Abstract:
An optical pickup apparatus which can be designed compact while having a plurality of semiconductor lasers. The optical pickup apparatus, which reads recorded information from an optical recording medium, comprises an optical system including light intensity detection element having a quarter-split light receiving surface, two semiconductor lasers for emitting light beams of different wavelengths, an objective lens for directing each of the light beams onto an optical recording medium to form a light spot on a recording surface, and a holographic optical element located between the quarter-split light receiving surface and the objective lens, whereby the holographic optical element eliminates coma aberration and spherical aberration of light beams launched from the two semiconductor lasers and traveled through the recording surface and the objective lens, thereby generating a predetermined amount of astigmatism. The holographic optical element further has a lens performance for converging light beams, launched from the two semiconductor lasers and traveled through the recording surface and the objective lens, onto the quarter-split light receiving surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pickup in an optical recording and reproducing apparatus. 
     2. Description of the Related Art 
     Optical recording and reproducing apparatuses include an optical disk player which reads recorded information from an optical recording medium or an optical disk, such as a so-called LD (Laser Disc), CD (Compact Disc) or DVD (Digital Video Disc). There also is a compatible disk player which reads information from those different kinds of optical disks. 
     An optical pickup of that compatible disk player like an ordinary one has an optical system which irradiates a light beam to an optical disk and reads return light from the optical disk. 
     Those optical disks as optical information recording media are designed with different specifications including the numerical aperture NA, the thickness of the substrate and the optimal wavelength of read light. Implementation of an optical pickup for a compatible player for LD, CD and DVD therefore requires that at least two differences in the numerical aperture NA and substrate thickness should be compensated. 
     For example, a two-focus pickup using a holographic lens (disclosed in Japanese Patent No. 2532818 and Japanese Patent Application Kokai No. Hei 7-98431) has a composite objective lens, which includes a convex objective lens and a holographic lens, and a diffraction grating with concentric ring-shaped recesses and projections, i.e., diffraction grooves, provided on a transparent plate of the holographic lens, and the performance of a concave lens is imparted on this transparent plate to form a focal point on the recording surface in accordance with each optical disk. At this time, the light beam directly passes through the area where the diffraction grooves are not formed, and converges, together with the zero-order diffraction light, onto the objective lens, resulting in differences in numerical aperture between the transmitted light and the zero-order diffraction light and the first-order diffraction light. The first-order diffraction light that has been diffracted by the diffraction grooves is used to read information from a CD which has a small numerical aperture, and the transmitted light and the zero-order diffraction light which have larger numerical apertures are used to read information from a DVD. 
     This conventional compatible player is designed to form read spots by means of a single, common light source. Generally, a light source which launches read light having a wavelength of 650 nm suitable for reproduction of a DVD is also used to play back a CD. To play back a CD-R (CD Recordable or R-CD (Recordable CD)), which can be written once by a light source with a wavelength of 780 nm, by using this read light, therefore, satisfactory reproduction signals cannot be acquired due to the insufficient sensitivity that results from a difference in wavelength. 
     To realize a compatible player capable of adequately recording and reproducing information on, and from, a CD-R as well as an LD, CD and DVD, it is essential to cope with at least three differences in numerical aperture NA, substrate thickness and the wavelength of the light source in use (780-nm type and 650-nm type). To implement a compatible player for an LD, CD, DVD and CD-R, therefore, it is necessary to design an optical pickup or head using a light source of multiple wavelengths suitable for the respective disks, not a light source of a single wavelength. 
     Constructing an optical system like a prism or lens using a plurality of light sources, however, complicates and enlarges the whole optical pickup or head. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an optical pickup apparatus which can be designed compact and has a holographic optical element suitable for an astigmatism scheme that employs light intensity detection means having a quarter-split light receiving surface. 
     According to this invention, an optical pickup apparatus for reading recorded information from an optical recording medium includes an optical system which comprises: light intensity detection means having a quarter-split light receiving surface; two semiconductor lasers for emitting light beams of different wavelengths; an objective lens for directing each of the light beams onto an optical recording medium to form a light spot on a recording surface; and a holographic optical element located between the quarter-split light receiving surface and the objective lens, wherein the holographic optical element eliminates coma aberration and spherical aberration of a light beam traveled through the recording surface and the objective lens, thereby generating a predetermined amount of astigmatism. 
     In the optical pickup apparatus, the holographic optical element may further have a lens performance for converging a light beam, traveled through the recording surface and the objective lens, onto the quarter-split light receiving surface. 
     According to another aspect of the invention, the optical pickup apparatus is characterized in that the holographic optical element passes a light beam of a first wavelength launched from one of the semiconductor lasers, guides zero-order diffraction light of the light beam to the objective lens, diffracts the zero-order diffraction light of the first wavelength traveled through the recording surface, and guides positive first-order diffraction light, acquired by diffraction, to the quarter-split light receiving surface. 
