Abstract:
To implement light quantity monitoring with high frequency responsivity and correction of astigmatic differences of a semiconductor laser with a simple configuration with fewer parts. Of the output from a semiconductor laser light source, a peripheral component is entered by a light reflection element into an anterior light monitoring photodetector formed in the vicinity of a semiconductor laser light source. Furthermore, the surface of the reflection sphere of the light reflection element is anamorphic, and thus condensed to an appropriate size on the photodetector without being focused, providing high frequency responsivity. Furthermore, the light reflection element is inclined at a predetermined angle to cancel out astigmatic differences of the optical semiconductor laser light source. In addition, the photodetector is placed so that reflected light is bent by an inclination of the light reflection element, reducing the amount of parallel displacement during adjustment of the light reflection element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical head apparatus that performs recording or reproduction of optical information recording media. 
     2. Description of the Prior Art 
     Generally, a rewritable type optical disc must monitor the quantity of incident light to the recording surface of the disc to secure the signal recording quality with high accuracy. For this reason, the accuracy of a system that monitors the light quantity using light output from the posterior end face of a laser chip used in a reproduction-only optical head is not high, and therefore it is necessary to monitor the light quantity using light radiated from the anterior end face of the laser chip (hereinafter referred to as “anterior light”). 
     On the other hand, while optical discs are attracting attention as large-capacity information memories, optical head apparatuses need to attend a demand for high-speed recording or reproduction of optical discs. To meet this demand, it is necessary to increase the speed of modulation of a semiconductor laser light source and at the same time improve responsivity of the above described monitoring of the anterior light. 
     A conventional optical pick up will be explained with reference to the attached drawings below. FIG. 14 shows an example of an outlined configuration of a conventional optical head, apparatus. A diverging beam  802  radiated from a semiconductor laser light source  801  passes through a parallel flat plate  803  placed diagonally to the optical axis and is converted to a parallel beam  805  by a collimate lens  804 . 
     This collimated beam  805  is partially reflected by a polarized beam splitter  806  and enters into a photodetector  809 . A beam  810 , the major portion of the collimated beam  805 , passes through the polarized beam splitter  806  and is converted to a circularly polarized beam by a ¼ wavelength plate  811 , and then condensed into an optical disc  814  through an objective lens  813  mounted on an actuator  812 . 
     The beam reflected by the optical disc  814  passes through the objective lens  813  and is converted by the ¼ wavelength plate  811  to a linearly polarized beam, which is orthogonal to the polarization plane of the outgoing radiation beam of the semiconductor laser light source  801  and entered into the polarized beam splitter  806 . 
     Since the polarization plane of the incident beam entered into the polarized beam splitter  806  is orthogonal to the first half of the optical path, the incident beam is reflected by the polarized beam splitter  806 , diffracted by a hologram element  815 , branched into a positive 1st-order diffracted light  817  and negative 1st-order diffracted light  818  with the optical axis of the incident light as an axis of symmetry, then condensed by a detection lens  817 , entered into signal detectors  820  and  821 , respectively, to detect control signals such as focusing and tacking, and RF signals. 
     On the other hand, photodetector  809  that detects light reflected by the polarized beam splitter  806  acts as an output light quantity monitor of the semiconductor laser light source  801 . 
     Here, the reason why the parallel plate  803  is placed diagonally to the optical axis of the incident beam between the semiconductor laser light source  801  and collimate lens  804  will be explained. Generally, as for a semiconductor laser used for a light source of the optical head apparatus, from the standpoint of an optical characteristic, mode west of an oscillated beam of a semiconductor laser element  901  differs between the semiconductor composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane) as shown in FIG.  15 . 
     That is, while the mode west is a point that matches a specular surface  902  within the perpendicular (Y-Z axial plane), it is a point inside an activated layer  903  of the semiconductor laser element  901 , that is, a point at a certain depth from the specular surface  902  into the resonator within the composition plane (X-Z axial plane). 
     Therefore, the converging point of the oscillated beam differs between the composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane), and thus an “astigmatic difference”  904  in optical terms is produced. 
     When an astigmatic difference occurs, the beam spot is distorted into a flat, vertically or horizontally oblong spot. Therefore, the beam spot spans mutually neighboring recording tracks of an optical disc, causing a problem of deteriorating a signal characteristic. 
     It is for this reason that in FIG. 14, the parallel plate  803  is placed inclined at a predetermined angle in the reverse direction in order to correct the astigmatism of the light beam radiated from the semiconductor laser  801 . 
     Moreover, another method proposed to correct such astigmatism of a light beam is canceling out the astigmatism of the light spot by inserting a cylindrical lens in the same optical path of the laser beam. 
     BRIEF SUMMARY OF THE INVENTION 
     Object of the Invention 
     The above described conventional optical head apparatus has the following problems: 
     Generally, when recording a signal on a rewritable type optical disc, it is necessary to secure sufficient optical power on the disc, and therefore the light utilization efficiency of the optical head must be secured. 
     However, the configuration of the above described conventional example performs no beam shaping, and therefore abandons a portion of light in the outer regions for reasons related to the design of the objective lens, which means a loss of light quantity. 
     Furthermore, a part of the beam within the effective aperture is reflected and used by the photodetector  809  to monitor the light quantity, which increases the loss all the more. To avoid this, lowering the light quantity to be conducted to the light quantity monitor and increasing the light quantity within the effective aperture will deteriorate the S/N ratio of the monitor signal. 
