Patent Publication Number: US-2011075526-A1

Title: Optical Element, Arm Mechanism, and Information Recording Device

Description:
TECHNICAL FIELD 
     The present invention relates to an information recording device and also relates to an arm mechanism and an optical element which are used for the same. More particularly, the present invention relates to, for example, an optical assist type magnetic recording device using a magnetic field and light for information recording and also relates to an arm mechanism and an optical element which are used for the same. 
     BACKGROUND ART 
     In magnetic recording, as the recording density increases, magnetic bits become more remarkably affected by an external temperature or the like. For this reason, a recording medium having high retention is needed, but when such a recording medium is used, the magnetic field required for recording becomes larger. Though the upper limit of the magnetic field generated by a recording head is determined by a saturation flux density, however, the value approximates to a material limit and cannot be expected to dramatically increase. Then, proposed is a method (thermal assist magnetic recording method) in which in a recording process, magnetic softening is caused by local heating, recording is performed while the retention is low, and then heating is stopped for natural cooling so that the stability of recorded magnetic bits may be ensured. 
     In the thermal assist magnetic recording method, it is desired to momentarily heat the recording medium and it is not permitted for the heating mechanism and the recording medium to come into contact with each other. For this reason, heating is generally performed by using absorption of light and the method in which heating is performed by using light is referred to as an optical assist type. As an optical assist type magnetic recording head, provided is a head which comprises an optical head portion having a planar waveguide with a light condensing function (PSIM) and a magnetic head portion for performing magnetic recording on a portion irradiated with light emitted from the optical head portion (in, for example, Patent Document 1). The PSIM proposed in Patent Document 1 is provided with a diffraction grating, and considering the ratio (light use efficiency) of the amount of light condensed by the PSIM to the amount of light emitted to the diffraction grating, for an incident angle to the diffraction grating, there is an angle range appropriate to the wavelength of incident light. 
     PRIOR-ART DOCUMENTS 
     Patent Documents 
     
         
         [Patent Document 1] U.S. Pat. No. 6,944,112 
       
    
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     Though Patent Document 1 shows emission of light from a light source with the light simply inclined with respect to the diffraction grating, however, no specific description is made on a method of guiding light from the light source to the diffraction grating. 
     The present invention is intended to solve the above problem, and it is an object of the present invention to propose a technique of guiding light to a waveguide, by which light use efficiency increases. 
     Means for Solving the Problems 
     In order to solve the above problem, the present invention is intended for an arm mechanism. According to a first aspect of the present invention, the arm mechanism comprises an arm portion, a head portion which includes a waveguide provided with a grating coupler and is attached to one end side of the arm portion, and an optical element which is provided on an optical path for light incident on the grating coupler, attached to the arm portion, and has a diffraction grating. 
     According to a second aspect of the present invention, in the arm mechanism of the first aspect, the optical element includes a prism. 
     According to a third aspect of the present invention, in the arm mechanism of the first or second aspect, the optical element has at least one reflection surface. 
     According to a fourth aspect of the present invention, in the arm mechanism of the third aspect, the diffraction grating includes a diffraction grating which consists of a plurality of reflection surfaces and causes a diffraction phenomenon of light by reflection of light on the plurality of reflection surfaces. 
     The present invention is also intended for an information recording device. According to a fifth aspect of the present invention, the information recording device comprises an arm mechanism as defined in any one of the first to fourth aspects, a light source unit for generating light to be emitted to the head portion through the optical element and a recording medium disposed to be opposed to the head portion, and in the information recording device of the fifth aspect, the head portion emits light to the recording medium to record information into the recording medium. 
     The present invention is further intended for an optical element. According to a sixth aspect of the present invention, the optical element is disposed on an optical path for light incident on a grating coupler provided in a waveguide, and the optical element has a diffraction grating which changes a direction in which light is emitted in correspondence with variation in an appropriate incident angle of the grating coupler in accordance with variation in a wavelength of light. 
     According to a seventh aspect of the present invention, the optical element of the sixth aspect includes a prism. 
     According to an eighth aspect of the present invention, the optical element of the sixth or seventh aspect has at least one reflection surface. 
     EFFECTS OF THE INVENTION 
     By the arm mechanism, the information recording device, and the optical element in accordance with any one of the first to eighth aspects, since the incident angle to the grating coupler is adjusted by the optical element provided on the optical path on the basis of the change in the range of an appropriate incident angle of light to the grating coupler in accordance with the variation in a wavelength, the light use efficiency can be increased. 
     By the arm mechanism in accordance with the third aspect, it becomes possible to adjust the optical path by the diffraction grating of the optical element, following the change in the range of an appropriate incident angle of light to the grating coupler. 
     By the information recording device in accordance with the fifth aspect, it is possible to ensure reduction in power consumption for recording of information. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing an exemplary schematic configuration of an information recording device in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a view showing an exemplary structure of a slider portion in accordance with the preferred embodiment of the present invention; 
         FIG. 3  is a view showing an exemplary structure of an optical assist unit having a waveguide; 
         FIG. 4  is a schematic diagram showing an exemplary structure of an arm mechanism; 
         FIG. 5  is a view showing an example of disposition of an optical element; 
         FIG. 6  is a table exemplarily showing characteristics of first to fifth light source units; 
         FIG. 7  is a graph exemplarily showing characteristics of the first to fifth light source units; 
         FIG. 8  is a schematic cross section showing an exemplary structure of a waveguide grating element; 
         FIG. 9  is a view for explanation of wavelength dispersion characteristics of the waveguide; 
         FIG. 10  is a schematic diagram showing an exemplary variation in an appropriate incident angle to the waveguide; 
         FIG. 11  is a graph showing an exemplary relation between efficiency of light incidence on the waveguide and an incident angle to the waveguide; 
         FIG. 12  is a schematic diagram showing an exemplary structure of a first optical element portion; 
         FIG. 13  is a view for explanation on adjustment of the incident angle by the first optical element portion; and 
         FIG. 14  is a schematic diagram showing an exemplary structure of an optical element portion in accordance with a variation. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the preferred embodiment of the present invention will be discussed with reference to figures. 
