Abstract:
An optical semiconductor device comprising an emitted beam branching section ( 61 ) which branches an emitted light beam from a laser device ( 51 ), a reflected light beam branching section ( 71 ) which branches a reflected light beam from an information recording medium ( 3 ) into light beams different from each other in focused state, servo signal sensing photodetectors ( 43, 45 ) which receive the branched reflected light beam in a defocused state, a first diffraction grating provided in the emitted light beam branching section for diffracting the reflected light beam having passed through the reflected light beam flux branching section, and a signal sensing photodetector ( 47 ) which receives the reflected light beam diffracted by the first diffraction grating.

Description:
This Application is a 371 of PCT/JP01/05822 Jul. 4, 2001. 
     TECHNICAL FIELD 
     The present invention relates generally to an optical information processing device that performs the recording, reproduction, erasure, etc. of information with respect to an information recording medium such as an optical disk. The present invention particularly relates to an optical semiconductor device employed in an optical head device, which has a function of detecting reproduction signals and various kinds of servo signals, and to an optical element used therein. 
     BACKGROUND ART 
     The following description will depict a configuration and principle of operation of a conventional optical semiconductor device used in an optical information processing device, which has a function of detecting reproduction signals and various kinds of servo signals, while referring to  FIG. 10 . A light beam emitted from a semiconductor laser element  101  as a light source is diffracted in a Y direction as viewed in the figure by a three-beam generating diffraction grating element  102 , so that a zeroth order light of the same becomes a main beam while the first order lights (±1) become sub beams. These three beams obtained by division are focused on an information recording medium  106  by an objective lens  105  and reflected by the information recording medium  106 , and then enter a hologram element  103 . 
     The hologram element  103  is a diffraction grating composed of gratings, each in a curved line form. A reflected light beam from the information recording medium  106  is divided by the hologram element  103 , where the +1-order diffracted light  107 A is subjected to a converging effect, while the −1-order diffracted light  107 B is subjected to a diverging effect, and they are guided to photodetector elements  104 A and  104 B, respectively. The +1-order diffracted light  107 A incident on the photodetector element  104 A is focused before a light-receiving surface thereof, whereas the −1-order diffracted light  107 B incident on the photodetector element  104 B is focused behind a light-receiving surface thereof. 
     Reproduction signals and focus error signals are detected from a main beam among the reflected light beams having been guided to the photodetector elements  104 A and  104 B, while tracking error signals are detected from sub beams among the same. Focus servo is performed so that the +1-order diffracted light  107 A and the −1-order diffracted light  107 B resulting from the division by the hologram element  103  have light spots of substantially the same size on the photodetector elements. Tracking servo is performed so that sub beams have equal quantities of light. The position of the objective lens is controlled by those servos, whereby an appropriate action of the optical semiconductor device as an optical information processing device can be achieved. 
     In the foregoing conventional optical semiconductor device, the same photodetector elements  104 A and  104 B are used for detecting reproduction signals and focus error signals. Since the ±1-order diffracted light having been subjected to the converging and diverging effects, respectively, at the hologram element  103  have to be received in a defocused state by the photodetector elements  104 A and  104 B, the photodetector elements  104 A and  104 B must have large light-receiving areas, approximately 30000 μm 2  each. In the case where the photodetector elements have large light-receiving areas, electric capacitances associated with the photodetector elements increase, thereby impairing the quick responsiveness significantly. This particularly has been a significant problem when CD-ROMs, DVD-ROMs, etc. are reproduced at a high speed, for instance, at several tens of times the normal speed. Furthermore, there has been the following problem as well: in the case where the photodetector element for detecting reproduction signals has a large light-receiving area, stray light components incident thereon (external light, unnecessary reflection) increase, thereby decreasing the signal-to-noise ratio (hereinafter referred to as S/N ratio) of the reproduction signals. 
     DISCLOSURE OF THE INVENTION 
     Therefore, with the foregoing in mind, it is an object of the present invention to solve the above-described problems of the prior art, and to provide an optical semiconductor device capable of reproducing an information recording medium at a high speed and obtaining reproduction signals with an excellent S/N ratio, and an optical information processing device employing the same. 
