Abstract:
An optical power splitter is disclosed that includes a semiconductor substrate, a core layered on the semiconductor substrate, functioning as a transmission medium for optical signals composed of multi channels according to a wavelength. The core includes an input waveguide for receiving the optical signals, and a plurality of output waveguides for outputting part of the optical signals whose powers are split. A cladding encompasses the core. At least one tapered waveguides, which connect a part of internal sides of the output waveguides have widths that gradually decrease along with a longitudinal direction thereof starting from one end of the output waveguide.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “OPTICAL POWER SPLITTER” filed in the Korean Industrial Property Office on Feb. 20, 2002 and assigned Serial No. 02-8954, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a planar lightwave circuit, and in particular, to an optical power splitter.  
           [0004]    2. Description of the Related Art  
           [0005]    A planar lightwave circuit (PLC) includes a semiconductor substrate, a core, and cladding encompassing the core. The core, which is layered on the semiconductor substrate, propagates input optical signals using total internal reflection. Typical examples of optical circuits using such planar lightwave circuits, i.e., waveguides, include optical power splitters/combiners for splitting or combining power of optical signal, and wavelength division multiplexers/demultiplexers for multiplexing or demultiplexing channels that compose optical signals according to wavelength.  
           [0006]    The structure of an optical power splitter is largely divided into a two-branch structures like a Y-branch waveguide, and a multi-branch structure like a star coupler. FIG. 1 is a schematic diagram of a conventional Y-branch waveguide. The Y-branch waveguide includes an input waveguide  110 , a branch waveguide  120 , and a first and a second out waveguides  130  and  140 .  
           [0007]    The input waveguide  110  is a rectilinear waveguide, into which optical signals are input through an input side edge  112 . The branch waveguide  120  receives the optical signals through the input side edge  112  that is connected to the input waveguide  110 . The width of the branch waveguide  120  increases towards the direction where the optical signals progress.  
           [0008]    The first and the second output waveguides  130  and  140  are extended symmetrically around a central line (not shown) of the branch waveguide  120  from an output side edge  124  of the branch waveguide  120 . The output of the branch waveguide  120  are split optical signals, respectively to the first and second output waveguides  130  and  140 . The optical signals that travel from the input waveguide  110  to the first or the second output waveguide  130  or  140  experience a continuous mode variation.  
           [0009]    Virtual edges  150  and  155  (which are perpendicular to a longitudinal direction of the first or the second output waveguide  130  or  140 ) on the borders of the first and the second output waveguides  130  and  140 , and the input waveguide  110  are not parallel to the output side edge  124  of the branch waveguide  120 . It is noted that when the output side edge  124  of the branch waveguide  120  and the virtual edges  150  and  155  are not parallel to each other, the mode becomes very unstable. For example, if the width of the input waveguide  110  is 8 μm, and the length of the first and the second output waveguides  130  and  140  is 1500 μm, and an optical signal having a wavelength of 1550 nm is inputted into the Y-branch waveguide. In this case, the optical signal loss amounts to 3.312 dB.  
           [0010]    [0010]FIG. 2 illustrates a beam profile of optical signals that progress to the Y-branch waveguide shown in FIG. 1. FIG. 3 diagrammatically shows mode profiles of the optical signals that were split on the output side edges  132  and  142  of the first and the second output waveguides  130  and  140  depicted in FIG. 1. From the beam profile of the split optical signals, it can be seen that the split optical signals progress unstably along with the first and the second output waveguides  130  and  140 . It is noted that the optical signals become perpendicularly incident on the input side edge  112  of the input waveguide  110 .  
           [0011]    Referring back to FIG. 3, a first and a second mode profiles  210  and  230  of the split optical signals are shown at the output side edges  132  and  142  of the first and the second output waveguides  130  and  140 . As shown, a mode center  215  or  235  of the first or the second mode profile  210  or  230  is separated from central lines  220  and  240  of the first or the second output waveguide  130  or  140  by a designated distance M 1  and M 2 . The mode variations M 1  and M 2  have the same value because the optical signals are perpendicularly incident on the input side edge  112  of the input waveguide  110 , and the first and the second output waveguides  130  and  140  are symmetrical around the central line of the branch waveguide  120 .  