       FIG. 1  is a view showing an exemplary schematic configuration of an information recording device  100 . In  FIG. 1  and the following figures, for clarification of the orientation relation, three axes, i.e., XYZ, which are orthogonal to one another are given as appropriate. 
     As shown in  FIG. 1 , the information recording device  100  is configured as a magnetic recording device (optical assist type magnetic recording device) equipped with first to fifth slider portions  31  to  35  which correspond to optical assist type magnetic recording heads, i.e., optical heads, and is applied to, for example, a hard disk device or the like. The information recording device  100  comprises a substantially rectangular parallelepiped case  1 , first to third recording disks (magnetic recording media)  2   a ,  2   b , and  2   c , and an arm mechanism  10  which are disposed in the case  1 . 
     The first to third recording disks  2   a ,  2   b , and  2   c  are disk-shaped recording media and arranged away from one another by a predetermined very short distance (e.g., 1 mm or less) so that disk surfaces thereof are substantially in parallel with one another. Specifically, from the upper side (from the +Z side toward the −Z side), the first recording disk  2   a , the second recording disk  2   b , and the third recording disk  2   c  are sequentially disposed in space in this order and supported rotatably with respect to the case  1  by a predetermined rotation axis and a motor. 
     The arm mechanism  10  mainly comprises the first to fifth slider portions  31  to  35 , first to fifth optical element portions  21  to  25 , first to third arm portions  41  to  43 , and a rotation shaft  5 . 
     The first to third arm portions  41  to  43  have the same shape, i.e., elongated thin plate-like shape, and are disposed substantially in parallel with one another. Specifically, from the upper side (from the +Z side toward the −Z side), the first arm portion  41 , the second arm portion  42 , and the third arm portion  43  are sequentially disposed in space in this order. The first to third arm portions  41  to  43  are connected to one another by the rotation shaft  5  at one end side (herein, the end portion in the −X direction) and supported rotatably with respect to the case  1  in a direction (tracking direction) indicated by the arrow mA with the rotation shaft  5  as a fulcrum. Herein, with rotation of the rotation shaft  5  by an actuator  6 , the first to third arm portions  41  to  43  are rotated in the direction indicated by the arrow mA with the rotation shaft  5  as a fulcrum. 
     The first arm portion  41  to the second arm portion  42  are so arranged as to sandwich the first recording disk  2   a  and the second arm portion  42  and the third arm portion  43  are so arranged as to sandwich the second recording disk  2   b . From another point of view, the second arm portion  42  is so disposed as to be sandwiched between the first recording disk  2   a  and the second recording disk  2   b  and the third arm portion  43  is so disposed as to be sandwiched between the second recording disk  2   b  and the third recording disk  2   c.    
     The first to fifth slider portions  31  to  35  have the same structure and serve as optical heads (which correspond to “head portions” of the present invention) for emitting light to the recording disks  2   a ,  2   b , and  2   c.    
     The first slider portion  31  is so provided at a lower surface of the first arm portion  41  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, as to face one main surface (herein, an upper surface) of the first recording disk  2   a , and the second slider portion  32  is so provided at an upper surface of the second arm portion  42  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, as to face the other main surface (herein, a lower surface) of the first recording disk  2   a . The third slider portion  33  is so provided at a lower surface of the second arm portion  42  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, as to face one main surface (herein, an upper surface) of the second recording disk  2   b , and the fourth slider portion  34  is so provided at an upper surface of the third arm portion  43  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, as to face the other main surface (herein, a lower surface) of the second recording disk  2   b . Further, the fifth slider portion  35  is so provided at a lower surface of the third arm portion  43  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, as to face one main surface (herein, an upper surface) of the third recording disk  2   c . Thus, the first to fifth slider portions  31  to  35  are held by the first to third arm portions  41  to  43 . 
     Surfaces (slider lower surfaces) of the slider portions  31  to  35 , each of which faces any one of the recording disks  2   a  to  2   c , each have a shape of air bearing surface (ABS). Herein, a motor (not shown) for rotating the first to third recording disks  2   a  to  2   c  in a direction indicated by the arrow mB is provided in the case  1 . When the first to third recording disks  2   a  to  2   c  are rotated, the first to fifth slider portions  31  to  35  can be moved relatively, being levitated from the first to third recording disks  2   a ,  2   b , and  2   c  by a substantially constant distance, above the first to third recording disks  2   a ,  2   b , and  2   c.    
       FIG. 2  is a schematic cross section showing an exemplary structure of each of the first to fifth slider portions  31  to  35 . As shown in  FIG. 2 , each of the first to fifth slider portions  31  to  35  has a recording head which uses light for information recording on any one of the first to third recording disks  2   a  to  2   c . Each of the first to fifth slider portions  31  to  35  is formed of, e.g., a substrate  50  made of ceramic or the like, and inside the substrate  50 , an optical assist unit  51 , a magnetic recording unit  52 , and a magnetic reproduction unit  53  are formed in this order from the inflow side toward the outflow side of a recorded portion of the corresponding one of the first to third recording disks  2   a  to  2   c  (in a direction indicated by the arrow mC). 
     The optical assist unit  51  mainly has an optical waveguide, and the optical waveguide is provided across a side surface and a lower surface of the substrate  50 . At an end portion (exit end) of the optical waveguide from which light is emitted, for example, disposed is a metal film provided with a very small rectangular opening. With this, light incident from an end portion (incident end) which is the reverse side of the exit end in the optical waveguide is guided from the side surface side of the substrate  50  up to the lower surface side thereof. It is preferable that the optical waveguide should have a tapered portion for condensing light to the exit end. Then, near-field light is generated at the exit end of the optical waveguide, and a light spot is formed on the corresponding one of the first to third recording disks  2   a  to  2   c  and the recorded portion of the corresponding one of the first to third recording disks  2   a  to  2   c  is heated by the light spot. The structure of the optical waveguide has a waveguide  60  (see  FIG. 3 ) described later. 
