JP5091072B2 - Y branch optical waveguide and design method thereof - Google Patents

Description

  The present invention relates to a substrate-type optical waveguide used in optical fiber communication and the like, and more particularly, to a Y-branch optical waveguide that branches light input to one input optical waveguide and outputs the light from two branched optical waveguides, and a design method thereof.

  2. Description of the Related Art Conventionally, an optical branching device using a substrate type optical waveguide has been widely used, and optical power distribution in an FTTH (Fiber To The Home) system and a monitor PD tap coupler in an ROADM (Reconfigurable Optical Add / Drop Multiplexer) system. It is used for etc.

  Usually, the structure is a Y-branch type or a directional coupler type, but the Y-branch type is advantageous from the viewpoint of manufacturing tolerance. For optical power distribution, a 1-input 2-output Y-branch type with an equal branch, that is, one-to-one output is used. On the other hand, a tap coupler having a large branching ratio such as 95 to 5, 99 to 1 is used.

  There is also a demand for an optical branching element having an intermediate branching ratio, that is, an optical branching element having an unequal branching ratio of 55:45, 6: 4, 7: 3, 9: 1. For example, an optical branching element that branches into an express path and a drop path in ROADM may have an unequal branching ratio of 7 to 3.

  In recent years, research on substrate-type optical waveguides having a high relative refractive index difference, such as silicon photonics devices, has been actively conducted, and these are different from conventional quartz-type optical waveguides or polymer substrate-type optical waveguides having a low relative refractive index difference. In comparison, the propagation loss is often large, and it may be preferable to adjust the branching ratio in accordance with the optical path design.

  However, it is difficult to realize such an optical branching element having a branching ratio lower than 9 to 1 and an intermediate branching ratio that is non-equal branching with a single-mode Y-branch substrate type optical waveguide. That is disclosed in Patent Document 1. Here, in the single-mode asymmetric Y-shaped optical branching path, most of the light flows into the optical waveguide side having a large propagation constant among the two branched optical waveguides, and almost no optical waveguide has a small propagation constant. It is described that it is difficult to control the branching ratio of the single-mode asymmetric Y-shaped optical branch because no light flows.

  As an optical branching element of a Y-branch substrate type optical waveguide having an unequal branching ratio, Patent Document 2 discloses an asymmetric branching by shifting the central axis of an input waveguide and the central axis between two branching waveguides. A Y-branch waveguide type optical tap realizing the ratio is disclosed.

As an optical branching element of a Y-branch substrate type optical waveguide having an unequal branching ratio, for example, the one disclosed in Patent Document 3 is known. In Patent Document 3, an asymmetric Y-branch waveguide optical tap having an unequal branching ratio is configured by making the core width of one branch optical waveguide smaller than the core width of the other branch waveguide.
JP-A-6-235842 Japanese Patent Laid-Open No. 9-33740 JP-A-8-122547

  However, in the case of the technique disclosed in Patent Document 2, as shown in FIG. 2, when the branching ratio is changed from 1: 1 to 1: 3.4, the shift amount is 0 to 1. It is necessary to change within a very narrow range of 5 μm. That is, the change in the branching ratio with respect to the change in the shift amount is very steep. Therefore, it is difficult to accurately control the branching ratio, and the yield may be lowered.

  Further, as in Patent Document 3, when the core width of one branch optical waveguide is made smaller than the core width of the other branch optical waveguide, the amount of light incident on the one branch optical waveguide is limited. Difficult to reduce excess loss. In addition, since the core width of one of the branch optical waveguides needs to be made smaller than the core width of a very small single mode optical waveguide, the yield of the branch optical waveguide may be reduced.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a Y-branch optical waveguide that can reduce excess loss and improve yield and a design method thereof.

In order to solve the above-described problems, a Y-branch optical waveguide according to an aspect of the present invention is formed so that the core width gradually increases and the narrow side end portion is connected to the input optical waveguide. A Y-branch optical waveguide comprising a tapered optical waveguide and two branched optical waveguides connected to each other at a wide-side end portion of the tapered optical waveguide with a gap having a predetermined width, the tapered optical waveguide and the branched optical waveguide The core widths of the two branched optical waveguides are formed to be equal to each other, the spread angle of one side of the core of the tapered optical waveguide is formed at 0 degree, and the spread angle of the other side is 0 The core width of the input optical waveguide is equal to the core width of the narrow end portion of the tapered optical waveguide .

  According to this aspect, the light incident on the input optical waveguide is gradually broadened in the optical distribution in the tapered optical waveguide, branched into two by the two branched optical waveguides, and output. In the Y-branch optical waveguide according to this aspect, in the cross section of the tapered optical waveguide, the light intensity on the side surface formed with the divergence angle larger than 0 degrees is larger than that on the side surface formed with the divergence angle 0 degrees. Large light distribution. Then, by connecting two branched optical waveguides having the same core width to the tapered waveguide, an unequal branched branched optical waveguide can be configured.

