Abstract:
An optical power splitter is provided that can stably operate even when there is a mode mismatch between an input optical signal and the optical power splitter. The optical power splitter of the present invention includes a semiconductor substrate, an optical waveguide stacked on the semiconductor substrate, and a clad surrounding the optical waveguide. The optical waveguide includes an input waveguide section through which the optical signal is input from an outer waveguide, a tapered waveguide section having a gradually increasing width, first and second waveguide branches extending from an output end of the tapered waveguide section and outputting first and second branched optical signals, and a stabilizing waveguide section disposed between the input waveguide section and the tapered waveguide section, the stabilizing waveguide section having length and width capable of stabilizing shaking of the optical signal which is generated by a mode mismatch between the optical signal and the input waveguide section.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “OPTICAL POWER SPLITTER HAVING A STABILIZING WAVEGUIDE,” filed in the Korean Industrial Property Office on Jan. 14, 2002 and assigned Serial No. 2002-2147, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a planar waveguide device, and more particularly to an optical power splitter utilizing a Y-branch optical waveguide.  
           [0004]    2. Description of the Related Art  
           [0005]    An optical waveguide device includes an optical waveguide core through which an optical signal is transmitted by experiencing total internal reflection in the optical waveguide core, and a clad surrounding the optical waveguide core. Representative examples of optical waveguide devices include a planar waveguide device fabricated in a semiconductor manufacturing process and an optical fiber manufactured by melting an optical fiber preform. Planar waveguide devices include optical power splitter/couplers that divide or couple the power of the optical signal and wavelength division multiplexer/demultiplexers that multiplex or demultiplex channels constituting the optical signal according to wavelengths of the optical signal. Further, the optical power splitter may include a tapered waveguide, that enlarges field distribution of an input optical signal, and a Y-branch optical waveguide having a pair of output waveguide branches, that splits the enlarged optical signal and outputs it through ends of the output waveguide branches.  
           [0006]    [0006]FIG. 1 is a schematic plan view of a conventional optical power splitter utilizing a Y-branch optical waveguide. The conventional optical power splitter  100  generally includes an input section  101 , a branching section  102 , and an output section  103 . Further, the optical power splitter  100  is constructed so that both sides are symmetric with reference to an axis  230  of the optical power splitter  100 .  
           [0007]    The input section  101  includes an input waveguide section  110 . The input waveguide section  110  receives an optical signal through its input end connected with an outer waveguide  250  and controls the device length of the optical power splitter  100 .  
           [0008]    In this case, in order to minimize the coupling loss of the optical signal, the input waveguide section  110  has a width  104  that can optimize the mode field diameter (MFD) of the outer waveguide  250  and the mode field diameter of the input waveguide section  110  and that enables single-mode propagation and low-loss insertion of the optical signal. That is, in order to minimize the coupling loss of the optical signal, the input waveguide section  110  enlarges the field distribution of the optical signal and functions as a coupling section that enables single-mode propagation and low-loss insertion of the optical signal. In this case, the outer waveguide  250  is a waveguide that constitutes an optical fiber or a planar waveguide device.  
           [0009]    The branching section  102  includes a first tapered waveguide section  140 , and first and second waveguide branches  150  and  160 .  
           [0010]    The first tapered waveguide section  140  receives the optical signal through its input end connected with the input waveguide section  110 , and the width of the first tapered waveguide section  140  gradually increases in the direction toward which the optical signal propagates.  
           [0011]    The first and second waveguide branches  150  and  160  extend from the output end of the first tapered waveguide section  140  symmetrically with reference to the axis  230 . The output section  103  includes second and third auxiliary waveguide sections  170  and  200 , second and third tapered waveguide sections  180  and  210 , and first and second output waveguide sections  190  and  220 .  
           [0012]    The second auxiliary waveguide section  170  controls the device length of the optical power splitter  100  and connects the first waveguide branch  150  and the second tapered waveguide section  180  with each other.  
