JPH04289803A - Multiple branch optical circuit - Google Patents

Abstract

PURPOSE:To provide a new circuit pattern for a multiple branch optical circuit of a tree structure capable of making the element length shorter than that of conventional element without increasing loss. CONSTITUTION:An extension CL of central axis of the first stage incident path 22 is assumed as a reference line. Of the branched portions in the second and later stages, each of branched portions 2, 3, 4 and 7 located in the outermost position is branched outward in a pattern in which the angles theta2 and theta3 formed between each branch center line and the reference line progressively increase toward the later stages.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention relates to a tree-structured multi-branch optical circuit having two or more stages of Y-branch sections, and in particular to a single optical circuit with miniaturized elements and reduced loss. The present invention relates to multi-mode multi-branch optical circuits. [0002] A conventional tree-structured multi-branch optical circuit has 8
The case of a branch optical circuit is shown in FIGS. 6 and 7. The one in FIG. 6 includes an optical circuit having three stages of Y-branching parts: a first-stage Y-branching part 1, a second-stage Y-branching part 2, 3, and a third-stage Y-branching part 4, 5, 6, 7. 31, and these branch parts are connected in multiple stages in a tree shape at connection parts 8, 9, 10, 11, 12, and 13, and the third stage, that is, the final stage, Y branch parts 4, 5.
, 6, 7 and the final stage output path 2 parallel to the normal line of the side surface of the substrate 31.
3, 24, 25, 26, 27, 28, 29, and 30 are smoothly connected by expanded portions 14, 15, 16, 17, 18, 19, 20, and 21. [0003] The conventional multi-branch optical circuit described above uses the central axis extension line CL of the first stage input path 22 as a reference line, and the branch centers of all the Y branch parts 1, 2, 3, 4, 5, 6, and 7. The line is parallel to the reference line CL and extends from the first stage entrance path 22 to the final stage exit path 23, 24, 25, 26, 27, 28, 29, 3.
The circuit pattern was made so that the waveguide path lengths up to 0 were equal. In another conventional example shown in FIG. Connection parts 8, 9, 10, 11,
12 and 13 are connected in multiple stages like a tree, and the third stage Y branch parts 4, 5, 6, 7 and the final stage output paths 23, 24, 25, 26, 27, 28 are parallel to the normal line of the side surface of the substrate 31. ,29,30
Expanding parts 14, 15, 16, 17, 18, 19, 20,
Connect smoothly at 21, Y branch part 1, 2, 3, 4, 5
, 6, and 7 are made parallel to the reference line CL, which is the same as in FIG. The difference from the one in FIG. 6 is that the arrangement of the Y branch parts is more dense than in the case of FIG.
15, 16, 17, 18, 19, 20, and 21, the patterns are developed parallel to the reference line CL at intervals smaller than the final stage exit path, and then redeployed to the interval of the final stage exit path. ing. [0006] Among the conventional multi-branch optical circuits described above, the one shown in FIG. ,29,3
Although it has the advantage that the waveguide paths to 0 are symmetrical and equal, and the design is very simple, the position of the Y branch part is determined by the interval between the final stage output paths, especially the branch center of the first stage Y part 1. Since the distance between the line and the branch center line of the second-stage Y branch parts 2 and 3 becomes large, it is necessary to use curved waveguides with large bending angles in the connecting parts 8 and 9. As a result, the element length of the optical circuit increases, propagation loss increases, and insertion loss increases. On the other hand, if the element length is designed to be short, it is necessary to reduce the radius of curvature of the curved waveguides of the connecting portions 8 and 9, resulting in an increase in bending loss and, as a result, a problem in that insertion loss also increases. The one in FIG. 7 includes a first stage entrance path 22, a final stage exit path 23, 24, 25, 26, 27, 28, 29,
The size of the device has been reduced by arranging the Y-branch portions at a higher density with a configuration in which up to 30 waveguide paths are unequal. However, although not as severe as in the case of FIG. 6, there was a problem in that the bending angle of the curved waveguides of the connecting portions 8 and 9 became large and the element length became long. In addition, in the deployment section, after being deployed parallel to the reference line CL at intervals smaller than the final stage exit path, the deployment section is redeployed to the interval of the final stage exit path, resulting in a problem that the deployment section becomes long. [0008] Regarding the length of the expanded portions in the optical circuit, the outermost expanded portions 14 and 21, where the interval between expansions is the largest, are the longest. Therefore, if the expansion section is designed to be as short as possible, the final stage (
It is determined by the angle β between the tangent of the expanded portion 14 at the connection point of the Y-branch portion 4 (in the illustrated example, the third stage) and the reference line CL, and the interval between the outermost expanded portions 14 and 21. In the case of FIG. 7, β is always 1/2 of the branching angle of the final Y-branch, so the length of the developed part is determined by the interval between the outermost developed parts 14 and 21, and the shortest Even with the design, miniaturization was insufficient. Furthermore, in order to shorten the length of the expanded portion, the branching angle of the final stage Y-branching portion must be increased, resulting in the problem of increased branching loss. Means for Solving the Problems In order to solve the above-mentioned conventional problems, in the present invention, from the first stage entrance path 22 whose end faces one side of the substrate, to the end which faces the opposite side of the substrate. It has a tree-structured multilayer structure having two or more Y-branch sections up to the final stage output path 23 facing the substrate, and the optical axes of the input and output paths are parallel to the normal line of the side surface of the substrate. In the branch optical circuit, the angle θ between the branch center line and the reference line for the outermost Y branch from the second stage to the final stage, with the central axis extension line CL of the first stage input path 22 as the reference line. However, a completely new waveguide circuit layout pattern was adopted in which the Y-branch portions are arranged so as to be inclined outward so as to gradually expand as one goes to the later stages. [Operation] The tangent line of the outermost expanded section 14 at the connection point of the Y-branch section 4 of the final stage and the central axis extension line CL of the first stage entrance path 22
