Optical combiner

Abstract

To provide an optical multiplexer with which it is possible to obtain an optical multiplexer, the waveguide shape of which is bilaterally asymmetrical with respect to the progression direction of light, and which is constructed exactly as designed even when heating treatment is applied.SOLUTION: Provided is an optical multiplexer including at least three input optical waveguides, at least two directional couplers and at least one output optical waveguide, in which the waveguide shape composed of the input optical waveguides, etc., is bilaterally asymmetrical with respect to the progression direction of light. For the directional couplers or a Mach-Zehnder interferometer constituted by a combination of two directional couplers, there is provided at least one instance of a dummy waveguide, the waveguide shape of which is laterally inverted from the bilaterally asymmetrical waveguides with respect to the center line of the directional couplers, etc., the waveguides in the optical coupling portions of the directional couplers being thereby prevented from collapsing inward.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical multiplexer having at least three input optical waveguides and at least one output optical waveguide, the optical multiplexer having a dummy waveguide that does not function as a waveguide. [Background technology]

[0002] In recent years, in a composite light generating device used as a light source for an image projection device such as a glasses-type terminal or a portable projector, an optical multiplexer is known that uses a plurality of laser diodes as a light source and multiplexes and outputs the light from the light sources via a waveguide (see Patent Document 1). The optical multiplexer is manufactured through a manufacturing process in which low-refractive index and high-refractive index silicon oxide films are laminated on a silicon substrate using a known chemical vapor deposition (CVD) method, sputtering method, or the like, and then patterned by a photolithography method using a photomask to form a waveguide and a directional coupler made of a high-refractive index silicon oxide film, and then a low-refractive index silicon oxide film is laminated.

[0003] Here, in the process of laminating a low refractive index silicon oxide film after forming the waveguide, the filling density of the low refractive index silicon oxide on the inside of the optical coupling part where the two waveguides of the directional coupler are close to each other may not be sufficient, and in the heat treatment process for making the low refractive index silicon oxide film transparent, which follows the laminating process of the low refractive index silicon oxide film, the two waveguides in the optical coupling part may collapse symmetrically inward, resulting in a defective product that cannot obtain optical coupling as designed. In order to prevent the occurrence of such defective products, it is known to provide dummy waveguides, which are pseudo waveguides that are not intended for guiding light, symmetrically outside the optical coupling part (see Patent Documents 2 and 3).

[0004] However, for the optical multiplexer in the composite light generating device used as the light source for the image projection device, at least two directional couplers are provided in close proximity for the purpose of miniaturization, and the waveguide shape consisting of the input optical waveguide etc. is asymmetric with respect to the direction of light propagation, so that there is a high possibility that the two waveguides in the optical coupling portion will tilt inward asymmetrically. Therefore, the simple idea of ​​providing dummy waveguides symmetrically outside the optical coupling portion cannot be adequately addressed, and it may be difficult to obtain the optical multiplexer as designed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-195603 A [Patent Document 2] Japanese Patent Application Publication No. 5-093813 [Patent Document 3] JP 2012-022273 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the above circumstances, and has an object to provide an optical multiplexer in which the waveguide shape is asymmetric with respect to the light propagation direction, and which makes it possible to obtain an optical multiplexer as designed even if a heat treatment is performed. [Means for solving the problem]

[0007] The present invention provides an optical multiplexer having at least three input optical waveguides, at least two directional couplers, and at least one output optical waveguide, in which the waveguide shape formed by the input optical waveguides etc. is asymmetric with respect to the traveling direction of light, and characterized in having at least one dummy waveguide having a shape obtained by mirror-inverting the waveguide shape of the asymmetric waveguide with respect to a center line of the directional coupler or a Mach-Zehnder interferometer formed by combining two of the directional couplers. Here, the dummy waveguide is a pseudo waveguide that is not intended to guide light, has a shape obtained by inverting the left-right asymmetric waveguide with respect to the center line of the directional coupler or the Mach-Zehnder interferometer, and is formed simultaneously with the formation of the waveguide, on the outer side opposite the center line of the waveguide in the optical coupling part of the directional coupler, at the same time as the formation of the waveguide, using the same components as the waveguide, in order to prevent the waveguide in the optical coupling part of the directional coupler from falling inward. Also, the distance from the dummy waveguide to the center line is designed to be symmetrical with the asymmetric waveguide with respect to the center line, or to be greater than the symmetrical distance.