  In this aspect, the core width of the two branched optical waveguides is made equal at the connection portion with the tapered optical waveguide, so that the light transmitted through the tapered optical waveguide is incident on the two branched optical waveguides with little loss. Therefore, the loss of light at the connecting portion between the tapered optical waveguide and the two branched optical waveguides is reduced, and a Y-branched optical waveguide with reduced excess loss can be configured.

  In the Y-branch optical waveguide according to this aspect, the core width of the branch optical waveguide may be formed to be equal, so that the core width can be easily controlled and the yield can be improved.

Furthermore, in the Y-branch optical waveguide according to this aspect, the branching ratio can be adjusted by changing the length of the tapered optical waveguide. Since the change in the branching ratio relative to the change in the shift amount in 2 is more gradual, the branching ratio can be easily controlled and the yield can be improved. Further, in the Y-branch optical waveguide according to this aspect, the core width of the input optical waveguide and the core width of the narrow side end portion of the tapered optical waveguide are formed to be equal, so that the input optical waveguide and the tapered optical waveguide are Since the loss of light at the connection portion is reduced, the excess loss of the Y branch optical waveguide can be reduced.

  The core width of the input optical waveguide and the core width of the two branch optical waveguides may be formed equal. Thereby, the input optical waveguide and the two branched optical waveguides can be made, for example, a single-mode optical waveguide, and a good Y-branched optical waveguide with reduced mode dispersion can be configured.

The core width of the wide end of the tape over path optical waveguide may be formed equal to the sum of the two widths of the core width and the gap portion of the branched optical waveguides. In this case, since the loss of light at the connecting portion of the branched optical waveguide and Te over path optical waveguide is reduced, thereby reducing the excess loss of the Y branch optical waveguide.

  The two branch optical waveguides may be formed to be symmetric with respect to the center line between the branch optical waveguides. In this case, the optical path lengths of both branch optical waveguides are equal, and the light loss of both branch optical waveguides is equal, so that the design of the optical circuit at the front stage or the rear stage becomes easy.

  In the relationship between the length of the tapered optical waveguide and the branching ratio of the two branched optical waveguides, the first range of the length of the first tapered optical waveguide in which the branching ratio increases as the length of the tapered optical waveguide becomes longer is the first range. When the range of the length of the tapered optical waveguide in which the branching ratio decreases as the length of the subsequent tapered optical waveguide becomes longer is set as the second range, the length of the tapered optical waveguide is changed from the first range and the second range. You may make it select.

  In this case, a Y-branch optical waveguide having a branching ratio lower than 9 to 1 and an intermediate branching ratio that is unequal branching can be suitably configured. By selecting the length of the tapered optical waveguide from the first range and the second range, it is possible to increase the adjustment range of the branching ratio.

  The length of the tapered optical waveguide may be selected from the second range. In this case, a Y-branch optical waveguide having a lower excess loss and a higher yield can be configured than when the length of the tapered optical waveguide is selected from the first range.

Another aspect of the present invention is a method for designing a Y-branch optical waveguide. The method includes an input optical waveguide,
Tapered optical waveguide formed so that the core width gradually increases, with the narrow side end connected to the input optical waveguide, and connected to the wide side end of the tapered optical waveguide side by side through a gap of a predetermined width A method of designing a Y-branch optical waveguide comprising two branched optical waveguides, the step of equalizing the core width of the input optical waveguide and the core width of the narrow side end of the tapered optical waveguide; The step of equalizing the core widths of the two branch optical waveguides at the connecting portion between the waveguide and the branch optical waveguide, the divergence angle of one side surface of the core of the tapered optical waveguide being set to 0 degree, and the divergence angle of the other side surface And a step of adjusting the branching ratio of the light output from the two branching optical waveguides by changing the length of the tapered optical waveguide.

  In the Y-branch optical waveguide designed by the method according to this aspect, the light incident on the input optical waveguide is gradually spread in the tapered optical waveguide, and is branched into two by the two branch optical waveguides. Is done. In the cross section of the tapered optical waveguide, the side of the side where the divergence angle is formed to be larger than 0 degrees has a light distribution having a greater light intensity than the side of the side where the divergence angle is formed to be 0 degrees. By connecting two branched optical waveguides having the same core width to the tapered waveguide, an unequal branched branched optical waveguide can be designed.

  In this aspect, the core width of the two branched optical waveguides is made equal at the connection portion with the tapered optical waveguide, so that the light transmitted through the tapered optical waveguide is incident on the two branched optical waveguides with little loss. Therefore, the loss of light at the connection portion between the tapered optical waveguide and the two branched optical waveguides is reduced, and a Y-branched optical waveguide with reduced excess loss can be designed.

