Abstract:
The present invention discloses a multiple-beam holographic optical pick-up head, which has the advantages of the multiple-beam optical pick-up head and the holographic optical pick-up head and can increase the light usage efficiency. This invention uses the property that a polarized diffractive optical element can generate different optical effects for laser beam with different polarizations and places a polarized multiple-beam grating and polarized hologram on the optical path for splitting the incident light toward the disc into multiple reading beams by the multiple-beam grating. The returning laser beams reflected from the disc are diffracted and diverted by the hologram to reach a photodetector. Both the incident and reflective beams are diffracted once only; thus the light usage efficiency can be increased.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention in general relates to a multiple-beam holographic optical pick-up head. More particularly, this invention combines the advantages of the multiple-beam optical pick-up head and the holographic optical pick-up head and can be the data retrieval pick-up head for the optical recording media. 
     2. Related Art 
     The data reading and writing on optical recording media such as the CD, CD-R and DVD are done with the help of an optical pick-up head. When writing, the laser beam converges on the data storage surface of the optical disk and changes its physical property; while when reading, the laser beam first converges on the data storage surface of the disk, then gets reflected and absorbed by a photodetector. This beam containing data signals is therein converted into the corresponding electrical signals. 
     Conventional optical disk drives use only a single beam to access data on the disk. Their speeds in reading/writing can only be increased by increasing the rotational speed of the motor, which is the way most high-speed optical disk drives adopt. However, the motor rotational speed has its own limit, this method is thus imperfect. 
     Multiple-beam reading/writing uses a special grating to split a laser beam into a plurality of separate laser beams that simultaneously accessing data in different tracks of the optical disk. With special control and a decoding circuit, data in different tracks are then recombined together. Therefore, this method can greatly increase the reading/writing speed of an optical disk drive. 
     FIG. 1 shows the optical path of an optical pick-up head with multiple beams. 
     A light source  101  generates an incident light, which is split by a grating  102  into a plurality of beams. After passing through a beam splitter  103 , a collimator lens  104 , and an objective lens  105 , the beams reach the optical disk surface and am reflected. The reflected beams then pass through the objective lens  105 , the collimator lens  104  and are deflected by the beam splitter  103 . Finally, after passing through another grating  106 , the reflected beams are absorbed by a photodetector  107 . 
     A conventional optical pick-up head comprises a beam splitter, a laser diode, a cylindrical lens, a photodetector, and etc. Since the elements are conventional separate ones, the package volume and weight of the elements are larger. 
     A holographic optical pick-up head replace the optical devices such as beam splitter and the cylindrical lens of a pick-up head in the prior art with a piece of tiny hologram. It shortens the distance between the laser diode and the photodetector. Therefore, all devices can be encapsulated within a tiny package called the holographic laser module. 
     FIGS.  2 ( a ) and  2 ( b ) show a conventional optical pick-up head and a holographic optical pick-up head, respectively. 
     The components described in FIGS.  2 ( a ) and  2 ( b ) are all well known in the art. They include: light source ( 201 ,  211 ), grating ( 207 ,  212 ), prism ( 203 ), collimator lens ( 204 ,  213 ), objective lens ( 205 ,  214 ), reflector ( 206 ,  215 ), another grating ( 202 ), and photodetector ( 208 ,  216 ). 
     Currently, holographic laser modules are very popular in single beam pick-up heads. But there is no multiple-beam optical pick-up head using the holographic laser module. The main reason is that the laser beam usage efficiency of the holographic optical pick-up head is low. For a single beam pick-up head, the energy efficiency of one that uses a holographic laser module is only {fraction (1/12)} of that of a conventional one. The multiple-beam pick-up head further splits one laser beam into a plurality of separate beams and thus diverse the laser beam energy. If one further combines both, the energy efficiency is even lower. Since the energy that reaches the photodetector decreases in such systems, the associated photoelectric current after photoelectric conversion is so weak that the S/N ratio is small. Then the gain of the I-to-V preamplifier has to be raised to meet the need. However, the frequency band of an amplifier is inversely proportional to the gain. Therefore, increasing the gain of the preamplifier will decrease the allowed frequency band of the pick-up head and thus limit the data access speed. 
