OPTICAL COUPLER AND METHOD OF MANUFACTURING THE SAME

Abstract: An optical coupler and a method of manufacturing the same are provided. The optical coupler includes an input optical waveguide (I) and N output optical waveguides (O1, O2, ...), and divides an optical signal received from the input optical waveguide (I) into N optical signals. The optical coupler further includes a plurality of Y-junction optical waveguides configured in m stages, for branching the received optical signal into two optical signals at each stage, and a plurality of curved optical waveguides alternately connected to the Y-junction optical waveguides, at least one curved optical waveguide connected between Y-junction optical waveguides, in the m-th stage and the output optical waveguides, wherein when the guiding direction of the optical signal is set to be the longitudinal axis, and the junction between the input optical waveguide and the first-stage Y-junction optical waveguide is set to be the starting point, the positions and sizes of the Y-junction waveguides and the curved optical waveguides are determined in order to minimize the longest path among N paths ranging from the starting point to each of the output optical waveguides. An optical coupler is manufactured using Y-junction optical waveguide modules (J11, J12, ...) and connection optical waveguide modules (B11, B21, ...), such that various Y-junction optical waveguide structures can be designed. Also, the degree of freedom upon layout of modules is increased, so that design of the optical coupler is easy.

Description: OPTICAL COUPLER AND METHOD OF MANUFACTURING THE SAME Technical Field The present invention relates to an optical coupler and a method of manufacturing the same, and more particularly, to an optical coupler having Y-junction optical waveguides between input and output optical waveguides, and curved optical waveguides connected to the Y-junction optical waveguides and a method of manufacturing the same Background Art Optical couplers optical devices for dividing an optical signal into a plurality of optical signals are the heart of an optical subscriber network Earlier optical couplers were manufactured by thermally uniting two optical fibers, which required many a large amount of manual work Thus, the manufacturing cost was high, and direct use of the optical fibers increased the size of the manufactured optical couplers In order to solve the above-described problems, an optical coupler using an integrated optics technique has been proposed The optical coupler using the integrated optics technique is manufactured by forming a fine pattern for the optical coupler on a plane substrate using a similar method to that of manufacturing semiconductor integrated circuits The method of manufacturing the optical coupler is suitable for mass production, thus lowering the manufacturing cost and downsizing the manufactured optical coupler Also, this method allows multi-output optical couplers (having 16 or more outputs) to be manufactured without serious difficulty An optical coupler having N outputs for one input is called a 1 xN optical coupler The 1 xN optical coupler can be formed by connecting 1 x2 optical couplers to each other in series In this case, the number (N) of outlets of the optical coupler is 2 m (where m denotes the number of stages of 1 χ2 optical couplers which are connected to each other in serial, and is expressed as a natural number) For example, a 1 χ4 optical coupler is formed by connecting a 1 x2 optical coupler to each of the two output ports of another 1 x2 optical coupler in series Here the 1 x2 optical coupler includes a Y-junction optical waveguide which is shaped like the English character "Y", and two curved waveguides for connecting the Y-junction optical waveguide to the two output waveguides At present, two types of optical couplers according to how Y- junction optical waveguides are arranged have been proposed One type is an on-axis optical coupler, which arranges Y-junction optical waveguides in parallel to the longitudial axis, that is, in the direction in which light travels The other type is an off-axis optical coupler, which arranges Y-junction optical waveguides so that the Y-junction optical waveguides are not parallel to the longitudinal axis FIG 1A shows the structure of an on-axis optical coupler which is disclosed in U S patent No 4,165,225 The optical coupler shown in FIG 1A is formed by laying out the Y-junction optical waveguides in a direction which is parallel to the longitudinal axis Z in accordance with the number of stages and connecting the inputs and outputs of the Y-junction optical waveguides to each other via connection optical waveguides which are curved waveguides In FIG 1 A, l^ denotes Y-junction optical waveguides, A^ denotes connection optical waveguides, and X x denotes output optical waveguides However, the optical coupler