Patent Publication Number: US-7899285-B2

Title: Leaky surface-plasmon mode directional coupler and polarization detection module for magneto-optical pickup head using the same

Description:
BACKGROUND OF THE INVENTION 
     This application claims the priority of Korean Patent Application No. 2003-64719, filed on Sep. 18, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     1. Field of the Invention 
     The present invention relates to a technique of miniaturizing a magneto-optical storage device, and more particularly, to a planar waveguide-type polarization detection module, which is very important for realizing an integrated optical pickup head (IOPH) into a magneto-optical pickup head. 
     2. Description of the Related Art 
     A typical magneto-optical storage device reads data from a recording medium by measuring a voltage/current signal that reflects polarization rotation rate variations, which are triggered by reflections of beams focused on the recording medium according to the Kerr effect. In order to detect such polarization rotation rate variations, a polarization detection module, which includes a polarization beam splitter and a photo diode, is generally used. 
       FIG. 1  illustrates an example of a conventional polarization detection module for a magneto-optical storage device. Referring to  FIG. 1 , the polarization detection module  20  includes an object lens  11 , which is located close to a disk  10 , and a partial polarization beam splitter  21 , which is placed between the object lens  11  and a light source  13 . A half waveplate  22  and a polarization beam splitter  23  are placed over a splitting path of the partial polarization beam splitter  21 . First and second photo diodes  24  and  25  are located along a beam transmission path and a beam reflection path, respectively, of the polarization beam splitter  23 . The disk  10  receives a beam from the light source  13  and then reflects the beam. The partial polarization beam splitter  21  polarizes the original beam emitted from the light source  13  and reflects the beam reflected from the disk  10  to the half waveplate  22 . The half waveplate  22  has a certain-directional principal axis. Light passing through the half waveplate  22  enters the polarization beam splitter  23  and then is split into a p-wave and an s-wave such that the p-wave and the s-wave enter the first and second photo diodes  24  and  25 , respectively. Each of the first and second photo diodes  24  and  25  generates current, the amount of which corresponds to the amount of light input thereinto. The current generated by each of the first and second photo diodes  24  and  25  is transmitted to a driving circuit (not shown) that extracts data or processes servo error signals. 
     However, there is a clear limit in integrating the above-mentioned elements of the conventional polarization detection module  20  into one body because each of the elements of the conventional polarization detection module  20  has a relatively large volume. If the polarization beam splitter  23  of the polarization detection module  20  could be manufactured into a waveguide-type polarization detection module, it would be easier to integrate the elements of the polarization detection module  20  into one body. 
     For example, an optical pickup head for a CD/CDR/CDRW, which is manufactured by patterning a focusing grating on a planar waveguide, has been reported by S. Ura et al., in “An Integrated-Optic Disk Pickup Device” (J. Lightware Technol. Vol. 4, pp. 913–918, 1986).  FIG. 2  illustrates an example of the optical pickup head. Referring to  FIG. 2 , an optical pickup head  30  includes a planar waveguide  32 , which is formed on a substrate  31 , and a laser diode  33 , a photo diode  34 , a beam splitter  35 , and a focusing grating  36 , which are integrated into one body on the planar waveguide  32 . In  FIG. 2 , reference numeral  37  represents a focusing grating coupler (FGC). The optical pickup head  30  detects an optical signal that indicates a difference between amounts of light reflected from a recording medium, i.e., an optical disk. Therefore, it is possible to manufacture an optical pickup head  30  with a small form factor by integrating all the necessary elements for detecting an optical signal into one body on the planar waveguide  32  with the help of the focusing grating  36 . 
     As an application of the above-described technique of manufacturing an optical pickup head, a polarization detection module, in which a grating is carved on a planar waveguide so as to realize a magneto-optical pickup head, has been disclosed by S. Ura et al., in “Focusing Grating Couplers for Polarization Detection” (J. Lightwave Technol. Vol 6, pp. 1028–1033, 1988).  FIG. 3A  illustrates an example of a conventional planar waveguide-type polarization detection module. Referring to  FIG. 3A , a polarization detection module  40  includes a planar waveguide  42 , which is formed on a substrate  41 , and a photo diode  43  and an input coupling focusing grating  44 , which are integrated into one body on the planar waveguide  42 . The polarization detection module  40  detects a polarization signal and a focusing/tracking error signal by coupling light input thereinto to the frequency of the input coupling focusing grating. The frequency of the input coupling focusing grating varies depending on whether a polarization mode is a transverse electric (TE) mode or a transverse magnetic (TM) mode. In  FIG. 3A , reference numeral  45  represents a focusing grating coupler. 
