Abstract:
A Y-branched optical waveguide with a uniform output characteristic for use in optical communication systems is disclosed. The optical waveguide, consisting of a core section serving as a transmission medium of an optical signal and a cladding section surrounding the core section, is formed on a semiconductor substrate. An input-tapered waveguide is configured to receive the optical signal through a first ending section and to output the optical signal through a second ending section, with a symmetrical structure with respect to a centered line of the input-tapered waveguide, so that a width of the input-tapered waveguide extends more widely along the centered line. A pair of first and second output-tapered waveguides each are configured to receive the optical signal branched through the second ending section, from which the pair of first and second output-tapered waveguides respectively extend downstream, with an asymmetrical structure with respect to the centered line, a respective width of the first and second output-tapered waveguides extending more widely along the centered line.

Description:
CLAIM OF PRIORITY 
   This application makes reference to and claims all the benefits accruing under 35 U.S.C. Section 119 from an application entitled “Y-Branched Optical Waveguide And Multi-Stage Optical Power Splitter Using The Same” filed in the Korean Industrial Property Office on Jul. 23, 2001 and there duly assigned Serial No. 2001-44094. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an optical power splitter for use in optical communication systems, and, in particular, to an improved structure of a Y-branched optical waveguide and a multi-stage optical power splitter using the same. 
   2. Description of the Related Art 
   Optical communication systems are fast-growing areas in communication networks. The “optical communication system” pertains to any system that uses optical signals to convey information across an optical waveguiding medium, such as an optical fiber. An optical waveguide generally consists of a core section configured to propagate an optical carrier signal within the core, and a cladding section surrounding the entire periphery of the core section. Optical elements employing such an optical waveguide include, i.e., an optical power splitter/coupler for splitting or coupling the optical power of the optical signals, and a wavelength division multiplexer/demultiplexer for multiplexing or demultiplexing multiple channels of the optical signal according to the wavelengths selected. A Y-branched optical waveguide is typically used for splitting optical power, and includes an input waveguide for receiving the optical signal, a tapered waveguide for extending the transfer mode of the input optical signal, and a pair of output waveguides for branching out the optical power of the extended optical signal to provide the branched optical power as an output optical signal. 
     FIG. 1  is a schematic diagram showing a prior art, Y-branched optical waveguide, which includes a substantially straight input waveguide  110  for receiving the optical signal through a first end section  112 ; a tapered waveguide  120 , the width of which increases along the direction of the propagation of the optical signal for receiving the optical signal through a second end section  122  that is coupled with the input waveguide  110 ; and, a first and a second output waveguide  130  and  140 , respectively, extending from third end sections  132  and  142  outwardly, being symmetrical to each other with respect to a center line  126  of the tapered waveguide  120 . The Y-branched optical waveguide may be a planar lightguide circuit (PLC) device formed of multiple layers of a high refractive index of the core section and a low refractive index of the cladding section surrounding the core section on a substrate. 
     FIG. 2  is a schematic diagram illustrating the waveguiding mode of the optical signal propagating in the Y-branched optical waveguiding medium shown in FIG.  1 . As shown in  FIG. 2 , it can be observed that the split optical signals propagate unstably along the length of the first and second output waveguides  130  and  140 , respectively.  FIG. 3  is a graphic diagram exhibiting the mode profile of the split optical signals in the third end sections  134  and  144  of the first and second output waveguides  130  and  140 . In particular, the first and second mode profiles  150  and  170  of the split optical signals indicate what appears in the third end sections  132  and  142  of the first and second output waveguide  130  and  140 , respectively. Note that the respective centering lines  160  and  180  of the first and second mode profiles  150  and  170  deviate by a given distance M 1  or M 2  from the respective center lines  136  and  146  of the first and second output waveguides  130  and  140 , thereby exhibiting the mode misalignment. Here, as the optical signal is perpendicularly incident upon the first end section  112  of the input waveguide  110  while the first and second output waveguides  130  and  140  are arranged symmetrically to each other with respect to the center line  126 , the amounts M 1  and M 2  of the above-mentioned mode misalignment are identical to each other. Therefore, this mode misalignment makes the output characteristic of the Y-branched waveguide unstable. As such, the connection of the Y-branched waveguide to other optical waveguiding elements or a subsequent stage of the Y-branched waveguide will influence the output characteristic of the corresponding Y-branched waveguide disadvantageously. 