           [0013]    The second tapered waveguide section  180  receives a first branched optical signal through its input end connected with the second auxiliary waveguide section  170 , and the width of the second tapered waveguide section  180  gradually increases in the direction toward which the first branched optical signal propagates.  
           [0014]    The first output waveguide section  190  receives the first branched optical signal through its input end and outputs the first branched optical signal through its output end.  
           [0015]    The third auxiliary waveguide section  200  controls the device length of the optical power splitter  100  and connects the second waveguide branch  160  to the third tapered waveguide section  210 .  
           [0016]    The third tapered waveguide section  210  receives a second branched optical signal through its input end connected with the third auxiliary waveguide section  200 , and the width of the third tapered waveguide section  210  gradually increases in the direction toward which the second branched optical signal propagates.  
           [0017]    The second output waveguide section  220  receives the second branched optical signal through its input end and outputs the second branched optical signal through its output end.  
           [0018]    [0018]FIGS. 2A and 2B are graphs describing the mode matching in the case where an optical signal is input in alignment with the axis of the optical power splitter  100 , that is, the case where the optical signal is input into the optical power splitter  100  in such a manner that the field distribution of the optical signal has a shape both sides of which are symmetric with reference to the axis  230 .  
           [0019]    The graph shown in FIG. 2A shows a first field distribution  310  in the input waveguide section  110  at the input end of the input waveguide section  110 , which means a field distribution of the optical signal propagating through the input waveguide section  110  directly after passing the input end of the input waveguide section  110 . As shown, the first field distribution  310  is arranged symmetrically with reference to the axis  230 . This mode matching maximizes the coupling efficiency between the input waveguide section  110  and the optical signal.  
           [0020]    The graph shown in FIG. 2B shows a third field distribution  320  in the first tapered waveguide section  140  and a fourth field distribution  330  in the first and second waveguide ranches  150  and  160 , at the output end of the first tapered waveguide section  140 . As shown, the first field distribution  310  and the fourth field distribution  330  are arranged symmetrically with reference to the axis  230 . These mode matches maximize the coupling efficiency of the optical signal between the first tapered waveguide section  140  and the first and second waveguide branches  150  and  160 .  
           [0021]    [0021]FIG. 3 is a view showing an intensity distribution of the optical signal propagating through the optical power splitter  100  in the mode match state. In FIG. 3, sections  301 ,  302 , and  303  represent the intensity distributions of the input section  101 , branching section  102 , and output section  103 , respectively.  
           [0022]    [0022]FIGS. 4A and 4B are graphs illustrating mode mismatch in the case where an optical signal is input in misalignment with the axis  230  of the optical power splitter  100 . That is, these figures illustrate the case where the optical signal is input into the optical power splitter  100  in such a manner that the field distribution of the optical signal has a shape in which the sides are nonsymmetrical with reference to the axis  230 .  
           [0023]    The graph illustrated in FIG. 4A shows a fifth field distribution  340  in the input waveguide section  110  at the input end of the input waveguide section  110 . As shown, a center line of the fifth field distribution  340  is not aligned with the axis  230 . This mode mismatch degrades the coupling efficiency between the input waveguide section  110  and the optical signal.  
           [0024]    The graph illustrated in FIG. 4B shows a sixth field distribution  350  in the first tapered waveguide section  140  and a seventh field distribution  360  in the first and second waveguide branches  150  and  160 , at the input end of the first tapered waveguide section  140 . As shown, the center lines of the sixth field distribution  350  and the seventh field distribution  360  are not aligned with each other. This mode mismatch degrades the coupling efficiency between the first tapered waveguide section  140  and the first and second waveguide branches  150  and  160 .  
           [0025]    [0025]FIG. 5 illustrates an intensity distribution of an optical signal propagating through the optical power splitter  100  in the mode mismatch state. In FIG. 5, sections  304 ,  305 , and  306  represent the intensity distributions of the input section  101 , branching section  102 , and output section  103 , respectively. As shown, the optical signal shakes while passing through the input section  101 , thereby having an effect on the branching section B, which consequently causes the mode mismatch as shown in FIG. 4B.  
