JP2006293171A - Optical branching circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make the entire length of waveguides substantially equalized regardless of the route, as well as, reducing the entire length, in an optical branching circuit using Y-branching waveguides. <P>SOLUTION: The optical branching circuit 1 is provided with mutually parallel and linear incident waveguide 10 for guiding light and a plurality of emitting waveguides 14, arranged with prescribed specific spaces p apart. The route, for guiding light from the incident waveguide 10 to each emission waveguide 14, is structured with three steps connecting a first, second and third Y-branching waveguides 21-23, in the order starting from the incident waveguide 10 end, which forms eight branching circuits. The Y-branching waveguides 21-23 are each provided with pre-branching linear waveguides 11-13 before the branching, wherein the waveguide, immediately after the branched part has a curve comprising a circular arc with a prescribed radius of curvature. Also, the first and the third Y-branching waveguides 21, 23 are arranged, with the pre-branching linear waveguides 11, 13 are set parallel with respect to the incident waveguide 10, while the second Y-branching waveguide 22 is arranged, with the pre-branching linear waveguide 12 inclined with respect to the incident waveguide 10. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical branch circuit using a Y branch waveguide.

  2. Description of the Related Art Conventionally, an optical branching circuit using a Y-shaped branching waveguide (Y branching waveguide) is known as means for branching input signal light into a plurality of parts when performing optical communication in an optical communication system or the like. An example of a conventional optical branch circuit is shown in FIG. The optical branching circuit 91 uses the Y branching waveguides 93 connected in four stages to branch the light input to the incident waveguide 10 into 16 branches and output them to the output waveguide 14. The incident waveguide 10 and the outgoing waveguide 14 are parallel to each other, and the outgoing waveguides 14 are arranged at equal intervals. Each Y-branch waveguide 93 is branched symmetrically, and its symmetrical central axis is also arranged parallel to the incident waveguide 10 and the outgoing waveguide 14. Each Y-branch waveguide 93 is branched into two by smoothly connecting a waveguide made of an arc to the pre-branch straight waveguide 93a before branching, and an arc having a curvature opposite to each of the two branched arcs is smoothly smoothed. A connected S-shaped waveguide is formed and connected to the pre-branch straight waveguide 93b in the next stage. In this optical branching circuit 91, the length of the waveguide from the incident waveguide 10 to each outgoing waveguide 14, that is, the total length of the waveguide for each branched light is the same. This is based on the symmetry of the optical branch circuit 91. In general, in an optical branch circuit, the optical waveguides are synchronized with each other, so that the total length of the waveguide for each branched light is the same.

  As described above, the curved portion of each Y-branch waveguide 93 is an S-shaped curve that combines arcs having a predetermined radius of curvature, and its shape depends on the radius of curvature of the arc and the spacing between the waveguides after branching. Determined. The total waveguide length is the sum of the incident waveguide length, the outgoing waveguide length, the S-shaped waveguide length, and the straight waveguide length before branching.

The total waveguide length is designed to be as short as possible in order to minimize the loss of light due to the waveguide material, and the overall length of the optical branch circuit is also shortened. For example, it is conceivable to shorten the overall length by reducing the radius of curvature of the waveguide curve portion. However, there is a limit as long as the waveguide length is the same for any waveguide. Therefore, as shown in FIG. 14, the number of branches required is reduced by using a three-branch waveguide 94 that branches three times at a time in place of the Y-branch waveguide. A branch circuit 92 has been proposed (see, for example, Patent Documents 1 and 2).
JP-A-8-271744 JP 2004-227015 A

  However, in the optical branch circuit as shown in FIG. 13 described above, the waveguide extension is long, and in the optical branch circuit using the three-branch waveguide as shown in FIG. There is a problem that the variation in uniformity due to wavelength becomes somewhat large, and when communication is performed in both the upstream and downstream directions, the loss in the upstream direction varies depending on the route.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems and to provide an optical branch circuit using a Y-branch waveguide in which the total length of the waveguide is substantially equal regardless of the path and the total length of the waveguide is shortened.

  In order to achieve the above object, an invention according to claim 1 includes linear incident waveguides parallel to each other for guiding light and a plurality of output waveguides arranged at a predetermined interval, and In an optical branch circuit in which a path for guiding light from a waveguide to each of the output waveguides is configured by connecting multi-stage Y branch waveguides, the Y branch waveguide is first, second from the incident waveguide side in order. , And a third Y-branch waveguide, each Y-branch waveguide has a pre-branch straight waveguide before branching, and the waveguide immediately after the branch has a predetermined radius of curvature. The first and third Y-branch waveguides are made of a curve made of an arc, and the straight branch waveguides before branching are arranged in parallel to the incident waveguides, and the first Y-branch waveguides are their Y-branches. An arc-shaped first having a predetermined radius of curvature smoothly connected to a branch point of the waveguide The second Y-branch waveguide has an arc waveguide, and the pre-branch straight waveguide consisting of a straight line of a predetermined length smoothly connected to the first arc waveguide, and the pre-branch straight waveguide An arc-shaped second arc waveguide having the same radius of curvature and the same bending direction as the first arc waveguide, and one end smoothly connected to the second arc waveguide, and An arc-shaped third arc waveguide having the other end smoothly connected to the pre-branch straight waveguide of the third Y-branch waveguide and having the same radius of curvature and a different bending direction as the second arc waveguide; In this way, an 8-branch circuit is formed by placing the pre-branch straight waveguide inclined with respect to the incident waveguide.