           [0012]    This mode instability consequently deteriorates output characteristic of the Y-branch waveguide. To overcome the problem, the first and the second output waveguides  130  and  140  were lengthened. While this may stabilize the mode somewhat, it also increased the size of the entire circuit, which in turn reduces the yield thereof. In addition, in such a configuration it is difficult to perform any process since the branching angle on the basis of a peak point  160  (shown in FIG. 1), a point where internal sides  134  and  144  of the first and the second output waveguides  130  and  140  meet, of the Y-branch waveguide is small. Moreover, depending on the process implementation of the peak point  160 , optical characteristics may vary severely.  
           [0013]    [0013]FIG. 4 is a schematic diagram explaining another conventional Y-branch waveguide. The Y-branch waveguide includes an input waveguide  310 , and a first and a second output waveguides  320  and  330 .  
           [0014]    The input waveguide  310  receives optical signals through an input side edge  312 , and outputs the optical signals through an output side edge  314  after splitting the signals. The input waveguide  310  gets broader along with the traveling direction of the optical signals.  
           [0015]    The first and the second output waveguides  320  and  330 , respectively, receive the split optical signals through an input side edge that is connected to the output side edge  314  of the input waveguide  310 . Inner sides  324  and  334  and outer sides of the first and second output waveguides  320  and  330  are bent at a corresponding curvature, forming an arc. The first or the second output waveguide  320  or  330  become gradually wider along with the traveling direction of the split optical signals. The internal sides  324  and  334  of the first and the second output waveguides  320  and  330  are separated from each other by a second space G 2 . The outer side of the first output waveguide  320  and the outer side of the input waveguide  310  are separated from each other by a first space G 1 . The outer side of the second output waveguide  330  and the outer side of the input waveguide  310  are also separated from each other by the first space G 1 . The first and the second output waveguides  320  and  330  are symmetrically formed around the central line of the input waveguide  310 .  
           [0016]    [0016]FIG. 5 is a diagram of a beam profile of optical signals that progress along with the Y-branch waveguide depicted in FIG. 4. FIG. 6 shows mode profiles of the split optical signals manifested on the output side edges  322  and  332  of the first and the second output waveguides  320  and  330  that are shown in FIG. 4. From the beam profile of the split optical signals, it can be seen that the split optical signals travel somewhat stably along with the longitudinal directions of the first and the second output waveguides  320  and  330 . It is noted that the input optical signals are perpendicularly incident on the input side edge  312  of the input waveguide  310 .  
           [0017]    [0017]FIG. 6 illustrates a first and a second mode profiles  410  and  430  of the split optical signals shown at the output side edges  322  and  332  of the first and the second output waveguides  320  and  330 . As depicted, a mode center  415  or  435  of the first or the second mode profiles  410  or  430 , and a central line  420  or  440  of the first or the second output waveguide  320  or  330  almost overlapped each other. The Y-branch waveguide, unlike the Y-branch waveguide of FIG. 1, does not have a peak point, so the process is more successfully reproduced, and the variation of optical characteristics due to process error is insignificant.  
           [0018]    Unfortunately however, the optical signals progressing from the input waveguide  310  to the first or the second output waveguide  320  or  330  experience a discontinuous mode variation, and because of that, some optical signals are lost. In addition, the process error in some parts cause light loss, i.e., at the boundaries of the input waveguide  310 , and the first and the second output waveguides  320  and  330 . More specifically, the increasing mode variation, according to changes in the width of a waveguide and refractive index, makes it difficult to design the boundary parts.  
         SUMMARY OF THE INVENTION  
         [0019]    One object of the present invention to provide an optical power splitter for improving output characteristic by minimizing mode instability and light loss.  
           [0020]    Another object of the present invention is to provide an optical power splitter for improving output characteristic by minimizing light loss, and by minimizing yield reduction due to a process error that can be overcome by relieving sensitivity to the process error.  