shown in FIG 1 A has a problem in that it is long Generally, a variation in the lateral displacement of a trajectory must be kept small to reduce the bending loss of the curved optical waveguide The on-axis optical coupler must make the terminal of the connection waveguide parallel to the longitudinal axis Z, in order to connect two laterally-separated Y-junction optical waveguides to each other in parallel to the longitudinal axis Here, if the variation in the lateral displacement is kept small, the longitudinal length of the connection optical waveguide increases Consequently, the total length of the device is increased by the length required to make the Y-junction optical waveguides parallel to each other Such an increase in the length of the device degrades the yield, thus increasing the manufacturing cost The pattern uniformity required within the device is increased, thus deteriorating the performance of the device FIG 1 B shows the structure of an off-axis optical coupler, which is disclosed in U S Patent No 5,222, 167 The optical coupler shown in FIG 1 B is manufactured by determining the path of an outermost optical waveguide for the optical coupler, determining the positions on the path at which Y- junction optical waveguides are placed, and forming Y-junction optical waveguides by drawing virtual symmetrical axes (for example, W) which contact the optical waveguide paths (for example, R 2 ) at the determined positions and drawing curved lines (for example, R 3 ) which are symmetrical about the virtual symmetrical axes with respect to the optical waveguide paths In FIG 1 B, K^ denotes a Y-junction optical waveguide, R x denotes the radius of a curved optical waveguide, and X x denotes an output optical waveguide The outermost optical waveguide generally adopts a circular arc optical waveguide, so that a Y-junction optical waveguide, to be formed along a curved line, is formed between the circular arcs R 2 and R 3 of two contacting circles as shown in FIG 1 B Here, the central axis of the Y-junction optical waveguide is no longer parallel to the longitudinal axis Z However, the optical coupler shown in FIG 1 B incorporates Y-junction optical waveguides and curved optical waveguides, such that the structures of the Y-junction optical waveguides are formed unilaterally by the curved optical waveguides Also, the positions of the Y-junction optical waveguides on the outermost curved optical waveguide must be determined so that the end of each path branched on the Y-junction optical waveguide accurately meets each output optical waveguide However, this position determination is also required by each of the Y-junction optical waveguides in the next stages, thus greatly restricting the position of each of the Y-junction optical waveguides on each path Therefore, a Y-junction optical waveguide must be located at a predetermined position, so that even a small amount of position adjustment is very difficult, and the degree of freedom in terms of design is thus lowered Such a difficulty becomes serious with an increase in the number of output optical waveguides In other words, more paths must be branched from the interior of the outermost optical waveguide in order to increase the number of output optical waveguides Here, the positions of the Y-junction optical waveguides on each branched optical waveguide must be determined so that the end of each path correctly meets the input of each of the output optical waveguides which are spaced a predetermined distance apart from each other. However, in the case of the off-axis optical coupler, the position determination of a Y-junction optical waveguide is achieved through trial and error due to intuitional judgement. Accordingly, the number of Y- junction optical waveguides in their position increases with an increase in the number of output optical waveguides. Thus, the position determination of the Y-junction optical waveguides using this method is difficult. Disclosure of the Invention An objective of the present invention is to provide an optical coupler in which the number of output optical waveguides can be easily increased by modularizing an off-axis optical coupler using Y-junction optical waveguides and curved optical waveguides for connecting the Y-junction optical waveguides, and a method of manufacturing the same. Accordingly, to achieve the above objective, the present invention provides an optical coupler including an input optical waveguide and N output optical waveguides, for dividing an optical signal received from the input optical waveguide into N optical signals, the optical coupler comprising: a plurality of Y-junction optical waveguides configured in m stages, for branching the received optical signal into two optical signals at each stage; and a plurality of curved optical waveguides alternately connected to the Y-junction optical waveguides, at least one