Patent Publication Number: US-2006002247-A1

Title: Optical pickup and optical recording and/or reproducing apparatus adopting the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of Korean Application No. 2004-51978, filed Jul. 5, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      An aspect of the present invention relates to an optical pickup and an optical recording and/or reproducing apparatus adopting the same, and, more particularly, to an optical pickup and an optical recording and/or reproducing apparatus adopting the same having a plurality of objective lenses.  
      2. Description of the Related Art  
      In an optical recording and/or reproducing apparatus to record arbitrary information and/or reproduce the recorded information to and/or from an optical disc, which is an optical information storage medium, using a light spot condensed by an objective lens, a recording capacity is determined by a size of a generated light spot. The light spot size S is given by 
 
S∝λ/NA   Equation 1 
 
 where λ is the wavelength of light used by the apparatus and NA is the numerical aperture of the objective lens. Therefore, a light source with a short wavelength such as a blue laser and an objective lens having an NA greater than 0.6 must be adopted in order to reduce the light spot size that is focused onto the optical disc when a high density optical disc is used. 
 
      Recently, a Blu-ray disc (BD) standard has been suggested that uses a light source with a wavelength of approximately 405 nm and an objective lens with an NA of 0.85. The BD standard uses an optical disc with a capacity of approximately 25 giga bytes (GB) and a thickness (i.e., the interval between a light-incident plane and an information storage plane, corresponding to the thickness of a protection layer in this case) of 0.1 mm, has been suggested. Further, a high definition DVD (HD DVD) standard has been suggested that uses a light source with a wavelength equal to that used by the BD standard, an objective lens with an NA of 0.65. The HD DVD standard uses an optical disc with a capacity of approximately 15 GB and a thickness (i.e., the interval between a light-incident plane and an information storage plane, corresponding to the thickness of a substrate in this case) of 0.6 mm.  
      In the field of digital versatile discs (DVDs), trends tend to occur in which diverse standards exist together. Occasionally, these trends are repeated. Therefore, a device to make two optical disc standards compatible each other in one system or apparatus is needed. For example, the DVD standards use a light source with similar wavelengths, an objective lens with similar NAs, and an optical disc substrate with similar thicknesses; only a track pitch or an optical disc structure is different. Therefore, since an operation of condensing light emitted from a light source onto an optical disc is almost the same regardless of the optical disc standard, a method of performing focusing and tracking in a compatible manner depending on track pitches has been considered. However, since a thickness of an optical disc is different in the case of next-generation DVD standards, such as the BD and HD DVD standards, the generation of spherical aberration due to differences in the thicknesses of the optical discs may be severe.  
      There exists a method of using a holographic optical element that compensates for spherical aberration. Use of a holographic optical element that compensates for the spherical aberration in a system that uses two light sources will be described below. In the case of a DVD/compact disc (CD) compatible apparatus, since the reflection ratio of a recordable optical disc is relatively remarkably reduced with respect to light sources having a wavelength different from a wavelength prescribed for each optical disc standard, two light sources must be used. For example, since reflection ratios of a DVD-R and a CD-R to be used in one time recording are remarkably reduced when light having a wavelength that is different from a standard wavelength is used, light sources having wavelengths of 650 nm and 780 nm must be used. In this case, compatibility may be effectively achieved by using a diffraction angle of the holographic optical element depending on the wavelength (650 nm and 780 nm) and a refraction angle difference resulting from a difference between the refraction indexes according to wavelength.  
      A first diffraction beam of the holographic optical element may have a high diffraction efficiency at the two wavelengths of 650 nm and 780 nm, as illustrated in  FIG. 1 . In the case that the recordable optical disc is excluded, there exists a method of making CD-family optical discs that are compatible using one light source, e.g., a DVD light source. In this case, zeroth-order light and first-order light, divided by the holographic optical element, may be effectively used to compensate for spherical aberration due to a difference of thicknesses between two optical disc&#39;s substrates. However, since light that is emitted from a single source is divided into two beams of light, optical efficiency is reduced by more than half.  
      Since the next-generation DVD standards, i.e., the BD and HD DVD standards use light sources with substantially similar wavelengths, using the above-described method to accomplish compatibility using a diffraction angle of the holographic optical element depending on the different wavelengths that are used and a refraction angle difference resulting from a difference between the refraction indexes according to wavelength is difficult. Further, the method of compensating for the spherical aberration due to a difference in thickness of two optical discs by allowing light emitted from one light source to be diffracted into the zeroth-order and the first-order light beams using the holographic optical element has a low optical efficiency. Therefore, applying the method to a high-speed apparatus that requires greater light quantity is difficult.  