           [0021]    One embodiment of the present invention is directed to an optical power splitter, including a semiconductor substrate and a core layered on the semiconductor substrate. The core functions as a transmission medium for optical signals composed of multi channels according to a wavelength. The core includes an input waveguide for receiving the optical signals and a plurality of output waveguides for outputting part of the optical signals whose powers are split. A cladding is used to encompass the core. The core also includes at least one tapered waveguide, which connects a part of internal sides of nearby output waveguides, and whose width gradually decreases along with a longitudinal direction thereof starting from one end of the output waveguide.  
           [0022]    Another aspect of the present invention is directed to an optical power splitter, including a semiconductor substrate, a core layered on the semiconductor substrate, functioning as a transmission medium for optical signals and a clad for encompassing the core. The core includes an input waveguide for receiving optical signals through the input side edge, and a first and a second output waveguides extended from an output side edge of the input waveguide, respectively, whose opposite internal sides having a designated curvature that meet together on the output side edge of the input waveguide and whose input side widths divide the output side widths of the input waveguide by two, output split optical signals, respectively. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0024]    [0024]FIG. 1 is a schematic diagram of a conventional Y-branch waveguide;  
         [0025]    [0025]FIG. 2 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 1;  
         [0026]    [0026]FIG. 3 is a diagram illustrating mode profiles of optical signals that are split on output side edges of the first and the second output waveguides shown in FIG. 1;  
         [0027]    [0027]FIG. 4 is a schematic diagram of another conventional Y-branch waveguide;  
         [0028]    [0028]FIG. 5 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 4;  
         [0029]    [0029]FIG. 6 is a diagram illustrating mode profiles of optical signals that are split on output side edges of the first and the second output waveguides shown in FIG. 4;  
         [0030]    [0030]FIG. 7 is a schematic diagram of a Y-branch waveguide in accordance with a first embodiment of the present invention;  
         [0031]    [0031]FIG. 8 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 7;  
         [0032]    [0032]FIG. 9 is a diagram illustrating mode profiles of optical signals that are split on output side edges of the first and the second output waveguides shown in FIG. 7;  
         [0033]    [0033]FIG. 10 is a schematic diagram of a Y-branch waveguide in accordance with a second embodiment of the present invention;  
         [0034]    [0034]FIG. 11 is an enlarged view of a portion “A” depicted in FIG. 10;  
         [0035]    [0035]FIG. 12 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 10;  
         [0036]    [0036]FIG. 13 is a diagram illustrating mode profiles of optical signals that are split on output side edges of the first and the second output waveguides shown in FIG. 10;  
         [0037]    [0037]FIG. 14 a -FIG. 14 d  are exemplary views showing loss variation due to curvature variation of the first and the second output waveguides depicted in FIG. 10;  
         [0038]    [0038]FIG. 15 is a schematic diagram of a Y-branch waveguide in accordance with a comparative example of the present invention;  
         [0039]    [0039]FIG. 16 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 15; and  
         [0040]    [0040]FIG. 17 is a diagram illustrating mode profiles of optical signals that are split on output side edges of the first and the second output waveguides shown in FIG. 15. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]    Various embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions, devices, elements or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0042]    [0042]FIG. 7 diagrammatically illustrates the structure of a Y-branch waveguide in accordance with a first embodiment of the present invention. The Y-branch waveguide is a PLC circuit, and is formed by layering a core having a high refractive index and a clad having a low refractive index for encompassing the core upon the substrate. The core includes an input waveguide  510 , and a first and a second output waveguides  520  and  530 .  
         [0043]    The input waveguide  510  receives optical signals through its input side edge  512 . The input optical signals are split and then output through an output side edge  514 . As shown in this embodiment, the input waveguide  510  is a rectilinear waveguide, whose width from the input side edge  512  to the output side edge  514  is constant.  