     Moreover, increasing the speed of laser modulation requires the responsivity of the anterior light monitor itself to be improved. For this reason, it is preferable to reduce the photoreception area of the photodetector and input a condensed beam in order to improve the response frequency characteristic of optical detection. 
     However, exposing the photodetector to an excessively condensed beam will increase the light intensity per unit area of the detector surface, increasing the carrier density on the photoreception surface of the detector, which then becomes saturated causing the traveling speed of carriers to slow down. That is, condensing the beam on the detector excessively may cause a problem of deteriorating the response frequency characteristic of optical detection. 
     Furthermore, all the above described methods to correct the astigmatism of a light beam produced by an astigmatic difference among the semiconductor laser elements above must provide special parts such as a transparent parallel plate and cylindrical lens separately, causing an additional problem of unavoidably increasing the number of parts, hence cost increase. 
     In addition, since the photodetector for an RF signal, focusing or tracking control signals is provided apart from the photodetector for laser light quantity monitoring, which increases the number of parts and complicates the optical system, making it difficult to reduce the size of the optical head. 
     The present invention has been implemented taking into account these problems of the conventional optical head apparatus and it is an object of the present invention to provide an optical head apparatus with high light utilization efficiency. 
     It is another object of the present invention to provide a compact optical head apparatus. 
     It is still another object of the present invention to provide an optical head apparatus with an excellent response frequency characteristic of optical detection. 
     SUMMARY OF THE INVENTION 
     Therefore one aspect of the present invention is an optical head apparatus, comprising: 
     a semiconductor laser light source; 
     a photodetector that receives at least one part of light from said semiconductor laser light source; 
     a light reflection element provided with a peripheral section that reflects peripheral light of to the light from said semiconductor laser light source and condenses it into said photodetector and a central section that transmits central light of the light from said semiconductor laser light source; and 
     a condenser lens that condenses the light that passes through said light reflection element onto an optical disc, 
     wherein: 
     each surface of the central section of said light reflection element has a flat shape; and 
     at least one surface of the peripheral section of said light reflection element has a spherical or non-spherical shape. 
     Therefore another aspect of the present invention is an optical head apparatus, comprising: 
     a semiconductor laser light source; 
     a photodetector that receives at least one part of light from said semiconductor laser light source; 
     a light reflection element provided with a function of reflecting peripheral light of the light from said semiconductor laser light source and condensing it into said photodetector and a function of transmitting the central light of the light from said semiconductor laser light source; and 
     a condenser lens that condenses the light that passes through said light reflection element onto an optical disc, 
     characterized in that said semiconductor laser light source and said photodetector are formed in one package. 
     Therefore still another aspect of the present invention is an optical head apparatus, comprising: 
     a semiconductor laser light source; 
     a plurality of photodetectors placed adjacent to said semiconductor laser light source; 
     a reflection type hologram element provided with a peripheral section that reflects and diffracts peripheral light of the light from said semiconductor laser light source and condenses it into one of said plurality of photodetectors and a central section that transmits central light of the light from said semiconductor laser light source; and 
     a condenser lens that condenses the light that passes through the central section of said reflection type hologram element onto an optical disc, 
     wherein: 
     said photodetector that receives said reflected and diffracted light is placed closer, with respect to said semiconductor laser light source, in the direction of the major axis of an ellipse than in the direction of the minor axis of the ellipse of an elliptic far field pattern of outgoing light from said semiconductor laser light source; and 
     the photodetector that receives signal light from said optical disc is placed closer, with respect to said semiconductor laser light source, in the direction of the minor axis of the ellipse than in the direction of the major axis of the ellipse of an elliptic far field pattern of outgoing light from said semiconductor laser light source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an outlined configuration of an optical head according to an embodiment of the present invention; 
     FIG. 2 is a drawing showing a configuration of elements used in the above embodiment of the present invention; 
     FIG. 3 a schematic drawing of spot shapes of the above embodiment of the present invention; 
     FIG. 4 an outlined configuration of an optical head according to another embodiment of the present invention; 
     FIG. 5 a schematic drawing of spot shapes of the above embodiment of the present invention; 
     FIG. 6 an outlined configuration of an optical head according to another embodiment of the present invention; 
     FIG. 7 a drawing showing a configuration of elements used in the above embodiment of the present invention; 
     FIG. 8 a configuration diagram of an optical head apparatus of another embodiment of the present invention; 
     FIG. 9 a plan view of a reflection type hologram according to another embodiment of the present invention; 
     FIG. 10 a layout of a reflection type hologram element, laser light source and photodetector according to another embodiment of the present invention; 
     FIG. 11 a configuration diagram of an optical head according to another embodiment of the present invention; 
     FIG. 12 a layout of a reflection type hologram element, laser light source and photodetector according to another embodiment of the present invention; 
     FIG. 13 a layout of a reflection type hologram element and photodetector according to another embodiment of the present invention; 
     FIG. 14 a drawing showing a conventional optical head apparatus; and 
     FIG. 15 a schematic drawing showing an astigmatic difference of a semiconductor laser. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 (Description of Symbols) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  1 
                 Laser light source 
               
