Abstract:
Disclosed is an optical pickup device having a variable refractive index, comprising a radiating element for radiating light beams; an objective lens for focusing the light beams onto an optical disc; a photodetector for receiving light beams reflected from the optical disc; and a variable refractive index element, connected to an external electrical field, for refracting and transmitting the light beams from the radiating element onto the objective lens.

Description:
INCORPORATION BY REFERENCE  
       [0001]     The present application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2004-0098716 filed on Nov. 29, 2004. The content of the application is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention is in the field of optical pickup devices. More particularly, the present invention relates to an optical pickup device with a variable refractive index, which can read information from an optical disc having multiple recordable layers.  
         [0004]     2. Description of the Related Art  
         [0005]     An optical pickup is a device that writes on or reads from optical discs by radiating light beams onto the optical discs and receiving beams reflected therefrom.  
         [0006]     With the rapid increase in demand for data storage, Blu ray, also known as Blu ray discs (BDs), have been developed as a substitute for CDs and DVDs. In this regard, many advances have been achieved for increasing the recording density of optical discs, and extensive research is actually in progress.  
         [0007]     The recording density of an optical disc is in direct proportion to the numerical aperture (NA) of the objective lens and in inverse proportion to the wavelength of incident beams. Thus, BDs onto which a light beam of 405 nm is radiated and which have an NA of 0.85, are of high interest.  
         [0008]     However, under the conditions of a large numerical aperture and a short wavelength, spherical aberrations due to variation in disc thickness tend to expand, therefore requiring compensation therefor. Reduction of spherical aberrations due to variation in disc thickness is disclosed in Japanese Pat. Publication No. 2004-103110, entitled “Objective lens and optical pickup device”, the structure of which is illustrated in  FIG. 12 .  
         [0009]     As depicted in  FIG. 12 , an objective lens module  200 , which concentrates light beams transmitted from a quarter wave plate (not shown) onto a signal-recording plane  103  on an optical disc  100 , consists of two objective lenses: a first objective lens  201  standing on the side of the quarter wave plate, and a second objective lens  202  positioned at a predetermined distance from the first objective lens  201 , facing the optical disc  100 . The shape and refractive index of the first and second objective lenses  201  and  202 , which determine the total refractive index of the objective lens  200 , are controlled in consideration of the thickness and refractive index of the transparent layer  102  of the optical disc  100  so as to focus the light beams transmitted from the quarter wave plate onto the recording plane  103  of the optical disc  100  without the occurrence of aberration.  
         [0010]     The first objective lens  201  is an aspherical convex lens which has a convex surface toward the quarter wave plate. Upon being incident on the first objective lens  201 , light beams are refracted at the interface between air and the convex part and therefore converge. Upon being emergent from the first objective lens  201 , the convergent light beams are refracted again at the interface between the surface of the lens on the side of the optical disc  100  and the air.  
         [0011]     The second objective lens  202  includes a plano-convex lens  203 , two glass plates  204  and  206 , and a liquid crystal element  205 . In the plano-convex lens  203 , the convex surface is directed toward the first objective lens  201  while the flat plane perpendicular to the optical axis stands on the side of the optical disc  100 . A pair of glass plates  204  and  205  with the liquid crystal element  205  disposed between is positioned in contact with the flat plane of the second objective lens  202 .  
         [0012]     The liquid crystal element  205  is arranged so that its incident and emergent planes are perpendicular to the optical axis of the objective lens module  200 . An electrical field can be applied across the liquid crystal element  205  via electrode films (not shown) provided to both its sides. Upon the application of an external electrical field, the refractive index of the liquid crystal element varies by an amount depending on the magnitude of the voltage. This change in refractive index allows the light beams passing through the liquid crystal element  205  to take a different path so as to correct spherical aberration due to variation in disc thickness.  
         [0013]     In the structure of the conventional optical pickup, however, the light beams refract to such a small extent as to compensate for spherical aberrations due to variation in disc thickness as small as approximately 3 μm because the refractive index variable member is a flat plate type. Thus, the conventional optical pickup device cannot be used where optical discs have large variations in thickness or where multiple recording layers are formed in BDs which are approximately 100 μm thick.  
         [0014]     Besides, most conventional optical pickup devices have complex structures and are heavy because they require additional objective lenses for correcting the spherical aberration caused by variation in disc thickness, in addition to a basic objective lens.  
       SUMMARY OF THE INVENTION  
       [0015]     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an optical pickup device which can expand the range of correction for spherical aberration due to variation in the thickness of optical discs by employing a variable refractive index element in which at least two sheets of liquid crystal portions with a curvature incident and/or emergent surface are included and the refractive index of which varies according to an external electric field applied to the element, in addition to being simple in structure and having a light weight because of requirement for no additional objective lenses.  
