Abstract:
An optical coupler including: a substrate; a cladding layer formed on the substrate; and a slab waveguide formed on the cladding layer, wherein the slab waveguide comprises a first waveguide area on which a laser beam is incident and a second waveguide area having an incident surface capable of converging and outputting the laser beam passing through the first waveguide in a width direction. The optical coupler may optically couple one of an optical fiber and a laser diode with the slab waveguide, and more particularly, and a photonic crystal waveguide, with high efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 2007-0098107 filed on Sep. 28, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical coupler, and more particularly, to an optical coupler for coupling light emitted from an optical fiber or a laser diode with a slab waveguide such as a photonic crystal waveguide. 
     2. Description of the Related Art 
     Photonic crystals indicate artificial crystal structures in which dielectrics are periodically modulated. Generally, materials having a crystal structure affect the motion of electrons since a periodic potential occurs due to regular arrangement of atoms or molecules forming the materials. An important phenomenon occurring due to this is the formation of a band gap. This concept is also applied to photons. In this case, dielectrics act as potentials to photons. In this case, a bad gap occurs, which is distinguished from an electronic band gap and called as a photonic band gap. 
     Such photonic crystal structures may be applied to embody micro photonic devices, and more particularly, used as basic structures for optical waveguide of next-generation optical printed circuit board (O-PCB). 
       FIG. 1  is a schematic diagram illustrating a laser beam incident on a photonic crystal slab waveguide. 
     Referring to  FIG. 1 , in the case of silicon-on-insulator (SOI) now generally used as a photonic crystal slab waveguide, a vertical waveguiding structure consists of a silicon oxide layer  12 , a silicon core layer  13 , and an air layer, sequentially formed on a silicon substrate  11 . Air holes h and line defects are formed in the silicon core layer  13 , thereby obtaining the photonic crystal waveguide WG. 
     A laser beam is incident on the photonic crystal waveguide WG, and an optical fiber or a laser diode is generally used as a laser light source  100 . 
     When light from the laser light source  100  is directly incident on the photonic crystal waveguide WG, there is shown much lower optical coupling efficiency than that of the case of a conventional dielectric waveguide. 
     The photonic crystal waveguide WG has a narrower width than other general waveguides. In detail, a diameter of light incident from an optical fiber or a laser diode is about 1.0 to 1.5 μm. On the other hand, since a width of the photonic crystal waveguide having the structure shown in  FIG. 1  is generally about 300 nm, it is difficult to obtain high optical coupling efficiency by using general optical coupling technologies. 
     Due to the structural problems, a ratio of light outputted to the outside via an output terminal  14  of the photonic crystal waveguide WG is just 1 to 2% of the light incident from the laser light source  100 . 
     As described above, when light emitted from an optical fiber or a laser diode is directly incident on a photonic crystal waveguide, optical coupling efficiency is just about 1 to 2%. Accordingly, there is required an optical coupler capable of improving optical coupling efficiency in the art. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides an optical coupler capable of optically coupling one of an optical fiber and a laser diode with a slab waveguide, and more particularly, a photonic crystal waveguide, with high efficiency. 
     According to an aspect of the present invention, there is provided an optical coupler including: a substrate; a cladding layer formed on the substrate; and a slab waveguide formed on the cladding layer, wherein the slab waveguide comprises a first waveguide area on which a laser beam is incident and a second waveguide area having an incident surface capable of converging and outputting the laser beam passing through the first waveguide in a width direction. 
     The first waveguide area and the second waveguide area may be integrated into one body. 
     The laser beam passing through the second waveguide area may be incident on a waveguide of an output terminal connected to the second waveguide area, and the slab waveguide and the waveguide of the output terminal may be integrated into one body. 
     The optical coupler may further include an optical converter converting and outputting the incident light into a plane laser beam and allowing the plane laser beam to be incident on the slab waveguide. 
     The optical coupler may further include a reflection mirror reflecting a laser beam that is not incident on the slab waveguide among the laser beam passing through the optical converter, to be turned toward the slab waveguide. 
     The laser beam incident on the optical converter may be oscillated from one of a terminal of an optical fiber and a laser diode. 
     The optical converter may have a lattice structure. 
     The optical converter may convert a laser beam longitudinally incident from the top into the plane laser beam and may output the plane laser beam in a lateral direction. 
     The first waveguide area and the second waveguide area may be integrated into one body. In addition, the slab waveguide and the optical converter may be integrated into one body. 