     Only when it is possible to monolithically integrate all necessary elements of a magneto-optical pickup head into one body, it is possible to manufacture the magneto-optical pickup head in large quantities and reduce the manufacturing costs. However, it is impossible to monolithically integrate the polarization detection module  40  of  FIG. 3A  and a focusing waveguide grating coupler into one body. Therefore, in order to realize a magneto-optical pickup head using the polarization detection module  40 , the polarization detection module  40  should be aligned in such a manner that the polarization detection module  40  and the optical pickup head  30  of  FIG. 2  overlap each other, as shown in  FIG. 3B . However, it is very difficult to make the polarization detection module  40  and the optical pickup head  30  overlap each other. 
     The polarization detection module  40  of  FIG. 3A  couples a beam to the planar waveguide  42  using the input coupling focusing grating  44  having a predetermined frequency, and thus it generally has a very low coupling efficiency. Moreover, if the polarization detection module  40  and the optical pickup head  30  are integrated so that they overlap each other, as shown in  FIG. 3B , the amount of light focused on a magneto-optical disk  51  may be undesirably small because of the higher-order refraction effect. 
     SUMMARY OF THE INVENTION 
     The present invention provides a coupler, which can effectively couple or split polarized beams without using a grating. 
     The present invention also provides a polarization detection module, which can be monolithically integrated into one body with a planar waveguide without causing the amount of light focused on a magneto-optical disk to decrease. 
     According to an aspect of the present invention, there is provided a planar waveguide-type leaky plasmon mode directional coupler, which includes a planar waveguide and a leaky plasmon mode waveguide. The planar waveguide comprises a substrate, a core layer, and a cladding layer that are sequentially stacked, the planar waveguide propagating a TM mode and a TE mode between an input port and an output port. The leaky plasmon mode waveguide comprises a lower cladding layer, a metal film, which is stacked on the lower cladding layer and generates a leaky plasmon mode, and an upper cladding layer. The lower cladding layer is the cladding layer of the planar waveguide. 
     According to another aspect of the present invention, there is provided a polarization detection module, which includes a planar waveguide, a leaky plasmon mode waveguide, a first photo diode, and a second photo diode. The planar waveguide comprises a substrate, a core layer, and a cladding layer that are sequentially stacked, the planar waveguide propagating a TM mode and a TE mode between an input port and an output port. The leaky plasmon mode waveguide comprises a lower cladding layer, a metal film, which is stacked on the lower cladding layer and generates a leaky plasmon mode, and an upper cladding layer, the lower cladding layer being the cladding layer of the planar waveguide. The first photo diode is located on a leaky plasmon mode directional coupler that comprises the planar waveguide and the leaky plasmon mode waveguide. The second photo diode is located at the output port of the core layer. A polarization angle is measured based on a difference between optical powers of the first and second photo diodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic view illustrating a conventional polarization detection module for a magneto-optical storage device; 
         FIG. 2  is a perspective view illustrating a conventional optical pickup head for a CD/CDR/CDRW, in which a focusing grating is carved on a planar waveguide; 
         FIG. 3A  is a perspective view illustrating a conventional planar waveguide-type polarization detection module; 
         FIG. 3B  is a perspective view illustrating a magneto-optical pickup head including the polarization detection module of  FIG. 3A ; 
         FIG. 4  is a cross-sectional view illustrating a leaky surface plasmon mode directional coupler according to a preferred embodiment of the present invention; 
         FIG. 5  is a diagram illustrating optical power variations in a longitudinal direction of a typical directional coupler and optical power exchange characteristics of the typical directional coupler; 
         FIG. 6  is a diagram illustrating effective refractive index variations according to the thickness of a metal film of a leaky plasmon mode waveguide; 