     SUMMARY OF THE INVENTION  
      Aspects of the present invention provide an optical pickup and an optical recording and/or reproducing apparatus capable of compatibly using two information storage media of different standards where wavelengths of light being radiated towards the media are substantially similar, and where thicknesses of the information storage media and NAs of objective lenses are different.  
      Further, the optical pickup and optical recording and/or reproducing apparatuses according to aspects of the present invention have high optical efficiency and are able to operate at a high speed.  
      According to an aspect of the present invention, there is provided an optical pickup, which includes: an optical unit to illuminate light onto an information storage medium and to receive light reflected back by the information storage medium to detect an information signal and/or an error signal; first and second objective lenses to focus incident light to focus to form a light spot onto an information storage surface of the information storage medium; and an optical path switching unit to electrically switch the proceeding direction of the light so that the light incident from the optical unit proceeds toward one of the first and second objective lenses.  
      According to an aspect of the invention, the optical unit includes a blue light source emitting light of a blue wavelength region and one of the first and the second objective lenses is formed in accordance with a BD standard and the other is formed in accordance with an HD DVD standard, whereby the BD and the HD DVD may be compatibly adopted.  
      According to an aspect of the invention, the optical unit further includes at least one of a red light source to emit light of a red wavelength region and an infrared light source to emit light of an infrared wavelength region, and the optical unit is configured to adopt compatibly at least one of a DVD and a CD.  
      According to an aspect of the invention, the objective lens appropriate for the HD DVD standard of the first and second objective lenses is formed to adopt compatibly at least one of a DVD and a CD.  
      According to an aspect of the invention, the optical pickup further includes a reflection mirror reflecting light that has transmitted the optical path switching unit.  
      According to an aspect of the invention, the first and second objective lenses are installed in a single lens holder along a radial direction of the information storage medium.  
      According to an aspect of the invention, the optical path switching unit includes a Bragg grating that can be electrically switched.  
      According to an aspect of the invention, the Bragg grating includes: a liquid crystal layer that is interposed between a pair of substrates, and having liquid crystal drops entangled with chains and a polymer filling a space between the liquid crystal drops, a refraction index difference between the liquid crystal drops and the polymer being adjusted by an electrical signal to selectively transmit or reflect/diffract incident light; and transparent electrodes formed on inner surfaces of the substrate producing an electric field in the liquid crystal layer.  
      According to an aspect of the invention, the Bragg grating is configured to perform a transmission operation when a voltage is applied and perform a diffraction operation when the voltage is not applied.  
      According to an aspect of the invention, the optical path switching unit includes: a selective half-wave plate selectively changing polarization of light of a specific wavelength region incident from the optical unit by being electrically driven; and a polarizing beam splitter selectively transmitting or reflecting the light incident from the selective half-wave plate depending on the polarization of the light.  
      According to an aspect of the invention, the light of the specific wavelength region is light of a blue wavelength region.  
      According to an aspect of the invention, the selective half-wave plate is a liquid crystal plate formed to selectively act as a half-wave plate by being electrically driven.  
      According to another aspect of the present invention, there is provided an optical recording and/or reproducing apparatus, which includes: an optical pickup that can move along a radial direction of an information storage medium to reproduce information recorded on the information storage medium or to record information; and a controller controlling the optical pickup, the optical pickup including the optical pickup of the present invention.  
