Abstract:
A polarized-dependent optical diffraction device includes a circular polarization beam generator and a diffraction grating. A diffraction grating is formed by a circular birefractive sheet having a corrugated surface with at least each groove filled with a mass of a material of a refractive index which is substantially equal to one of the circular birefractive indices. A circular polarization beam is supplied to the diffraction grating that the circularly-polarized beams traveling in both directions along a specific optical axis have different diffraction effects so as to enhance the utilization efficiency of the light. In particular, an external magnetic field is used to change the circular birefractive indices in such a manner diffraction efficiency is changed, featuring the merit of adjustability.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The present invention relates to a polarized-dependent optical diffraction device and, in particular, to a diffraction device of the optical products such as the optical pickup heads.  
           [0003]    2. Related Art  
           [0004]    Data stored on optical recording media such as the CD, CD-R, and DVD is retrieved with an optical pickup head. The optical pickup head focuses a laser beam into a light spot on the data storage surface of the optical disk and converts the light beam reflected from the optical disk surface carrying data signals into recognizable electrical signals for further processing.  
           [0005]    Since the laser beam passes through the diffractive elements, the diffraction results in low efficiency of the light. It is primary technical subject for the diffraction devices used in optical pickup heads to enhance the utilization efficiency of the light in order to increase the accuracy of signal retrieval and data recording.  
         SUMMARY OF THE INVENTION  
         [0006]    It is the object of the present invention to produce different diffraction effects on circularly-polarized beams traveling in both directions along an optical axis so as to increase the utilization efficiency of the light. In particular, an external magnetic field is used to change the circular birefractive indices in such a manner diffraction efficiency is changed, featuring the merit of adjustability.  
           [0007]    In accordance with the technology disclosed in the invention, a polarized-dependent optical diffraction device is provided to produce different physics effects on left- and right-circularly-polarized beams using the optical active material in such a way that circularly-polarized beams traveling in both direction along an optical axis have different diffraction. The optical device includes a circular polarization beam generator for converting an incident beam into a circularly-polarized beam and a grating for producing diffraction on the circularly-polarized beam passing therethrough. This grating is formed by isotropic and optical active materials interweaved on the same plane.  
           [0008]    When an incident linearly-polarized beam passes through the circular polarization beam generator, it is converted into a circularly-polarized beam. After reflection, the polarization of the circularly-polarized beam is inversed with respect to the incident one (i.e., from left-circular polarization to right-circular polarization, and vice versa). Due to the special characteristics that the optical active material on the grating produces different physics effects on the left- and right-circularly-polarized beams, the utilization efficiency of the light can be increased.  
           [0009]    In particular, when the magneto-optical material is employed in the grating an external magnetic field can be used to change the diffraction effects on the light, featuring the merit of adjustability.  
           [0010]    These and additional objects and advantages, as well as other embodiments of the invention, will be more readily understood after a consideration of the drawings and the detailed description of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a first embodiment of the polarized-dependent optical diffraction device of the present invention;  
         [0012]    [0012]FIG. 2 is a second embodiment of the polarized-dependent optical diffraction device of the present invention;  
         [0013]    [0013]FIG. 3 is a first embodiment of the optical pickup head of the present invention; and  
         [0014]    [0014]FIG. 4 is a second embodiment of the optical pickup head of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Referring to FIG. 1, the first embodiment of the polarized-dependent optical diffraction device according to the present invention is shown.  
         [0016]    The circular polarization beam generator  102  converts an incident beam into a circularly-polarized beam.  
         [0017]    The grating  104  is composed of isotropic and optical active materials interweaved on the same plane, with the optical active material as a stuffed material provided on the isotropic material as a substrate, or alternatively with the isotropic material as a stuffed material provided on the optical active material as a substrate, by film coating or heavy penetration. The grating  104  diffracts the reflective circularly-polarized beam.  
         [0018]    When an incident beam  101  passes through the circular polarization beam generator  102 , it turns into the circularly-polarized beam  103 , which further passes through the grating  104  and forms the beam  105 . The beam  105  is reflected by a reflection surface  106  and becomes a reflective circularly-polarized beam  107 . The polarization direction of the reflective beam  107  is opposite to that of the incident one  105  (i.e., from left-circular polarization to right-circular polarization, and vice versa). The reflective circularly-polarized beam  107  passes the grating  104  again and produces diffraction, forming the diffracted circularly-polarized beam  108 . Since the optical active material on the grating produces different physics effects on the left- and right-circularly-polarized beams, the utilization efficiency of the light increases.  
         [0019]    With reference to FIG. 2, the second embodiment of the polarized-dependent optical diffraction device of the invention is shown.  
         [0020]    The ¼ λ waveplate  202  for converts the incident beam  201  into the circularly-polarized beam.  
         [0021]    The grating  203  is composed of isotropic and optical active materials interweaved on the same plane, with the optical active material as a stuffed material provided on the isotropic material as a substrate, or alternatively with the isotropic material as a stuffed material provided on the optical active material as a substrate, by film coating or heavy penetration, and the isotropic material can be glass and the optical active material is selected from the group comprising liquid crystal, ferromagnetic materials, and cholesterics materials. The grating  203  closely connected to the ¼ λ waveplate  202  for diffracting the reflective circularly-polarized beam.  