curved optical waveguide connected between Y-junction optical waveguides in the m-th stage and the output optical waveguides, wherein when the guiding direction of the optical signal is set to be the longitudinal axis, and the junction between the input optical waveguide and the first-stage Y-junction optical waveguide is set to be the starting point, the positions and sizes of the Y-junction optical waveguides and the curved optical waveguides are determined in order to minimize the longest path among N paths ranging from the starting point to each of the output optical waveguides. To achieve the above objective, the present invention provides a method of designing a pattern mask for an optical coupler including an input optical waveguide, N output optical waveguides a plurality of Y-junction optical waveguides configured in m stages for branching an optical signal received by the input optical waveguide into two optical signals at each stage, and a plurality of curved optical waveguides alternately connected to the Y- junction optical waveguides, at least one curved optical waveguide connected between Y-junction optical waveguides at the m-th stage and the output optical waveguides, and manufacturing the optical coupler according to the mask pattern, the method comprising the steps of (a) determining the lengths, widths and branching angles oftheY-junction optical waveguides, and forming Y-junction optical waveguide modules having sizes depending on the determined lengths, widths and branching angles of the Y-junction optical waveguides, (b) forming connection optical waveguide modules having sizes depending on the radius of curvature and inside angle of each of the curved optical waveguides, for connecting the Y-junction optical waveguides to each other and connecting the m-th Y-junction optical waveguide modules to the output optical waveguides, (c) arranging the Y-junction optical waveguide modules and the curved optical waveguide modules between the input optical waveguide and each of the output optical waveguides, after determining the lengths of the connection optical waveguide modules and the positions of the Y-junction optical waveguide modules and the connection optical waveguide modules so that the longitudinal distance between the starting point and each of the output optical waveguides is minimized, when the junction between the input optical waveguide and the first-stage Y-junction optical waveguide is set to be the starting point, and the guiding direction of the optical signal is set to be along the longitudinal axis, and (d) manufacturing the pattern mask according to the arranged structure Brief Description of the Drawings FIGS 1 A and 1 B are views illustrating the structures of conventional optical couplers, FIG 2 is a view illustrating the structure of an optical coupler according to the present invention FIG 3 is a view illustrating the structure of the Y-junction optical waveguide module of FIG 2, FIG 4 is a view illustrating a connection optical waveguide module using a circular arc optical waveguide, FIGS 5A through 5D are views illustrating first through fourth paths, respectively, from the start point I of the optical coupler of FIG 2 to each of the end points E, through E 4 thereof, FIG 6 is a view illustrating a 1 x8 optical coupler according to the present invention, FIG 7 is a view illustrating the comparison in length between a 1 x32 optical coupler according to the present invention and a 1 χ32 optical coupler according to the prior art, FIG 8 is a view illustrating part of a P-curved optical waveguide, and FIG 9 is a view illustrating a circular arc optical waveguide having a radius of curvature (R) of 5000μm and an inside angle of 90°, and a polynomial arc (PA) optical waveguide Best mode for carrying out the Invention Referring to FIG 2 showing a 1 xN optical coupler according to the present invention, the optical coupler has a 1 x8 structure provided with 8 output waveguides The optical coupler according to the present invention has a longitudinally symmetrical structure, and FIG 2 shows only four upper optical waveguides The optical coupler shown in FIG 2 includes an input optical waveguide (I), N output optical waveguides O x , a plurality of Y-junction optical waveguide modules J^, and a plurality of connection optical waveguide modules B^ Shown circles (o) denote junctions The Y-junction optical waveguide modules Y^ are connected to each other in m stages For example, in the case of a 1 χ8 optical coupler, the Y-junction optical waveguide modules J^ are connected to each other in three stages The input optical waveguide (I) is a straight line optical waveguide which is placed on the longitudinal axis Z The output optical waveguides (O x ) straight line optical waveguides separated by equal intervals (Δx) transmit an optical signal to an output optical fiber The lateral positions of the