  Further, in this aspect, since the core widths of the branched optical waveguides are formed to be equal, the core width can be easily controlled and the yield can be improved.

Further, in this aspect, the branching ratio can be adjusted by changing the length of the tapered optical waveguide. However, the change in the branching ratio with respect to the change in the length of the tapered optical waveguide is the change in the shift amount in the above-mentioned Patent Document 2. Therefore, the branching ratio can be easily controlled and the yield can be improved. Further, in this aspect, by making the core width of the input optical waveguide equal to the core width of the narrow end portion of the tapered optical waveguide, light loss at the connection portion between the input optical waveguide and the tapered optical waveguide is reduced. Therefore, the excess loss of the Y branch optical waveguide can be reduced.

  In the relationship between the length of the tapered optical waveguide and the branching ratio of the two branched optical waveguides, the first range of the length of the first tapered optical waveguide in which the branching ratio increases as the length of the tapered optical waveguide becomes longer is the first range. In the step of adjusting the branching ratio, when the range of the length of the tapered optical waveguide in which the branching ratio decreases as the length of the subsequent tapered optical waveguide becomes longer is the second range, You may make it select from a 1st range and a 2nd range.

  In this case, it is possible to suitably design a Y-branch optical waveguide having a branching ratio lower than 9 to 1 and an intermediate branching ratio that is unequal branching. By selecting the length of the tapered optical waveguide from the first range and the second range, it is possible to increase the adjustment range of the branching ratio.

  The length of the tapered optical waveguide may be selected from the second range. In this case, it is possible to design a Y-branch optical waveguide with a lower excess loss and a higher yield than when the length of the tapered optical waveguide is selected from the first range.

  According to the present invention, it is possible to provide a Y-branch optical waveguide having a branching ratio lower than 9 to 1 and having an intermediate branching ratio that is unequal branching and a design method thereof. This Y-branch optical waveguide can reduce the excess loss and improve the yield.

  FIG. 1 is a plan view of a Y-branch optical waveguide 10 according to an embodiment of the present invention. The Y branch optical waveguide 10 is an embedded substrate type optical waveguide whose core shape has a rectangular cross section. The Y-branch optical waveguide 10 includes one input optical waveguide 12, a tapered optical waveguide 14 whose core width gradually increases, and a first branched optical waveguide 16 and a second branched optical waveguide 18 connected to the tapered optical waveguide 14. With. The first branch optical waveguide 16 includes a first S-shaped curved optical waveguide 20 and a first output optical waveguide 24. The second branch optical waveguide 18 includes a second S-shaped curved optical waveguide 22 and a second output optical waveguide 26.

  The input optical waveguide 12 is a straight optical waveguide, and an optical fiber, for example, is connected to the input end 12a. The output end 12 b of the input optical waveguide 12 is connected to the narrow side end 14 a of the tapered optical waveguide 14. An input end 20 a of the first S-shaped curved optical waveguide 20 and an input end 22 a of the second S-shaped curved optical waveguide 22 are connected to the wide end 14 b of the tapered optical waveguide 14. A detailed configuration around the tapered optical waveguide 14 will be described later.

  The first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 are bent optical waveguides bent into an S-shape. The first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 are formed such that the distance from each other increases as the distance from the tapered optical waveguide 14 increases. The tangent direction of the input end 20a of the first S-shaped curved optical waveguide 20 and the input end 22a of the second S-shaped curved optical waveguide 22 are parallel to the optical axis of the output end 12b of the input optical waveguide 12, respectively. Is formed.

  The output end 20 b of the first S-shaped curved optical waveguide 20 is connected to the input end 24 a of the first output optical waveguide 24, and the output end 22 b of the second S-shaped curved optical waveguide 22 is connected to the output end 22 b. The input end portion 26a of the two-output optical waveguide 26 is connected. The output end 20b of the first S-shaped curved optical waveguide 20 and the output end 22b of the second S-shaped curved optical waveguide 22 are tangential in the input end 24a of the first output optical waveguide 24, and the second output optical waveguide, respectively. 26 is preferably formed so as to be parallel to the optical axis of the input end portion 26a. In this case, the loss of light at the connection portion can be reduced. As shown in FIG. 1, the first output optical waveguide 24 and the second output optical waveguide 26 may be straight optical waveguides or curved optical waveguides.

  The S-shaped bent shape of the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 can take a plurality of shapes depending on how the center line is taken, but the relative refractive index between the core and the clad. Any S-shape may be used as long as it is designed using a sufficiently gentle radius of curvature according to the difference. In the present embodiment, an S-shaped curved optical waveguide based on an arc shape is formed. The separation distance between the output end 20b of the first S-shaped curved optical waveguide 20 and the output end 22b of the second S-shaped curved optical waveguide 22 is the same as that of the monolithic integrated optical component or the first output optical waveguide 24, When the two-output optical waveguide 26 is a curved optical waveguide, it is defined by the contents and dimensions of the optical circuit at the next stage. Further, when the linear first output optical waveguide 24 and the second output optical waveguide 26 are directly connected to the optical output coupling component such as an optical fiber array as in the present embodiment, the optical output coupling It is defined by the input interval of parts. For example, the interval is 127 μm for a half pitch optical fiber array and 250 μm for a full pitch optical fiber array.