     According to a further aspect of the invention, the optical pickup apparatus is characterized in that the other one of the semiconductor lasers is located at such a position that the holographic optical element passes a light beam of a second wavelength launched from the other semiconductor laser, guides zero-order diffraction light of the light beam to the objective lens, diffracts the zero-order diffraction light of the second wavelength traveled through the recording surface, and guides positive first-order diffraction light, acquired by diffraction, to the quarter-split light receiving surface. 
     According to a still further aspect of the invention, the optical pickup apparatus is characterized in that the other one of the semiconductor lasers is located at such a position that the holographic optical element passes a light beam of a second wavelength launched from the other semiconductor laser, guides negative first-order diffraction light of the light beam to the objective lens, receives return light from a light spot on the recording surface, produced by the negative first-order diffraction light of the second wavelength, from the objective lens, diffracts the return light, and guides positive firstorder diffraction light of the second wavelength, acquired by diffraction, to the quarter-split light receiving surface; and 
     that the optical system further includes an aberration correcting element, located between the holographic optical element and the objective lens, for passing the light beam of the second wavelength launched from the other semiconductor laser without acting on the light beam of the first wavelength, and guiding the light beam of the second wavelength to the objective lens while eliminating aberration from the light beam of the second wavelength. 
     According to a further aspect of the invention, the optical pickup apparatus is characterized in that the other one of the semiconductor lasers is located at such a position that the holographic optical element passes a light beam of a second wavelength launched from the other semiconductor laser, guides negative first-order diffraction light of the light beam to the objective lens, receives return light from a light spot on the recording surface, produced by the negative first-order diffraction light of the second wavelength, from the objective lens, diffracts the return light, and guides positive first-order diffraction light of the second wavelength, acquired by diffraction, to the quarter-split light receiving surface; and 
     that the optical system further includes a light-source side aberration correcting element, located between the other semiconductor laser and the holographic optical element, for passing the light beam of the second wavelength launched from the other semiconductor laser, giving the light beam of the second wavelength such aberration as to cancel aberration, which occurs at a time the light beam of the second wavelength passes the holographic optical element, and guiding the light beam of the second wavelength to the holographic optical element, thereby eliminating aberration from the light beam of the second wavelength having passed the holographic optical element. 
     According to a further aspect of the invention, the optical pickup apparatus may be characterized in that the optical system further includes a second quarter-split light receiving surface; and 
     that the other one of the semiconductor lasers is located at such a position that the holographic optical element passes a light beam of a second wavelength launched from the other semiconductor laser, guides zero-order diffraction light of the light beam to the objective lens, diffracts the zero-order diffraction light of the second wavelength traveled through the recording surface, and guides positive first-order diffraction light, acquired by diffraction, to the second quarter-split light receiving surface. 
     According to a further aspect of the invention, the optical pickup apparatus may be characterized in that the optical system further includes a second quarter-split light receiving surface; 
     that the other one of the semiconductor lasers is located at such a position that the holographic optical element passes a light beam of a second wavelength launched from the other semiconductor laser, guides negative first-order diffraction light of the light beam to the objective lens, receives return light from a light spot on the recording surface, produced by the negative first-order diffraction light of the second wavelength, from the objective lens, diffracts the return light, and guides positive first-order diffraction light of the second wavelength, acquired by diffraction, to the quarter-split light receiving surface; and 
     that the optical system further includes an aberration correcting element, located between the holographic optical element and the objective lens, for passing the light beam of the second wavelength launched from the other semiconductor laser without acting on the light beam of the first wavelength, and guiding the light beam of the second wavelength to the objective lens while eliminating aberration from the light beam of the second wavelength. 
     According to a further aspect to the invention, the optical pickup apparatus is characterized in that the optical system further includes a second quarter-split light receiving surface; 
     that the other one of the semiconductor lasers is located at such a position that the holographic optical element passes a light beam of a second wavelength launched from the other semiconductor laser, guides negative first-order diffraction light of the light beam to the objective lens, receives return light from a light spot on the recording surface, produced by the negative first-order diffraction light of the second wavelength, from the objective lens, diffracts the return light, and guides positive first-order diffraction light of the second wavelength, acquired by diffraction, to the second quarter-split light receiving surface; and 
     that the optical system further includes a light-source side aberration correcting element, located between the other semiconductor laser and the holographic optical element, for passing the light beam of the second wavelength, giving the light beam of the second wavelength launched from the other semiconductor laser such aberration as to cancel aberration, which occurs at a time the light beam of the second wavelength passes the holographic optical element, and guiding the light beam of the second wavelength to the holographic optical element, thereby eliminating aberration from the light beam of the second wavelength having passed the holographic optical element. 