output optical waveguides (OJ are x 1 τ x 2 , x 3 and x 4 , respectively The Y-junction optical waveguide modules J^ each have one inlet and two outlets, and are defined by the length, the width and the branching angle Each of the Y-junction optical waveguide modules J^ is located on a path between the input optical waveguide (I) and each of the output optical waveguides (OJ, and branches an input optical signal into two optical signals Here, the Y-junction optical waveguide modules J^ may not be parallel to the The connection optical waveguide modules B^ connect the Y-junction optical waveguide modules J^ to each other, and at least one connection optical waveguide module is connected between the m-th stage and each of the output optical waveguides (OJ In the present embodiment, two connection optical waveguide modules are connected between the m-th stage and each of the output optical waveguides (O x ) Each of the connection optical waveguide modules B^ outputs the optical signals branched by the Y- junction optical waveguide modules J^, to a Y-junction optical waveguide module J^ on the next stage or the output optical waveguides (OJ E x denotes the end point of each of the connection optical waveguide modules which are connected to the output optical waveguides The junctions (o) include junctions between the input optical waveguide and each of the Y- junction optical waveguide modules J^, junctions between the Y-junction optical waveguide modules J^ and the connection optical waveguide modules B^, and junctions between the connection optical waveguide modules B^ The junctions (o) should meet the following restriction conditions 1 ) coordinate matching between the outlet of a module and the inlet of a module on the next stage, 2) tangent inclination matching between the outlet of a module and the inlet of a module on the next stage, and 3) curvature matching between the outlet of a module and the inlet of a module on the next stage, in order to prevent scattering of guided light and distortion of the wave plane of the guided light In order to manutacture an optical coupler having the above-described structure, first, the number (N) of output optical waveguides for the optical coupler and the interval (Δx) between adjacent output optical waveguides should be determined As shown in FIG 3, the length Lj, the half width P j , and the branching angle θj of the Y-junction optical waveguide module J^ are determined to design the Y-junction optical waveguide modules J xy FIG 3 shows the structure of a Y-junction optical waveguide module J^ Here, a denotes an inlet, and b and c denote the outlets The branching angle θ j denotes the intermediate angle between the axis of symmetry W and a tangent line at the upper outlet point b of a Y-junction module The outlet coordinate (b , c) of a Y-junction module rotated by Θ R from the longitudinal axis is determined by the following Equation 1 LJCOSΘ }{ - P j SmQ R L J smθ R + P J cosθ R/ (1 ) cosθ Λ - sιnθ Λ L j Cosd R + P j m R c = a + sιnθ Λ cosθ Λ > χ L j Smθ R - PJCOSΘ RJ wherein the outlet angle at the outlet b of the Y-junction module rotated by Θ R is θ R +θ j , and the outlet angle at the outletc is θ R -θ j Next, the radii (R) of circular arc optical waveguides are determined to design connection optical waveguide modules The radii of the circular arc optical waveguides can be set to be different from each other or the radii themselves can be set to be unknown variables However, the present embodiment assumes that all of the circular arc optical waveguides have the same radius A circular arc optical waveguide can be used as the connection optical waveguide according to the present invention Preferably, a polynomial arc (PA) waveguide is used The PA waveguide is a curved waveguide which has the merits of a circular arc optical waveguide and also has corrections to make up for the defects thereof The circular arc optical waveguide is formed by adopting part of a circle having a radius of curvature of R as an optical waveguide, and can easily be expressed by path equations However, in a case where circular arc optical waveguides having different curvatures are connected, transition loss and wave plane distortion due to curvature mismatch occur at the junctions The paths of the PA optical waveguide can be defined by a longitudinal axis polynomial z(t) and a lateral axis polynomial x(t) each expressed as a fifth degree polynomial with respect to a parameter (t) The coefficients of each term of each equation are determined by the boundary conditions at the inlet/outlets of a curved optical waveguide, that is, by the coordinate matching, the tangent inclination matching, and the curvature matching However, many parameters to be derived make it difficult to use the PA optical waveguide as the connection optical waveguide module for the optical coupler Accordingly, in the present embodiment, a