Abstract:
An objective lens apparatus is disclosed. The apparatus includes: a first objective lens with a first numerical aperture, which is capable of focusing a laser light on a first optical record medium with a first cover layer of a first thickness; a second objective lens with a second numerical aperture smaller than the first numerical aperture, which is capable of focusing the laser light on a second optical record medium with a second cover layer of a second thickness larger than the first thickness; a third objective lens with a third numerical aperture smaller than the second numerical aperture, which is capable of focusing the laser light on a third optical record medium with a third cover layer of a third thickness larger than the second thickness; and a lens holder which holds the first objective lens, the second objective lens, and the third objective lens.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2006-130108 filed in the Japanese Patent Office on May 9, 2006, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup apparatus that optically records a signal to an optical record medium and/or reproduces a signal therefrom. The present invention also relates to an objective lens apparatus mounted in the optical pickup apparatus. The present invention also relates to an optical disc driving apparatus that mounts the optical pickup apparatus. 
     2. Description of the Related Art 
     As objective lenses mounted in optical pickups of related art that deal with three formats, for example, BD (Blu-ray Disc, trademark), DVD (Digital Versatile Disc), CD (Compact Disc), and so forth, the following objective lens are known (for example, Japanese Patent Application Laid-Open Nos. 2005-302163 and 2005-293770 referred to as Patent Document 1 and Patent Document 2, respectively). 
     In the optical pickup disclosed in Patent Document 1, a first objective lens ( 24 ) that focuses laser light having wavelengths of 400 to 410 nm to a BD ( 100   c ) and a second objective lens ( 25 ) that focuses laser light having wavelengths of 650 to 780 nm to a CD ( 100   b ) or a DVD ( 100   a ) are held by a movable block ( 12 ). In Patent Document 1, to decrease the thickness of the pickup, for example as shown in  FIG. 7 , in consideration of the working distance of each disc ( 100 ), namely the distance from the front surface of each disc to each objective lens, the neutral position in the focus direction of the movable block ( 12 ) is set. In other words, a two-wavelength compatible objective lens is used for a CD and a DVD because their focal lengths are nearly the same. The compatible objective lens is used, so it is a matter of course that their focal lengths are the same. Although their focal lengths are the same, since the thickness of the cover layer of the CD that laser light enters is different from that of the cover layer of the DVD that laser light enters, it is necessary to consider the working distances. As a result, the neutral position (that is a position apart from each disc by the working distance) of the focus stroke of the CD is different from that of the DVD. Thus, a total stroke takes place. 
     In this case, “stroke” is a range in which an objective lens is moved by an actuator. In Patent Document 1, to deal with the three formats including the BD, the objective lenses are designed so that the neutral position of the objective lens for the BD is in the middle between the neutral position of the objective lens for the CD and the neutral position of the objective lens for the DVD. As a result, the thickness in the focus direction of the optical pickup is decreased. 
     On the other hand, in Patent Document 2, a three-wavelength compatible objective lens that deals with the foregoing three formats is used. 
     SUMMARY OF THE INVENTION 
     Although the thickness of the optical pickup of Patent Document 1 is decreased, since the neutral position of the objective lens for the CD is different from that for the DVD, there is a difference between their working distances (ΔWD). As a result, the thickness in the optical axis direction of the objective lens of the optical pickup is increased by the difference of the working distances. 
     In Patent Document 2, the differences of optical path lengths in air corresponding to the differences of the cover layers of the BD, DVD, and CD that laser light enters (Δ cover thickness/refractive index of cover layer) become the differences of the center positions of the actuator strokes in the focus direction of the three-wavelength compatible lens (differences of neutral positions). Thus, in this structure, the stroke amounts become large, resulting in preventing the thickness of the optical pickup from being decreased. 
     In Patent Document 1, as described above, the focal length of the two-wavelength compatible objective lens is constant. However, the NA (Numerical Aperture) of the two-wavelength compatible objective lens for laser light used for the DVD is different from that for the CD. When the NA for the DVD is larger than that for the CD, the diameter of a laser beam for the DVD becomes unnecessarily large. In other words, when the two-wavelength compatible objective lens transmits laser light, the effective diameter of laser light for the CD is different from that for the DVD. Particularly, in Patent Document 2, when the three-wavelength compatible objective lens is used, since the NA for the BD is the largest, the beam diameter for the BD is larger than that for the DVD. Thus, in this case, the objective lens becomes huge. 
     In view of the foregoing, it would be desirable to provide an objective lens apparatus, an optical pickup apparatus, and an optical disc driving apparatus that deal with record media corresponding to three different types of formats and that allow the thicknesses of the apparatuses to be decreased. 
     According to an embodiment of the present invention, there is provided an objective lens apparatus including a first objective lens, a second objective lens, a third objective lens, and a lens holder. The first objective lens has a first numerical aperture and is capable of focusing laser light on a disc-shaped first optical record medium having a first cover layer which has a first thickness. The second objective lens has a second numerical aperture and is capable of focusing laser light on a disc-shaped second optical record medium having a second cover layer which has a second thickness, the second thickness being larger than the first thickness, the second numerical aperture being smaller than the first numerical aperture. The third objective lens has a third numerical aperture and is capable of focusing laser light on a disc-shaped third optical record medium having a third cover layer which has a third thickness, the third thickness being larger than the second thickness, the third numerical aperture being smaller than the second numerical aperture. The lens holder integrally holds the first objective lens, the second objective lens, and the third objective lens. 
