Abstract:
A pickup module comprises a plurality of optical components along an optical path, wherein at least two of the optical components are provided with coating layers to change a polarization of a light beam and collectively circularly polarize the light beam. The circularly-polarized beam is projected on an optical storage medium and converted to a signal beam received by an optical detector. A quarter wavelength plate (QWP) or the other phase retarded plate for required purpose (½λ, ¼λ, ⅛λ . . . ) is not included in the optical component set.

Description:
BACKGROUND  
       [0001]     The invention relates to an optical recorder and, in particular, to an optical pickup module.  
         [0002]     Information access by an optical pickup module is accomplished by focusing a laser beam on a surface of a storage medium(disc) and converting the reflected beam to an electronic signal via a photo detector. Circularly polarized laser beams incident on the surface of the disc provide improved signal accuracy, and are thus required in optical pickup module.  
         [0003]      FIG. 1A  shows a conventional pickup module, comprising a first laser diode  101  for DVD, a second laser diode  103  for CD, first polarized beam splitter  105  and second polarized beam splitter  107 , a folding mirror  109 , a quarter wavelength plate  111 , and a photo detector  113 . The first laser diode  101  and second laser diode  103  generate laser beams. The laser beams from first laser diode  101  and second laser diode  103  are linearly polarized, i.e., having no phase difference between orthogonal components S-wave and P-wave. The first polarized beam splitter  105  and the second polarized beam splitter  107  transmit or reflect different components (S-wave and P-wave). The folding mirror  109  reflects a light beam and changes the propagation direction of the light beam. The quarter wavelength plate  111  changes the polarization of the light beam and phase difference between the S-wave and P-wave. When the phase difference between the S-wave and P-wave reaches 90 or 270° (−90°), the light beam is converted to a circularly polarized light for optimum signal accuracy. The photo detector  113  receives the light beam reflected from a surface of a disc.  
         [0004]     For a CD system, operation thereof is almost the same as that of a DVD system. In order to explain the invention, we use the CD system to illustrate. After the laser diode  103  for CD emits a laser beam, orthogonal S-wave and P-wave components are generated, having initial intensities of respectively I S1  and I P1 . If an initial phase difference is δ S−P =0°, as shown in  FIG. 1B , when the CD light beam enters the second polarized beam splitter  107 , the reflectivity of the S-wave and the P-wave are respectively 0% and 90%, as shown by solid arrows in  FIG. 1C . In  FIG. 1C , the dashed arrow represents the orthogonal components of the light beam before the CD light beam passes the second polarized beam splitter  107 , and the dotted arrow the orthogonal components of the CD light beam after the CD light beam passes the second polarized beam splitter  107 . When the light beam reaches the folding mirror  109 , reflectivity of the S-wave and the P-wave are respectively 70% and 20%, as shown by solid arrows in  FIG. 1D . Optical characteristics of the second polarized beam splitter  107  and the folding mirror  109  are shown in Table I.  
                               TABLE I                                       the second                   polarized beam   Folding           reflectivity   splitter 107   mirror109                           S-wave    0%   70%           reflectivity           P-wave   90%   20%           reflectivity                      
 