         FIG. 7  is a diagram illustrating a plasmon mode and an input TM mode obtained by superposing a TM even mode and a TM odd mode; 
         FIG. 8  is a diagram illustrating the variation of TM and TE modes when the TM and TE modes enter the asymmetric leaky plasmon mode directional coupler of  FIG. 4  from the right to the left; 
         FIG. 9  is a cross-sectional view illustrating a polarization detection module for a magneto-optical pickup head, which uses a leaky plasmon mode directional coupler according to a preferred embodiment of the present invention; and 
         FIG. 10  is diagram illustrating extinction and generation of a plasmon mode and an input TM mode in the module of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. In the drawings, the forms of elements are exaggerated for clarity, and the same reference numerals represent the same elements. 
     Electrons on the surface of a metal collectively vibrate in a normal direction to the surface of the metal, and this phenomenon is called a surface plasmon wave. Quantized electrons that vibrate in such manner are called surface plasmons or surface plasmon polaritons. 
     A typical planar waveguide has two different modes, i.e., a TE mode and a TM mode, which orthogonally cross each other. Therefore, polarization can be split by spatially guiding the TE mode and the TM mode to two different places. Resonance of surface plasmons is very sensitive to polarization characteristics (orientation of light polarization) of light. In other words, plasmon mode coupling is characterized by the fact that a plasmon mode is not coupled to a latitudinal TE wave or the TE mode but coupled to a longitudinal TM wave or the TM mode. The present invention provides a plasmon mode directional coupler and a polarization detection module, which can spatially separate the TE and TM modes from each other by taking advantage of the characteristics of the plasmon mode coupling. More specifically, leaky plasmon mode coupling is adopted in the present invention. 
       FIG. 4  is a cross-sectional view of a leaky plasmon mode directional coupler  90  according to a preferred embodiment of the present invention, taken along a direction in which modes proceed. Referring to  FIG. 4 , a core layer  110  having a refractive index n f  is formed on a substrate  105  having a refractive index n s , and a dielectric middle layer  115  having a refractive index n d  is formed on the core layer  110  to have a thickness of d. Here, n s  is not larger than n f , and n d  is not larger than n f . The dielectric middle layer  115  is coated with a metal film  120  so as to generate surface plasmons. An absolute value of a refractive index n m  of the metal film  120  is larger than the refractive index n f  of the core layer  110 . A dielectric cover layer  125  having a refractive index n c  is thickly formed on the metal film  120 . The dielectric cover layer  125  has almost the same refractive index as the dielectric middle layer  115 . In short, as illustrated in the right portion of  FIG. 4 , n d , n s , n c &lt;n f &lt;|n m |. 
     A multilayered film consisting of the dielectric middle layer  115 , the metal film  120 , and the dielectric cover layer  125  constitutes a plasmon mode waveguide  130 , and a multilayered film consisting of the substrate  105 , the core layer  110 , and the dielectric middle layer  115  constitutes a planar waveguide  100 , which supports TE and TM modes. In other words, the dielectric middle layer  115  contributes to both the planar waveguide  100  as an upper cladding layer for the core layer  110  and the plasmon mode waveguide  130  as a lower cladding layer for the metal film  120 . The dielectric middle layer  115  and the dielectric cover layer  125  may be formed of metal oxide or fluoride. The metal film  120  may be formed of gold or silver. As descried above, a plasmon mode is not coupled to the TE mode but coupled to the TM mode. Therefore, it is possible to spatially separate the TE mode from the TM mode by taking advantage of the characteristics of the plasmon mode coupling. In addition, the plasmon mode directional coupler  90  can be used as a polarization beam splitter. The plasmon mode directional coupler  90  is an asymmetric leaky plasmon mode directional coupler. 
     Coupling of a plasmon mode and a TM mode can be described based on a coupled-mode theory. In general, a directional coupler is manufactured so that two independent waveguides are located very close to each other. Therefore, modes existing in the waveguides share their electromagnetic fields with each other and periodically exchange their optical powers and amounts of light while proceeding along their respective waveguides. For example, assuming that there is a directional coupler formed of a combination of two waveguides having the same refractive index and the two waveguides have modes A and B, respectively, optical powers A(z) and B(z) of the modes A and B, respectively, can be expressed using Equation (1):
 