circular arc optical waveguide is selected as the connection optical waveguide and parameters are derived, in the parameter deriving step to design the optical coupler, and the circular arc optical waveguide is then substituted by a curvature-matched PA optical waveguide FIG 4 shows a connection optical waveguide module using a circular arc optical waveguide Here, a denotes an inlet, and b denotes an outlet The connection optical waveguide module is defined by the inside angle (Θ B ) and the radius of curvature (R) of a circle of which part of the arc is the connection optical waveguide Here, the inside angle (Θ B ) is obtained by subtracting the tangent angle (θ,) of the optical waveguide inlet from the tangent angle (θ 0 ) of the optical waveguide outlet The outlet position b of a connection optical waveguide rotated by θ, is determined by R and Θ B with respect to the inlet position a, as shown in the following Equation 2 (2) wherein r denotes the radius of curvature which has a sign, which is defined as Resign (Θ B ) The sign of r the sign of a curvature is positive if the tangent angle of the connection optical waveguide increases along the path, and negative if the tangent angle decreases, which is determined by the following Equation 3 Θ A >0 »S«(Θ A ) θ β =0 (3) Θ Λ <0 The paths of an optical coupler formed by sequentially connecting Y- junction optical waveguide modules and connection optical waveguide modules can be expressed by equations derived from Equations 2 and 3 According to Equations 2 and 3 the equations relate to the inside angle of each circular arc optical waveguide Thus, the size of the connection optical waveguide can be obtained by calculating the solution of the equations, and the positions of the Y-junction optical waveguide modules according to the paths, and the positions of the connection optical waveguide modules can be determined FIG 5A shows the first path from the start point (I) to the end point E 1 Referring to FIG 5A, the first path is J„ to B^ to J 21 to B 21 to J 31 to B 31 to B 41 to E., The end point coordinates (z x of the first path are defined by the following Equation 4, from the radii of curvature R B11 l R B21 , R B31 and R B41 of circular arcs and the inside angles Θ B11 , Θ B21 , Θ B31 and Θ B41 thereof with respect to the connection optical waveguide modules B^, B 21 , B 31 and B 41 > "B31> B 1> ^os^+θ^sinfO^+θ,) + 2r B2/ cos(^ + θ βy;+ 2θ in(-^) + L j ∞s B2] + d B11 +2θ / n(θ β27 +Q B1! + 2Θ,) fl fl + 2^ 7 cos(-^ + θ £2;+ θ β7 + 3θ,)sin(^) fl fl + 2^cos(-^ + θ S3y+ θ^ + θ β;;+ 3θ in(-^) l r B2P r B31' V B4 "BU> "B21> "B31> "B41' + L j S (Q BJJ +θ J )+P J cos(θ Bn +θ y ) fl fl + 2r 5 n(- ^ + Q B3] +d B2] + Q BJJ +3θ in(-^) Since the input optical waveguide and the output optical waveguide are parallel to the longitudinal axis Z, the angle θ, accumulated along the first path must be zero as shown in the following Equation 5: θ l = i( θ BU> θ B2 θ B31> θ B4l) = ®Bπ +Q B21 + Sl^B41 + Wj -<fi) = 0 FIG.5B shows the second path from the start point (I) to the end point E 2 . Referring to FIG.5B, the second path is J^ to B to J 21 to B 21 to J 31 to B to B 42 to E 2 . The end point coordinates (z 2 , x 2 ) of the second path are defined by the following Equation 6, from the radii of curvature R B11 , R B21 , R B32 and R^ of each circular arc and the inside angles Θ B11 , Θ B21 , Θ B32 and β^ thereof with respect to the connection optical waveguide modules B^, B 21 , B 32 and B 2 - Σ 2 + E J cos(θ βy/ +θ J )- J sin(θ B;;+ θ J ) fl fl + 2r B2 cos(^+θ βyy+ 2θ in(-^) +L j ∞s B21 + θ £yy + 2θ P (θ ω +θ flyy + 2Θ,) fl fl + 2r 52 cos(-^ + θ B2;+ θ £;;+ θ J )sin(-^) ^r^cos^θ^d^θ^θ^n^) X 2 82 r BU> r B21-> r B32> T B42 "-377' "B21> 73J ' "R 42 ) + L (θ £yy+ θ Pcos(θ Byy+ θ J ) fl fl + 2 r B2y sin(-^ + θ By;+ 2θ e( -^) (6) + L m(β B2} +Θ B]] +2Q j )-P jC os(θ B2] + Θ B]] +2Θ,) θ ''S n i„^ „ „ „ . θ. + 2 W 552"i"n^(--^+θ i?2y + θ βy + θ n(-^) + 2^ 2 sin(^ + θ^ + θ β2y+ θ β y+ θ,)sin(^) Since the input optical waveguide and the output optical waveguide are parallel to the longitudinal axis Z, the angle θ 2 accumulated along the second path must be zero as shown in the following Equation 7: θ 2 = Ψ 2 ( Θ S77> Θ B27> Θ B32> Θ B«) = Θ BJJ +Θ B21 +Q B32 + Θ B42 + Θ J (?) = 0 FIG.5C shows the third path from the start point (I) to the end point E 3 . Referring to FIG.5C, the third path is J„ to B to J 21 to B 22 to J 32 to B 33 to B 43 to E 3 . The terminal point coordinates (z 3 , x 3 ) of the third path are defined by the following Equation 8, from the radii of curvature R B11 , R B22 , R^ and R B43 of each circular arc and the inside angles Θ B11 , Θ B22 , and Θ B43 thereof with respect to the connection optical waveguide modules B^, B 22 , B 33 and B 43 : z 3 = JfS(r'B]]' r B22> <r B33> r ' B43> θ u 7577> θ υ 52 ' θ 7555' β ^B 3 )l fl fl = L J+ 2r Bn cos(- - + θ,)sin(-^) + I J cos(θ £;y+ θ J ) + J