     An optical semiconductor device of the present invention includes: a laser element; an emitted beam dividing portion for dividing an emitted light beam from the laser element into a plurality of light beams; a reflected beam dividing portion for dividing a reflected light beam from an information recording medium into light beams in different focused states; servo-signal-detecting photodetector elements for receiving the reflected light beams obtained by the division by the reflected beam dividing portion in a defocused state; a first diffraction grating that is provided in the emitted beam dividing portion and that diffracts the reflected light beam having passed through the reflected beam dividing portion; and a signal-detecting photodetector element for receiving reflected light beams having been subjected to the diffraction by the first diffraction grating. 
     This configuration allows the diffracted light obtained by the diffraction of the reflected light beam from the information recording medium by the first diffraction grating to substantially focus on the reproduction-signal-detecting photodetector element, thereby making it possible to reduce a light receiving area of the reproduction-signal-detecting photodetector element. This allows reduction of the capacitance associated with the photodetector element, thereby ensuring high-speed response of the reproduction signals. Furthermore, the reduction of the area of the reproduction-signal-detecting photodetector element leads to a decrease in stray light components incident on the detecting portion for detecting reproduction signals, thereby allowing reproduction signals with an excellent S/N ratio to be obtained. 
     An optical semiconductor device of the present invention that has another configuration includes: a laser element; a first optical element through which an emitted light beam from the laser element passes; a second optical element for dividing the reflected light beam from an information recording medium into light beams in different focused states; and a first diffraction grating that is provided in the first optical element and that diffracts the reflected light beam having passed through the second optical element. 
     An optical element of the present invention includes: a first optical element that is provided on one surface of a transparent member and that includes first and second diffraction gratings; and a second optical element that is provided on the other surface of the transparent member and that divides a reflected light beam into light beams in different focused states, wherein the first and second diffraction gratings are juxtaposed in a first direction, and gratings of the first diffraction grating are arranged in a direction different from the first direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a schematic configuration of an optical semiconductor device according to a first embodiment of the present invention and an optical information processing device employing the same. 
         FIG. 2  is a plan view of an emitted beam dividing portion in the optical semiconductor device shown in  FIG. 1 . 
         FIG. 3  is an enlarged cross-sectional view specifically illustrating a first diffraction grating in the emitted beam dividing portion shown in  FIG. 2 . 
         FIG. 4  is an enlarged cross-sectional view specifically illustrating another embodiment of the first diffraction grating. 
         FIG. 5  is an enlarged plan view specifically illustrating an arrangement of detecting sections included in the photodetector element in the optical semiconductor device shown in  FIG. 1   
         FIG. 6  is a plan view illustrating an emitted beam dividing portion in an optical semiconductor device according to a second embodiment of the present invention. 
         FIG. 7  is a plan view illustrating an emitted beam dividing portion in an optical semiconductor device according to a third embodiment of the present invention. 
         FIG. 8  is an enlarged plan view specifically illustrating an arrangement of detecting sections included in a photodetector element in the optical semiconductor device according to the third embodiment of the present invention. 
         FIG. 9  is an enlarged plane view specifically illustrating another embodiment of an optical element in the optical semiconductor device according to the third embodiment of the present invention. 
         FIG. 10  is a cross-sectional view illustrating a conventional optical semiconductor device and an optical information processing device employing the same. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     First Embodiment 
       FIG. 1  schematically illustrates an optical semiconductor device  1  (basic cross-sectional configuration) according to the first embodiment of the present invention, and an optical information processing device  2  in which the optical semiconductor device  1  is employed. The optical information processing device  2  includes an optical semiconductor device  1 , an objective lens  4  for focusing a light beam emitted from the optical semiconductor device  1  into an information recording medium  3  (for instance, an optical disk, a magneto-optical disk, etc.), and a mechanism controlling actions of the same (a servo mechanism, a signal processing circuit, etc., though not shown). 
