Patent Publication Number: US-2007116399-A1

Title: Optical element, manufacturing method of the optical element and driving method of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is based on and claims priority of Japanese Patent Application No. 2005-336713 filed on Nov. 22, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an optical element used in an optical signal transmission system, a manufacturing method of the optical element and a driving method of the same. In particular, the present invention relates to an optical element which controls light by magnetism, a manufacturing method of the optical element and a driving method of the same.  
      2. Description of the Prior Art  
      With the advent of advanced information society, an amount of information transmission in optical communications has been growing steadily in recent years. In response to this trend, the technique of wavelength multiplexing and the like have been developed in order to improve transmission efficiency, so that speed and capacity in an optical signal transmission system have been increasing steadily.  
      Meanwhile, in the optical signal transmission system, an optical element such as an optical modulation element and an optical switch, which controls light, is required, so that various kinds of optical elements have heretofore been developed. One of such optical elements is a mechanically-driven optical element having a movable mirror. However, in the mechanically-driven optical element, there is difficulty in meeting the demand for an increase in a signal transmission speed and miniaturization of element.  
      Japanese Patent Laid-Open Official Gazette No. 9-318978 (patent document 1) describes an optical element which partially changes a refractive index of an optical waveguide by an electrical field to switch light. The optical element of this kind has an advantage which enables not only a high speed switching of optical signals but also integration on a substrate, and various studies are carried out. However, in the optical element of this kind, a metal thin film electrode must be formed on the optical waveguide and this leads to a drawback that a large optical loss occurs. Various contrivances are needed to reduce the optical loss, causing a problem in which the structure becomes complicated to increase manufacturing cost.  
      On the other hand, an optical element which controls light by magnetism has been also developed. The optical element of this kind has advantages in which there is no optical loss due to electrodes, a structure is simple and a reaction speed is high. For example, Japanese Patent Laid-Open Official Gazette No. 9-230298 (patent document 2) describes an optical element using a magnetic layer and a polarizer as an optical element of this kind. This optical element rotates a plane of polarization of light, which propagates through the magnetic layer, by a magnetic field applied to the magnetic layer, to control an amount of light passing through the polarizer. This optical element has a drawback in which an optical loss is increased when a region corresponding to a magnetic driving portion is expanded. However, Japanese Patent Laid-Open Official Gazette No. 2004-309700 (patent document 3) describes that the use of a magnetic multilayered film can solve the above drawback.  
      Moreover, Japanese Patent Laid-Open Official Gazette No. 2000-347135 (patent document 4) describes an optical waveguide in which a clad and a core are formed of a magnetic crystal thin film. However, the optical waveguide described in the patent document 4 is used in a Faraday rotator that forms an optical isolator. Since the Faraday rotator is placed at the subsequent stage of a polarizing element to rotate a plane of polarization of light transmitted through the polarizing element, the optical waveguide described in the patent document 4 does not control an intensity of light, which propagates through the optical waveguide, by a magnetic field.  
      The optical elements described in the patent document 2 and the patent document 3 are useful to apply to a relatively large-scale optical switch. However, these optical elements cannot be integrated on the substrate since the polarizer must be placed in the optical path.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention is to provide an optical element capable of controlling light by magnetism and of being integrated on a substrate, a manufacturing method of the optical element and a driving method of the same.  
      According to one aspect of the present invention, there is provided an optical element comprising: a substrate; an optical waveguide including a clad made of material showing a magnetic resistance effect and a core made of material having a higher refractive index than that of the clad, the optical waveguide being placed on the substrate; and magnetic field applying means which applies a magnetic field to the optical waveguide to control an intensity of light propagating through the optical waveguide.  
      When no magnetic field is applied to the optical waveguide of the optical element of the present invention, little loss of light, which propagates through the optical waveguide, occurs. However, in the present invention, since the clad is formed of the material showing the magnetic resistance effect, a part of the light, which propagates through the optical waveguide, is absorbed by the clad according to a change in the magnetic resistance of the clad when the magnetic field is applied, so that a large optical loss occurs. Accordingly, for example, at the time when no magnetic field is applied is made to correspond to an on-state and at the time when the magnetic field is applied is made to correspond to an off-state, thereby making it possible to cause the optical element of the present invention to function as an optical switch.  