sin(θ yjyy+ θ,) fl fl + 2r β22 cos(^ + θ βyy )sin(-^) + Ecos(θ S2 + θ βy )- J sin(θ i?22 +θ βyy ) fl fl + 2r, i os(-^ + θ /322+ θ 5y;+ θ J )sin(-^i) fl fl + 2r B43 co (- + Q B33 +Q B22 + n +θ in(-^i) 3 S3VBW r B22> r B33-> r B43> BJl> B22> 7555' *B43/ fl fl = ^ + 2γ β ,,sin(^+θ n(-^) + I,sin(θ Byy+ θ)- J cos(θ Byy+ θ J ) fl fl + 2γ B22 sin(-^H + θ £yy )sin(-^) (8) + I J sin(θ B22 +θ β y )+ J cos(θ £22 +θ 5;y ) + 2y B33 sm(d B33 OVER2 + 6 B22 +β B]1+ Q J )sm(^≡) + 2γ^sin(- θ ^ + θ^ + θ B22+ θ β7y+ θ)sin(^) Since the input optical waveguide and the output optical waveguide are parallel to the longitudinal axis Z, the angle θ 3 accumulated along the third path must be zero as shown in the following Equation 9: θ 3 = Ψ 3 ( θ 577> θ 7522> Q B33> θ 5«) = U^B22^B33^B43^J -0) FIG.5D shows the fourth path from the start point (I) to the end point E 4 . Referring to FIG.5D, the fourth path is J^ to B^ to J 21 to B 22 to J 32 to B^ to B 44 to E 4 . The end point coordinates (z 4 , x 4 ) of the fourth path are defined by the following Equation 10, from the radii of curvature R B11 , R B22 , R B34 and R B44 of each circular arc and the inside angles Θ B11 , Θ B22 , Θ B34 and Θ B44 thereof with respect to the connection optical waveguide modules B^, B 22 , B 34 and B 44 : Z 4 ~ J 4 f B1P f B22> } B34> K B44> °7577' ^7 2 ' ^B34' ^ B44' = L j +2 B ιι C0S (— - + θ in(θ Byy over2) + L J cos(θ B]] + θ J ) + P J sm(θ B11 + θ j ) + 2γ^cos(- θ ^ + θ 5yy )s (-^) + J L y cos(θ β22 + θ βyy )+/ J J sin(θ B22+ θ Byy ) fl fl + 2γ^cos(^ + θ β22 + θ Byy -θ J )sin(^) X A S4VBJI' r B22> Y B34-> T B44 B1Ϊ> B22' B34' B44l + L j Sm(d B]] + Q J )-P J cos(θ B]] +Θ,) fl fl + 2r B22 ή n(- + θ BJ] )sm(-f-) ...(10) + L J sm(θ B22 +β B]J )-P J cos(θ B22+ β B1] ) fl fl + 2^sin(- + θ j522+ θ i3yy -θ,)sin(- l) fl fl + 2r B44 sm(- + Q B34+ θ B22+ θ BU -θ J )sm(- ) Since the input optical waveguide and the output optical waveguide are parallel to the longitudinal axis Z, the angle θ 4 accumulated along the fourth path must be zero as shown in the following Equation 11 : θ 4 = Ψ 4 ( θ 7577> θ 7522> θ 7554' θ 75 = n ^B22 + 34^B44^J -(H ) = 0 Eight non-linear simultaneous equations having 11 unknown quantities are obtained from Equations 4 through 11. The 11 unknown quantities are the inside angles of each connection optical waveguide module and a set (U) of the 11 unknown quantities is determined to be expressed as {Θ B11 , Θ B21 , Θ B22 θ B ι Θ B32 . Θ B33 , Θ B34 , Θ B41 , Θ B42 , Θ B43 , Θ B44 } Four of the eight non-linear simultaneous equations are lateral moving distance equations, that is, the lateral coordinate values of the end points, x=g, (••• )=((N+1 )/2- ι)»Δx which are obtained from the boundary conditions with respect to the lateral displacement of the output optical waveguide The remaining four non-linear simultaneous equations are accumulated angle equations θ, = ψ, (••• )=0 which are obtained from the condition in which each accumulated angle value from the input optical waveguide to the output optical waveguide is zero In this case, the number of unknown quantities is greater than the number of equations, so that several solution sets are obtained A set (U,) of solutions among the several sets of solutions which minimizes the longitudinal moving distance equations z=f,(»" ) of Equations 4 through 11 , can be obtained by several methods In the present embodiment, the solution set (U is obtained by a mathematical optimization technique After solution sets U 1 t U 2 , U 3 and U 4 for minimizing each path (ι=1 , 2, 3 and 4) is obtained, paths each having the maximum longitudinal moving distance, among the four resultant paths obtained by substituting the solution sets \J U 2 , U 3 and U 4 into each of the equations, are compared to select a solution set having the minimum length That is, a solution set is selected, which satisfies minimum { maxιmum[f 1 (UJ, f 2 (UJ, f 3 (UJ, f 4 (UJ], maximumt U , f 2 (U 2 ), f 3 (U 2 ), f 4 (U 2 )], f 2 (U 3 ), f 3 (U 3 ), f 4 (U 3 )j, maximum [f.(U 4 ), f 2 (U 4 ), f 3 (U 4 ), f 4 (U 4 )]} The sizes of the connection optical waveguide modules are determined from the selected solution set, thus connecting and laying out the Y-junction optical waveguide modules and the connection optical waveguide modules The lower portion of the optical coupler is formed by laying out the obtained Y-junction modules and connection optical waveguide modules so that they are symmetrical about the longitudinal axis FIG 6 shows a 1 x8 optical coupler according to the present invention The input optical waveguide and the output optical waveguides are parallel to the longitudinal axis, and the interval between adjacent output optical waveguides is 250 m Each Y-junction optical waveguide module has a length (LJ of 1700μm, a half width (PJ of 14 8μm, and a branching angle (ΘJ of 1 028° The radius of curvature of the connection optical waveguide