Abstract:
Provided is an optical device configured to makes it possible to enhance use efficiency of light. An optical device is provided with an optical element to deflect incident light and a fixation member on which the optical element is fixed. The optical element includes a reflective surface and a diffraction grating surface that deflects the incident light. The optical element is fixed on the fixation member to restrain displacement in accordance with temperature changes at portions thereof other than the reflective surface and the diffracting grating surface in such a condition that displacement is caused without restraint be temperature changes at the reflective surface and the diffraction grating surface. A change of the incident light in the deflection angle due to an inclination change by the reflective surface and the diffracting grating surface is suppressed by a change in a diffraction angle due to a periodical change in the diffraction grating by the displacement of the diffraction grating surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an optical device, optical recording head and optical recording device. 
       DESCRIPTION OF RELATED ART 
       [0002]    In recent years, there is a trend moving toward increasingly greater densities in an information recording medium. Various forms of recording methods have been proposed. The heat-assisted magnetic recording method is one of such proposed methods. To increase densities in the magnetic recording method, the size of each magnetic domain must be reduced. To ensure stable saving of data, it is necessary to use a recording medium made of the material characterized by greater coercive force. Such a recording medium requires a strong magnetic field to be generated at the time of writing. However, the size of the magnetic field is limited in a smaller head corresponding to the magnetic domain reduced in size. 
         [0003]    In the heat-assisted magnetic recording method, the recording medium is locally heated at the time of recording so that magnetic weakening occurs. When the coercive force has been reduced, recording is performed. After that heating is suspended, and the stability of the recorded magnetic bit is ensured by natural cooling. 
         [0004]    The recording medium is preferably heated instantaneously in the heat-assisted magnetic recording method. Contact between the healing mechanism and recording medium is not permitted. This requires heating to be conducted by absorption of light in most cases. The method of using light for heating is referred to as a photo-assisted method. When high-density recording is used in the photo-assisted method, it is necessary to utilize a minute optical spot with a wavelength without exceeding the wavelength of the light to be used. 
         [0005]    To meet this requirement, a proposal has been made of an optical head that uses the nearby field light (also referred to as near field light) generated from an optical opening with a size without exceeding the wavelength of an incident light (Patent Literature 1). 
         [0006]    The optical recording head described in Patent Literature 1 is provided with a writing magnetic pole, and a waveguide having a core layer adjacent to this writing magnetic pole and a clad layer. The core layer is provided with a diffraction grating for introducing light into this core layer. If a laser beam is applied to this core layer, the laser beam is coupled to the core layer. The laser beam coupled to the core layer converges to the focal point located close to the tip end of the core layer. The recording medium is heated by the light emitted from the tip end and writing is performed by the writing magnetic pole. The element having the waveguide with light converging function is called the Planar Solid Immersion Mirror (PSIM). The PSIM described in Patent Literature 1 is equipped with a diffraction grating, as described above. When consideration is given to the percentage of the amount of light (light usage efficiency) converged on the PSIM relative to the amount of light entering this diffraction grating, an appropriate angle is present as the incident angle of the light entering the diffraction grating. 
       EARLIER LITERATURE 
     Patent Literature 
       [0007]    Patent Literature 1: U.S. Pat. No. 6,944,112 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0008]    However, the Patent Literature 1 merely describes that the light of the light source is applied by being inclined with respect to the diffraction gating, without any reference to a specific method of leading the light of the light source to the diffraction grating. 
         [0009]    In view of the problems described above, it is an object of the present invention to provide an optical device, optical recording head and optical recording device capable of enhancing the efficiency of using light. 
       Means for Solving the Problems 
       [0010]    The above problems can be solved by the following Structures: 
         [0011]    1. An optical device including; an optical element for deflecting an incident light and a fixing member for fixing the optical element; wherein the optical element has a reflective surface for deflecting the incident light and a diffraction grating surface for deflecting the incident light; the optical element is fixed on the fixing member so as to restrain displacement due to a temperature change at portions thereof other than the reflective surface and the diffracting grating surface of the optical element, under a condition that a displacement due to the temperature change on the reflective surface and the diffraction grating surface is kept in a free state; and, a change in the deflecting angle of the incident light caused by a change in inclination due to the displacement on the reflective surface and the diffraction grating surface is suppressed by a change in the diffraction angle caused by a change of the period of the diffraction grating due to the displacement in the diffraction grating surface. 
         [0012]    2. The optical device described in Structure 1 wherein a thermal expansion coefficient of a material constituting the fixing member is smaller than a thermal expansion coefficient of a material constituting the optical element. 
         [0013]    3. The optical device described in Structure 2 wherein the material of the fixing member is metallic and the material of the optical element is resinous. 
         [0014]    4. The optical device described in any one of the aforementioned Structures 1 through 3 wherein a surface of the optical element allowing entry of the incident light is fixed to the fixing member. 
         [0015]    5. The optical device described in any one of the aforementioned Structures 1 through 3 wherein the optical element further comprises a columnar member with a surface allowing entry of the incident light; the reflective surface and the diffraction grating surface is provided at a position for receiving the light which passes through the columnar member; and the optical element is fixed to the fixing member on a side surface of the columnar member. 
         [0016]    6. The optical device described in Structure 5 wherein, around the columnar member, a frame member made of the material having a thermal expansion coefficient smaller than a thermal expansion coefficient of the material constituting the optical element is covered in contact with the side surface of the columnar member. 
         [0017]    7. An optical recording head for optically recording information on a recording medium, wherein the optical recording head includes; a slider provided with a light propagation element for irradiating the recording medium with light a suspension for supporting the slider so as to move the slider relative to the recording medium; and an optical device described in Structure 4; wherein the fixing member is fixed to the suspension, and the optical element allows a deflected light to enter the light propagation element. 