     FIG. 4  shows a schematic diagram of the structure of a two-stage optical power splitter using the prior art Y-branched waveguide.  FIGS. 5   a  and  5   b  each shows a graphic diagram of the respective mode profiles of the optical signals propagating through the two-stage stage optical power splitter. As shown in  FIG. 4 , the two-stage optical power splitter includes a first Y-branched waveguide  200  having a first input waveguide  210 , a first tapered waveguide  220 , and a first and a second output waveguides  230  and  240 ; a second Y-branched waveguide  250  having a second input waveguide  260 , a second tapered waveguide  270 , and a third and a fourth output waveguides  280  and  290 ; and, a third Y-branched waveguide  300  having a third input waveguide  310 , a first tapered waveguide  320 , and a fifth and a sixth output waveguides  330  and  340 . 
     FIG. 5   a  shows the mode profile  350  of the optical signal appearing in the first end section  222  of the first tapered waveguide  220 , in which the input optical signal is perpendicularly incident upon the first end section  212  of the first input waveguide  210 . Thus, the alignment between the center mode of the mode profile  350  and a first center line  226  of the first tapered waveguide  220  is achieved.  FIG. 5   b  shows the mode profiles  360  and  380  of the first-branched optical signals appearing in the first end sections  222  and  322  of the second and third tapered waveguides  270  and  320 , respectively. The mode centers  370  and  390  of the mode profiles  360  and  380  are arranged to deviate by a fixed distance M 3  or M 4  respectively from the first and second center lines  276  and  326 , thereby exhibiting the mode misalignment. 
   As the two-stage optical power splitter has a symmetrical structure with respect to the first center line  226 , the mode profiles of the second-branched optical signals appearing in the second end sections  284 ,  294 ,  334 , and  344  of the third to sixth output waveguides  280 ,  290 ,  330 , and  340  are formed in symmetry with respect to the first center line  226 . 
     FIG. 6  schematically shows the waveguiding mode of the optical signal propagating through the two-stage optical power splitter. As shown in  FIG. 6 , it is noted that the first-branched optical signal unstably propagates along the longitudinal direction of the first and second output waveguides  230  and  240 , and in a similar way, the second-branched optical signal propagates even more unstably along the longitudinal direction of the third to the sixth output waveguides  280 ,  290 ,  330 , and  340 . 
     FIG. 7  shows a graphic diagram of the first to the fourth mode profiles  410 ,  420 ,  430 , and  440  of the second-branched optical signals appearing in the second end sections  284 ,  294 ,  334 , and  344  of the third to the sixth output waveguides  280 ,  290 ,  330 , and  340 , respectively. Note that the optical intensity in the center of the second and third mode profiles  420  and  430  of the first to the fourth mode profiles  410 ,  420 ,  430 , and  440  is much higher than that in the center of the first and the fourth mode profiles  410  and  440 . That is to say, the input optical signal has been subject to a first modal misalignment passing through the first stage of the two-stage optical power splitter and then a second modal misalignment passing through the second stage of the two-stage optical power splitter. Thus, it is also noted that the first to the fourth mode profiles  410 ,  420 ,  430 , and  440  represent the result of those two successive modal misalignments overlapped. 
   As appreciated from the foregoing, the prior art, Y-branched optical waveguide of  FIG. 1  has some disadvantages in that it generates undesirably uneven output characteristic as the input optical signal propagating thereof has the modal misalignment in the interim. Furthermore, the multi-stage optical power splitter using the prior art, Y-branched optical waveguide as shown in  FIG. 4  may also have the same problem in that it will be undesirably subject to the generation of such uneven output characteristic because the input optical signal passing through the optical power splitter will be effected by the successive modal misalignment caused by a multiplicity of Y-branched optical waveguides. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a Y-branched optical waveguide with a uniform output characteristic for use in optical communication systems. 
   It is another object of the present invention to provide a multi-stage optical power splitter with a uniform output characteristic suitable for use in the optical communication systems. 