         [0016]     In accordance with an aspect of the present invention, there is provided an optical pickup device with a variable refractive index, comprising: a radiating element for radiating light beams; an objective lens for focusing the light beams onto an optical disc; a photodetector for receiving light beams reflected from the optical disc; and a variable refractive index element, connected to an external electrical field, for refracting and transmitting the light beams from the radiating element onto the objective lens, said variable refractive index element having incident and emergent surfaces at least one of which has a predetermined curvature radius and showing a refractive index which varies by an amount depending on the magnitude of the external electrical field applied thereto.  
         [0017]     In accordance with another aspect of the present invention, there is provided an optical pickup device with a variable refractive index, comprising: a radiating element for radiating light beams; a collimate lens for collimating the light beams from the radiating element; an objective lens for focusing the light beams onto an optical disc; a photodetector for receiving light beams reflected from the optical disc; and a variable refractive index element, built in the collimate lens and connected to an external electrical field, for refracting and transmitting the light beams from the radiating element onto the objective lens, said variable refractive index element having incident and emergent surfaces at least one of which has a predetermined curvature radius and showing a refractive index which varies by an amount depending on the magnitude of the external electrical field applied thereto.  
         [0018]     In the optical pickup device, the variable refractive index element is a multilayer structure comprising a plurality of transparent protecting portions and a plurality of liquid crystal portions, in which the transparent protecting portions alternate with the liquid crystal portions while constituting the outermost surfaces of the variable refractive index element with regard to the propagation direction of the light beams, said liquid crystal portions having incident and emergent surfaces at least one of which has a predetermined curvature and showing a refractive index which varies by an amount depending on the magnitude of the external electrical field applied thereto.  
         [0019]     In the optical pickup device, the plurality of liquid crystal portions comprises at least one spherical or aspherical concave lens type-liquid crystal portion, the concave surface of which serves as an incident plane and at least one spherical or aspherical convex lens type-liquid crystal portion, the convex surface of which serves as an emergent plane.  
         [0020]     In the optical pickup device, the plurality of liquid crystal portions comprises a pair of convex lens type-liquid crystal portions which are spherical or aspherical, each having a convex surface serving as an incident plane.  
         [0021]     In the optical pickup device, the plurality of liquid crystal portions comprises at least one meniscus liquid crystal portion which is spherical or aspherical.  
         [0022]     In the optical pickup device, each of the liquid crystal portions has a thickness ranging from 6 to 40 μm and a curvature range of 45.145 to 280 mm, and its refractive index may be linearly changed in the range of 1.5 to 1.72. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0024]      FIG. 1  is a schematic view showing a structure of an optical pickup device according to an embodiment of the present invention;  
         [0025]      FIG. 2  is a schematic cross sectional view showing a variable refractive index element according to the embodiment of  FIG. 1 ;  
         [0026]      FIG. 3  is a schematic cross sectional view showing liquid crystal portions of the variable refractive index element of  FIG. 2 ;  
         [0027]      FIG. 4A  is a schematic cross sectional view showing a state of a molecular arrangement of the liquid crystal portions in the absence of an external electric field;  
         [0028]      FIG. 4B  is a schematic cross sectional view showing a state of a molecular arrangement of the liquid crystal portions in the presence of an external electric field;  
         [0029]      FIG. 5  is a schematic view showing optical paths of the light beams passing through the variable refractive index element of  FIG. 2 ;  
         [0030]      FIG. 6  is a schematic view showing optical paths of the light beams passing through a variable refractive index element according to another embodiment of the optical pickup device of  FIG. 1 ;  
         [0031]      FIG. 7  is a schematic view showing optical paths of the light beams passing through a variable refractive index element according to a further embodiment of the optical pickup device of  FIG. 1 ;  
         [0032]      FIG. 8  is a schematic view showing a structure of an optical pickup device according to another embodiment of the present invention;  
         [0033]      FIG. 9  is a schematic cross sectional view showing a collimate lens according to an embodiment of the optical pickup device of  FIG. 8 ;  
         [0034]      FIG. 10  is a schematic view showing optical paths of the light beams passing through the collimate lens of  FIG. 9 ;  
         [0035]      FIG. 11  is a schematic view showing optical paths of the light beams passing through a collimate lens according to another embodiment of the optical pickup device of  FIG. 8 ; and  
         [0036]      FIG. 12  is a schematic view showing a part of a conventional optical pickup device.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]     Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.  
         [0038]     A description will be given of embodiments of optical pickup devices which are adapted to BDs. If different optical discs, CDs or DVDs, are used, necessary parts such as objective lenses may be modified or supplemented.  
         [0039]     With reference to  FIG. 1 , an optical pickup device  100  according to an embodiment of the present invention is shown which comprises a radiating element  110 , a collimate lens  120 , a polarized beam splitter  130 , a quarter wave plate  140 , variable refractive index elements  10 ,  20  and  30 , an objective lens  150 , a focal lens  170 , and a photodetector  180 .  