      Additional and/or other aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a graph of the diffraction efficiency of zeroth-order light and first-order light according to wavelength in a general holographic optical element;  
       FIG. 2  is a schematic view of an optical pickup according to an aspect of the present invention;  
       FIGS. 3A and 3B  are views illustrating the structure of a liquid crystal layer of a switchable Bragg grating and selective transmitting and reflecting/diffracting in the switchable Bragg grating according to an aspect of the present invention;  
       FIGS. 4A and 4B  are schematic views illustrating proceeding light paths switched by an optical path switching unit in the optical pickup illustrated in  FIG. 2  according to an aspect of the present invention;  
       FIG. 5  is a schematic view of optical composition of an optical pickup according to an embodiment of the present invention;  
       FIG. 6  is a perspective view of the optical pickup illustrated in  FIG. 5 ;  
       FIG. 7  is a perspective view of an optical pickup according to another embodiment of the present invention;  
       FIG. 8  is a schematic view of optical composition of an optical pickup according to another embodiment of the present invention;  
       FIGS. 9A and 9B  are schematic views illustrating proceeding light paths switched by an optical path switching unit in the optical pickup illustrated in  FIG. 8 ; and  
       FIG. 10  is a schematic view of an optical recording and/or reproducing apparatus adopting an optical pickup according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
       FIG. 2  is a schematic view of an optical pickup according to an embodiment of the present invention. Referring to  FIG. 2 , the optical pickup includes: an optical unit  10  to illuminate light onto an information storage medium (i.e., an optical disc  1 ), and to receive light reflected from the optical disc  1  so as to detect an information signal and/or an error signal; first and second objective lenses  60  and  70  to condense incident light to form a light spot onto an information storage surface of the optical disc  1 ; and an optical path switching unit  50  to electrically switch the path of the light that is incident from the optical unit  10  so as to allow the light to selectively progress to one of the first and the second objective lenses  60  and  70 . The optical disc  1  can be a Blu-RAY (BD) disc  1   a  or a HIGH DEFINITION DVD (HD DVD) disc  1   b . However, it is understood that the optical disc  1  can also be other types of discs compatible with the wavelength and/or the N/A substantially that of the BD or the HD DVD.  
      The optical unit  10  includes at least one light source, where that at least one light source includes a light source emitting light in a blue wavelength region (e.g., with a wavelength of 405 nm) appropriate for a BD  1   a  and an HD DVD  1   b , which are next-generation DVD. Various embodiments of the optical unit  10  will be described below.  
      The first objective lens  60  may be formed in accordance with a BD standard and the second objective lens  70  may be formed in accordance with an HD DVD standard.  
      That is, the first objective lens  60  may be optimized for the optical disc  1  having a thickness of approximately 0.1 mm and for light in the blue wavelength region, and have an NA of 0.85. The second objective lens  70  may be optimized for the optical disc  1  having a thickness of approximately 0.6 mm and for light of the blue wavelength region, and have an NA of 0.65. As an alternative, the first objective lens  60  may be formed in accordance with the HD DVD standard and the second objective lens  70  may be formed in accordance with the BD standard. Further, as described below, the objective lens  60  or  70  formed in accordance with the HD DVD standard may be adaptable so as to be compatible with the DVD, or CD or the like standard. However, it will be assumed that the first and the second objective lenses  60  and  70  are appropriate for the BD and the HD DVD standards, respectively.  
      In this case, the optical pickup of the present invention is compatibly adapted to be usable with the BD  1  a and the HD DVD  1   b . While not required in all aspects, the first and the second objective lenses  60  and  70  are disposed along a radial direction of the optical disc  1  in a single lens holder  90 , and are driven in a focus direction, tracking direction and/or tilt direction by an actuator  80 . When the first objective lens  60  is formed appropriately for the BD standard, the first objective lens  60  is disposed farther from the center of the optical disc  1  than the second objective lens  70 . As an alternative, the first objective lens  60  can be disposed closer from the center of the optical disc  1  than the second objective lens  70 .  
      While not required in all aspects, the first and the second objective lenses  60  and  70  are installed in the single lens holder  90  considering the difference in thicknesses and the difference in working distances between the BD  1   a  and the HD DVD  1   b . For example, when information is recorded onto or reproduced from the HD DVD  1   b , which is relatively thick, the first objective lens  60  may be installed in the lens holder  90  so as to be separated from the optical disc  1   b  y a greater distance than an original working distance of the first objective lens  60  so that a collision between the first objective lens  60  and the HD DVD  1   b  may be prevented. The first and the second objective lenses  60  and  70 , the single lens holder  90  mounting the objective lenses  60  and  70 , and a magnetic circuit portion of the actuator  80  installed in the single lens holder  90  constitute a moving unit of an optical pickup assembly.  
      The optical path switching unit  50  switches between operations of transmitting and reflecting incident light using an electrical signal. For example, the optical path switching unit  50  operates as a reflection mirror in a power-off state to direct the incident light to the first objective lens  60  and to transmit the incident light in a power-on state to direct the incident light to the second objective lens  70 .  