     In this embodiment, the first, second, and third objective lenses are disposed corresponding to the first, second, and third optical record media, respectively. Thus, the positions in the focus directions of these objective lenses can be relatively changed corresponding to the working distances of these objective lenses and these objective lenses can be held by the lens holder. When these objective lenses are held at their optimum positions in their focus directions, an objective lens apparatus whose thickness is more decreased than related art can be accomplished. 
     In addition, since the first, second, and third objective lenses are disposed, the problem of which a lens such as a compatible objective lens of related art becomes large can be solved. Thus, this embodiment contributes to decreasing the thickness of the objective lens apparatus. 
     The “is capable of focusing” includes the state of which a signal is capable of being recorded or reproduced. 
     In this embodiment, the first objective lens has a first focal length and a first lens principal point. The second objective lens has a second focal length and a second lens principal point. The third objective lens has a third focal length and a third lens principal point. The lens holder holds the second objective lens such that the second lens principal point is placed at a position in a focus direction apart from the first lens principal point of the first objective lens by a distance of which a difference between the first focal length and the second focal length and an optical path length in air corresponding to a difference between the first thickness and the second thickness are added, and holds the third objective lens such that the third lens principal point is placed at a position in a focus direction apart from the first lens principal point by a distance of which a difference between the first focal length and the third focal length and an optical path length in air corresponding to a difference between the first thickness and the third thickness are added. Thus, the initial positions in the focus directions of the objective lenses (hereinafter these initial positions are referred to as “initial focus positions” or “center positions of strokes”) are fixed by the holder. In other words, the neutral positions of the objective lenses described in Patent Documents 1 and 2 become constant. Thus, the differences of the neutral positions become zero. As a result, the thickness of the objective lens apparatus can be decreased. 
     In this embodiment, the difference of thicknesses of any two of the first cover layer, second cover layer, and third cover layer that laser light enters are replaced with optical path lengths in air and the initial focus positions of the objective lenses are offset with these optical path lengths. Thus, their differences are converted into and referred to as “optical path lengths in air”. 
     The “focus direction” is a direction in which the objective lens apparatus approaches to or goes apart from the front surface of one of the first to third record media. In this embodiment, it is obvious that an objective lens having a larger numerical aperture is placed at a position closer to an optical record medium. 
     This embodiment is substantially the same as an embodiment of which the lens holder holds the first objective lens such that the first lens principal point is placed at a position in a focus direction apart from the second lens principal point of the second objective lens by a distance of which a difference between the second focal length and the first focal length and an optical path length in air corresponding to a difference between the second thickness and the first thickness are added and holds the third objective lens such that the third lens principal point is placed at a position in a focus direction apart from the second lens principal point by a distance of which a difference between the second focal length and the third focal length and an optical path length in air corresponding to a difference between the second thickness and the third thickness are added. In other words, in this embodiment, the second lens principal point of the second objective lens is set to a reference position. 
     This applies to the case that the third lens principal point of the third objective lens is set to a reference position. In this case, the lens holder holds the first objective lens such that the first lens principal point is placed at a position in a focus direction apart from the third lens principal point of the third objective lens by a distance of which a difference between the third focal length and the first focal length and an optical path length in air corresponding to a difference between the third thickness and the first thickness are added and holds the second objective lens such that the second lens principal point is placed at a position in a focus direction apart from the third lens principal point by a distance of which a difference between the third focal length and the second focal length and an optical path length in air corresponding to a difference between the third thickness and the second thickness are added. 
     In this embodiment, at least two of the first objective lens, the second objective lens, and the third objective lens are integrally cast. Thus, the distance of at least two objective lenses can be decreased. As a result, the size of the objective lens apparatus can be decreased. Thus, when the objective lens apparatus is manufactured, since the objective lenses are integrally cast, the mounting position accuracies and tilt accuracies of at least two objective lenses are improved. 
     In this embodiment, the first numerical aperture is in a range from 0.8 to 0.9, the second numerical aperture is in a range from 0.6 to 0.7, and the third numerical aperture is in a range from 0.45 to 0.55. In other words, this embodiment describes that the first optical record medium is a BD, the second optical record medium is a DVD or a HD (High Definition) DVD, and the third optical record medium is a CD. The ranges of the numeric values of these numerical apertures represent manufacturing tolerances of the first to third optical record media or those of the first to third objective lenses. 
     According to an embodiment of the present invention, there is provided an optical pickup apparatus including a light source, a first objective lens, a second objective lens, a third objective lens, a lens holder, and an actuator. The light source emits first laser light having a first wavelength, second laser light having a second wavelength larger than the first wavelength, and third laser light having a third wavelength larger than the second wavelength. The first objective lens has a first numerical aperture and is capable of focusing the first laser light on a disc-shaped first optical record medium having a first cover layer which has a first thickness. The second objective lens has a second numerical aperture and is capable of focusing the second laser light on a disc-shaped second optical record medium having a second cover layer which has a second thickness, the second thickness being larger than the first thickness, the second numerical aperture being smaller than the first numerical aperture. The third objective lens has a third numerical aperture and is capable of focusing the third laser light on a disc-shaped third optical record medium having a third cover layer which has a third thickness, the third thickness being larger than the second thickness, the third numerical aperture being smaller than the second numerical aperture. The lens holder holds the first objective lens, the second objective lens, and the third objective lens. The actuator drives the lens holder. 