     According to a further aspect of the invention, the optical pickup apparatus having the aberration correcting element is characterized in that the aberration correcting element may be a liquid crystal type aberration correcting element having transparent electrodes formed on inner surfaces of a pair of transparent glass substrates in a pattern corresponding to an aberration correcting wave surface, and a liquid crystal layer provided between the transparent electrodes, and can be enabled or disabled selectively by applying a voltage to the transparent electrodes in association with switching between the semiconductor lasers. 
     According to a further aspect of the invention, the optical pickup apparatus having the aberration correcting element is characterized in that the aberration correcting element is an aberration correcting element of a non-linear optical material type having a transparent substrate of a wavelength-selectable non-linear optical material and an isotropic optical material, filled in recesses formed on the transparent substrate and having a pattern corresponding to an aberration correcting wave surface, and having a refractive index equal to a refractive index of extraordinary ray or a refractive index of ordinary ray of the non-linear optical material, and can be enabled or disabled selectively by applying a voltage to the transparent electrodes in association with switching between the semiconductor lasers. 
     As this invention can permit light paths to be shared by a holographic optical element common to a plurality of semiconductor lasers, the optical system of the optical pickup can be simplified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic structural diagram showing the interior of an optical pickup apparatus according to a first embodiment of this invention; 
     FIGS. 2A through 2C are plan views of a quarter-split light receiving surface of a photodetector in this embodiment; 
     FIG. 3 is a flow chart illustrating how to design a wave surface of a holographic optical element of the optical pickup apparatus according to the first embodiment of this invention; 
     FIG. 4 is a schematic diagram showing the design of a wave surface of the holographic optical element of the optical pickup apparatus according to the first embodiment of this invention; 
     FIG. 5 is a schematic diagram depicting the design of a wave surface of the holographic optical element of the optical pickup apparatus according to the first embodiment of this invention; 
     FIG. 6 is a schematic diagram showing the design of a wave surface of the holographic optical element of the optical pickup apparatus according to the first embodiment of this invention; 
     FIG. 7 is a schematic diagram showing the design of an optical system of the optical pickup apparatus according to the first embodiment of this invention; 
     FIG. 8 is a schematic perspective view of the interior of an optical pickup apparatus according to a second embodiment of this invention; 
     FIG. 9 is a schematic diagram showing the design of an optical system of the optical pickup apparatus according to the second embodiment of this invention; 
     FIG. 10 is a flow chart illustrating the design of the optical system of the optical pickup apparatus according to the second embodiment of this invention; 
     FIG. 11 is a schematic perspective view of the interior of an optical pickup apparatus according to a third embodiment of this invention; 
     FIG. 12 is a schematic diagram showing the design of an optical system of the optical pickup apparatus according to the third embodiment of this invention; 
     FIG. 13 is a schematic perspective view depicting the interior of an optical pickup apparatus according to a fourth embodiment of this invention; 
     FIG. 14 is a schematic diagram showing the design of an optical system of the optical pickup apparatus according to the fourth embodiment of this invention; 
     FIG. 15 is a schematic perspective view of the interior of an optical pickup apparatus according to a fifth embodiment of this invention; 
     FIG. 16 is a schematic diagram showing the design of an optical system of the optical pickup apparatus according to the fifth embodiment of this invention; 
     FIG. 17 is a schematic partly cross-sectional view of an aberration correcting element of the optical pickup apparatus according to the second embodiment of this invention; and 
     FIG. 18 is a schematic partly cross-sectional view showing another aberration correcting element of the optical pickup apparatus according to the second embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of this invention will now be described with reference to the accompanying drawings. 
     First Embodiment 
     FIG. 1 schematically shows an optical pickup of a recording and reproducing apparatus according to a first embodiment. The pickup body accommodates a semiconductor laser LD 1  (wavelength of, for example, 650 nm; λ1) for reading information from a DVD and a semiconductor laser LD 2  (wavelength of, for example, 780 nm; λ2) for reading information from a CD, both mounted on a common heat sink (not shown) in such a way as to launch light beams upward. Further provided in the pickup body are a holographic optical element  50 , an objective lens  4  whose optical axis is common to that of the holographic optical element  50  and which converges a light beam onto an optical disk  5  to form a light spot, and a quarter-split light receiving surface PD 1  of a photodetector which receives reflected light from the light spot. The objective lens  4 , the holographic optical element  50  and a substrate  10  are arranged approximately in parallel to one another. 
     The heat sink for the semiconductor lasers LD 1  and LD 2  is secured onto the substrate  10  on which the quarter-split light receiving surface PD 1  of the photodetector for detecting the intensity of light is formed under the holographic optical element  50 . 