  According to a second aspect of the present invention, in the optical branch circuit according to the first aspect, each of the Y branch waveguides is formed by a straight waveguide in the vicinity of the branch portion. In addition, the straight waveguide after the branching is formed so as to smoothly transition to a curved waveguide formed of an arc having a predetermined radius of curvature.

  According to a third aspect of the present invention, in the optical branch circuit according to the first or second aspect, the third Y branch waveguide connected to the inner waveguide after branching of the second Y branch waveguide is: The straight waveguide before branching is inclined with respect to the incident waveguide and disposed closer to the incident waveguide.

  According to a fourth aspect of the present invention, in the optical branch circuit according to the first or second aspect, the inner waveguide after branching of the second Y-branch waveguide is extended until it becomes parallel to the incident waveguide. The straight waveguide before branching of the third Y branching waveguide is connected to the stretched waveguide.

  According to a fifth aspect of the present invention, in the optical branch circuit according to the fourth aspect, a straight waveguide length before branching of each of the Y branch waveguides is set to a predetermined length and is connected to the stretched waveguide. The pre-branch straight waveguide of the third Y branch waveguide is formed longer than the predetermined length.

  The invention of claim 6 is an optical branch circuit in which a circuit having 16 branches or more is formed by using the 8-branch optical branch circuit according to any one of claims 1 to 5 as a part of the circuit.

  Since the optical branch circuit having such a configuration inclines the pre-branch straight waveguide of the second Y branch waveguide, the second Y branch waveguide is disposed without going through the S-shaped waveguide portion. Therefore, the total length of the waveguide and the length of the optical branch circuit can be shortened, and an optical branch circuit with reduced insertion loss can be realized.

  Hereinafter, an optical branch circuit using a Y branch waveguide according to an embodiment of the present invention will be described with reference to the drawings. 1 shows an optical branching circuit 1 of the present invention, FIG. 2 (a) shows an optical branching circuit 90 as an example of the prior art, and FIG. 2 (b) shows an optical branching circuit 1 and a conventional optical branching circuit 90. Shown in comparison. As shown in FIG. 1, the optical branch circuit 1 includes linear incident waveguides 10 that are parallel to each other for guiding light and a plurality of output waveguides 14 that are arranged at a predetermined constant interval p. A path for guiding light from the waveguide 10 to each output waveguide 14 has a three-stage configuration in which the first, second, and third Y branch waveguides 21, 22, and 23 are connected in order from the incident waveguide 10 side. Thus, an 8-branch circuit is formed.

  Each of the Y branch waveguides 21, 22, and 23 has pre-branch straight waveguides 11, 12, and 13 before branching, and the waveguide immediately after the branch portion is formed by a curve formed by an arc having a predetermined curvature radius. . The first and third Y branch waveguides 21 and 23 are arranged so that the straight waveguides 11 and 13 before branching are parallel to the incident waveguide 10, and the second Y branch waveguide 22 is before branching. The straight waveguide 12 is disposed so as to be inclined with respect to the incident waveguide 10.

  The waveguide constituting the optical branch circuit 1 described above is formed by a clad and a core having different refractive indexes. In the present specification, a core portion having a linear shape as viewed macroscopically, which is a main component for guiding light, is illustrated as a path for guiding light. Further, the waveguide is composed of a path in which straight lines and curves are smoothly connected as described above, and a point on the path is referred to by appropriately adding a symbol a, b, c, etc. to a geometric change point on the path. To do. The direction is referred to with the direction from the incident waveguide 10 toward the output waveguide 14 as the x direction and the direction within the waveguide formation plane and perpendicular to the x direction as the y direction.

  The optical branch circuit 1 has a symmetric structure with respect to the incident waveguide 10 (which is also the pre-branch straight waveguide 11 of the first Y branch waveguide 21) and the incident optical axis 3 which is an extension thereof. . In this specification, the side closer to the incident optical axis 3 is referred to as the inner side, and the far side is referred to as the outer side. The distance W between the straight waveguide 13 before branching of the third Y-branch waveguide 23 and the incident optical axis 3 in the outermost waveguide is referred to in the following description.