               
                  2 
                 Reflection type hologram element 
               
               
                  3 
                 Collimate lens 
               
               
                  4 
                 Objective lens 
               
               
                  5 
                 Polarized hologram element 
               
               
                  6 
                 Anterior light monitoring photodetector 
               
               
                  7 
                 Signal detection photodetector 
               
               
                  8 
                 Optical disc plane 
               
               
                  9 
                 Actuator 
               
               
                  10 
                 Reflection hologram 
               
               
                 101 
                 Semiconductor laser light source 
               
               
                 102 
                 Divergent light 
               
               
                 103 
                 Photodetector 
               
               
                 104 
                 Collimate lens 
               
               
                 105 
                 Parallel light 
               
               
                 106 
                 Polarized beam splitter 
               
               
                 107 
                 Light reflection element 
               
               
                 108 
                 Reflected light 
               
               
                 109 
                 Optical integrated module 
               
               
                 110 
                 Transmission light 
               
               
                 111 
                 ¼ wavelength plate 
               
               
                 112 
                 Actuator 
               
               
                 113 
                 Objective lens 
               
               
                 114 
                 Optical disc 
               
               
                 115 
                 Hologram element 
               
               
                 116 
                 Detection lens 
               
               
                 117 
                 Positive 1st order diffracted light 
               
               
                 118 
                 Negative 1st order diffracted light 
               
               
                 119 
                 Signal detector 
               
               
                 120 
                 Signal detector 
               
               
                 201 
                 Laser optical axis 
               
               
                 202 
                 Transmission plane 
               
               
                 203 
                 Aluminum-evaporated plane 
               
               
                 407 
                 Light reflection element 
               
               
                 607 
                 Reflection type hologram element 
               
               
                 608 
                 Reflected/diffracted light 
               
               
                 701 
                 Laser optical axis 
               
               
                 702 
                 Transmission plane 
               
               
                 703 
                 Reflection hologram plane 
               
               
                 801 
                 Semiconductor laser light source 
               
               
                 802 
                 Divergent light 
               
               
                 803 
                 Parallel plate 
               
               
                 804 
                 Collimate lens 
               
               
                 805 
                 Parallel light 
               
               
                 806 
                 Polarized beam splitter 
               
               
                 807 
                 Reflected light 
               
               
                 809 
                 Optical integrated module 
               
               
                 810 
                 Transmission light 
               
               
                 811 
                 ¼ wavelength plate 
               
               
                 812 
                 Actuator 
               
               
                 813 
                 Objective lens 
               
               
                 814 
                 Optical disc 
               
               
                 815 
                 Hologram element 
               
               
                 816 
                 Detection lens 
               
               
                 817 
                 Positive 1st order light 
               
               
                 818 
                 Negative 1st order light 
               
               
                 819 
                 Signal detector 
               
               
                 820 
                 Signal detector 
               
               
                   
               
             
          
         