      According to another aspect of the present invention, there is provided a manufacturing method of an optical element, comprising the steps of: forming an optical waveguide which includes a clad made of material showing a magnetic resistance effect and a core made of material having a higher refractive index than that of the clad, on a substrate; and placing magnetic field applying means which applies a magnetic field to the optical waveguide, on at least one of upper and lower sides of the substrate.  
      According to the manufacturing method of the present invention, it is possible to easily manufacture the optical element which controls an amount of loss of light, which propagates through the optical waveguide, by magnetism. Unlike an optical element which controls a refractive index of an optical waveguide by an electric field, the optical element manufactured by the manufacturing method of the present invention has no thin-film metallic electrode on the optical waveguide. Consequently, the optical loss due to the thin-film metal can be avoided. Moreover, since the clad and the core are film-formed on the substrate, integration is easily carried out. Still moreover, since the amount of loss of light, which propagates through the optical waveguide, is controlled by a magnetic field, the high speed operation can be carried out as compared with a mechanically-driven optical element.  
      According to further another aspect of the present invention, there is provided a driving method of an optical element, in which a magnetic field is applied to an optical waveguide including a clad made of material showing a magnetic resistance effect and a core made of material having a higher refractive index than that of the clad, the optical waveguide being placed on a substrate to control the intensity of light propagating through the optical waveguide.  
      In the present invention, since the magnetic field is applied to the optical waveguide including the clad made of the material showing the magnetic resistance effect and the core made of the material having the higher refractive index than that of the clad, the optical waveguide being placed on the substrate to control the intensity of light propagating through the optical waveguide, it is possible to switch an optical signal to be input to the optical element and modulate the optical signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view illustrating one example of an optical waveguide used in an optical element according to the present invention;  
       FIG. 2  is a view illustrating an examination result of each of the relationship between a length of an optical waveguide and a light intensity when no magnetic field is applied to the optical waveguide and the relationship between the length of the optical waveguide and the light intensity when a magnetic field of 3500G is applied to the optical waveguide by an end surface diffuse reflection method;  
       FIG. 3  is a schematic view illustrating the end diffuse reflection method;  
       FIG. 4  is a plane view illustrating an optical element (optical switch) according to a first embodiment of the present invention;  
       FIG. 5  is a cross-sectional view taken along the line of I-I of  FIG. 4 ;  
       FIGS. 6A and 6B  are schematic views each illustrating an operation of the optical element according to the first embodiment of the present invention;  
       FIGS. 7A  to  7 C are cross-sectional views illustrating a manufacturing method of the optical element according to the first embodiment of the present invention, in the order of processes;  
       FIG. 8A  is a perspective view (partially notched state) illustrating an example of a commercial surface-mount type stacked coil, and  FIG. 8B  is a perspective view illustrating an example of a commercial surface-mount type winding coil;  
       FIG. 9  is a plane view illustrating an optical element (optical switch) according to a second embodiment of the present invention;  
       FIG. 10  is a cross-sectional view taken along the line of II-II of  FIG. 9 ;  
       FIG. 11  is a cross-sectional view illustrating an optical element (optical switch) according to a third embodiment of the present invention;  
       FIG. 12  is a plane view illustrating an optical element (optical modulator) according to a fourth embodiment of the present invention; and  
       FIG. 13  is a cross-sectional view taken along the line of III-III of  FIG. 12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The following will explain the principle of the present invention before explaining the embodiments of the present invention.  
       FIG. 1  is a cross-sectional view illustrating one example of an optical waveguide used in an optical element according to the present invention. An optical waveguide  10  is formed by layering a clad  12  made of La 0.5 Sr 0.5 CoO 3 , and a core  13  made of PLZT (La-doped lead zirconate titanate), on a SrTiO 3  single crystal substrate  11 . The core  13  must be formed of material having a high light transmittance and a refractive index higher than that of the clad  12 . Moreover, in the present invention, the clad  12  must be formed of material showing a magnetic resistance effect.  