     The laser beam passing through the second waveguide area may be incident on the waveguide of the output terminal connected to the second waveguide area, and the slab waveguide, the optical converter, and the waveguide of the output terminal may be integrated into one body. 
     The laser beam passing through the second waveguide area may be incident on the waveguide of the output terminal, and the waveguide of the output terminal may be a photonic crystal. 
     To converge to a width direction of the plane laser beam, the second waveguide area may have a convex-lens shape. 
     The optical coupler of claim  1 , wherein the first waveguide area and the second waveguide area are formed of the same material and have a different thickness from each other. 
     To have effective refractive indexes different, the second waveguide area may have a thickness greater than a thickness of the first waveguide. 
     The first waveguide area may have a thickness with one to three laser beam modes. 
     The second waveguide area may have a thickness with one to five laser beam modes. 
     The second waveguide area may have an effective refractive index greater than an effective refractive index of the first waveguide area. 
     The laser beam passing through the second waveguide area may be incident on the waveguide of the output terminal connected to the second waveguide area, and the second waveguide area and the waveguide of the output terminal may have the same thickness. 
     The first waveguide area may be formed of a different material from the second waveguide area. A material forming the second waveguide area may have a refractive index greater than a refractive index of a material forming the first waveguide area. 
     The first waveguide area and the second waveguide area may have the same thickness. 
     As described above, according to an exemplary embodiment of the present invention, there is provided an optical coupler capable of optically coupling one of an optical fiber and a laser diode with a slab waveguide, and more particularly, a photonic crystal waveguide, with high efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram illustrating a laser beam incident on a photonic crystal slab waveguide; 
         FIG. 2  is a perspective view illustrating an optical coupler according to an exemplary embodiment of the present invention; 
         FIG. 3A  is a top view of a slab waveguide of the optical coupler of  FIG. 2 ; 
         FIG. 3B  is a side view of the slab waveguide of  FIG. 3A ; 
         FIG. 4A  is a perspective view of a slab waveguide including a silicon oxide cladding layer; 
         FIG. 4B  is a side view of the slab waveguide of  FIG. 4A ; 
         FIG. 5  is a diagram illustrating refractive indexes according thicknesses of silicon core layers in a waveguide having a silicon-on-insulator (SOI) structure as shown in  FIG. 2 ; 
         FIG. 6  is a graph illustrating light widths of an output laser beam, according to thicknesses of a second waveguide area; 
         FIG. 7  is a diagram illustrating a more improved optical coupler according to an embodiment of the present invention than that of  FIG. 2 ; 
         FIG. 8A  is a top view illustrating a slab waveguide employed in an optical coupler according to another embodiment of the present invention; 
         FIG. 8B  is a perspective view of the slab waveguide of  FIG. 8A ; 
         FIGS. 9A and 9B  illustrate results of a two-dimensional finite difference time domain (FDTD) simulation to show to what degree an output laser beam is focused in an optical coupler designed based on results of  FIGS. 4 and 6 ; 
         FIG. 10  is a diagram illustrating light emitting intensity of an output laser beam obtained as the results of the simulation of  FIG. 9 ; 
         FIG. 11  is a diagram illustrating an output terminal of a slab waveguide used in the simulation of  FIG. 9 , which is coupled with a photonic crystal waveguide, and a time-averaged Poynting vector; and 
         FIG. 12  is a diagram illustrating a reflectance according to a wavelength of an incident laser beam based on the result of the simulation of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The invention may however be embodied in many different forms and should not be construed as 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 shapes and dimensions may be exaggerated for clarity, and the same reference numerals are used throughout to designate the same or similar components. 
       FIG. 2  is a perspective view illustrating an optical coupler according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the optical coupler includes an optical converter  104  with a lattice structure and a slab waveguide  105 . The slab waveguide  105  includes a first waveguide area and a second waveguide area, which will be described in detail with reference to  FIGS. 3 and 4 . 
     A laser light source  100  is vertically disposed above the optical converter  104  with the lattice structure, and a photonic crystal waveguide  106  on which a laser beam passing through the slab waveguide  105  is incident disposed on an output terminal of the second waveguide area. As shown in  FIG. 2 , in the photonic crystal waveguide  106 , air holes h and a waveguide are formed for a photonic crystal structure, which corresponds to a slab structure similar to the slab waveguide  105 . 
     The optical converter  104  converts an incident laser beam having a spherical surface shape into a plane shape and provides the converted laser beam to the slab waveguide  105 . In detail, a laser beam emitted from an optical fiber or a laser diode vertically disposed above is converted into a plane laser beam and the plane laser beam is incident on the slab waveguide  105  disposed laterally adjacent to the optical converter  104 . 