   To achieve the above and other objects of the invention, according to one aspect of the present invention, a Y-branched optical waveguide with a core section serving as a transmission medium of an optical signal and a cladding section surrounding the core section, being formed on a semiconductor substrate, includes an input-tapered waveguide configured to receive the optical signal through a first ending section and to output the optical signal through a second ending section, having a symmetrical structure with respect to a centered line of the input-tapered waveguide, a width of the input-tapered waveguide gradually extending more widely along the centered line; and, a pair of first and second output-tapered waveguides each configured to receive the optical signal branched through the second ending section, from which the pair of first and second output-tapered waveguides respectively extend downstream, with an asymmetrical structure with respect to the centered line, a respective width of the first and second output-tapered waveguides gradually extending more widely along the centered line. 
   Preferably, the entrances of the first and second output-tapered waveguides are arranged in the second ending section in such a way that their inner edges are spaced apart from each other by a first specified width of gap stopped with a part of the second ending section. 
   Preferably, the entrances of the first and second output-tapered waveguides are arranged in the second ending section, in such a way that their respective outer edges are spaced apart from a respective upper or lower edge of the outer surfaces of the input-tapered waveguide, respectively by a second or third specified width of gaps different from each other. 
   More preferably, the first and second output-tapered waveguides are respectively formed in an arc with a specified curvature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a schematic diagram of a prior art, Y-branched optical waveguide; 
       FIG. 2  is a schematic diagram showing the waveguiding mode of an optical signal propagating through the Y-branched optical waveguide of  FIG. 1 ; 
       FIG. 3  is a graphic diagram illustrating the mode profiles of the branched optical signals appearing in the respective second end sections of the first and second output waveguides; 
       FIG. 4  is a schematic diagram showing the structure of a two-stage optical power splitter using the prior art, Y-branched optical waveguide; 
       FIGS. 5   a  and  5   b  are graphic diagrams illustrating the mode profiles of the split optical signals propagating through the two-stage optical power splitter according to  FIG. 4 ; 
       FIG. 6  is a schematic diagram showing the waveguiding mode of the optical signals propagating through the two-stage optical power splitter of  FIG. 4 ; 
       FIG. 7  is a graphic diagram illustrating the first to the fourth mode profiles of the second branched optical signals appearing in the second end sections of the third to the sixth output waveguides; 
       FIG. 8  is a schematic diagram showing the structure of the Y-branched optical waveguide according to a preferred embodiment of the present invention; 
       FIG. 9  is a schematic diagram illustrating the waveguiding mode of the optical signals propagating through the Y-branched optical waveguide of  FIG. 8 ; 
       FIG. 10  is a graphic diagram illustrating the mode profiles of the split optical signals appearing in the second end sections of the first and second tapered output waveguides of  FIG. 8 ; 
       FIG. 11  is a schematic diagram showing the structure of a two-stage optical power splitter having the Y-branched optical waveguide of  FIG. 8  according to a preferred embodiment of the present invention; 
       FIGS. 12   a  and  12   b  are graphic diagrams illustrating the mode profiles of the optical signals propagating through the two-stage optical power splitter of  FIG. 11 ; 
       FIG. 13  is a schematic diagram illustrating the waveguiding mode of the optical signals propagating through the two-stage optical power splitter according to  FIG. 11 ; 
       FIG. 14  is a graphic diagram illustrating the first to the fourth mode profiles of the second-branched optical signals appearing in the second end sections of the third to the sixth tapered output waveguides; 
       FIG. 15  is a schematic diagram showing the structure of the Y-branched optical waveguide according to a preferred embodiment of the present invention; 
       FIG. 16  is a schematic diagram illustrating the waveguiding mode of the optical signals propagating through the Y-branched optical waveguide according to  FIG. 15 ; 
       FIG. 17  is a graphic diagram illustrating the first and second mode profiles of the split optical signals appearing in the second end sections of the first and second tapered output waveguides according to  FIG. 15 ; 
       FIG. 18  is a schematic diagram showing the structure of a two-stage optical power splitter having the Y-branched optical waveguide of  FIG. 15  according to a preferred embodiment of the present invention; 
       FIGS. 19   a  and  19   b  are graphic diagrams illustrating the mode profiles of the optical signals propagating through the two-stage optical power splitter of  FIG. 18 ; 
       FIG. 20  is a schematic diagram illustrating the waveguiding mode of the optical signals propagating through the two-stage optical power splitter according to  FIG. 18 ; and, 
       FIG. 21  is a graphic diagram illustrating the first to the fourth mode profiles of the second-branched optical signals appearing in the second end sections of the third to the sixth tapered output waveguides. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. For the purposes of simplicity and clarity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail. 