  As shown in FIG. 1, the first branch optical waveguide 16 and the second branch optical waveguide 18 are preferably formed so as to be symmetric with respect to the center line A between the two branch optical waveguides. In this case, the optical path lengths of both branch optical waveguides are equal, and the light loss of both branch optical waveguides is equal, so that the design of the optical circuit at the front stage or the rear stage becomes easy.

  FIG. 2 is an XX cross-sectional view of the Y-branch optical waveguide 10 shown in FIG. Although the cross section of the input optical waveguide 12 is shown here, the cross sections of the first branch optical waveguide 16 and the second branch optical waveguide 18 are the same. Also in the tapered optical waveguide 14, only the width w of the core 34 is changed, and the basic configuration is the same.

  In the Y branch optical waveguide 10, a lower cladding layer 32 is formed of a material having a low refractive index on a substrate 30 made of, for example, silicon or quartz having a thickness of about 0.7 to 1 mm. A core layer is formed on the lower cladding layer 32 with a material having a high refractive index, and the core layer is formed into a desired shape by a photolithography process and an etching process, thereby forming the core 34. Thereafter, the Y-branch optical waveguide 10 is formed by forming the upper cladding layer 36 with a material having a low refractive index and covering the core 34. Various organic / inorganic materials that are transparent to the wavelength of light used in the Y-branch optical waveguide 10 can be applied to the core material and the cladding material.

  The core width w is set to be a single mode optical waveguide based on the refractive index of the core material and the refractive index of the cladding material. The core width of the input optical waveguide 12 and the core widths of the first branch optical waveguide 16 and the second branch optical waveguide 18 are formed to be equal. Thereby, a good Y-branch optical waveguide with reduced mode dispersion can be configured. From the viewpoint of reducing the propagation loss due to scattering due to the roughness of the core side wall and the connection loss at the input / output portion of the Y-branch optical waveguide 10, the core width w is preferably large, but it is too large and deviates from the single mode condition. This will cause multimode interference and multimode dispersion.

  FIG. 3 is an enlarged view of a main part in which the periphery of the tapered optical waveguide 14 is enlarged. As shown in FIG. 3, the taper optical waveguide 14 has a core formed in a trapezoidal shape in plan view. The first side wall 14 c of the core of the tapered optical waveguide 14 is connected in parallel to the first side wall 12 c of the core of the input optical waveguide 12. That is, the first side wall 14 c of the tapered optical waveguide 14 is formed in parallel with the optical axis of the output end portion 12 b of the input optical waveguide 12. Further, the second side wall 14 d of the core of the tapered optical waveguide 14 is connected to the second side wall 12 d of the input optical waveguide 12 at an angle θ. That is, the second side wall 14 d of the core of the tapered optical waveguide 14 is formed to be inclined at an angle θ with respect to the optical axis of the output end portion 12 b of the input optical waveguide 12. Hereinafter, an angle formed by the optical axis of the output end portion 12b of the input optical waveguide 12 and the side wall of the tapered optical waveguide 14 is referred to as “a spread angle”. The first sidewall 14c of the tapered optical waveguide 14 has a divergence angle of 0 degrees. The spread angle θ of the second side wall 14d is determined by the taper optical waveguide length L and the difference between the core widths of the narrow side end portion 14a and the wide side end portion 14b.

  The core width w 1 of the narrow side end portion 14 a of the tapered optical waveguide 14 is formed to be equal to the core width w of the input optical waveguide 12. Thus, by forming the core width w 1 of the narrow side end portion 14 a equal to the core width w of the input optical waveguide 12, it is possible to reduce light loss at the connection portion between the input optical waveguide 12 and the tapered optical waveguide 14.

  The taper optical waveguide 14 gradually increases in core width from the input optical waveguide 12 side to the first branch optical waveguide 16 side and the second branch optical waveguide 18 side, and the first S-shaped curved optical waveguide 20 at the wide side end portion 14b. The second S-shaped curved optical waveguide 22 is connected. The first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 are arranged side by side via a gap portion 28 having a predetermined width g. The first side wall 14c of the core of the tapered optical waveguide 14 is continuously connected to the outer wall 20c of the core of the first S-shaped curved optical waveguide 20, and the second side wall 14d of the core of the tapered optical waveguide 14 is The 2S-shaped curved optical waveguide 22 is continuously connected to the outer wall 22c.