      When the first objective lens  60  is formed in accordance with the BD standard, the optical path switching unit  50  is operated to direct the incident light to the first objective lens  60  when information is recorded onto or reproduced from the BD  1   a  and to direct the incident light to the second objective lens  70  when information is recorded onto or reproduced from an optical disc of the other standards (e.g., one of the HD DVD  1   b , DVD, and CD).  
      According to another aspect, the optical path switching unit  50  is an electrically switchable Bragg grating (ESBG)  150  as shown in  FIGS. 3A and 3B . Unlike the holographic optical element, refractive index modulation is possible with the ESBG  150 . The ESBG  150  includes a liquid crystal layer  151  that is interposed between transparent glass or plastic substrates  152  and selectively transmits or diffracts incident light according to an electrical signal. The ESBG  150  further includes an indium tin oxide (ITO) transparent electrode  153  coated on an inside surface of the substrate  152  to apply an electric field to the liquid crystal layer  151 .  
      Referring to  FIGS. 3A and 3B , the liquid crystal layer  151  includes: fine liquid crystal drops  151   a  entangled with chains disposed at predetermined intervals; and a polymer  151   b  filling the space between the fine liquid crystal drops  151   a . That is, the fine liquid crystal drops  151   a  are densely distributed between the transparent ITO-coated substrates  152  and the polymer  151   b  fills between the fine liquid crystal drops  151   a  to form a Bragg surface.  
      As is described above, the ESBG  150  includes the polymer  151   b  which is relatively pure as well as planes of the dense fine liquid crystal drops  151   a  alternately distributed with constant intervals between the adjacent planes of the fine liquid crystal drops. Therefore, when the refractive index n LCM  of the fine liquid crystal drops  151   a  is different from the refractive index n p  of the polymer  151   b , a volume phase hologram is generated so that a diffraction operation is generated. That is, the fine liquid crystal drops  151   a  that are disposed at constant interval act like a diffraction grating to diffract and reflect the incident light. In the liquid crystal layer  151  having the above-described structure, selective transmission and reflection of the incident light are performed using the following principle. If a voltage is not applied to the liquid crystal layer  151  (voltage-off state), since the refractive index n LCM  of the fine liquid crystal drops  151   a  is different from the refractive index np of the polymer  151   b , the liquid crystal layer  151  diffracts/reflects the incident light as illustrated in  FIG. 3A .  
      Referring to  FIG. 3B , when a predetermined voltage is applied to the liquid crystal layer  151 , since an optical axis of liquid crystal molecules constituting the liquid crystal drops  151   a  is aligned so that the refractive index nLCM of the liquid crystal molecules is substantially similar as that of the refractive index np  151   b  of the polymer, the diffraction grating resulting from a difference between the refractive indexes of the liquid crystal drops  151   a  and the polymer  151   b  disappears. Thus, the liquid crystal layer  151  becomes transparent and transmits almost all of the incident light without diffraction. As is described above, the ESBG  150  is configured to transmit light when a voltage is applied thereto and to diffract light when the voltage is not applied thereto.  
      The optical pickup further includes a reflection mirror  55  to reflect light that has passed through the optical path switching unit  50  to the second objective lens  70 .  
      The optical pickup according to the embodiment of the present invention as described above selectively transmits or reflects incident light by electrically driving the optical path switching unit  50 . For example, referring to  FIG. 4A , when information is recorded onto or reproduced from the BD  1   a , if the optical path switching unit  50  receives a first voltage V 1  (e.g., the optical patch switching unit is in a power-off or a lower power state), the optical path switching unit  50  reflects all of the light that is incident from the optical unit  10  to the first objective lens  60 . Further, referring to  FIG. 4B , when information is recorded onto or reproduced from the HD DVD  1   b , if the optical path switching unit  50  receives a second voltage V 2  (e.g., the optical path switching unit is in a power-on or higher power state), the optical path switching unit  50  may transmit all of the light that is incident from the optical unit  10  to the second objective lens  70 .  
      As is described above, in the optical pickup of an aspect of the present invention, since quantities of light directed to the BD  1   a  and the HD DVD  1   b  respectively may be maximized, high optical efficiency is achieved. Thus, high-speed recording/reproducing, which requires a large quantity of light, may be obtained. The reason a large quantity of light is required for high-speed recording/reproducing is that as the speed increases a time during which a light spot, which is illuminated to a predetermined position of the optical disc  1 , decreases the light quantity illuminated per unit time must be increased to obtain the optical power required to perform recording and/or reproducing operations.  