  Here, looking at the 8-branch optical branch circuit 90 according to the conventional method of constructing the waveguide, as shown in FIG. 2A, secondly, the straight waveguide 12 before branching of the Y-branch waveguide 22 is incident and guided. It is parallel to the waveguide 10. When the optical branch circuit 1 and the conventional optical branch circuit 90 are overlapped, as shown in FIG. 2B, it can be seen that the optical branch circuit 1 is shorter by the length ΔX. This is the effect of tilting the pre-branch straight waveguide 12 of the second Y branch waveguide 22 in the optical branch circuit 1.

  That is, unlike the conventional optical branch circuit 90 shown in FIG. 2A in which the second Y branch waveguide 22 is connected via the S-shaped waveguide 15, the optical branch circuit 1 By tilting the pre-branch straight waveguide 12, the second Y branch waveguide 22 can be connected to the first Y branch waveguide 21 without going through the S-shaped waveguide portion. Thereby, the total waveguide length and the optical branch circuit length can be shortened, and an optical branch circuit with reduced insertion loss can be realized. Such an effect is that the two S-shaped waveguides 15 and 16 in the outermost waveguide shown in FIG. 2A are located in the vicinity of the second Y-branch waveguide 22 in the optical branch circuit 1. This is because they are combined into one S-shaped waveguide.

  Next, further shortening of the total waveguide length of the optical branching circuit 1 from the viewpoint of the above-described S-shaped waveguide will be described by further adding FIG. FIG. 3 shows a third Y-branch waveguide 23. As shown in FIG. 2A, the conventional optical branch circuit 90 using the waveguide pattern is such that the center line of the Y branch waveguide, and hence the straight waveguide before branching, is always parallel to the incident waveguide 10. ing. For this reason, it is necessary to return using the S-shaped curved waveguide until the branched curved waveguide becomes parallel to the incident waveguide 10, resulting in a longer waveguide. Accordingly, the center line of the Y branch waveguide, that is, the straight branch waveguide before branching is not parallel to the incident waveguide 10, and the Y branch waveguide is arranged while being inclined, thereby shortening the total length of the waveguide, It is considered possible to shorten the length of the optical branch circuit.

  First, the first Y-branch waveguide 21 is the first branch, so it is not necessary to incline it and keep the shape as before. Next, the third Y-branch waveguide 23 is the last branch in an eight-branch circuit. Therefore, even if it is tilted, the lengths of the two paths after branching are as shown in FIG. The outer path 23a becomes longer than the inner path 23b. When trying to eliminate this difference in length, on the contrary, the waveguide length becomes longer than that in the case of symmetry as shown in FIG. From the above, in the 8-branch circuit, the overall length can be shortened with a pattern in which the pre-branch straight waveguide 12 of the second Y-branch waveguide 22 is inclined with respect to the direction of the incident waveguide 10. . Note that tilting the pre-branch waveguide means tilting the entire Y branch waveguide. This is because the two waveguides after branching need to be symmetric with respect to the straight waveguide before branching in order to maintain the quality of guided light.

  In a general optical branch circuit, as shown in FIG. 2A, the intervals between the output waveguides 14 are arranged at a predetermined constant interval. Here, it is assumed that the incident waveguide 10 coincides with the middle position of the fourth and fifth waveguides from the outside of the eight output waveguides 14 with respect to the y-direction position of the output waveguide 14. Considering the paths from the incident waveguide 10 to the eight outgoing waveguides 14, the moving distance in the x direction is the same, and the moving distance in the y direction is the maximum for the outermost outgoing waveguide 14. Therefore, it is important to shorten the path to the outermost output waveguide 14 as much as possible.

  Next, the structure of the inclined Y-branch waveguide will be described by further adding FIGS. 4 (a), 4 (b), and 5. FIG. FIG. 4A shows a state in which the waveguide paths L1 and L2 that are parallel to each other with a distance D are connected in a shortest and smooth manner by an S-shaped curve with an arc having the same curvature, and FIG. FIG. 5 shows how the second Y-branch waveguide 22 is formed by inserting a straight waveguide into the arc-shaped waveguide portion of the waveguide shown in FIG. Show shape. In consideration of the fact that the third Y-branch waveguide 23 is formed in parallel to the incident waveguide 10, the overall length of the waveguide is shortened from the point b on the outermost waveguide shown in FIG. This is done so as to minimize the route to the point f. When the second Y-branch waveguide 22 is inclined, as shown in FIG. 1, the points ab, cd, fg, and hi are straight (the straight waveguide 11 before branching). , 12, 13 and the output waveguide 14, which are hereinafter also referred to as straight line portions 11 to 14, respectively), and between the points bc, df, and gh are arcuate curves. The total waveguide length is formed by a combination of these.

