Abstract:
An optical pickup head which utilizes two or three laser beams of different wavelengths for reading or writing data on different kinds of optical recording media through a same optical output path is disclosed. According to the present invention, a beam shaper located in optical paths of multiple laser beams is used. The beam shaper is composed of two prisms in which several laser beam interfaces are formed. Each laser beam interface is furnished with a specific coating for reflecting laser beam of a specific wavelength and transmitting and refracting laser beams of other wavelengths. Therefore, laser beams in different wavelengths can be shaped into a same optical output path for reading and writing data from optical recording media. The laser beams passing through the beam shaper can be eliminated of their chromatic aberrations, and improved their cross-sectional shapes to get higher usage efficiencies of the laser beams. And, the optical pickup head is constructed as a compact unit with small size.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to an optical pickup head, and more particularly to an optical pickup head which utilizes several laser beams of different wavelengths for reading or writing data on different kinds of optical disc. 
     An optical pickup records and reproduces information such as video or audio data onto/from recording media, e.g., laser discs. A disc has a structure that an information-bearing surface is formed on a substrate. Regular compact disc drivers are designed subject to the specifications of the laser discs. Currently, a DVD (digital versatile disc) driver is designed to read/write data on different kinds of optical disc, such as CD (compact disc) or DVD. The reading or writing is made by an optical pickup head which can provide two focusing points on an information-bearing surface of the disc which has a specific thickness. The thickness of the disc is defined as a distance from the surface of the disc to the information-bearing surface. When reading, the laser beam reflected from the disc and bearing the data signal is transformed by a photo detector into recognizable electrical signal. A similar process is generated for data writing. 
     The following is a table listing the specifications of the disc and the laser light for a CD and a DVD: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                   
                 NA 
               
               
                   
                 THICKNESS 
                 TRACK 
                 WAVELENGTH 
                 (numerical 
               
               
                   
                 (mm) 
                 PITCH (μm) 
                 (nm) 
                 aperture) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 CD 
                 1.2 
                 1.6 
                 780 
                 0.45 
               
               
                 DVD 
                 0.6 * 2 
                 0.74 
                 635-650 
                 0.6 
               
               
                   
               
             
          
         
       
     