     The optical semiconductor device  1  includes a semiconductor element  11 , a package  21  incorporating the same, and an optical element  31  mounted on the package  21 . By sealing the semiconductor element  11  in the package  21  and the optical element  31 , the optical semiconductor device  1  has an integrated configuration. Increasing the effectiveness of the sealing improves the reliability of the semiconductor element  11 . 
     The semiconductor element  11  includes a photodetector element  41  having a plurality of detecting portions  43 ,  45 , and  47  that are provided by thermal diffusion or the like on a silicon substrate, and a semiconductor laser element  51  formed on the same silicon substrate. The semiconductor laser element  51  is made of a chemical compound provided at a bottom of a recessed part (not shown) having an inclined surface, formed in the silicon substrate by wet etching. The semiconductor laser element  51  emits a laser beam with a wavelength of approximately 300 to 800 nm. A light beam is emitted in a Y direction from the semiconductor laser element  51 , reflected by the inclined surface, so as to be directed in a Z direction. It should be noted that such an inclined surface is unnecessary in the case where the semiconductor laser element  51  is of a plane emission type. 
     Thus, by integrating the semiconductor laser element  51  and the photodetector element  41  on one silicon substrate, it is possible to obtain a smaller-size optical semiconductor device. Furthermore, in the case where the relative positional relationship between the semiconductor laser element  51  and the photodetector element  41  is determined in the semiconductor process for processing the silicon substrate (for instance, by forming the detecting portions and the recessed part by using mask alignment), it is possible to reduce the number of steps, thereby shortening the time required for assembling and reducing the cost, as compared with a case where these elements are assembled separately and then they are fixed after their positional relationship is adjusted. Furthermore, the detecting portions and the integrated circuit either for current-voltage conversion of electric signals obtained from the detecting portions or for calculation of the same may be provided integrally on a silicon substrate. This makes it possible to further miniaturize the device. It should be noted that the light source is not limited to the semiconductor laser, and a laser of another type (for instance, a laser in which SHG is used) may be used as a light source. 
     The optical element  31  is composed of a base made of a glass or a resin having substantial transparency with respect to a wavelength used, an emitted beam dividing portion  61  is provided on one of surfaces of the base, for dividing a light beam emitted from the semiconductor laser element  51 , and a reflected beam dividing portion  71  is provided on a surface opposite to the foregoing surface, for dividing a light beam reflected from the information recording medium  3 . In the emitted beam dividing portion  61 , a diffraction grating is provided that, as described later, divides a light beam having passed through the reflected beam dividing portion  71  among the reflected light beam from the information recording medium  3 . The emitted beam dividing portion  61  and the reflected beam dividing portion  71  may be provided in the form of separated first and second optical elements. Furthermore, the emitted beam dividing portion  61  may be configured simply to transmit the light beam emitted from the semiconductor laser  51 , without having a function to divide the emitted light beam from the semiconductor laser element  51 . 
     A light beam emitted from the semiconductor laser element  51  passes through the emitted beam dividing portion  61  and the reflected beam dividing portion  71 , and is focused on the information recording medium  3  by the objective lens  4 . A reflected light beam from the information recording medium  3  is divided by diffraction in the X direction by the reflected beam dividing portion  71 , and ±1-order diffracted light beams  83  and  85  thus obtained are guided to the detecting portions  43  and  45  of the photodetector element  41 . Furthermore, the 0-order diffracted light (transmitted light) again enters the emitted beam dividing portion  61 , and is diffracted by a first diffraction grating provided on the emitted beam dividing portion  61 , so as to be guided to the detecting portion  47 . 
     As shown in  FIG. 2 , the emitted beam dividing portion  61  includes regions serving as a first diffraction grating  63 , a second diffraction grating  65 , and a third diffraction grating  67 , thereby composing a three-beam-generating diffraction grating element. In the drawing, respective patterns of the diffraction gratings are illustrated with a plurality of lines. Furthermore, effective regions on the emitted beam dividing portion  61  corresponding to the beams to be collected into the objective lens  4  are indicated as a main beam  81  (r representing a radius of the same), and sub beams  82  and  84 . 