     In this embodiment, the “light source” may be light sources each of which emits laser light. Instead, the “light source” may be a light source that is composed of one physical structure. The “first wavelength” is for example in the range from 400 to 410 nm, the “second wavelength” is for example in the range from 650 to 660 nm, and the “third wavelength” is for example in the range from 770 to 830 nm. However, they are not limited to these ranges. 
     The “actuator” is a driving mechanism that drives the lens holder holding each objective lens at least in the tracking direction and the focus direction to record or reproduce a signal. The actuator may be of any type as long as it is driven for example electromagnetically, electrostatically, or piezoelectrically. 
     In this embodiment, the lens holder holds the first objective lens and the second objective lens such that ΔST 1 &lt;L 1  is satisfied where ΔST 1  is a difference in a focus direction between a center position of a stroke of the first objective lens and a center position of a stroke of the second objective lens by the actuator, and L 1  is an optical path length in air corresponding to a difference between the first thickness and the second thickness. Instead, in this embodiment, the lens holder holds the second objective lens and the third objective lens such that ΔST 2 &lt;L 2  is satisfied where ΔST 2  is a difference in a focus direction between a center position of a stroke of the second objective lens and a center position of a stroke of the third objective lens by the actuator, and L 2  is an optical path length in air corresponding to a difference between the second thickness and the third thickness. Instead, in this embodiment, the lens holder holds the third objective lens and the first objective lens such that ΔST 3 &lt;L 3  is satisfied where ΔST 3  is a difference in a focus direction between a center position of a stroke of the third objective lens and a center position of a stroke of the first objective lens by the actuator, and L 3  is an optical path length in air corresponding to a difference between the third thickness and the first thickness. 
     According to an embodiment of the present invention, there is provided an optical disc driving apparatus, including a rotating and driving mechanism, a first objective lens, a second objective lens, a third objective lens, a lens holder, an actuator, and a recording/reproducing process section. The rotating and driving mechanism rotates and drives a disc-shaped first optical record medium having a first cover layer which has a first thickness, a disc-shaped second optical record medium having a second cover layer which has a second thickness, the second thickness being larger than the first thickness, or a disc-shaped third optical record medium having a third cover layer which has a third thickness, the third thickness being larger than the second thickness. The first objective lens has a first numerical aperture and is capable of focusing a laser light on the first optical record medium. The second objective lens has a second numerical aperture smaller than the first numerical aperture and is capable of focusing the laser light on the second optical record medium. The third objective lens has a third numerical aperture smaller than the second numerical aperture and is capable of focusing the laser light on the third optical record medium. The lens holder integrally holds the first objective lens, the second objective lens, and the third objective lens. The actuator drives the lens holder. The recording/reproducing process section records a signal to the first optical record medium, the second optical record medium, or the third optical record medium rotated and driven by the rotating and driving mechanism or reproduces a signal therefrom with the first objective lens, the second objective lens, or the third objective lens. The “recording/reproducing process section” means a member, a function, a process circuit, or the like that is necessary to record or reproduce a signal. 
     According to an embodiment of the present invention, there is provided a method of driving objective lenses. A first objective lens having a first numerical aperture is caused to focus laser light on a disc-shaped first optical record medium having a first cover layer having a first thickness. A second objective lens having a second numerical aperture smaller than the first numerical aperture is caused to focus laser light on a disc-shaped second optical record medium having a second cover layer having a second thickness larger than the first thickness. A third objective lens having a third numerical aperture smaller than the second numerical aperture is caused to focus laser light on a disc-shaped third optical record medium having a third cover layer having a third thickness larger than the second thickness. A lens holder which integrally holds the first objective lens, the second objective lens, and the third objective lens is driven so that a signal is recorded or reproduced. 
     As described above, according to these embodiments, record media of three formats can be handled and the thicknesses of the apparatuses can be decreased. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein similar reference numerals denote similar elements, in which: 
         FIG. 1  is an exploded perspective view showing an optical disc driving apparatus according to an embodiment of the present invention; 
         FIG. 2  is a plan view schematically showing an optical pickup apparatus mounted in the optical disc driving apparatus shown in  FIG. 1 ; 
         FIG. 3  is a block diagram showing a structure of the optical disc driving apparatus shown in  FIG. 1 ; 
         FIG. 4  is a side view showing a light collecting device contained in an optical system of the optical pickup apparatus shown in  FIG. 3 ; 
         FIG. 5  is a perspective view showing a laser coupler; 
         FIG. 6  is a table that lists φ (effective diameter), f (focal length), and NA of each of objective lenses for a CD, a DVD, and a BD; 
         FIG. 7  is a table that lists thicknesses of cover layers of the CD, DVD, and BD; 
         FIG. 8  is a plan view showing an optical pickup apparatus according to another embodiment of the present invention; 
         FIG. 9  is a side view showing a peripheral portion of the light collecting device of the optical pickup apparatus shown in  FIG. 8 ; 
         FIG. 10  is a plan view showing an optical pickup apparatus according to another embodiment of the present invention; 
         FIG. 11A  is a sectional view taken along line A-A of  FIG. 10 ; 
         FIG. 11B  is a sectional view taken along line B-B of  FIG. 10 ; and 