         [0040]     After being emitted from the radiating element  110 , 405 nm blue light beams with a plane of polarization are transmitted to the collimate lens  120 .  
         [0041]     In the collimate lens  120 , all of the optical paths that the light beams follow after the light source are made to be almost parallel to the optical axis. The parallel light is directed to the polarized beam splitter  130 .  
         [0042]     The polarized beam splitter  130  allows the parallel light, which is linearly polarized light with a plane of polarization, to pass therethrough, but reflects at a right angle the light reflected from an optical disc. After passing through the polarized beam splitter  130 , the parallel light goes to the quarter wave plate  140  while the light reflected at a right angle from the polarized beam splitter  130  is directed to the focal lens  170 .  
         [0043]     The quarter wave plate  140  rotates the electrical field component of the parallel light incident thereon to turn linearly polarized light into circularly polarized light and vice versa. Thus, the linearly polarized light incident on the quarter wave plate  140  is changed into circularly polarized light which is then transmitted to the variable refractive index elements  10 ,  20  and  30 . On the other hand, the light reflected from the optical disc  160  changes from circularly polarized light to linearly polarized light in the quarter wave plate  140  before being sent to the polarized beam splitter  130 .  
         [0044]     Before traveling to the objective lens  150 , the circularly polarized light incident on the variable refractive index elements  10 ,  20  and  30  undergoes variable refraction because refractive indices of the variable refractive index elements  10 ,  20  and  30  change in accordance with an externally applied electric field.  
         [0045]     Through the objective lens  150 , the circularly polarized light beams transmitted from the variable refractive index elements  10 ,  20  and  30  converge onto a recording plane of a corresponding layer formed on the optical disc  160 . Also, the objective lens  150  serves to collimate the light beams reflected from the optical disc. The collimated light beams pass through the variable refractive index elements  10 ,  20  and  30 , followed by being linear polarized in the quarter wave plate  140 .  
         [0046]     The optical disc BD has a multilayer recording plane structure. Light beams, which are refracted at different indices depending on the external electric field applied to the variable refractive index elements  10 ,  20  and  30 , are focused on an optical spot on the multilayer recording planes in the BD. By controlling the electric field, the optical spot can be accurately formed on a desired recording plane. Thus, it is possible to read the information stored on the multilayer recording planes.  
         [0047]     Positioned between the polarized beam splitter  130  and the photodetector  180 , the focal lens  170  converges the light reflected from the polarized beam splitter  130  into the photodetector  180  which converts the reflected light into electrical signals to read the information.  
         [0048]     The variable refractive index elements applied to the optical pickup device of  FIG. 1  can be structured in various formats which are explained in detail with reference to FIGS.  2  to  7 .  
         [0049]      FIG. 2  shows a liquid crystal module  10 , which comprises triple glass plates  11  and two liquid crystal portions  12  and  13 , each being interposed between the glass plates  11 . In this embodiment, the liquid crystal portions  12  and  13  are concave and convex, respectively.  
         [0050]     On both sides of each of the concave lens type- and the convex lens type-liquid crystal portions  12  and  13  are formed optically transparent electrode films  14  with which an external electric field can be applied across each of the liquid crystal portions  12  and  13 .  
         [0051]     The concave lens type-liquid crystal portion  12 , as shown in  FIG. 3 , has a total thickness of t 1  and is a plano-concave type with one concave surface having a curvature radius of ρ 1  and facing the light source, and one flat surface facing the objective lens. Thus, the concave surface serves as an incident plane while the flat surface serves as an emergent plane.  
         [0052]     On the other hand, the convex lens type-liquid crystal portion is a plano-convex type with a total thickness of t 2  and a curvature radius of ρ 2 . It is disposed in such a manner that the flat surface faces the light source, serving as an incident plane while the convex surface faces the objective lens, serving as an emergent plane.  
         [0053]     Each of the thicknesses t 1  and t 2  of the concave lens type- and the convex lens type-liquid crystal portion  12  and  13  falls into the range of 6 to 40 μm and is preferably 10 μm. As for the curvature radii ρ 1  and ρ 2 , each of them ranges from 45.145 to 280 mm and is preferably 144.5 mm. The thickness of each of the liquid crystal portions  12  and  13  is preferably dependent on the external electric field applied to the liquid crystal portions  12  and  13 . For instance, the thickness is preferably set to be 10 μm if 10V is applied. The thickness is preferably increased to 20 μm with an external electric field of 20 V. Of course, the refractive indices of the liquid crystal portions  12  and  13  vary with the thickness.  
         [0054]     The following descriptions will be made on the basis of liquid crystal portions that are 10 μm thick when account is taken of the fact that an external electric field applied to an optical pickup device generally has a voltage of 10V.  
         [0055]     In the absence of external electric fields, as shown in  FIG. 4A , liquid crystal particles are in a stable state. Under this condition, the liquid crystal portions  12  and  13  show refractive indices n 1 =n 2 =1.72 while the refractive index n a  of air is 1 and the refractive index n g  of the glass plates  11  is 1.5.  