  The width g of the gap portion 28 is selected from a range in which the gap portion 28 can be formed stably from the capability values of the photolithography process and the etching process. When the width g of the gap portion 28 is reduced, excess loss can be reduced, and when the width g is increased, stable processing is possible.

  In the present embodiment, the core width w2 of the input end portion 20a of the first S-shaped curved optical waveguide 20 and the core width w3 of the input end portion 22a of the second S-shaped curved optical waveguide 22 are formed to be equal. ing. The core width w2 and the core width w3 are formed to be equal to the core width w of the input optical waveguide 12.

  The core width of the wide end portion 14b of the tapered optical waveguide 14 is the core width w2 of the input end portion 20a of the first S-shaped curved optical waveguide 20 and the core width of the input end portion 22a of the second S-shaped curved optical waveguide 22. It is formed to be equal to the sum of w3 and the width g of the gap portion 28. Thereby, it is possible to reduce the light loss at the connection portion between the tapered optical waveguide 14, the first S-shaped curved optical waveguide 20, and the second S-shaped curved optical waveguide 22.

  In the Y-branch optical waveguide 10 formed as described above, when light is incident on the input end 12a of the input optical waveguide 12, the light is propagated through the input optical waveguide 12 in a single mode. Light distribution of the light incident on the tapered optical waveguide 14 is gradually expanded in the tapered optical waveguide 14. In the Y-branch optical waveguide 10, the spread angle of the first side wall 14 c of the core of the taper optical waveguide 14 is set to 0 degree, and the spread angle of the second side wall 14 d is set to θ degrees larger than 0 degree. The light intensity distribution in the cross section of the wide-side end portion 14b is asymmetric between the first side wall 14c side and the second side wall 14d side. As a result of a simulation by the present inventor to be described later, the light intensity distribution at the wide-side end portion 14b of the tapered optical waveguide 14 is a light intensity distribution with a higher light intensity on the second side wall 14d side than on the first side wall 14c. I know that. In the present embodiment, since the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 having the same core width are connected to the wide-side end portion 14b of the tapered optical waveguide 14, the second sidewall 14d side The amount of light incident on the second S-shaped curved optical waveguide 22 is larger than that of the first S-shaped curved optical waveguide 20 on the first side wall 14c side. That is, the light is split into unequal parts and is incident on the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22. The light that is branched in an undivided manner is emitted from the output end 24 b of the first output optical waveguide 24 and the output end 26 b of the second output optical waveguide 26.

  In the Y branch optical waveguide 10, the core width w2 of the input end portion 20a of the first S-shaped curved optical waveguide 20 and the core width w3 of the input end portion 22a of the second S-shaped curved optical waveguide 22 are made equal. The light transmitted through the tapered optical waveguide 14 is incident on the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 with little loss. Therefore, according to the present embodiment, a Y-branch optical waveguide with reduced excess loss can be realized. In the Y-branch optical waveguide 10, the core widths of the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 may be formed to be equal, so that the core width can be easily controlled and the yield can be improved.

(First embodiment)
Next, the result of having performed the simulation about the Y branch optical waveguide 10 of the above structures using the three-dimensional beam propagation method (BPM: Beam Propagation Method) is shown. Here, a ratio in which the core is formed of SiON (silicon oxynitride) whose refractive index is adjusted to 1.5, and the lower clad 32 and the upper clad 36 are formed of SiO ₂ (silica) having a refractive index of 1.465. A simulation was performed for a high relative refractive index difference Y-branch optical waveguide with a refractive index difference Δ = 2.3%. The lower clad 32 was set to a thickness of 5 μm, the core 34 was set to a width w of 3.2 μm × height of 3.2 μm, and the upper clad 36 was set to a thickness of 8.2 μm. Since the width g of the branch gap is 0.4 μm, the width of the wide end portion 14b of the tapered optical waveguide 14 is 6.8 μm. The radius of curvature of the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 was 1000 μm, and the separation distance between the first output optical waveguide 24 and the second output optical waveguide 26 was 127 μm.

  FIG. 4 is a diagram showing the relationship between the relative output light intensity of each branch optical waveguide and the tapered optical waveguide length L in the first embodiment. The horizontal axis of FIG. 4 represents the tapered optical waveguide length L, and the vertical axis represents the relative output light intensity of each branch optical waveguide with respect to the incident light intensity of the input optical waveguide 12. The relative output light intensity of each branch optical waveguide is obtained by calculating the overlap integral between the light intensity distribution of the output light from each branch optical waveguide and the light intensity distribution of the basic propagation mode of each branch optical waveguide, and calculating this with respect to the incident light intensity. The relative value was obtained.

  In FIG. 4, a solid line 40 is a characteristic curve of the output from the first branch optical waveguide 16, and a broken line 42 is a characteristic curve of the output from the second branch optical waveguide 18. As shown in FIG. 4, it can be seen that the first branched optical waveguide 16 and the second branched optical waveguide 18 are asymmetric in relative output light intensity. From FIG. 4, it can be seen that the relative output light intensity is higher in the second branch optical waveguide 18 in the most range of the tapered optical waveguide length L.