           [0026]    [0026]FIG. 6 is a graph illustrating loss corresponding to the mode mismatch of the optical signal input to the optical power splitter  100 , and FIG. 7 is a graph illustrating uniformity according to the mode mismatch of the optical signal input to the optical power splitter  100 .  
           [0027]    [0027]FIG. 6 illustrates a first output curve  370  representing the output of the first output waveguide section  190  according to the mode mismatch of the optical signal and a second output curve  380  representing the output of the second output waveguide section  220  according to the mode mismatch of the optical signal. As shown in FIG. 6, as the mode mismatch grows larger, the difference of the outputs between the first output curve  370  and the second output curve  380  becomes significantly larger.  
           [0028]    [0028]FIG. 7 illustrates a uniformity curve  390  representing the difference between the outputs of the first and second output waveguide sections  190  and  220  according to the mode mismatch of the optical signal. As shown, the difference between the outputs of the first and second output waveguide sections  190  and  220  abruptly increases as the mode mismatch increases.  
           [0029]    As described above, the conventional optical power splitter utilizing a Y-branch optical waveguide is problematic in that, the larger the mode mismatch, the larger the difference between the outputs of the first and second output waveguide sections  190  and  220 . Moreover, the difference between the outputs of the first and second output waveguide sections  190  and  220  according to the mode mismatch of the optical signal further increases when the optical signal is a multi-channel signal.  
         SUMMARY OF THE INVENTION  
         [0030]    Thus, there is a need for an optical splitter that minimizes the difference in outputs when there is mode mismatch. The present invention provides an optical power splitter  400  that can operate stably even when there is a mode mismatch between an input optical signal and the optical power splitter.  
           [0031]    Referring now to FIG. 8, the present invention is an optical power splitter  400  comprising a semiconductor substrate, an optical waveguide stacked on the semiconductor substrate, and a clad surrounding the optical waveguide. In a preferred embodiment of the present invention, the optical waveguide functions as a medium through which an optical signal transmits, the signal having a plurality of channels according to wavelengths. The optical waveguide comprises several components. At the input is an input waveguide section  410  through which the optical signal is input from an outer waveguide  550 . Connected to the input waveguide section  410  is a tapered waveguide section  440  that gradually increases in width. First  450  and second  460  waveguide branches in a branching section  403  extend from an output end of the tapered waveguide section  440  and output first and second branched optical signals. A stabilizing waveguide section  430  is disposed between the input waveguide section  410  and the tapered waveguide section  440 , the stabilizing waveguide section having a length  407  and a width  406  capable of being preset to values which enable stabilizing of a shaking of the optical signal that is generated by a mode mismatch between the optical signal and the input waveguide section  410 . 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a schematic plan view of a prior art optical power splitter utilizing a Y 15  branch optical waveguide.  
         [0033]    [0033]FIGS. 2A and 2B illustrate graphs that describe the mode matching in the case where an optical signal is input in alignment with the axis of the optical power splitter shown in FIG. 1.  
         [0034]    [0034]FIG. 3 is an illustration of an intensity distribution of the optical signal propagating through the optical power splitter shown in FIG. 1 in a mode mismatch state.  
         [0035]    [0035]FIGS. 4A and 4B are graphs that describe mode mismatch in the case where an optical signal is input in misalignment with the axis of the optical power splitter shown in FIG. 1.  
         [0036]    [0036]FIG. 5 illustrates an intensity distribution of the optical signal propagating through the optical power splitter shown in FIG. 1 in a mode mismatch state.  
         [0037]    [0037]FIG. 6 is a graph illustrating the loss according to the mode mismatch of the optical signal input to the optical power splitter shown in FIG. 1.  
         [0038]    [0038]FIG. 7 is a graph illustrating uniformity according to the mode mismatch of the optical signal input to the optical power splitter shown in FIG. 1.  