         [0018]      8 . An optical recording head for optically recording information on a recording medium, wherein the optical recording head includes; a slider provided with a light propagation element for irradiating the recording medium with light; a suspension for supporting the slider so that the slider can be moved relative to the recording medium; and an optical device described in Structure 5 or 6; wherein the fixing member is the suspension, and the optical element allows a deflected light to enter the light propagation element. 
         [0019]    9. The optical recording head described in Structure 7 or 8 wherein the light propagation element further includes; a waveguide for propagating light; and a grating coupler for coupling light into the waveguide; and the optical element allows light to enter the grating coupler. 
         [0020]    10. An optical recording device includes; a light source; the optical recording head described in any one of the aforementioned Structures 7 through 9, allowing the light of the light source to enter the optical element; and the recording medium. 
       Effects of the Invention 
       [0021]    The present invention suppresses the deflecting angle of light being changed by the temperature change, thereby enhancing the efficiency of using light and ensures stable optical recording. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a schematic configuration diagram representing an optical recording device provided with a photo-assisted magnetic recording head in an embodiment of the present invention; 
           [0023]      FIG. 2  is a diagram representing the schematic configuration of an optical recording head; 
           [0024]      FIG. 3  is a front view representing the light propagation element; 
           [0025]      FIG. 4  is a cross sectional view representing the light propagation element; 
           [0026]      FIG. 5  is a cross sectional view representing the prism  50 A and its surroundings; 
           [0027]      FIG. 6  is a cross sectional view representing the prism  50 B and its surroundings; 
           [0028]      FIG. 7  is a cross sectional view representing the prism  50 C and its surroundings; 
           [0029]      FIG. 8  is a cross sectional view representing the prism  50 D and its surroundings; 
           [0030]      FIG. 9  is a diagram showing the prism  50 A as viewed from the light inputting direction; 
           [0031]      FIG. 10  is a diagram showing an example of the plasmon antenna; 
           [0032]      FIG. 11  is a cross sectional view explaining the color correction at the time of wavelength fluctuation due to the prism equipped with a diffraction grating; 
           [0033]      FIG. 12  is a cross sectional view showing the prism  50 L and the periphery thereof in an reference example; and 
           [0034]      FIG. 13  is a diagram showing the schematic configuration of an optical recording head in a reference example. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    The following describes reference examples with reference to  FIG. 13  prior to describing the embodiments of the present invention: 
         [0036]      FIG. 13  is a diagram showing the schematic configuration of an optical recording head and periphery thereof in a reference example. 
         [0037]    In  FIG. 13 , the reference numeral  2  is a recording medium, the reference numeral  4  is a suspension supported by the arm  5  provided rotatably in the direction of tracking, and the reference numeral  3  is an optical recording head provided on the tip end of the suspension  4 . A light source  10  such as an optical fiber and a lens  12  are fixed on the arm  5 , and the light of the light source  10  is emitted as parallel light from the lens  12 . 
         [0038]    The optical recording head  3  is provided with a slider  30  moving relative to a disk  2  as a recording medium. A light propagation element  20  such as a PSIM that propagates to the disk  2  the light  10   a  from the light source  10  is installed on the side surface of the slider  30 . Light  10   a  is applied to the slider  30  equipped with the light propagation element  20 , approximately from the lateral direction. 
         [0039]    To ensure effective propagation of the light  10   a  to the disk  2 , the light  10   a  from the light source  10  must be coupled effectively to the light propagation element  20 . A diffraction grating is installed at the position where the light of the light propagation element  20  is applied. The light entering the diffraction grating is coupled to the waveguide. To ensure effective connection of the light entering the diffraction grating to the waveguide, the incident angle of the light entering the diffraction grating must be set to a prescribed optimum angle. This requires the prism  50  to be arranged on the optical path of the light  10   a.  The light  10   a  is deflected by the prism  50  to have a prescribed optimum angle. 
         [0040]    The light source  10  emits light from the outgoing end of the optical fiber, i.e., from a semiconductor laser (not illustrated). In the semiconductor laser, in the Fabry-Perot reflection type, for example, a so-called mode hop phenomenon occurs if there is a temperature change, and the oscillation waveform undergoes fluctuation. The diffraction angle will be changed if there is a change in the wavelength of the light entering the diffraction grating of the light propagation element  20 . This will cause the optical coupling efficiency of the light to be reduced. To prevent the optical coupling efficiency from being reduced, the incident angle of the light propagation element  20  to the diffraction grating can be changed in response to waveform changes. 
         [0041]    The prism  50  is provided with a diffraction grating to modify the incident angle of the light entering the light propagation element  20  appropriately in response to the change in wavelength. If there is a change in the wavelength of the light entering the diffraction grating of the prism  50 , the diffraction grating is changed in response to the wavelength, and the angle of the light emitted from the prism  50  can be changed. This can be used to make the change in the outgoing angle depending on the wavelength of the prism  50  and the change in the incoming angle depending on the wavelength of the light propagation element  20  match. This matching ensures that the light entering the prism  50  is coupled with the light propagation element  20  as if there were no fluctuation in wavelength. 
         [0042]    However, if there is a change in the ambient temperature, the configuration of the prism  50  may be subject to change in response to the thermal expansion coefficient. If the configuration of the prism  50  is changed, there will be a change in the deflecting angle of the output light relative to the incoming light  10   a.  This may reduce the efficiency of the light  10   a  being coupled to the light propagation element  20 . 
         [0043]    The embodiment of the present invention to be described below solves the problem in the reference example. 
         [0044]    The following describes the optical device, optical recording head and optical recording device as embodiments of the present invention. It should be noted that the present invention is not restricted to these embodiments. The same or corresponding portions in embodiments will be assigned with the same numerals of reference, and will not be described as appropriate to avoid duplication. 