  In the Y branch optical waveguide 10, the core width w2 of the input end 20a of the first S-shaped curved optical waveguide 20 and the core width w3 of the input end 22a of the second S-shaped curved optical waveguide 22 are formed to be equal. Therefore, the light intensity of the portion connected to the second S-shaped curved optical waveguide 22 is already higher than the portion connected to the first S-shaped curved optical waveguide 20 at the wide-side end portion 14 b of the tapered optical waveguide 14. It can be seen that the light intensity distribution is large. In the tapered optical waveguide 14, the higher-order mode light generated in the tapered optical waveguide 14 interferes with the fundamental mode light, so that the waveform of the propagating light is deformed. As a result of this simulation, the spread angle θ is provided only on the second side wall 14d of the tapered optical waveguide 14, so that the higher-order mode light and the fundamental mode light are generated at the portion on the second side wall 14d side of the tapered optical waveguide 14. This is presumably due to the fact that the conditions for strengthening each other were established.

  FIG. 5 is a diagram showing the relationship between the branching ratio and the taper optical waveguide length L in the first embodiment. The vertical axis in FIG. 5 is the branching ratio obtained by dividing the output light intensity of the branch optical waveguide having the larger output by the output light intensity of the smaller branch waveguide. FIG. 6 is a diagram showing the relationship between excess loss and tapered optical waveguide length L in the first embodiment.

  As shown in FIG. 5, the branching ratio increases as the taper optical waveguide length L increases in the range of L = 40 to 60 μm, and increases from 1.8 when L = 40 μm to 3.4 when L = 60 μm. To do. In the range of L = 40 to 60 μm, the excess loss is 0.02 to 0.03 dB as shown in FIG. This range of L = 40-60 μm is called the first range. As shown in FIG. 5, the branching ratio starts to decrease from L = 60 μm, and becomes 1.1 at L = 140 μm. In the range of L = 60 to 140 μm, the excess loss is 0.013 to 0.02 dB as shown in FIG. This range of L = 60 to 140 μm is called the second range. When L = 140 μm or more, the branching ratio repeatedly increases and decreases, but the maximum branching ratio gradually decreases.

  As can be seen from FIG. 5, the tapered optical waveguide length L is preferably selected from the first range and the second range. By selecting the tapered optical waveguide length L from the first range and the second range, it is possible to increase the adjustment range of the branching ratio. Even when the tapered optical waveguide length L is selected from either the first range or the second range, the change in the branching ratio with respect to the change in the tapered optical waveguide length L is the change in the branching ratio with respect to the change in the shift amount in the above-mentioned Patent Document 2. Therefore, the branching ratio can be easily controlled and the yield can be improved.

  Further, as shown in FIG. 6, since the excess loss is smaller in the second range than in the first range, it is more preferable to select the tapered optical waveguide length L from the second range. In the second range, as shown in FIG. 5, the change in the branching ratio with respect to the change in the taper optical waveguide length L is gradual. Therefore, the selection of the taper optical waveguide length L from the second range corresponds to the processing error during manufacturing. It also has the advantage of high tolerance. By selecting the taper optical waveguide length L from the second range, the Y-branch optical waveguide 10 having a branch ratio of 1.1 to 3.4 having a very small excess loss of 0.013 to 0.02 dB and a large manufacturing tolerance. Can be obtained. Thus, according to this embodiment, it is possible to realize a single-mode Y-branch substrate type optical waveguide having a branching ratio lower than 9 to 1 and having an intermediate branching ratio that is unequal branching. it can.

  When the Y-branch optical waveguide 10 according to the first embodiment is manufactured, for example, a silica lower cladding layer is formed with a thickness of 5 μm on a silicon wafer having a thickness of 1 mm using a PECVD apparatus. Subsequently, the core layer is formed to a thickness of 3.2 μm by adjusting the oxygen / nitrogen ratio so that silicon oxynitride has a refractive index of 1.5. The core layer is processed by using a photolithography process and an etching process to form a core, and a silica upper clad is formed with a thickness of 8.2 μm. At this time, by setting the tapered optical waveguide length L to 80 μm, a Y-branch optical waveguide having a branching ratio of 3.1 can be obtained. Also, by setting the taper optical waveguide length L to 100 μm, a Y-branch optical waveguide with a branching ratio of 1.9 can be obtained.