     As described above, the laser beam having the spherical surface shape is converted into the plane laser beam by the optical converter  104  before incident on the photonic crystal waveguide  106 , thereby more improving optical coupling efficiency than a case in which the laser beam having the spherical surface shape is directly incident on the slab waveguide  105 . 
     However, the optical converter  104  with the lattice structure is not an essential element in the present invention. Other optical converter capable of converting a laser beam having a spherical surface shape into a plane laser beam and outputting the plane laser beam may be used. 
     In addition, when a plane laser beam is capable of being directly incident on the slab waveguide  105 , it is not required to use an optical converter. 
     The slab waveguide  105  is formed on a silicon substrate  101  where a silicon oxide layer  102  is formed. An air layer and the silicon oxide layer  102 , which are an upper and lower cladding layers, respectively, perform an optical waveguide function. In the present embodiment, considering an aspect of convenience of process and a refractive index, the slab waveguide  105  may be formed of silicon. 
     On the other hand, the optical converter  104 , the slab waveguide  105 , and the photonic crystal waveguide  106  are integrated to form one body. 
     That is, as shown in  FIG. 2 , the optical converter  104 , the slab waveguide  105 , and the photonic crystal waveguide  106  correspond to a silicon core layer  103 . 
     In an aspect of function, the slab waveguide  105  converges an incident plane laser beam to a width direction of a waveguide connected to an output terminal, that is, the photonic crystal waveguide  106  in such a way that an optical width of the plane laser beam becomes identical to a width of the photonic crystal waveguide  106 . That is, the slab waveguide  105  allows the width of the plane laser beam not to be changed and a size of the plane laser beam in the width direction to be reduced, thereby improving coupling efficiency with the photonic crystal waveguide. This is, due to the one body, the second waveguide area of the slab waveguide  105 , which has a convex lens shape, has a thickness identical to a thickness of the photonic crystal waveguide  106 . 
     To perform such function, in the structure of the slab waveguide  105 , as shown in  FIG. 2 , since the first waveguide area has a thickness different from the thickness of the second waveguide area, there is a step between the first waveguide area and the second waveguide area. 
       FIG. 3A  is a top view of the slab waveguide  105 , and  FIG. 3B  is a side view of the slab waveguide  105 .  FIG. 4A  is a perspective view illustrating a configuration including a silicon oxide cladding layer.  FIG. 4B  is a side view of the configuration of  FIG. 4A . 
     As shown in  FIGS. 3A to 4B , the slab waveguide  105  includes a first waveguide area  31  on which a plane laser beam passing through the optical converter  104  is incident and a second waveguide area  32  allowing an optical width of the plane laser beam passing through the first waveguide area  31  to be identical to the photonic crystal waveguide  106 . In this case, a surface of the second waveguide area  32 , on which the plane laser beam passing through the first waveguide area  31  is incident has a convex lens shape. 
     Also, in the present embodiment, the first and second waveguide areas  31  and  32  are formed on the silicon oxide layer  102  and formed of silicon, which are integrated to form one body. 
     As described above, since the first waveguide area  31  and the second waveguide area  32  are formed of the same material, to converge the plane laser beam to a width direction of a waveguide of an output terminal, there is required a structure capable of making an effective refractive index of the first waveguide area  31  different from an effective refractive index of the second waveguide area  32 . In the present embodiment, the effective refractive index may be controlled by making a thickness t 1  of the first waveguide area different from a thickness t 2  of the second waveguide area  32 . 
     In detail, since the plane laser beam passing through the first waveguide area  31  should be converged while incident on the second waveguide area  32 , the thickness t 2  of the second waveguide area  32  may be greater than the thickness t 1  of the first waveguide area  31 . 
     Referring to  FIGS. 5 and 6 , a condition to design the thicknesses of the first waveguide area  31  and the second waveguide area  32  will be described in detail. 
       FIG. 5  illustrates effective refractive indexes according to a thickness of a silicon core layer in a waveguide having a silicon-on-insulator structure as shown in  FIG. 2 . 
     In this case, each effective refractive index is illustrated according to an optical mode and an incident laser beam has a center wavelength of 1.55 μm. That is, as shown as a dotted line, when the thickness of the silicon core layer is about 150 nm, the number of allowable optical modes is one (m=1) and an effective refractive index is about 2.53. Similarly, when the thickness of the silicon core layer is approximately from 300 to 400 nm, the number of the allowable optical modes is two (m=0, m=1) and the respective effective refractive indexes are previously determined. 
     On the other hand, an optical width of the laser beam incident on the waveguide of the output terminal, that is, a photonic crystal waveguide by the slab waveguide may be determined according to Scalar diffraction theory as shown in following Equation 1. 
     