         [0056]     When 5V are applied across each of the liquid crystal portions  12  and  13 , the liquid crystal particles, as shown in  FIG. 4B , are in an unstable state. In this condition, the refractive indices of the liquid crystal portions  12  and  13  are changed to n 1 =n 2 =1.61 while n a =1 and n g =1.5. In the presence of 10V, their, refractive indices are decreased to n 1 =n 2 =1.5 while n a =1 and n g =1.5. The respective change of refractive index from 1.72 to 1.61 and 1.5 at external voltages of 5V and 10V is attributed to the fact that the refractive indices of the liquid crystal portions  12  and  13  change with the applied external voltages in corresponding local regions.  
         [0057]      FIG. 5  depicts the refraction of light beams depending on the external voltages applied to the liquid crystal portions  12  and  13  of the variable refractive index element  10  in the structure of the optical pickup device according to one embodiment of the present invention.  
         [0058]     When no external electric field is applied, that is, when the glass plates  11 , the concave lens type-liquid crystal portion  12  and the convex lens type-liquid crystal portion  13  have refractive indices of 1.5, 1.72 and 1.72, respectively, the parallel light beams incident on the variable refractive index element  10 , as indicated by solid lines, are diverged by the concave lens type-liquid crystal portion  12  and then converged by the convex lens type-liquid crystal portion  13 , followed by the concentration of the light beams by the objective lens  150  to form an optical spot on a recording plane at position L 0  in the multilayer structure of the optical disc  160 .  
         [0059]     On the other hand, when 10 V are applied across the concave lens type-liquid crystal portion  12 , its refractive index is changed to 1.5 while the refractive indices of the glass plates  11  and the convex lens type-liquid crystal portion  13  remain at 1.5 and 1.72, respectively. In this circumstance, the parallel light beams incident on the variable refractive index element  10 , as indicated by dotted lines, travel the concave lens type-liquid crystal portion  12  without alternation of the optical path and then are converged by the convex lens type-liquid crystal portion  13 . Thereafter, the convergent light beams are focused by the objective lens  150  onto an optical spot on a recording plane at position L −1  in the multilayer structure of the optical disc  160 .  
         [0060]     Application of 5V to the concave lens type-liquid crystal portion  12  causes its refractive index to change to 1.61. In this case (not shown), the parallel light beams incident on the variable refractive index element  10  diverge at a smaller angle than when 10 V are applied, and then converge while passing through the convex lens type-liquid crystal portion  13 . The convergent light beams are further converged by the objective lens  150  to form an optical spot on a virtual recording plane present between positions L 0  and L −1  in the optical disc  160 .  
         [0061]     Where an external electric field of 10 V is applied across the convex lens type-liquid crystal portion  13 , that is, where the convex lens type-liquid crystal portion  13  is set to have a refractive index of 1.5 while the refractive indices of the glass plates  11  and the concave lens type-liquid crystal portion  12  remain at 1.5 and 1.72, respectively, the parallel light beams incident on the variable refractive index element  10 , as indicated by dot and dashed lines, are diverged by the concave lens type-liquid crystal portion  12  and then converged by the convex lens type-liquid crystal portion  13  at a smaller angle in relation to when no external electric field is applied. Thereafter, the convergent light beams are focused by the objective lens  150  onto an optical spot on a recording plane at position L +1  in the multilayer structure of the optical disc  160 .  
         [0062]     The application of 5V to the convex lens type-liquid crystal portion  13  causes its refractive index to change to 1.61. In this case (not shown), the parallel light beams incident on the variable refractive index element  10  are diverged by the concave lens type-liquid crystal portion  12  and then converged by the convex lens type-liquid crystal portion  13  at a larger angle than when 10 V are applied. The convergent light beams are further converged by the objective lens  150  to form an optical spot on a virtual recording plane present between positions L 0  and L +1  in the optical disc  160 .  
         [0063]     In  FIG. 6 , a different type of a variable refractive index element  20  is introduced which comprises a pair of convex lens type-liquid crystal portions  22  and  23  in accordance with another embodiment. The variable refractive index element  20  has the same structure as in  FIG. 5 , with the exception that all of the liquid crystal portions are a plano-convex type and thus, a description of this variable refractive index element  20  is omitted.  FIG. 6  depicts the refraction of light beams depending on the external voltages applied to the liquid crystal portions  22  and  23  of the variable refractive index element  20  in the structure of the optical pickup device.  
         [0064]     In the case of employing a pair of convex lens type-liquid crystal portions  22  and  23 , the optical pickup device is preferably designed so that the light beams are slightly divergent upon being incident on the variable refractive index element  20 . To this end, an additional diverging means may be installed or the positional relationship of other related parts may be changed.  