[0008] The degree of inward tilt of the optical waveguide in the optical coupling section is not constant, but varies depending on the design, manufacturing conditions, and other circumstances of the optical multiplexer. When the degree of tilt is greatest, the distance from the dummy waveguide to the center line is preferably a distance that is symmetrical with the asymmetrical waveguide with respect to the center line. However, when the degree of tilt is small, the symmetrical distance may cause the effect of preventing tilting of the dummy waveguide to be excessively effective, and tilting toward the dummy waveguide may occur, making optical coupling difficult. Therefore, it is necessary to adjust the effect of preventing tilting by making the distance of the dummy waveguide from the center line greater than the symmetrical distance depending on the degree of inward tilting. Here, the effect of preventing tilting decreases as the distance of the dummy waveguide from the center line increases.

[0009] In the optical multiplexer, it is preferable that the shape of the dummy waveguide is a shape obtained by partially omitting the left-right inverted shape. Here, the shape that is partially omitted from the left-right inverted shape means a shape in which, in the left-right inverted shape, dummy waveguides are omitted from a portion where the inward collapse of the waveguide hardly occurs due to circumstances such as the design and manufacturing conditions of the optical multiplexer.

[0010] It is preferable that the distance from the dummy waveguide to the center line of the directional coupler or the Mach-Zehnder interferometer is 1 to 2.5 times the distance symmetrical with the asymmetrical waveguide with respect to the center line. Here, if it is less than 1, it is preferable for preventing the waveguide from falling inward, but since it is close to the waveguide, unnecessary optical coupling may occur between the waveguide and the dummy waveguide, which may impair the multiplexing characteristics. If it exceeds 2.5 times, there is almost no effect of preventing the waveguide from falling inward in the optical coupling part.

[0011] Preferably, in the optical multiplexer, the input light in the at least three input optical waveguides includes at least red light, green light and blue light.

[0012] The optical multiplexer preferably includes three directional couplers.

[0013] The optical multiplexer preferably includes two directional couplers.

[0014] The present invention is preferably an image projection device using the optical multiplexer. Effect of the Invention

[0015] According to the present invention, in an optical multiplexer having at least three input optical waveguides, at least two directional couplers, and at least one output optical waveguide, in which the waveguide shapes formed by the input optical waveguides, etc. are asymmetric with respect to the traveling direction of light, the optical multiplexer has at least one dummy waveguide having a shape obtained by mirror-inverting the waveguide shape of the asymmetric waveguide with respect to the center line of the directional coupler or a Mach-Zehnder interferometer formed by combining two of the directional couplers. This prevents the waveguides from tilting inward at the optical coupling portion where the two waveguides of the directional coupler are close to each other, and makes it possible to obtain an optical multiplexer as designed, thereby improving the yield rate and reducing manufacturing costs. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a top view of the optical multiplexer according to the first embodiment. [Diagram 2] 2 is an XY sectional view of the optical multiplexer of the first embodiment. FIG. [Diagram 3] 2 is a cross-sectional view taken along line AB of the optical multiplexer according to the first embodiment. FIG. [Figure 4] FIG. 2 is a top view of an optical multiplexer according to second to fourth embodiments. [Diagram 5] FIG. 1 is a top view of the directional coupler of the first conventional example. [Figure 6] FIG. 11 is a QR cross-sectional view of the directional coupler of the conventional example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, examples for carrying out the present invention will be described with reference to the drawings, however, the present invention is not limited to these examples.

[0018] FIG. 5 is a top view of a directional coupler according to a first conventional example, in which dummy waveguides are provided symmetrically outside the optical coupling portion as described in Patent Document 2. The solid lines indicate the optical waveguides, and the dashed lines indicate the dummy waveguides.

[0019] Fig. 6 is a QR cross-sectional view of the directional coupler shown in Fig. 5, which is the conventional example 1, and (a) shows an ideal waveguide shape in design. (b) shows a state after a heat treatment process in the case where a dummy waveguide is not provided and the packing density of silicon oxide is insufficient, and shows a waveguide shape in which the two waveguides in the optical coupling part collapse inward, making it impossible to perform optical coupling as designed. (c) shows a case where a dummy waveguide is provided in addition to (b), and shows a waveguide shape in which the dummy waveguides are provided on the outsides of the two waveguides in the optical coupling part, preventing the two waveguides from collapsing inward, making it possible to perform optical coupling as designed.

