Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2006-0075305, filed on Aug. 9, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to an optical pickup apparatus and an optical recording/reproducing system employing the same. 
     2. Description of the Related Art 
     As the industry develops, the amount of data to be processed and recorded increases, and an optical recording medium and optical recording/reproducing system having higher recording density are required. According to such requirement, optical recording/reproducing systems, for example, the Blu-ray Disc (BD) system and HD-DVD system have been suggested. However, these systems need to be compatible with an existing system such as CD or DVD. In particular, since light having a wavelength of 405 nm which is significantly shorter than conventional system is used in the BD system, an objective lens having a higher numerical aperture (NA) is required. Therefore, in order for all CD/DVD/HD-DVD/BD systems to be compatible, an optical pickup apparatus of the optical recording/reproducing system includes two objective lenses. In other words, an objective lens for a CD/DVD/HD-DVD system and an additional objective lens for a BD system is included in the optical pickup apparatus. 
     The method of using two objective lenses in one optical pickup apparatus includes preparing an additional actuator for each objective lens, and mounting all two objective lenses in one actuator. In addition, when mounting all two objective lenses in one actuator, two objective lenses may be arranged in a radial direction or in a tangential direction of a corresponding optical disc. When two objective lenses are arranged in a tangential direction of the optical disc due to a various structural reasons, one of two objective lenses falls off from the center line of the optical disc, which intersects the center of the optical disc, to the tangential direction, that is, offset of the lens occurs. 
     For example, as shown in  FIGS. 1A and 1B , it is assumed that a first objective lens  15  disposed in the left side of an actuator  10  is arranged on the center line of a disk D. In  FIG. 1A , although the first objective lens  15  moves along a radial direction of the disk D and tracks T 1 , T 2 , and T 3  at any position, angles with respect to each track are not changed. However, since a second objective lens  15 ′ disposed in the right side of the actuator  10  is not arranged on an axis that passes through the center of the disk D, as shown in  FIG. 1B , directions of tracks T 1 , T 2 , and T 3  are changed according to the position of the second objective lens  15 ′ determined based on a radial direction of the disk D. Consequently, with respect to optical system combined with the second objective lens  15 ′, wherein the second objective lens  15 ′ is disposed in the right side of the actuator  10 , accurate tracking error signals cannot be obtained using a conventional Differential Push-Pull (DPP) method. 
     Therefore, a conventional method of forming two or more sub beams on the tracks or shifting phases of all or a part of the sub beams by providing a specially shaped diffraction grating or a hologram optical element (HOE) to the optical system having the objective lens which is offset from the central line of the disk is used so that theoretically only a direct current (DC) exists in a sub push-pull signal. In addition, in order to reduce an effect due to a change of a track direction in the inner/outer circumference of discs, a direction of the diffraction grating is adjusted based on the center track T 2 , and the rest of the configuration of the optical system is made as if there is no offset of the lens. 
     However, in the conventional method, while the diffraction grating is rotated, an optical axis of a light source and an axis of the diffraction grating do not coincide, and thus, a difference in the amount of light is generated between sub-beams separated from the diffraction grating. Subsequently, an alternating current (AC) is substantially generated in sub push-pull (SPP) signals. In addition, when an optical axis of a light source and an axis of the diffraction grating coincide with each other, the centers of the sub-beams do not coincide with the boundary of the diffraction grating, and thus, an AC is generated in the SPP signal for each of two sub-beams. Moreover, in the conventional optical system, light emitted from the light source and incident on a track of the disk is only considered to control a direction of diffraction grating. Consequently, for light reflected from the disk track and incident on the photodetector, an angle of incidence may still be inaccurate. In other words, the main beam and a number of sub-beams are controlled to be accurately arranged along the track at the center of the disk, however, angles between the main beam and the sub-beams reflected from the track and photodetectors used to detect such beams may still be off. Accordingly, an AC is generated in the SPP signal, and phases of the SPP signal and a Main Push-Pull (MPP) signal are changed according to the radial position of the objective lens disposed on the disk. As a result, a tracking error signal is changed according to a radius of the disk as shown in  FIG. 2 . Since the tracking error signal is changed according to a radius of the disk, accurate servo-control of the optical pickup apparatus is difficult. 
     SUMMARY 
     Accordingly, an aspect of the present invention is to provide an optical pickup apparatus including an optical system having two objective lenses disposed in a tangential direction of a disc which accurately performs a servo-control, even when one of the objective lenses are offset. 
     Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the invention. 