Since the angle β formed by the Y-branch can be made larger than 1/2 of the branch angle α3 of the Y-branch, the expanded portion can be shortened without increasing the branch angle of the Y-branch. [0012] Furthermore, Y branch parts 1, 2, 3, 4, 5, 6,
Since the branch center line of No. 7 and the central axis extension of the first stage entrance path 22 are not arranged in parallel, there is no need to use curved waveguides with large bending angles for the connecting portions 8 and 9, and the connecting portions can be shortened. These shorten the element length and reduce propagation loss. Furthermore, if the element length is designed to be the same as that of the conventional one, it becomes possible to use a curved waveguide with a larger radius of curvature than that of the conventional one, and bending loss can be reduced. [Embodiments] Hereinafter, embodiments of the present invention shown in the drawings will be explained in detail. FIG. 1 is a plan view showing a first embodiment of an eight-branch optical circuit according to the present invention, and FIG. 2 is a plan view showing an enlarged main part of FIG. First stage Y branch part 1, second stage Y branch part 2, 3, and third stage (final stage) Y branch part 4, 5 formed on the substrate 31.
, 6, 7 are connection parts 8, 9 composed of only straight waveguides.
, 10, 11, 12, 13 are connected in multiple stages in a tree shape, with third stage Y branch parts 4, 5, 6, 7 and final stage emission paths 23, 24, 25 parallel to the normal line of the side surface of the substrate 31. ,26,27
, 28, 29, 30 are development parts 14, 15, 16, 17
, 18, 19, 20, and 21 are smoothly connected. As an example, the branch angle α1 of the first stage Y branch part
= 1°, second stage Y branching angle α2 = 1°, third stage Y
Assuming that the branching angle α3 of the branching portion is 1°, and using the central axis extension line CL of the first stage entrance path 22 as a reference line, the angle θ formed between the centerline of the outermost Y branching portion and the reference line CL is the second θ2 = α2 /2 at Y-branches 2 and 3, θ at 3rd Y-branch 4 and 7
3=(α1+α2)/2, and it is gradually expanded toward the later stages. In the above configuration, the Y branch portions 1, 2, 3, 4,
Since the branch center lines 5, 6, and 7 are not arranged in parallel with the reference line CL, the connecting portions 8 and 9 can be shortened without using curved waveguides with large bending angles. In addition, the tangent line of the outermost developed part 14 at the connection point of the third stage Y branch part 4, which is the final stage, and the central axis extension line CL of the first stage entrance path 22
Angle β=(α1+α2+α3)/2>α3/
2, and the expanded portion can be shortened without increasing α3. These effects make it possible to downsize the device and reduce loss. In this example, first stage entrance path length = 5 mm, final stage exit path length = 5 mm, final stage exit path interval = 250 μm, element length = 4
2 mm, and the radius of curvature of the curved waveguide was 150 mm. In the present invention, the branching angles α1 and α of the Y-branching portion of each stage are
2, α3... is desirably 1° or less so as not to increase branching loss. Further, it is desirable that the minimum interval between the developed parts 17 and 18 located at the innermost side with the first stage incidence path 22 at the center is such an interval that mode coupling does not occur between the waveguides. It is not necessary that all the branching angles of the Y branching parts from the first stage to the final stage are the same, but it is preferable that the branching angles of the Y branching parts located in line-symmetrical positions with respect to the reference line CL are at least the same. desirable. The single-mode 8-branch optical circuit of the first embodiment of the present invention described above and the single-mode 8-branch optical circuit of FIG. 6 as a comparative example were fabricated by a two-stage natural ion exchange method. The first stage input path length, final stage output path length, final stage output path interval, and element length of the eight-branch optical circuit of this comparative example were the same as those of the first example. The curved waveguide has a maximum radius of curvature of 80 mm that can be used under these conditions.
mm was used. For details of the two-stage natural ion exchange method, Sugawara et al.
This is stated in a paper (Paper No. 369) presented at the Materials Division National Conference, etc., but to briefly explain, the surface of the glass substrate is coated with a mask film to prevent ion transmission, and this mask film is coated with a predetermined waveguide pattern. An opening is formed in advance, and the mask film-coated glass substrate is brought into contact with a molten salt containing monovalent cations that increase the refractive index of the glass, thereby exchanging ions in the salt with ions in the glass. , forming a high refractive index region having a substantially semicircular cross section with a distribution in which the refractive index gradually decreases from the mask opening toward the inside. Then,