       FIG. 3  is a view showing an exemplary structure of the optical assist unit  51  having the waveguide  60 . As the waveguide  60 , a waveguide type solid immersion mirror or a PSIM (planar solid immersion mirror) proposed in, for example, Patent Document 1 can be applied. The waveguide  60  has parabolic inner surfaces  70  and  72  of which the thickness is thin and the width becomes narrower from the upper side toward the lower side. A side surface of an upper portion of the waveguide  60  is provided with a grating coupler which is an optical coupler having a diffraction grating portion  68 . When a laser beam is emitted to the diffraction grating portion  68  (specifically, a laser irradiation area inside a portion indicated by the thick broken line) and the laser beam is introduced in the waveguide  60 , the laser beam is reflected on the inner surfaces  70  and  72  as indicated by the thick arrow  64  and the laser beam is condensed on a focus F at the lowermost portion of the waveguide  60 . Then, an electromagnetic wave is generated toward the corresponding one of the first to third recording disks  2   a  to  2   c  and a very small region of the corresponding one of the first to third recording disks  2   a  to  2   c  is heated. In  FIG. 2 , the outline of an optical path PR of the laser beam emitted to the optical assist unit  51  is represented by a one-dot chain line. For increasing the efficiency of incidence of the laser beam to the waveguide  60 , there is an appropriate angle (appropriate incident angle) for the incident angle of the laser beam to the diffraction grating portion  68 . The appropriate incident angle will be discussed later. 
     The magnetic recording unit  52  is formed of a magnetic recording element for writing magnetic information on the recorded portion of the corresponding one of the first to third recording disks  2   a  to  2   c . In each of the slider portions  31  to  35 , the optical assist unit  51  emits light and the magnetic recording unit  52  records information on the corresponding one of the first to third recording disks  2   a  to  2   c . Though not shown in  FIGS. 1 to 3 , disposed are first to fifth light source units P 1  to P 5  (see  FIG. 4 ) for supplying light rays to the first to fifth slider portions  31  to  35 , respectively. Each of the first to fifth light source units P 1  to P 5  is formed of, e.g., a semiconductor laser chip or the like and attached to any one of the arm portions  41  to  43  as appropriate. 
     The magnetic reproduction unit  53  is formed of a magnetic reproduction element for reading magnetic information recorded in the corresponding one of the first to third recording disks  2   a  to  2   c.    
     Referring to  FIG. 1  again, discussion will continue. 
     The first to fifth optical element portions  21  to  25  have the same structure, and each of the optical element portions  21  to  25  has a diffraction grating and is attached to any one of the first to third arm portions  41  to  43 . The optical element portions  21  to  25  are disposed near the slider portions  31  to  35 , respectively, and on the optical paths for light emitted from the light source units P 1  to P 5  ( FIG. 4 ), respectively, to be incident on the respective diffraction grating portions  68 . 
     Specifically, the first optical element portion  21  is provided on a side of the first slider portion  31  which is closer to the rotation shaft  5 , at the lower surface of the first arm portion  41  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, and the second optical element portion  22  is provided on a side of the second slider portion  32  which is closer to the rotation shaft  5 , at the upper surface of the second arm portion  42  on the other end side which is different from the one end side to which the rotation shaft  5  is connected. The third optical element portion  23  is provided on a side of the third slider portion  33  which is closer to the rotation shaft  5 , at the lower surface of the second arm portion  42  on the other end side which is different from the one end side to which the rotation shaft  5  is connected, and the fourth optical element portion  24  is provided on a side of the fourth slider portion  34  which is closer to the rotation shaft  5 , at the upper surface of the third arm portion  43  on the other end side which is different from the one end side to which the rotation shaft  5  is connected. Further, the fifth optical element portion  25  is provided on a side of the fifth slider portion  35  which is closer to the rotation shaft  5 , at the lower surface of the third arm portion  43  on the other end side which is different from the one end side to which the rotation shaft  5  is connected. Thus, the first to fifth optical element portions  21  to  25  are held by the first to third arm portions  41  to  43 . 
     &lt;Structure of Arm Mechanism&gt; 
       FIG. 4  is a schematic diagram showing an exemplary structure of the arm mechanism  10 .  FIG. 4  is a schematic diagram as the structure of the arm mechanism  10  and in the vicinity thereof is viewed from the upper side (the +Z direction), and in  FIG. 4 , an arrangement relation of the first to fifth light source units P 1  to P 5 , the first to fifth optical element portions  21  to  25 , and the first to fifth slider portions  31  to  35  with respect to the first arm portion  41  is schematically represented by broken lines. In  FIG. 4 , the optical paths L 1  to L 5  for light emitted from the light source units P 1  to P 5  and introduced to the slider portions  31  to  35  through the optical element portions  21  to  25 , respectively, are represented by a one-dot chain line.  FIG. 5  is a schematic diagram showing an example of disposition of the optical element portion  21 . In  FIG. 5 , the optical path L 1  for a laser beam from the first light source unit P 1  to the first slider portion  31  is represented by a one-dot chain line. 
     The first arm portion  41  is constituted of an arm body  41   a  and a suspension portion  41   b . The arm body  41   a  is formed of a material which is thicker and more rigid than that of the suspension portion  41   b  and the suspension portion  41   b  is formed of a material having flexibility. The arm body  41   a  and the suspension portion  41   b  extend substantially in the same direction. One end (end portion on the −X side) of the arm body  41   a  is fixed to the rotation shaft  5  and an upper surface (surface on the +Z side) of one end of the suspension portion  41   b  is connected to a lower surface (surface on the −Z side) of the other end (end portion on the +X side) of the arm body  41   a.    
     The first slider portion  31  and the first optical element portion  21  are attached to a lower surface of the suspension portion  41   b , which is near the other end reverse to the one end connected to the arm body  41   a . In more detail, the first slider portion  31  is attached to the suspension portion  41   b  by using a predetermined spring member and the first optical element portion  21  is attached to the suspension portion  41   b  with a resin adhesive or the like. Further, the first light source unit P 1  is disposed on a lower surface (surface on the −Z side) of the arm body  41   a  near one end (end portion on the −X side) thereof which is fixed to the rotation shaft  5 , and the light emitted from the first light source unit P 1  is supplied to the first slider portion  31  through the first optical element portion  21 . In other words, the first optical element portion  21  is disposed between the first light source unit P 1  and the first slider portion  31 , on the optical path of the laser beam which is emitted from the first light source unit P 1  and led to the first slider portion  31 . 
     The same configuration consisting of the first light source unit P 1 , the first optical element portion  21 , and the slider portion  31  which are disposed as above is also disposed on the second arm portion  42  and the third arm portion  43 . 