     Because the thickness of a CD is different from that of a DVD, for the requirement of compatibility, a DVD driver must be designed to read discs of different thickness. There are several methods which achieve this requirement. These methods include: 
     1) using two objective lenses with different focuses, and selecting one of them corresponding to the CD or DVD by a driving mechanism such as a rotary mechanism. But the two objective lenses greatly increase the weight of the pickup head and the price of the product; 
     2) providing a diffraction member for the objective lens and performing two focuses; 
     3) using a HOE (holographic optical element) to provide two focusing points. But the efficiency of usage of laser light is decreased, and the HOE is difficult and expensive to be manufactured; 
     4) using an LCD (liquid crystal display) shutter as an NA controller which produces two numerical apertures from a single wavelength laser. However, it is difficult to be installed since the polarization of the laser light source must be properly arranged relative to the LCD and polarizer. Moreover, the LCD needs continuous power supply to maintain shutter effect; 
     5) providing an objective lens having a near axial region which includes a center of the light path and a far axial region located radially outward from the intermediate region, using electric signals corresponding to the near axial region converted in only the inner photodetector when the light is reflected from a CD; and using electric signals corresponding to both near and far axial regions converted in both the inner and outer photodetector when the light is reflected from a DVD. This is disclosed in U.S. Pat. No. 5,665,957. It has the disadvantages of higher manufacturing cost, lower acceptable tolerance or higher precision demand of installation, and decreasing the laser efficiency about 15%; 
     6) as disclosed in U.S. Pat. No. 5,777,970, providing a first optical system which leads the laser beam output from a first laser diode and a second optical system which leads the laser beam output from a second laser diode, providing an optical element which has refractivity to either the first optical system or the second optical system, and driving either the first laser diode or the second laser diode according to the type of the loaded optical disc. The drawbacks are that it is expensive and complicated in structure; and 
     7) adjusting an electronic aperture ring to change the numerical aperture, and change the distance between the disc and the object lens subject to type of the disc used. Similar designs are disclosed in U.S. Pat. No. 5,659,533 and U.S. Pat. No. 5,281,797. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide an optical pickup head which utilizes two or three laser beams of different wavelengths for reading or writing data on different kinds of optical recording media through a same optical output path. 
     According to the present invention, a beam shaper located in optical output paths of multiple laser beams is used. The beam shaper is composed of two prisms in which several laser beam interfaces are formed. Each laser beam interface is furnished with a specific coating for reflecting laser beam of a specific wavelength and transmitting and refracting laser beams of other wavelengths. By accommodating the angles and the material of the prisms, laser beams in different wavelengths can be shaped into a same optical output path for reading and writing data from optical recording media of different densities. 
     Another objective of the present invention is to provide an optical pickup head which utilizes multiple laser sources and is constructed as a compact unit with small size. 
     According to the invention, several laser beams of different wavelengths are incident to a beam shaper from different planes individually, then reflected or refracted via some optical interfaces and composed into an output path, therefore, no complicated components or complicated optical paths are needed. 
     A further objective of the present invention is to provide an optical element of a pickup head in which laser beams can be enlarged, and a higher usage efficiency of laser beam can be obtained. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The above objects and advantages of the present invention will become more apparent from the following detailed description preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 is a sectional view of a beam shaper for pickup head according to the present invention showing two laser beams of different wavelengths composed into a same output path; 
     FIG. 2 is another usage view of the beam shaper of FIG. 1 showing three laser beams of different wavelengths composed into a same output path; 
     FIG. 3 is a sectional view of a beam shaper as a second embodiment showing two laser beams of different wavelengths composed into a same output path; 