         [0045]      FIG. 1  is a schematic configuration diagram representing an optical recording device (e.g., hard disk device) provided with a photo-assisted magnetic recording head in an embodiment of the present invention. This optical recording device  100  has the following items 1 through 6 incorporated in an enclosure  1 : 
         [0046]    (1) Recording disk (recording medium)  2   
         [0047]    (2) Suspension  4  supported by the arm  5  mounted rotatably in the direction of arrow A (tracking direction) using the spindle  6  as a fulcrum 
         [0048]    (3) Tracking actuator  7  mounted on the arm  5   
         [0049]    (4) Photo-assisted magnetic recording head (hereinafter referred to as “optical recording head  3 ”) mounted on the tip end of the suspension  4  through coupling member  4   a    
         [0050]    (5) Motor for rotating the disk  2  in the direction of arrow B (not illustrated)  9736   
         [0051]    (6) Control section  8  for controlling the tracking actuator  6 , motor, light being applied in response to the writing information to be recorded on the disk  2 , and optical recording head  3  for generation of magnetic field. 
         [0052]    In the optical recording device  100 , the optical recording head  3  makes a relative movement levitating on the disk  2 . 
         [0053]      FIG. 2  schematically shows the configuration of the optical recording head  3  as viewed from the side surface. The optical recording head  3  is an optical recording head that uses light to record information on the disk  2 , and includes a slider  30 , light propagation element  20 , magnetic recording section  40 , magnetic reproduction section  41 , and prism  50  as an optical element The aforementioned PSIM is used as a light propagation element  20 . 
         [0054]    The slider  30  moves relative to the disk  2  as a magnetic recording medium while levitating. The presence of dust or damage on the disk  2  may cause contact between them. To minimize the abrasion possibly caused by the contact, the slider is preferably made of a highly abrasion-resistant hard material. For example, it is preferred to use a ceramic material containing Al 2 O 3  characterized by smaller thermal expansion coefficient such as an AltiC, zirconium or TiN. To prevent abrasion, it is good practice to provide surface treatment, thereby increasing the abrasion resistance on the surface on the side of the disk  2  of the slider  30 . For example, use of a DLC (Diamond Like Carbon) coat enhances the light transmittance, and provides the highest degree of hardness (Hv greater than 3000) second only to diamond. 
         [0055]    The surface facing the disk  2  is provided with an air bearing surface  32  (also referred to as “ABS”) to enhance levitation characteristics. 
         [0056]    Levitation of the slider  30  requires stability to be ensured in close proximity to the disk  2 . An adequate pressure to suppress the force of levitation must be applied to the slider  30 . Thus, the suspension  4  used to support the slider  30  is provided with a function of applying an adequate pressure for suppressing the force of levitation of the slider  30 , in addition to the function of tracking the optical recording head  3 . 
         [0057]    At the optical fiber output end, the light source  10  is fixed to the arm  5  together with the lens  12  equipped with a plurality of lenses for ensuring that the light emitted from the light source  10  is converted into parallel light. A laser element (semiconductor laser) that emits parallel light can be used as the light source  10 . 
         [0058]    In the optical recording head  3 , a light propagation element  20  is installed on the side surface of the slider  30  facing the light source  10 , approximately perpendicular to the recording surface of the disk  2 . 
         [0059]    The light  10   a  enters the prism  50  from the lens  12 . The incoming light is deflected to a prescribed angle by the prism  50  so that light effectively enters the light propagation element  20 . The light deflected to a prescribed angle as the light  10   b  coming from the prism  50  enters the light propagation element  20  and is coupled to the light propagation element  20 . The light coupled to the light propagation element  20  goes to the bottom end surface  24  of the light propagation element  20  and is led to the disk  2  as irradiating light for heating the disk  2 . 
         [0060]    When the light from the bottom end surface  24  is applied to the disk  2  as a small optical spot, there is a temporary increase in temperature on the light-exposed portion of the disk  2 , with the result that coercive force of the disk  2  is reduced. Then the magnetic recording section  40  allows magnetic information to be written on the portion wherein coercive force is reduced. The magnetic reproduction section  41  for reading the magnetic recorded information written to the disk  2  is provided immediately after the magnetic recording section  40 . However, this magnetic reproduction section  41  can be mounted immediately before the light propagation element  20 . 
         [0061]      FIG. 3  schematically represents the front view of the light propagation element  20 .  FIG. 4  schematically shows the cross sectional view in the axis C of  FIG. 3 . The light propagation element  20  includes a core layer  21  constituting the waveguide, a lower clad layer  22 , and an upper clad layer  23 . The core layer  21  is provided with the diffraction grating  20   a  (also called the grating coupler) for ensuring that the light  10   b  corning from the prism  50  is coupled with the core layer  21 . In  FIG. 3 , the light  10   b  is shown as an optical spot. The waveguide can be formed of a plurality of layers of substances having different refractive index. The refractive index of the core layer  21  is greater than that of the lower clad layer  22  or upper clad layer  23 . The difference in the refractive indexes is used to form the waveguide. The light in the core layer  21  is trapped inside the core layer  21 , and is efficiently led to the arrow mark  25  to reach the bottom end surface  24 . 
         [0062]    The refractive index of the core layer  21  preferably lies in the range of about 1.45 through 4.0, and the refractive indexes of the lower clad layer  22  and upper clad layer  23  preferably lie in the range of about 1.0 through 2.0. 
         [0063]    The core layer  21  is formed of Ta 2 O 5 , TiO 2 , ZnSe and others, and may have a thickness ranging from about 20 nm through 500 nm. The lower clad layer  22  and upper clad layer  23  are formed of SiO 2 , air, Al 2 O 3  and others, and may have a thickness ranging from about 200 nm through 2000 nm. 
         [0064]    The core layer  21  is provided with side surfaces  26  and  27  wherein the contour of the outer peripheral surface is formed in a parabola, these side surfaces  26  and  27  being formed to reflect the light coupled by the diffraction grating  20   a,  toward the focal point F for the purpose of converging this light to the focal point F. In  FIG. 3 , the bilaterally symmetric center axis of the parabola is represented by axis C (a line passing through the focal point F perpendicular to the directrix (not illustrated)), and the focal point of the parabolic line is shown as the focal point F. The side surfaces  26  and  27  can be provided, for example, with such a reflecting substance as gold, silver, aluminum or the like, thereby reducing the loss of light reflection. 