         [0044]    The first and the second output waveguide  520  and  530  are extended from the output side edge  514  of the input waveguide  510 , respectively. More specifically, they are extended bilaterally and symmetrically around a central line (not shown) of the input waveguide  510 . The first or the second output waveguides  520  and  530  gradually becomes wider starting from an input side edge which is fed by the output side edge  514  of the input waveguide  510  to an outside edge  522  or  532 . The growth should be substantially constant along the length of the first or second output waveguide  520  and  530 . The first and second output waveguides  520  and  530  have an internal sides  524  and  534 , respectively. The outer side of the first and second output waveguides  520  and  530  are bent to a corresponding curvature, and form an arc The opposite internal sides  524  and  534  of the first and the second output waveguides  520  and  530  meet together at the output side edge  514  of the input waveguide  510 . A peak point  540  of the opposite internal sides  524  and  534  is located on the output side edge  514  of the input waveguide  510 . The input side widths of the first and the second output waveguides  520  and  530  divide the width of the input waveguide  510  by two, and output split optical signals, respectively.  
         [0045]    It is noted that the optical signals traveling from the input waveguide  510  to the first or the second output waveguide  520  or  530  experience a continuous mode variation. Moreover, the virtual edges (not shown) on the boundaries of the first and the second output waveguide  520  and  530 , and the input waveguide  510  (here, the virtual edges are perpendicular to a longitudinal direction of the first or the second output waveguide  520  or  530 ) overlap with the input side edges of the first and the second output waveguides  520  and  530 . The input side edges of the first and the second output waveguides  520  and  530  are parallel to the output side edge  514  of the input waveguide  510 . Therefore, no further loss is resulted from the inconsistency between the virtual edge and the input side edge of the first or the second output waveguide  520  or  530 . For example, in case where the width of the input waveguide  510  is 8 μm, and the length of the first and the second output waveguides  520  and  530  is 1500 μm, and an optical signal having a wavelength of 1550 nm is input into the Y-branch waveguide shown in FIG. 7, the optical signal loss measured is 3.010 dB.  
         [0046]    [0046]FIG. 8 diagrammatically illustrates a beam profile of optical signals that travel at the Y-branch waveguide depicted in FIG. 7. FIG. 9 diagrammatically illustrates mode profiles of optical signals that are split on output side edges  522  and  532  of the first and the second output waveguides  520  and  530  shown in FIG. 7. It can be seen from the beam profile of the split optical signals that the split optical signals stably progress along with the longitudinal direction of the first and the second output waveguides  520  and  530 . It is noted that the input optical signals are perpendicularly incident upon the input side edge  512  of the input waveguide  510 .  
         [0047]    Depicted in FIG. 9 are a first and a second mode profiles  610  and  620  of the split optical signals that are shown on the output side edges  522  and  532  of the first and the second output waveguides  520  and  530 . As shown in the drawing, a mode center  615  or  625  of the first or the second mode profile  610  or  620  is consistent with the central line of the first or the second output waveguide  520  or  530 , i.e., mode matching occurs.  
         [0048]    [0048]FIG. 10 is a schematic diagram of a Y-branch waveguide according to a second embodiment of the present invention. FIG. 11 is an enlarged view of a portion “A” shown in FIG. 10. In this embodiment, the Y-branch waveguide includes an input waveguide  910 , a branch waveguide  920 , a tapered waveguide  950 , and a first and a second output waveguides  930  and  940 .  
         [0049]    Similar to the first embodiment, the input waveguide  910  receives optical signals through an input side edge  912 . The input optical signals output through an output side edge  914 . The input waveguide  910  is a rectilinear waveguide, whose width from the input side edge  912  to the output side edge  914  is constant.  
         [0050]    The branch waveguide  920  receives optical signals through an input side edge connected to the output side edge  914 . The input optical signals are split and output through an output side edge  922 . The branch waveguide  920  has a designated length, L 2 , and its width is gradually increased toward the traveling direction of the optical signals. The increase in width is substantially constant along the length of the branch waveguide  920 .  