     Specifically, the second arm portion  42  comprises an arm body  42   a , an upper suspension portion  42   b , and a lower suspension portion  42   c  all of which extend substantially in the same direction. One end (end portion on the −X side) of the arm body  42   a  is fixed to the rotation shaft  5 , and a lower surface (surface on the −Z side) of one end of the upper suspension portion  42   b  is connected to an upper surface (surface on the +Z side) of the other end (end portion on the +X side) of the arm body  42   a  and an upper surface (surface on the +Z side) of one end of the lower suspension portion  42   c  is connected to a lower surface (surface on the −Z side) of the other end (end portion on the +X side) of the arm body  42   a.    
     The second slider portion  32  and the second optical element portion  22  are attached to an upper surface near the other end reverse to one end of the upper suspension portion  42   b  which is connected to the arm body  42   a . In more detail, the second slider portion  32  is attached to the upper suspension portion  42   b  by using a predetermined spring member and the second optical element portion  22  is attached to the upper suspension portion  42   b  with a resin adhesive or the like. The second light source unit P 2  is disposed on an upper surface (surface on the +Z side) of the arm body  42   a  near one end (end portion on the −X side) thereof which is fixed to the rotation shaft  5 , and the light emitted from the second light source unit P 2  is supplied to the second slider portion  32  through the second optical element portion  22 . In other words, the second optical element portion  22  is disposed between the second light source unit P 2  and the second slider portion  32 , on the optical path of the laser beam which is emitted from the second light source unit P 2  and led to the second slider portion  32 . 
     Further, the third slider portion  33  and the third optical element portion  23  are attached to a lower surface near the other end reverse to one end of the lower suspension portion  42   c  which is connected to the arm body  42   a . In more detail, the third slider portion  33  is attached to the lower suspension portion  42   c  by using a predetermined spring member and the third optical element portion  23  is attached to the lower suspension portion  42   c  with a resin adhesive or the like. The third light source unit P 3  is disposed on a lower surface (surface on the −Z side) of the arm body  42   a  near one end (end portion on the −X side) thereof which is fixed to the rotation shaft  5 , and the light emitted from the third light source unit P 3  is supplied to the third slider portion  33  through the third optical element portion  23 . In other words, the third optical element portion  23  is disposed between the third light source unit P 3  and the third slider portion  33 , on the optical path of the laser beam which is emitted from the third light source unit P 3  and led to the third slider portion  33 . 
     The third arm portion  43  comprises an arm body  43   a , an upper suspension portion  43   b , and a lower suspension portion  43   c , and the arm body  43   a , the upper suspension portion  43   b , and the lower suspension portion  43   c  extend substantially in the same direction. One end (end portion on the −X side) of the arm body  43   a  is fixed to the rotation shaft  5 , and a lower surface (surface on the −Z side) of one end of the upper suspension portion  43   b  is connected to an upper surface (surface on the +Z side) of the other end (end portion on the +X side) of the arm body  43   a  and an upper surface (surface on the +Z side) of one end of the lower suspension portion  43   c  is connected to a lower surface (surface on the −Z side) of the other end (end portion on the +X side) of the arm body  43   a.    
     The fourth slider portion  34  and the fourth optical element portion  24  are attached to an upper surface near the other end reverse to one end of the upper suspension portion  43   b  which is connected to the arm body  43   a . In more detail, the fourth slider portion  34  is attached to the upper suspension portion  43   b  by using a predetermined spring member and the fourth optical element portion  24  is attached to the upper suspension portion  43   b  with a resin adhesive or the like. The fourth light source unit P 4  is disposed on an upper surface (surface on the +Z side) of the arm body  43   a  near one end (end portion on the −X side) thereof which is fixed to the rotation shaft  5 , and the light emitted from the fourth light source unit P 4  is supplied to the fourth slider portion  34  through the fourth optical element portion  24 . In other words, the fourth optical element portion  24  is disposed between the fourth light source unit P 4  and the fourth slider portion  34 , on the optical path of the laser beam which is emitted from the fourth light source unit P 4  and led to the fourth slider portion  34 . 
     Further, the fifth slider portion  35  and the fifth optical element portion  25  are attached to a lower surface near the other end reverse to one end of the lower suspension portion  43   c  which is connected to the arm body  43   a . In more detail, the fifth slider portion  35  is attached to the lower suspension portion  43   c  by using a predetermined spring member and the fifth optical element portion  25  is attached to the lower suspension portion  43   c  with a resin adhesive or the like. The fifth light source unit P 5  is disposed on a lower surface (surface on the −Z side) of the arm body  43   a  near one end (end portion on the −X side) thereof which is fixed to the rotation shaft  5 , and the light emitted from the fifth light source unit P 5  is supplied to the fifth slider portion  35  through the fifth optical element portion  25 . In other words, the fifth optical element portion  25  is disposed between the fifth light source unit P 5  and the fifth slider portion  35 , on the optical path of the laser beam which is emitted from the fifth light source unit P 5  and led to the fifth slider portion  35 . 
     &lt;Characteristics of Light Source Unit&gt; 
     The first to fifth light source units P 1  to P 5  emit light to the first to fifth slider portions  31  to  35  through the first to fifth optical element portions  21  to  25 , and are each formed of a cheap Fabry-Perot laser diode which is used for, for example, general-type CD players and DVD recorders. 
       FIGS. 6 and 7  are views each exemplarily showing characteristics of the first to fifth light source units P 1  to P 5 . In  FIG. 6 , shown are respective characteristics of laser light sources used for general-type CD players, DVD recorders, and Blu-ray Disc (BD) recorders. Specifically,  FIG. 6  shows, with respect to each laser light source, the wavelength of a laser beam at a room temperature (25° C.), the degree of variation in the wavelength (the degree of variation in the wavelength) in response to the change in the temperature of the laser light source, the amount of variation in the wavelength (the amount of variation in the wavelength) in a predetermined range of temperature (0 to 60° C.), and the amount of variation in the wavelength (the amount of variation in the wavelength) in a predetermined range of temperature (−40 to 70° C.).  FIG. 7  exemplarily shows a temperature dependency of the wavelength of a laser beam emitted from a laser light source used for general-type CD players. 
