Abstract:
Disclosed is an optical power splitter that maximizes uniformity of the power split ratio, while minimizing the output differences between channels. The optical power splitter 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, wherein the core comprises an input waveguide for receiving the optical signals and a plurality of output waveguides for outputting respective portions of the optical signals whose powers are split; a clad for encompassing the core; and a rectilinear assistant waveguide coupled between the input waveguide and a plurality of output waveguides, having a designated width and length to uniformize mode profiles of the multi-channels that are manifested on the output side edge of the rectilinear assistant waveguide.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “OPTICAL POWER SPLITTER WITH ASSISTANCE WAVEGUIDE” filed in the Korean Industrial Property Office on Dec. 17, 2001 and assigned Serial No. 01-79907, 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 planer lightwave circuit, and in particular, to an optical power splitter.  
           [0004]    2. Description of the Related Art  
           [0005]    In general, a planar lightwave circuit includes a semiconductor substrate, a core, which is layered on the semiconductor substrate, for propagating inputted optical signals using total internal reflection, and a clad encompassing the core. An optical circuit using such waveguide would also 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. Moreover, the structure of an optical power splitter is largely divided into a two-branch structure, a so-called Y-branch waveguide, and a multi-branch structure, a so-called star coupler.  
           [0006]    [0006]FIG. 1 diagrammatically illustrates a Y-branch waveguide. The Y-branch waveguide includes an input waveguide  110  whose input edge receives optical signals and whose width is gradually larger from the input edge to an output side edge  115 , and a first and a second output waveguides  120  and  130  that are symmetrically extended around a central line  140  from the input waveguide  110  to the output side edge  115 . The Y-branch waveguide is a planar lightwave circuit and is formed by layering a core having a high refractive index and clad having a low refractive index to encompass the core upon a semiconductor substrate.  
           [0007]    The power of the combined optical signals through the input side edge of the Y-branch waveguide splits and is outputted through the first and the second output waveguides  120  and  130 . It is important to make the power of the optical signals outputted from the first and the second output waveguides  120  and  130  equal, that is, to make the power split ratio uniform for the Y-branch waveguide. In addition, the power uniformization is required for optical signals of a single channel as well as optical signals of multi-channels.  
           [0008]    [0008]FIG. 2 a  is a diagram illustrating mode profiles of the input waveguide  110  according to a wavelength, on the basis of the output side edge  115  of the input waveguide  110 . FIG. 2 b  is a diagram illustrating mode profiles of the first and the second output waveguides  120  and  130  according to a wavelength on the basis of the output side edge  115  of the input waveguide  110 .  
           [0009]    Depicted in FIG. 2 a  are a first mode profile  210  for a first channel, and a second mode profile  220  for a second channel. The first channel has a wavelength of 1250 nm, and the second channel has a wavelength of 1650 nm. As shown in the drawing, the first mode profile  210  for a short wavelength is sharper than the second mode profile  220  for a long wavelength.  
           [0010]    Similarly, FIG. 2 b  shows a third mode profile  230  for the first channel and a fourth mode profile  240  for the second channel. As illustrated in the drawing, the third mode profile  230  for a short wavelength is sharper than the fourth mode profile  240  for a long wavelength.  
           [0011]    [0011]FIG. 3 a  is a diagram explaining mode inconsistency of the Y-branch waveguide for the first channel and FIG. 3 b  is a diagram explaining mode inconsistency of the Y-branch waveguide for the second channel.  
           [0012]    Depicted in FIGS. 3 a  are the first mode profile  210  of the input waveguide  110  for the first channel and the third mode profile  230  of the first and the second output waveguides  120  and  130 . As shown in the drawing, the first mode profile  210  and the third mode profile  230  are not consistent with each other. As a result, this mode inconsistency causes the output of the split optical signals to the first and the second output waveguides  120  and  130 .  
           [0013]    [0013]FIG. 3 b  shows the second mode profile  220  of the input waveguide  110  for the second channel and the fourth mode profile  240  of the first and the second output waveguides  120  and  130 . As depicted in the drawing, the second mode profile  220  and the fourth mode profile  240  are not consistent with each other. As a result, this inconsistency causes the output of the split optical signals to the first and the second output waveguides  120  and  130 .  