     The objective lens  4  can be constructed to absorb differences in the disk&#39;s thickness and numerical aperture in accordance with the specifications of a CD and DVD by, for example, selecting a combination of a condenser lens and a Fresnel lens or a holographic lens, switching two objective lens, which match for the respective specifications, from one to the other as needed, or providing a condenser lens for a DVD with some means for restricting the aperture at the time of playing back a CD. Alternatively, the condenser lens itself may be a two-focus objective lens designed for both a CD and DVD. 
     The pickup body is further provided with an objective lens drive mechanism  26  which includes a tracking actuator and a focus actuator. The focus actuator of this objective lens drive mechanism  26  moves the objective lens  4  in a direction perpendicular to the recording surface of the optical disk  5 , and the tracking actuator moves the objective lens  4  in the radial direction of the optical disk  5 . The objective lens drive mechanism  26  includes a slider mechanism for coarse movement in the radial direction. 
     The optical pickup apparatus, as apparent from the above, has a common optical system which guides the light beam from each semiconductor laser on the optical disk  5  via the finite objective lens  4  to form a light spot on the recording surface, converges the return light from the light spot via the objective lens  4  and guides the light to the quarter-split light receiving surface PD 1 . 
     The holographic optical element  50  is so designed and arranged as to direct the light beams, launched from the semiconductor lasers LD 1  and LD 2  along a substantially common light path. The holographic optical element  50  has a general shape of a plate with a transparent diffraction grating  50   b  (which may be a refractive index distribution type or relief type and will be called “diffraction relief” hereinafter) defined on one major surface of a plate  50   a  which is made of a transparent isotropic or anisotropic material. The diffraction relief  50   b  of the holographic optical element  50  directly passes the light beams from the semiconductor lasers LD 1  and LD 2  without demonstrating anything but a diffracting action on the light beams, eliminates coma aberration and spherical aberration of the light reflected on the information recording surface of the optical disk (i.e., return light) and generates a predetermined amount of astigmatism. In addition, the holographic optical element  50  has a lens performance to convert the return light to have a wave surface with varied image forming distances. 
     Recording and reproduction of this optical pickup apparatus will now be briefly explained. As shown in FIG. 1, in playing back a DVD, the laser beam from the semiconductor laser LD 1  is condensed on the optical disk  5  with the numerical aperture NA of 0.6 according to the set distance between the objective lens  4  and the optical disk  5 , thereby forming a small light spot. At the time of playing back a CD, or recording or playing back a CD-R, the laser beam from the semiconductor laser LD 2  forms a light spot on the optical disk  5  with the numerical aperture NA of 0.47 through the objective lens  4 , according to the set distance between the objective lens  4  and the optical disk  5 . 
     Each return light from the light spot on the recording surface of the optical disk  5  enters the holographic optical element  50  via the objective lens  4  and is diffracted there, and an acquired first-order diffraction light component reaches the quarter-split light receiving surface PD 1  of the photodetector. 
     The quarter-split light receiving surface PD 1 , as shown in FIG. 2, consists of four elements DET 1  to DET 4  of the first to fourth quadrants, which are arranged adjacent to one another with respect to two perpendicular segmenting lines L 1  and L 2  and are independent of one another. The quarter-split light receiving surface PD 1  is arranged, together with the semiconductor lasers, on the substrate  10  in line so that one of the segmenting lines is parallel to the track direction (also called tangential (TAN) direction) of the recording surface and the other in parallel to the radial (also called RAD direction) of the optical disk. The segmenting lines coincide with the segmenting directions in the case where tracking servo is carried out according to a retardation method (time difference method). 
     With the light beam in focus on the recording surface, a light spot SP of a complete circle whose intensity distribution is symmetrical with respect to the center O of the quarter-split light receiving surface PD 1  or symmetrical in the track direction and radial direction as shown in FIG. 2A is formed on the quarter-split light receiving surface PD 1 . Therefore, a value acquired by adding the photoelectrically converted outputs of the elements on one diagonal line becomes equal to a value obtained by adding the photoelectrically converted outputs of the elements on the other diagonal line, so that the focus error component becomes “0”. With the light beam in out of focus, on the other hand, a light spot SP of an ellipse in a diagonal direction as shown in FIG. 2B or  2 C is formed on the quarter-split light receiving surface PD 1 . Therefore, focus error components acquired by adding the photoelectrically converted outputs of the elements on each diagonal line are opposite in phase. In other words, with DET 1  to DET 4  denoting the corresponding outputs, (DET 1 +DET 3 )−(DET 2 +DET 4 ) becomes a focus error signal. Further, (DET 1 +DET 4 )−(DET 2 +DET 3 ) becomes a tracking error signal, and DET 1 +DET 2 +DET 3 +DET 4  becomes an RF signal. 