         [0051]    The first and the second output waveguides  930  and  940 , respectively, receive the split optical signals through an input side edge that is connected to the output side edge  922  of the branch waveguide  920 . The first and second output waveguides  930  and  940  have internal sides  934  and  944  and outer sides that are bent to a corresponding curvature, and form an arc together. The width of the first or the second output waveguide  930  or  940  is gradually increased toward the progress direction of the split optical signals. The internal sides  934  and  944  of the first and the second output waveguides  930  and  940  are separated from each other by a fourth space G 4 .The first and the second output waveguides  930  and  940  are also symmetric around a central line (not shown) of the input waveguide  910 . If the internal sides  934  and  944  of the first and the second output waveguides  930  and  940  are extended toward the input waveguide  910  along with the corresponding curvature, they meet together or converge at the output side edge  914  of the input waveguide  910 . A virtual peak point  960  is formed on the output side edge  914  of the input waveguide  910 .  
         [0052]    The tapered waveguide  950  connects part of the internal sides  934  and  944  of the first and the second output waveguides  930  and  940 . The tapered waveguide  950  has a designated length, L 3 , and its width is gradually decreased (see, e.g., FIG. 11) from the input side edge of the first and the second output waveguides  930  and  940  along with the longitudinal direction thereof. The decrease in width is substantially constant along the length of the tapered waveguide  950 .  
         [0053]    As shown in FIG. 11, the tapered waveguide  950  has a tilted shape to reduce its width continuously. The optical signals traveling at the branch waveguide  920  are gradually branched toward the first and the second output waveguides  930  and  940  by the tapered waveguide  950 . The optical signals progressing from the branch waveguide  920  to the first or the second output waveguide  930  or  940  experience a gradual mode variation. Accordingly, the mode variation owing to the process error, i.e., the changes in the width and the refractive index of the waveguide, at the boundaries of the branch waveguide  920 , and the first and the second output waveguides  930  and  940  is small. For example, in the case where the width of the input waveguide  510  is 8 μm, and the length of the first and the second output waveguides  520  and  530  is 1500 μm, and an optical signal having a wavelength of 1550 nm is input into the Y-branch waveguide, then the optical signal loss measured is 3.025 dB.  
         [0054]    [0054]FIG. 12 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 10. FIG. 13 is a diagram illustrating mode profiles of optical signals that are split at output side edges  932  and  942  of the first and the second output waveguides  930  and  940  shown in FIG. 10. From the beam profile of the split optical signals it can be seen that the split optical signals stably progress along with the longitudinal direction of the first and the second output waveguides  930  and  940 . It is noted that the inputted optical signals are perpendicularly incident upon the input side edge  912  of the input waveguide  910 .  
         [0055]    Illustrated in FIG. 13 are a first and a second mode profiles  1010  and  1030  of the split optical signals that are shown at the output side edges  932  and  942  of the first and the second output waveguides  930  and  940 . As shown in FIG. 13, a mode center  1015  or  1035  of the first or the second mode profile  1010  or  1030  is almost consistent with the central line ( 1020  or  10040 ) of the first or the second output waveguide  930  or  940 .  
         [0056]    [0056]FIG. 14 a  through FIG. 14 d  are exemplary views showing loss variation due to the curvature variation of the first and the second output waveguides depicted in FIG. 10. The internal sides  934  and  944  of the first and the second output waveguides  930  and  940  depicted in FIG. 14 b  have a designated curvature, C 1 , and the internal sides  934  and  944  depicted in FIG. 14 c  have a designated curvature, C 2 , and the internal sides  934  and  944  depicted in FIG. 14 d  have a designated curvature, C 3 (C 3 &gt;C 2 &gt;C 1 ). In FIG. 14 a  through FIG. 14 d , the minimum width, G 6  and the maximum width, G 7  of the tapered waveguide  950  are constant. As the curvature of the internal sides  934  and  944  is increased(C 1 →C 2 →C 3 ), the length of the branch waveguide  920  is increased(L 4 →L 6 →L 8 , wherein L 4 &lt;L 6 &lt;L 8 ), and the length of the tapered waveguide  950  is increased then decreased(L 5 →L 7 →L 9 , wherein L 9 &lt;L 5 &lt;L 7 ). According to the above variations, light loss of the Y-branch waveguide varies as depicted in FIG. 14 a . That is, the optimum curvature, C 2 , exits for the first and the second output waveguides  930  and  940  for minimizing light loss of the Y-branch waveguide.  