         [0005]     Thus, intensity I S  of the S-wave(reflected by the second polarized beam splitter  107 ) becomes 0 and intensity I P  of the P-wave which passing the second polarized beam splitter  107  equals I P1 ×90%, i.e., 0.9I P1 . Thereafter, the S-wave is reflected by the folding mirror  109  and intensity I S  of the S-wave equals 0×70%, i.e., 0. Intensity I P  of the P-wave reflected by the folding mirror  109  equals 0.9I P1 ×20%, i.e., 0.18I P1 . The CD light beam is converted to a circularly polarized state by a quarter wavelength plate  111  before reaching a disc, as shown in  FIG. 1E . As a result, although the light beam is eventually converted to a circularly polarized state, energy stored in the S-wave component is never used.  
       SUMMARY  
       [0006]     An embodiment of a pickup module comprises a plurality of optical components along an optical path, wherein at least two of the optical components are provided with coating layers with phase design to change polarization of a light beam and collectively circularly polarize the light beam. The circularly polarized beam is projected onto an optical storage medium and converted to a signal beam received by an optical detector. A quarter wavelength plate(QWP) is not included in the optical component set.  
         [0007]     An embodiment of a method of fabricating an optical component set comprising a plurality of optical components, excluding a quarter wavelength plate, comprises coating a first coating layer on a first polarized beam splitter, coating a second coating layer on a second polarized beam splitter, and coating an nth coating layer on an nth optical component. The first, second and nth coating layers collectively convert a non-circularly polarized light beam to a circularly polarized light beam projected on an optical storage medium and converted to a signal beam received by an optical detector.  
         [0008]     The pickup module according to the embodiment of the invention requires no quarter wavelength plate or the other phase retarded plate for the required purpose(½λ, ¼λ, ⅛λ . . . ), thus reducing cost thereof and avoiding problems resulting from poor quality or poor assembly thereof. In addition, phase difference is induced by combinations of different optical components with coating layers. As a result, coating layers on different optical components change polarization of a light beam such that a better transmittance or reflectivity is optimized and signal intensity improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     FIGS.  1 A˜ 1 E are schematic diagrams illustrating optical components of a conventional pickup module and characteristics thereof.  
         [0010]     FIGS.  2 A˜ 2 E are schematic diagrams illustrating optical components of a pickup module according to an embodiment of the invention and characteristics thereof.  
         [0011]     FIGS.  3 A˜ 3 D show curves of phase difference generated by the optical components versus wavelength of a light beam.  
         [0012]      FIG. 4  is a schematic diagram illustrating optical design of a pickup module according to an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]     A CD system is taken for example.  FIG. 2A  is a schematic diagram of a pickup module according to an embodiment of the invention. The pickup module comprises a first laser diode  201  for DVD, a second laser diode  203  for CD, the first polarized beam splitter  205  with a reflective coating layer  204 , the second polarized beam splitter  207  with a reflective coating layer  206 , a folding mirror  209  with a reflective coating layer  208 , and a photo detector  211 . The first laser diode  201  and second laser diode  203  generate laser beams for DVD and CD. Referring to  FIG. 2A , after the laser diode  203  for CD emits a CD laser beam, orthogonal S-wave and P-wave components are generated, having initial intensities of respectively I S1  and I P1 . If an initial phase difference is δ S−P =0°, as shown in  FIG. 2B , when the light beam enter the second polarized beam splitter  207  with the coating layer  206 , the reflectivity of the S-wave and the P-wave are respectively 0% and 90%, as shown-by solid arrows in  FIG. 2C . When the light beam enters the folding mirror  209  with the coating layer  208 , Reflectivity of the S-wave and the P-wave are respectively 70% and 20%, as shown by solid-arrows in  FIG. 2D . In the embodiment of the invention, coating layers of  206  and  208  respectively generate phase differences of 70° and 200°, as shown by dashed ellipses in  FIGS. 2C and 2D . Optical characteristics of the second polarized beam splitter  207  and the folding mirror  209  are shown in Table II.  
                               TABLE II                                       The second                   polarized               beam   Folding               splitter 206   mirror 208               with coating   with coating           reflectivity   layer 207   layer 209                           S-wave reflectivity    0%   70%           P-wave reflectivity   90%   20%           Phase difference δ S−P     70°   200°                      
 