      A variety of embodiments for optical construction of the optical pickup according to aspects of the present invention will be described below.  FIG. 5  is a schematic view of optical composition of an optical pickup according to an embodiment of the present invention and  FIG. 6  is a perspective view of the optical pickup illustrated in  FIG. 5 . The same reference numerals are used for the elements substantially performing the same or similar operations as those illustrated in  FIG. 2  and descriptions thereof will be omitted.  
      Referring to  FIGS. 5 and 6 , the optical pickup according to the present embodiment includes a blue light source  11  to emit a first light  11   a  in a blue wavelength region (e.g., with a wavelength of 405 nm in accordance with standards of next-generation DVD&#39;s, e.g., the BD  1   a  and the HD DVD  1   b ) and may be adapted to be compatible with the BD  1   a  and the HD DVD  1   b . Further, the first objective lens  60  may be optimized for the BD  1   a  and the second objective lens  70  may be optimized for the HD DVD  1   b . In that case, the optical path switching unit  50  is formed and operated to reflect the first light  11   a  incident from the optical unit  10  when the BD  1   a  is applied and to transmit the first light  11   a  incident from the optical unit  10  when the HD DVD  1   b  is applied.  
      In the optical pickup according to the embodiment of  FIGS. 5 and 6 , the optical unit  10  includes: the blue light source  11  to emit the first light  11   a  in the blue wavelength (i.e., with the wavelength of 405 nm in accordance with the next-generation DVDs); a first optical path changing unit to change the proceeding path of the incident first light  11   a ; and a photo detector  20  to receive the first light  11   a  reflected from the optical disc  1  so as to detect an information reproducing signal and/or an error signal. Further, the optical unit  10  further includes a grating  13  to divide the first light  11   a  emitted from the blue light source  11  into at least three light beams. The optical unit  10  further includes a collimating lens  19  to collimate the first light  11   a  emitted from the blue light source  11 . The optical unit  10  further includes a detection lens  18  arranged between a polarizing beam splitter  15  and the photo detector  20 . However, it is understood that the optical pickup  10  can be otherwise constructed.  
      The illustrated first optical path changing unit includes: the polarizing beam splitter  15  transmitting or reflecting the incident first light  11   a  according to the light  11   a &#39;s polarization state; and a quarter-wave plate  17  formed in accordance with the wavelength of the first light  11   a  to change the polarization of the first light  11   a . The grating  13  is disposed along an optical path between the blue light source  11  and the polarizing beam splitter  15 . The grating  13  is configured so that tracking error signal detection using a 3-beam method or a differential push-full method is possible. However, it is understood that the first optical path changing unit can be otherwise constructed.  
      While not required in all aspects, the detection lens  18  may be an astigmatism lens (e.g., a cylindrical lens), generating astigmatism in the incident first light  11   a  to direct a focus error signal using an astigmatism method.  
      In order to control the optical power of the blue light source  11 , the illustrated optical unit  10  further includes a photo detector  16  for monitoring the first light by detecting the first light  11   a  emitted from the blue light source  11  and partially reflected by the polarizing beam splitter  15 .  
      In the above-described optical pickup, the optical path switching unit  50  is operated to reflect the first light  11   a  incident from the optical unit  10  to the first objective lens  60  when the BD  1   a  is applied and transmit the first light  11   a  incident from the optical unit  10  to the reflection mirror  55  when the HD DVD  1   b  is applied. The light is then reflected by the reflection mirror  55  to the second objective lens  70 . The first light  11   a  reflected by the BD  1   a  or the HD DVD  1   b  is reincident on the optical unit  10  by following reverse path. Thus, the optical pickup of the present embodiment may be compatible and applied to operate with both the BD and the HD DVD.  
       FIG. 7  is a perspective view of an optical pickup according to another embodiment of the present invention. The optical pickup shown in  FIG. 7  has an optical construction capable of being compatible with a DVD and a CD as well as the BD  1   a  and the HD DVD  1   b  in comparison with the optical pickup embodiment of  FIGS. 5 and 6 . Here, the same reference numerals are used for the elements performing substantially the same operations as those of  FIGS. 2 and 5  and descriptions thereof will not be repeated.  