  The waveguide elements related to the second Y-branch waveguide 22 are represented by a combination of symbols such as a and b indicating geometric change points on the waveguide, and the configuration of the waveguide will be described in more detail. . The first Y-branch waveguide 21 has an arc-shaped first arc waveguide bc having a predetermined radius of curvature (referred to as R) smoothly connected to the branch point b of the Y-branch waveguide. The second Y-branch waveguide 22 is smoothly connected to the straight straight waveguide cd (12) before branching having a predetermined straight line length L and smoothly connected to the first arcuate waveguide bc, and to the straight waveguide bc before branching. An arc-shaped second arc waveguide de having the same radius of curvature R and the same bending direction (convex inward) as the first arc waveguide bc, and one end smoothly connected to the second arc waveguide de And the other end is smoothly connected to the pre-branch straight waveguide fg of the third Y-branch waveguide 23 and has the same radius of curvature R and a different bending direction (convex outward) as the second arcuate waveguide de. An arc-shaped third arc waveguide ef. Point e is an inflection point, and second arc waveguide de and third arc waveguide ef form an S-shaped curve.

FIGS. 4A and 4B show the concept of determining the above-described waveguide elements (first arc waveguide bc, etc.), which will be described. In FIG. 4 (a), an S-shaped curve be-f is composed of a part of two circles having a radius R in contact with each other, and smoothly connects parallel linear portions L1 and L2 with a distance D therebetween. Yes. At this time, the tangent slope β at the inflection point e of the S-shaped curve is determined as β = atan ((4RD−D ² ) ^1/2 / (2R−D)). Further, the x-direction linear distance Sx of the curved portion from the point b to the point f is determined as Sx = (4RD−D ² ) ^1/2 .

  Here, the arc be is divided at the point d on the arc curve be shown in FIG. 4 (a) (the angle of the tangent is θ), and the tangent at the point d as shown in FIG. 4 (b). A curved portion including the point b is translated by a distance L along the direction. Assuming that the straight portions L1 and L2 in FIG. 4A are the straight portions 12 and 13, respectively, and the point separated from the point d is a new point c, the points b and c from the straight portion 12 shown in FIG. , D, e, and f, the path from the straight line portion 12 to the straight line portion 13 of the waveguide shown in FIG. In order to determine this route, parameters such as the straight line length L and the inclination angle θ must be determined. Note that the point d becomes a branch point of the second Y-branch waveguide 22, and the waveguide from the point e to the point u branches.

  In FIG. 1, the parameters for determining the above-described path are the straight line length L of the straight line portion 12 between the points cd, the inclination angle θ of the straight line portion, the straight line portion 13 between the points bb and the incident waveguide 10. The distance W from the extension line (incident optical axis 3) and the radius of curvature R of each arc curve.

  Of these parameters, the distance W is determined by W = 3 × p from the interval p of the output waveguide 14. The radius of curvature R is determined as the minimum radius of curvature that causes no bending loss from the cross-sectional shape of the waveguide and the difference in refractive index. The straight line length L of the straight line portion 12 is such that the ratio of the light branching in the second Y-branch waveguide 22 connected to the straight line portion 12 is stably 1: 1, so that the light incident on the branch portion is For the purpose of dampening the vibration, the length is a predetermined length or more. Therefore, the straight line length L is determined as the minimum necessary length. Therefore, if the remaining inclination angle θ is determined so as to minimize the path from point b to point f, the overall shape of the waveguide is determined.

  For example, if the parameter is p = 250 μm, W = 750 μm. Therefore, the inclination angle θ that minimizes the x-direction length is determined under the condition that the y-direction displacement of the point on the waveguide is 750 μm from the point b to the point f. Here, the displacement in the y direction is decomposed into y components Ly and Ly = L · sin θ of the straight line cd (straight line portion 12) and y components of the curves bc and df. The curves bc and df are divided by the straight line cd, but the slopes at the points c and d are the same. If these curves bc and df are considered to be integrated, FIG. ), An S-shaped curve be-f passes through the inflection point e.

As shown in FIGS. 4A and 4B, the distance W is expressed as W = L · sin θ + D. Further, the x-direction length Sx of the S-shaped curve bef is obtained analytically,
Sx = (4RD-D ² ) ^1/2
= (4 (W · L · sin θ) R− (W · L · sin θ) ² ) ^1/2
It becomes. Further, the length Lx in the x direction of the straight line cd is
Lx = L · cos θ
It is. Therefore, both Sx and Lx decrease as the inclination angle θ increases, and their sum (Sx + Lx) also decreases (shortens) as the inclination angle increases. It will be explained that Sx decreases as θ increases. When θ increases, L · sin θ increases, and D = W−L · sin θ decreases. Here, if Sx is deformed, Sx = (4R ² − (2R−D) ² ) ^1/2 , and therefore, (2R−D) ² increases due to a decrease in D, and Sx decreases. become. If attention is paid only to the x direction in this way, the distance from the point b to the point f can be shortened as the inclination angle θ is increased and the second Y-branch waveguide 22 is inclined more greatly. However, the inclination angle θ has an upper limit, which will be described with reference to FIG.