     The emitted light beam from the semiconductor laser element  51  is diffracted by the emitted beam dividing portion  61  in a Y direction shown in the drawing. As a result of division, a 0-order diffracted light obtained from the first diffraction grating is obtained as the main beam  81 , while ±1-order diffracted lights obtained from the second and third diffraction gratings  65  and  67  are obtained as the sub beams  82  and  84 , respectively, thereby being collected into the objective lens  4  as a focusing means. 
     On an optical axis extending between an emission point of the semiconductor laser element  51  and a main beam spot incident on the information recording medium  3 , let an air-equivalent distance from the emission point of the semiconductor laser element  51  to the emitted beam dividing portion  61  be represented by d. Here, the air-equivalent distance means a value obtained by dividing a distance of light transmission through a medium by a refractive index of the medium. Further, let a numerical aperture of the objective lens  4  be represented by NA. Furthermore, let a distance in an X direction measured from a point at which the optical axis and the emitted beam dividing portion  61  cross on the emitted beam dividing portion  61  be represented by r. Here, the first diffraction grating  63  is formed so as to have an area that includes at least an area satisfying:
 
 r≦d ×tan(sin −1 ( NA ))  [Formula 1]
 
     The reflected light dividing portion  71  is made of a hologram element composed of gratings, each in a curved line form. Among the three beams reflected from the information recording medium  3 , as to each of the main beam  81  and sub beams  82  and  84 , the +1-order diffracted light beam  83  is subjected to a converging effect while the −1-order diffracted light beam  85  is subjected to a diverging effect when they are divided, and they are guided to the detecting portions  43  and  45  of the photodetector element  41 . On the other hand, the 0-order diffracted light beam  87  of the main beam  81  of the reflected light beam, obtained by the division by the reflected beam dividing portion  71 , again enters the emitted beam dividing portion  61 . This light beam is diffracted in the X direction by the first diffraction grating  63  of the emitted beam dividing portion  61 . The first diffraction grating  63  and the like are configured so that the +1-order diffracted light beam  88  at the first diffracted grating  63  assumes a substantially focused state at the reproduction-signal-detecting detecting portion  47  provided on the photodetector element  41 . This makes it possible to decrease the light-receiving area of the reproduction-signal-detecting detecting portion  47 , thereby allowing reproduction signals to be detected at a higher speed. In examples, it was possible to set the light-receiving area of the reproduction-signal-detecting detecting portion  47  to be approximately 400 to 2500 μm 2 . 
       FIG. 3  is an enlarged cross-sectional view specifically illustrating the first diffraction grating  63  in the emitted beam dividing portion  61 . In each diffraction grating in the first diffraction grating  63 , gratings  63 A in a step-like form are provided further. This configuration makes it possible to increase the diffraction efficiency of the +1-order diffracted light beam  88  incident on the reproduction-signal-detecting detecting portion  47  with priority to the diffraction efficiency of the −1-order diffracted light beam  89 . Therefore, the quantity of the signal light incident on the reproduction-signal-detecting detecting portion  47  increases, thereby allowing reproduction signals with an excellent S/N ratio to be obtained. The step-like grating  63 A can be formed by, for instance, carrying out photolithography and etching a plurality of times. 
       FIG. 4  is an enlarged cross-sectional view specifically illustrating another configuration of the first diffraction grating  64 . Gratings  64 A, each having a triangular cross section, are provided as diffraction gratings composing the first diffraction grating  64 . It is possible to directly form the triangular gratings  64 A with an electron beam (EB) whose intensity can be varied stepwise, or alternatively, it is possible to form the same by exposing and developing a photosensitive material and using the obtained photosensitive material as a die. This configuration also makes it possible to increase the diffraction efficiency of the +1-order diffracted light beam  88  incident on the reproduction-signal-detecting detecting portion  47  with priority to the −1-order diffracted light beam  89 , thereby allowing reproduction signals with an excellent S/N ratio to be obtained. 