     FIG. 4 is another usage view of the beam shaper of FIG. 3 showing three laser beams of different wavelengths composed into a same output path; 
     FIG. 5 shows an arrangement of a pickup head according to a first embodiment of the present invention applying the beam shaper of FIG. 1; 
     FIG. 6 shows an arrangement of a pickup head according to a second embodiment of the present invention applying the beam shaper of FIG. 2; 
     FIG. 7 shows an arrangement of a pickup head modified from the arrangement of FIG. 6; 
     FIG. 8 shows another arrangement of a pickup head modified from the arrangement of FIG. 6; and 
     FIG. 9 is a cross sectional view of a laser beam enlarged by a beam shaper according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the beam shaper  10  applicable to a pickup head according to the invention is consisted of a first prism  20  and a second prism  30  which have different refraction indexes. The beam shaper  10  is formed with several planes locating in different angles. The planes include at least an incident laser plane  101 , a first beam-composing interface  102  and a second beam-composing interface  103 . The incident plane  101  and one of the first and second beam-composing interfaces are formed on a same prism, while the other beam-composing interface is formed on the conjunction plane of the first prism  20  and the second prism  30 . The beam shaper  10  will be installed in the optical path of a multiple laser source pickup head for composing several laser beams of different wavelengths into a single optical output path (P O ), therefore enables the pickup head using several different laser beams to read and write data from the optical recording media of different densities, e.g. CD and DVD, through a same optical path. 
     In the embodiment shown in FIG. 1, the beam shaper  10  is composed of a quadrilateral prism as a first prism  20  and a right triangular prism as a second prism  30 . The incident laser plane  101  enables a second laser beam (Beam 2) coming into the beam shaper  10  and refracted to the first beam-composing interface  102 . The first beam-composing interface  102  is processed with a specific coating for refracting a first laser beam (Beam 1) coming into the prism  20 , but reflecting the second laser beam (Beam 2) coming into the beam shaper  10 . Therefore, the first laser beam (Beam 1) and the second laser beam (Beam 2) are composed into the same optical path P 1  and coming to the second beam-composing interface  103 . The second beam-composing interface  103  is processed with specific coating for passing the first laser beam (Beam 1) and the second laser beam (Beam 2), but reflecting the third laser beam which is the signal beam (Return Beam) returning from the media or actually a third incident laser beam (Beam 3, as shown in FIG. 2) composed into the same optical output path P O . 
     The beam shaper  10  in FIG. 1 is an example to be used for a two laser source pickup head. The first laser beam (Beam 1) and the second laser beam (Beam 2) are coming from two laser generators, such as laser diodes, of different wavelengths. The Return Beam is the laser beam carrying data signal, reflected from the unshown optical recording media reversely via the optical output path P O  of the first laser beam (Beam 1) and the second laser beam (Beam 2), and reflected by the second beam-composing interface  103  to an unshown photo detector for retrieving the data signal. 
     Referring to FIG. 2, the beam shaper  10  may also be used for a three laser source pickup head. The first laser beam (Beam 1), the second laser beam (Beam 2) and the third laser beam (Beam 3) are coming from three laser generating elements of different wavelengths, and composed by the first beam-composing interface  102  and the second beam-composing interface  103  into the optical output path P O  for coming to the optical recording media and retrieving the data signal therefrom. 
     To accomplish the composition of different laser beams to the same output path P O  as described above, the angles and the materials of the first prism  20  and the second prism  30  have to be carefully designed, so that the laser beams of different wavelengths coming to the incident plane  101 , reflected or refracted by the first beam-composing interface  102  and the second beam-composing interface  103  can all be lead into the same optical output path P O . 
     The first prism  20  and the second prism  30  of the present invention are made of materials of different refraction indexes. According to the achromatic theory, matching of an F-series lens with a C-series lens will eliminate the chromatic aberration. And, by the Snell&#39;s law, as the formulas listed below, the angle A c  of the first prism  20  and the angle A f  of the second prism  30  can be determined (referring to FIG.  1 ): 
     