  Here, regarding the second Y-branch waveguide 22, conditions for the shape of the branched waveguide, which generally come from considering the characteristics of light at the time of branching in the Y-branch waveguide, will be described. As shown in FIG. 5, the curved portion of the second Y-branch waveguide 22 has a shape in which the straight portion 12 having a length L enters the middle of the S-curve and the S-curve is divided. In FIG. 5, the radius of curvature of the arc forming each curved portion is R, and the direction of the center of curvature is indicated by an arrow Q. The waveguide of the straight line portion 12 is branched by two circular arc curves connected to the branch point d. The two waveguides branched in this way keep the optical signal branching ratio at one unit 1 from the branching point d to a predetermined distance. 12 must be symmetrical. The predetermined distance is a distance that is separated to an interval δ at which light propagating through the two waveguides does not affect each other.

  Up to the point e where the distance between the two waveguides becomes δ, the arc shape is convex on the inside (incident optical axis 3 side), and this point e is the inflection point, and the arc convex on the outside is the boundary. The shape is changed and the inclination of the straight portion 12 is returned. When the waveguide has such a shape, the maximum inclination of the curved portion is not increased more than necessary. When the interval between the two branched waveguides is opened beyond the interval δ, the maximum inclination angle of the curved portion is increased, and the length in the x direction up to the point f is extended (described later). Therefore, the inflection point e which gives the maximum inclination angle of the curved portion is determined as the point where the interval between the two branched waveguides becomes δ as described above.

As shown in FIG. 5, the angle α, which is an increase in the inclination angle of the tangent line of the curve de, from the point d to the point e on the waveguide, and the tangent angle of the curved portion at the inflection point e. A maximum tilt angle β is defined. The angle α is calculated using R and δ.
α = atan ((4Rδ−δ ² ) ^1/2 / (2R−δ)) (1)
It is expressed. In addition, the angle β is determined using R and D,
β = atan ((4RD-D ² ) ^1/2 / (2R-D)) (2)
It is expressed. In addition, the inclination angle θ is a relational expression of α, β, θ,
β = θ + α (3)
And a relational expression of D, W, L, θ,
D = W−L · sin θ (4)
And solving the simultaneous equations consisting of four equations (1) to (4) to which the above equation is added.

  The inclination angle θ obtained based on the above formulas (1) to (4) and specific numerical values such as R = 50 mm, p = 250 μm, W = 750 μm, L = 3 mm, and δ = 100 μm is θ≈3. .5 degrees. Further, the total length of the optical branching circuit 1 based on these numerical values is about 27.6 mm. Since the total length of the optical branch circuit 90 using the conventional pattern is about 34.1 mm, it can be shortened by about 7 mm.

  Next, with reference to FIGS. 6A, 6B, and 6C, the total waveguide length of the outermost waveguide of the optical branch circuit according to the inclination of the straight waveguide before branching of the second Y branching waveguide 22 is large. Qualitatively explain the impact on 6 (a), 6 (b), and 6 (c), when the inclination angles θ1, θ2, and θ3 of the pre-branching straight waveguide 12 of the second Y branching waveguide 22 are too small, the optimum Indicates a case that is too large. Assuming that the radius of curvature R of the arc forming the curved portion of the waveguide is constant, the distance required to separate the waveguide interval after branching by a predetermined interval δ (not shown) depends on the inclination angles θ1, θ2, and θ3. Without being constant. Accordingly, the inclination of the tangent line at the inflection point e of the S-shaped curve is larger than the inclination angle of the straight line portion 12 by the angle α. Here, the relationship of the inclination angle of the straight line portion 12 is θ1 <θ2 <θ3, and the inclination of the tangent at the inflection point e increases in the order of (θ1 + α) <(θ2 + α) <(θ3 + α). After the inflection point e, the waveguide inclination is returned so as to be parallel to the incident optical axis 3, but the length in the x direction necessary for the return becomes longer according to the inclination angle. Assuming that the point returned to parallel is the point f1, the length of the point e-f1 becomes longer as the inclination angle becomes larger.

  That is, as shown in FIG. 6A, in the case of the inclination angle θ1, the length of the point e−f1 is the shortest, but the condition that “the position of the point f1 is the distance W from the incident optical axis 3” is satisfied. Since this condition is not satisfied, an extra S-curve is required to satisfy this condition, and the entire waveguide length becomes longer than that in the case of the inclination angle θ2. In the case of a large inclination angle, as shown in FIG. 6C, the inclination angle θ3 is too large to satisfy the condition that “the position of the point f1 is the distance W from the incident optical axis 3”. After all, an extra S-shaped curve is required. As a result, as shown in FIG. 6B, the optimum inclination angle is the case of the inclination angle θ2 where no extra S-curve is added. In this case, the point f1 is coincident with the point f.