     Since each laser light source generates heat to raise the temperature thereof when the laser light source continuously or intermittently emits a laser beam, the wavelength of the laser beam emitted from the laser light source varies. As shown in  FIGS. 6 and 7 , the amount of variation in the wavelength is somewhat large, i.e., about ±7 mm, for example, even in a normal use environment (in a range of temperature from 0 to 60° C.). The variation in the wavelength causes variation in the appropriate incident angle of the laser beam to the diffraction grating portion  68 . Then, in the information recording device  100  of the present preferred embodiment, in order to compensate the variation in the appropriate incident angle, the first to fifth optical element portions  21  to  25  are provided. 
     Hereinafter, discussion will be sequentially made on characteristics of the diffraction grating portion  68 , structure of the first to fifth optical element portions  21  to  25 , and compensation of the variation in the appropriate incident angle by using the first to fifth optical element portions  21  to  25 . 
     &lt;Characteristics of Diffraction Grating Portion&gt; 
       FIG. 8  is a schematic cross section showing an exemplary structure of an element (waveguide grating element) of the waveguide  60  provided with the diffraction grating portion  68 . 
     The waveguide  60  is constituted of a lower clad layer  603 , a core layer  602 , and an upper clad layer  601  which are layered in this order. The diffraction grating portion  68  for coupling light is provided in the core layer  602  which is sandwiched between the upper clad layer  601  and the lower clad layer  603 . The core layer  602  is formed of a material having a refractive index higher than that of each of the clad layers  601  and  603 , and the light emitted to the diffraction grating portion  68  is coupled by the diffraction grating portion  68  and propagated in the core layer  602 , travelling toward the lower side (in the −Z direction in  FIG. 8 ). 
       FIG. 9  is a view for explanation of characteristics of wavelength dispersion (wavelength dispersion characteristics) of the waveguide  60 . Herein, discussion will be made on characteristics of light emitted from the diffraction grating portion  68  when the light is propagated from the lower side (the −Z direction) toward the upper side (the +Z direction) of the core layer  602 . In this discussion, for easy understanding, light is propagated in a direction reverse to that in an actual case where the optical assist unit  51  is used. 
     Herein, assuming that a propagation constant of a waveguide mode in the core layer  602  is β 0 , an angle of a luminous flux emitted to the outside of the waveguide  60  with respect to an XY plane (emission angle to the outside of the substrate) is θ 0 , an angle of a luminous flux emitted from the core layer  602  to the upper clad layer  601  with respect to the XY plane (emission angle in the clad layer) is θc, a refractive index in the outside of the waveguide  60  is n 0  (n 0 =1 in the air), a refractive index in the upper clad layer  601  is n c , a refractive index in the core layer  602  is n f , an effective refractive index (also referred to as an equivalent refractive index) is N, a cycle of projections and depressions of the diffraction grating portion  68  is Λ, a wavelength of light emitted to the outside of the waveguide  60  is λ, and an order of diffraction is q (herein, only −1), the following equation (1) is true: 
       β q   =n   0 ×sin θ 0   =n   c ×sin θ c   =N+qλ/Λ   (1)
 
     Herein, assuming that the refractive index n 0  of the air in the outside of the waveguide  60  is 1.0, the refractive index n c  of SiO 2  which is a material of the upper clad layer  601  is 1.47, the refractive index n f  of Ta 2 O 5  which is a material of the core layer  602  is 2.1, the effective refractive index N is 1.72, the cycle Λ is 0.8462 μm, and the wavelength λ is 785 nm, the emission angle θ 0  to the outside of the substrate is 52.4° from the above equation (1). Therefore, when a light ray having the wavelength λ enters the diffraction grating portion  68  at the emission angle θ 0 , the light use efficiency becomes highest and the appropriate incident angle of the light ray having the wavelength λ with respect to the diffraction grating portion  68  is 52.4°. The appropriate incident angle refers to an appropriate value of the angle (the incident angle achieving the highest efficiency of light incidence) of the incident light with respect to the XY plane. As shown in  FIGS. 6 and 7 , in a case where the wavelength varies by about ±7 nm, the appropriate incident angle also varies by about ±0.78°. 
     As discussed above, from the effective refractive index N of the waveguide mode in the core layer  602 , the cycle Λ of the diffraction grating portion  68 , and the like, the appropriate incident angle from the outside of the waveguide  60  to the waveguide  60  can be determined. The appropriate incident angle also depends on the wavelength λ of the light (incident light) incident on the waveguide  60 , the appropriate incident angle becomes θ 11  when the wavelength is λ i  and the appropriate incident angle becomes θ 12  when the wavelength is λ 2 . 
     When the wavelength λ 1  and the wavelength λ 2  have a relation represented by the following expression (2), as shown in  FIG. 10 , the appropriate incident angle θ 11  and the appropriate incident angle θ 12  have a relation represented by the following expression (3). 
       λ 1 &gt;λ 2   (2)
 
       θ 11 &lt;θ 12   (3)
 
     As to the cycle Λ of the diffraction grating portion  68 , considering light coupling efficiency, it is preferable to adopt such a cycle as to generate secondary light and tertiary light, and it is preferable to set the cycle to, for example, about half to five times of the wavelength λ. Under such a condition, however, it is preferable that the incident angle of the light incident on the waveguide  60  falls within ±0.1° of the appropriate incident angle. 
       FIG. 11  is a graph showing an exemplary relation between efficiency of light incidence which corresponds to the ratio of the amount of light incident on the waveguide  60  (the amount of incident light) to the amount of light emitted to the waveguide  60  (the amount of emitted light) and an incident angle of the light incident on the waveguide  60 .  FIG. 11  shows a case where the appropriate incident angle is 52.4° as discussed above, wherein the horizontal axis indicates the incident angle and the vertical axis indicates the efficiency of light incidence (relative efficiency) in a case where the maximum value of the efficiency of light incidence is 1. 
     As shown in  FIG. 11 , when it is intended to maintain the relative efficiency, e.g., not lower than 0.9, it is necessary to set the incident angle of the light incident on the waveguide  60  within ±0.1° of the appropriate incident angle of 52.4°. 