After removing the mask film, the glass substrate is brought into contact with a molten salt containing monovalent cations that are effective in reducing the refractive index of glass. By this second stage ion exchange treatment, the center of the maximum refractive index of the high refractive index region moves from the substrate surface to a deep portion, and the cross-sectional shape of the entire high refractive index region becomes approximately circular. The results of measuring the excess loss of the 8-branch optical circuit of the first embodiment are shown in the table below. [0022] According to the present invention, the loss can be significantly reduced at all measurement wavelengths compared to the conventional method. Since bending loss increases on the long wavelength side, the loss reduction effect is effective in the case of a single mode waveguide where the refractive index difference of the waveguide is small and bending loss is likely to occur, or on the long wavelength side (1.55 μm). becomes larger. [0023] In the first embodiment described above, the connection between the branch parts is constructed of only straight waveguides, but it may also be constructed of only curved waveguides or a combination of straight waveguides and curved waveguides. can. FIG. 3 is a plan view of essential parts showing a second embodiment of the eight-branch waveguide according to the present invention. The connecting parts 8 and 9 are constructed by combining a straight waveguide and a curved waveguide having a bending angle θR, and the other connecting parts are constructed only by straight waveguides. In this example, β=(α1+α2+α3+2θ
R)/2, the expanded portion can be made even shorter than in the first embodiment, and miniaturization and low loss can be achieved. It is desirable to set θR such that the interval between the innermost expanded portions 17 and 18 is such that mode coupling does not occur. Furthermore, it is desirable that θR be 10° or less so as not to increase bending loss. In this example, the branch angle α1 of the first stage Y branch part is
= 0.5°, the branching angle α2 of the second stage Y branch part = 1°, the branching angle α3 of the third stage Y branch part = 1°, and θR = 0.25°. Further, the first stage entrance path length = 5 mm, the final stage exit path length = 5 mm, the final stage exit path interval = 250 μm, and the radius of curvature of the curved waveguide = 150 mm. When curved waveguides are used for the connection part, the two curved waveguides constituting the connection part between the first stage Y branch part and the second stage Y branch part are connected to the standard as shown in the second embodiment. They need to be convex so that they are separated from each other when viewed from the line CL. However, there is generally no need for curved waveguides constituting the connection sections after the second stage Y branch section. Regarding the connection parts after the fourth stage Y-branch part, which are required for 16 branches or more, it is preferable that the curved waveguides of the connection parts are all convex with respect to the reference line CL. Next, we will discuss the 8-branch optical circuits of the first and second embodiments of the present invention and the 8-branch optical circuits of the conventional structure (FIGS. 6 and 7).
FIGS. 4 and 5 show an example of the results of calculating the element length for the device shown in FIG. [0028] As a calculation condition, the length of the Y branch part is 2 m.
m, the waveguide spacing at the end = 20 μm, the branching angle = 1°, the first stage entrance path length = 0 μm, and the final stage exit path length = 0 μm. Furthermore, for both embodiments of the present invention and the conventional configuration shown in FIG. 7, the waveguide interval at the end of the connection portion = 40 μm;
The minimum waveguide spacing in the developed section was set to 40 μm or more. Furthermore, in the configuration of the second embodiment of the present invention, the bending angle θR of the curved waveguide
= 0.5°. The interval between the final stage exit paths and the radius of curvature of the curved waveguide were all the same, and the element length was calculated by varying these. From the results shown in FIGS. 4 and 5, it is clear that the eight-branch optical circuit can be significantly miniaturized by the present invention. In the first embodiment, when the radius of curvature is 150 mm and the interval between the final stage exit paths is 500 μm, the element length is 36.1 mm, which is 19.8 mm shorter than the conventional structure shown in FIG. The 8-branch optical circuit in FIG. 7 has a greater effect of shortening the element length if the interval between the final stage output paths is larger than that in FIG. It turns out that it is enough. Furthermore, in the second embodiment, the element length is approximately 2 mm shorter than that in the first embodiment;
The effect increases as the number of Y-branch sections from the first-stage entrance path to the final-stage exit path increases, and the Y-branch section
A multi-branch optical circuit with 16 or more branches, which has more than 16 stages, has a great effect. Due to the above-described effects, a multi-branch optical circuit with reduced element size and reduced loss can be realized. [0030] According to the present invention, in a multi-branch optical circuit, it is possible to shorten the length of the expanded portion and the connecting portion, so that the element can be miniaturized and excess loss can be reduced. Furthermore, if the element length is the same as that of the conventional structure, it is possible to use a curved waveguide with a larger radius of curvature than that of the conventional structure, and bending loss can be reduced. Due to the above effects, it is possible to realize a multi-branched waveguide with a smaller element and reduced loss. In addition to glass waveguides, the present invention also provides quartz-based waveguides, Ti-diffused LiNbO3
It can also be applied to optical branch circuits using waveguides, compound semiconductor waveguides, plastic waveguides, etc.