     The foregoing and/or other aspects of the present invention are achieved by providing an optical pickup apparatus including at least two optical systems for different types of optical recording media, one of objective lenses of the optical systems being offset from a central line of the optical recording medium, wherein the optical system including the offset objective lens includes a diffraction grating diffracting light emitted from a light source to form a main beam and a number of sub-beams, wherein the diffraction grating includes first and second diffraction regions having different grating patterns arranged alternately thereon, and a center of each sub-beam is arranged at the boundary of the first and second diffraction regions on the surface of the diffraction grating, and wherein a center of the diffraction grating and an optical axis of the light source are adjusted to be coincided with each other, thereby preventing a generation of an alternating current (AC) in a Push-Pull signal of the sub-beams. 
     According to an aspect of the present invention, the diffraction grating is rotated for the main beam and the sub-beams formed by the diffraction grating to be arranged along a track direction of the optical recording medium. 
     According to an aspect of the present invention, a rotational angle of the diffraction grating is determined based on a direction of a track positioned at a middle of the optical recording medium in a radial direction. 
     According to an aspect of the present invention, the optical pickup apparatus further includes a photodetector unit and an astigmatic lens in the optical system including the offset objective lens, both rotated corresponding to a track of the optical recording medium, wherein the photodetector unit measures the amounts of light of the main beam and the sub-beams reflected from the optical recording medium and the astigmatic lens introduces astigmatism into the main beam and the sub-beams incident onto the photodetector unit. 
     According to an aspect of the present invention, rotational angles of the astigmatic lens and the photodetector unit are determined based on a track position at the middle of the optical recording medium in the radial direction. 
     According to an aspect of the present invention, the areas of the first and second diffraction regions for each sub-beam are the same on the surface of the diffraction grating. 
     According to an aspect of the present invention, the grating patterns of the first and second diffraction regions may be inclined in opposite directions. 
     It is another aspect of the present invention to provide an optical recording/reproducing system including the optical pickup apparatus described above so as to make tracking servo possible for different types of optical recording media. 
     It is another aspect of the present invention to provide a method of preventing a generation of an AC in a Push-Pull signal of sub-beams in an optical pickup apparatus, wherein the optical pickup apparatus includes at least two optical systems for different types of optical recording media, one of objective lenses in the optical systems being offset from a central line of the optical recording medium, the method including disposing a diffraction grating diffracting light emitted from a light source to form a main beam and a number of sub-beams in the optical system including the offset objective lens, the diffraction grating comprising first and second diffraction regions having different grating patterns arranged alternately on the diffraction grating; and positioning a center of each of the sub-beams at a boundary of the first and second diffraction regions on the surface of the diffraction grating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1A  is a diagram of a conventional optical pickup apparatus including an optical system having two objective lenses illustrating tracking with a first objective lens of optical system; 
         FIG. 1B  is a diagram of a conventional optical pickup apparatus including an optical system having two objective lenses illustrating tracking with a second objective lens of optical system; 
         FIG. 2  is a graph showing changes according to disc radius with respect to track error signals generated from the second objective lens of optical system in the conventional optical pickup apparatus including an optical system having two objective lenses of  FIGS. 1A and 1B ; and 
         FIG. 3  is a diagram schematically illustrating an optical system having an objective lens used in an optical pickup apparatus according to an embodiment of the present invention; 
         FIG. 4  is a diagram of a diffraction grating according to an embodiment of the present invention; 
         FIG. 5A  is a diagram illustrating positions of main beam and sub-beam generated from a conventional diffraction grating; 
         FIG. 5B  is a diagram illustrating positions of main beam and sub-beam generated from the diffraction grating according to an embodiment of the present invention; 
         FIG. 6A  is a diagram illustrating positions of main beam and sub-beam in which light is accepted to four partitioned light detectors in the conventional optical pickup apparatus; 
         FIG. 6B  is a diagram illustrating positions of main beam and sub-beam in which light is accepted to four partitioned photodetectors in the optical pickup apparatus according to an embodiment of the present invention; and 
         FIG. 7  is a graph showing changes according to disc radius with respect to track error signals generated from the second objective lens of optical system in the optical pickup apparatus according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
       FIG. 3  is a diagram schematically illustrating an optical system  20  having an objective lens used in a general optical pickup apparatus according to an embodiment of the present invention. As shown in  FIG. 3 , the optical system  20  comprises a light source  21 , a diffraction grating  22 , a polarization beam splitter  23 , a collimating lens  24 , a reflective mirror  25 , a ¼ wave plate  26 , an objective lens  27 , an astigmatic lens  28 , and a photodetector unit  29 . 