module is R=50mm The length (from I to EJ of the optical coupler is determined by the outermost path, which is 15mm This is shorter than the length (22mm) of an optical coupler according to the prior art FIG 7 shows the comparison of the length of a 1 x32 optical coupler according to the present invention with the length of a conventional 1 x32 optical coupler Here, (a) is the case of a conventional optical coupler having a length of 67mm, and (b) is the case of an optical coupler according to the present invention having a length of 31 mm In the present invention, a PA optical waveguide is used as the connection optical waveguide module, which is formed by replacing the start and end points of a circular arc optical waveguide by polynomial curve waveguides (hereinafter, referred to as P-curve waveguides) The P-curved optical waveguide, which has been proposed by F Ladoucer, can change the curvature along the path of the optical waveguide FIG 8 shows part of the P-curved optical waveguide As shown in FIG 8, in order to prevent a curved waveguide 802 connecting two straight line optical waveguides 800 and 804 from having transition loss, the coordinates, inclination and curvature of the first optical waveguide 800 should be consistent with those of the curved optical waveguide 802 at the junction (A) between the end point of the first straight line optical waveguide 800 and the start point of the curved optical waveguide 802 Also, the coordinates, inclination and curvature of the curved optical waveguide 802 must be consistent with those of the second straight line optical waveguide 804 at the junction (B) between the end point of the curved optical waveguide 802 and the start point of the second straight line optical waveguide 804 In order to apply the above-described boundary conditions, a path vector R(t) is expressed by the following polynomial 12 with respect to a parameter (t) R(l) = =(ή ϊ7^ x(t) u χ ...(12) wherein it and ?7 denote the unit vectors of the Z axis and the X axis, respectively. Since the above-described boundary condition provides 6 boundary conditions for each coordinate, the polynomial 12 is preferably a fifth degree polynomial. Accordingly, z(t) and x(t) are obtained by the following Equation 13: n=0 (13) x( = ∑ b n t" n=0 When t is zero at the junction A, and t is L at the junction B, the following algebraic Equation 14 is obtained from the above-described boundary conditions: Here, dotting over each term in the right matrix indicates differentiation with respect to (t), and each term is shown by the following Equation 15: z/=Z p i^cos β z f =-xsm j - wherein x, and x f are curvatures of the start and end points of the circular arc optical waveguide, respectively a 0 through a 5 can be obtained from Equations 14 and 15 b 0 through b 5 are also obtained by the same method, as shown in the following Equation 17 A PA optical waveguide is a curved optical waveguide formed by connecting the start and end points of a circular optical waveguide in the shape of a P-curve FIG 9 shows a PA optical waveguide and a circular arc optical waveguide having a radius of curvature (R) of 50000μm and an inside angle of 90° Here, a dotted line indicates the circular arc optical waveguide, and a solid line indicates the PA optical waveguide The boundary condition at the junctions A and B is determined by Equation 15, as in the following Equation 18: r =0, =cosθ = l , z =- κ sinθ =0 x =0, x =sinθ =0, =κ cosθ =0 (18) r =50000, _y=cosθ =0, Jr^-κ^inθ^O x.= 50000, ,=sinθ,= l , x,=κ £osθ =0 Since L is equal to Rxθ, z(t) and x(t) are determined from Equations 13, 16 and 17, as in the following Equation 19: z(t) = t+5.936 χ l0- π t 3 -3.198 χ l 0- ]5 t 4 +2.154 χ l0- 20 t 5 , 0<t<7.854 χ l0 4 x(t) = 3.836 χ ] 0 " 10 t 3 - 5.262 χ l 0- 15 t 4 +2.154 χ l 0- 20 t 5 , 0<t<7.854><10 4 ' " * ' The PA optical waveguide shown in FIG. 9 is obtained by drawing the trajectory of the path vector R(t) of Equation 12 while increasing the parameter t of Equation 19 from the start point of a connection optical waveguide, that is, 0, to L (7.854x10 4 ). The optical coupler is manufactured by fabricating a patterned mask including an input optical waveguide, a plurality of output optical waveguides, and Y-junction optical waveguide modules and connection optical waveguide modules having positions and lengths designed as above, and forming the mask pattern on a substrate. Industrial Applicability In the present invention, an optical coupler is manufactured using Y- junction optical waveguide modules and connection optical waveguide modules, such that various Y-junction optical waveguide structures can be designed. Also, the degree of freedom upon layout of modules is increased, so that design of the optical coupler is easy. Therefore, an optical coupler which is shorter than a conventional optical coupler can be manufactured, which increases the yield.