         [0065]     In this embodiment, as shown in  FIG. 6 , the convex lens type-liquid crystal portions  22  and  23  are arranged in such a manner that both of the convex surfaces thereof are directed toward the radiating element (not shown). Of course, other arrangements may be used. For example, an arrangement design may be used in which the convex surface of one convex lens type-liquid crystal portion faces the radiating element while the convex surface of the other convex lens type-liquid crystal portion is directed toward the objective lens  150 . Alternatively, both of the convex surfaces may be arranged to face the objective lens  150 . In order to expand the range of compensation for spherical aberrations due to variation in disc thickness, however, it is preferred that both of the convex surfaces of the convex lens type-liquid crystal portions  22  and  23  stand facing the radiating element.  
         [0066]     In the absence of the application of an external electric field, that is, when glass plates  21  and the pair of convex lens type-liquid crystal portions  22  and  23  have refractive indices of 1.5, 1.72 and 1.72, respectively, slightly divergent light beams incident on the variable refractive index element  20 , as indicated by solid lines, are converged several times by the convex lens type-liquid crystal portions  22  and  23 , followed by the concentration of the light beams by the objective lens  150  to form an optical spot on a recording plane at position L 0  in the multilayer structure of the optical disc  160 .  
         [0067]     On the other hand, when 10 V are applied across the convex lens type-liquid crystal portion  22 , its refractive index is changed to 1.5 while the refractive indices of the glass plates  21  and the convex lens type-liquid crystal portion  23  remain at 1.5 and 1.72, respectively. In this circumstance, the divergent light beams incident on the variable refractive index element  20 , as indicated by dotted lines, travel the convex lens type-liquid crystal portion  22  without alternation of the optical path and then are converged by the convex lens type-liquid crystal portion  23 . Thereafter, the convergent light beams are focused by the objective lens  150  onto an optical spot on a recording plane at position L +1  in the multilayer structure of the optical disc  160 .  
         [0068]     The application of 5V to the convex lens type-liquid crystal portion  22  causes the refractive index to change to 1.61. In this case (not shown), the divergent light beams incident on the variable refractive index element  20  converge at a smaller angle relative to when no external electric field is applied, and then further converge while passing through the convex lens type-liquid crystal portion  23 . The convergent light beams are concentrated by the objective lens  150  to form an optical spot on a virtual recording plane between positions L 0  and L +1  in the optical disc  160 .  
         [0069]     Where an external electric field of 10 V is applied across both of the convex lens type-liquid crystal portions  22  and  23 , that is, where all the glass plates  21  and the convex lens type-liquid crystal portions  22  and  23  have the same refractive index of 1.5, the divergent light beams incident on the variable refractive index element  20 , as indicated by dot and dashed lines, pass through the convex lens type-liquid crystal portions  22  and  23  without alteration of the optical path and then converge outside the convex lens type-liquid crystal portion  23 . Thereafter, the convergent light beams are focused by the objective lens  150  onto an optical spot on a recording plane at position L +2  in the multilayer structure of the optical disc  160 .  
         [0070]     Both of the convex lens type-liquid crystal portions  22  and  23  have a refractive index of 1.61 if 5V are applied thereto. In this case (not shown), the divergent light beams incident on the variable refractive index element  20  are converged by the convex lens type-liquid crystal portion  22  at a smaller angle than when no external electric field is applied. This convergence at a relatively smaller angle is repeated by the convex lens type-liquid crystal portion  23 . The convergent light beams are further converged by the objective lens  150  to form an optical spot on a virtual recording plane present between positions L 1  and L +2  in the optical disc  160 .  
         [0071]      FIG. 7  shows a variable refractive index element  30  in accordance with another embodiment of the present invention, which has the same structure as in  FIG. 5 , with the exception that the liquid crystal portions of the variable refractive index element  30  are aspherical and thus, a description of this variable refractive index element  30  is omitted. In  FIG. 7 , the refraction of light beams depending on the external voltages applied to the aspherical liquid crystal portions  32  and  33  of the variable refractive index element  30  in the structure of the optical pickup device is depicted according to another embodiment.  
         [0072]     When no external electric field is applied, that is, when glass plates  31 , the aspherical concave lens type-liquid crystal portion  32 , and the aspherical convex lens type-liquid crystal portion  33  has refractive indices of 1.5, 1.72 and 1.72, respectively, the parallel light beams incident on the variable refractive index element  30 , as indicated by solid lines, are diverged by the aspherical concave lens type-liquid crystal portion  32  and then converged by the aspherical convex lens type-liquid crystal portion  33 , followed by the concentration of the light beams by the objective lens  150  to form an optical spot on a recording plane at position L 0  in the multilayer structure of the optical disc  160 .  