     Here, according to an embodiment of the present invention, the light source  21  is an infrared laser light source having a wavelength of approximately 780 nm, red laser light source having a wavelength of approximately 650 nm, or a blue laser light source having a wavelength of approximately 405 nm when used for CD, DVD, and BD, respectively. However, the light source  21  of the present invention is not limited hereto and may vary as necessary. 
     The diffraction grating  22  comprises a fine grating pattern formed on the surface thereof and diffracts light emitted from the light source  21  to form a zero-order diffracted main beam and ±1st or more order diffracted sub-beams, the sub-beams having smaller amount of light than the main beam. The polarization beam splitter  23  allows light to pass through or to reflect depending on a polarization direction of the incident light. The collimating lens  24  collimates light, progressing from the light source  21 . The reflective mirror  25  bends a progressive direction of light from the light source  21  in order for the light to be incident on a disk D through the objective lens  27 . The ¼ wave plate  26  changes a polarization direction of the light reflected from the disk D after incident on the disk D. The objective lens  27  focuses the main beam and the sub-beam to form a main spot and sub spot on the track of the disk D. The astigmatic lens  28  introduces astigmatism of approximately 45° direction into light reflected by the polarization beam splitter  23  toward the photodetector unit  29 . The photodetector unit  29  comprises a number of photodetectors to measure a respective amount of light of the main beam and the sub-beam which is reflected from the disk D. For example, each photodetector of the photodetector unit  29  is a quad-detector. However, the present invention is not limited hereto, therefore, the photodetector unit  29  may be other photodetectors such as six divided or eight divided photodetector, for example. 
     In an optical pickup apparatus in which all CD, DVD, HD-DVD, and BD are compatible, two optical systems having the above structure are installed respectively for CD/DVD/HD-DVD and BD. In this case, two objective lenses  27  can be installed in one actuator. However, when two objective lenses  27  are installed in a tangential direction of the disk D, problems described above may arise. In order to solve these problems, in the optical system comprising objective lenses  27  which are not arranged at the center line of the disk D, the diffraction grating  22  is simply rotated in the conventional optical pickup apparatus. However, while the diffraction grating  22  was rotated, an additional process to make the optical axis of the light source and the axis of diffraction grating  22  coincide was not performed, and thus, a difference in the amount of light generated among the sub-beams separated from the diffraction grating  22  occurs. 
       FIG. 4  is a diagram of the diffraction grating  22  according to an embodiment of the present invention. In  FIG. 4 , in the diffraction grating  22 , two diffraction regions  22 A and  22 B having grating patterns inclined in an opposite direction to each other are alternately arranged in parallel. As shown in  FIG. 4 , the two diffraction regions  22 A and  22 B are also inclined at a predetermined angle. The inclined angle of the diffraction regions  22 A and  22 B can be determined to correspond with a track direction, for example, at a center track T 2 . According to an embodiment of the present invention, a width of the diffraction regions  22 A and  22 B, a pitch of the grating patterns in the diffraction regions  22 A and  22 B, and a number of the grating patterns are determined so that the amount of light of the sub-beams are same when the centers of the light source  21  and the diffraction grating  22  are arranged to coincide with each other. For example, according to an embodiment of the present invention, when a blue laser diode having a wavelength of 405 nm is used as the light source  21 , the pitch of the grating patterns in the diffraction regions  22 A and  22 B is approximately 18 μm and the number of the grating patterns is approximately 6. However, the present invention is not limited hereto and may vary, as necessary. 
       FIG. 5A  is a diagram illustrating positions of a main beam and a sub-beam generated from a conventional diffraction grating  22 ′ and  FIG. 5B  is a diagram illustrating positions of main beam and sub-beam generated from the diffraction grating  22  according to an embodiment of the present invention.  FIGS. 5A and 5B  are illustrated to explain the principle that the amount of light of the sub-beams becomes the same by the diffraction grating  22  according to an embodiment of the present invention. The diffraction regions in  FIGS. 5A and 5B  are horizontal for convenience but are substantially inclined along a track direction. 