[0020] Figure 1 shows a top view of the optical multiplexer of Example 1. It has a Mach-Zehnder interferometer consisting of two directional couplers (first and second directional couplers from the left) connected in series at the top, and a third directional coupler (third directional coupler) that uses the lower waveguide between the two directional couplers of the Mach-Zehnder interferometer as an optical coupling waveguide. Three types of light with different wavelengths are input from the three input optical waveguides on the left, and the multiplexed light multiplexed by the Mach-Zehnder interferometer and the third directional coupler is output from the central waveguide of the three output optical waveguides on the right.

[0021] Here, which of the three output optical waveguides the multiplexed light is output from can be changed by the design of the optical multiplexer, such as the length and position of the directional coupler, etc. In addition, the output waveguide that does not output the multiplexed light may have a termination within the multiplexer, since the amount of output light is small. In this case, it is desirable that the output direction of the termination be different from the output direction of the multiplexed light so as not to affect the output multiplexed light.

[0022] 1, the waveguide shape is asymmetric with respect to the light traveling direction, and as described below, the optical multiplexer has a dummy waveguide path having a shape obtained by inverting the waveguide shape of the waveguide that is asymmetric with respect to the center line of the Mach-Zehnder interferometer and the center line of the third directional coupler.

[0023] A dummy waveguide (dashed line) is provided at the top, and has a shape that is the left-right inversion of the waveguide shape of the lower waveguide in the third directional coupler, which is asymmetric with respect to the center line indicated by the dashed dotted line of the Mach-Zehnder interferometer.

[0024] In addition, a dummy waveguide (dashed line) is provided at the bottom, which has a shape obtained by left-right inversion of the upper waveguide shape between the two directional couplers of the Mach-Zehnder interferometer, which is asymmetric with respect to the center line of the third directional coupler, indicated by the dashed line. Here, it is sufficient for the dummy waveguide corresponding to the third directional coupler to have only straight and bent portions, and it may also have only straight portions.

[0025] FIG. 2 is an XY cross-sectional view of the optical multiplexer shown in FIG. 1, and shows a dummy waveguide that is provided so that the left and right of the waveguide shape of the asymmetric waveguide is reversed with respect to the center line of the Mach-Zehnder interferometer. (a) shows the ideal waveguide shape in the design. (b) shows the waveguide shape after the heat treatment process when the dummy waveguide is not provided and the packing density of the silicon oxide is insufficient. The waveguide on the right side of the optical coupling part has an asymmetric waveguide on the outside right, which prevents it from falling inward of the optical coupling part, but the waveguide on the left side of the optical coupling part falls inward of the optical coupling part, and the two waveguides in the optical coupling part are asymmetric, so optical coupling as designed cannot be performed. (c) shows the case where a dummy waveguide is provided for the above (b). By providing a waveguide whose shape is asymmetrical on the outside and further to the left of the left waveguide of the optical coupling portion, and a dummy waveguide whose shape is mirror imaged with respect to the center line of the optical coupling portion and located at a distance symmetrical with respect to the center line, the left waveguide of the optical coupling portion is prevented from falling inward, the two waveguides of the optical coupling portion become symmetrical, and optical coupling as designed can be achieved.

[0026] Figure 3 is an AB cross-sectional view of the optical multiplexer shown in Figure 1, and shows two types of dummy waveguides: a dummy waveguide that is provided so that the left and right sides of the waveguide shape of the asymmetric waveguide are reversed with respect to the center line of the Mach-Zehnder interferometer, and a dummy waveguide that is provided so that the left and right sides of the waveguide shape of the asymmetric waveguide are reversed with respect to the center line of the third directional coupler. Here, the right waveguide of the Mach-Zehnder interferometer also serves as the left waveguide of the optical coupling part of the third directional coupler. (a) shows the ideal waveguide shape in design. (b) shows the waveguide shape after the heat treatment process when no dummy waveguide is provided and the packing density of silicon oxide is insufficient. Although a waveguide exists to the left of the left waveguide of the optical coupling part of the third directional coupler, both of the two waveguides of the optical coupling part are inclined inward, and optical coupling as designed cannot be performed. Furthermore, asymmetric inclination occurs in the two waveguides of the Mach-Zehnder interferometer, which may cause deterioration of characteristics. (c) is a case where a dummy waveguide is provided so that the right waveguide of the optical coupling part of the third directional coupler, which is asymmetric with respect to the center line of the Mach-Zehnder interferometer, has a shape that is reversed left and right with respect to the center line of the Mach-Zehnder interferometer. In this case, the inclination of the two waveguides in the Mach-Zehnder interferometer becomes symmetric, and deterioration of characteristics can be reduced. Here, the left waveguide of the optical coupling part of the third directional coupler, which also serves as the right waveguide of the Mach-Zehnder interferometer, has a smaller degree of inclination, so the two waveguides of the optical coupling part become asymmetric. (d) is a comparison with (c) above, in which a dummy waveguide is provided so that the left and right of the left waveguide of the Mach-Zehnder interferometer, which is asymmetric with respect to the center line of the third directional coupler, are inverted. By providing the dummy waveguide on the outer right side of the right waveguide of the optical coupling portion, the right waveguide of the optical coupling portion is prevented from collapsing.