           [0014]    As explained above, the outputs of the first and the second output lightwaves  120  and  130  are similar to each other. In particular, the first and the second output lightwaves  120  and  130  in the Y-branch waveguide have a bilaterally symmetrical structure around the central line  140 , shown in FIG. 1. This property may be helpful to uniformize the power split ratio, however, the performance of the Y-branch waveguide is deteriorated due to output differences between the first and the second output lightwaves  120  and  130 . FIG. 4 is an explanatory diagram of outputs per wavelength in the Y-branch waveguide. FIG. 4 shows output curves 250 per wavelength of the first or second output waveguide  110  or  130 . As shown in FIG. 4, the output power decreases as the wavelength increases. Moreover, the variation range A also increases as the wavelength increases.  
           [0015]    [0015]FIG. 5 is a schematic diagram of a prior art star coupler. The star coupler includes an input waveguide  310  for receiving optical signals through an input side edge, an oval-shaped slab waveguide  320  that is connected to the input waveguide  310 , and the first through the fourth output waveguides  330 ,  340 ,  350  and  360  that are extended symmetrically around a central line  370  from an output side edge  325  of the slab waveguide  320 . Here, the star coupler is a planar lightwave circuit, and is formed by layering a core having a high refractive index and clad having a low refractive index to encompass the core upon a semiconductor substrate.  
           [0016]    The combined optical signals through the input side edge of the input waveguide  310  are outputted through the first through the fourth output waveguides  330 ,  340 ,  350  and  360  via the slab waveguide  320 . It is important to make the power split ratio uniform for the star coupler, thereby allowing the output powers of the optical signals from the first through the fourth output waveguides  330 ,  340 ,  350  and  360  to also be uniform. In addition, the power uniformization is required for optical signals of a single channel as well as optical signals of multi-channels.  
           [0017]    [0017]FIG. 6 a  is a diagram illustrating mode profiles of the slab waveguide  320  according to a wavelength, based on the output side edge  325  of the slab waveguide  320 . FIG. 6 b  is a diagram illustrating mode profiles of the first through the fourth output waveguides  330 ,  340 ,  350  and  360  according to a wavelength, based on the output side edge  325  of the slab waveguide  320 .  
           [0018]    Depicted in FIG. 6 a  are a first mode profile  410  for a first channel and a second mode profile  420  for a second channel. The first channel has a wavelength of 1250 nm, and the second channel has a wavelength of 1650 nm. As shown in the drawing, the first mode profile  410  for a shorter wavelength is sharper than the second mode profile  420  for a longer wavelength.  
           [0019]    Similarly, FIG. 6 b  shows a third mode profile  430  for the first channel and a fourth mode profile  440  for the second channel. As illustrated in the drawing, the third mode profile  430  for a shorter wavelength is sharper than the fourth mode profile  440  for a longer wavelength.  
           [0020]    [0020]FIG. 7 a  is a diagram explaining mode inconsistency of the star coupler for the first channel and FIG. 7 b  is a diagram explaining mode inconsistency of the star coupler for the second channel.  
           [0021]    Depicted in FIG. 7 a  is the first mode profile  410  of the slab waveguide  320  for the first channel and the third mode profile  430  of the first through the fourth output waveguides  330 ,  340 ,  350  and  360 . As shown in the drawing, the first mode profile  410  and the third mode profile  430  are not consistent with each other, and this mode inconsistency causes the split optical signals to be outputted to the first through the fourth output waveguides  330 ,  340 ,  350  and  360 .  
           [0022]    [0022]FIG. 7 b  shows the second mode profile  420  of the slab waveguide  320  for the second channel and the fourth mode profile  440  of the first through the fourth output waveguides  330 ,  340 ,  350  and  360 . As depicted in the drawing, the second mode profile  420  and the fourth mode profile  440  are not consistent with each other and this inconsistency causes the split optical signals to be outputted to the first through the fourth output waveguides  330 ,  340 ,  350  and  360 .  