 A ( z )=[ a   1   e   ikz   +a   2   e   −jkz] 
 
B( z )=[ b   1   e   ikz   +b   2   e   −jkz]   (1)
 
     where k represents a coupling coefficient, a 1  and b 1  represent modal amplitudes of the modes A and B when the modes A and B proceed in a positive direction, respectively, and a 2  and b 2  represent modal amplitudes of the modes A and B, respectively, when the modes A and B proceed in a negative direction. Supposing that there is no loss of light between the modes A and B, a propagation constant β, which is obtained by multiplying 
               2   ⁢           ⁢   π     λ         
by an effective refractive index, is a real number, and the modes A and B satisfy Equation (2) below.
 
     
       
         
           
             
               
                 
                   
                     
                       
                         ⅆ 
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                     = 
                     
                       
                         
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                         kB 
                       
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                         j 
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                         A 
                       
                     
                   
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                         ⅆ 
                         B 
                       
                       
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                         B 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
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     As shown in Equations (1) and (2), the modes A and B periodically exchange their optical powers as they proceed in a z direction. The righthand first term of Equation (2), in particular, shows that the modes A and B exchange optical powers of the same proportion while proceeding in the z direction. Solutions of Equations (1) and (2) can be expressed by Equation (3) below. 
     
       
         
           
             
               
                 
                   
                     
                       
                         P 
                         a 
                       
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                         z 
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                     = 
                     
                       
                         
                           
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                           2 
                         
                         
                           
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                           2 
                         
                       
                       = 
                       
                         1 
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                             2 
                           
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                             ( 
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                           2 
                         
                       
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       FIG. 5  illustrates optical power variations according to the length of a coupler in a case where the modes A and B exchange optical powers, as shown in Equation (3), while proceeding in the z direction. 
     Even though two waveguides are not symmetric, they can completely exchange their optical powers at intervals of a predetermined amount of time as long as they have the same propagation constant β. This type of coupling is called synchronous directional coupling. Since it is assumed that there is no loss of light between the two waveguides, a coupling length 
                 L   c     ⁡     (     =     π     2   ⁢   k         )       ,         
with which the two waveguides completely exchange their optical powers, is determined solely depending on the coupling coefficient k. However, if there is a loss of light between the two waveguides, the optical powers of the two waveguides cannot be preserved, and thus the coupling length L c  is hardly determined solely based on the coupling coefficient k. In other words, the propagation constant β is a complex number. Therefore, the equations for calculating optical powers exchanged between the two waveguides should be appropriately modified by reflecting a loss of light and the coupling coefficient k.
 
     Referring to  FIG. 4 , the plasmon mode directional coupler  90  couples a leaky plasmon mode of the plasmon mode waveguide  130  and a TM mode of the planar waveguide  100 . Preferably, a propagation constant for the leaky plasmon mode and a real number portion of a propagation constant for the TM mode are set to be the same so that the leaky plasmon mode and the TM mode can exchange as much optical power as possible. 
       FIG. 6  illustrates the variation of effective refractive indices of a leaky plasmon mode and a TM mode according to the thickness of the metal layer  120  in order to present an example of a method for designing the plasmon mode waveguide  130  and the planar waveguide  100  to have the same propagation constant. Referring to  FIG. 6 , a dotted line represents the variation of the effective refractive index of the TM mode. The TM mode is determined independently of the thickness of the metal layer  120 . On the other hand, the effective refractive index of the leaky plasmon mode varies according to the thickness of the metal film  120 . Therefore, the effective refractive index of the leaky plasmon mode is adjusted to be the same as that of the TM mode by varying the thickness of the metal film  120 , increasing the refractive index of the dielectric cover layer  125 , or decreasing the refractive index of the dielectric middle layer  115 . The effective refractive index of the leaky plasmon mode is higher than that of a non-leaky plasmon mode or that of the TM mode, and thus coupling of the leaky plasmon mode rarely occurs. Therefore, the effective refractive index of the leaky plasmon mode can be reduced and desirable conditions for a more efficient coupling can be attained by increasing the refractive index of the dielectric cover layer  125  and decreasing the refractive index of the dielectric middle layer  115 . 
     At an early stage of coupling, the TM mode proceeds without any loss of light. When the TM mode encounters the leaky plasmon mode directional coupler  90 , the TM mode begins to transfer its optical power to the leaky plasmon mode. The leaky plasmon mode slightly leaks light toward the dielectric cover layer  125 . In other words, the leaky plasmon mode leaks some of the optical power given from the planar waveguide  100  toward the dielectric cover layer  125 . As time goes by, the TM mode transfers more of its optical power to the leaky plasmon mode. Then, the leaky plasmon mode leaks all the optical power given from the TM mode toward the dielectric cover layer  125  with a loss of optical power, while proceeding. Therefore, all the optical power of the TM mode is absorbed into the dielectric cover layer  125 . The TE mode, unlike the TM mode, does not exchange optical power with the leaky plasmon mode because of the characteristics of a plasmon mode. 
     The two waveguides  100  and  130  of the leaky plasmon mode directional coupler  90  share the dielectric middle layer  115  with each other. Therefore, the leaky plasmon mode and the TM mode can share their magnetic and electric fields and can exchange as much optical power as possible when their propagation constants in the z direction are the same.  FIG. 7  illustrates the distributions of field intensities of the leaky plasmon mode and the TM mode in a direction of the X-axis. More specifically,  FIG. 7  illustrates superposed modes  1  ( 211 ) and  2  ( 212 ). The superposed modes  1  ( 211 ) and  2  ( 212 ) correspond to the leaky plasmon mode and the TM mode, respectively. The superposed mode  1  ( 211 ) is asymmetric with respect to its peak. The electric field intensity of the superposed mode  1  ( 211 ) exponentially decays in the left portion of the superposed mode  1  ( 211 ) from the peak, which shows an evanescent wave, such as a near field. The electric field intensity of the superposed mode  1  ( 211 ) fluctuates taking the form of a sine function curve in the right portion of the superposed mode  1 , which corresponds to the plasmon mode waveguide  130 . Therefore, optical power does not propagate or leak to the planar waveguide  100  but propagates to the plasmon mode waveguide  130 . Coupling of the superposed modes  1  ( 211 ) and  2  ( 212 ), i.e., coupling of the plasmon mode and the input TM mode, can be expressed by Equation (4): 
     