     When a spot image is formed near the center of the quarter-split light receiving surface PD 1 , the photodetector supplies an electric signal according to the spot image formed on the individual four receiving surfaces to a demodulating circuit  32   a  and an error detecting circuit  32   b . The demodulating circuit  32   a  produces a record signal based on that electric signal. The error detecting circuit  32   b  generates a focus error signal, a tracking error signal, other servo signals, etc. based on the electric signal, and supplies the individual drive signals to the respective actuators via an actuator driving circuit  33  to carry out servo control on the objective lens  4  and other associated components in accordance with those drive signals. 
     Design of Holographic Optical Element 
     The holographic optical element may be designed in the following manner by a computer aided scheme. 
     One way of determining the wave surface of the holographic optical element will be explained with reference to a flow chart in FIG.  3 . 
     First, in step S 1 , it is assumed that a plane-parallel plate  70  (refractive index n) having a thickness t 1  is placed, with its optical axis set perpendicularly, in the light path of rays of light (wavelength λ1), emitted from one point A corresponding to a semiconductor laser, as shown in FIG.  4 . The coordinates of the point A and the initial values for the parameters λ1, t 1  and n are set. 
     For the diverging rays after passing the plane-parallel plate  70 , its wave surface including spherical aberration at the coordinates of a position B is computed and the result is saved. 
     The spherical aberration of the diverging rays produced by the plane-parallel plate  70  is corrected and removed in the next step S 2 . The amount of produced astigmatism can be adjusted by changing the thickness t 1  of the plane-parallel plate  70 . 
     In step S 2 , as the rays are returned in the converging or reverse direction from the saved wave surface at the position B, the rays converge on one point A after passing the plane-parallel plate of the thickness t 1 , as shown in FIG.  5 . 
     With two plane-parallel plates  71  (refractive index n) of a thickness t 2 , instead of the plane-parallel plate  70 , set apart from each other and inclined at angles of ⊖ degrees and −⊖ degrees with respect to a plane perpendicular to the optical axis in such a way as to have a mirror image relation to that plane, the wave surface of rays of light at a position C after passing the plane-parallel plates is computed. For this purpose, the coordinates of the position C and the parameters t 2 , ⊖ and −⊖ are introduced. 
     In this case, as the rays are returned in the reverse direction from the position B, the wave surface that passes those plane-parallel plates  71  arranged in a pattern like the inverted “V” do not include coma aberration though they have astigmatism and spherical aberration. The spherical aberration can be adjusted by changing the thickness t 2  of the plane-parallel plates  71 . It is thus possible to cancel out the spherical aberration that has been produced in step S 1 . This way, it is possible to calculate a wave surface of the rays of light with a predetermined amount of astigmatism but without any coma aberration or spherical aberration at a position C after passing the two plane-parallel plates  71  arranged in a pattern like the inverted “V”. The obtained wave surface at the position C will be saved. 
     In the next step S 3 , a wave surface at a certain inclined (angle α) position H of rays of light that are returned again in the reverse direction from the saved wave surface at the position C to diverge, as shown in FIG. 6, is computed. Here, a point on which rays of light converge from the saved wave surface at the position C corresponds to the position of the quarter-split light receiving surface PD 1 . 
     At this position H (holographic optical element), interference is caused between the saved wave surface and the wave surface of rays of light (wavelength λ1) which diverge from a given point 0 (semiconductor laser LD 1 ), producing interference fringes at that position H. The grating pattern of the holographic optical element of this embodiment can be designed by saving the interference fringes. For this purpose, the coordinates of H and 0 and the parameter a are introduced. 
     Forming this grating pattern on the transparent substrate can provide the holographic optical element  50  (shown in FIG. 1) which eliminates coma aberration and spherical aberration, produces a predetermined amount of astigmatism and has its image forming distance varied by the lens performance given to the element  50 . 
     In this design example, various optimal values for the holographic optical element can be set by changing the specifications of the semiconductor lasers and the parameters like the wavelength, the distances among the points and the refractive index besides t 1  and t 2 . 
     As apparent from the above, it is possible to determine the grating pattern of the holographic optical element  50  which passes incident light unaberrated except for astigmatism in a process up to step S 3 . 
     The next step S 4  designs a system of guiding the diverging light beam from the semiconductor laser LD 1  to the objective lens  4  through the holographic optical element  50 , then converging the light beam on the optical disk  5 , allowing the reflected light to reach the quarter-split light receiving surface PD 1  via the objective lens  4  and the holographic optical element  50 , and likewise allowing the diverging light beam from the additional semiconductor laser LD 2  to reach the quarter-split light receiving surface PD 1  in a similar manner. 