       
         n sin θ 1 =n c sin θ 1′   (1) 
       
     
     
       
         n sin θ 2 =n c sin θ 2′   (2) 
       
     
     
       
         n c sin θ 3 =n f sin θ 3′   (3) 
       
     
     in which, 
     n c  is the refraction index of the first prism  20 ; and 
     n f  is the refraction index of the second prism  30 . 
     Further according to the achromatic theory,                    A   c          (       n   c     -   1     )         Vd   c       =         A   f          (       n   f     -   1     )         Vd   f               (   4   )                                
     in which, 
     A c  is the angle of the first prism  20  between the first beam-composing interface  102  and the second beam-composing interface  103 ; 
     A f  is the angle of the second prism  30  between the second beam-composing interface  103  and the output plane  104 ; 
     Vd c  is the chromatic dispersion ratio, i.e. the Abbe number, of the first prism  20 ; and 
     Vd f  is the chromatic dispersion ratio, i.e. the Abbe number, of the second prism  30 . 
     Supposing the first prism  20  and the second prism  30  are made of F8 and BK7 glass respectively, and the angles A f =45°, θ 3′ =45°, then the angle A c  can be calculated from the formula (4); then 
     the angle θ 3  can be calculated from the formula (3), and the angle θ 1′  can be calculated from triangle geometry; further 
     the angle θ 1  can be calculated from the formula (1), and the angle θ 2′  can be calculated from triangle geometry; and finally 
     the angle θ 2  can be calculated from the formula (2). 
     If A f ≠45°, the angle θ 3′  can still be found out from the output plane  104 , then the rest angles can be calculated in the same way as described above. 
     The composition of the first prism  20  and the second prism  30  not only combines different laser beams into an optical output path P O , but also eliminates the chromatic aberration of the first laser beam (Beam 1) and the second laser beam (Beam 2) when they passing through the beam shaper  10 . Furthermore, when the first laser beam (Beam 1) and the second laser beam (Beam 2) are generated from laser diodes, the cross-sectional shape of the laser beam is originally of elongate ellipses as shown with dashed ellipse in FIG. 9 which will decrease the usage efficiency of the laser beams. But after the first laser beam (Beam 1) and the second laser beam (Beam 2) passing through he first prism  20 , they are enlarged into round circular shapes as shown with the rigid circle in FIG. 9, and the usage efficiencies of the laser beams are improved. The enlargement rates of the first laser beam (Beam 1) and the second laser beam (Beam 2) can be calculated from the following formulas:                Γ   1     =       cos                   θ     1   ′           cos                   θ   1                 (   6   )                 Γ   2     =       cos                   θ     2   ′           cos                   θ   2                 (   7   )                                
     in which, 
     Γ 1  is the enlargement rate of the first laser beam (Beam 1), and 
     Γ 2  is the enlargement rate of the second laser beam (Beam 2). 
     Referring now to FIG. 3, a second embodiment of a beam shaper  10   a  applicable to a pickup head according to the invention is shown. The beam shaper  10   a  is also consisted of a first prism  20   a  and a second prism  30   a . Differing from the quadrilateral prism  20  shown in FIG.  1  and FIG. 2, the first prism  20   a  shown in FIG. 3 is also of a triangular prism. 
     The beam shaper  10   a  is also formed with several planes locating in different angles. The planes include at least an incident laser plane  101   a , a first beam-composing interface  102   a  and a second beam-composing interface  103   a.    
     In the same way, the angle A c  of the first prism  20   a  and the angle A f  of the second prism  30   a  can also be calculated from the aforesaid formulas. 
     The beam shaper  10   a  shown in FIG. 3 is used for a two laser source pickup head. The first laser beam (Beam 1) and the second laser beam (Beam 2) are coming from two laser generators, such as laser diodes, of different wavelengths. The Return Beam is the laser beam carrying data signal, reflected from the unshown optical recording media via the optical output path P O  of the first laser beam (Beam 1) and the second laser beam (Beam 2), and reflected by the second beam-composing interface  103   a  to an unshown photo detector for retrieving the data signal. The incident laser plane  101   a  enables the second laser beam (Beam 2) coming into the beam shaper  10   a  with an incident angle θ 2  and refracting to the first beam-composing interface  102   a  with a refraction angle θ 2 ′. The first beam-composing interface  102   a  is processed with a specific coating for refracting a first laser beam (Beam 1) coming into the prism  20   a , but reflecting the second laser beam (Beam 2) coming into the beam shaper  10   a . Therefore, the first laser beam (Beam 1) and the second laser beam (Beam 2) are composed into the same optical path P 1   a  and coming to the second beam-composing interface  103   a . The second beam-composing interface  103   a  is processed with specific coating for passing the first laser beam (Beam 1) and the second laser beam (Beam 2), but reflecting the third laser beam which is the signal beam (Return Beam) returning from the media or actually a third incident laser beam (Beam  3   a , as shown in FIG. 4) composing into the same optical output path P O . 
     Referring to FIG. 4, the beam shaper  10   a  may also be used for a three laser source pickup head. The first laser beam (Beam 1), the second laser beam (Beam 2) and the third laser beam are coming from three laser generating elements of different wavelengths, and composed by the first beam-composing interface  102   a  and the second beam-composing interface  103   a  into the optical output path P O  for coming to the optical recording media and retrieving the data signal therefrom. 