           [0023]    As explained above, the outputs of the first and the fourth output waveguides  330  and  360  are similar to each other, and the outputs of the second and the third output waveguides  340  and  350  are similar to each other. Further, the star coupler has a bilaterally symmetrical structure around the central line  370  shown in FIG. 5, and the first and the fourth waveguides  330  and  360  and the second and the third output waveguides  340  and  350 , respectively, share similarities with each other.  
           [0024]    Therefore, unlike the Y-branch waveguide, the star coupler has known limitations with un-uniform power split ratios and output differences between channels. This inconsistency in the power split ratio and the severe output differences between channels consequently deteriorate the performance of the star coupler.  
           [0025]    [0025]FIG. 8 is an explanatory diagram of outputs per wavelength of the star coupler. Depicted in the drawing are a first output curve per wavelength  450  of the first or the fourth output waveguide  330  or  360 , and a second output curve per wavelength  460  of the second or the third output waveguide  340  or  350 . From the drawing it is shown that the output power of the first output curve per wavelength  450  tends to increase for longer wavelengths, while the output power of the second output curve per wavelength  460  tends to decrease for longer wavelengths. Moreover, the entire variation range B of the first and the second output curves per wavelength  450  and  460  is very large.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention reduces or overcome many of the above limitations by providing an optical power splitter for maximizing uniformization of the power split ratio, while minimizing the output differences between channels.  
           [0027]    In accordance with principals of the present invention, an optical power splitter is provided, which includes: a semiconductor substrate, a core on the semiconductor substrate that is used as a transmission medium for optical signals, which are composed of multi-channels according to a wavelength, and a clad for encompassing the core. The core includes an input waveguide for receiving an optical signal and a plurality of output waveguides for outputting respective portions of the optical signal (whose power has been split). The optical power splitter further includes a rectilinear assistant waveguide coupled between the input waveguide and the plurality of output waveguides, having a predetermined width and length that substantially uniforms the mode profiles of the multi-channels (e.g. the respective portions of the optical signal) that are manifested on the output side edge of the rectilinear assistant waveguide, which multi-channels are thereafter provided to the plurality of output waveguides.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0029]    [0029]FIG. 1 is a schematic diagram of a prior art Y-branch waveguide;  
         [0030]    [0030]FIGS. 2 a  through  3   b  are diagrams of mode inconsistency of the Y-branch waveguide depicted in FIG. 1;  
         [0031]    [0031]FIG. 4 is a diagram of the output per wavelength of the Y-branch waveguide;  
         [0032]    [0032]FIG. 5 is a schematic diagram of a prior art star coupler;  
         [0033]    [0033]FIGS. 6 a  through  7   b  are diagrams of mode inconsistency of the star coupler depicted in FIG. 5;  
         [0034]    [0034]FIG. 8 is a diagram of the output per wavelength of the star coupler;  
         [0035]    [0035]FIG. 9 is a schematic diagram of an optical power splitter in accordance with a preferred embodiment of the present invention;  
         [0036]    [0036]FIG. 10 is a diagram of the output per wavelength of the Y-branch waveguide;  
         [0037]    [0037]FIG. 11 is a schematic diagram of an optical power splitter in accordance with another embodiment of the present invention;  
         [0038]    [0038]FIGS. 12 through 15 are diagrams of the output changes of the star coupler according to changes in width or length of the assistant waveguide depicted in FIG. 11; and  
         [0039]    [0039]FIG. 16 is a diagram of the output per wavelength of the star coupler.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0040]    In the following description of the present invention, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. Moreover, it will be recognized that certain aspects of the figures are simplified for explanation purposes and that the full system environment for the invention will comprise many known functions and configurations all of which need not be shown here. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings.  
         [0041]    [0041]FIG. 9 is a schematic diagram of an optical power splitter in accordance with a preferred embodiment of the present invention. In this illustrative embodiment, the optical power splitter includes a Y-branch waveguide having an input waveguide  510 , an assistant waveguide  520 , and a first and a second output waveguide  530  and  540 .  
         [0042]    The input waveguide  510  receives optical signals through the input side edge and its width gets broader towards output side edge  515 .  