      In an optical pickup according to the embodiment of  FIG. 7 , an optical unit  110  includes: first and second optical modules  130  and  120  to emit second and third light with wavelengths appropriate for a DVD and a CD, respectively, and to receive the second and the third light after the first and second light beams are reflected from the optical disc  1  so as to detect an information reproducing signal and/or an error signal; a beam splitter  141  to separate and combine optical paths of the second and third light emitted from the first and second optical modules  130  and  120 ; a collimating lens  143  to collimate the second and third light emitted from the first and second optical modules  130  and  120 ; and a beam splitter  145  to cause the second and third light emitted from the first and second optical modules  130  and  120  to proceed to the optical path switching unit  50  by way of the same optical path as the first light  11   a  emitted from the blue light source  11 .  
      The first and the second optical modules  130  and  120  are respectively a red light source to emit light in a red wavelength region (e.g., with a wavelength of 650 nm in accordance with information recording and/or reproducing standards of a DVD), and an infrared light source to emit light in an infrared wavelength region (e.g., with a wavelength of 780 nm in accordance with information recording and/or reproducing standards of a CD).  
      while not required in all aspects, a hologram optical module for a DVD and a hologram optical module for a CD may be provided for the first and the second optical modules  130  and  120 , respectively. A hologram optical module includes: a light source; a photo detector; and a hologram to transmit light that is emitted from the light source, and to transmit diffracted light that is reflected from the optical disc  1  toward the photo detector. As the hologram optical module is well known in the art, a more detailed description and illustration thereof are omitted. Moreover, it is understood that elements of the module  120  or  130  can be separately provided and need not be provided in a single module.  
      In the illustrated embodiment of  FIG. 7 , the second objective lens  70  is formed in accordance with information recording and/or reproducing standards of an HD DVD and to be able to be compatible with a DVD and a CD. Since the HD DVD standard is the same as or similar to the DVD standard in terms of the thickness of the optical disc and the NA of the objective lens used, compatibility between the HD DVD and the DVD may be achieved using one objective lens. Further, designing the objective lens to be compatible with a CD is similarly possible.  
      In the embodiment of  FIG. 7 , the optical pickup is able to be compatible with a DVD and a CD as well as a BD and an HD DVD.  
      An optical pickup according to the present invention may also be formed to be compatible with a BD, an HD DVD, and a DVD. Referring to  FIG. 7 , the optical pickup need not include one optical module of the first and the second optical modules  130  and  120  and the beam splitter  141  in  FIG. 7 . Indeed, the second objective lens  70  may be configured in accordance with information recording and/or reproducing standards of an HD DVD and may be adapted to be compatible with one of a DVD and a CD. The optical pickup according to aspects of the present invention may be formed to include an optical system that is compatible with a DVD and/or a CD, in addition to an optical system for a BD and an HD DVD. For example, in  FIG. 7 , the optical system may be modified such that the beam splitter  145  is excluded, an objective lens for a DVD and/or a CD is separately provided, the optical paths of the second and the third light emitted from the first and the second optical modules  130  and  120  are separated/combined by the beam splitter  141 , and the second and third light is collimated by the collimating lens  143  and is incident on the objective lens for the DVD and/or the CD. In that case, the optical pickup according to the present invention includes at least three objective lenses. Since such an optical arrangement can be sufficiently inferred from the above description, detailed description thereof will be omitted.  
      In the above description, it has been has been assumed that the first objective lens  60  on which the light reflected by the optical path switching unit  50  is incident is appropriate for the BD  1   a  and the second objective lens  70  on which the light that has passed through the optical path switching unit  50  is incident is appropriate for the HD DVD  1   b . However, it is possible that the first objective lens  60  is formed appropriately for the HD DVD  1   b  and the second objective lens  70  is formed appropriately for the BD  1   a.    
      The operation of the optical pickup according to an aspect of the present invention as described above will now be described using the optical configuration illustrated in  FIG. 7  as an example. When a BD  1   a  is mounted in the optical recording and/or reproducing apparatus, the blue light source  11  is operated to emit the first light  11   a  and the optical path switching unit  50  is operated to reflect the incident first light  11   a . For example, the optical path switching unit  50  is maintained in a power-off state. A linearly polarized component of the first light  11   a  emitted from the blue light  11  passes through the polarizing beam splitter  15  and is converted into circularly polarized light by the quarter-wave plate  17 . The first light  11   a  is collimated by the collimating lens  19 , passes through the beam splitter  145  and is incident on the optical path switching unit  50 . The incident first light  11   a  is reflected by the optical path switching unit  50  and is focused in the form of a light spot on the information storage surface of the BD  1   a  by the first objective lens  60 . The first light  11   a , converted into different circularly polarized light by being reflected by the information storage surface of the BD  1   a , follows the original optical path in the opposite direction back to the quarter-wave plate  17 , gets converted into different linearly-polarized light while passing through the quarter-wave plate  17  and, having been reflected by the polarizing beam splitter  15 , is then received by the optical detector  20 .  