       
         
           
             
               
                 
                   
                     
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                       ⅈ 
                     
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                         ⅆ 
                         
                           a 
                           1 
                         
                       
                       
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                           ( 
                           
                             
                               β 
                               1 
                             
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                               ⅈ 
                               ⁢ 
                               
                                   
                               
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                                 1 
                               
                             
                           
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                         ka 
                         2 
                       
                       ⁢ 
                       
                         
 
                       
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                               a 
                               2 
                             
                           
                           
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                           2 
                         
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                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where β 1 +iα 1  represents a propagation constant for the leaky plasmon mode, which is a complex number, β 2  represents a propagation constant for the TM mode, a 1  and a 2  represent amplitudes of the leaky plasmon mode and the TM mode, respectively, and k represents a linear coupling coefficient between the leaky plasmon mode and the TM mode. Since the propagation constant for the leaky plasmon mode is a complex number, a loss of light occurs. In order to completely transfer optical power from the core layer  110  to the metal film  120  through a perfect coupling of the leaky plasmon mode and the TM mode, the coupling length L c  should be longer than a typical lossless coupling length 
               π     2   ⁢   k       .         
An optimal coupling length L of  FIG. 4  or the optimal coupling coefficient k can be obtained using Equation (4). In calculating the optimal coupling length L or the optimal coupling coefficient k, a real number portion of the propagation constant for the superposed mode  1  ( 211 ), i.e., the leaky plasmon mode, is set to be equal to the propagation constant for the superposed mode  2  ( 212 ), i.e., the TM mode, so that β 1 =β 2 . By doing so, a coupling efficiency of 100% can be achieved. In order to determine parameters for the leaky plasmon mode and the TM mode according to the propagation constants for the leaky plasmon mode and the TM mode and the optimal coupling coefficient k, solutions of the leaky plasmon mode and the TM mode are obtained using a transfer matrix for an electric field of a film, which is taught by E. Anemogiannis et al., J. Lightware Technol. Vol. 17, No. 5, pp. 929˜941, 1999. The solutions are given in terms of the TM even mode  214  and the TM odd mode  215 , as shown in  FIG. 7 . More specifically, the coupling coefficient k is obtained from different propagation constants for the TM even mode  214  and the TM odd mode  215 . Then, the superposed modes  1  ( 211 ) and  2  ( 212 ) are obtained by superposing the TM even mode  214  and the TM odd mode  215 . In otherwords, the superposed mode  1  ( 211 ) is obtained by negative-superposing the TM even mode  214  and the TM odd mode  215 , and the superposed mode  2  ( 212 ) is obtained by positive-superposing the TM even mode  214  and the TM odd mode  215 .
 