         [0043]    The assistant waveguide  520  is coupled between the input waveguide  510  and the first and the second output waveguides  530  and  540 . The assistant waveguide  520  has a larger width C than the input waveguide  510 , and a designated length D.  
         [0044]    The first and the second output waveguides  530  and  540  are extended symmetrically around a central line  550  from an output side edge  525  of the assistant waveguide  520 .  
         [0045]    The Y-branch waveguide is a planar lightwave circuit and is formed by layering a core having a high refractive index and clad having a low refractive index to encompass the core upon a semiconductor substrate.  
         [0046]    Importantly, the maximum mode field diameter of optical signals that are combined through the input side edge of the input waveguide  510  is extended while passing the assistant waveguide  520 . Moreover, the local mode field diameter and phase of the optical signals are continuously changed along with the longitudinal direction of the assistant waveguide  520 . The mode field diameter indicates the width of a mode profile for the optical signal at an arbitrary position on the assistant waveguide  520 . Advantageously, the assistant waveguide  520  has a designated length for uniformizing mode profiles based on the output side edge  525 . The first and second output waveguides  530  and  540  split the power of the inputted optical signals through the output side edge  525  of the assistant waveguide  520 , and output the split optical signals.  
         [0047]    [0047]FIG. 10 is a diagram of the output per wavelength of the Y-branch waveguide. Output curve per wavelength  610  is shown for the first or the second output waveguide  530  or  540 , where the length of the assistant waveguide  520  is ‘0’. Output curve per wavelength  620  is shown for the first or the second output waveguides  530  or  540 , where the length of the assistant waveguide  520  is 225 μm. It is noted that the width of the assistant waveguide  520  is 12.5 μm, and if the length of the assistant waveguide  520  is ‘0’, the Y-branch waveguide has the same structure with that of FIG. 1.  
         [0048]    It is also found that the outputs of the first output curve per wavelength  610  gradually decrease towards longer wavelengths, and the variation range E is relatively large. On the other hand, the second output curve per wavelength  620  is uniform, and the variation range F is also relatively small.  
         [0049]    [0049]FIG. 11 is a schematic diagram of an optical power splitter in accordance with another embodiment of the present invention. More specifically, FIG. 11 shows a star coupler, which includes an input waveguide  710  for receiving optical signals through an input side edge, an assistant waveguide  620 , an oval-shaped slab waveguide  730  and a first through fourth output waveguides  740 ,  750 ,  760  and  770 .  
         [0050]    The assistant waveguide  720  is coupled between the input waveguide  710  and the first and the slab waveguide  730 . The assistant waveguide  720  has a larger width G than the input waveguide  710 , and a designated length H.  
         [0051]    The oval-shaped slab waveguide  730  is coupled to the assistant waveguide  720  and the first through the fourth output waveguides  740 ,  750 ,  760  and  770  are extended symmetrically around a central line  780  from an output side edge  735  of the slab waveguide  730 .  
         [0052]    The star coupler is a planar lightwave circuit and is formed by layering a core having a high refractive index and clad having a low refractive index to encompass the core upon a semiconductor substrate.  
         [0053]    The maximum mode field diameter of optical signals that are combined through the input side edge of the input waveguide  710  is extended while passing the assistant waveguide  720 . Also, the local mode field diameter and phase of the optical signals are continuously changed along with the longitudinal direction of the assistant waveguide  720 . Advantageously, the assistant waveguide  720  has a designated length for uniformizing mode profiles on the basis of the output side edge  725 . Thereafter, the optical signals are outputted through the first through the fourth output waveguides  740 ,  750 ,  760  and  770  via the slab waveguide  730 .  
         [0054]    [0054]FIGS. 12 through 15 are diagram of the output changes of the star coupler according to changes in width G or length H of the assistant waveguide depicted  720 .  