         [0022]    An external magnetic field  208  controls the physical properties of the magneto-optical material on the left- and right-circularly-polarized beams and thus adjusting the utilization efficiency of the light.  
         [0023]    When an incident beam  201  passes through the ¼ λ waveplate  202 , it turns into a circularly-polarized beam, which further passes through the grating  203  and produces diffraction, forming the beam  204 . The beam  204  is reflected by a reflection surface  205  and becomes a reflective circularly-polarized beam  206 . The polarization direction of the reflective beam  206  is opposite to that of the incident one  204  (i.e., from left-circular polarization to right-circular polarization, and vice versa). The reflective circularly-polarized beam  206  passes the grating  203  again and produces diffraction, forming a second diffracted circularly-polarized beam  207 . Since the optical active material on the grating produces different physics effects on the left- and right-circularly-polarized beams, the two diffractions have different effects. The physics effects of the optical active material on the left- and right-circularly-polarized beams are controlled and changed by the external magnetic field  208  so that the utilization efficiency of the light is adjusted.  
         [0024]    Refer to FIG. 3, the first embodiment of the optical pickup head according to the invention is shown.  
         [0025]    The laser source  301  generates the linearly-polarized beam  302  as the laser source for reading optical recording media.  
         [0026]    The circular polarization beam generator  303  converts the linearly-polarized beam  302  into the circularly-polarized beam  304 .  
         [0027]    The grating  305  is composed of isotropic and optical active materials interweaved on the same plane, with the optical active material as a stuffed material provided on the isotropic material as a substrate, or alternatively with the isotropic material as a stuffed material provided on the optical active material as a substrate, by film coating or heavy penetration. The grating  305  diffracts the reflective circularly-polarized beam.  
         [0028]    The photodetector  311  receives and converts the reflective circularly-polarized beam  310  into the corresponding electrical signals.  
         [0029]    The laser source  301  generates an incident beam  302 , which passes through the circular polarization beam generator  303  and turns into a circularly-polarized beam. The incident beam  304  passes through the grating  305  to produce diffraction, forming a first diffracted circularly-polarized beam  306 . After reflecting from the reflection surface  307 , a reflective circularly-polarized beam  308  with the opposite polarization to the incident circularly-polarized beam (i.e., from left-circular polarization to right-circular polarization, and vice versa) passes the grating  305  again and has another diffraction pattern, forming a second diffracted circularly-polarized beam  309 . This diffracted beam  309  is still circularly-polarized after passing through the circular polarization beam generator  303 . Finally, the circularly-polarized beam  310  is detected by the photodetector  311  and its optical signals are converted into the corresponding electrical signals. Since the optical active material on the grating of the optical pickup head has different physics effects on the left- and right-circularly-polarized beams, the utilization efficiency of the light is increased.  
         [0030]    Referring to FIG. 4, the second embodiment of the optical pickup head disclosed by the invention is shown.  
         [0031]    The laser source  401  generates the incident beam  402 .  
         [0032]    The ¼ λ waveplate  403  converts the incident beam  402  into the circularly-polarized beam.  
         [0033]    The grating  404  is composed of isotropic and optical active materials interweaved, with the optical active material as a stuffed material provided on the isotropic material as a substrate, or alternatively with the isotropic material as a stuffed material provided on the optical active material as a substrate, by film coating or heavy penetration, and the isotropic material can be glass and the optical active material is selected from the group comprising liquid crystal, ferromagnetic materials, and cholesterics materials. The grating  404  is closely connected to the ¼ λ waveplate  403  for diffracting the reflective circularly-polarized beam. The external magnetic field  408  controls the physical properties of the magneto-optical material on the left- and right-circularly-polarized beams and thus adjusts the utilization efficiency of the light.  
         [0034]    The photodetector  410  receives and converts the reflective circularly-polarized beam  409  into the corresponding electrical signals.  
         [0035]    The laser source  401  generates the incident beam  402 , which passes through the ¼ λ waveplate  403  and turns into the circularly-polarized beam. The incident circularly-polarized beam passes through the grating  404  and forms the beam  405 . After reflecting from the reflection surface  406 , a reflective circularly-polarized beam with the opposite polarization to the incident circularly-polarized beam (i.e., from left-circular polarization to right-circular polarization, and vice versa) passes the grating  404  again and has another diffraction pattern, forming the diffracted circularly-polarized beam  409 . This diffracted beam  409  is detected by the photodetector  410  and the optical signals contained therein are converted into the corresponding electrical signals. Since the optical active material on the grating of the optical pickup head has different physics effects on the left- and right-circularly-polarized beams, the two diffractions differ from each other. With the external magnetic field  408  to change the direction or magnitude of the magnetic field, the diffraction efficiency of the optical active material on the left- and right-circularly-polarized beams can be adjusted. Through this adjustment of diffraction effects the utilization efficiency of the light can be modified.  
         [0036]    The polarized-dependent optical diffraction device disclosed in the present invention allows different diffraction effects on the circularly-polarized beams traveling in both directions along a specific optical axis so as to increase the utilization efficiency of the light. This device solves the problem of low utilization efficiency when the light is diffracted twice. More particularly, when the magneto-optical material is adopted in the grating, the diffraction effects can be varied with the external magnetic field, featuring the merit of adjustability.  
         [0037]    Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.