         [0039]    [0039]FIG. 8 illustrates an optical power splitter utilizing a Y-branch optical waveguide according to a preferred embodiment of the present invention.  
         [0040]    [0040]FIGS. 9A and 9B are graphs illustrating a process of stabilizing the field distribution of an optical signal in the case where the optical signal is input in misalignment with the axis of the optical power splitter shown in FIG. 8.  
         [0041]    [0041]FIG. 10 illustrates an intensity distribution of the optical signal propagating through the optical power splitter shown in FIG. 8.  
         [0042]    [0042]FIG. 11 is a graph illustrating the change of outputs according to the mode mismatch of the optical signal input to the optical power splitter shown in FIG. 8.  
         [0043]    [0043]FIG. 12 is a graph illustrating the change of uniformity according to the mode mismatch of the optical signal input to the optical power splitter shown in FIG. 8.  
         [0044]    [0044]FIG. 13 is a graph illustrating the change of uniformity according to the mode mismatch of the optical signal input to the optical power splitter and the width of the stabilizing waveguide section shown in FIG. 8.  
         [0045]    [0045]FIG. 14 is a graph illustrating the change of uniformity according to the width of the stabilizing waveguide section shown in FIG. 8.  
         [0046]    [0046]FIG. 15 is a graph illustrating the change of uniformity according to the length of the stabilizing waveguide section shown in FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0047]    [0047]FIG. 8 illustrates an optical power splitter utilizing a Y-branch optical waveguide according to a preferred embodiment of the present invention. The optical power splitter  400  generally includes an input section  401 , a stabilizing section  402 , a branching section  403 , and an output section  404 . Further, the optical power splitter  400  has a construction both sides of which are symmetric with respect to axis  530  of the optical power splitter  400 .  
         [0048]    The input section  401  includes an input waveguide section  410  having width  405  and a first tapered waveguide section  420 .  
         [0049]    The input waveguide section  410  receives an optical signal through its input end that is connected with an outer waveguide  550 . In a preferred embodiment, the input waveguide section  410  has characteristics of a single-mode propagation and low-loss insertion. The first tapered waveguide section  420  receives the optical signal through its input end connected with the input waveguide section  410 , and the width of the first tapered waveguide section  420  gradually decreases in the direction toward which the optical signal propagates. In this case, in order to minimize the coupling loss of the optical signal, the input waveguide section  410  has a width  405  that optimizes mode field diameters of the outer waveguide  550  and the input waveguide section  410 . In this case, the outer waveguide  550  is a waveguide that constitutes an optical fiber or a planar waveguide device.  
         [0050]    The stabilizing section  402  comprises stabilizing waveguide section  430  having a predetermined width  406  and length  407 .  
         [0051]    The first tapered waveguide section  420  is disposed between the input waveguide section  410  and the stabilizing waveguide section  430 . The coupling efficiency is highly dependent on alignment and the tapering of the first tapered waveguide section  420  gradually decreases its width thus allowing gradual confinement of the propagating light which relieves alignment concerns. The stabilizing waveguide section  430  is disposed between the first tapered waveguide section  420  that tapers increasingly and a second tapered waveguide section  440  that tapers decreasingly and has a width  406  and length  407  which in combination stabilize the shaking of the optical signal, which shaking may be generated due to mode mismatch between the optical signal and the input waveguide section  410 . In a preferred embodiment, the width  406  of the stabilizing waveguide section  430  is smaller than the width  105  of  130 , since the smaller the width  406  of the stabilizing waveguide section  430 , within a predetermined limit, the greater the stabilization of the optical signal.  
         [0052]    The branching section  403  includes the second tapered waveguide section  440 , and a first and a second waveguide branche  450  and  460 .  
         [0053]    The second tapered waveguide section  440  receives the optical signal through its input end connected with the stabilizing waveguide section  430 , and the width of the second tapered waveguide section  440  gradually increases in the direction toward which the optical signal propagates.  