     In contrast to this, in a case where each of the first to fifth light source units P 1  to P 5  for emitting the light to be incident is formed of a Fabry-Perot laser diode, as discussed above, as the temperatures of the first to fifth light source units P 1  to P 5  rise, the wavelength λ of the laser beam emitted from each of the first to fifth light source units P 1  to P 5  becomes larger. Since an angle of diffraction of light in the diffraction grating portion  68  becomes larger, for example, when the wavelength λ of the laser beam becomes larger, the appropriate incident angle of the laser beam becomes smaller, as represented by the above expressions (2) and (3). When a range of operating temperature of the first to fifth light source units P 1  to P 5  is from 0 to 60° C., for example, the amount of variation in the wavelength λ is about ±7 nm and the appropriate incident angle of the laser beam varies by about ±0.78°. The amount of variation in the appropriate incident angle is much higher than an allowable error of the incident angle (hereinafter, referred to as “allowable error” and in this case, the allowable error is within ±0.1°) in the case where the relative efficiency shown in  FIG. 11  is considered. When the incident angle of the laser beam simply becomes higher than the allowable error, this causes a decrease in the efficiency of light incidence even if there is no variation in the positional relation or the angle relation between the luminous flux of the laser beam and the diffraction grating portion  68 . 
     In the information recording device  100  of the present preferred embodiment, however, adjustment is made by the first to fifth optical element portions  21  to  25  so that the incident angle of the laser beam to the diffraction grating portion  68  becomes the appropriate incident angle in accordance with the variation in the wavelength λ of the laser beam while the positional relation between the first to fifth light source units P 1  to P 5  and the waveguide  60  is maintained. 
     &lt;Structure of Optical Element Portion and Adjustment of Incident Angle&gt; 
     The first to fifth optical element portions  21  to  25  have the same structure, and so hereinafter discussion will be made taking the first optical element portion  21  as an example, 
       FIG. 12  is a schematic diagram showing an exemplary structure of the first optical element portion  21 .  FIG. 12(   a ) is a perspective view schematically showing an exemplary structure of the first optical element portion  21 ,  FIG. 12(   b ) is a schematic cross section with the first optical element portion  21  cut by the XZ plane, and  FIG. 12(   c ) is a view for explanation of a shape of a reflective diffraction grating portion  212  of the first optical element portion  21 . Since an actual shape of the reflective diffraction grating portion  212  is very fine, for clarification of the shape of the reflective diffraction grating portion  212 , the projections and depressions of the reflective diffraction grating portion  212  are emphasized as a matter of convenience in  FIG. 12 . 
     As shown in  FIG. 12(   a ), the first optical element portion  21  is an optical element (reflective diffraction grating prism) in which a prism portion  211  having a shape of substantially triangular prism, extending along the Y axis, of which the XZ plane has a shape of substantially right triangle which is substantially constant and the reflective diffraction grating portion  212  of which the surface on the +Z side has sawtooth-like projections and depressions along the +X direction are formed integrally. The first optical element portion  21  is entirely formed of a resin, except a reflection film  212 M described below. Herein, it is assumed, for example, that the lengths H 21  and W 21  of two sides sandwiching the right angle of a side surface are set to 240 μm and 222 μm, respectively, and the length D 21  extending along the Y axis is set to 1 mm. 
     As shown in  FIG. 12(   b ), the prism portion  211  has a light incident surface  211   a  on the −X side on which the laser beam emitted from the first light source unit P 1  is incident and an internal reflection surface  211   b  for reflecting the laser beam travelling in the +X direction to deflect the light by about 90° so that the travelling direction of the laser beam may be changed to the +Z direction. The reflective diffraction grating portion  212  has a so-called blaze shape, of which the XZ section has a sawtooth-like shape in which a plane (inclined plane) ascending linearly in the +Z direction as it travels in the +X direction and a plane (vertical plane) which is substantially in parallel with the YZ plane are repeatedly disposed, and the reflection film  212 M is formed on each inclined plane. Therefore, a plurality of inclined planes formed on an upper surface of the reflective diffraction grating portion  212  serve as reflection surfaces, and the laser beam travelling in the +Z direction is reflected on the plurality of inclined planes to cause a light diffraction phenomenon. 
     The reflection film  212 M is formed of, for example, a metal reflection film or a dielectric multilayer film made of aluminum (Al), silver (Ag), or the like. The section of one projection constituted of an inclined plane and a vertical plane in the reflective diffraction grating portion  212  has a shape of substantially right triangle of which the length of the bottom side is W a  (herein, 0.69 μm) and the height is H a  (herein, 0.348 μm). Hereinafter, a ratio of the length W a  of the bottom side and the height H a  of one projection in the sawtooth-like shape, specifically a value obtained by dividing the height H a  by the length W a  of the bottom side, is referred to as an aspect ratio (herein, 0.504). 
       FIG. 13  is a view for explanation on adjustment of the incident angle by the first optical element portion  21 .  FIG. 13(   a ) is a schematic diagram exemplarily showing the variation in the incident angle of the laser beam from the first optical element portion  21  in accordance with the variation in the wavelength λ of the laser beam. In  FIG. 13(   a ), a light exit path through which the laser beam emitted from the first light source unit P 1  at a predetermined reference temperature (e.g., a room temperature of 25° C.) exits from the reflective diffraction grating portion  212  is represented by a one-dot chain line L 1   a  and a light exit path through which the laser beam emitted from the first light source unit P 1  at a low temperature (e.g., 0° C.) exits from the reflective diffraction grating portion  212  is represented by a broken line L 1   b .  FIG. 13(   b ) is a view showing exemplary set values in a manner of reflection of the laser beam inside the first optical element portion  21 . 
     As shown in  FIG. 13(   b ), for example, an angle made by a luminous flux L 1  emitted from the first light source unit P 1  at a predetermined reference temperature and the normal of the internal reflection surface  211   b  is set to 42.775° and an angle (diffraction angle) α 31  made by a luminous flux incident on an upper surface of the first optical element portion  21  from the internal reflection surface  211   b  and a luminous flux exiting from the upper surface of the first optical element portion  21  is set to 41.03°. 