     To sum up, the multiple-beam optical pick-up head has the advantage of greatly increasing the access speed of the optical disk drive, the holographic optical pick-up head has the merits of a small volume, easy to design and produce. If one wants to combine both, i.e., to have a holographic pick-up head with multiple beams, how to increase the laser energy efficiency is an important technical issue to be solved. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the present invention to provide a multiple-beam holographic optical pick-up head that can increase the laser beam usage efficiency. 
     The disclosed technology uses a polarized diffractive optical element to diffract laser light polarized in a specific direction while leaving the laser light polarized in the other direction unaffected at all. Two polarized diffractive optical elements are provided on the optical path of the system. One is a multiple-beam grating which can diffract one incident beam into several beams, but let reflective beams to completely pass through. The other is a hologram which allows the incident light to completely pass through without diffraction and diffracts the reflective beams. A quarterwave plate is provided on the optical path of the pick-up head for changing the polarization directions of the incident light and reflected light. Since both the incident beams and reflected beams experience diffraction once only, there would not be such problems as low efficiency. The multiple-beam holographic optical pick-up head is formed by combining both the multiple-beam optical pick-up head and the holographic optical pick-up head, and thus has the advantages of both systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a schematic view of the optical path of a multiple-beam optical pick-up head; 
     FIG. 2 compares a normal optical pick-up head with a holographic optical pick-up head; 
     FIG. 3 illustrates how a polarized diffractive optical element functions; 
     FIG. 4 is a first embodiment of the multiple-beam holographic optical pick-up head according to the present invention; 
     FIG. 5 shows a second embodiment of the multiple-beam holographic optical pick-up head according to the present invention; 
     FIG. 6 shows a third embodiment of the multiple-beam holographic optical pick-up head according to the present invention; and 
     FIG. 7 shows a fourth embodiment of the multiple-beam holographic optical pick-up head according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Please refer to FIG. 3 for an illustration of how a polarized diffractive optical element functions. 
     A polarized diffractive optical element uses the birefringent property of the material to diffract laser light polarized in a specific direction, leaving laser light polarized in the other direction unaffected and completely passing through. 
     The diffractive optical element is manufactured by filling a material with one refraction index over a surface-etched birefringent material substrate. Assume that the material with one refraction index has an index of n i  and the refraction indices of the birefringent material on an ordinary ray (hereinafter as O-ray) polarized incident light and an extraordinary ray (hereinafter as E-ray) polarized incident light are n o  and n e , respectively. 
     The birefringent material and the single index material are selected so that n o  is very close to n i . When the incident light is an O-ray  301 , it will completely pass through this diffractive optical element without diffraction because of the index matching between n o  and n i . When the incident light is an E-ray  302 , there will be diffraction due to the difference between n e  and n i . 
     This type of polarized diffractive optical elements can be formed by filling a birefringent material over a surface-etched single index material. 