(Second embodiment)
Next, another embodiment will be described. Here, a relative refractive index difference Δ using a fluorinated polyimide whose refractive index is adjusted to 1.534 as a core material and a fluorinated polyimide whose refractive index is adjusted to 1.529 is used for the lower clad 32 and the upper clad 36. = A 0.3% low relative refractive index difference Y-branched optical waveguide was simulated using a two-dimensional beam propagation method. The lower clad 32 was set to a thickness of 15 μm, the core 34 was set to a width w of 7 μm × height of 7 μm, and the upper clad 36 was set to a thickness of 22 μm. Since the width g of the branch gap is 2 μm, the width of the wide side end portion 14b of the tapered optical waveguide 14 is 16 μm. The radius of curvature of the first S-shaped curved optical waveguide 20 and the second S-shaped curved optical waveguide 22 was 40 mm, and the separation distance between the first output optical waveguide 24 and the second output optical waveguide 26 was 127 μm.

  FIG. 7 is a diagram showing the relationship between the relative output light intensity of each branch optical waveguide and the tapered optical waveguide length L in the second embodiment. The calculation method of the relative output light intensity on the vertical axis is the same as that in FIG. 4 of the first embodiment. In FIG. 7, the solid line 44 is a characteristic curve of the output from the first branch optical waveguide 16, and the broken line 46 is a characteristic curve of the output from the second branch optical waveguide 18. Also in the second embodiment, the first branched optical waveguide 16 and the second branched optical waveguide 18 are asymmetric in relative output light intensity, and the second branched optical waveguide 18 is almost in the range of the tapered optical waveguide length L. It can be seen that the relative output light intensity is increased.

  FIG. 8 is a diagram showing the relationship between the branching ratio and the taper optical waveguide length L in the second embodiment. FIG. 9 is a diagram showing the relationship between excess loss and the taper optical waveguide length L in the second embodiment. The setting of each axis is the same as in FIGS. 5 and 6 of the first embodiment.

  As shown in FIG. 8, the branching ratio increases as the taper optical waveguide length L increases in the range of L = 150 to 400 μm, and increases from 1.1 when L = 150 μm to 5.2 when L = 400 μm. To do. In the range of L = 150 to 400 μm, the excess loss is 0.11 to 0.16 dB as shown in FIG. This range of L = 150 to 400 μm is called the first range. When the tapered optical waveguide length L is equal to or longer than the first range, the branching ratio decreases, and becomes 1.1 when L = 900 μm. In the range of L = 400 to 900 μm, the excess loss is 0.04 to 0.16 dB as shown in FIG. This range of L = 400 to 900 μm is called a second range. When L = 900 μm or more, the branching ratio repeatedly increases and decreases, but the maximum branching ratio gradually decreases.

  As can be seen from FIG. 8, the tapered optical waveguide length L is preferably selected from the first range and the second range. By selecting the tapered optical waveguide length L from the first range and the second range, it is possible to increase the adjustment range of the branching ratio. Even when the tapered optical waveguide length L is selected from either the first range or the second range, the change in the branching ratio with respect to the change in the tapered optical waveguide length L is the change in the branching ratio with respect to the change in the shift amount in the above-mentioned Patent Document 2. Therefore, the branching ratio can be easily controlled and the yield can be improved.

  Also, as shown in FIG. 9, since the excess loss is less in the second range than in the first range, it is more preferable to select the tapered optical waveguide length L from the second range. In the second range, as shown in FIG. 8, the change in the branching ratio with respect to the change in the taper optical waveguide length L is gradual. It also has the advantage of high tolerance. By selecting the tapered optical waveguide length L from the second range, the Y branch optical waveguide 10 having a branch ratio of 1.1 to 5.2 with a small excess loss of 0.04 to 0.16 dB and a high manufacturing tolerance is obtained. be able to. Thus, according to this embodiment, it is possible to realize a single-mode Y-branch substrate type optical waveguide having a branching ratio lower than 9 to 1 and having an intermediate branching ratio that is unequal branching. it can.

  When the Y-branch optical waveguide 10 according to the second embodiment is manufactured, a fluorinated polyimide clad layer is formed with a thickness of 15 μm on a silicon wafer having a thickness of 1 mm, for example, using a spin coater and an inert gas oven. To do. Subsequently, a fluorinated polyimide core layer is formed to a thickness of 7 μm using a material blended so that the refractive index is slightly higher. The core layer is processed using a photolithography process and an etching process to form a core, and a fluorinated polyimide upper clad is formed to a thickness of 22 μm. At this time, by setting the taper optical waveguide length L to 450 μm, a Y-branch optical waveguide having a branching ratio of 5.0 can be obtained. Further, by setting the taper optical waveguide length L to 650 μm, a Y-branch optical waveguide having a branching ratio of 2.0 can be obtained.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are also within the scope of the present invention. is there.

  The input optical waveguide 12 is not limited to a straight optical waveguide, and may be a curved optical waveguide, for example. In this case, it is preferable to form a straight line in the vicinity of the output end 12b. By forming the output end portion 12b of the input optical waveguide 12 in a straight line, the loss of light at the connection portion between the input optical waveguide 12 and the tapered optical waveguide 14 can be reduced.