Claims: What is claimed is 1 An optical coupler including an input optical waveguide and N output optical waveguides, for dividing an optical signal received from the input optical waveguide into N optical signals and outputting the N optical signals to the output optical waveguides, the optical coupler comprising a plurality of Y-junction optical waveguides configured in m stages, for branching the received optical signal into two optical signals at each stage, and a plurality of curved optical waveguides alternately connected to the Y- junction optical waveguides, at least one curved optical waveguide connected between Y-junction optical waveguides in the m-th stage and the output optical waveguides, wherein when the guiding direction of the optical signal is set to be the longitudinal axis, and the junction between the input optical waveguide and the first-stage Y-junction optical waveguide is set to be the starting point, the positions and sizes of the Y-junction optical waveguides and the curved optical waveguides are determined in order to minimize the longest path among N paths ranging from the starting point to each of the output optical waveguides 2 The optical coupler of claim 1 , wherein each of the Y-junction optical waveguides has one inlet and two outlets, and the structure of the Y- junction optical waveguides is determined by the length and width of each of the Y-junction optical waveguides and the branching angle between two outlets 3 The optical coupler of claim 1 , wherein the trajectory of each of the curved optical waveguides is determined by the radius of curvature and inside angle of each of the curved optical waveguides 4 The optical coupler of claim 3, wherein the trajectory of each of the curved optical waveguides is formed according to values obtained from longitudinal and lateral axis polynomials having coefficients, which satisfy the predetermined boundary conditions to prevent the transition loss of an optical signal from occurring at the junctions of the curved optical waveguides, while changing the parameter value of the polynomials from zero to the length value of each of the connection optical waveguides 5 A method of designing a pattern mask for an optical coupler including an input optical waveguide, N output optical waveguides, a plurality of Y-junction optical waveguides configured in m stages for branching an optical signal received by the input optical waveguide into two optical signals at each stage, and a plurality of curved optical waveguides alternately connected to the Y-junction optical waveguides, at least one curved optical waveguide connected between Y-junction optical waveguides at the m-th stage and the output optical waveguides, and manufacturing the optical coupler according to the mask pattern, the method comprising the steps of (a) determining the lengths, widths and branching angles of the Y- junction optical waveguides, and forming Y-junction optical waveguide modules having sizes depending on the determined lengths, widths and branching angles of the Y-junction optical waveguides, (b) forming connection optical waveguide modules having sizes depending on the radius of curvature and inside angle of each of the curved optical waveguides, for connecting the Y-junction optical waveguides to each other and connecting the m-th Y-junction optical waveguide modules to the output optical waveguides, (c) arranging the Y-junction optical waveguide modules and the curved optical waveguide modules between the input optical waveguide and each of the output optical waveguides, after determining the lengths of the connection optical waveguide modules and the positions of the Y-junction optical waveguide modules and the connection optical waveguide modules so that the longitudinal distance between the starting point and each of the output optical waveguides is minimized, when the junction between the input optical waveguide and the first-stage Y-junction optical waveguide is set to be the starting point, and the guiding direction of the optical signal is set to be along the longitudinal axis, and (d) manufacturing the pattern mask according to the arranged structure 6 The method of claim 5, wherein the step (c) comprises the substeps of determining the radius of curvature of each of the connection optical waveguide modules, and obtaining the lengths of the connection optical waveguide modules from the determined radii of curvature and the unknown inside angle of each of the connection optical waveguide modules, obtaining the longitudinal and lateral distances from the starting point to each of the output optical waveguides, from the length of each of the Y- junction optical waveguide modules and the length of each of the connection optical waveguides, obtaining the inside angles which minimize the longitudinal distances while satisfying a predetermined restriction condition, when the lateral distances to each of the output optical waveguides are determined according to the interval between adjacent output optical waveguides, and obtaining the position of each of the Y-junction optical waveguide modules, and the position and length of each of the connection optical waveguide modules from the obtained inside angles 7 The method of claim 6, wherein the restriction condition is that the sum of the branching angles of Y-junction modules and the inside angles of the connection optical waveguide modules included in each of the paths ranging between the starting point and each of the output optical waveguides is zero on each of the paths 8 The method of claim 7, wherein the step of calculating the inside angles comprises obtaining a plurality of inside angle sets which satisfy the minimization step and determining an inside angle set which minimizes the longest path among paths obtained from the inside angle sets 9 The method of claim 8, wherein the layout of the connection optical waveguide modules includes the step of laying out the connection optical waveguides so that the connection optical waveguides satisfy a second restriction condition which minimizes the transition loss of light which passes through the junction of each of the connection optical waveguide modules 10 The method of claim 9, wherein the second restriction condition includes the coordinate match, the tangent inclination match, and the curvature match between the inlet and outlet of each module connected at each of the junctions 11 The method of claim 10, wherein the trajectory of each of the connection optical waveguides is determined according to values obtained from longitudinal and lateral polynomials having coefficients, which are determined by the second restriction condition at the junctions of each of the connection optical waveguides, while changing the parameter value of the polynomials from zero to the length value of each of the connection optical waveguides

Inventors: CHO JUNG-HWAN, KIM DUG-BONG, RHEE TAE-HYUNG, YI SANG-YUN, CHO, JUNG-HWAN, KIM, DUG-BONG, RHEE, TAE-HYUNG, YI, SANG-YUN
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: G02B6
IPC codes: G02B6