         [0065]    The bottom end surface  24  of the core layer  21  of the waveguide is manufactured in a planer shape with the tip end of the parabola apparently trimmed off. The light  60  from the focal point F exhibits abrupt diffusion. Thus, if the bottom end surface  24  is manufactured in a planer form, the focal point F is preferably arranged closer to the disk  2 . Further, the focal point F can be formed on the bottom end surface  24 . 
         [0066]    A plasmon antenna  24   d  for nearby field light generation is installed at the focal point F of the core layer  21  or in the vicinity thereof.  FIG. 10  shows a specific example of the configuration of the plasmon antenna  24   d.    
         [0067]    In  FIG. 10 , (a) indicates the plasmon antenna  24   d  made up of a triangular planer metallic thin film (wherein the material is exemplified by aluminum, gold and silver), and (b) represents the plasmon antenna  24   d  made up of a bow-tie type planer metallic thin film (wherein the material is exemplified by aluminum, gold and silver). Both of these antennas have an apex P with a curvature radius of 20 nm or less. Further, (c) shows a plasmon antenna  24   d  made up of a planer metallic thin film (wherein the material is exemplified by aluminum, gold and silver) equipped with an opening. This antenna has an apex P with a curvature radius of 20 nm or less. 
         [0068]    If light acts on the plasmon antenna  24   d,  nearby field light is generated close to the apex P, and recording or reproduction can be performed using the light of very small spot size. To be more specific, when a plasmon antenna  24   d  is provided at the focal point F of the core layer  21  or in the vicinity thereof, and a local plasmon is generated, the size of the optical spot formed at the focal point can be further reduced. This provides advantages in high density recording. It should be noted that the apex P of the plasmon antenna  24   d  is preferably located at the focal point F. 
         [0069]    In case of the light  10   b  emitted from the diffraction grating  20   a  and photo-coupled to the waveguide, the optimum incident angle of the light entering the diffraction grating  20   a  characterized by the highest photo-coupling efficiency is determined by the effective refractive index of the waveguide mode of the core layer  21  and the period of the diffraction grating  20   a.  The optimum incident angle depends on the wavelength of the incident light as well.  FIG. 4  shows this angle as incident angle θ 11  for wavelength λ 1  and as incident angle θ 12  for wavelength λ 2 . In  FIG. 4 , the symbol Z indicates the normal line on the light entering surface of the diffraction grating  20   a.  The same normal line will be used in the following drawings. Here assume that λ 1 &gt;λ 2  . . . (1). This gives θ 11 &lt;θ 12  . . . (2). To be more specific, an increase in the wavelength increases the diffraction grating, and this reduces the optimum incident angle to the diffraction grating  20   a.    
         [0070]    When consideration is given to the photo coupling efficiency, the period of the diffraction grating  20   a  to be used is preferably such that the second- or third-order light is generated. The period is approximately in the range from 0.5 through five times the wavelength. In this case, the permissible incident angle range in a certain wavelength is preferably about ±0.1 degree, when consideration is given to reduction in the photo-coupling efficiency. 
         [0071]    In the meantime, when the Fabry-Perot type semiconductor laser is used as the light emitted from the light source  10 , the wavelength of the light will be increased with a rise of temperature. If the temperature to be used lies in the range from 0 through 60 degrees Celsius and the fluctuation in the wavelength of the semiconductor laser lies in the range between ±10 nm, the fluctuation of the aforementioned optimum incident angle will be about 0.3 degrees, and this exceeds the aforementioned permissible incident angle range. 
         [0072]    If the fluctuation in the optimum incident angle has exceeded the permissible incident angle range due to fluctuation of the wavelength, the photo coupling efficiency will be reduced, even if there is no fluctuation in the positional relationship between the diffraction grating  20   a  due to mechanical fluctuation and the light  10   b  to be applied thereto, for example. To solve this problem, the incident angle of the light entering the diffraction grating  20   a  must be changed in response to the fluctuation in wavelength. To permit this change, the prism  50  is provided with a diffraction grating. 
         [0073]    The reference numeral  50  is used to collectively indicate the prism that deflects the light  10   a  from the light source  10  and emits the light  10   b  to be coupled with the light propagation element  20 . To illustrate the specific examples of the prism  50 ,  FIGS. 5 through 8  give the cross sectional views each representing the prisms  50 A,  50 B,  50 C and  50 D with the peripheral portions thereof. 
         [0074]    Prisms  50 A,  50 B,  50 C and  50 D can be produced by the injection molding method or press molding method using the thermoplastic resin as the material, for example. Thermoplastic resin can be exemplified by ZEONEX (registered trademark) 480R (with a refractive index of 1.525, made by Nippon Zeon Co., Ltd.), PMMA (polymethyl methacrylate, e.g., SUMIPEX (registered trademark) MGSS with a refractive index of 1.49, made by Sumitomo Chemical Co., Ltd.), and PC (polycarbonate, e.g., PANLITE (registered trademark) AD5503 with a refractive index of 1.585, made by Teijin Chemicals Ltd.). Further, these prisms can be manufactured by press molding technique using glass as a material. 
         [0075]    Referring to  FIG. 11  showing the cross section, the following describes the color correction in an ideal prism  50 K having the same configuration as the prism  50 A without any thermal expansion coefficient. The description using the prism  50 A without any thermal expansion coefficient refers to the state prior to temperature rise in the subsequent description of the prisms  50 L and  50 A through  50 D. 