     The first diffraction grating  63  may be composed of substantially rectangular diffraction gratings. 
       FIG. 5  is an enlarged plan view specifically illustrating the detecting portions  43  and  45  and the reproduction-signal-detecting detecting portion  47  of the optical element  41 . The detecting portion  43  includes three detecting sections  433 ,  435 , and  437  extending in an X direction. The detecting section  433  is divided into three portions  433   a ,  433   b , and  433   c  in a Y direction. The detecting portion  45 , like the detecting portion  43 , includes the detecting sections  453 ,  455 , and  457 . The detecting section  453  is divided into three portions  453   a ,  453   b , and  453   c  in the Y direction. Focus error signals are detected from a main beam  81  among the reflected light beams having been guided to the detecting portions  43  and  45 , while tracking error signals are detected from sub beams  82  and  84  among the same. Spots formed on the detecting sections are shown schematically in  FIG. 5 . 
     Here, let signal quantities detected at the detecting sections  433   a ,  433   b ,  433   c ,  435 ,  437 ,  453   a ,  453   b ,  453   c ,  455 , and  457  be expressed as S( 433   a ), S( 433   b ), S( 433   c ), S( 435 ), S( 437 ), S( 453   a ), S( 453   b ), S( 453   c ), S( 455 ), and S( 457 ), respectively. 
     The focus servo is performed so that the spot on the detecting portion for the main beam has substantially the same size, that is, the following is satisfied:
 
{S( 433   a )+S( 433   c )+S( 453   b )}−{S( 433   b )+S( 453   a )+S( 453   c )}=0
 
The tracking servo is performed so that the sub beams have equal light quantities, that is, the following is satisfied:
 
{S( 435 )+S( 455 )}−{S( 437 )+S( 457 )}=0
 
     As described above, the 0-order diffracted light beam  87  from the reflected beam dividing portion  71  is incident on the first diffraction grating  63  of the emitted beam dividing portion  61 , and the +1-order diffracted light beam  88  obtained therefrom is incident in a substantially focused state on the reproduction-signal-detecting detecting portion  47 . A spot formed by the +1-order diffracted light beam  88  on the reproduction-signal-detecting detecting portion  47  is shown schematically in  FIG. 5 . Since the +1-order diffracted light beam  88  is incident thereon in the substantially focused state, the reproduction-signal-detecting detecting portion  47  may have a reduced area for receiving light, as compared with the other detecting sections  433 ,  435 , and  437  for servo. The reduction of the light-receiving area of the reproduction-signal-detecting detecting portion  47  allows the capacitance associated with the photodetector element to be remarkably reduced, thereby ensuring high-speed response of the reproduction signals. Furthermore, this leads to a decrease in stray light components incident on the reproduction-signal-detecting detecting portion  47 , thereby allowing reproduction signals with an excellent S/N ratio to be obtained. 
     Second Embodiment 
       FIG. 6  is a plan view illustrating an emitted beam dividing portion  62  according to the second embodiment. The present embodiment is an example of another embodiment of the emitted beam dividing portion  61  used in the first embodiment. 
     The emitted beam dividing portion  62  is characterized in that a first diffraction grating  68  is composed of gratings, each in a curved line form. The other parts of the configuration are the same as those of the emitted beam dividing portion  61 , and the same elements are designated with the same reference numerals. The configuration with gratings in the curved line form is able to apply a converging or diverging effect to light beams when being diffracted by the first diffraction grating  68 . Therefore, it is possible to change freely a position of a focus of the reflected light beam from the information recording medium  3  by varying the curvature of the grating lines. This makes it possible to make the reflected light beam from the information recording medium  3  in a substantially focused state on the reproduction-signal-detecting detecting portion  47 , irrespective of the distance between the surface of the photodetector element and the emitted beam dividing portion  62 . 
     The same effect as that of the first embodiment can be achieved by providing step-like gratings similar to those in the first embodiment in each grating of the first diffraction grating  68 . 