         [0073]     On the other hand, when 10 V are applied across the aspherical concave lens type-liquid crystal portion  32 , its refractive index is changed to 1.5 while the refractive indices of the glass plates  31  and the convex lens type-liquid crystal portion  33  remain at 1.5 and 1.72, respectively. In this circumstance, the parallel light beams incident on the variable refractive index element  30 , as indicated by dotted lines, travel through the aspherical concave lens type-liquid crystal portion  32  without alteration of the optical path and then are converged by the aspherical convex lens type-liquid crystal portion  33 . Thereafter, the convergent light beams are focused by the objective lens  150  onto an optical spot on a recording plane at position L −1  in the multilayer structure of the optical disc  160 .  
         [0074]     Application of 5V to the aspherical concave lens type-liquid crystal portion  12  causes the refractive index to change to 1.61. In this case (not shown), the parallel light beams incident on the variable refractive index element  30  diverge at a smaller angle then when 10 V are applied and then converge while passing through the aspherical convex lens type-liquid crystal portion  33 . By the objective lens  150 , the convergent light beams are further converged to form an optical spot on a virtual recording plane present between positions L 0  and L −1  in the optical disc  160 .  
         [0075]     Where an external electric field of 10 V is applied across the aspherical convex lens type-liquid crystal portion  33 , that is, where the aspherical convex lens type-liquid crystal portion  33  is set to have a refractive index of 1.5 while the refractive indices of the glass plates  31  and the aspherical concave lens type-liquid crystal portion  32  remain at 1.5 and 1.72, respectively, the parallel light beams incident on the variable refractive index element  30 , as indicated by dot and dashed lines, are diverged by the aspherical concave lens type-liquid crystal portion  32  and then converged by the aspherical convex lens type-liquid crystal portion  33  at a smaller angle in relation to when no external electric field is applied. Thereafter, the convergent light beams are focused by the objective lens  150  onto an optical spot on a recording plane at position L +1  in the multilayer structure of the optical disc  160 .  
         [0076]     The application of 5V to the aspherical convex lens type-liquid crystal portion  33  causes the refractive index to change to 1.61. In this case (not shown), the parallel light beams incident on the variable refractive index element  10  are diverged by the aspherical concave lens type-liquid crystal portion  32  and then converged by the aspherical convex lens type-liquid crystal portion  33  at a larger angle than when 10 V are applied. By the objective lens  150 , the convergent light beams are further converged to form an optical spot on a virtual recording plane present between positions L 0  and L +1  in the optical disc  160 .  
         [0077]     Instead of the concave or convex lenses described in the above embodiments, meniscus lenses may be employed. In this case, the focus distance may be shortened using the same thickness lens, that is, 10 μm, but the aberration compensating ability may be somewhat degraded.  
         [0078]      FIG. 8  depicts a structure of an optical pickup device  200  in accordance with another embodiment of the present invention, which employs an integrated collimate lens with a variable refractive index element built therein. A description will be given of embodiments of optical pickup devices which are adapted to BDs. If different optical discs, CDs or DVDs, are used, necessary parts such as objective lenses may be modified or supplemented.  
         [0079]     The optical pickup device  200 , as depicted in  FIG. 8 , comprises a radiating element  210 , a collimate lens  220 , a polarized beam splitter  230 , a quarter wave plate  240 , an objective lens  250 , a focal lens  270 , and a photodetector  280 .  
         [0080]     After being emitted from the radiating element  210 , 405 nm blue light beams with a plane of polarization are transmitted to the collimate lens  220 .  
         [0081]     In the collimate lens  220 , all of the optical paths that the light beams keep after the light source, are made to be almost parallel to the optical axis. The parallel light is directed to the polarized beam splitter  230 . At this time, the light beams incident on the collimate lens  220  are refracted by the variable refractive index element built in the collimate lens  220 , so that the traveling path of the parallel light beams depends on the refractive index of the element.  
         [0082]     The polarized beam splitter  230  allows the parallel light to pass therethrough, but reflects at a right angle the light reflected from an optical disc. After passing through the polarized beam splitter  230 , the parallel light goes to the quarter wave plate  240  while the light reflected at a right angle from the polarized beam splitter  230  is forwarded to the focal lens  270 .  
         [0083]     The quarter wave plate  240  changes the parallel light incident thereon from linearly polarized light into circularly polarized light and vice versa. Thus, the linearly polarized light incident on the quarter wave plate  240  is changed into circularly polarized light which is then transmitted to the objective lens  250 . On the other hand, the light reflected from the optical disc  260  changes from circularly polarized light to linearly polarized light in the quarter wave plate  240  before being forwarded to the polarized beam splitter  230 .  
         [0084]     Through the objective lens  250 , the parallel light beams transmitted from the quarter wave plate  240  are converged into a recording plane of a corresponding layer formed in the optical disc  260 . Also, the objective lens  250  serves to collimate the light beams reflected from the optical disc before the reflected light beams travel to the quarter wave plate  240 . As for the spot position of the light beams, it is determined by the variable refractive index element integrated in the collimate lens  220 . Because the light beams from the light source are refracted many times by the variable refractive index element, their optical path varies depending on the refraction extent at the variable refractive index element, thus determining the position of the optical spot in the optical disc  260 . By controlling the refractive index of the variable refractive index element, therefore, optical spots can be accurately formed on a desired recording layer in a multilayer-structured BD so as to write or read information.  