  In FIG. 6A, the waveguide indicated by the dotted line passing through each point e-f2-f-g connects the straight line e-f2 to the inflection point e to ensure a movement distance in the y direction. The distance W is secured together with the subsequent circular arc curve f2-f. Thus, by connecting the straight waveguide portion to the inflection point e, a waveguide path shorter than the path shown by the solid line in FIG. 6A can be formed.

  Next, referring to FIGS. 7 and 8 in addition to FIG. 1, the structure of the Y branch waveguide in the optical branch circuit 1 of the present invention will be described. The optical branch circuit 1 includes a plurality of Y branch waveguides 21, 22, and 23 as shown in FIG. As shown in FIG. 7, these Y branch waveguides 21, 22, and 23 are formed by straight waveguides in which the waveguides 2b and 2b branch from the pre-branch straight waveguide 2a are formed in the vicinity of the branch portion. The branched straight waveguide is formed so as to smoothly transition to a curved waveguide formed by an arc having a predetermined radius of curvature.

  Here, the manufacture of the waveguide will be described. The waveguide of the optical branch circuit 1 is manufactured using, for example, a lithography technique. In the case of this manufacturing technique, after a photosensitive resin is applied on a substrate on which a waveguide is to be formed, a part of the resin is cured by exposure and an unnecessary part is removed by development to form a waveguide of the photosensitive resin. In these manufacturing processes, the amount of resin to be removed varies depending on the development time. Therefore, a waveguide having the same shape is not always obtained, and the width (core width) of the waveguide varies. The tip shape of the partial waveguide also changes. In FIG. 7, in the actual shape 41 shown by a broken line, the core width is thicker than the target shape (design shape) 4 of the waveguide core portion shown by a solid line. Even if such a core width variation occurs, when a straight waveguide is used as the branched waveguide as shown in FIG. 7, the actual angle is relative to the target angle γ1 at the tip of the branch. γ2 can maintain the relationship γ2 = γ1. Therefore, the branching angle is constant, and an increase or fluctuation in branching loss or branching ratio (uniformity) can be suppressed.

  On the other hand, as shown in FIG. 8, in the case of a conventional waveguide in which a Y-branch waveguide is formed using a circular arc curve, when the core width fluctuates, the actual angle γ2 with respect to the target angle γ1 at the tip of the branching portion. Fluctuate and γ2 ≠ γ1. That is, due to manufacturing variations in the development process, the branch angle in the Y-branch waveguide changes, and there is a possibility that fluctuations in branching loss and branching ratios to each waveguide may occur.

  As described above, since the waveguide branched from the straight waveguide before branching is formed by the straight waveguide in the vicinity of the branched portion, the angle between the straight lines, that is, the branch angle in the Y-branch waveguide portion becomes constant. Therefore, it is possible to suppress a branch loss and a branch ratio fluctuation caused by an inappropriate branch angle. This is because when the branching angle is composed of an angle sandwiched by curves, the branching angle varies due to fluctuations in the radius of curvature of the curve caused by process condition variations in the waveguide manufacturing process. This branching angle is due to the fact that the radius of curvature of the straight line is infinite and is stable against process condition fluctuations in the waveguide manufacturing process.

  Next, an optical branching circuit 1 according to another embodiment of the present invention will be described with reference to FIGS. In the optical branching circuit 1 shown in FIG. 9, the third Y branching waveguide 23 connected to the inner waveguide after the branching of the second Y branching waveguide 22 is incident on the straight waveguide 13 before branching. It is inclined with respect to the waveguide 10 and is disposed closer to the incident waveguide 10. In the optical branch circuit 1 having such a structure, it is possible to prevent the entire length of the inner waveguide from becoming longer than the outer side. Therefore, the total length of the optical branch circuit 1 is shortened, and the insertion loss of the optical branch circuit 1 is reduced. be able to.

  Further explanation will be given. In the description of the optical branching circuit 1 up to FIG. 8 described above, the total length is shortened so that the total length of the waveguide reaching the outermost output waveguide 14 is shortened. This is based on the premise that since the outermost output waveguide 14 is displaced in the y direction more, the length in the x direction is required to be shorter than the inner output waveguide 14.

  However, as shown in FIG. 10, if the second Y-branch waveguide 22 is tilted and the path to the outermost output waveguide 14 is shortened preferentially, the path to the inner output waveguide 14 is not necessarily the same. It is not always shorter than the outermost route. When the inclination angle of the second Y-branch waveguide 22 is large, an extra S-curve is inserted in the path to the inner waveguide as shown in a circled area A. This leads to the waveguide length. Becomes longer. Then, the start position of the third Y-branch waveguide 23 is shifted backward from the start position of the third Y-branch waveguide 23 in the outermost path. Therefore, even if the outermost path is shortened, the effect of shortening the entire length cannot be obtained due to the influence of the inner path length.