     Herein, when the wavelength λ 1  and the wavelength λ 2  of the laser beams have such a relation as represented by the above expression (2) with the variation in the temperature of the first light source unit P 1 , as shown in  FIG. 13(   a ), the diffraction angle α 31  in the case of the wavelength λ i  and a diffraction angle α 32  in the case of the wavelength λ 2  have such a relation as represented by the following expression (4). 
       α 31 &gt;α 32   (4)
 
     As to the incident angle θ of light incident on the diffraction grating portion  68  of the waveguide  60 , as shown in  FIG. 13(   a ), an angle (incident angle) θ 31  made by a laser beam having the wavelength λ 1  and the XY plane and an angle (incident angle) θ 32  made by a laser beam having the wavelength λ 2  and the XY plane have such a relation as represented by the following expression (5). 
       θ 31 &lt;θ 32   (5)
 
     The variation in the incident angle θ caused by the variation in the temperature of the first light source unit P 1  (in other words, the variation in the wavelength λ), which is represented by the expression (5), corresponds to the variation in the appropriate incident angle caused by the variation in the temperature of the first light source unit P 1  (in other words, the variation in the wavelength λ), which is represented by the above expression (3). Therefore, by adjusting the shape of the reflective diffraction grating portion  212  (for example, the length W a  of the bottom side and the height H a  in the section thereof, in other words, the aspect ratio) as appropriate, the variation in the appropriate incident angle in response to the variation in the wavelength λ of the laser beam caused by the variation in the temperature of the first light source unit P 1  can be cancelled (i.e., counteracted) by the diffraction phenomenon produced by the first optical element portion  21 . 
     Thus, in the information recording device  100  of the preferred embodiment of the present invention, though the appropriate range of incident angle of the laser beam incident on the diffraction grating portion  68  varies in response to the variation in the wavelength λ of the laser beam emitted from the first to fifth light source units P 1  to P 5 , the incident angle to the diffraction grating portion  68  can be adjusted by the first to fifth optical element portions  21  to  25  each provided on the optical path of the laser beam. This increases the light use efficiency and ensures reduction in power consumption. 
     Further, in the information recording device  100 , generally, the laser beam is emitted from the side to the first to fifth slider portions  31  to  35  and each of gaps between the suspension portions  41   b ,  42   b ,  42   c ,  43   b , and  43   c  and the recording disks  2   a ,  2   b , and  2   c  is set to 0.5 mm or less, being very narrow, for thinning of the device. In contrast to this, the appropriate incident angle to the diffraction grating portion  68  is generally in a slanting direction. Therefore, it is preferable to adopt a thin optical system which can deflect the light incident on the diffraction grating portion  68  to the slanting direction and compensate the variation in the appropriate incident angle. In order to satisfy this need, the first to fifth optical element portions  21  to  25  of the preferred embodiment are provided as elements which use the reflection therein to compensate the variation in the appropriate incident angle while being thin. 
     In the first to fifth optical element portions  21  to  25 , since the reflective diffraction grating portion  212  is provided on the upper surface of each of the first to fifth optical element portions  21  to  25  and the surface area of the upper surface increases, the adhesive strength increases in bonding the first to fifth optical element portions  21  to  25  onto the suspension portions  41   b ,  42   b ,  42   c ,  43   b , and  43   c . Therefore, the stability can be increased by fixation of the first to fifth optical element portions  21  to  25 . 
     &lt;Variations&gt; 
     The present invention is not limited to the above-discussed preferred embodiment but numerous modifications and variations can be devised without departing from the scope of the invention. 
     For example, though the first to fifth optical element portions  21  to  25  are each formed of a reflective diffraction grating prism having the reflective diffraction grating portion  212  in the above preferred embodiment, this is only one exemplary case. The first to fifth optical element portions  21  to  25  may be each formed of a prism (transmission diffraction grating prism) of which the light exit surface for a laser beam is a transmission diffraction grating. In other words, the first to fifth optical element portions  21  to  25  each have simply to use an optical element having the wavelength dispersion characteristics which cancels the wavelength dispersion characteristics of the diffraction grating portion  68 . Hereinafter, a specific example of the first to fifth optical element portions  21 A to  25 A which are constituent elements of an arm mechanism  10 A in an information recording device  100 A and each use a transmission diffraction grating prism. 
       FIG. 14  is a schematic diagram showing an exemplary structure of the first optical element portion  21 A in accordance with this variation. Specifically,  FIG. 14  is a schematic cross section with the first optical element portion  21 A cut by the XY plane. Since an actual shape of a transmission diffraction grating portion  215  is very fine, for clarification of the shape of the transmission diffraction grating portion  215 , the projections and depressions of the transmission diffraction grating portion  215  are emphasized as a matter of convenience in  FIG. 14 . 
     The first optical element portion  21 A is an optical element in which a prism portion  213  having a shape of substantially square pole, extending along the Y axis, of which the XY plane has a shape of substantially trapezoid which is substantially constant and the transmission diffraction grating portion  215  of which the bottom surface (surface on the −Z side) has sawtooth-like projections and depressions along the +X direction are formed integrally. 
     As shown in  FIG. 14 , the prism portion  213  has a light incident surface  213   a  on the −X side on which the laser beam emitted from the first light source unit P 1  is incident, an upper surface  213 U which is substantially in parallel with the XY plane, and a tilted reflection surface  213 M for reflecting the laser beam travelling in the +X direction on a covered reflection film (fowled of, for example, Al, Ag, or the like) to deflect the light by about 60° so that the travelling direction of the laser beam may be changed to diagonally downward (diagonally downward toward left in  FIG. 14 ). The transmission diffraction grating portion  215  has a so-called blaze shape, of which the XZ section has a sawtooth-like shape in which a plane (inclined plane) descending linearly in the −Z direction as it travels in the +X direction and a plane (vertical plane) which is substantially in parallel with the YZ plane are repeatedly disposed, and has a lower surface (light exit surface)  215 G through which the laser beam is transmitted to exit. The light exit surface  215 G produces a diffraction phenomenon of the laser beam. 
     As to the incident angle θ of light incident on the diffraction grating portion  68  of the waveguide  60 , when the wavelength λ 1  and the wavelength λ 2  of the laser beams have such a relation as represented by the above expression (2) with the variation in the temperature of the first light source unit P 1 , as shown in  FIG. 14 , an angle (incident angle) θ 41  made by a laser beam having the wavelength λ i  and the XY plane and an angle (incident angle) θ 42  made by a laser beam having the wavelength λ 2  and the XY plane have such a relation as represented by the following expression (6). 