[0027] In (d) of Fig. 3, the dummy waveguide on the left side is a dummy waveguide having a left-right inverted shape of the waveguide shape that is asymmetric with respect to the center line of the Mach-Zehnder interferometer, and the dummy waveguide on the right side is a dummy waveguide having a left-right inverted shape of the waveguide shape that is asymmetric with respect to the center line of the third directional coupler. The two dummy waveguides provided on the left and right reduce the collapse of the two waveguides in the optical coupling portion of the third directional coupler, making it possible to perform optical coupling as designed.

[0028] FIG. 4(a) is a top view of an optical multiplexer of the second embodiment, and the second embodiment has a shape in which the dummy waveguide with respect to the center line of the Mach-Zehnder interferometer in the first embodiment is partially omitted. Specifically, the straight portion of the dummy waveguide closest to the Mach-Zehnder interferometer is omitted, and unnecessary optical coupling between the dummy waveguide and the Mach-Zehnder interferometer does not occur in the adjacent straight portion, so that the possibility of deterioration of the multiplexing characteristics due to the optical coupling can be eliminated. Although the effect of preventing the collapse of the waveguide corresponding to the omitted portion is reduced, the effect of the dummy waveguide in the other portions is effective. In this way, when it is necessary to prioritize the multiplexing characteristics over the effect of the dummy waveguide due to the design and manufacturing conditions of the optical multiplexer, the dummy waveguide can be partially omitted.

[0029] 4(b) is a top view of an optical multiplexer according to the third embodiment. In the third embodiment, when the distance from the dummy waveguide to the center line of the Mach-Zehnder interferometer in the first embodiment is α1, which is the distance symmetrical with the asymmetrical waveguide with respect to the center line, the entire dummy waveguide is translated so that the distance to the center line is α2, and the distance of the entire dummy waveguide is multiplied by "α2 / α1". Here, 1≦α2 / α1≦2.5.

[0030] 4(c) is a top view of an optical multiplexer according to a fourth embodiment. In the fourth embodiment, when the distance from the dummy waveguide to the center line of the Mach-Zehnder interferometer in the first embodiment is β1, which is a distance symmetrical with the asymmetric waveguide with respect to the center line, a part of the dummy waveguide is partially translated so that the distance to the center line is β2, and the distance of the part of the dummy waveguide is multiplied by "β2 / β1". Here, 1≦β2 / β1≦2.5.

Claims

1. an optical multiplexer having at least three input optical waveguides, at least two directional couplers, and at least one output optical waveguide, wherein a waveguide shape consisting of the input optical waveguides, the directional couplers, and the output optical waveguide is asymmetric with respect to a traveling direction of light, and wherein the optical multiplexer has at least one dummy waveguide having a shape obtained by inverting a waveguide shape of a waveguide that is asymmetric with respect to a center line of the directional coupler or a Mach-Zehnder interferometer formed by combining two of the directional couplers, in a direction crossing the directional coupler of the Mach-Zehnder interferometer along which the center line is drawn.

2. 2. The optical multiplexer according to claim 1, wherein the shape of said dummy waveguide is a shape obtained by partially omitting said left-right inverted shape.

3. 3. The optical multiplexer according to claim 2, wherein a distance from the dummy waveguide to a center line of the directional coupler or the Mach-Zehnder interferometer is 1 to 2.5 times a distance symmetrical with respect to the center line of the asymmetric waveguide.

4. 4. The optical multiplexer according to claim 1, wherein the input light in the at least three input optical waveguides includes at least red light, green light and blue light.

5. 5. The optical multiplexer according to claim 1, wherein the number of said directional couplers is three.

6. 5. The optical multiplexer according to claim 1, wherein the number of said directional couplers is two.

7. An image projection device using the optical multiplexer according to any one of claims 1 to 6.

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