       
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to FIG.  1  through FIG. 7, embodiments of the present invention will be explained below. Detailed explanations of the parts that have the same functions as those in the conventional example will be omitted. 
     FIG. 1 shows an outlined configuration of the optical head apparatus of an embodiment of the present invention and FIG. 2 shows the configuration of a light reflection element, laser and photodetector that are used as components of optical head apparatus. A light reflection element  107  used in the configuration in FIG. 1 is made of glass and provided with a flat shaped transmission plane  202  at its center section (corresponding to the central section in the present invention) with a centered laser optical axis  201 , and a non-spherically formed aluminum-evaporated plane  203  at a ring-figured area (corresponding to the peripheral section of the present invention) surrounding said transmission plane  202  as shown in FIG. 2 . 
     Among the light beam radiated from the semiconductor laser light source  101  in FIG. 1, the peripheral beam component is reflected and condensed by the light reflection element  107  and condensed into a photodetector  103  for anterior light monitoring formed in the vicinity of the semiconductor laser light source  101 . The semiconductor laser light source  101  and photodetector  103  for anterior light monitoring are integrated into an optical integrated module  109  for the purpose of reducing the size and weight. 
     On the other hand, the beam at the central section is collimated by a collimate lens  104  into a parallel beam, passes through a polarized beam splitter  106 , and is condensed by an objective lens  113  mounted on an actuator  112  onto the surface of an optical disc  114 . 
     The beam reflected by the optical disc  114  passes through the objective lens  113  and is converted by a ¼ wavelength plate  111  to linearly polarized light orthogonal to the polarization plane of the semiconductor laser outgoing radiation beam and is entered into the polarized beam splitter  106 . 
     Since the polarization plane of the incident beam entered into the polarized beam splitter  106  is orthogonal to that in the case of the first half of the optical path, the incident beam is reflected by the polarized beam splitter  106  and diffracted by a hologram element  115 . The diffracted beam is branched into a positive 1st-order diffracted light  117  and negative 1st-order diffracted light  118  with the optical axis of the incident light as an axis of symmetry, then is condensed by a detection lens  116 , entered into signal detectors  119  and  120 , respectively, and used for control signals such as focusing and tacking, and RF signals. 
     The astigmatism of the light beam radiated from the semiconductor laser  101  is compensated by inclining the light reflection element  107  at a predetermined angle. 
     Moreover, as shown in FIG. 1, due to an inclination of the light reflection element  107 , the optical axis of the reflected light  108  is inclined with respect to the optical axis of the light from the semiconductor laser light source. The photodetector  103  is placed in the direction of the reflected light  108 . Here, when manufacturing, it is necessary to adjust so that the reflected light  108  enters into the photodetector  103 . For this purpose, it is necessary to adjust the central axis of the reflection spherical plane of the light reflection element  108  through parallel displacement of the central axis in the direction indicated by an arrow in FIG. 1 as appropriate. The above described inclination of the photodetector  103  reduces the amount of parallel displacement compared to the case without the inclination. 
     The ability to reduce the amount of parallel displacement for adjustment can reduce the area of the central flat section of the light reflection element  107 , form a wider light reflection surface of the peripheral section and capture more reflected light. This is because larger parallel displacement for adjustment requires the central flat section of the light reflection element  107  to be designed with more allowance beforehand. 
     Furthermore, since it is possible to design a smaller angle of reflection of the reflection spherical surface, the reflection plane of the light reflection element  107  becomes a spherical plane with smaller curvature, making it easier to shape the reflection plane. 
     Furthermore, if the peripheral section of the light reflection element  107  is formed in an anamorphic, non-spherical shape, as shown in FIG. 2, the focus onto which light is reflected and condensed by the light reflection element  107  differs between the x-z axial plane and the y-z axial plane normal thereto in FIG.  2 . That is, the focus of spots on the surface of the photodetector  103  has an astigmatic difference as shown in FIG.  3 . Even if defocusing occurs due to errors in mounting optical parts, etc., the astigmatic difference increases the light intensity per unit area of the surface of the photodetector, and prevents deterioration of frequency responsivity caused by concentration of carriers. 