     First, the light beam from the semiconductor laser LD 1  of the wavelength λ passes the holographic optical element  50  undiffracted, as zero-order diffraction light along the forward path, and in the return path after reflection, the diffraction light is diffracted by the holographic optical element  50  and its positive first-order diffraction light converges on the quarter-split light receiving surface PD 1 . Here, the holographic optical element  50  passes the light beam of the wavelength λ1, launched from the semiconductor laser LD 1 , guides its zero-order diffraction light to the objective lens  4 , receives from the objective lens  4  the return light from the light spot on the recording surface  5 , formed by the zero-order diffraction light of the wavelength λ1, diffracts the return light, and guides the diffracted first-order diffraction light of the wavelength λ1 to the quarter-split light receiving surface PD 1 . 
     Meanwhile, the light beam from the semiconductor laser LD 2  of the wavelength λ2 likewise passes the holographic optical element  50 , undiffracted, as zero-order diffraction light in the forward path, but in the return path, the diffraction angle of the diffraction light is changed by the holographic optical element  50  because its wavelength λ2 differs from that of the semiconductor laser LD 1 . 
     Therefore, with the position of the quarter-split light receiving surface PD 1  fixed, the light path of the semiconductor laser LD 2  of the wavelength λ2 is set by altering the position of the semiconductor laser LD 2  on the substrate  10  in such a way that the light beam from the semiconductor laser LD 2  comes to the fixed position of the quarter-split light receiving surface PD 1 . 
     In short, after setting the positions of the semiconductor laser LD 1  and the quarter-split light receiving surface PD 1  in the optical system in advance, the position of the semiconductor laser LD 2  is set in the above-discussed manner. 
     Second Embodiment 
     An optical pickup apparatus which is the same as that of the first embodiment, except for the position of the semiconductor laser LD 2  on the substrate  10  and an aberration correcting element  80  provided between the holographic optical element  50  and the objective lens  4  as shown in FIG. 8, can be provided by the combination of the semiconductor laser LD 1 , the quarter-split light receiving surface PD 1  and the holographic optical element  50 , which are designed by the calculations in the steps S 1  to S 3 . 
     Let us first consider the case where the light from the semiconductor laser LD 2  is diffracted by the holographic optical element  50  to become negative first-order diffraction light and no aberration correcting element  80  is present. 
     As shown in FIG. 9, the semiconductor laser LD 2  is arranged on the substrate at such a position that the holographic optical element  50  passes the light beam of the wavelength λ2, launched from the semiconductor laser LD 2 , guides its negative first-order diffraction light to the objective lens  4 , receives from the objective lens  4  the return light from the light spot on the recording surface  5 , formed by the negative first-order diffraction light of the wavelength λ2, diffracts the return light, and guides the resulting positive first-order diffraction light of the wavelength λ2 to the quarter-split light receiving surface PD 1 . 
     Because the holographic optical element  50  is designed by causing interference between the light of the wavelength λ1 from the semiconductor laser LD 1  and the return light which converges on the quarter-split light receiving surface PD 1 , the negative first-order diffraction light of the light of the wavelength λ2 from the semiconductor laser LD 2  will have aberration in the forward path between the holographic optical element  50  and the objective lens  4 . 
     To correct this aberration, the aberration correcting element  80  is arranged between the holographic optical element  50  and the objective lens  4 . This aberration correcting element  80  does not act at all when the light beam from the semiconductor laser LD 1  passes there, and converts the light from the semiconductor laser LD 2  which has passed the holographic optical element  50  to unaberrated light. That is, the aberration correcting element  80  passes the light beam of the wavelength λ2, launched from the semiconductor laser LD 2 , eliminates aberration from the light beam and guides the resultant light beam to the objective lens  4 , while taking no action on the light beam of the wavelength λ1. 
     This aberration correcting element  80  can be realized by electrically switching a liquid crystal device as shown in FIG.  17 . 
     The liquid crystal type aberration correcting element  80  in FIG. 17 has transparent electrodes  83  and  84  a pattern corresponding to an aberration correcting wave surface on the respective inner surfaces of a pair of transparent glass substrates  81  and  82 , and a liquid crystal layer  85  provided between the transparent electrodes  83  and  84 . When a voltage is applied to the liquid crystal layer  85  via the transparent electrodes  83  and  84 , liquid crystal molecules are inclined from the state where no voltage is applied. Using this phenomenon, the aberration correcting element  80  can selectively be rendered to act, or not to act, on the rays of light of the wavelengths λ1 and λ2 from the semiconductor lasers LD 1  and LD 2 . When the polarization direction of the incident light is perpendicular to the alignment of the liquid crystal molecules, for example, no diffraction occurs and the element  80  does not operate as a liquid crystal type aberration correcting element. When the polarization direction of the incident light is parallel to the alignment of the liquid crystal molecules, on the other hand, diffraction occurs and the aberration correcting element  80  acts as a liquid crystal type aberration correcting element. That is, selective application of a voltage or no voltage sets the inclined and uninclined portions of the liquid crystal molecules into a pattern corresponding to the aberration correcting wave surface, resulting in variations in the refractive index of the liquid crystal layer and the length of the light path of the incident light, so that the element  80  works as an aberration correcting element. The reverse setting is also possible. Furthermore, because the inclination of the liquid crystal molecules can be controlled in accordance with the voltage applied to the liquid crystal layer, the amount of aberration correction of the aberration correcting element can be controlled arbitrarily. 