      On the other hand, if an HD DVD  1   b  is mounted in the optical recording and/or reproducing apparatus, the blue light source  11  is operated to emit the first light  11   a  and the optical path switching unit  50  is operated to transmit the incident first light  11   a . For example, the optical path switching unit  50  is maintained in a power-on state. In this time, the optical path and polarization of the first light  11   a  are changed in the same manner as when the BD  1   a  is used. After that, the first light  11   a  is transmitted through the optical path switching unit  50 , reflected to the second objective lens  70  by the reflection mirror  55 , condensed by the second objective lens  70 , and focused in the form of a light spot on the information storage surface of the HD DVD  1   b . The first light  11   a  reflected by the information storage surface of the HD DVD  1   b  follows the original optical path in the opposite direction and is reflected by the polarizing beam splitter  15  and is received at the photo detector  20 .  
      When a DVD is mounted in the optical recording and/or reproducing apparatus, the first optical module  130  is operated to emit the second light and the optical path switching unit  50  is operated to transmit the incident second light. For example, the optical path switching unit  50  is maintained in a power-on state. The second light that is emitted from the first optical module  130  passes through the beam splitter  141 , is collimated by the collimating lens  143  and is incident on the beam splitter  145 . The incident second light is reflected by the beam splitter  145 , passes through the optical path switching unit  50 , is reflected by the reflection mirror  55 , and is incident on the second objective lens  70 . The incident second light is condensed by the second objective lens  70  and focused in the form of a light spot on the information storage surface of the DVD. The second light reflected by the information storage surface of the DVD follows the original optical path back to the first optical module  130 . When a CD is mounted in the optical recording and/or reproducing apparatus, the second optical module  120  is operated to emit the third light and the optical path switching unit  50  is operated to transmit the incident third light. For example, the optical path switching unit  50  is maintained in a power-on state. The third light emitted from the second optical module  120  is reflected by the beam splitter  141 , collimated by the collimating lens  143  and incident on the beam splitter  145 . The incident third light is reflected by the beam splitter  145 , passes through the optical path switching unit  50 , is reflected by the reflection mirror  55  and is incident on the second objective lens  70 . The incident third light is condensed by the second objective lens  70  and focused in the form of a light spot on the information storage surface of the CD. The third light reflected by the information storage surface of the CD follows the original optical path back to the second optical module  120 .  
       FIG. 8  is a schematic view of an optical pickup according to another embodiment of the present invention. Unlike the optical pickup of  FIGS. 5 and 6 , the optical pickup illustrated in  FIG. 8  includes an optical path switching unit  250  different from the optical path switching unit  50  and wavelength plates  17  are arranged between the optical path switching unit  250  and the first and the second objective lenses  60  and  70 , respectively, instead of being disposed in the optical unit  10 ′. Here, the same reference numerals are used for elements performing substantially the same functions as those of  FIGS. 2 and 5 , and description thereof will not be repeated.  
      Referring to  FIG. 8 , the optical path switching unit  250  includes: a selective half-wave plate  251  to selectively change the polarization of light in a specific wavelength region to which light whose optical path is to be switched belongs by an electrical driving; and a polarizing beam splitter  255  to selectively transmit or reflect light incident from the selective half-wave plate  251  depending on the polarization of the light. As with the embodiments of  FIGS. 5-7 , when the blue light source  11  to emit the first light  11   a  in a blue wavelength region (i.e., with a wavelength of 405 nm) appropriate for next-generation DVD (i.e., the BD  1   a  and the HD DVD  1   b ), is provided, the light of the specific wavelength region is the light of the blue wavelength region and the selective half-wave plate  251  selectively acts as a half-wave plate for the light of the blue wavelength region. The selective half-wave plate  251  may be a liquid crystal plate configured to selectively act as a half-wave plate according to which it is electrically driven.  