     FIG. 4 shows an embodiment using a light reflection element  407  with reflected and converged light having a spherical aberration instead of the light reflection element  107  with reflected and converged light having an astigmatic difference in the embodiment of FIG.  1 . In this case, the spot focus on the surface of the photodetector  103  has a spherical aberration as shown in FIG.  5 . With this, even if defocusing occurs due to errors in mounting optical parts, etc., it is possible to maintain lower light intensity per unit area of the surface of the photodetector than spots without spherical aberration, and prevent deterioration of frequency responsivity caused by concentration of carriers. 
     FIG. 6 shows a schematic drawing of another embodiment of the present invention. A reflection type hologram element  607  used in the configuration in FIG. 6 is provided with a reflection hologram plane  703  formed in a ring-figured area (corresponding to the peripheral section of the present invention) at the peripheral section for a laser optical axis  701  as a center position as shown in FIG.  7  and there is a circular light transmission area  702  (corresponding to the central section in the present invention) in the vicinity of the laser optical axis  701 . 
     Among the light beam radiated from the semiconductor laser light source  101  in FIG. 6, the peripheral beam component is reflected and diffracted by the reflection hologram  607 . The pitch and groove orientation of this reflection type hologram are different depending on the incident light and incident position of the laser beam and the reflected/diffracted beam  608  is condensed into the photodetector  103  placed in the vicinity of the semiconductor light source  101 . 
     Furthermore, since this reflection type hologram element  607  is formed to have an astigmatic difference as in the case of the above described light reflection element  107 , a spot beam does not form a focus but focal line. With this, even if defocusing occurs due to errors in mounting optical parts, etc., it is possible to maintain lower light intensity per unit area of the surface of the photodetector than spots without astigmatic differences, and prevent deterioration of frequency responsivity caused by concentration of carriers. 
     Furthermore, for manufacturing adjustment, it is necessary to enter the reflected/converged light from the reflection type hologram element  607  into the photodetector through parallel displacement of the reflection type hologram element  607  in the direction indicated by an arrow in FIG.  6 . In this case, as shown in FIG. 6, placing the photodetector  103  in the direction that the light is reflected by the inclined reflection type hologram element  607  can reduce the amount of parallel displacement to adjust the reflection type hologram element  607 . 
     This can reduce the angle of diffraction of the reflection type hologram element  607  compared to the case where the reflection type hologram element  607  is not inclined and allows the reflection type hologram element  407  to be designed with a wider pitch, securing an advantage in respect of hologram elaboration pitch limitations. 
     Here, in the embodiment in FIG. 6, it goes without saying that using a light reflection element with the reflected/diffracted light having a spherical aberration instead of the reflection hologram element  607  with the reflected/diffracted light having an astigmatic difference will also obtain effects similar to those in the embodiment in FIG.  4 . 
     FIG.  8 ( a ) shows an outlined configuration of the optical head apparatus according to another embodiment of the present invention and FIG.  8 ( b ) shows a layout of the reflection type hologram element, laser and photodetector used as its components. 
     The reflection type hologram element  2  used in the configuration in FIG.  8 ( a ) consists of a reflection type hologram  10  formed in a ring-figured area in the outer circumference for the laser optical axis  15  as a central position and a circular light transmission area  11  in the vicinity of the laser optical axis  15  as shown in FIG.  8 ( b ) 
     Of the light beam radiated from the semiconductor laser light source  1  in FIG.  8 ( a ), the peripheral beam component  22  is reflected and diffracted by the reflection type hologram  10 . The pitch and groove orientation of this reflection hologram are different depending on the incident light and incident position of the laser beam and the reflected/diffracted beam  23  is condensed into the photodetector  6  for anterior light monitoring placed in the vicinity of the semiconductor light source  1 . 
     On the other hand, among the beam radiated from the semiconductor laser light source, the inner beam component passes through the reflection type hologram element  2  with restricted aperture, polarized hologram element  5  and ¼ wavelength plate  20 , and is converted to a parallel beam by a collimate lens  3 , and then condensed into an optical disc  8  through an objective lens  4  mounted on an actuator  9 . 