   Referring to  FIG. 8 , a description will be made relating to the structure of a Y-branched optical waveguide according to a preferred embodiment of the present invention. As shown in  FIG. 8 , the Y-branched optical waveguide is provided with an input-tapered waveguide  450 , a first output-tapered waveguide  460 , and a second output-tapered waveguide  470 . The input-tapered waveguide  450  is configured to receive an input optical signal through a first end section  452  and supply the optical signal branched through its other end sections. The input-tapered waveguide  450  is also configured to have its width spreading out more widely as the optical signal propagates further into the input waveguide, with its outer surface formed in an S-shaped structure. Each of the first and second output-tapered waveguides  460  and  470  is configured to receive the branched optical signals at the first end sections  462  and  472  that are coupled the other end section of the input-tapered waveguide  450 . Both tapered waveguides  460  and  470  have their respective outer surface formed in an arc at specified curvatures and configured to have their widths spreading out more widely as the optical signals propagate therethrough. The two receiving ends of the inner surfaces  468  and  478  of the first and second output-tapered waveguides  460  and  470  are spaced apart from each other by a specified gap G 1 , while the two receiving ends of the outer surfaces  467  and  477  of the first and second output-tapered waveguides  460  and  470  are spaced apart from the respective outer surfaces  458  of the input waveguide  450  by another specified gap G 2 . Furthermore, the first and second output-tapered waveguides  460  and  470  are disposed in symmetry with respect to a centered line  456  of the input waveguide  450 . 
     FIG. 9  shows the waveguiding mode of the optical signals propagating through the Y-branched optical waveguide of  FIG. 8  during simulation.  FIG. 10  shows the mode profiles of the branched optical signals appearing in the second end sections  464  and  474  of the first and second tapered output waveguides  460  and  470  shown in FIG.  8 . As shown in the waveguiding modes of the branched optical signals, note that the optical signals propagate in a relatively stable manner along the longitudinal direction of the first and second output-tapered waveguides  460  and  470  when the input optical signal is incident on the first end section  452  of the input waveguide  450 . 
   Referring to  FIG. 10 , the first and second mode profiles  480  and  490  of the branched optical signals appearing in the second end sections  464  and  474  of the first and second tapered output waveguides  460  and  470  are shown respectively. As shown in  FIG. 10 , the respective centered lines  485  and  495  of the first and second mode profiles  480  and  490  are respectively spaced apart by a specified interval M 5  or M 6  from the centered lines  466  and  476  of the first and second tapered output waveguides  460  and  470 , thereby resulting in the modal misalignment. Here, as the input optical signal is perpendicularly incident on the first end section  452  of the input waveguide  450  while the first and second output-tapered waveguides  460  and  470  are disposed in symmetry to the centered line  456  of the input waveguide  450 , those intervals M 5  and M 6  in modal misalignment are the same to each other. Although such modal misalignment often makes the output characteristic of the Y-branched optical waveguide uneven, it should be appreciated that the extent of such unevenness in the modal misalignment in the case where the Y-branched optical waveguide is structured as shown in  FIG. 9  may be negligible when compared to the prior art structure of FIG.  1 . 
     FIG. 11  schematically shows the structure of a two-stage optical power splitter utilizing the Y-branched optical waveguide of  FIG. 8  according to another preferred embodiment of the present invention.  FIGS. 12   a  and  12   b  each shows the mode profiles of the optical signals propagating through the two-stage optical power splitter. As shown in  FIG. 11 , the two-stage optical power splitter includes a first Y-branched optical waveguide  500 , which constructs a first stage of the two-stage optical power splitter, having a first input-tapered waveguide  510 ,a first output-tapered waveguide  520 , and a second output-tapered waveguide  530 . The two-stage optical power splitter further includes a second Y-branched optical waveguide  550 , which constructs a second stage of the two-stage optical power splitter, having a second input-tapered waveguide  560  coupled to a third and a fourth output-tapered waveguides  570  and  580  respectively, and a third Y-branched optical waveguide  600  having a third input-tapered waveguide  610  coupled to a fifth output-tapered waveguide  620  and a sixth output-tapered waveguide  630 , respectively. 