      When no magnetic field is applied to the optical waveguide  10 , light, which was input to the core  13  from one end side of the optical waveguide  10 , propagates in the core  13  while being reflected at a boundary surface between the core  13  and the clad  12  or air, thereafter being output from the other end side.  
      When the magnetic field is applied to the optical waveguide  10 , a part of light, which propagates through the optical waveguide  10 , is absorbed by the clad  12  with a change in a magnetic resistance of the clad  12 , so that a large loss occurs in light propagating through the optical waveguide  10 .  FIG. 2  is a view illustrating an examination result of each of the relationship between a length of an optical waveguide and a light intensity at the time when no magnetic field is applied to the optical waveguide, and the relationship between the length of the optical waveguide and the light intensity at the time when a magnetic field of 3500G is applied to the optical waveguide by an end surface diffuse reflection method where the length of the optical waveguide is plotted in abscissa and an absolute value of light intensity (relative intensity) is plotted in ordinate.  
       FIG. 3  is a schematic view illustrating the end surface diffuse reflection method. As illustrated in  FIG. 3 , a detector  20  was placed on an end surface of the optical waveguide  10  and light was introduced into the optical waveguide  10  (core  13 ) through a prism  21 . Moreover, the prism  21  was moved in a length direction of the optical waveguide  10  and light diffusely reflected at the end surface of the optical waveguide  10  was detected by the detector  20 . Then, the relationship between a length of the optical waveguide  10  and a light intensity was examined in each of the cases at the time when the magnetic field was applied and at the time when no magnetic field was applied where a distance L, which was from the end surface of the optical waveguide  10  to the prism  21 , was a length of the optical waveguide  10  and an output of the detector  20  was a light intensity.  
      As a result, as illustrated in  FIG. 2 , an amount of optical loss was about 4.8 dB/cm when no magnetic field was applied to the optical waveguide  10 , whereas the amount of optical loss was about 12 dB/cm when a magnetic field of 3500G was applied to the optical waveguide  10 . Namely, this showed that an optical loss of about 7.2 dB/cm was controlled by the magnetic field of 3500G.  
       FIG. 2  shows that at the time when no magnetic field is applied (little optical loss occurs) is made to correspond to an on-state and at the time when the magnetic field is applied (a large optical loss occurs) is made to correspond to an off-state, thereby making it possible to cause the optical waveguide shown in  FIG. 1  to function as an optical switch. Moreover, this shows that the intensity of an applied magnetic field is changed with time, thereby making it possible to change a light intensity with time and cause the optical waveguide shown in  FIG. 1  to function as an optical modulator.  
     First Embodiment  
       FIG. 4  is a plane view illustrating an optical element according to a first embodiment of the present invention.  FIG. 5  is a cross-sectional view taken along a line of I-I of  FIG. 4 . In addition, the present embodiment shows an example in which the present invention is applied to an optical switch with one input and two outputs (1×2).  
      As illustrated in  FIG. 5 , an optical waveguide including a clad  52  made of La 0.5 Sr 0.5 CoO 3  and a core  53  made of PLZT, is formed on a SrTiO 3  substrate  51 . In the present embodiment, the clad  52  is formed on the entire upper surface of the substrate  51 , and the core  53  is branched in a Y shape as illustrated in  FIG. 4  and is formed. The optical waveguide, which extends from a signal input terminal IN (left end portion of the core  53  in  FIG. 4 ) to a branch point A, is hereinafter called an input side optical waveguide  54   a,  and two optical waveguides, which extend from signal output terminals OUT 1  and OUT 2  (right end portion of the core  53  in  FIG. 4 ) to the branch point A, are hereinafter called output side optical waveguides  54   b  and  54   c,  respectively.  