         [0076]    The surface S 3  of the prism  50 K is formed in a blazed reflective diffraction grating, and is provided with a metallic reflective film and dielectric multi-layered film made of such a material as aluminum or silver. The light  10   a  entering the prism  50 K is reflected by the surface S 2  to enter the surface S 3  having a reflective diffraction grating approximately in the perpendicular direction. The light having entered the surface S 3  is emitted from the diffracted surface S 2 . Assume that the wavelengths of the light  10   a  entering the surface S 1  are wavelengths λ 1  and λ 2  satisfying the formula (1). Then the diffraction angle α is given as α 21 &gt;α 22  . . . (3). Thus, the incident angle of light entering the diffraction grating  20   a  of the light propagation element  20  will be given as θ 21 &lt;θ 22  . . . (4). 
         [0077]    The above description shows that, when the period of the reflective diffraction grating is adjusted, it is possible to cancel the formula (2) representing the relationship of the incident angle depending on the wavelength of the diffraction grating  20   a  of the light propagation element  20  (i.e., it is possible to correct colors). To be more specific, it is possible to adjust and set at least one of the period of the reflective diffraction grating and the period of the grading of the diffraction grating  20   a  provided on the surface S 3  so that θ 11 =θ 21  for wavelength λ 1  and θ 12 =θ 22  for wavelength λ 2 . 
         [0078]    In actual practice, however, the prism  50  is made of the glass or resin whose thermal expansion coefficient is not zero. The shape is changed in response to the ambient temperature by the thermal expansion coefficient of the material. 
         [0079]    As a reference example,  FIG. 12  shows the cross section of the periphery of the prism  50 K fixed onto the suspension  4  of  FIG. 13  by the adhesive agent  55 . The prism  50 L of  FIG. 12  has the same shape as the prism  50 A. 
         [0080]    In  FIG. 12 , the resin-made prism  50 L is fixed onto the lower surface  4   d  of the suspension  4  made of metal such as stainless steel by means of the adhesive agent  55 . The broken line indicates the shape before the ambient temperature rises. The solid line indicates the shape after the ambient temperature has risen. The surface S 3  provided with the diffraction grating is fixed onto the metallic suspension  4  having a thermal expansion coefficient smaller than that of the resin. Accordingly, there is no displacement in the shape wherein a problem may be raised by temperature rise. In actual practice, a displacement in the shape due to temperature rise also occurs to other than the positions indicated by the solid line. However, for ease of explanation, the characteristic displacement in the shape is shown in a simplified manner. In the following description, the broken line is also used to indicate the shape before the ambient temperature rises, and the solid line is used indicate the shape after the ambient temperature has risen. Further, the characteristic displacement in the shape is shown in a simplified manner, similarly to the above. 
         [0081]    As illustrated in  FIG. 12 , when there is a temperature rise, the surface S 3  is fixed in position without displacement in the shape. However, the prism  50 L expands. As shown by the dotted line and solid line, there is an increase in the inclination of the surface S 2  as a reflective surface, and the incident angle is reduced. Thus, the light  10   a  entering the surface S 1  undergoes a change in the angle when reflected from the surface S 2 . The incident angle which was approximately perpendicular (zero) to the diffraction grating surface before temperature rise changes to β 10  after temperature rise. Since there is no change in the pitch between diffraction gratings of the surface S 3 , the diffraction angle α is the same as the value shown in  FIG. 11 , and remains unchanged. To be more specific, α 31 =α 21 , and α 32 =α 22 . Thus, if 0-th order light direction R due to the diffraction grating has inclined by β 10  as compared to the level before temperature rise, the incident angles θ 31  and θ 32  with respect to wavelengths λ 1  and λ 2  of the light entering the light propagation element  20  are increased over the angles before temperature rise. To be more specific, θ 31 &gt;θ 21 , and θ 32 &gt;θ 22 . An increase of these incident angles θ 31  and θ 32  over the angles before temperature rise is applicable to all the wavelengths of the light  10   a  entering the prism  50 L, without being restricted to a specific wavelength of the light  10   a . As a result, even if the light  10   a  from the light source is subjected to color-correction by the prism  50 L, the incident angle of the light entering the light propagation element  20  may deviate from the optimum angle, and light coupling efficiency may be reduced. This may cause a failure in stable optical recording. 
         [0082]    The present embodiment has been obtained from the concentrated study efforts made by the present inventors to find out the way of fixing the prism equipped with a diffraction grating to the suspension  4  to ensure that the incident angle of the light entering the light propagation element  20  does not deviate from the optimum level. 
         [0083]    In the prism  50 A of  FIG. 5 , the light  10   a  enters the surface S 1 , and the incoming light is reflected by the surface S 2  as a reflective surface. The reflected light is diffracted by the surface S 3  as the diffraction grating surface equipped with the diffraction grating (reflective diffraction grating), and is emitted from the surface S 2 . The light  10   b  emitted from the surface S 2  enters the diffraction grating  20   a  for coupling light with the light propagation element  20 , at a prescribed incident angle. This light is convey led into the waveguide light, which is propagated to the lower portion of  FIG. 4  (in the direction of arrow mark  25 ). 
         [0084]    From the formula (1) representing the relationship between wavelengths λ 1  and λ 2 , the rotation angle θ is: 
         [0000]      α51&gt;α52   (5)
 
         [0085]    Thus, the incident angle of the light entering the diffraction grating  20   a  is given as: 
         [0000]      θ51&lt;θ52   (6)
 
         [0086]    The dependency of the incident angle on the wavelength of the diffraction grating  20   a,  shown in the formula (2), can be cancelled by adjusting the period of the diffraction grating provided on the surface S 3  of the prism  50 A. 