[Brief explanation of the drawing]

[Fig. 1] A plan view showing a first embodiment of the present invention.

[Figure 2] Figure 1
A plan view showing an enlarged view of the main parts of the optical circuit.

[Fig. 3] A plan view of main parts showing a second embodiment of the present invention.

FIG. 4 is a diagram showing an example of the results of element length calculations for an 8-branch optical circuit according to the present invention and an 8-branch optical circuit with a conventional structure.

[Fig. 5] 8-branch optical circuit according to the present invention and 8-branch optical circuit according to the conventional structure
Diagram showing another example of the result of calculating the element length for a branch optical circuit

[Fig. 6] A plan view showing an example of a conventional multi-branch optical circuit.

[Figure 7
] A plan view showing another example of a conventional multi-branch optical circuit.

[Explanation of symbols]

1. ．． ．． ．． First stage Y branch 2, 3. ．． Second stage Y branch part 4, 5, 6, 7. ．． ．． Third stage Y branch part 8, 9, 10, 11, 12, 13. ．． ．． Connection part 14,1
5, 16, 17, 18, 19, 20, 21. ．． ．． ．． Expanding section 22. ．． ．． First stage entrance path 23, 24, 25, 26, 27, 28, 29, 30. ．．
．． ．． Final stage exit path 31. ．． ．． substrate

Claims (1)

[Claims] Claim 1: A Y branch part is provided in two or more stages from a first stage entrance path whose end faces one side of the substrate to a final stage exit path whose end faces the opposite side of the substrate. , in a multi-branch optical circuit with a tree structure in which the optical axes of the input and output paths are parallel to the normal line of the side surface of the substrate, from the second stage to the final stage, using the central axis extension of the first stage input path as a reference line. For the outermost Y branch up to the step, the angle θ between the branch center line and the reference line
The multi-branch optical circuit is characterized in that the Y-branch portions are inclined outward in a pattern that gradually expands toward the later stages.