         FIG. 12  is a side view showing a light collecting device according to another embodiment of the present invention, the objective lens for the BD being disposed in the middle of the light collecting device. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, with reference to the accompanying drawings, embodiments of the present invention will be described. 
       FIG. 1  is an exploded perspective view schematically showing an optical disc driving apparatus  50  according to an embodiment of the present invention.  FIG. 2  is a plan view schematically showing an optical pickup apparatus mounted in the optical disc driving apparatus  50  shown in  FIG. 1 . 
     The optical disc driving apparatus  50  is an apparatus that records and reproduces information to and from an optical disc (DVD±R/RW, CD-R/RW, or BD)  1  as an optical record medium. The optical disc  1  may have a single signal record layer or a plurality of signal record layers. In the following description, the optical disc  1  may be referred to as a “BD  100 ”, a “DVD  200 ”, or a “CD  300 ”. These three optical discs or four optical discs including a HD DVD may be collectively referred to as the “optical disc  1 ”. 
     The optical disc driving apparatus  50  has for example a disc table  3  on which the optical disc  1  is loaded, an optical pickup apparatus  6  in which an optical system and so forth that will be described later are mounted, and a housing  7  that houses the optical disc  1  and the optical pickup apparatus  6 . 
     The disc table  3  has a chucking mechanism with which the optical disc  1  is loaded on the disc table  3 . The chucking mechanism allows the optical disc  1  to be loaded and rotated on the disc table  3 . 
     The optical pickup apparatus  6  has a movable base  4 , an optical system  30  mounted on the movable base  4 , and an actuator  8  that drives a light collecting device (objective lens apparatus)  20 . The movable base  4  is connected to a rotating shaft of a thread motor (not shown) and is slidable in the radial direction of the optical disc  1  along guide shafts  5  disposed on both ends of the optical pickup apparatus  6 . The actuator  8  is a two-axis actuator that drives and controls for example focus servo and tracking servo mechanisms that move the light collecting device  20  in the focus direction and the tracking direction. Instead of the two-axis actuator, the actuator  8  may be a three-axis actuator that is capable of moving the light collecting device  20  in a tilt angle direction to the optical disc  1  along with the focus direction and the tracking direction. 
       FIG. 3  is a block diagram showing a structure of the optical disc driving apparatus  50  shown in  FIG. 1 . 
     As shown in  FIG. 2 , together with the optical pickup apparatus  6 , the optical disc driving apparatus  50  has a spindle motor  9 , a feed motor  10 , a system controller  11 , a servo control circuit  12 , a preamplifier  13 , a signal modulator/demodulator/ECC (error correction coding) section  14 , an interface  15 , a D/A and A/D converter  16 , an audio visual process section  17 , an audio visual signal input/output section  18 , and a laser control section  19 . 
     The spindle motor  9  is a motor that rotates and drives the optical disc  1 . The disc table  3  and the spindle motor  9  compose a rotating and driving mechanism. 
     The feed motor  10  is a motor that moves the movable base  4  shown in  FIG. 1  in the radial direction of the optical disc  1 . Thus, the feed motor  10  moves the optical pickup apparatus  6  in the radial direction of the optical disc  1 . 
     The system controller  11  is disposed to control the overall optical disc driving apparatus  50  and independently performs a signal process and controls servo mechanisms. 
     The servo control circuit  12  generates a focus servo signal and a tracking servo signal based on signals (focus error signal and tracking error signal) obtained from the preamplifier  13  and supplies these signals to the optical pickup apparatus  6  and the feed motor  10 . 
     The preamplifier  13  generates the focus error signal, the tracking error signal, and an RF signal with signals obtained by the optical pickup apparatus  6 . 
     The signal modulator/demodulator/ECC (error correction coding) section  14  demodulates the RF signal, obtains a record signal, and performs an error correction coding process for the record signal. For example, the signal modulator/demodulator/ECC section  14  adds an ECC to a record signal and corrects an error of a reproduction signal (RF signal). 
     The interface  15  exchanges signals with an external computer  27 . 
     The D/A and A/D converter  16  converts the reproduction signal as a digital signal into an analog signal and converts the record signal as an analog signal into a digital signal. 
     The audio visual process section  17  and the audio visual signal input/output section  18  exchange an audio signal and a video signal with an external device. 
     The laser control section  19  controls an output and a wavelength of a semiconductor laser mounted in the optical pickup apparatus  6  depending on record mode, a reproduction mode, type of the optical disc  1 , and so forth. 
     The optical disc driving apparatus  50  causes the spindle motor  9  to rotate the optical disc  1  and drive and control the feed motor  10  with the control signal supplied from the servo control circuit  12 . By moving the optical pickup apparatus  6  to a position corresponding to a desired record track of a selected signal record layer of the optical disc  1 , the optical disc driving apparatus  50  records and reproduces information to and from the selected signal record layer. 
     Referring to  FIG. 2 , the optical system  30  has a single wavelength laser diode  90 , a laser coupler  92 , a photo detector  93 , a first adjustment lens  99 , a λ/2 plate  96 , a first polarizing beam splitter  94 , a second polarizing beam splitter  95 , a second adjustment lens  97 , a mirror  98 , a grating  24 , a collimator lens  84 , and the light collecting device  20 . 
     The single wavelength laser diode  90  emits laser light having wavelengths of 400 to 410 nm corresponding to the BD  100  (hereinafter this laser light is referred to as the first laser light). The laser coupler  92  has a light receiving and emitting device that emits laser light having wavelengths of 650 to 660 nm corresponding to the DVD  200  (this laser light is referred to as the second laser light) and laser light having wavelengths of 770 to 830 nm corresponding to the CD  300  (hereinafter this laser light is referred to as the third laser light) and receives reflected lights of the second and third laser lights from the optical discs. 
     The photo detector  93  detects return light of the first laser light reflected from the optical disc  1 . The second adjustment lens  97  adjusts the beam diameter with which the photo detector  93  properly detects the return light. The mirror  98  guides the first laser light emitted from the second adjustment lens  97  to the photo detector  93 . 