      A coil (electrical magnet)  55   b  is placed on the output side optical waveguide  54   b  and a coil (electrical magnet)  55   c  is placed on the output side optical waveguide  54   c.  When current is supplied from a driving circuit (not shown), these coils  55   b  and  55   c  generate a magnetic field to change transmittance (optical loss amount) of the optical waveguides  54   b  and  54   c.    
      The following will explain an operation of the optical switch of the present embodiment. It is assumed that no current is supplied to the coils  55   b  and  55   c  in an initial state. In this state, an optical signal input to the input side optical waveguide  54   a  from the signal input terminal IN is branched at the branch point A, and one optical signal is output from the output terminal OUT 1  through the output side optical waveguide  54   b  and the other optical signal is output from the output terminal OUT 2  through the output side optical waveguide  54   c.  Namely, in the initial state, both the output side optical waveguides  54   b  and  54   c  are in an on-state and the optical signals each having a predetermined intensity are output from the output terminal OUT 1  and OUT 2 .  
      Then, as illustrated in  FIG. 6A , when current supply to the coil  55   c  is started, a magnetic resistance of a lower portion of the coil  55   c  of the clad  52  is changed by a magnetic field generated by the coil  55   c,  and a part of the optical signal, which propagates through the output side optical waveguide  54   c,  is absorbed by the clad  52  to generate loss in the optical signal. Namely, the output side optical waveguide  54   c  is in an off-state and intensity of the optical signal output from the signal output terminal OUT 2  is considerably reduced. On the other hand, since no current is supplied to the coil  55   b,  the output side optical waveguide  54   b  remains in an on-state, and an optical signal with a predetermined intensity is output from the signal output terminal OUT 1 .  
      Next, as illustrated in  FIG. 6B , when a current supply to the coil  55   c  is stopped and a current supply to the coil  55   b  is started, the magnetic resistance of the lower portion of the coil  55   b  of the clad  52  is changed by a magnetic field generated by the coil  55   b,  and a part of the optical signal, which propagates through the output side optical waveguide  54   b,  is absorbed by the clad  52  to generate loss in the optical signal. Namely, the output side optical waveguide  53   b  becomes in an off-state and intensity of the optical signal output from the signal output terminal OUT 1  is considerably reduced. On the other hand, since no current is supplied to the coil  55   c,  the output side optical waveguide  54   c  becomes in an on-state and an optical signal with a predetermined intensity is output from the signal output terminal OUT 2 . In this way, switching of the optical signals is performed.  
       FIGS. 7A  to  7 C are cross-sectional views illustrating a manufacturing method of the optical switch of the aforementioned embodiment, in the order of processes.  
      First, as illustrated in  FIG. 7A , the substrate  51  is prepared. As the substrate  51 , for example, a single crystal substrate, which contains any one of compounds of SrTiO 3 , MgO, LaAlO 3 , LiNbO 3 , and LiTaO 3  as a principal component, is used. In the present embodiment, an Nb-doped SrTiO 3  (111) single crystal conductive substrate is used. The size of the substrate  51  is, for example, 10 mm in width, 50 mm in length, and 0.5 mm in thickness.  
      Next, as illustrated in  FIG. 7B , the clad  52  made of the La 0.5 Sr 0.5 CoO 3  is formed to have a thickness about, for example, 100 nm. In the present embodiment, the clad  52  is film-formed in an oxygen atmosphere at substrate temperature of 300° C.˜800° C. and 1 mTorr to 500 mTorr (0.133 Pa to 66.5 Pa) pressure by a laser ablation method using an Nd-YAG laser (this is also called PLD (Pulsed Laser Deposition)). It is noted that the clad  52  may be formed by a method other than the laser ablation method.  
      The clad  52  must be formed of material having a refractive index lower than that of the core  53  and showing a magnetic resistance effect. Compounds such as perovskite structured (Ca x Sr 1-x )RuO 3  (in this case, 0≦x≦1), (Ba x Sr 1-x )RuO 3  (in this case, 0≦x≦1), (La x S 1-x )CoO 3  (in this case, 0≦x≦1), and (La x Sr 1-x )MnO 3  (in this case, 0≦x≦1) can be used as such material.  