     FIG.  4 ( a ) is a first embodiment of the multiple-beam holographic optical pick-up head according to the present invention. The pick-up head comprises: 
     a light source generator  401  for generating a linearly polarized beam as the incident light; 
     a multiple-beam grating  402  installed in front of the light source generator  401  for diffracting the incident beam into a plurality of beams and allowing reflected beams to completely pass through, which grating  402  is formed by filling an etched substrate with a filling material, wherein the substrate and filling material can be a single index material and a birefringent material, respectively, or vice versa, with the refraction index of the single index material n i  being close to one of the two refraction indices n o  and n e  of an the birefringent material(e.g., |n i −n o |/n i &lt;10% or |n i −n e |/n i &lt;10%); 
     a quarterwave retardation plate  404  installed in front of an optical recording medium  407  for making the polarization direction of the incident beam perpendicular to that of the reflected beam by having the phase of the incident light retarded by 90° after passing through the quarterwave retardation plate, the linearly polarized light being converted into a circularly polarized light, and having the phase of the reflected light retarded by 90° after passing through the quarterwave retardation plate, the circularly polarized light being converted back into a linearly polarized light but with a phase difference of 180° from that of the incident light (i.e., the polarization directions are perpendicular); 
     a hologram  403  as a polarized diffractive optical element installed between the multiple-beam grating  402  and the quarterwave retardation plate  404  for allowing the incident light to completely pass through while making the reflected light diffracted to produce deflected light, the hologram being comprised of a substrate and a filling material for filling the substrate once it is etched, wherein the substrate and filling material can be a single index material and a birefringent material, respectively, or vice versa, with the refraction index of the single index material m i  being close to one of the two refraction indices m o  and m e  of the birefringent material (e.g., |m i −m o |/m i &lt;10% or |m i −m e |/m i &lt;10%); and 
     a photodetector  408  installed on the optical path of the deflected light generated by the hologram  403  for receiving the reflected light and converting it into the corresponding electrical signals. 
     The light source generator  401  generates a linearly polarized light as the incident light. When the incident light enters the multiple-beam grating  402 , it will be diffracted into a plurality of beams due to the different refraction indices of the substrate and the filling material upon the incident light. When the incident light enters the hologram  403 , it will not be diffracted and completely pass through because the refraction indices of the substrate and the filling material are close. When the incident light enters the quarterwave retardation plate  404 , its phase will be retarded by 90° and the linearly polarized light becomes a circularly polarized light. The incident light passes through a parallel lens  405 , an objective lens  406  and projects onto the optical recording medium  407 . When the reflected light from the optical recording medium  407  enters the quarterwave retardation plate  404 , its phase is further retarded by 90° and the circularly polarized light turns back into a linearly polarized light, but with a phase difference of 180° from the original incident light (i.e., their polarization directions are perpendicular to each other). When the reflected light enters the hologram  403 , it is diffracted to generate deflected light due to the different refraction indices of the substrate and the filling material upon the reflected light. When the reflected light enters the multiple-beam grating  402 , it completely passes through without diffraction because the refraction indices of the substrate and the filling material are close for the reflected light. Once the reflected light is received by the photodetector  408 , it is converted into the corresponding electrical signals for obtaining the data on the optical recording medium. Since both the incident and reflected beams are diffracted just once, the problem of low efficiency in laser light uses can be conquered. 
     The incident and reflected beams are shown as  411  and  412 , respectively, in FIG.  4 ( b ). 
     FIG.  5 ( a ) shows a second embodiment of the multiple-beam holographic optical pick-up head according to the present invention. It comprise: 
     a light source generator  501  for generating a linearly polarized beam as the incident light; 
     a multiple-beam hologram  502  as a polarized diffractive optical element installed in front of the light source generator  501 , which hologram  502  is composed of a first layer of substrate  502 A disposed closer to the light source generator  501 , a second layer of substrate  502 B disposed closer to an optical recording medium  506  and a filling material  502 C filling in between, the first layer of substrate being a birefringent material with indices n o1  and n e1 , the filling material being a single index material with the index n i , and the second layer of substrate being a birefringent material with indices n o2  and n e2 , wherein when n i  is close to both n e2  and n o1  the incident light is diffracted by the first layer or substrate  502 A into a plurality of beams and the beams completely pass through the second layer of substrate  502 B and the filling material  502 C, whereas the reflected light is diffracted by the second layer of substrate  502 B into deflected light, which completely passes through the first layer of material  502 A and the filling material  502 C; 
     a quarterwave retardation plate  503  installed in front of the optical recording medium  506  for making the polarization directions of the incident and reflected beams perpendicular to each other; and 
     a photodetector  507  installed on the optical path of the deflected light generated by the multiple-beam hologram  502  for receiving the reflected light and converting it into the corresponding electrical signals. 