       FIG. 8  illustrates the variation of optical powers of the TM and TE modes when the TM and TE modes enter the leaky plasmon mode directional coupler  90  of  FIG. 4  from the right to the left. While the TE mode proceeds, a pattern of the distribution of optical powers of the TE mode does not change. Therefore, optical powers  311   a ,  311   b , and  311   c  that the TE mode has at an input port, a middle portion, and an output port, respectively, of the leaky plasmon mode directional coupler  90  have the same distribution pattern. On the other hand, the optical power of the TM mode propagates to the leaky plasmon mode when the TM mode is coupled to the leaky plasmon mode. Therefore, optical power  312   b  of the TM mode at the middle portion, i.e., optical power of the TM mode when the TM mode is coupled to the leaky plasmon mode, is proportionately distributed between the planar waveguide  100  and the leaky plasmon mode waveguide  130 . An optical power  312   a  of the TM mode at the input port has almost the same distribution pattern as the optical power  311   a  of the TE mode at the input port. When the TM mode arrives at the output port of the leaky plasmon mode directional coupler  90 , the TM mode practically has no optical power ( 312   a ) left because most of the optical power of the TM mode has been transferred to the leaky plasmon mode. Thus, in the planar waveguide  100 , only the TE mode remains. Therefore, the TM and TE modes, which orthogonally cross each other, are concentrated on different films, i.e., the metal film  120  and the core layer  110 , respectively. As illustrated in  FIG. 7 , the optical power  312   a  at the input port of the leaky plasmon mode directional coupler  90  resembles the superposed mode  2  ( 212 ), and the optical power  312   c  at the output port of the leaky plasmon mode directional coupler  90  resembles the superposed mode  1  ( 211 ). 
     As described above, the leaky plasmon mode directional coupler  90  can effectively split polarizations, which orthogonally cross each other, without using a grating structure. In addition, the leaky plasmon mode directional coupler  90  can be monolithically integrated into one body with other elements because the leaky plasmon mode directional coupler  90  itself is a waveguide. In other words, all necessary elements of, for example, a detection module can be monolithically into one body on a substrate, and thus additional connection modules are not necessary. Therefore, it is possible to make a polarization detection module lighter and even more compact-sized and considerably reduce the manufacturing costs. 
       FIG. 9  is a cross-sectional view illustrating a polarization detection module  400  that can measure optical powers of two different modes, which can be physically separated from each other, by using the leaky plasmon mode directional coupler  90  of  FIG. 4 , and  FIG. 10  illustrates extinctions and generations of the superposed modes  1  ( 211 ) and  2  ( 212 ) of  FIG. 7 , i.e., the plasmon mode and the TM mode. 
     Referring to  FIG. 9 , the leaky plasmon mode directional coupler  90  has the same structure as the one illustrated in  FIG. 4  except that the leaky plasmon mode directional coupler  90  of  FIG. 9  is connected to a first photo diode  410  so that it can detect the amount of light leaking to the dielectric cover layer  125  from the leaky plasmon mode. Therefore, the amount of light emitted from the leaky plasmon mode is detected by the first diode  410 . As shown in  FIG. 10 , the optical power of the superposed mode  1  ( 211 ), i.e., the leaky plasmon mode, completely disappears at an end of the leaky plasmon mode directional coupler  90 . Since only the TE mode can pass through the leaky plasmon mode directional coupler  90 , only the superposed mode  2  ( 212 ), i.e., the TE mode, is detected at the end of the leaky plasmon mode directional coupler  90 . The TE mode is detected by a second photo diode  420  located at a sidewall of the leaky plasmon mode directional coupler  90 . The amounts of light respectively detected by the first and second diodes  410  and  420  are transmitted to an amplification circuit  430 , thus reading data recorded on a magneto-optic disk. 
     As described above, the leaky plasmon mode directional coupler  90  can be monolithically integrated into one body with other elements because the leaky plasmon mode directional coupler  90  itself is a waveguide. Therefore, it is possible to manufacture an ultra-slim, ultra-small magneto-optical pickup head by integrating the leaky plasmon mode directional coupler  90  and a focusing waveguide grating coupler (FWGC), which is a planar waveguide-type pickup head, into one body. Focusing beams, generated by such magneto-optical pickup head, do not distort optical power or optical aberration. In addition, it is possible to completely split polarizations by taking advantage of the characteristics of plasmon mode coupling that a plasmon mode is coupled only to a TM mode. Therefore, it is possible to effectively split polarizations and considerably enhance a polarization extinction ratio. 
     In the present invention, two polarizations, which orthogonally cross each other, are split and detected using a leaky plasmon mode directional coupler. 
     Since the leaky plasmon mode directional coupler itself is a waveguide, it can be monolithically integrated into one body with other elements. In other words, since the leaky plasmon mode directional coupler can be integrated into one body with other elements on a substrate, additional connection modules for passively aligning or connecting elements are not necessary. Therefore, it is possible to make a polarization detection module lighter and even more compact-sized and considerably reduce the manufacturing costs. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.