      Next, as shown in  FIG. 7C , a core layer  53   a  made of PLZT is formed on the clad  52  to have a thickness about, for example, 2 μm and a core layer  53   a  is patterned to form a core  53 . The following shows a method of forming the core layer  53   a  by a sol-gel process. However, the core layer  53   a  may be formed by a method other than the so-gel process.  
      First of all, chemicals as raw materials are prepared. In this embodiment, Pd(CH 3 COO) 2 .3H 2 O (lead acetate) is used as an organic compound for Pd, La(i-OC 3 H 7 ) 3  (lanthanum isopropoxide) is used as an organic compound for La, Ti(i-OC 3 H 7 ) 4  (titanium isopropoxide) is used as an organic compound for Ti, and Zr(OC 3 H 7 ) 4  (zirconium propoxide) is used as an organic compound for Zr. Moreover, CH 3 COCH 2 COCH 3  (2,4-pentanedione) is used as a stabilizer, and CH 3 OC 2 H 4 OH (2-methoxyethanol) is used as a solvent.  
      Subsequently, a PLZT solution is synthesized by reflux using materials including the aforementioned organic compounds, stabilizer, and the solvent. In order for a composition ratio among La, Zr and Ti of PLZT to be 9:65:35, a mole ratio between Pb(CH 3 COO) 2 .3H 2 O and La(i-OC 3 H 7 ) 3  may be 101:9, and a mole ratio between Zr(OC 3 H 7 ) and Ti(i-OC 3 H 7 ) 4  may be 65:35.  
      Next, the clad  52  is coated with the PLZT solution, which was synthesized by the aforementioned method, by a spin coat method. Then, temporary burning is performed at temperature of 350° C. and thereafter burning is further performed at temperature of 750° C. in an oxygen atmosphere. This forms a PLZT film with a thickness of about 120 μm. Since only the PLZT film with a thickness of about 120 μm can be formed in one PLZT film forming process by the sol-gel process, the respective step processes of coating, temporary burning, and burning are repeated a plurality of times, thereby layering the PLZT films to form the core layer  53   a  with a desired thickness (for example, 2 μm).  
      After forming the core layer  53   a  on the clad  52  as mentioned above, an etching mask (not shown) is formed on the core layer  53   a  by a photolithography method. Then, a wet etching or dry etching is carried out to pattern the core layer  53   a  to a desired shape. Since the optical switch with one input and two outputs is formed in the present embodiment, the core layer  53   a  is patterned in a Y shape to form the core  53  as illustrated in  FIG. 4 . When the core layer  53   a  is patterned by the wet etching, a mixed solution of, for example, HCl and HF is used as an etchant. When the core layer  53   a  is patterned by the dry etching, a mixed gas of, for example, Ar (argon) and Cl (chlorine) is used as an etching gas. Then, the core layer  53   a  is patterned to form the core  53 , thereafter the etching mask is removed therefrom. In this manner, a Y-shape optical waveguide including the clad  52  and the core  53 , namely, the input side optical waveguide  54   a  and the output side optical waveguides  54   b  and  54   c  are formed.  
      Subsequently, as illustrated in  FIGS. 4 and 5 , the coils  55   b  and  55   c  are bonded onto the output side optical waveguides  54   b  and  54   c,  thereby completing the optical switch of the present embodiment. In the present embodiment, a surface-mount type stacked coil or winding coil, which is marketed for use in a surface-mount onto a print board, is used as coils  55   b  and  55   c.    
       FIG. 8A  illustrates an example (perspective view showing a partially notched state) of a commercial surface-mount type stacked coil, and  FIG. 8B  illustrates an example (perspective view) of a commercial surface-mount type winding coil. A surface-mount type stacked coil  61  shown in  FIG. 8A  has a structure, in which a coil  62  made of a thin film conductive material, is buried in a ferrite  63 . Both ends of the coil  62  are connected to electrodes  64   a  and  64   b,  respectively, and power is supplied from a driving circuit (not shown) through these electrodes  64   a  and  64   b.    