       
         
           
             
               
                 
                   
                     d 
                     FWHM 
                   
                   = 
                   
                     λ 
                     
                       2.25 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         n 
                         out 
                       
                       ⁢ 
                       
                         sin 
                         ⁡ 
                         
                           [ 
                           
                             
                               π 
                               2 
                             
                             - 
                             
                               
                                 sin 
                                 
                                   - 
                                   1 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     n 
                                     in 
                                   
                                   
                                     n 
                                     out 
                                   
                                 
                                 ) 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     where λ indicates a wavelength of an incident laser beam, n in  indicates an effective refractive index of the first waveguide area, and n out  indicates an effect refractive index of an emitting portion. 
     Referring to Equation 1, it may be known that a size of the optical width of the laser beam passing through the second waveguide area and incident on the waveguide of the output terminal is inversely proportional to a difference between effective refractive indexes of the first and the second waveguide area. That is, to improve optical coupling efficiency with the photonic crystal waveguide that is the waveguide of the output terminal in the present embodiment, a small size of the optical width of the output laser beam of the slab waveguide is advantageous, which is capable of being obtained by making the effective refractive index n out  greater than the effective refractive index n in  of the first waveguide area. 
     Considering such condition in association with  FIG. 5 , the smaller thickness of the silicon core layer, the smaller effective refractive index. Accordingly, the thickness t 1  of the first waveguide area may be smaller. 
     However, when a thickness of a waveguide is excessively smaller, coupling efficiency of light incident on the first waveguide area via the optical converter shown in  FIG. 2  is decreased, the first waveguide area may have a thickness of a certain degree or more. 
     On the other hand, the greater thickness of the waveguide, the more difficult to reduce the size of the optical width. In addition, since the number of allowable optical modes is increased, an aberration may occur due to the increased multi modes. 
     Considering this, the thickness t 1  of the first waveguide area, which is capable of being employed in the present embodiment, may be limited in such a way that the number of the allowable optical modes is 1 to 3. Referring to  FIG. 5 , in the present embodiment, the thickness t 1  may correspond to from about 150 to 170 nm. In this case, when a problem of the coupling efficiency with the laser beam incident on the first waveguide area is solved, the thickness t 1  of the first waveguide area may be small. Accordingly, in the present embodiment, the thickness t 1  is 150 nm. 
     Based on the described above, referring to  FIG. 6 , a process of determining a thickness of the second waveguide area will be described. 
       FIG. 6  illustrates a size of the optical width of the output laser beam according to the thickness t 2  of the second waveguide area. This is induced by applying the wavelength of the laser beam according to the present embodiment and a result of  FIG. 5  to Equation 1, which is divided according to a thickness t 1  of the first waveguide area. 
     Referring to  FIG. 6 , when the thickness t 1  of the first waveguide area is determined to be 150 nm as in the present embodiment, the thickness t 2  of the second waveguide area may be about 350 nm. This is, as the thickness t 2  of the second waveguide area increases, aberrations may occur due to multi modes. At a thickness of 350 nm or more, an effect of reducing the optical width of the output laser beam is not great. 
     As below, when the thickness t 1  of the first waveguide area is 150 nm and the thickness t 2  of the waveguide area is 350 nm, the optical width is about 400 nm, which is capable of improving optical coupling efficiency. 
       FIG. 7  illustrates a more improved optical coupler according to an embodiment of the present invention than that of  FIG. 2 . 
     In the present embodiment, a reflection mirror  200  is added to the configuration of  FIG. 2 , in which the same reference numerals designate the same elements. 
     The reflection mirror  200  reflects a plane laser beam from the optical converter  104  with a lattice structure, which is not incident on the slab waveguide  105 , in such a way that the plane laser beam is incident on the slab waveguide  105  and optical coupling efficiency is more improved. 
     In this case, as shown in  FIG. 7 , the reflection mirror  200  may be disposed opposite to the slab waveguide  105 , interposing the optical converter  104  therebetween. 
       FIG. 8A  is a top view illustrating a slab waveguide  205  employed in an optical coupler according to another embodiment of the present invention, and  FIG. 8B  is a perspective view illustrating the slab waveguide  205 . 
