 47  when the optical beam of the 660 nm for the DVD is used. Thus, in this case the object distance of 142 mm is used so as to minimize the wavefront aberration. 
   For the aperture switching means  22 , not only the one that uses diffraction but also the elements that use reflection or absorption or polarization may be used. 
   In the case of using reflection, a dielectric multilayer film having wavelength selectivity may be provided similarly to the case of the wavelength-selective diffraction element  43 . 
   More specifically, it is possible to construct the aperture switching means  22  such that the central part where the dielectric multilayer film is not provided shows high transmissivity to any of the blue, red and infrared wavelength bands while the region outside the central region shows a high transmissivity only to the blue and red wavelength bands and shows low transmissivity to the infrared optical wavelength band. 
   Similarly, it is possible to change the beam diameter in the aperture switching means  22  in response to the wavelength thereof by using optical absorption. 
   Further, it is possible to use polarization in the aperture switching element  22  for changing the beam diameter in response to the wavelength. In this case, the red optical beam and the infrared optical beam are disposed such that the polarization directions intersect perpendicularly with each other. Thereby, the aperture is switched by switching the polarization direction between the mutually perpendicular polarization directions. 
   For the aperture switching means  22 , it is also possible to provide a coating or diffraction grating on the surface of the objective lens  2  ( 2 A,  2 B,  2 C), not in the form of a separate element as shown in  FIGS. 41A-41C . 
   For example,  FIGS. 48A-48C  show the case of forming the transmission/reflection coating  44  having wavelength selectivity on the lens surface of the objective lens  2  at the side facing the optical source. 
   Thus,  FIG. 48A  shows the case the optical beam having the wavelength of 405 nm is irradiated upon the blue optical recording medium  1   a , while  FIG. 48B  shows the case the optical beam having the wavelength of 660 nm is irradiated upon the DVD medium  1   b  and  FIG. 18C  shows the case the optical beam having the wavelength of 785 nm is irradiated upon the CD medium  1   c.    
   Here, the coating  44  passes the optical radiation at the wavelength of 405 nm and 660 nm while it reflects the optical radiation at the wavelength of 785 nm selectively. 
   Tenth Embodiment 
   Next, the construction of an optical information processing apparatus according to a tenth embodiment of the present invention will be explained with reference to  FIG. 49 . 
   Referring to  FIG. 49 , the information processing apparatus  51  is an apparatus carrying out at least one of recording, playback and erasing of information to and from an optical recording medium by using an optical pickup  52  shown in  FIG. 39 . 
   In the present embodiment, it should be noted that the optical recording medium  1  is implemented in the form of a disk accommodated in a cartridge  53  or protective case. Thus, the optical recording medium  1  is loaded to the information recording apparatus  51  by inserting the disk  1  into an insertion opening  54  of the information recording apparatus  51  in the direction of an arrow together with the protective case  53 . The disk  1  thus loaded is rotated by a spindle motor  55 , and recording, playback or erasing of information is achieved by the optical pickup  52 . Further, it should be noted that the optical recording medium  1  ( 1   a ,  1   b  or  1   c ) is not necessarily accommodated in the protective case but may be used in the bare state. 
   For the optical pickup  52 , it is possible to use the optical pickup described in the foregoing embodiments, and the optical information processing apparatus  51  can achieve the three-generation compatibility between blue/DVD/CD technologies without using an aperture switching element (or two-generation compatibility between blue/DVD technology) by using the objective lens or optical pickup in which the spherical aberration is effectively suppressed. 
   Further, the present invention is by no means limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.