         [0085]     Positioned between the polarized beam splitter  230  and the photodetector  280 , the focal lens  270  converges the light reflected from the polarized beam splitter  230  onto the photodetector  280  which converts the reflected light into electrical signals to read the information.  
         [0086]     The variable refractive index elements built in the collimate lens  220  applied to the optical pickup device of  FIG. 8  can be structured in various formats which are explained in detail with reference to FIGS.  9  to  11 .  
         [0087]      FIG. 9  shows an integral collimate lens  220  in which the same variable refractive index element  10  as described in FIGS.  2  to  5  is built. Even if the variable refractive index element  10  comprising the concave lens type-liquid crystal portion  12  and the convex lens type-liquid crystal portion  13  is depicted in  FIG. 9 , the variable refractive index element  20  comprising the convex lens type-liquid crystal portions  22  and  23 , or the variable refractive index element  30  comprising the spherical liquid crystal portions  32  and  33 , which are described in  FIGS. 6 and 7 , may be employed, instead.  
         [0088]     The variable refractive index element  10  comprising the concave lens type-liquid crystal portion  12  and the convex lens type-liquid crystal portion  13  differs in the extent of refraction from the variable refractive index element  30  comprising the aspherical liquid crystal portions  32  and  33 . However, the variable refractive index elements  10  and  30  allow light beams passing therethrough to take similar optical paths in correspondence with the refractive index change due to the external electric field applied thereto. For this reason, the optical paths based on the variable refractive index element  30  may refer to that based on the variable refractive index element  10 , which is described in detail below with reference to  FIG. 10 .  
         [0089]     When no external electric field is applied, that is, when the glass plates  11 , the concave lens type-liquid crystal portion  12  and the convex lens type-liquid crystal portion  13  have refractive indices of 1.5, 1.72 and 1.72, respectively, the light beams incident on the collimate lens  220 , as indicated by solid lines, are refracted many times at predetermined angles and then are emergent as parallel light beams, followed by the concentration of the parallel light beams by the objective lens  250  to form an optical spot on a recording plane at position L 0  in the multilayer structure of the optical disc  260 .  
         [0090]     On the other hand, when 10 V are applied across the concave lens type-liquid crystal portion  12 , its refractive index is changed to 1.5 while the refractive indices of the glass plates  11  and the convex lens type-liquid crystal portion  13  remain at 1.5 and 1.72, respectively. In this circumstance, the light beams incident on the collimate lens  220 , as indicated by dotted lines, are refracted many times at predetermined angles and then are emergent as parallel light beams. Thereafter, the parallel light beams are focused by the objective lens  250  onto an optical spot on a recording plane at position L −1  in the multilayer structure of the optical disc  260 .  
         [0091]     Where an external electric field of 10 V is applied across the convex lens type-liquid crystal portion  13 , that is, where the convex lens type-liquid crystal portion  13  is set to have a refractive index of 1.5 while the refractive indices of the glass plates  11  and the concave lens type-liquid crystal portion  12  remain at 1.5 and 1.72, respectively, the light beams incident on the collimate lens  220 , as indicated by dot and dashed lines, are refracted many times at predetermined angles and then are emergent as parallel light beams. Thereafter, the parallel light beams are focused by the objective lens  250  onto an optical spot on a recording plane at position L +1  in the multilayer structure of the optical disc  260 .  
         [0092]     With reference to  FIG. 11 , a description is given of optical paths of the light beams passing through the collimate lens  220  in which a pair of convex lens type-liquid crystal portions  22  and  23 .  
         [0093]     In the absence of the application of an external electric field, that is, when glass plates  21  and the pair of convex lens type-liquid crystal portions  22  and  23  have refractive indices of 1.5, 1.72 and 1.72, respectively, light beams incident on the collimate lens  220 , as indicated by solid lines, are refracted many times at predetermined angles and then are emergent as parallel light beams, followed by the concentration of the parallel light beams by the objective lens  250  to form an optical spot on a recording plane at position L 0  in the multilayer structure of the optical disc  260 .  
         [0094]     On the other hand, when 10 V are applied across the convex lens type-liquid crystal portion  22 , its refractive index is changed to 1.5 while the refractive indices of the glass plates  21  and the convex lens type-liquid crystal portion  23  remain at 1.5 and 1.72, respectively. In this circumstance, the light beams incident on the collimate lens  220 , as indicated by dotted lines, are refracted many times at predetermined angles and then are emergent as parallel light beams. Thereafter, the parallel light beams are focused by the objective lens  250  onto an optical spot on a recording plane at position L +1  in the multilayer structure of the optical disc  260 .  