  Therefore, as shown in FIG. 9, the third Y-branch waveguide 23 of the inner path is also inclined so that the inner path is not longer than the outermost path. In the optical branch circuit shown in FIG. 10, the S-shaped curve and the subsequent straight line portion existed independently. However, in the optical branch circuit 1 shown in FIG. After 23, there is a part where the interval is widened by a curve. Therefore, by adjusting the height in the y direction after the third branch, generation of an extra S-curve part can be prevented. By adopting such a waveguide shape for the optical branch circuit 1, it is possible to prevent the inner waveguide length from becoming longer than the outer side, shorten the overall length of the optical branch circuit 1, and reduce insertion loss. Can be made.

  Next, an optical branch circuit 1 according to still another embodiment of the present invention will be described with reference to FIGS. The optical branch circuit 1 shown in FIG. 11A is stretched until the inner waveguide after branching of the second Y-branch waveguide 22 becomes parallel to the incident waveguide 10, and the stretched waveguide 13 is In this example, the pre-branch straight waveguide 13 of the three Y branch waveguides 23 is connected. In such an optical branch circuit 1, since it is possible to prevent the entire length of the inner waveguide from becoming longer than the outer side, the total length of the optical branch circuit 1 can be shortened, and the insertion loss of the optical branch circuit 1 can be reduced. Can do.

  The optical branch circuit 1 has the same structure as the optical branch circuit 1 shown in FIG. 9 so that the inner path is not longer than the outermost path. A straight waveguide portion is provided after the curve after the second Y-branch waveguide 22 is parallel to the incident optical axis 3, and the interval after the curve after the third Y-branch waveguide 23 is widened in the y direction. By doing so, the S-curve portion can be omitted. However, in this case, the position in the y direction of the straight waveguide 13 before branching does not coincide with the intermediate position between the two subsequent output waveguides, and therefore the path to the output waveguide after the third branch waveguide 23. Is not symmetric. However, by adopting the structure of the optical branch circuit 1 as shown in FIG. 11A, it is possible to prevent the inner waveguide length from becoming longer than the outer side, shortening the total length of the optical branch circuit, Insertion loss can be reduced.

  Further, the pre-branch straight waveguide length of each Y branch waveguide of the optical branch circuit 1 is set to a predetermined length, and the optical branch circuit 1 shown in FIG. 11B is shown in FIG. The straight branch waveguide 13 before branching of the third Y branching waveguide 23 connected to the extended waveguide of the optical branching circuit 1 is formed longer than a predetermined length. In such an optical branching circuit 1, there is a situation in which there is a margin for moving the position of the third Y branching waveguide 23 back and forth as in the optical branching circuit 1 shown in FIG. Thus, by making the length of the straight waveguide 13 before branching longer than a predetermined length, it is possible to further suppress the vibration of light incident on the branching portion and reduce the fluctuation of the branching ratio.

  In general, light that has passed through a curved waveguide or a branching portion of the waveguide propagates while vibrating in the waveguide. When light oscillating in this way propagates through the straight waveguide, this vibration can be attenuated to some extent. Further, when the vibrating light is incident on the Y branch waveguide, the branching ratio is changed. Therefore, a straight waveguide is disposed in front of the Y branch waveguide. This makes it difficult for the branching ratio (uniformity) to fluctuate and improves the waveguide characteristics. If attention is paid only to the uniformity, the straight waveguide portion should be as long as possible. However, if the straight waveguide portion is lengthened, the entire length of the waveguide is lengthened, so that the insertion loss is deteriorated. In the case of the optical branch circuit 1 shown in FIG. 11 (b), only the inner waveguide is provided, but the length of the pre-branch straight waveguide of the third Y branch waveguide 23 can be increased. It is possible to stabilize the branching ratio (homogeneity) by attenuating the vibration of light.

  Next, with reference to FIG. 12, a 16-branched optical branching circuit 1 according to still another embodiment of the present invention will be described. This optical branching circuit 1 is formed by using the above-described 8-branching optical branching circuit 1 as a part of a circuit to form a 16-branch circuit. According to such an optical branch circuit 1, since the effect of shortening the total waveguide length and the optical branch circuit length can be obtained in the same manner as the above-described eight-branch optical branch circuit, insertion loss of the optical branch circuit can be reduced. it can. In this 16-branch optical branch circuit 1, a Y-branch waveguide 20 is provided in front of the first Y-branch waveguide in the 8-branch optical branch circuit, and two 8-branch optical splitter circuits are provided in the Y-branch waveguide 20. Connected. In this case, the waveguide branched by the Y branching waveguide 20 is separated by a necessary minimum distance. Further, the inclination of the pre-branching straight waveguide of the second Y branching waveguide 22 is inclined toward the incident waveguide 10 by increasing the inclination toward the outer side as indicated by a circle B and a square C in the drawing.

  The present invention is not limited to the above-described configuration, and various modifications can be made. In the above, examples of 8 branches and 16 branches have been shown. However, according to the present invention, optical branch circuits having other numbers of branches can be configured. Not only a symmetric optical branch circuit with respect to the incident waveguide 10 but also an asymmetric optical branch circuit can be configured.