     The collimator lens  84  collimates laser light having each wavelength that the first polarizing beam splitter  94  has transmitted or reflected. The collimator lens  84  is supported by a lens holder  51 . In addition, both ends of the lens holder  51  are supported by a pair of guide shafts  52  that extend in the optical axis direction. In addition, since the lens holder  51  is fit to a lead screw  55  of a drive motor  54 , the lens holder  51  is movable in the optical axis direction. The lead screw  55  of the drive motor  54  is rotated and driven corresponding to the format of the optical disc  1 . As a result, the lens holder  51  is moved in the optical axis direction. Thus, spherical aberration of the collimator lens  84 , which occurs due to the thickness of the cover layer on the front surface that differs in each format of the optical disc  1  that laser light enters, can be compensated. 
       FIG. 4  is a side view showing a peripheral portion of the light collecting device contained in the optical system  30  of the optical pickup apparatus  6  shown in  FIG. 2 . The optical system  30  also has a prism  86 , a mirror  83 , a first λ/4 plate  87 , and a second λ/4 plate  85 . 
     The prism  86  transmits the first laser light that the collimator lens  84  has transmitted and reflects the second or third laser light that the collimator lens  84  has transmitted to the optical disc  1  side. The mirror  83  reflects the first laser light that the collimator lens  84  and the prism  86  have transmitted to the optical disc  1  side. The first λ/4 plate  87  converts linearly polarized light of the second or third laser light that the prism  86  has reflected into circularly polarized light. The second λ/4 plate  85  converts linearly polarized light of the first laser light that the prism  86  has transmitted into circularly polarized light. 
     The prism  86  has separation films  86   a  and  86   b  that have wavelength dependencies and reflection and transmission film characteristics depending on the order of the corresponding objective lens. However, as will be described later, when a DVD objective lens  22  is used as an HD DVD objective lens, since the wavelength of the laser light for the BD is the same as that for the HD DVD, it is necessary to optimally set the ratio of reflection and transmission of the separation films  86   a  and  86   b  of the prism  86 . 
     A light emitting section of the single wavelength laser diode  90  emits the first laser light to the first polarizing beam splitter  94 . The laser light emitted from the single wavelength laser diode  90  is rotated by the grating  24  having a function of a λ/2 plate for the first laser light such that S polarized light of the first laser light enters the first polarizing beam splitter  94 . In addition, the grating  24  divides the first laser light into three beams to generate a tracking error signal according to the differential push-pull method. Thereafter, the three beams enter the first polarizing beam splitter  94 . 
       FIG. 5  is a perspective view showing the laser coupler  92 . In  FIG. 5 , a member such as a package that covers the laser coupler  92  is not shown. The laser coupler  92  has a silicon chip  103  on its surface area. The silicon chip  103  has photo detectors  101  and  102  that detect return lights of the second and third laser lights, respectively. Mounted on the silicon chip  103  is a photo diode chip  109 . Mounted on the photo diode chip  109  is a two-wavelength laser diode  104 . The two-wavelength laser diode  104  is mounted on the silicon chip  103  through the photo diode chip  109  having a PIN photo diode  108  on its surface area. The PIN photo diode  108  mounted on the photo diode chip  109  monitors laser light emitted from the rear surface of the two-wavelength laser diode  104  to control the output of the two-wavelength laser diode  104 . 
     Mounted on the silicon chip  103  is a prism  105  having a tilt plane  105   a  that reflects the second or third laser light emitted from the two-wavelength laser diode  104  nearly at right angles. The laser light reflected on the tilt plane  105   a  travels to the second polarizing beam splitter  95 . On the other hand, the return light reflected on the signal record surface of the optical disc  1  passes through the tilt plane  105   a  of the prism  105 . As a result, the return light is detected by the photo detectors  101  or  102 . 
     With reference to  FIG. 2 , the first and second polarizing beam splitters  94  and  95  each have a wavelength selection function. In other words, the first and second polarizing beam splitters  94  and  95  are devices that transmit or reflect incident light depending on its wavelength. The first and second polarizing beam splitters  94  and  95  are composed of an optical thin film having a predetermined structure formed on bonded planes  94   a  and  95   a  of the prisms. Thus, the first polarizing beam splitter  94  transmits the second and third laser lights regardless of their polarized states. In contrast, the first polarizing beam splitter  94  transmits or reflects the first laser light depending on its polarized state. Likewise, the second polarizing beam splitter  95  transmits the second and third laser lights regardless of their polarized states. In contrast, the second polarizing beam splitter  95  transmits or reflects the first laser light depending on its polarized state. 
     More specifically, when the single wavelength laser diode  90  emits the first laser light and the incident angle of the first laser light as the S polarized light generated by the grating  24  is a designed center value, the first polarizing beam splitter  94  totally reflects the first laser light to the collimator lens  84  side through the bonded plane  94   a . On the other hand, when the first laser light is reflected on the signal record surface of the optical disc  1  and the incident angle of the return light of the first laser light as the P polarized light generated by the second λ/4 plate  85  is a designed center angle, the first polarizing beam splitter  94  totally transmits the first laser light to the second polarizing beam splitter  95  side. 
     The λ/2 plate  96  is disposed between the first polarizing beam splitter  94  and the second polarizing beam splitter  95 . The λ/2 plate  96  changes the return light of the first laser light as the P polarized right that the first polarizing beam splitter  94  has transmitted to the S polarized light. The S polarized light enters the second polarizing beam splitter  95 . Like the first polarizing beam splitter  94 , when the incident angle of the first laser light as the S polarized light is a designed center value, the second polarizing beam splitter  95  totally reflects the first laser light through the bonded plane  95   a . The return light reflected on the bonded plane  95   a  enters the light reception plane of the photo detector  93  through the second adjustment lens  97  and the mirror  98 . 