   Referring now to  FIG. 12   a , note that the mode profile  650  of the optical signals appearing in the second end section  514  of the first input-tapered waveguide  510 . Here, the arrangement is made in such a way that the input optical signal is perpendicularly incident to the first end section  512  of the first input waveguide  510 , thus the center of the mode profile  650  coincides with a first centered line  516  of the input waveguide  510 . 
   Referring then to  FIG. 12   b , note that the mode profiles  660  and  670  of the first branched optical signals appearing in the second end section  564  and  614  of the second and third input-tapered waveguides  560  and  610 . As illustrated, note that the center lines  665  and  675  of the mode profiles  660  and  670  are spaced apart, by a specified gap M 7  or M 8 , from the second and third center lines  566  and  616  of the input waveguide  510 , thereby resulting in the modal misalignment. As the two-stage optical power splitter has a symmetrical structure with respect to the first center line  516 , the mode profiles of the second branched optical signals appearing in the second end section  578 ,  584 ,  624 , and  634  of the third to the sixth output-tapered waveguides  570 ,  580 ,  620 , and  630  are respectively formed symmetrically with respect to the first center line  516 . 
     FIG. 13  schematically shows the waveguiding mode of the optical signals propagating through the two-stage optical power splitter shown in FIG.  11 . Referring to the waveguiding mode of the branched optical signals, note that the first branched optical signals propagate through the first and second output waveguides  520  and  530  along their longitudinal directions in a relatively stable manner, while the second branched optical signals propagate through the third to the sixth output waveguides  570 ,  580 ,  620 , and  630  along their longitudinal directions in a relatively unstable manner. 
   Referring to  FIG. 14 , note that the first to the fourth mode profiles  650 ,  660 ,  670 , and  680  of the second-branched optical signals appearing in the second end sections  574 ,  584 ,  624 , and  634  of the third to the sixth output-tapered waveguides  570 ,  580 ,  620 , and  630 . As illustrated, note that the peak intensity in the second and third mode profiles  660  and  670  of the above first to the fourth mode profiles  650 ,  660 ,  670 , and  680  is formed slightly lower than that in the first and the fifth mode profiles  650  and  680 . Thus, it will be appreciated by those skilled in the art that the input optical signal will be subject to a first modal misalignment passing through the first stage of the two-stage optical power splitter and then a second misalignment that is a similar modal misalignment passing through the second stage of the two-stage optical power splitter. Although such modal misalignment often makes the output characteristic of the Y-branched optical waveguide uneven, it should be appreciated that the extent of such unevenness in the modal misalignment in the case where the Y-branched optical waveguide is structured as shown in  FIG. 11  may be negligible when compared to the prior art structure of FIG.  1 . 
     FIG. 15  will make a description relating to the structure of the Y-branched optical waveguide according to another preferred embodiment of the present invention. As shown in  FIG. 15 , the Y-branched optical waveguide is provided with an input-tapered waveguide  710  and a first and a second output-tapered waveguides  720  and  730 . The input-tapered waveguide  710  is configured to receive the input optical signal through its first end section  712  and supply the optical signal branched through its second end sections. The input-tapered waveguide  710  is also configured to have its width spreading out more widely as the optical signal propagates therethrough. The first and second output tapered waveguides  720  and  730  are each configured to receive the branched optical signals at the first end sections  722  and  732  coupled with the second end section of the input-tapered waveguide  710 , and have their respective outer surface formed in an arc with a specified curvature. The first and second output-tapered waveguides  460  and  470  each are also configured to have their widths spreading out more widely as the optical signals propagate therethrough in a similar way as the above. The two inputting edges of the inner surfaces  728  and  738  of the first and second output-tapered waveguides  720  and  730  are spaced apart from each other by a specified gap G 4 , while the other two inputting edges of the outer surfaces  727  and  737  of the first and second output-tapered waveguides are spaced apart from the respective outer surfaces  718 , i.e., either an upper edge or a lower edge, of the input waveguide  710  by another specified gap G 5 . Here, the gap G 6  is arranged preferably to be larger than the gap G 5 , in such a way that the value of (G 5 +offset) is substantially equal to, or very similar to, the value of (G 6 −offset). 