         [0087]    To ensure that the deflecting angle of the light  10   b  emitted from the prism  50 A will not be deviated by temperature fluctuation, the surface S 1  is fixed by adhesive agent or the like on the fixing plate  42  which is the fixing member provided with a suspension  4 , as shown in  FIG. 5 . To put it more specifically, part of the suspension  4  is slit and bent to form a fixing plate  42 , to which the surface S 11  of the prism  50 A is fixed. In this case, the fixing plate  42  can be another member fixed onto the suspension  4 . In this example, the fixing plate  42  is a metallic plate having a surface perpendicular to the optical axis of the light  10   a.  The surface S 3  and lower surface  4   d  of the suspension  4  are in contact with each other in such a way that they are slightly in touch with each other, without the surface S 3  being fixed to the suspension  4 . Thus, the prism  50 A is fixed to the suspension  4  in such a way that only the surface S 1  restricts the displacement due to the temperature change. The surface S 3  as a diffraction gating surface, and the surface S 2  as a reflective surface can be freely displaced by the temperature change, without being fixed. 
         [0088]    The fixing plate  42  fixed with the surface S 1  is provided with an opening that allows the flux of light  10   a  to pass through the prism  50 A without being adversely affected.  FIG. 9  shows the prism  50 A fixed to the fixing plate  42  as viewed from the side of the fixing plate  42 . 
         [0089]    The fixing plate  42  is a frame-like member equipped with an opening  43  for allowing entry of the light  10   a.  To minimize the displacement of the surface SI due to temperature change, means are preferably taken to ensure that, except for the portion other than the opening  43 , the surface S 1  will not be exposed, and the surface S 1  is preferably fixed to the fixing plate  42  wherever possible. Minimizing the displacement of the surface S 1  provides the sufficient displacement effect of the freely displaceable reflective surface and diffraction grating surface. 
         [0090]    The thermal expansion coefficient of the material constituting the fixing plate  42  minimizes the geometrical displacement of the surface S 1 , and is preferably smaller than the thermal expansion coefficient of the material constituting the prism  50 A. Further, the fixing plate  42  is preferred to have rigidity higher than that of the resin. Thus, the fixing plate  42  is preferably made of metals such as stainless steel. The rigidity can be enhanced by increasing the thickness of the plate or by designing such a structure that both edges of the fixing surface wherein the surface S 1  is fixed are bent approximately perpendicular to the fixing surface. Enhanced rigidity of the fixing plate  42  prevents fixing plate  42  from being deformed due to the thermal expansion of the prism  50 A. Enhanced rigidity also prevents the surface S 1  as the fixing surface of the prism  50  from being inclined with respect to the optical axis. To prevent the fixing plate  42  itself from being inclined, the fixing plate  42  is preferably fixed to the suspension  4  with a high degree of rigidity. 
         [0091]    In the prism  50 A, as described above, the surface S 1  as a light input surface is fixed to the fixing plate  42 , while the surface S 3  equipped with a diffraction grating and surface S 2  as a reflective surface are not fixed. If the ambient temperature has arisen under this condition and the thermal expansion coefficient of the fixing plate  42  is less than the thermal expansion coefficient of the prism  50 A, the prism  50 A will be displaced in such a way as to expand in the direction of optical axis of the light  10   a,  as illustrated by the dotted line representing the state before temperature rise, and the solid line representing the state after temperature rise. The prism  50 A does not expand in the direction perpendicular to the optical axis of the light  10   a,  as shown in  FIG. 12 . Thus, differently from the case of  FIG. 12 , the inclination of the surface S 2  as a reflective surface is reduced, and the incident angle of the light  10   a  is increased. The incident angle having been approximately perpendicular to the surface S 3  equipped with a diffraction grating before temperature rise changes to an angle β 5  in the direction opposite that in the case of  FIG. 12 . This causes the 0-th order light direction R due to diffraction grating to be inclined by β 5  with reference to the level before temperature rise. This change reduces the incident angles θ 51  and θ 52  of the light entering the light propagation element  20 . 
         [0092]    In the meantime, the surface S 3  equipped with a diffraction grating expands and extends in the direction of grating pitch along the lower surface  4   d  of the suspension  4 , whereby the period of the diffraction grating is increased and the diffraction angle α is reduced. To be more specific, α 51 &lt;α 21 , and α 52 &lt;α 22 . The incident angles θ 51  and θ 52  of the light entering the light propagation element  20  are increased. 
         [0093]    Thus, the amount of change in the inclination of the surface S 2  and the amount of change in the period of the diffraction grating of the surface S 3  can be made to have such relationship as to mutually cancel out the impact on the change in the incident angle of light entering the light propagation element  20 . 
         [0094]    Accordingly, in the incident angle of the light entering the light propagation element  20  due to temperature change, if the amount of inclination of the surface S 2  and the amount of cyclic change of the diffraction grating of the surface S 3  are set so as to cancel out each other, the deflecting angle in the prism  50 A does not fluctuate, without depending on temperature change. Thus, the incident angles θ 51  and θ 52  of the light entering the light propagation element  20  are not changed by temperature change, and θ 51 =θ 21  and θ 52 =θ 22  can be ensured. This arrangement provides stable photo-coupling to the light propagation element  20 , and allows stable optical recording to be performed by the optical recording head  3 . 
         [0095]    When selecting the material for constituting the prism  50 , this arrangement eliminates the need of giving special consideration to select a material characterized by the smallest thermal expansion coefficient. This increases the range of material selection and provides designing and manufacturing advantages. 
         [0096]    The prism  50 B of  FIG. 6  is another specific example of the prism equipped with reflective diffraction grating. In the prism  50 B, the light  10   a  enters the surface S 1 , and the incoming light is reflected by the surface S 2 . The reflected light is diffracted by the surface S 3  equipped with the diffraction grating (reflective diffraction grating), and is outputted from the surface S 2 . The light  10   b  coming out of the surface S 2  enters the diffraction grating  20   a  of the light propagation element  20  at a prescribed incident angle, and is converted into the waveguide light, which is then propagated downward (in the direction of arrow mark  25 ) in  FIG. 4 . 