       θ 41 &lt;θ 42   (6)
 
     The variation in the incident angle θ caused by the variation in the temperature of the first light source unit P 1  (in other words, the variation in the wavelength λ), which is represented by the expression (6), corresponds to the variation in the appropriate incident angle caused by the variation in the temperature of the first light source unit P 1  (in other words, the variation in the wavelength λ), which is represented by the above expression (3). Therefore, by adjusting the blaze shape of the transmission diffraction grating portion  215  as appropriate, the variation in the appropriate incident angle in response to the variation in the wavelength λ of the laser beam caused by the variation in the temperature of the first light source unit P 1  can be cancelled (i.e., counteracted) by the diffraction phenomenon produced by the first optical element portion  21 A. 
     Thus, though adopting the first optical element portion  21 A produces the same effect as that in the above preferred embodiment, from the viewpoint that it is intended to facilitate the manufacturing operation by reducing the ratio (aspect ratio) between the length of the bottom surface and the height of one projection in the sawtooth-like shape, the reflective diffraction grating prism is preferable to the transmission diffraction grating prism. On the other hand, as the distance between the diffraction grating portion of the optical element portion and the diffraction grating portion  68  of the waveguide  60  becomes shorter, the position at which the laser beam is emitted to the diffraction grating portion  68  becomes less deviated by the variation in the wavelength λ of the laser beam. Therefore, from this point of view, in the first optical element portion  21 A using the transmission diffraction grating portion  215 , it is easier to make preferable position setting. Further, as to covering of a material (reflection material) for the reflection film, the first optical element portion  21 A formed by covering a flat surface with the reflection material is preferable because of easier manufacturing operation. 
     Further, though the laser beam is reflected on the reflection film in the above preferred embodiment and the above specific example, there may be a case where no reflection film is used and reflection of the laser beam is made by adjusting the refractive index of a material for the prism and using total reflection. 
     Though the first to fifth optical element portions  21  to  25  each including the reflective diffraction grating portion  212  are adopted in the above preferred embodiment and the first to fifth optical element portions each including the transmission diffraction grating portion  215  are adopted in the above specific example, these are only exemplary cases. Without providing the diffraction grating portion, various optical elements each having wavelength dispersion characteristics, such as a prism using a material having a refractive index which is much higher than that in the air, may be adopted. 
     Though the information recording devices  100  and  100 A are each an optical assist type magnetic recording device which gives heat to the recording medium by using light to magnetically record and read information in the above preferred embodiment and the specific example, this is only one exemplary case. The present invention may be applied to, for example, general information recording devices which record and read information by using light irradiation, such as an information recording device which records and reads information by irradiating a recording medium with light, not magnetically. In other words, the present invention may be applied to general information recording devices which record information into a recording medium by using an optical head for irradiating the recording medium with light. 
     Though a laser beam is reflected twice inside the first to fifth optical element portions  21  to  25  of the above preferred embodiment and a laser beam is reflected once inside the first optical element portions  21 A to  25 A of the specific example, these are only exemplary cases. 
     Even if a laser beam is not reflected and the diffraction grating simply deflects the travelling direction of the laser beam by using the diffraction phenomenon in front of the diffraction grating portion  68 , however, the travelling direction of the laser beam cannot be deflected to a direction where the variation in the appropriate incident angle in response to the variation in the wavelength λ of the laser beam which is caused by the variation in the temperature of the light source unit can be cancelled (i.e., counteracted) by the diffraction phenomenon produced by the front diffraction grating. Therefore, it is necessary to reflect the laser beam once or more before the laser beam is incident on the diffraction grating portion  68  and provide one or more reflection surfaces in each of the first to fifth optical element portions  21  to  25  or  21 A to  25 A. Depending on the direction of the inclined plane in the sawtooth-like shape of the diffraction grating portion included in each of the first to fifth optical element portions  21  to  25  or  21 A to  25 A, the number of reflections of the laser beam (odd number or even number) is determined, and following the change of the range of the appropriate incident angle to the diffraction grating portion  68 , the optical path can be adjusted by the first to fifth optical element portions  21  to  25  or  21 A to  25 A. 
     Though the first to fifth light source units P 1  to P 5  are each formed of a semiconductor laser chip in the above preferred embodiment, this is only one exemplary case. The first to fifth light source units P 1  to P 5  may be each formed of any one of various optical elements, such as a light emitting diode (LED) or the like. 
     Though the incident angle to the diffraction grating portion  68  may be adjusted by adjusting the position and angle of each of the first to fifth light source units P 1  to P 5  as appropriate, since changing the position and angle of each of the first to fifth light source units P 1  to P 5  disadvantageously causes an unstable operation of the information recording device  100  and an increase in the power consumption in moving units, it is preferable to adjust the incident angle by the first to fifth optical element portions  21  to  25  or  21 A to  25 A. 
     Though the first optical element portion  21  is entirely formed of a resin, except the reflection film  212 M in the above preferred embodiment, the material is not limited to the resin. The first optical element portion  21  except the reflection film  212 M may be formed of, for example, glass or the like. 
     Further, though the prism portion  211  and the reflective diffraction grating portion  212  which constitute the first optical element portion  21  are formed of a resin in the above preferred embodiment, this is only one exemplary case. There may be a case, for example, where the prism portion  211  is formed of glass while the reflective diffraction grating portion  212  is formed of a resin. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           2   a  to  2   c  first to third recording disks 
           10 ,  10 A arm mechanism 
           21  to  25 ,  21 A to  25 A first to fifth optical element portions 
           31  to  35 ,  31 A to  35 A first to fifth slider portions 
           41  to  43  first to third arm portions 
           51  optical assist unit 
           60  waveguide 
           68  diffraction grating portion 
           100 ,  100 A information recording device 
           211 ,  213  prism portion 
           211   a ,  213   a  light incident surface 
           212  reflective diffraction grating portion 
           212 M reflection film 
           213 M tilted reflection surface 
           215  transmission diffraction grating portion 
           211   b ,  215 G light exit surface 
         P 1  to P 5  first to fifth light source units