      The incident light of a specific wavelength region (e.g., the incident light  11   a  of blue wavelength region), may be selectively sent to the first or the second objective lens  60  or  70  as illustrated in  FIGS. 9A and 9B  by the optical path switching unit  250 . Referring to  FIG. 9A , when information is recorded and/or reproduced with respect to the BD  1   a , if the selective half-wave plate  251  receives a first voltage V 1  (e.g., the selective half-wave plate  25  is in a power-off or lower power state), the p-polarized component of the light of the blue wavelength region incident from the optical unit  10  passes through the selective half-wave plate  251 , becomes s-polarized light and is reflected by the polarizing beam splitter  255 , and directed to the first objective lens  60 .  
      Referring to  FIG. 9B , when information is recorded and/or reproduced with respect to the HD DVD  1   b , if the selective half-wave plate  251  receives a second voltage V 2  (e.g., the selective half-wave plate  25  is in a power-on or higher power state), the p-polarized component of the light  11   a  of the blue wavelength region passes through the selective half-wave plate  251  without a change in polarization, passes through the polarizing beam splitter  255 , and is directed to the second objective lens  70 .  
      Since the amount of light respectively directed to the BD  1   a  and the HD DVD  1   b  may be maximized even when the optical path switching unit  250  is used, an optical efficiency is increased and thus high-speed operations requiring a large quantity of light may be obtained.  FIGS. 8 through 9 B illustrate an embodiment in which the optical path switching unit  250  is applied to the BD and HD DVD-compatible type optical pickup described with reference to  FIGS. 5 and 6 , and is intended for exemplary purpose only. The optical path switching unit  250  may be applied to the optical construction of  FIG. 7  in place of the optical path switching unit  50 . In this case, the wave plate  17  is arranged on the optical path between the optical path switching unit  250  and the first and the second objective lenses  60  and  70  instead of being arranged in the optical unit  110 .  
      In  FIGS. 8 through 9 B, the optical path switching unit  250  includes a cubic-type polarizing beam splitter  255 . Alternately, the polarizing beam splitter  255  may be a plate-type polarizing beam splitter.  
       FIG. 10  is a schematic view of an optical recording and/or reproducing apparatus adopting an optical pickup according to the present invention. Referring to  FIG. 10 , the optical recording and/or reproducing apparatus includes: a spindle motor  312  to rotate an optical disc  1 ; an optical pickup  300  installed to move along a radial direction of the disc to reproduce information recorded on the optical disc  1  and/or record information to the optical disc; a driving unit  307  to drive the spindle motor  312  and the optical pickup  300 ; and a controller  309  to control focus, tracking and/or tilt servos of the optical pickup  300 . The optical recording and/or reproducing apparatus further includes a turn table  352  and a clamp chucking the optical disc  353 .  
      The optical pickup  300  has the optical structure of an optical configuration according to aspect of the present invention shown in  FIGS. 2-9B .  
      Light reflected from the optical disc  1  is detected by a photo detector included in the optical pickup  300  and is then transformed into an electrical signal through photoelectric-conversion. The electrical signal is inputted to the controller  309  through the driving unit  307 . The driving unit  307  controls a rotational speed of the spindle motor  312 , amplifies an inputted signal, and drives the optical pickup  300 . The controller  309  sends a focus servo, a tracking servo and/or a tilt servo, and a command controlled based on the signal inputted from the driving unit  307  to the driving unit  307  to perform focusing, tracking and/or tilt operations of the optical pickup  300 . The optical recording and/or reproducing apparatus adopting the optical pickup  300  can adopt the BD and HD DVD standards in a compatible manner (and possibly other media such as CDs and/or DVDs) and maximize light quantities directed to the BD  1   a  and the HD DVD  1   b , respectively, and thus realize high speed operations.  
      Although the above descriptions describe the case where the optical pickup according to the present invention uses the next-generation DVD standards (i.e., the BD and the HD DVD standards), in a compatible manner, the present invention is not limited to this case. That is, the optical pickup of the present invention can be applied to a variety of systems requiring a plurality of objective lenses of different design specifications besides the BD and the HD DVD systems and be properly modified as required by those systems.  
      According to aspects of the present invention, since the light quantities respectively directed to the plurality of objective lenses may be maximized, optical efficiency is increased and thus a high speed operation can be performed.  
      Further, the optical pickup may adopt the information storage media of two different standards where the wavelength of the light used is the same, and the thickness of the information storage media and the NA are different, e.g., the BD and the HD DVD standards.  
      Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.