     If, as shown in FIG. 18, a wavelength-selectable non-linear optical material like lithium niobate is used for a transparent substrate  181 , and the pattern corresponding to the aberration correcting wave surface is etched to form recesses  182 , which are then filled with an isotropic optical material  183  whose refractive index is equal to the refractive index of extraordinary ray or the refractive index of ordinary ray of the non-linear optical material, it is possible to select the enabled state or the disabled state of the non-linear optical material type aberration correcting element  80  depending on the difference in wavelength, λ1 or λ2, between the semiconductor lasers LD 1  and LD 2 . 
     Specific procedures for designing this optical system will be discussed below. First, the optimal shape of the grating pattern of the holographic optical element  50  is designed through the calculations in the steps S 1  to S 3  among the steps shown in FIG. 10 using the semiconductor laser LD 1  and the quarter-split light receiving surface PD 1 . 
     In step S 5 , the wave surface of the diverging rays of light from the semiconductor laser LD 2 , located at a given coordinate position, on the holographic optical element  50  (negative first-order diffraction) is obtained by ray-tracing using the high refractive index method and the phase function method, followed by the acquisition of the wave surface on the aberration correcting element  80 , to thereby compute the amount of aberration correction of the aberration correcting element  80 . 
     In step S 6 , the wave surface of the diverging rays of light from the semiconductor laser LD 2  on the holographic optical element  50  (negative first-order diffraction), the wave surface of the diverging rays on the aberration correcting element  80 , the wave surface of the diverging rays on the objective lens, the wave surface of the diverging rays on the recording surface of the optical disk, the wave surface of the reflected light on the objective lens, the wave surface of the reflected light on the aberration correcting element  80 , the wave surface of the reflected light on the holographic optical element  50  (positive first-order diffraction), and the wave surface of the reflected light on the quarter-split light receiving surface PD 1  are computed in order. 
     In step S 7 , with the coordinate position of the semiconductor laser LD 2  changed, the amount of aberration on the quarter-split light receiving surface PD 1  is computed and is saved. In step S 8 , the computed aberration amount is compared with a predetermined threshold value, and when the former exceeds the latter, the flow returns to step S 5  to repeat the computation with the coordinate position of the semiconductor laser LD 2  changed. When the computed aberration amount becomes equal to or smaller than the threshold value, the process is terminated. The position of the semiconductor laser LD 2  which minimizes the unnecessary aberration on the quarter-split light receiving surface PD 1  is acquired in the above manner. 
     Further, the optical system of the second embodiment may also be implemented by setting a loop R 1  extending from this step S 8  to step S 1  so that while the position of the semiconductor laser LD 1  on the optical axis and/or the position of the quarter-split light receiving surface PD 1  is changed in steps S 1 -S 3 , the position of the semiconductor laser LD 2  which minimizes the unnecessary aberration on the quarter-split light receiving surface PD 1  is acquired in steps S 5 -S 7 , and the steps S 1 -S 8  are repeated until the acquired position of the semiconductor laser LD 2  leads to the minimized aberration or until the amount of aberration becomes equal to or smaller than the predetermined threshold value. 
     Third Embodiment 
     An optical pickup apparatus which is the same as that of the second embodiment, except for a light-source side aberration correcting element  80   a  instead of the aberration correction element  80  provided between the semiconductor laser LD 2  and the holographic optical element as shown in FIG. 11, can be provided as per the second embodiment. 
     As shown in FIG. 12, in the forward path, the light-source side aberration correcting element  80   a , located between the semiconductor laser LD 2  and the holographic optical element  50 , passes the light beam of the wavelength λ2, launched from the semiconductor laser LD 2 , and gives this light beam such aberration as to cancel aberration, which occurs at the time the light beam passes the holographic optical element  50 , to thereby eliminate aberration from the light beam of the wavelength λ2 which has passed the holographic optical element  50 . When this light-source side aberration correcting element  80   a  is placed between the semiconductor laser LD 2  and the holographic optical element  50 , the parameters of the semiconductor laser LD 1  become irrelevant at the design phase. 
     Fourth Embodiment 
     An optical pickup apparatus which is the same as that of the first embodiment, except for the additional provision of a quarter-split light receiving surface PD 2  as shown in FIG. 13, can be provided by the combination of the semiconductor laser LD 1 , the quarter-split light receiving surface PD 1  and the holographic optical element  50 , which are designed by the calculations in the steps S 1  to S 3 . 