     The light collecting device  20  has a BD objective lens  21  that focuses the first laser light on the signal record surface of the optical disc  1 , a DVD objective lens that focuses the second laser light on the signal record surface of the optical disc  1 , a CD objective lens  23  that focuses the third laser light on the signal record surface of the optical disc  1 , and a lens holder  26  that integrally holds the BD, DVD, and CD objective lenses  21 ,  22 , and  23 . The lens holder  26  is typically made of a resin. Instead, the lens holder  26  may be made of a light metal such as aluminum. Instead, the lens holder  26  may be made of other than these materials. 
     Since the BD, DVD, and CD objective lenses  21 ,  22 , and  23  corresponding to the three disc formats are independently disposed, they can be held by the lens holder  26  such that the positions in the focus directions of the objective lenses  21 ,  22 , and  23  are relatively changed depending on their working distances. When the objective lenses  21 ,  22 , and  23  are held at optimum positions in their focus directions, an objective lens apparatus whose thickness is more decreased than related art can be accomplished. The optimum positions at which the objective lenses  21 ,  22 , and  23  are held will be described later. 
     In addition, when the BD, DVD, and CD objective lenses  21 ,  22 , and  23  are independently disposed, the problem of which lenses such as compatible objective lenses of related art become large can be solved. Thus, this structure contributes to decreasing the thickness of the objective lens apparatus. This point will be described later in detail. 
     In the optical system  30 , the first laser light emitted from the single wavelength laser diode  90  is changed to S polarized light by the grating  24 . Thus, the first laser light as the S polarized light is totally reflected on the bonded plane  94   a  of the first polarizing beam splitter  94 . The first laser light that has been totally reflected is focused on a signal record surface  100   b  through the collimator lens  84  that has been set for a predetermined focal length that allows spherical aberration of the BD  100  to be removed, the objective lens  21 , and a cover layer  100   a  of the BD  100 . On the other hand, the return laser light reflected on the signal record surface  100   b  of the BD  100  is changed to P polarized light by the second λ/4 plate  85 . As a result, the laser light as the P polarized light is totally transmitted by the first polarizing beam splitter  94 . The return first laser light that has been totally transmitted is changed to S polarized light by the λ/2 plate  96  disposed immediately before the second polarizing beam splitter  95 . As a result, the first laser light as the S polarized light is totally reflected on the bonded plane  95   a  of the second polarizing beam splitter  95  and then focused on the light receiving surface of the photo detector  93 . 
     In addition, the second or third laser light emitted from the laser coupler  92  is divided into three beams by a grating (not shown) disposed at an upper portion of the package that covers the laser coupler and then totally transmitted by the second polarizing beam splitter  95 , the λ/2 plate  96 , and the first polarizing beam splitter  94 . The second or third laser light that has been totally transmitted is focused on the signal record surface  200   b  or  300   b  through the collimator lens  84  that has been set for a predetermined focal length that allows spherical aberration of the DVD  200  or CD  300  to be removed, and the DVD objective lens  22  or the CD objective lens, and a cover layer  200   a  or  300   a  of the DVD  200  or CD  300 . On the other hand, the return second or third laser light reflected on the signal record surface  200   b  or  300   b  of the DVD  200  or CD  300  enters the laser coupler  92  through the same optical path as the going path. The return second or third laser light is transmitted on the tilt plane  105   a  of the prism  105  and then focused on the photo detector  101  or  102 . 
     In this embodiment, as the tracking servo method, the three-beam method was exemplified. Instead, any known tracking servo method may be used. In the optical system  30 , only the first laser light is selectively transmitted or reflected. Instead, with the polarized light separation films (bonded planes  94   a  and  95   a ) of the first and second polarizing beam splitters  94  and  95 , only the second laser light may be selectively transmitted or reflected. 
       FIG. 6  is a table that lists φ (effective diameter) (mm), f (focal length) (mm), and NA of each of the CD objective lens  23 , DVD objective lens  22 , and BD objective lens  21  according to this embodiment of the present invention.  FIG. 7  is a table that lists t (thickness) (mm) of each of the cover layers  300   a ,  200   a , and  100   a  of the CD  300 , DVD  200 , and BD  100 . With respect to φ and f, the values of the table are just examples. These values depend on the designed sizes of the objective lenses. With respect to NA, the values of the table may have a range to some extent. In particular, NA of the CD objective lens  23  has a range from 0.45 to 0.55. With respect to t, the values of the table may deviate within a margin of error. Hereinafter, the thickness of the cover layer of the BD is represented by t 1 , the thickness of the cover layer of the DVD is represented by t 2 , and the thickness of the cover layer of the CD is represented by t 3  (see  FIG. 4 ). 
     As shown in  FIG. 4 , the lens principal points of the BD, DVD, and CD objective lenses  21 ,  22 , and  23  are referred to as a first lens principal point  21   a , a second lens principal point  22   a , and a third lens principal point  23   a , respectively. The lens principal point is the optical center of a lens and is the center point on the basis of which the focal length f is defined. The BD, DVD, and CD objective lenses  21 ,  22 , and  23  have a first focal length f 1 , a second focal length f 2 , and a third focal length f 3 , respectively. 
     In this example, the difference between the first focal length f 1  and the second focal length f 2  is represented by Δfa, the difference between the second focal length f 2  and the third focal length f 3  is represented by Δfb, and the difference between the third focal length f 3  and the first focal length f 1  is represented by Δfc. In addition, the difference between the thicknesses of the cover layers  100   a  and  200   a  of the BD  100  and the DVD  200  that laser light enters is represented by Δta (=t 2 −t 1 ), the difference between the thicknesses of the cover layers  200   a  and  300   a  of the DVD  200  and the CD  300  that laser light enters is represented by Δtb (=t 3 −t 2 ), and the difference between the thicknesses of the cover layers  300   a  and  100   a  of the CD  300  and the BD  100  that laser light enters is represented by Δtc (=t 3 −t 1 ). 
     The example shown in  FIG. 6  denotes that Δfa=0.385, Δfb=0.380, Δfc=0.765, Δta=0.5, Δtb=0.6, and Δtc=1.1. 
     The optical path length L 1  of laser light in air corresponding to Δta, the optical path length L 2  of laser light in air corresponding to Δtb, and the optical path length L 3  of laser light in air corresponding to Δtc can be expressed by the following formulas.
 