     The slab waveguide  205  may perform approximately similar functions to the slab waveguide  105  employed in the previous embodiment. Only, a first waveguide area  31 ′ and a second waveguide area  32 ′ are formed of a different material from each other. Also, different from the case of  FIG. 2 , since it is not required to adjust an effective refractive index according to a thickness, the first waveguide area  31 ′ and the second waveguide area  32 ′ may have the same thickness. 
     In the present embodiment, as shown in  FIG. 8 , an interface between different materials having different refractive indexes, respectively, may be in the shape of a convex lens, thereby reducing a size of an optical width of a plane laser beam and obtaining high optical coupling efficiency with a photonic crystal waveguide. 
     On the other hand, in an aspect of convenience of process, the first waveguide area and the second waveguide area may be formed of the same material as in the embodiment corresponding to  FIG. 2 . However, when different materials with different refractive indexes between which difference is great may be obtained, it is not required to precisely adjust the thicknesses of the first waveguide area and the second waveguide area as in the embodiment of  FIG. 8 . 
     To provide high optical coupling efficiency of the optical coupler according to the embodiments of the present invention, the present inventors designed an optical coupler based on the results shown in  FIGS. 5 and 6  and simulated optical coupling efficiency thereof. 
       FIGS. 9A and 9B  illustrate results of two-dimensional finite difference time domain (FDTD) simulation to check to what degree an output laser beam is focused in the optical coupler designed based on the results shown in  FIGS. 5 and 6 . Also,  FIG. 10  illustrates distribution of light emitting intensity of the output laser beam obtained by the results of the simulation of  FIGS. 9A and 9B , according to a width direction (X) of a waveguide of an output terminal, which is for calculating an optical width of the output laser beam according to the result of the simulation. 
     As shown in  FIG. 9A , based on  FIGS. 5 and 6 , the thickness t 1  of the first waveguide area  31  is set to be 150 nm and the thickness t 2  of the second waveguide area  32  is set to be 350 nm. Accordingly, the effective refractive indexes n in  and n out  of the first waveguide area  31  and the second waveguide area  32  are 2.53 and 3.15, respectively. Also, a lens shape of the second waveguide area  32  is designed to be an oval. 
       FIG. 9B  illustrates a result of FDTD simulation of electric field strength obtained at intervals to check a process of focusing a plane wave incident on the second waveguide area  32 . An image on the right of the  FIG. 9B  shows that light is focused on a focal length position of the lens shape. 
     Also, as shown in  FIG. 10 , there is obtained a result in which an optical width of the output laser beam passing through the slab waveguide is about 400 nm, which is approximately near to about 380 nm that is the result of applying the condition to Equation 1. Considering that light directly emitted from an optical fiber has an optical width of about 1000 nm, it may be known that the output laser beam has an optical width greatly smaller than that of the light directly emitted from the optical fiber. Also, the result indicates that optical coupling efficiency is notably improved than conventional arts though the optical width of about 400 nm is a little greater than the width of the photonic crystal waveguide, which is 300 nm. 
       FIG. 11  illustrates the photonic crystal waveguide  106  coupled with the output terminal of the slab waveguide  105  used in the simulation of  FIGS. 9A and 9B , in which a bottom shows a time-averaged Poynting vector according to a proceeding direction Z of the laser beam. 
     In this case, for convenience of experiment, different from the case of  FIG. 2 , a plane laser beam incident on the first waveguide area  31  does not pass through an optical converter with a lattice structure. That is, in this experiment, a plane laser beam in a basic mode, which has a wavelength of 1.55 μm is incident on the first waveguide area  31 . 
     The second waveguide area  32  is scaled down to a suitable size to calculate FDTD, the photonic crystal waveguide  106  has a photonic crystal structure with a lattice constant of 397.24 nm and a width of about 297 nm. Also, for convenience of calculating a laser beam transmittance, a final output terminal Out having an effective refractive index identical to that of a silicon core layer is connected to the photonic crystal waveguide  106 . 
     Referring to the bottom of  FIG. 11 , optical operation characteristics according to employing the slab waveguide  105  may be known and it may be checked that a considerable amount of reflection occurs in input and output parts. 
       FIG. 12  illustrates reflectivity at the slab waveguide  105  according to a wavelength of an incident laser beam based on the result of the simulation of  FIG. 9 . 
     As shown in  FIG. 12 , it may be known that the slab waveguide  105  shows a reflectivity of about 5% with respect to an incident laser beam with a center wavelength of 1.55 μm and shows a low reflectivity, that is, high optical coupling efficiency when the center wavelength is within a range from 1.50 to 1.60 μm. 
     It may be known that the coupling efficiency of the optical coupler according to the embodiments of the present invention is about 95% or more. 
     While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.