     The beam reflected by the optical disc  8  is diffracted by the objective lens  4 , collimate lens  3 , ¼ wavelength plate  20  and polarized hologram element  5  and entered into a signal detection photodetector  7  formed around the semiconductor laser light source  1  for detection of signals such as RF signal, focusing and tracking control signals. 
     With such a configuration using outer circumferential light component that is originally not used for anterior light monitoring, the present embodiment can improve the light utilization efficiency of the optical head and integrate all the semiconductor laser light source  1 , signal detection detector  7 , detector  6  for anterior light monitoring in a single unit, thus reducing the number of parts of the optical head apparatus. 
     Furthermore, the reflection type hologram  10  having a condensing function allows light beam to be condensed into a photodetector with a small area without another condensing means such as a lens, simplifying and reducing the size of the optical head apparatus while securing high-speed responsivity of anterior light monitoring. 
     Here, as shown in FIG.  8 ( b ), the semiconductor laser light source  1 , anterior light monitoring photodetector  6  and signal detection photodetector  7  are configured as follows: 
     That is, for a laser outgoing radiation far field pattern  12  as shown by dotted line in FIG.  8 ( b ), the anterior light monitoring photodetector  6  is placed in the direction close to the direction  13  of the major axis of the ellipse and signal detection photodetector  7  to detect signals from the optical disc is placed in the direction  14  close to the direction of the minor axis of the ellipse. 
     That is, the anterior light monitoring photodetector  6  is placed close to the direction of the major axis of the ellipse rather than the minor axis with respect to the optical axis of the light from the semiconductor laser light source. On the other hand, the photodetector  7  for signal detection is placed close to the direction of the minor axis of the ellipse. For example, it is preferable that they be placed in the direction of the major axis and minor axis of the ellipse, respectively. 
     That is, such a configuration has the following effects. The reflection hologram  10  produces a beam with the order which diffracts toward the anterior light monitoring photodetector  6 , a beam with the order which diffracts in its opposite side and a 0-order diffracted beam, but since unnecessary beams other than the beam with the order which diffracts toward the anterior light monitoring photodetector  6  travel in the direction of the major axis of the ellipse of the far field pattern, these beams are not entered into the photodetector  7  to detect a signal from the optical disc as stray light. 
     Furthermore, since each photo detector can be placed close to the laser chip, requiring only a small angle of diffraction by the reflection hologram  10  or polarized hologram element  5 , it is possible to have a large hologram pitch and secure sufficient allowance for hologram elaboration pitch limitations. 
     FIG. 9 shows a plan view of the reflection type hologram element  2  according to another embodiment of the present invention which sets the spreading angle of a laser light source, the hologram area and a relative distance between the light source and hologram so that more light in the major axis of the ellipse of the outgoing radiation far field pattern  12  of the semiconductor laser light source is reflected and diffracted. 
     Generally, a density distribution of a semiconductor laser changes due to temperature variations more in the direction of the minor axis than in the direction of the major axis. This change affects the linearity of the light quantity of anterior light monitoring and the light quantity of light passing through the reflection hologram element  2 . Therefore, for a system requiring control of light quantity with very high accuracy as in the case of the present embodiment, it is preferable to use only light in the direction of the major axis as the light for anterior light monitoring. That is, the hologram  10  formation area of the reflection type hologram element  2  is formed more widely in the direction of the major axis of the ellipse with respect to the center of the axis of the elliptic far field pattern of the above described semiconductor laser. 
     FIG. 10 shows a plan view of the reflection type hologram and the location of the photodetector in another embodiment of the present invention. In the present embodiment, the area of the reflection type hologram  10  is formed asymmetric with respect to a point centered on the laser optical axis  15 . That is, if a laser beam is reflected/diffracted by the anterior light monitoring photodetector  6 , which is deviated from the laser optical axis  15 , the hologram pitch varies sequentially depending on its incidence angle and position. However, since this pitch also has elaboration limitations, the area is subject to these limitations. 