         [0097]    The prism  50 B is fixed to the fixing plate  42  by the surface S 1  as a light input surface, similarly to the case of the prism  50 A. A gap is provided between the surface S 3  as a diffraction grating and suspension  4  to ensure that the surface S 3  will not touch the suspension  4 , even when the surface S 3  is extended and displaced by the thermal expansion. 
         [0098]    As described above, in the prism  50 B, the surface S 1  as a light input surface is fixed to the fixing plate  42 , while the surface S 3  as a diffraction grating surface and the surface S 2  as a reflective surface are not fixed. This structure allows free displacement to be performed by temperature change. If the ambient temperature has arisen under this condition and the thermal expansion coefficient of the fixing member is less than the thermal expansion coefficient of the prism  50 B, the prism  50 B will be displaced in shape in such a way as to incline toward the suspension  4  and to extend in the direction of optical axis of the light  10   a,  as illustrated by the dotted line representing the state before temperature rise, and the solid line representing the state after temperature rise. 
         [0099]    Thus, differently from the case of  FIG. 5 , the inclination of the surface S 2  as a reflective surface is increased, and the incident angle of the light  10   a  is decreased. The incident angle having been approximately perpendicular to the surface S 3  as a diffraction grating before temperature rise changes in the direction opposite that in the case of  FIG. 5 . This increases the incident angles θ 51  and θ 52  of the light entering the light propagation element  20 . 
         [0100]    The surface S 3  provided with a diffraction grating is extended in the direction of grating pitch by thermal expansion, and is inclined toward the suspension  4  at the same time. The surface S 3  as a diffraction grating surface which the light  10   a  reflected by the surface S 2  enters is inclined in the counterclockwise direction toward the surface of paper, as shown in  FIG. 6 . This change reduces the incident angles θ 51  and θ 52  of the light entering the light propagation element  20 . Reference numeral β 6  is used to indicate the incident angle of the light reflected from the surface S 2  with reference to the surface S 3  as the inclined diffraction grating surface. 
         [0101]    In the specific example, the amount of change due to the inclination of the surface S 3  is set to be greater than the amount of change due to the inclination of the surface S 2  in the incident angles θ 51  and θ 52 . To be more specific, the amount of change in the inclination of the surface S 3  is greater than the amount of change in the inclination of the surface S 2 . Thus, the changes in the inclinations of surface S 2  and surface S 3  reduce incident angles θ 61  and θ 62  of the light entering the light propagation element  20 , as a result. 
         [0102]    As the diffraction grating of the surface S 3  expands and extends in the direction of grating pitch, the period of the diffraction grating is increased, and the diffraction angle α is reduced. To be more specific, α 61 &lt;α 21  and α 62 &lt;α 22 . Thus, the incident angles θ 61  and θ 62  of the light entering the light propagation element  20  are increased. 
         [0103]    Thus, the amounts of changes in the inclinations of the surface S 2  and surface S 3  and the amount of change in the period of the diffraction grating can be made to have such relationship as to mutually cancel out the impact on the change in the incident angle of light entering the light propagation element  20 . 
         [0104]    Accordingly, in the incident angle of the light entering the light propagation element  20  due to temperature change, the amounts of changes in the inclinations of the surface S 2  and surface S 3  and the amount of change in the period of the diffraction grating of the surface S 3  are set so as to cancel out each other. This ensures that the deflecting angle in the prism  50 B does not change without depending on the temperature change. Thus, θ 61 =θ 21  and θ 62 =θ 22  can be obtained without incident angles θ 61  and θ 62  of the light entering the light propagation element  20  being changed by temperature changes. This arrangement ensures stable photo-coupling on the light propagation element  20 , and allows stable optical recording to be performed by the optical recording head  3 . 
         [0105]    The prism  50 C of  FIG. 7  represents another specific example of the prism equipped with a transmission type diffraction grating. In the prism  50 C, the light  10   a  enters the surface S 1  and the inputted light is reflected from the surface S 2 . The reflected light is diffracted and emitted from the surface S 3  provided with a diffraction grating (transmission type diffraction grating). The light  10   b  emitted from the surface S 3  enters the diffraction grating  20   a  of the light propagation element  20  at a prescribed incident angle, and is converted to a waveguide light, which is then propagated downward (in the direction of arrow mark  25 ) in  FIG. 4 . 
         [0106]    The prism  50 C is fixed to the fixing plate  42  by the surface S 1  as a light input surface, similarly to the case of the prism  50 A. Further, the surface S 4  facing the surface S 3  equipped with the diffraction grating is fixed to the lower surface  4   d  of the suspension  4 . 
         [0107]    As described above, in the prism  50 C, the surface S 1  as a light input surface is fixed to the fixing plate  42 , and the surface S 4  is fixed to the suspension  4 , whereas the surface S 3  as a diffraction grating surface and surface S 2  as a reflective surface are not fixed. This structure allows free displacement to be performed by temperature change. If the ambient temperature has arisen under this condition and the thermal expansion coefficient of the fixing member and suspension  4  is less than the thermal expansion coefficient of the prism  50 C, the prism  50 C will be displaced in shape in such a way as to be extended in the direction of optical axis of the light  10   a,  as illustrated by the dotted line representing the state before temperature rise, and the solid line representing the state after temperature rise in  FIG. 7 . 
         [0108]    This increases the inclination of the surface S 2  as the reflective surface, and reduces the incident angle of the light to the surface S 3  as the diffraction grating surface by angle β 7 . This change reduces the incident angles θ 1  and θ 72  of the light entering the light propagation element  20 . 
         [0109]    The diffraction grating of the surface S 3  expands and extends in the direction of grating pitch, whereby the period of the diffraction grating is increased and the diffraction angle α is reduced. To be more specific, α 71 &lt;α 21 , and α 72 &lt;α 22 . The incident angles θ 71  and θ 72  of the light entering the light propagation element  20  are increased. 