     In this case, the light beam from the semiconductor laser LD 2  of the wavelength λ2 likewise passes the holographic optical element  50  unaberrated as zero-order diffraction light. The semiconductor laser LD 2  and the second quarter-split light receiving surface PD 2  are arranged in such a manner that in the return path, the holographic optical element  50  diffracts the zero-order diffraction light of the second wavelength of the return light that has traveled through the recording surface and guides the positive first-order diffraction light, acquired by the diffraction, to the second quarter-split light receiving surface PD 2 . 
     As apparent from the above, the positions of the set of the f first semiconductor laser LD 1  and the quarter-split light receiving surface PD 1  in the optical system are set first, followed by the setting of the positions of the set of the semiconductor laser LD 2  and the second quarter-split light receiving surface PD 2 . 
     It is also possible to provide a pickup apparatus that has the aberration correcting element  80  as shown by a broken line in FIG. 13, which is located between the holographic optical element and the objective lens and does not act on the light beam of the wavelength λ1, as per the second embodiment, except for the additional provision of the second quarter-split light receiving surface PD 2 . The second quarter-split light receiving surface PD 2  is arranged on the substrate  10  at such a position that the holographic optical element  50  receives from the objective lens  4  the return light from the light spot on the recording surface  5 , which has been formed by the negative first-order diffraction light of the wavelength λ2, diffracts the return light and guides the resulting first-order diffraction light of the wavelength λ2 to the quarter-split light receiving surface PD 2 . 
     With the aberration correcting element  80  provided, as shown in FIG. 14, the semiconductor laser LD 1 , the quarter-split light receiving surface PD 1  and the holographic optical element  50  are arranged in such a way that the holographic optical element  50  passes the light beam of the wavelength λ1, launched from the semiconductor laser LD 1 , guides its zero-order diffraction light to the objective lens  4 , receives from the objective lens  4  the return light from the light spot on the recording surface  5  formed by the zero-order diffraction light of the wavelength λ1, diffracts the return light, and guides the resulting first-order diffraction light of the wavelength λ1 to the quarter-split light receiving surface PD 1 . 
     The semiconductor laser LD 2  is arranged on the substrate  10  at such a position that the holographic optical element  50  passes the light beam of the wavelength λ2, launched from the semiconductor laser LD 2 , and guides its negative first-order diffraction light to the objective lens  4  along substantially the same light path as the zero-order diffraction light of the light beam of the wavelength λ1. 
     As in the second embodiment, the aberration correcting element  80  which does not act on the light beam of the wavelength λ1 is arranged between the holographic optical element  50  and the objective lens  4 . This aberration correcting element  80  passes the light beam of the wavelength λ2, launched from the semiconductor laser LD 2 , eliminates aberration from this light beam and guides the resultant light to the objective lens  4 . 
     As is readily understood from the above, the light from the semiconductor laser LD 2  is diffracted by the holographic optical element  50  to be negative first-order diffraction light, which travels along the same light path as the light from the semiconductor laser LD 1  that travels, undiffracted, toward the objective lens  4 . The return light of the light of the wavelength λ2 from the semiconductor laser LD 2  converges on the second quarter-split light receiving surface PD 2  because the diffraction angle at the holographic optical element  50  differs from that of the light from the semiconductor laser LD 1  of the wavelength λ1. 
     Specific procedures for designing this optical system are similar to those of the second embodiment illustrated in FIG. 10, except that the optimal designs of the amount of aberration and the wave surface of the return light on the second quarter-split light receiving surface PD 2  are carried out in the steps S 5  and S 6  instead of those on the quarter-split light receiving surface PD 1 . 
     Fifth Embodiment 
     An optical pickup apparatus which is the same as that of the fourth embodiment, except for the provision of a light-source side aberration correcting element  80   a  between the semiconductor laser LD 2  and the holographic optical element  50 , as shown in FIG.  15 . 
     As shown in FIG. 16, in the forward path, the light-source side aberration correcting element  80   a , located between the semiconductor laser LD 2  and the holographic optical element  50 , passes the light beam of the wavelength λ2 launched from the semiconductor laser LD 2 , gives this light beam such aberration as to cancel aberration, which occurs at the time the light beam passes the holographic optical element  50 , to thereby eliminate aberration from the light beam of the wavelength λ2 which has passed the holographic optical element  50 . When this light-source side aberration correcting element  80   a  is placed between the semiconductor laser LD 2  and the holographic optical element  50 , the parameters of the semiconductor laser LD 1  become irrelevant at the design phase. 
     Although it is premised in the foregoing description that the objective lens of the optical pickup apparatus of this invention is a finite type, it should be apparent to those skilled in the art that such an objective lens may be replaced with an infinite objective lens, implemented by arranging a condenser lens on an optical recording medium side and arranging a collimator lens on the semiconductor laser side, without sacrificing the advantages.