     However, some directions allow a large distance from the optical axis to the pitch elaboration limitations, and therefore it is possible to increase the light quantity of reflected/diffracted light for anterior light monitoring by forming the reflection type hologram  10  up to the boundary  17  of the elaboration pitch limitations indicated by the area asymmetric with respect to a point as shown in FIG.  10 . 
     FIG.  11 ( a ) shows an optical head apparatus in another embodiment of the present invention and FIG.  11 ( b ) shows a plan view of a reflection type hologram element used for its configuration. 
     As shown in FIG.  11 ( b ), the reflection type hologram element  2  has an oval or slotted-hole shaped light transmission area  11  at the center. In FIG.  11 ( a ), a polarized hologram element  5  and ¼ wavelength plate  20  to diffract the reflected light from the optical disc and lead it to the photodetector  7  for detection of signals such as focusing and tracking are mounted together with an objective lens  4  on the movable part of an objective lens actuator  9 . 
     Therefore, when the objective lens  4  moves in the direction orthogonal to the track in order to follow up tracking errors due to eccentricity of the optical disc, the light  25  diffracted by the polarized hologram element  5  also moves together (solid arrow  25  → dotted line arrow  25 ′). 
     According to the reflection type hologram element  2  in FIG.  11 ( b ), the light transmission area  11  of the reflection hologram  10  extends widely in this direction of movement, and therefore it is possible to implement a structure that prevents shading the signal detection light minimizing the reduction of light quantity of reflected/diffracted light for anterior light monitoring. FIG.  11 ( a ) and FIG.  11 ( b ) depict tracking operation directions with the vertical and horizontal directions reversed. 
     FIG. 12 shows a part of the optical head apparatus of another embodiment of the present invention. In FIG. 12, a polarized hologram element is formed with a polarized hologram layer  26  and ¼ wavelength film  19  sandwiched between two glass plates. Furthermore, a reflection type hologram  10  is formed on a glass substrate on the other side. 
     This allows the elements to be integrated and simplifies the configuration of the optical head and at the same time allows, when the reflected/diffracted light spot of the reflection type hologram  10  is positioned on the monitoring photodetector, the signal detection hologram to be positioned simultaneously, making it possibly to simplify adjustment in the optical head manufacturing process. 
     FIG. 13 shows a reflection type hologram element  2 , anterior light monitoring photodetector  6  and light beam diffracted by the reflection type hologram of the optical head apparatus according to another embodiment of the present invention. 
     As shown in FIG. 13, the condensing point of the light beams reflected and diffracted by the reflection type hologram element  2  is defocused before and after the photodetector  6  because the wavelength of the laser light source fluctuates due to temperature variations, etc. 
     In order to prevent the light beam from going off the edge of the anterior light monitoring photodetector  6  due to such defocusing, it is preferable to design so that the condensing point matches the plane of the photodetector at a midpoint  32  between a focus point  31  at the minimum temperature in the operating temperature range of the optical head and a focus point  30  at the maximum temperature. This reduces variations of the monitoring light quantity even with variations in the laser wavelength, allowing stable control of light quantity within the guaranteed temperature range of the product. 
     As described above, the optical head apparatus of the present invention can implement anterior light monitoring with high-speed responsivity by condensing a beam to a predetermined size on a photodetector, correct an astigmatic difference of a semiconductor laser using this light reflection element and integrate the semiconductor laser and photodetector in a single unit, thus making it possible to simplify and reduce the size of the optical head. 
     Furthermore, the configuration according to the present invention monitors the light quantity of laser radiating beams by effectively utilizing light beams outside the aperture, making it possible to reduce loss of light quantity, increase the monitoring light quantity by optimizing the area and location of the reflection/diffraction grating, thus providing a high S/N ratio of monitor signals. 
     Furthermore, since the reflection type hologram itself provides high-level condensing, it is possible not only to reduce the size of optical spots on the photodetector but also reduce the optical detection area, making it possible to implement anterior light monitoring with high-speed responsivity and stabilize the recording quality by the recording type optical head such as DVD-RAM. 
     Furthermore, the present invention can simplify and reduce the size of the optical head by integrating the anterior light monitoring photodetector, laser chip and signal detection photodetector, etc. in a single unit.