         [0095]     Where an external electric field of 10 V is applied across both of the convex lens type-liquid crystal portions  22  and  23 , that is, where all the glass plates  21  and the convex lens type-liquid crystal portions  22  and  23  have the same refractive index of 1.5, the light beams incident on the collimate lens  220 , as indicated by dot and dashed lines, are refracted many times at predetermined angles and then emergent as parallel light beams. Thereafter, the parallel light beams are focused by the objective lens  250  onto an optical spot on a recording plane at position L +2  in the multilayer structure of the optical disc  260 .  
         [0096]     Instead of the concave or convex lenses described in the above embodiments, meniscus lenses may be employed. In this case, the focus distance may be shortened even with the same thickness, that is, 10 μm, but aberration compensating ability may be somewhat degraded.  
         [0097]     Results of aberration correction obtained by use of the variable refractive index elements in the above-described optical pickup devices are summarized in the following tables. In the optical pickup devices, blue light beams with a wavelength of 400±5 nm were used in combination with an NA of 0.85 and an the element having an effective radius of 3.0 mm and a thickness of 0.1 mm performed at a working distance of 0.675 mm.  
         [0098]     The variable refractive index elements used have the specifications shown in Table 1, below.  
                                             TABLE 1                       Variable                   Refractive   Curvature   Thick.       Index Elements   Radii(mm)   (μm)   Shapes                                1 st     144.5   10   Concave + Convex       2 nd     72.25   20   Concave + Convex       3 rd     144.5   10   Aspherical                  
 
         [0099]     Spherical aberrations due to the variation in thickness of BDs are given in Table 2, below.  
                           TABLE 2                                   Thick.(mm)   Abberations(RMS)                           0.1 ± 0.00   0.0003           0.1 ± 0.01   0.0973           0.1 ± 0.02   0.1944           0.1 ± 0.03   0.2917           0.1 ± 0.04   0.3890           0.1 ± 0.05   0.4865                      
 
         [0100]     The aberration correction capacities of the 1 st , the 2 nd  and the 3 rd  variable refractive index element of Table 1 are given in Table 3, below.  
                           TABLE 3                                   Variable Refractive   Max. Aberration           Index Elements   Corrections(RMS)                           1 st     0.0740           2 nd     0.1411           3 rd     0.5432                      
 
         [0101]     Results from the correction for the spherical aberration due to the variation in thickness by the use of the 1 st , the 2 nd  and the 3 rd  variable refractive index element are given in Table 4, below.  
                       TABLE 4                       Variable Refractive   Thickness   Aberrations       Index Elements   Variation (mm)   Corrected(RMS)                   1 st     0.1 + 0.01   0.0209       1 st     0.1 − 0.01   0.0202       2 nd     0.1 + 0.02   0.0433       2 nd     0.1 − 0.02   0.0389       3 rd     0.1 + 0.05   0.0754       3 rd     0.1 − 0.05   0.0683                  
 
         [0102]     It is evident from the results of Tables 1 to 4 that when account is taken of the fact that a BD usually has recording planes partitioned into two layers with an interlayer distance of approximately 25 μm, the first variable refractive index element having a curvature radius of 144.5 mm and a thickness of 10 μm can be adapted to BDs having double recording layers because its correction for spherical aberration is in the range of approximatey ±20 μm (0.002 mm).  
         [0103]     In addition, the second variable refractive index element with a curvature radius of 72.25 mm and a thickness of 20 μm has a spherical aberration correction range of approximately ±40 μm (0.004 mm) so that it can be applied to BDs which have at least two recording layers. In consideration of the fact that the distance between recording layers of BDs is not fixed at 25 μm but differs from one manufacturer of BDs to another, the second variable refractive index element can be applied to BDs having triple recording layers with an interlayer distance of about 13 μm. Furthermore, when a spherical element, like the first or the second variable refractive index element, is used, its correction for spherical aberration can be in the range of ±50 μm by increasing the thickness of the liquid crystal portion to 30-40 μm, which makes it possible to adapt the variable refractive index element to BDs having as many as five recording layers.  
         [0104]     In the case of employing the third variable refractive index element which is aspherical, the correction for spherical aberrations is in the range of ±50 μm so that the element can be adapted to BDs having five recording layers. Moreover, an increase in thickness of the liquid crystal portion of the aspherical element may result in expanding the range of spherical aberration correction to more than ±50 μm.  
         [0105]     Including at least two liquid crystal portions each of which has an incident or emergent plane with a curvature, as described hereinbefore, the variable refractive index element used has a more useful refractive index than conventional flat variable refractive index elements. Therefore, the optical pickup devices according to the present invention can write on and read from a 100 μm-thick BD having multi-recording layers, thereby greatly enhancing the recording density of optical discs.  
         [0106]     In contrast to conventional ones, the optical pickup device of the present invention needs no additional objective lenses by virtue of the variable refractive index element, thus enjoying the advantage of being simple in structure and having a light weight.  
         [0107]     Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.