     FIG.  5 ( a ) also shows a parallel lens  504  and an objective lens  505 , similar to the parallel lens  405  and objective lens  406 , respectively, shown in FIG.  4 ( a ). The incident and reflected beams are shown as  511  and  512 , respectively, in FIG.  5 ( b ). 
     The main difference between the current embodiment and the first embodiment is in that the multiple-beam grating and the hologram has been combined to form a multiple-beam hologram. 
     FIG.  6 ( a ) shows a third embodiment of the multiple-beam holographic optical pick-up head according to the present invention. It comprises: 
     a light source generator  601  for generating a linearly polarized beam as the incident light; 
     a multiple-beam hologram  602  as a polarized diffractive optical element installed in front of the light source generator  601 , which hologram  602  is composed of a first layer of substrate  602 A disposed closer to the light source generator  601 , a second layer of substrate  602 B disposed closer to an optical recording medium  606  and a filling material filling  602 C in between, the first layer of substrate being a single index material with the index n 1 , the filling material being a birefringent material with the indices n o  and n e , and the second layer of substrate being a single index material with the index n 2 , wherein when n 2  is close to n e  and n 1  is close to n o  the incident light is diffracted by the first layer or substrate  602 A into a plurality of beams and the beams completely pass through the second layer of substrate  602 B and the filling material  602 C, whereas the reflected light is diffracted by the second layer of substrate  602 B into deflected light, which completely passes through the first layer of material  602 A and the filling material  602 C; 
     a quarterwave retardation plate  603  installed in front of the optical recording medium  606  for making the polarization directions of the incident and reflected beams perpendicular to each other; and 
     a photodetector  607  installed on the optical path of the deflected light generated by the multiple-beam hologram  602  for receiving the reflected light and converting it into the corresponding electrical signals. 
     FIG.  6 ( a ) also shows a parallel lens  604  and an objective lens  605 , similar the parallel lens  405  and objective lens  406 , respectively, shown in FIG.  4 ( a ). The incident and reflected beams are shown as  611  and  612 , respectively, in FIG.  6 ( b ). 
     The main difference between the current embodiment and the second embodiment is in that the second embodiment uses birefringent materials as the first and second layers of substrate and a single index material as the filling material in the multiple-beam hologram, while the current embodiment uses a single index material as the first and second layers of substrate and a birefringent material as the filling material in the multiple-beam hologram. 
     FIG.  7 ( a ) shows a fourth embodiment of the multiple-beam holographic optical pick-up head according to the present invention. Similar to the previous three embodiments, it also contains a light source generator  701 , a first layer of substrate  702 , a second layer of substrate  704 , a filler material between the two layers of substrate  703 , a quarterwave retardation plate  705 , a parallel lens  706  and an objective lens  707 . The main difference between tho fourth embodiment and previous embodiments is that the multiple-beam hologram (which comprises the first layer of substrate  702 , the second layer of substrate  704 , and the filler material  703 ) is very closely connected to the quarterwave retardation plate  705  in the fourth embodiment. The connection can be done by glue or by directly etching a diffraction pattern on the quarterwave retardation plate. Similar to other embodiments, the incident and reflected beams are shown as  711  and  712 , respectively, in FIG.  7 ( b ). 
     EFFECTS OF THE INVENTION 
     The present invention discloses a multiple-beam holographic optical pick-up head, which combines the technologies of the multiple-beam optical pick-up head and the holographic optical pick-up head. It can be applied in data reading/writing on optical recording media. It has such advantages as increasing data access speed, small volume, simple design and ideal for mass production. 
     While the present invention has been shown and described with reference to the foregoing preferred embodiments, it is to be understood by those skilled in the art that other changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.