 L 1 =Δta /refractive index of cover layer  (1)
 
 L 2 =Δtb /refractive index of cover layer  (2)
 
 L 3 =Δtc /refractive index of cover layer  (3)
 
     When the cover layers of the BD  100 , DVD  200 , and CD  300  that laser light enters are made of for example polycarbonate resin, since the refractive index thereof is 1.6, in the example shown in  FIG. 6 , the optical paths are as follows.
 
 L 1=0.5/1.6≈0.31  (4)
 
 L 2=0.6/1.6≈0.38  (5)
 
 L 3=1.1/1.6≈0.69  (6)
 
     When the cover layers of the BD, DVD, and CD objective lenses  21 ,  22 , and  23  are made of other than polycarbonate resin, since the refractive index thereof is different from 1.6, the optical paths L 1 , L 2 , and L 3  are different from the foregoing values. 
     In this embodiment, the differences of the thicknesses of the cover layers  100   a ,  200   a , and  300   a  that laser light enters are replaced with optical path lengths in air and the initial focus positions of the objective lenses  21 ,  22 , and  23  are offset with the optical path lengths. The initial focus position is the center position of the stroke of the lens holder  26  by the actuator  8 . In this embodiment, the differences of the center positions of the strokes of the BD, DVD, and CD objective lenses  21 ,  22 , and  23  can be set to zero. 
     It can be thought that formulas (1), (2), and (3) represent the differences of the center positions of the strokes of the optical pickup using the two-wavelength compatible objective lens of related art. 
     (a) in the case of formula (1), it is assumed that a BD/DVD compatible objective lens and an independent CD objective lens are used. In this case, the difference between the center positions of the strokes of the BD/DVD compatible objective lens and the CD objective lens is L 1 . 
     (b) in the case of formula (2), it is assumed that a DVD/CD compatible objective lens and an independent BD objective lens are used. In this case, the difference between the center positions of the strokes of the DVD/CD compatible objective lens and the BD objective lens is L 2 . 
     (c) in the case of formula (3), it is assumed that a BD/CD compatible objective lens and an independent DVD objective lens are used. In this case, the difference between the center positions of the strokes of the BD/CD compatible objective lens and the DVD objective lens is L 3 . 
     As will be described later, in this embodiment, the stroke differences of the objective lenses  21 ,  22 , and  23  are smaller than any of (a) to (c) of related art. Specifically, the BD, DVD, and CD objective lenses  21 ,  22 , and  23  are held at the following positions by the lens holder  26 . 
     The lens holder  26  holds the BD objective lens  21  at a predetermined position in the focus direction thereof. Although the predetermined position is not limited, it becomes a reference position. The lens holder  26  holds the DVD objective lens  22  such that the second lens principal point  22   a  is placed at a position in the focus direction apart from the first lens principal point  21   a  as the reference position by a distance of which Δfa and L 1  are added. The focal lengths are considered because the BD objective lens  21 , the DVD objective lens  22 , and the CD objective lens  23  are independently disposed and their focal lengths are different from each other. 
     Of course, in this case, it is necessary to align the objective lenses  21 ,  22 , and  23  in the focus direction such that the stroke of the DVD objective lens  22  and the stroke of the BD objective lens  21  are contained in the stroke of the CD objective lens  23  as shown in  FIG. 4 . In other words, the BD objective lens  21 , the DVD objective lens  22 , and the CD objective lens  23  are successively aligned on the near side of the optical disc  1 . 
     In addition, the lens holder  26  holds the CD objective lens  23  such that the third lens principal point  23   a  is placed at a position in the focus direction apart from the first lens principal point  21   a  as the reference position by Δfc+L 3 . 
     Thus, since the lens holder allows the maximum value (ΔST) of the differences of the center positions of the strokes of the objective lenses  21 ,  22 , and  23  to be zero, the thickness of the light collecting device  20  can be decreased. As a result, the thicknesses of the optical pickup apparatus  6  and the optical disc driving apparatus  50  can be decreased. 
     The “maximum value” means the largest value of the difference between the center positions of the strokes of the BD objective lens  21  and the DVD objective lens  22 , the difference between the center positions of the strokes of the DVD objective lens  22  and the CD objective lens  23 , and the difference between the center positions of the strokes of the CD objective lens  23  and the BD objective lens  21 . 
     In this case, the effective diameters of the objective lenses  21 ,  22 , and  23  are nearly the same. The relationship of effective diameter φ, NA, and focal length f is given as follows:
 
φ=2 ×NA×f  
 
Thus, as disclosed in Patent Documents 1 and 2, when a compatible objective lens is used, since f is fixed, the effective diameter becomes large in proportion to NA. However, when a compatible objective lens is not used as in this embodiment, the effective diameters of the BD and DVD do not unnecessarily become large. Therefore, a disadvantage is not caused for decreasing the size and thickness of the optical pickup. This is because in this embodiment, the three objective lenses  21 ,  22 , and  23  are independently disposed and their focal lengths are selectable.
 
     In this embodiment, without necessity of a special prism (31) shown in FIG. 2 of Japanese Patent Application Laid-Open No. 2005-100513, the thickness of the optical pickup apparatus  6  can be decreased. 
     In this embodiment, since the three objective lenses  21 ,  22 , and  23  are independently disposed, although the light collecting device  20  becomes heavy, the total weight of the three objective lenses  21 ,  22 , and  23  is nearly the same as the giant lens such as the three-wave compatible objective lens of related art. 
     In this embodiment, since the differences of the center positions of the strokes are removed and the strokes become short, the actuator  8  can be easily designed. In other words, a DC gain and a frequency band of the servo operation of the actuator  8  can be suppressed. In addition, since the strokes become short, a signal can be recorded and reproduced at high speed. 
     As described above, it is most preferred that the maximum value ΔST of the differences of the center positions of the strokes of the objective lenses  21 ,  22 , and  23  be zero. However, it is not necessary that the maximum value Δ be zero. In this embodiment, when the difference of the center positions of the strokes of the BD objective lens  21  and the DVD objective lens  22  is represented by ΔST 1 , it is preferred that the lens holder  26  hold the BD objective lens  21  and the DVD objective lens  22  such that the following relationship is satisfied.
 
ΔST1&lt;L1  (7)
 
     Instead, when the difference between the center positions of the strokes of the DVD objective lens  22  and the CD objective lens  23  is represented by ΔST 2 , it is preferred that the lens holder  26  hold the DVD objective lens  22  and the CD objective lens  23  such that the following relationship is satisfied.
 