         [0014]     Thus, when CD laser beam enters the second polarized beam splitter  207  and the coating layer  206 , the coating layer  206  redistributes energy of the light beam and generates a phase difference of  700  between the S-wave and the P-wave. As a result, the CD laser beam is elliptically polarized and a long axis thereof modified to the proximity of the P-wave due to higher reflectivity of the P-wave for coating design. For example, if the phase difference δ S−P  generated by coating layer  206  on the second polarized beam splitter  207  equals 70°, intensities I S  and I P  respectively equal 0.15I S1  and 0.85I P1  after the CD light beam passes through the second polarized beam splitter  207  with the coating layer  206 , generating phase difference. If there is no coating layer with phase shift design, no S-wave component energy is retained after the light beam passes the optical component  207 .  
         [0015]     Subsequently, the same principle can be utilized such that the folding mirror  209  with coating layer  208  redistributes and focuses most energy to the P-wave component. In the absence off coating layer  208  with phase-shift design, since no S-wave component energy is retained after the light beam passes through the second polarized beam splitter  207 , the energy in the original S-wave component is never used. For example, if the phase difference δ S−P  of the second polarized beam splitter  207  with the coating layer  206  is 70°, the intensities I S  and I P  respectively equal 0.15I S1  and 0.85I P1  after the CD light beam passes the second polarized beam splitter  207  with coating layer  206  for CD system, generating a phase difference. The coating layer  208  on the folding mirror  209  generates a phase difference δ S−P  of 200 degrees, with most CD light beam energy focused on the P-wave component. As a result, the intensities I S  and I P  respectively equal 0.13I S1  and 0.10I P1  after the CD light beam passes the folding mirror  209  with the coating layer  208 , generating a phase difference. Intensities I S1  and I P1  of the S-wave and P-wave components of the original light beam are equivalent. Thus, the total energy of the light beam through the pickup module according to an embodiment of the invention is higher than in a conventional configuration. In addition, the optical components generates a total phase difference of δ S−P =70°+200°=270° or −90°, as shown in  FIG. 2E . As a result, a circularly polarized state is obtained.  
         [0016]     The same principle is also used for DVD system. The folding mirror  209  reflects a DVD light beam and changes propagation direction thereof. The coating layers  204 ,  206 , and  208  on the first polarized beam splitter  205  and second polarized beam splitter  207  and the folding mirror  209  change polarization of the DVD light beam and phase difference between the S-wave and P-wave. When the phase difference between the S-wave-and P-wave reaches 90° or 270° (−90°), the light beam is converted to a circularly polarized light. The photo detector  211  receives the DVD light beam reflected from a surface of a disc.  
         [0017]     The wavelength of a laser diode  201  for DVD is typically 660 nm. The coating layers  204 ,  206 , and  208  on the first polarized beam splitter  205  the second polarized beam splitter  207  and the folding mirror  209  can provide phase difference with wavelength as shown in  FIGS. 3A, 3B  and  3 C. Thus, when the DVD laser beam passes through the first polarized beam splitter  205  with the coating layer  204  and the second polarized beam splitter  207  with the coating layer  206 , and the folding mirror  209  with the coating layer  208 , the total phase difference is as shown by the curve in  FIG. 3D . When the wavelength of the DVD laser beam is 660 nm, the coating layers  204 ,  206 , and  208  respectively generates phase differences of θ 1 , θ 2 , and θ 3  between the S-wave and P-wave such that θ 1 , θ 2 , and θ 3  are: 
 
θ 1 +θ 2 +θ 3 =90° or 270°  (1) 
 
         [0018]     Accordingly, the pickup module converts light to a circularly polarized light beam. The effect thereof is the same as that of a quarter wavelength plate. As a result, no quarter wavelength plate is needed. Those skilled in the art can add or remove optical components in the pickup module according to needs. Optical components commonly used in the pickup module can be a laser diode, a beam splitter, a cubic, a grating, a folding mirror, a polarizer, and a collimator. The optical components respectively generate a phase difference of θ 1 , θ 2 , . . . , and collectively generate a total phase difference of ±90° to obtain a circularly polarized light beam. In summary, the principle is used to take both the phase difference and efficiency into consideration by coating design to reach the better efficiency and phase shift.  
         [0019]     In addition, the invention converts a light beam to a state with higher transmissive or reflective efficiency by generating phase difference. In other words, selection of material, number of coating layers and thicknesses thereof are made according to the required phase difference and reflectivity/transmittance. During design of the optical components, as shown in  FIG. 4 , a formula as follows can be used. 
 
Tan 2α=2 I   S0   I   P0  cos θ SP   /I   S0   2   −I   P0   2    (2) 
 
         [0020]     I S0  and I P0  respectively represent reflectivity or transmittance of the optical components with coating layers generating a phase difference. θ SP  represents the phase difference between the S-wave and P-wave components, generated by the optical components with coating layers. The angle a represents an angle between a long axis of an elliptically polarized light and an S-wave axis.  
         [0021]     The optical component set according to the embodiment of the invention does not require a quarter wavelength plate or the other phase retarded plate for required purpose (½λ, ¼λ, ⅛λ . . . ), reducing cost thereof and avoiding problems resulting from poor quality or poor assembly thereof. In addition, phase difference is induced by combinations of different optical components with coating layers. As a result, coating layers on different optical components change polarization of a light beam such that transmittance or reflectivity is optimized, and signal intensity improved.  
         [0022]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and the advantages would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.