         [0054]    The first and second waveguide branches  450  and  460  extend from the output end of the second tapered waveguide section  440  symmetrically with reference to the axis  530 .  
         [0055]    The output section  404  includes a first and a second auxiliary waveguide section  470  and  500 , a third and a fourth tapered waveguide section  480  and  510 , and a first and a second output waveguide section  490  and  520 .  
         [0056]    The first auxiliary waveguide section  470  controls the device length of the optical power splitter  400  and links the first waveguide branch  450  and the third tapered waveguide section  480  with each other.  
         [0057]    The third tapered waveguide section  480  receives a first branched optical signal through its input end connected with the first auxiliary waveguide section  470 , and the width of the third tapered waveguide section  480  gradually increases in the direction toward which the first branched optical signal propagates.  
         [0058]    The first output waveguide section  490  receives the first branched optical signal through its input end and outputs the first branched optical signal through its output end.  
         [0059]    The second auxiliary waveguide section  500  controls the device length of the optical power splitter  400  and links the second waveguide branch  460  and the fourth tapered waveguide section  510  with each other.  
         [0060]    The fourth tapered waveguide section  510  receives a second branched optical signal through its input end connected with the second auxiliary waveguide section  500 , and the width of the fourth tapered waveguide section  510  gradually increases in the direction toward which the second branched optical signal propagates.  
         [0061]    The second output waveguide section  520  receives the second branched optical signal through its input end and outputs the second branched optical signal through its output end.  
         [0062]    [0062]FIGS. 9A and 9B are graphs illustrating a process of stabilizing the field distribution of an optical signal in the case where the optical signal is input in misalignment with the axis  530  of the optical power splitter  400 , that is, the case where the optical signal is input into the optical power splitter  400  in such a manner that the field distribution of the optical signal has a shape both sides of which are nonsymmetrical with reference to the axis  530 .  
         [0063]    The graph illustrated in FIG. 9A shows a first field distribution  610  in the input waveguide section  410  at the input end of the input waveguide section  410 , which is a field distribution of the optical signal propagating through the input waveguide section  410  directly after passing through the input end of the input waveguide section  410 . As shown, a center line of the first field distribution  610  is not aligned with the axis  530 . This mode mismatch degrades the coupling efficiency between the input waveguide section  410  and the optical signal.  
         [0064]    The graph illustrated in FIG. 9B shows a second field distribution  620  in the second tapered waveguide section  440  and a third field distribution  630  in the first and second waveguide branches  450  and  460 , at the output end of the second tapered waveguide section  440 . As shown, the second field distribution  620  and the third field distribution  630  are arranged symmetrically with reference to the axis  530 . These mode matches maximize the coupling efficiency of the optical signal between the second tapered waveguide section  440  and the first and second waveguide branches  450  and  460 .  
         [0065]    [0065]FIG. 10 illustrates an intensity distribution of the optical signal propagating through the optical power splitter  400 . In FIG. 10, sections  1001 ,  1002 ,  1003 , and  1004  represent the intensity distributions of the input section  401 , stabilizing section  402 , branching section  403 , and output section  404 , respectively. FIG. 10 illustrates the optical signal shaking while passing through the input section  401  but stably propagating through the stabilizing section  402 . That is, the optical signal initially propagating while shaking is gradually stabilized while passing through the stabilizing section  402 . This stabilization of the optical signal results in the mode match illustrated in FIG. 9B.  
         [0066]    [0066]FIG. 11 is a graph illustrating the change of outputs according to the mode mismatch of the optical signal input to the optical power splitter  400 , and FIG. 12 is a graph illustrating the change of uniformity according to the mode mismatch of the optical signal input to the optical power splitter  400 . In these cases, the stabilizing waveguide section  430  of the present invention has a width  406  of 3 μm and a length  407  of 2000 μm.  