  In the above embodiment, the input optical waveguide 12 and the like are single mode optical waveguides, but may be multimode optical waveguides.

It is a top view of the Y branch optical waveguide which concerns on embodiment of this invention. It is XX sectional drawing of the Y branch optical waveguide shown in FIG. It is the principal part enlarged view to which the periphery of the taper optical waveguide was expanded. It is a figure which shows the relationship between the relative output light intensity of each branch optical waveguide in 1st Example, and the taper optical waveguide length L. FIG. It is a figure which shows the relationship between the branching ratio and taper optical waveguide length L in a 1st Example. It is a figure which shows the relationship between the excess loss and taper optical waveguide length L in a 1st Example. It is a figure which shows the relationship between the relative output light intensity of each branch optical waveguide in 2nd Example, and the taper optical waveguide length L. FIG. It is a figure which shows the relationship between the branching ratio and the taper optical waveguide length L in a 2nd Example. It is a figure which shows the relationship between the excess loss and taper optical waveguide length L in a 2nd Example.

Explanation of symbols

  10 Y branch optical waveguide, 12 input optical waveguide, 14 tapered optical waveguide, 16 first branched optical waveguide, 18 second branched optical waveguide, 20 first S-shaped curved optical waveguide, 22 second S-shaped curved optical waveguide, 24 second 1 output optical waveguide, 26 second output optical waveguide, 28 gap portion, 30 substrate, 32 lower cladding layer, 34 core.

Claims (9)

An input optical waveguide;
A tapered optical waveguide formed so that the core width gradually increases, and the narrow side end portion is connected to the input optical waveguide;
Two branched optical waveguides connected to each other on the wide-side end of the tapered optical waveguide, with a gap having a predetermined width,
A Y-branch optical waveguide comprising:
In the connection portion between the tapered optical waveguide and the branched optical waveguide, the core widths of the two branched optical waveguides are formed equally,
The spread angle of one side surface of the core of the tapered optical waveguide is formed at 0 degree, and the spread angle of the other side surface is formed at an angle larger than 0 degree ,
A Y-branch optical waveguide characterized in that a core width of the input optical waveguide and a core width of a narrow side end portion of the tapered optical waveguide are formed to be equal .   2. The Y-branch optical waveguide according to claim 1, wherein a core width of the input optical waveguide is equal to a core width of the two branch optical waveguides. The core width of the wide end of the tapered optical waveguide, according to claim 1 or 2, characterized in that the core width of two of said branch optical waveguides and the are equally formed on the sum of the width of the gap portion Y branch optical waveguide. Two of said branched optical waveguides, Y-branch optical waveguide according to any one of claims 1 to 3, characterized in that it is formed so as to be symmetrical with respect to the center line between the branched optical waveguides. In the relationship between the length of the tapered optical waveguide and the branching ratio of the two branched optical waveguides, the first range of the length of the tapered optical waveguide in which the branching ratio increases as the length of the tapered optical waveguide becomes longer is When the range of the length of the tapered optical waveguide in which the branching ratio decreases as the length of the subsequent tapered optical waveguide becomes longer is set as the second range, the length of the tapered optical waveguide is Y branch optical waveguide according to any one of claims 1 to 4, characterized in that it has to select from the first range and the second range. 6. The Y-branch optical waveguide according to claim 5 , wherein a length of the tapered optical waveguide is selected from the second range. An input optical waveguide;
A tapered optical waveguide formed so that the core width gradually increases, and the narrow side end portion is connected to the input optical waveguide;
Two branched optical waveguides connected to each other on the wide-side end of the tapered optical waveguide, with a gap having a predetermined width,
A Y branch optical waveguide design method comprising:
Equalizing the core width of the input optical waveguide and the core width of the narrow side end of the tapered optical waveguide;
Equalizing the core width of the two branched optical waveguides at the connection between the tapered optical waveguide and the branched optical waveguide;
Setting the divergence angle of one side surface of the core of the tapered optical waveguide to 0 degree and the divergence angle of the other side surface to an angle larger than 0 degree;
Adjusting the branching ratio of the light output from the two branched optical waveguides by changing the length of the tapered optical waveguide;
A method for designing a Y-branch optical waveguide, comprising: In the relationship between the length of the tapered optical waveguide and the branching ratio of the two branched optical waveguides, the first range of the length of the tapered optical waveguide in which the branching ratio increases as the length of the tapered optical waveguide becomes longer is In the step of adjusting the branching ratio when the range of the length of the tapered optical waveguide in which the branching ratio decreases as the length of the tapered optical waveguide following the first range becomes a second range, 8. The method of designing a Y-branch optical waveguide according to claim 7 , wherein the length of the tapered optical waveguide is selected from the first range and the second range. The method of designing a Y-branch optical waveguide according to claim 8 , wherein the length of the tapered optical waveguide is selected from the second range.

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