     FIG. 16  schematically shows the waveguiding mode of the optical signals propagating through the Y-branched optical waveguide according to FIG.  15 .  FIG. 17  shows the mode profiles of the split optical signals appearing in the second end sections  724  and  734  of the first and second tapered output waveguides  720  and  730 . As seen in the waveguiding mode of the branched optical signals, it is noted that the optical signals propagate in a relatively stable manner along the longitudinal direction of the first and second output-tapered waveguides, wherein the input optical signal is perpendicularly incident to the first end section  712  of the input-tapered waveguide  710 . 
     FIG. 17  shows the first and second mode profiles  750  and  760  of the branched optical signals appearing in the second end sections  724  and  734  of the first and second tapered output waveguides  720  and  730 , respectively. As illustrated, it is noted that the respective centered lines  726  and  736  of the first and second mode profiles  750  and  760  respectively coincide with the respective centered lines  726  and  736  of the first and second tapered output waveguides  720  and  730 . Here, the offset value should be set, for example, so as to compensate the interval M 7  or M 8  in modal misalignment, in case where all the physical conditions of this Y-branched optical waveguide are adapted to be identical to those of the other Y-branched optical waveguide of  FIG. 8 , except for the offset condition only. 
     FIG. 18  schematically shows the structure of a two-stage optical power splitter having the Y-branched optical waveguide of  FIG. 15  according to another preferred embodiment of the present invention.  FIGS. 19   a  and  19   b  each show the mode profiles of the optical signals propagating through the two-stage optical-power splitter according to FIG.  18 . This two-stage optical power splitter includes a first Y-branched optical waveguide  800 , which constructs a first stage of the two-stage optical power splitter, having a first input-tapered waveguide  810 , and a first and a second output-tapered waveguides  820  and  830 . The two-stage optical power splitter further includes a second Y-branched optical waveguide  850 , which constructs a second stage of the two-stage optical power splitter, having a second input-tapered waveguide  860 , and a third and a fourth output-tapered waveguides  870  and  880  respectively, and a third Y-branched optical waveguide  900  having a third input-tapered waveguide  910 , and a fifth and a sixth output-tapered waveguides  920  and  930 , respectively. 
     FIG. 19   a  shows the mode profile  940  of the optical signal appearing in the second end section  814  of the first input-tapered waveguide  810 . Here, the arrangement is made in such a way that the input optical signal is perpendicularly incident to the first end section  812  of the first input waveguide  810 , and thus the center of the mode profile  940  coincides to a first centered line  816  of the first input waveguide. 
     FIG. 19   b  shows the mode profiles  945  and  950  of the first branched optical signals appearing in the second end section  864  and  914  of the second and third input-tapered waveguides  860  and  910 . As illustrated, it is noted that the center lines of the mode profiles  945  and  950  are formed to coincide with the center lines  866  and  916  of the second and third input-tapered waveguide  860  and  910 . 
     FIG. 20  schematically illustrates the waveguiding mode of the optical signals propagating through the two-stage optical power splitter according to FIG.  18 . As seen in the waveguiding mode of the branched optical signals, it is observed that the first branched optical signals propagate through the first and second output-tapered waveguides  820  and  830  along their longitudinal directions in a relatively stable manner, and the second branched optical signals propagate through the third to the sixth output waveguides  870 ,  880 ,  920 , and  930  along their longitudinal directions in a relatively unstable manner likewise. 
   Referring to  FIG. 21 , there are illustrated the first to the fourth mode profiles  960 ,  970 ,  980 , and  990  of the second-branched optical signals appearing in the second end sections  874 ,  884 ,  924 , and  934  of the third to the sixth tapered output waveguides  870 ,  880 ,  920 , and  930 . The illustrated diagram shows that all the center intensities in these first to the fourth mode profiles  960 ,  970 ,  980 , and  990  are substantially the same to each other. 
   As apparent from the foregoing description, the Y-branched optical waveguide according to the present invention would secure a more even output characteristic in an optical waveguiding medium by way of disposing the respective entrance of the two, i.e., first and second, output-tapered waveguides on a dismal end section of the input-tapered waveguide, asymmetrically with respect to a centered line of the input-tapered waveguide. Furthermore, it will be appreciated that the multi-stage optical power splitter according to the present invention is particularly advantageous in securing a more even output characteristic by means of connecting a plurality of Y-branched optical waveguides in series or parallel. 
   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 or modifications in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.