     Third Embodiment 
     The third embodiment is an example in which the emitted beam dividing portion  61  in the first embodiment is modified so as to have still another configuration.  FIG. 7  is a plan view illustrating an emitted beam dividing portion  62 A according to the present embodiment.  FIG. 8  is an enlarged plan view specifically illustrating the photodetector element. Elements other than a first diffraction grating  69  and detecting portions  48   a ,  48   b ,  48   c , and  48   d  for detecting reproduction signals have the same configurations as those of the first embodiment, and they are designated with the same reference numerals. 
     In the emitted beam dividing portion  62 A according to the present embodiment, the first diffraction grating  69  is composed of a plurality of diffraction grating regions with grating arrangement directions different from each other. In the first diffraction grating  69  shown in  FIG. 7 , four diffraction grating regions  69   a ,  69   b ,  69   c , and  69   d  with the grating arrangement directions different from each other are provided so as to equally divide a spot of a reflected light beam from the information recording medium  3 . 
     As shown in  FIG. 8 , a plurality of the detecting portions  48   a ,  48   b ,  48   c , and  48   d  are provided to receive light beams diffracted by the diffraction grating regions  69   a ,  69   b ,  69   c , and  69   d . They are arranged so that light beams diffracted by the diffraction grating regions  69   a ,  69   b ,  69   c , and  69   d  substantially focus on the detecting portions  48   a ,  48   b ,  48   c , and  48   d  for detecting reproduction signals, respectively. 
     With this configuration, the reflected light beam of the main beam  81  from the information recording medium  3  is diffracted by the plurality of diffraction grating regions  69   a ,  69   b ,  69   c , and  69   d  composing the first diffraction grating  69 , and is incident on the detecting portions  48   a ,  48   b ,  48   c , and  48   d  for detecting reproduction signals, respectively. Thus, the use of signals detected by the plurality of detecting portions  48   a ,  48   b ,  48   c , and  48   d  corresponding to the respective spots makes it possible to obtain a push-pull signal or a phase difference signal. Therefore, this makes it possible to subject the tracking error signal to an optimal method selected from among the three-beam method, the push-pull method, and the phase difference method according to the type of the information recording medium  3  used. 
     Furthermore, as shown in  FIG. 9 , which is an enlarged plan view specifically illustrating the photodetector element, the detecting portions  48   a ,  48   b ,  48   c , and  48   d  for detecting reproduction signals may be arranged at equal distances from the emission point. This configuration allows the first diffraction grating  69  to have the same grating periodic interval at all the plurality of diffraction grating regions  69   a ,  69   b ,  69   c , and  69   d  composing the first diffraction grating  69 . Therefore, when the first diffraction grating is formed by etching or the like, variation in manufacture, such as variation of depth of the gratings, is minimized. Consequently, it is possible to manufacture the same with stable properties. 
     It should be noted that in the optical information processing device of  FIG. 1 , the objective lens  4  may be fixed in the optical semiconductor device  1  so as to be actuated integrally. This by no means leads to the impairment of optical characteristics, such as a decrease in the signal quantity, that tends to occur when the objective lens is actuated independently. Therefore, it is possible to obtain excellent reproduction signals and focus/tracking error signals. 
     The optical semiconductor device according to the present invention can be defined in a broader sense. For instance, a so-called optical pickup device in which the optical semiconductor device  1 , the objective lens  4 , and a part of the control mechanisms are modularized also is categorized as the optical semiconductor device of the present invention. On the other hand, in the case where the semiconductor element  11  is dealt with independently, it is called a semiconductor laser device or a light-receiving device, which also is categorized in a broad sense as the optical semiconductor device of the present invention. 
     Furthermore, although the above-described embodiments show an example in which an area of the detecting portions for detecting reproduction signals is reduced, this invention is also applicable to a detecting portion that is required to perform high-speed processing for a different reason. 
     INDUSTRIAL APPLICABILITY 
     With the present invention, it is possible to reduce an area of a reproduction-signal-detecting photodetector element in an optical semiconductor device. This makes it possible to provide an optical information processing device that is capable of performing high-speed reproduction and obtaining reproduction signals with an excellent SIN ratio.