         [0067]    [0067]FIG. 11 shows a first output curve  640  representing the output of the first output waveguide section  490  according to the mode mismatch of the optical signal and a second output curve  650  representing the output of the second output waveguide section  520  according to the mode mismatch of the optical signal. As shown in FIG. 11, as the mode mismatch grows larger, the difference of the outputs shown by the first output curve  640  and the second output curve  650  becomes only slightly larger for an optical power splitter having a stabilizing waveguide according to the present invention.  
         [0068]    [0068]FIG. 12 is a graph illustrating a first uniformity curve  660  representing the difference between the outputs of the first and second output waveguide sections  490  and  520  according to the mode mismatch of the optical signal. As shown in FIG. 12, the difference between the outputs of the first and second output waveguide sections  490  and  520  slightly increases as the mode mismatch increases for an optical power splitter having a stabilizing waveguide according to the present invention.  
         [0069]    [0069]FIG. 13 is a graph illustrating the change of uniformity according to the mode mismatch of the optical signal input to the optical power splitter  400  and the width  406  of the stabilizing waveguide section  430 . FIG. 13 shows first to sixth uniformity curves  660 ,  670 ,  680 ,  690 ,  700 , and  710  representing the difference between the outputs of the first and second output waveguide sections  490  and  520  according to the mode mismatch of the optical signal, respectively. In FIG. 13, the first to sixth uniformity curves  660 ,  670 ,  680 ,  690 ,  700 , and  710  represent the difference between the outputs of the first and second output waveguide sections  490  and  520  in the case where the width  406  of the stabilizing waveguide section  430  is 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, and 8 μm, respectively. In each case, the length  407  of the stabilizing waveguide section  430  is 2000 μm. FIG. 13 shows that the smaller the width  406  of the stabilizing waveguide section  430  according to the present invention, the smoother the corresponding uniformity curve.  
         [0070]    [0070]FIGS. 14 and 15 are graphs illustrating the change of uniformity according to the width  406  and length  407  of the stabilizing waveguide section  430  according to the present invention.  
         [0071]    [0071]FIG. 14 shows seventh to twelfth uniformity curves  720 ,  730 ,  740 ,  750 ,  760 , and  770  representing the difference between the outputs of the first and second output waveguide sections  490  and  520  according to the width  406  of the stabilizing waveguide section  430 . In FIG. 14, the seventh to twelfth uniformity curves  720 ,  730 ,  740 ,  750 ,  760 , and  770  represent the difference between the outputs of the first and second output waveguide sections  490  and  520  in the case where the length  407  of the stabilizing waveguide section  430  is 0 μm, 1000 μm, 2000 μm, 3000 μm, 4000 μm, and 5000 μm, respectively. In each case, the error in the mode matching of the optical signal is 0.3 μm. Thus, the smaller the width  406  of the stabilizing waveguide section  430  according to the present invention, the substantially smaller the difference between the outputs of the first and second output waveguide sections  490  and  520 .  
         [0072]    [0072]FIG. 15 illustrates thirteenth to seventeenth uniformity curves  780 ,  790 ,  800 ,  810 , and  820  representing the difference between the outputs of the first and second output waveguide sections  490  and  520  according to the length  407  of the stabilizing waveguide section  430 . In FIG. 15, the thirteenth to seventeenth uniformity curves  780 ,  790 ,  800 ,  810 , and  820  represent the difference between the outputs of the first and second output waveguide sections  490  and  520  in the case where the width  406  of the stabilizing waveguide section  430  is 3 μm, 4 μm, 5 μm, 6 μm, and 7 μm, respectively. In each case, the error in the mode matching of the optical signal is 0.3 μm. FIG. 15 illustrates that the larger the length  407  of the stabilizing waveguide section  430  according to the present invention, the substantially smaller the difference between the outputs of the first and second output waveguide sections  490  and  520 .  
         [0073]    As described above, in an optical power splitter having a stabilizing wave guide according to the present invention, the width and length of the stabilizing wave guide can be adjusted, thereby enabling the optical power splitter to stably operate even when there is a mode mismatch between an input optical signal and the optical power splitter.  
         [0074]    While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.