ΔST2&lt;L2  (8)
 
     Instead, when the difference between the center positions of the strokes of the CD objective lens  23  and the BD objective lens  21  is represented by ΔST 3 , it is preferred that the lens holder  26  hold the CD objective lens  23  and the BD objective lens  21  such that the following relationship is satisfied.
 
ΔST3&lt;L3  (9)
 
     When expressions (7), (8), and (9) are applied to the example shown in  FIGS. 6 and 7 , the value of L 1 ≈0.31 is the minimum value. Thus, it is most preferred that expression (7) be satisfied. 
     When the center positions of the strokes of the BD objective lens  21  and the DVD objective lens  22  deviate from the center position of the stroke of the CD objective lens  23  for example by around 0.1 mm, the performance of the actuator  8  and the effect of which the thickness of the actuator  8  is decreased are the same as those in the case that ΔST is zero. In other words, the specified warping values of the BD and DVD are smaller than that of the CD. Thus, the amount of surface fluctuation of each of the BD and DVD is smaller than that of the CD. The maximum specified warping value of the BD and DVD is ±0.3 mm, whereas the maximum specified warping value of the CD is ±0.4 mm. In other words, although the BD objective lens  21  or the DVD objective lens  22  deviates from the center position of the stroke of the CD objective lens  23  by 0.1 mm, the center position of the stroke of the BD objective lens  21  or DVD objective lens  22  is in the stroke range of the CD whose amount of surface fluctuation is the largest. 
       FIG. 8  is a plan view showing an optical pickup apparatus  106  according to another embodiment of the present invention.  FIG. 9  is a side view showing a peripheral portion of a light collecting device  35  of the optical pickup apparatus  106 . Description of members and functions of the optical pickup apparatus  106  similar to those of the optical pickup apparatus  6  shown in  FIG. 2  and  FIG. 4  will be simplified or omitted. Only members and functions of the optical pickup apparatus  106  different from those of the optical pickup apparatus  6  will be described. 
     In the light collecting device  35  of the optical pickup apparatus  106 , a DVD objective lens  32  and a CD objective lens  33  are integrally cast as a DVD/CD objective lens unit  34 . In this case, the objective lenses  32  and  33  may be made of resin or glass. The DVD/CD objective lens unit  34  and the BD objective lens  21  are independent members. These objective lenses are integrally held by a lens holder  36 . In this structure, the distance between the two objective lenses  32  and  33  can be shortened. As a result, the sizes of the light collecting device  35  and the optical pickup apparatus  106  can be decreased. In addition, since they are integrally cast, when the light collecting device  35  is manufactured, the mounting position accuracies and tilt accuracies of the two objective lenses  32  and  33  are improved. 
     Thus, not only the DVD objective lens  32  and the CD objective lens  33  are integrally cast, but all the BD, DVD, and CD objective lenses may be integrally cast. Instead, the BD and CD objective lenses may be integrally cast, whereas the DVD objective lens may be independently disposed. 
       FIG. 10  is a plan view showing an optical pickup apparatus  206  according to another embodiment of the present invention.  FIG. 11A  is a sectional view taken along line A-A of  FIG. 10 .  FIG. 11B  is a sectional view taken along line B-B of  FIG. 10 . In a light collecting device  40  of the optical pickup apparatus  206 , a BD objective lens  21  and a CD objective lens  23  are aligned in the radial direction of the optical disc  1 . In addition, the CD objective lens  23  and a DVD objective lens  22  are aligned in the tangential direction of the optical disc  1 . The BD, DVD, and CD objective lenses  21 ,  22 , and  23  are integrally held by a lens holder  46 . 
     In the optical pickup apparatus  206 , the positions of the single wavelength laser diode  90  and the laser coupler  92  are reversed from those shown in  FIG. 2  due to the positions of the objective lenses  21 ,  22 , and  23 . In addition, in the optical pickup apparatus  206 , a light collecting lens  41 , a mirror  42 , and so forth are disposed. 
     In this embodiment, depending on designing conditions such as the structure, arrangement, and so forth of each part that composes the light collecting device  40  or designing conditions of an optical disc driving apparatus that mounts the optical pickup apparatus  206 , the positions of the objective lenses  21 ,  22 , and  23  of the light collecting device  40  can be laid out. For example, in  FIG. 10 , the positions of the BD objective lens  21  and the CD objective lens  23  may be reversed. Likewise, the positions of the CD objective lens  23  and the DVD objective lens  22  may be reversed. Likewise, the positions of the BD objective lens  21  and the DVD objective lens  22  may be reversed. Instead, the objective lenses  21 ,  22 , and  23  may be aligned at an angle to the radial direction. 
       FIG. 12  is a side view showing a light collecting device according to another embodiment of the present invention. In the light collecting device, a BD objective lens is disposed at the center of objective lenses. In this example, a CD objective lens  23 , a BD objective lens  21 , and a DVD objective lens  22  are held by a lens holder  26  such that they are successively aligned on the near side of a light source (not shown). In this case, as shown in  FIG. 4 , a prism  86  and a mirror  83  may be disposed. In  FIG. 12 , one prism  75  may be disposed. A λ/4 plate  76  is disposed between the prism  75  and each of the objective lenses  21 ,  22 , and  23 . 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alternations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 
     In the optical system  30  shown in  FIG. 2  and  FIG. 4 , a part of optical paths of the first, second, and third laser lights is shared. Instead, the optical system  30  may be structured such that the optical paths of the first, second, and third laser lights are independent from each other. 
     The DVD objective lens  22  of the light collecting device  20  shown in  FIG. 4  may be used for an HD DVD objective lens. In other words, the optical pickup apparatus  6  can deal with optical discs corresponding to four formats with three objective lenses  21 ,  22 , and  23 . The NA and thickness of the cover layer that are important design conditions for the DVD objective lens are nearly the same as those of the HD DVD objective lens. Thus, when a wavelength selective hologram is used, the DVD objective lens  22  can be used in common with the HD DVD objective lens.