     As shown in  FIG. 5A , position relation between a main beam M, sub-beams S 1  and S 2 , both generating from the conventional diffraction grating  22 ′, and diffraction regions  22 A′ and  22 B′ are illustrated. Although the center of the main beam coincides with a boundary of the diffraction regions  22 A′ and  22 B′ in the diffraction grating  22 ′, the center of the first sub-beam S 1  and the center of the second sub-beam S 2  are disposed in relative positions in different diffraction regions of each other on the diffraction grating  22 ′. Accordingly, the area A of the first diffraction region  22 A′ in the first sub-beam S 1  and the area B of the second diffraction region  22 B′ in the first sub-beam S 1  are different from each other and the area A of the first diffraction region  22 A′ in the second sub-beam S 2  and the area B of the second diffraction region  22 B′ in the second sub-beam S 2  are different from each other. Subsequently, the amounts of light of the two sub-beams S 1  and S 2  are different from each other. Therefore, an AC is generated in each sub push-pull signal for the first sub-beam S 1  and the second sub-beam S 2 , and thus, an AC is generated in the whole SPP signal. If the main beam M in  FIG. 5A  does not coincide with the boundary of the diffraction regions  22 A′ and  22 B′ of the diffraction grating  22 ′, the area A of the first diffraction region  22 A′ in the first sub-beam S 1  and the second sub-beam S 2  cannot be the same as the area B of the second diffraction region  22 B′ in the first sub-beam S 1  and the second sub-beam S 2 , and thus, an AC is generated in the SPP signal. 
     As shown in  FIG. 5B , the center of the first sub-beam S 1  and the center of the second sub-beam S 2  are both disposed at the boundary of a diffraction region  22 A and a second diffraction region  22 B on the surface of the diffraction grating  22  according to an embodiment of the present invention. Therefore, the area A of the first diffraction region  22 A and the area B of the second diffraction region  22 B in the first sub-beam S 1  are same and the area A of the first diffraction region  22 A and the area B of the second diffraction region  22 B in the second sub-beam S 2  are same. Therefore, when the center of the diffraction grating  22  according to an embodiment of the present invention accurately coincides with the optical axis of the light source  21 , the areas A and B of the first and second diffraction regions  22 A and  22 B in the first and second sub-beams S 1  and S 2  divided by the diffraction grating  22  are exactly same as each other. Consequently, the amounts of light of the two sub-beams S 1  and S 2  are the same. 
     According to the principle described above, when the center of the diffraction grating  22  and the optical axis of the light source  21  are arranged to accurately coincide with each other, an AC in the SPP signal can be reduced by rotating the diffraction grating  22  and then accurately arranging the main beam and the sub-beams along a track direction. Accordingly, since a direction of the track is changed according to a radial direction of the disk D, a rotational angle of the diffraction grating  22  may be determined corresponding to a track direction at the center track T 2  of the disk D in order to minimize an error. 
     On the other hand, in the conventional optical system in which the objective lens is not arranged on the center line of the disk D described above, light emitted from the light source  21  which is incident on the disk D is only considered. That is, light reflected from the disk D and incident on the photodetector unit  29  is not considered. Therefore, as illustrated in  FIG. 6A , the main beam M and the sub-beams S 1  and S 2  which are received by three quad-photodetectors  29   a ,  29   b , and  29   c  are inclined as much as a direction of the track at which the objective lens  27  is positioned. Consequently, a baseball pattern of each beam, an astigmatism direction, and division angles of the photodetectors  29   a ,  29   b , and  29   c  are off each other and thus it is hard to accurately determine a tracking error signal. 
     In order to improve such problems, both an astigmatic lens  28  and a photodetector unit  29  which are indicated by a box A in  FIG. 3 , are rotated at a predetermined angle so that the quad-photodetectors  29   a ,  29   b , and  29   c  and beams received by the quad-photodetectors  29   a ,  29   b , and  29   c  coincide with each other. Here, rotation of the astigmatic lens  28  is to introduce a 45° astigmatism into baseball pattern of the main beam M and the sub-beams S 1  and S 2  generated from the disk D. Here, the direction of the tracks varies according to the position of the objective lens  27  based on the radial direction of the disk D as described above. Therefore, in order to minimize an error, the astigmatic lens  28  and the photodetector unit  29  are rotated based on the center track T 2  of the disk D. 
     Consequently, SPP signals not having an AC can be obtained. Then, as illustrated in  FIG. 7 , a tracking error signal without deviation according to a radial direction can be obtained. For example, in the conventional optical pickup apparatus, the deviation of the tracking error signal according to a radial direction is approximately 40% as illustrated in  FIG. 2 . However, in the present invention, the deviation of the tracking error signal according to a radial direction is approximately 5% as illustrated in  FIG. 7 . 
     As described above, in the optical recording/reproducing system according to the present invention which is compatible with respect to an optical recording medium having various kinds such as CD/DVD/BD/HD-DVD, even two objective lenses can be installed along a tangential direction in one actuator, and accurate tracking error signals can be obtained using either two optical systems. Therefore, accurate tracking servo is possible for various kinds of optical recording medium. 
     Although a few embodiments 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.

Technology Category: 3