     Referring to FIG. 5, an arrangement of a pickup head according to the present invention applying the beam shaper of FIG. 1 is shown. The pickup head is applied in a DVD driver capable of reading/writing CD and DVD. The pickup head includes a first laser beam generator  40  for providing the first laser beam (Beam 1) and a second laser beam generator  50  for providing the second laser beam (Beam 2). The two laser beam generators are laser diodes or other similar elements. The wavelength of the first laser beam is 430 nm or 780 nm for reading/writing data of a DVD. The wavelength of the second laser beam is 650 nm or 635 nm for reading/writing data of a CD. 
     The first laser beam (Beam 1) first passes through a first collimator lens  41  to become a parallel beam, then refracted by the first beam-composing interface  102 , further refracted by the second beam-composing interface  103 , then comes out along the output path P O , and finally focused by an objective lens  60  onto the information-bearing surface of a optical recording media  70  as an reading/writing spot. The beam containing data and reflected from the optical recording media  70  reversely goes along the output path P O , then reflected by the second beam-composing interface  103  and focused by a lens  61  to a photo detector  62 . The photo detector  62  will transform the reflected beam into corresponding signals which represents data retrieval from the optical recording media  70 . The writing procedure is done through the same optical path. 
     As for the second laser beam (Beam 2), it will first pass through a second collimator lens  41   a  to become a parallel beam, refracted by the incident plane  101 , reflected by the first beam-composing interface  102 , then, in the same way, refracted by the second beam-composing interface  103 , comes out along the output path P O , and finally focused by the objective lens  60  onto the information-bearing surface of the optical recording media  70  as an reading/writing spot. The beam containing data and reflected from the optical recording media  70  reversely goes along the output path P O , then reflected by the second beam-composing interface  103  and focused by the lens  61  to the photo detector  62  for accomplishing data retrieval. 
     FIG. 6 shows another arrangement of a pickup head for reading/writing CD or DVD according to the present invention in which three laser sources are applied. The pickup head includes a first laser beam generator  40  for providing the first laser beam (Beam 1), a second laser beam generator  50  for providing the second laser beam (Beam 2) and a third laser beam generator  80  for providing the third laser beam (Beam 3). The three laser beam generators are laser diodes or other similar elements. The wavelength of the first laser beam is 430 nm for reading/writing data of a DVD. The wavelength of the second laser beam is 650 nm or 635 nm for reading/writing data of a CD. The wavelength of the third laser beam is 780 nm for reading/writing data of a DVD also. 
     The optical paths of the first laser beam (Beam 1) and the second laser beam (Beam 2) are the same as that of FIG.  5 . The third laser beam (Beam 3) first passes through a third collimator lens  41   b  to become a parallel beam, then comes to the second beam-composing interface  103 , refracted by the second beam-composing interface  103 , then comes out along the same output path P O  as that of the first laser beam (Beam 1) and the second laser beam (Beam 2), and finally focused by the objective lens  60  onto the information-bearing surface of the optical recording media  70  as an reading/writing spot. 
     The physical design for retrieving data from the reflective beam from the optical recording media  70  along the output path P O  can be accomplished by the following examples. 
     First, as shown in FIG. 6, a beam splitter cube  65  is arranged in the optical path P O  between the beam shaper  10  and the objective lens  60 . The beam containing data and reflected from the optical recording media  70  along the output path P O  will be reflected by the beam splitter cube  65  and focused by the lens  61  to the photo detector  62  for data retrieval. 
     Second, as shown in FIG. 7, two or three holographic optical elements  66   b ,  66   a  and  66   c  are incorporated respectively with the first laser beam generator  40   a , the second laser beam generator  50   a  and the third laser beam generator  80   a . Then, two or three photo detectors  62   b ,  62   a  and  62   c  are used to received respectively the corresponding laser beam separated by the holographic optical elements  66   b ,  66   a  and  66   c  for data retrieval. 
     Third, as shown in FIG. 8, a polarizing beam splitter (P.B.S.)  67  and a quarter wavelength (λ/4) plate  68  are arranged in the optical path between the first laser beam generator  40  and the first collimator lens  41 . Therefore, the laser beam reflected from the optical recording media  70  can be splitted and reflected to the lens  61   a  to the photo detector  62   d  for data retrieval. 
     As shown in FIGS. 5 to  8 , in any arrangement of pickup heads with two or three laser sources, a mirror  63  for changing the direction of optical path, or a filter  64  for changing the numerical aperture can be arranged in the optical path P O  between the beam shaper  10  and the objective lens  60  as required. Of course, the filter  64  can also be replaced by an apparatus having interchangeable objective lenses. 
     The advantages of the present invention are as follows: 
     1) Two or three laser sources are incorporated into a same optical path. So, the multiple laser source pickup head is compact in structure; 
     2) Applicable to pickup heads with two or three laser sources; and 
     3) The cross-sectional ellipse shapes of two laser beams can be enlarged into round circular shapes, as shown in FIG. 9, to improve the usage efficiencies of the laser beams. 
     It is to be understood that the drawings are designed for purposes of illustration only, and are not intended as a definition of the limits and scope of the invention disclosed.