         [0057]    [0057]FIG. 15 is a schematic diagram of a Y-branch waveguide in accordance with a comparative example of the present invention. The Y-branch waveguide includes an input waveguide  710 , a branch waveguide  720 , and a first and a second output waveguides  730  and  740 .  
         [0058]    The input waveguide  710  receives optical signals through an input side edge  712 . The input optical signals are split and output through an output side edge  714 . The input waveguide  710  is a rectilinear waveguide, whose width from the input side edge  712  to the output side edge  714  is constant.  
         [0059]    The branch waveguide  720  receives optical signals through an input side edge connected to the output side edge  714 . The input optical signals are split and output through an output side edge  722 . The width of the branch waveguide  920  is gradually increased toward the traveling direction of the optical signals. The increase in width is substantially constant along the length of the branch waveguide  920 .  
         [0060]    The first and the second output waveguides  730  and  740 , respectively, receive the split optical signals through an input side edge that is connected to the output side edge  722  of the branch waveguide  720 , and their internal side  734  or  744  and an outer side are bent to a corresponding curvature, and form an arc together. The width of the first or the second output waveguide  730  or  740  is gradually increased toward the progress direction of the split optical signals. The internal sides  734  and  744  of the first and the second output waveguides  730  and  740  are separated from each other by a third space G 3 . The first and the second output waveguides  730  and  740  are symmetric around the central line (not shown) of the input waveguide  710 . If the internal sides  734  and  744  of the first and the second output waveguides  730  and  740  are extended toward the input waveguide  710  along with the corresponding curvature, they meet together or converge on the output side edge  714  of the input waveguide  710 . A virtual peak point  750  is formed on the output side edge  714  of the input waveguide  710 .  
         [0061]    At the input waveguide  710 , the optical signals progressing to the first or the second output waveguides  730  or  740  experience a discontinuous mode variation, and as the result thereof, some optical signals are lost. For example, in the case where the width of the input waveguide  710  is 8 μm, and the length of the first and the second output waveguides  730  and  740  is 1500 μm, and an optical signal having a wavelength of 1550 nm is inputted into the Y-branch waveguide, then the optical signal loss measured is 3.062 dB.  
         [0062]    [0062]FIG. 16 is a diagram illustrating a beam profile of optical signals that travel the Y-branch waveguide depicted in FIG. 15. FIG. 17 is a diagram illustrating mode profiles of optical signals that are split at output side edges of the first and the second output waveguides  730  and  740  shown in FIG. 15. From the beam profile of the split optical signals, it can be seen that the split optical signals stably progress along with the longitudinal direction of the first and the second output waveguides  730  and  740 . It is noted that the inputted optical signals are perpendicularly incident upon the input side edge  712  of the input waveguide  710 .  
         [0063]    Illustrated in FIG. 17 are a first and a second mode profiles  810  and  830  of the split optical signals that are shown on the output side edges  734  and  744  of the first and the second output waveguides  730  and  740 . As shown in FIG. 17, a mode center  815  or  835  of the first or the second mode profile  810  or  830  is almost consistent with the central line  820  or  840  of the first or the second output waveguide  730  or  740 .  
         [0064]    In conclusion, the described optical power splitter embodiments of the present invention allow the input waveguide&#39;s mode and the first and the second output waveguides to be consistent to one another. This allow the embodiments of the present invention to improve output characteristics by minimizing mode instability and light loss that are usually caused by the mode inconsistency.  
         [0065]    In addition, the optical power splitter embodiments of the present invention allow for the separation the first output waveguide and the second output waveguide using a tapered waveguide, which relieves the optical power splitter&#39;s sensitivity to the process error, and further minimizes the yield reduction due to the process error. In this manner, the light loss is also minimized, and the output characteristics are greatly improved.  
         [0066]    While the invention has been shown and described with reference to a certain preferred embodiment 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.