         [0055]    [0055]FIG. 12 illustrates a case where the width of the assistant waveguide  720  is 9 μm. In particular, the drawing shows a first and a second output curves per length  814  and  818  of the first and the second output waveguides  740  and  750  for the first channel, a third and a fourth output curves per length  824  and  828  of the first and the second output waveguides  740  and  750  for the second channel, and a fifth and a sixth output curves per length  834  and  838  of the first and the second output waveguides  740  and  750  for the third channel. The first channel has a wavelength of 1250 nm, the second channel 1450 nm and the third channel 1650 nm. In addition, it is known that the first through the sixth output curves per length  814 ,  818 ,  824 ,  828 ,  834  and  838  have a designated converging region I.  
         [0056]    [0056]FIG. 13 illustrates a case where the width of the assistant waveguide  720  is 10 μm. In particular, the drawing shows a first and a second output curves per length  844  and  848  of the first and the second output waveguides  740  and  750  for the first channel, a third and a fourth output curves per length  854  and  858  of the first and the second output waveguides  740  and  750  for the second channel, and a fifth and a sixth output curves per length  864  and  868  of the first and the second output waveguides  740  and  750  for the third channel. The first channel has a wavelength of 1250 nm, the second channel 1450 nm and the third channel 1650 nm. In addition, it is known that the first through the sixth output curves per length  844 ,  848 ,  854 ,  858 ,  864  and  868  have a designated converging region J.  
         [0057]    [0057]FIG. 14 illustrates a case where the width of the assistant waveguide  720  is 11 μm. In particular, the drawing shows a first and a second output curves per length  874  and  878  of the first and the second output waveguides  740  and  750  for the first channel, a third and a fourth output curves per length  884  and  888  of the first and the second output waveguides  740  and  750  for the second channel, and a fifth and a sixth output curves per length  894  and  898  of the first and the second output waveguides  740  and  750  for the third channel. The first channel has a wavelength of 1250 nm, the second channel 1450 nm and the third channel 1650 nm. In addition, it is known that the first through the sixth output curves per length  874 ,  878 ,  884 ,  888 ,  894  and  898  have a designated converging region K.  
         [0058]    [0058]FIG. 15 illustrates a case where the width of the assistant waveguide  720  is 12 μm. In particular, the drawing shows a first and a second output curves per length  914  and  918  of the first and the second output waveguides  740  and  750  for the first channel, a third and a fourth output curves per length  924  and  928  of the first and the second output waveguides  740  and  750  for the second channel, and a fifth and a sixth output curves per length  934  and  938  of the first and the second output waveguides  740  and  750  for the third channel. The first channel has a wavelength of 1250 nm, the second channel 1450 nm and the third channel 1650 nm. In addition, it is known that the first through the sixth output curves per length  914 ,  918 ,  924 ,  928 ,  934  and  938  have a designated converging region L.  
         [0059]    Advantageously, by selecting an optimal length and width for the assistant waveguide  720 , it is possible to maximize uniformization of the power split ratio of the star coupler and to minimize differences in outputs between channels thereof.  
         [0060]    [0060]FIG. 16 is a diagram of the output per wavelength of the star coupler. In particular, FIG. 16 illustrates a first and a second output curves per wavelength  940  and  950  for the first and the second output waveguides  740  and  750  in case the length of the assistant waveguide  720  is 0, and a third and a fourth output curves per wavelength  960  and  970  for the first and the second output waveguides  740  and  750  in case the assistant waveguide  720  has a width of 11 μm and a length of 255 μm. It is noted that when the length of the assistant waveguide  720  is 0, the star coupler has the same structure with that of FIG. 5.  
         [0061]    As shown in FIG. 16, the output powers on the first output curve per wavelength  940  tend to increase for longer wavelengths, while the output powers on the second output curve per wavelength  950  tend to decrease for longer wavelengths. On the other hand, the third and the fourth output curves per wavelength  960  and  970  are uniform. Accordingly, it is found that the variation range N of the third and the fourth output curves per wavelength  960  and  970  is relatively small to the variation range M of the first and the second output curves per wavelength  940  and  950 .  
         [0062]    The optical power splitter mounted with the assistant waveguide, according to the present invention, is beneficial in maximizing the uniformity of the power split ratio and minimizing differences in output between channels by disposing an assistant waveguide having an optimal width and length between an input waveguide and a plurality of output waveguides.  
         [0063]    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.