      A surface-mount type winding coil  65  shown in  FIG. 8B  has a structure in which an electric wire  66  is wound around a center of a core  67  having an H-shape cross-section. Both ends of the electric coil  66  are connected to electrodes  68   a  and  68   b,  respectively, and power is supplied from a driving circuit (not shown) through these electrodes  68   a  and  68   b.    
      The optical switch of the present embodiment is structured to have the coils  55   b  and  55   c  on the Y-shape optical waveguide, so that the manufacture is easily carried out. Furthermore, since the optical switch of the present embodiment does not have to form thin film electrodes on the optical waveguide unlike an optical element which changes a refractive index of the optical waveguide by an electric field (optical element as described in the aforementioned patent document 1), the optical loss due to electrodes does not occur. Moreover, since the optical switch of the present embodiment switches the optical signals, which propagate through the optical waveguide, by the magnetism, the operation speed is higher than that of a mechanically-driven optical switch. Still moreover, since the optical switch of the present embodiment is manufactured by film-forming the clad  52  and the core  53  on the substrate  51 , integration is easily carried out.  
      In addition, although the present embodiment uses the commercial core coils shown in  FIGS. 8A and 8B  as magnetic field applying means, it is possible to use various kinds of coils such as an air-core coil, an iron-core coil or a thin film coil formed by a thin film technique as the magnetic field applying means. When a magnetic material is used as the core material of a core coil, there is a case where the magnetic field is always applied to the optical waveguide, so that the coil must be designed taking this point into consideration.  
     Second Embodiment  
       FIG. 9  is a plane view illustrating an optical element (optical switch) according to a second embodiment of the present invention, and  FIG. 10  is a cross-sectional view taken along the line of II-II of  FIG. 9 . In addition, this embodiment is different from the first embodiment in the point that the core  53  is surrounded by the clad while the other components are basically the same as those of the first embodiment. Accordingly, the components of  FIGS. 9 and 10  identical to those of  FIGS. 4 and 5  are assigned the same reference numerals and their detailed explanation is omitted.  
      As illustrated in  FIG. 10 , the lower clad (first clad layer)  52   a  made of La 0.5 Sr 0.5 CoO 3  is formed on the SrTiO 3  substrate  51  and the core  53  made of PLZT is formed on the lower clad  52   a.  The optical switch of this embodiment is also an optical switch with one input and two outputs and the core  53  is patterned in a Y-shape, similar to the first embodiment (see  FIG. 9 ).  
      An upper clad (second clad layer)  52   b  made of La 0.5 Sr 0.5 CoO 3  is formed on the lower clad  52   a  and the core  53  and the upper surface and side surface of the core  53  are covered with the upper clad  52   b.  The optical waveguide includes the lower clad  52   a,  the core  53  and the upper clad  52   b.  Likewise, in this embodiment, the optical waveguide, which extends from the signal input terminal IN (left end portion of the core  53  in  FIG. 9 ) to the branch point A, is hereinafter called the input side optical waveguide  54   a,  and two optical waveguides, which extend from the branch point A to the signal output terminals OUT 1  and OUT 2  (right end portion of the core  53  in  FIG. 9 ), are hereinafter called output side optical waveguides  54   b  and  54   c,  respectively. The coil  55   b  is placed on the output side optical waveguide  54   b  and the coil  55   c  is placed on the output side optical waveguide  54   c.    
      The optical switch of this embodiment can also obtain the same effect as that of the first embodiment since the lower clad  52   a  and the upper clad  52   b  are formed of the materials showing the magnetic resistance effect. Moreover, since the core  53  is surrounded by the lower clad  52   a  and the upper clad  52   b,  the present embodiment has an advantage in which the amount of optical loss of the optical waveguide due to the magnetic field is large as compared with the first embodiment, thereby making it possible to reduce the lengths of the output side optical waveguides  54   b  and  54   c.    