The top view of the optical branch circuit which concerns on one Embodiment of this invention. (A) is a plan view of a conventional optical branching circuit, and (b) is a plane in which the optical branching circuit shown in FIG. 1 is overlapped for comparison with the conventional optical branching circuit shown in FIG. 2 (a). Figure. The top view of a Y branch waveguide. (A) is a plan view for explaining the dimensional relationship when parallel waveguide paths are connected in a shortest and smooth manner by an S-shaped curve with an arc having the same curvature, and (b) is a plan view in the waveguide shown in FIG. The top view which inserted the linear waveguide in the circular-arc-shaped waveguide part, and formed the Y branch waveguide. The top view explaining the shape of the 2nd Y branch waveguide in the optical branch circuit of this invention. (A) is a plan view of the outermost waveguide of the optical branching circuit when the inclination of the pre-branch linear waveguide of the second Y branching waveguide is too small, and (b) is the optimum inclination of the straight waveguide before branching The top view of the outermost waveguide of the optical branch circuit in the case of (2), (c) is the top view of the outermost waveguide of the optical branch circuit when the inclination of the straight waveguide before branching is too large. The top view explaining the structure of the Y branch waveguide in the optical branch circuit of this invention. The top view explaining the structure of the conventional Y branch waveguide circuit. The top view of the optical branch circuit which concerns on other embodiment of this invention. The top view of the comparative example with respect to the optical branch circuit shown in FIG. (A) is a top view of the optical branch circuit which concerns on further another embodiment of this invention, (b) is a top view of the optical branch circuit which moved the 3rd Y branch waveguide in the same optical branch circuit. The top view of the 16-branch optical branching circuit which concerns on other embodiment of this invention. The top view of the conventional 16 branch optical branch circuit. FIG. 6 is a plan view of a conventional 16-branch optical branching circuit including a branching structure that branches three times at once.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical branch circuit 10 Incident waveguide 11, 12, 13 Pre-branch straight waveguide 14 Outgoing waveguide 20, 21, 22, 23 Y branch waveguide bc, de, ef Arc waveguide

Claims (6)

A path for guiding light from the incident waveguide to each of the output waveguides has a linear incident waveguide parallel to each other for guiding light and a plurality of output waveguides arranged at predetermined intervals. In an optical branch circuit configured by connecting Y branch waveguides in multiple stages,
The Y branch waveguide has a three-stage configuration in which the first, second, and third Y branch waveguides are connected in order from the incident waveguide side, and each Y branch waveguide is linearly guided before branching before branching. The waveguide having a waveguide and immediately after the branch portion is composed of a curve made of an arc having a predetermined radius of curvature,
The first and third Y-branch waveguides are arranged such that the straight waveguide before branching is parallel to the incident waveguide,
The first Y-branch waveguide has an arc-shaped first arc waveguide having a predetermined radius of curvature smoothly connected to a branch point of the Y-branch waveguide;
The second Y-branch waveguide is smoothly connected to the pre-branch straight waveguide consisting of a straight line having a predetermined length and smoothly connected to the first arcuate waveguide, and to the first branch straight waveguide. An arcuate second arcuate waveguide having the same radius of curvature and the same bending direction as the one arcuate waveguide, one end smoothly connected to the second arcuate waveguide, and the other end to the third arcuate waveguide An arc-shaped third arc waveguide that is smoothly connected to the pre-branch straight waveguide of the Y branch waveguide and has the same radius of curvature and a different bending direction as the second arc waveguide; An optical branch circuit comprising an 8-branch circuit in which the straight waveguide before branching is inclined with respect to the incident waveguide.   Each of the Y branch waveguides is formed by a straight waveguide in the vicinity of the branch portion, and the straight waveguide after the branch is formed from an arc having a predetermined radius of curvature. The optical branch circuit according to claim 1, wherein the optical branch circuit is formed so as to smoothly transition to a curved waveguide.   The third Y branch waveguide connected to the inner waveguide after branching of the second Y branch waveguide is inclined closer to the incident waveguide and closer to the incident waveguide. 3. The optical branch circuit according to claim 1, wherein the optical branch circuit is disposed at a position.   The inner waveguide after branching of the second Y-branch waveguide is extended until it becomes parallel to the incident waveguide, and the straight waveguide before branching of the third Y-branch waveguide is extended to the extended waveguide. The optical branch circuit according to claim 1, wherein the optical branch circuit is connected.   The pre-branch straight waveguide length of each Y branch waveguide is set to a predetermined length, and the pre-branch straight waveguide of the third Y branch waveguide connected to the extended waveguide is the predetermined branch The optical branch circuit according to claim 4, wherein the optical branch circuit is longer than   6. An optical branch circuit comprising a circuit having 16 branches or more formed by using the 8-branch optical branch circuit according to claim 1 as a part of the circuit.

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