         [0110]    Thus, the amount of change in the inclination of the surface S 2  and the amount of change in the period of the diffraction grating of the surface S 3  can be made to have such relationship as to mutually cancel out the impact on the change in the incident angle of light entering the light propagation element  20 . 
         [0111]    Accordingly, in the incident angle of the light entering the light propagation element  20  due to temperature change, the amount of change in the inclination of the surface S 2  and the amount of change in the period of the diffraction grating of the surface S 3  are set so as to cancel out each other. This ensures that the deflecting angle in the prism  50 C does not change without depending on the temperature change. Thus, θ 71 =θ 21  and θ 72 =θ 22  can be obtained without incident angles θ 71  and θ 72  of the light entering the light propagation element  20  being changed by temperature changes. This arrangement ensures stable photo-coupling on the light propagation element  20 , and allows stable optical recording to be performed by the optical recording head  3 . 
         [0112]    The prism  50 D of  FIG. 8   a  represents the prism  50 A of  FIG. 5  additionally provided with the columnar section  50 D- 1  wherein the portion having the same cross section (quadrilateral section) as the surface S 1  of the prism  50 A extends in the direction of the inputted light  10   a.  The light  10   a  enters the surface S 1  and the inputted light and passes through the columnar section  50 D- 1 . This light is reflected from the surface S 2  located at the rear on the light path, and the reflected light is diffracted by the diffraction grating (transmission type diffraction grating) installed on the surface S 3 , and is emitted from the surface S 2 . The light  10   b  emitted from the surface S 2  enters the diffraction grating  20   a  of the light propagation element  20  at a prescribed incident angle, and is converted to a waveguide light, which is then propagated downward (in the direction of arrow mark  25 ) in  FIG. 4 . 
         [0113]    In the prism  50 D, the lower surface  4   d  of the suspension  4  as a fixing member in this case fixes the surface S 3 - 1  of the part not provided with the diffraction grating of the surface S 3  perpendicular to the deflecting surface (surface parallel to the surface of paper in  FIG. 8   a ) for deflecting the light  10   a  on the side surface of the columnar section  50 D- 1 . The diffraction grating surface of the surface S 3  and surface S 2  as a reflective surface are not fixed in position. This structure allows free displacement to be performed by temperature change. 
         [0114]      FIG. 8   b  shows the prism  50 D as viewed from the side that the light  10   a  enters. Around the columnar section  50 D- 1 , as shown in  FIG. 8   b , the frame member  44  made of such a metal as stainless steel, the same material as that of the suspension  4 , having the thermal expansion coefficient smaller than that of the material constituting the prism  50 D, is covered with the columnar section  50 D- 1  in contact therewith. This arrangement reduces the thermal expansion of the columnar section  50 D- 1 . The columnar section  50 D- 1  and frame member  44  can be fixed with each other using an adhesive agent or the like. 
         [0115]    If the ambient temperature has arisen under this condition and the thermal expansion coefficient of the suspension  4  is less than the thermal expansion coefficient of the prism  50 D, the prism  50 D will be displaced in such a way as to expand in the direction of optical axis, as illustrated in  FIG. 8   a  by the dotted line representing the state before temperature rise, and the solid line representing the state after temperature rise. Thus, the inclination of the surface S 2  as a reflective surface is reduced, and the incident angle of the light  10   a  is increased. The incident angle having been approximately perpendicular to the diffraction grating of the surface S 3  before temperature rise changes to an angle β 8 . This change reduces the incident angles θ 81  and θ 82  of the light entering the light propagation element  20 . 
         [0116]    The diffraction grating of the surface S 3  expands and extends in the direction of grating pitch along the lower surface  4   d  of the suspension  4 , whereby the period of the diffraction grating is increased and the diffraction angle α is reduced. To be more specific, α 81 &lt;α 21 , and α 82 &lt;α 22 . The incident angles θ 81  and θ 82  of the light entering the light propagation element  20  are increased. 
         [0117]    Thus, the amount of change in the inclination of the surface S 2  and the amount of change in the period of the diffraction gating of the surface S 3  can be made to have such relationship as to mutually cancel out the impact on the change in the incident angle of light entering the light propagation element  20 . 
         [0118]    Accordingly, in the incident angle of the light entering the light propagation element  20  due to temperature change, the amount of change in the inclinations of the surface S 2  and the amount of change in the period of the diffraction grating of the surface S 3  are set so as to cancel out each other. This ensures that the deflecting angle in the prism  50 D does not change without depending on the temperature change. Thus, θ 81 =θ 21  and θ 82 =θ 22  can be obtained without incident angles θ 81  and θ 82  of the light entering the light propagation element  20  being changed by temperature changes. This arrangement ensures stable photo-coupling on the light propagation element  20 , and allows stable optical recording to be performed by the optical recording head  3 . 
         [0119]    The aforementioned embodiments relate to the photo-assisted magnetic recording head and photo-assisted magnetic recording device. The major structures of these embodiments can be used in the optical recording head and optical recording device wherein an optical recording disk is used as a recording medium. In this case, the magnetic recording section  40  and magnetic reproduction section  41  provided on the slider  30  are not necessary. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0000]    
       
         
           
               1 . Enclosure 
               2 . Disk 
               3 . Optical recording head 
               4 . Suspension 
               5 . Arm 
               10 . Light source 
               10   a.    10   b.  Light 
               12 . Lens 
               20 . Light propagation element 
               21 . Core layer 
               22 . Lower clad layer 
               23 . Upper clad layer 
               24 . Bottom end surface 
               24   d.  Plasmon antenna 
               26 ,  27 . Side surface 
               20   a.  Diffraction grating 
               30 . Slider 
               32 . Air bearing surface 
               40 . Magnetic recording section 
               41 . Magnetic reproduction section 
               42 . Fixing plate 
               50 ,  50 A,  50 B,  50 C,  50 D,  50 K,  50 L. Prism 
               100 . Optical recording device 
             C. Axis 
             F. Focal point 
             R. 0-th order light direction