      Additionally, in this embodiment, both the lower clad  52   a  and the upper clad  52   b  are formed of the materials showing the magnetic resistance effect. However, if there is no need to reduce the lengths of the output side optical waveguides  54   b  and  54   c,  only one of the lower clad  52   a  and the upper clad  52   b  may be formed of the material showing the magnetic resistance effect and the other may be formed of material showing no magnetic resistance effect.  
     Third Embodiment  
       FIG. 11  is a cross-sectional view illustrating an optical element (optical switch) according to a third embodiment of the present invention. In addition, this embodiment is different from the second embodiment in the point that coils are placed on not only the upper sides of the output side optical waveguides  54   b  and  54   c  but also the lower portions thereof. Since the other components are basically the same as those of the second embodiment, the components of  FIG. 11  identical to those of  FIG. 10  are assigned the same reference numerals and their detailed explanation is omitted.  
      In the present embodiment, concave portions  51   a  are formed at positions conforming to the output side optical waveguides  54   b  and  54   c  of the back surface of the substrate  51 , respectively. Then, coils  58   b  and  58   c  are placed in these concave portions  51   a.  Namely, in this embodiment, the optical loss of the output side optical waveguide  54   b  is controlled by the coils  55   b  and  58   b  placed on the upper and lower sides of the output side optical waveguide  54   b,  and the optical loss of the output side optical waveguide  54   c  is controlled by the coils  55   c  and  58   c  placed on the upper and lower sides of the output side optical waveguide  54   c.    
      In this embodiment, since the core  53  is surrounded by the lower clad  52   a  and the upper clad  52   b  formed of the materials showing the magnetic resistance effect similar to the optical switch of the second embodiment, the same effect as that of the second embodiment can be obtained. Moreover, the optical switch of this embodiment controls the optical loss of the output side optical waveguides  54   b  and  54   c  by each two coils (coils  54   b  and  58   b  or coils  54   c  and  58   c ), so that the amount of optical loss of the optical waveguide due to the magnetic field is further increased as compared with the optical switch of the second embodiment. Accordingly, the optical switch of this embodiment has an advantage in which the lengths of the output side optical waveguides  54   b  and  54   c  can be further reduced as compared with the optical switch of the second embodiment.  
     Fourth Embodiment  
       FIG. 12  is a plane view illustrating an optical element according to a fourth embodiment of the present invention.  FIG. 13  is a cross-sectional view taken along the line of III-III of  FIG. 12 . This embodiment shows an example in which the present invention is applied to an optical modulator.  
      A clad  72  made of La 0.5 Sr 0.5 CoO 3  is formed on a single crystal substrate  71  made of Nb-doped SrTiO 3 . Moreover, a core  73  made of PLZT is formed on the clad  72  to construct an optical waveguide  74 . Furthermore, a surface-mount type stacked coil  75  is placed on the core  73 .  
      In the above-structured optical modulator of this embodiment, when light with a predetermined intensity is input from the input terminal IN and current, which is supplied to the coil  75  from a driving circuit (not shown) according to a modulation signal, is on/off controlled, transmittance (optical loss) of the optical waveguide  74  is changed according to the modulation signal and a modulation light is output from the output terminal OUT.  
      Similar to the first embodiment, the clad  72  made of La 0.5 Sr 0.5 CoO 3  is formed on the substrate  71  by the laser ablation method and the core  73  made of PLZT is formed thereon by the sol-gel process, and thereafter the commercial surface-mount type stacked coil  75  is placed on the core  73 , thereby manufacturing the optical modulator of this embodiment.  
      The optical modulator of this embodiment on/off-controls the magnetic field to be applied to the optical waveguide  74  to change an intensity of light, which propagates through the optical waveguide  74 , and to generate the modulation light. In this embodiment, since the loss of light, which propagates through the optical waveguide  74 , is also controlled by the magnetism, the high speed operation is possible and the manufacture is easily carried out. Moreover, since the optical modulator of the present embodiment is manufactured by film-forming the clad  72  and the core  73  on the substrate  71 , integration is easily carried out.