WO2015147276A1

WO2015147276A1 - Optical waveguide device

Info

Publication number: WO2015147276A1
Application number: PCT/JP2015/059668
Authority: WO
Inventors: 徳一宮崎; 勝利近藤
Original assignee: 住友大阪セメント株式会社
Priority date: 2014-03-28
Filing date: 2015-03-27
Publication date: 2015-10-01
Also published as: JP2015191140A; JP6084177B2

Abstract

The purpose of the present invention is to provide an optical waveguide device that has little optical insertion loss even while achieving a decrease in size. Provided is an optical waveguide device having optical waveguides (5) formed on a substrate (4), and a merging/branching unit where optical waveguides branch or merge in at least a part of the optical waveguide, wherein: the merging/branching unit is configured so that a number N of optical waveguides (51) merge and then branch into a number M, greater than N, of optical waveguides (53, 54); at a position where the N optical waveguides and the M optical waveguides are directly connected to one another, an overall width (W1) of the N optical waveguides is smaller than an overall width (W2) of the M optical waveguides; and at this connection position, discontinuous stepped differences are formed on the outside contour of the optical waveguides.

Description

Optical waveguide device

The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device provided with an optical waveguide having a junction.

In the technical field such as optical communication or optical measurement, optical waveguide devices such as optical modulators or optical switches are often used. In an optical waveguide device, an optical waveguide is formed on a substrate such as a semiconductor substrate or a substrate having an electrooptic effect such as lithium niobate.

As the pattern shape of the optical waveguide, for example, a branching part where the optical waveguide branches into a plurality of parts, or a combining part where a plurality of optical waveguides join together, such as a Mach-Zehnder type optical waveguide, is used. In particular, in order to realize large-capacity and high-speed transmission, an optical modulator or the like corresponding to a multilevel modulation format or a polarization multiplexing system is widely used together with a digital coherent technology. For this reason, the pattern shape of the optical waveguide is complicated and enlarged as in a configuration in which a plurality of Mach-Zehnder optical waveguides are integrated.

In recent years, with the demand for miniaturization and high-density mounting of optical communication devices, market needs for miniaturization of optical waveguide devices are increasing. In order to reduce the size of the optical waveguide device, it is important to reduce the size of the branching or merging portion of the optical waveguide, that is, the so-called merging / branching portion, which is high in the element size.

In order to reduce the size of the joining / branching portion, it is conceivable to sharply bend the optical waveguide (decrease the radius of curvature), or increase the angle between the branched optical waveguides or the angle between the joined optical waveguides. However, in such a method, the light wave propagating from the optical waveguide into the substrate leaks at the bent portion or the coupling portion of the optical waveguide, and the optical insertion loss of the optical waveguide device increases.

In order to reduce the size of the joining / branching portion, a structure of an optical waveguide as in

Patent Document

1 or 2 has been proposed. In Patent Document 1, as shown in FIG. 1, an optical waveguide (joining / branching portion) 1, an input / output waveguide 11 that propagates in a single mode, and a pair of branching waveguides that branch from the input / output waveguide are provided. . The shape of the branching waveguide is provided with a multimode propagation part 13 in which the width of the optical waveguide is wider than that of the input / output waveguide 11 and a connection part 12 in which the width of the optical waveguide increases from the branching end toward the multimode propagation part. Yes.

In the case of the structure as shown in FIG. 1, light leakage occurs due to insufficient light confinement in the optical waveguide at the bent portion near the branch start portion (branch end). In addition, since the extending direction of the optical waveguide and the width of the optical waveguide simultaneously change in the connection portion, a large structural change occurs with respect to light propagation, and loss due to light mode conversion is also likely to occur.

If the width of the input / output waveguide 11 is increased to the branch start portion and radiation due to bending is suppressed, the thickened optical waveguide portion becomes an optical waveguide having a higher order mode larger than the second order. . In such a case, if the optical waveguide structure before and after the branch start part is different, for example, if there is a difference in the number of optical waveguides before and after the joint branch part, the electric field distribution before and after the branch start becomes inconsistent, and the higher-order mode Conversion to is likely to occur, and excess loss increases or wavelength dependence of the optical branching ratio occurs.

Also, in order to reduce the driving voltage or increase the bandwidth of the optical waveguide device, the substrate on which the optical waveguide is formed is thinned to a thickness of 20 μm or less, for example. In the case of an optical waveguide structure using such a thin plate, confinement of the optical electric field distribution in the cross-sectional lateral direction (direction perpendicular to the thickness direction) is weak in a cross-section perpendicular to the light propagation direction. In order to reduce the loss due to the bent portion of the optical waveguide using a thin plate, it is necessary to further strengthen the confinement of the optical waveguide. However, if the width of the optical waveguide is widened for that purpose, the optical waveguide condition that a higher order mode is more likely to be generated, and the wavelength dependency of the optical branching ratio at the coupling / branching portion may be a serious problem. In addition, in the thin plate structure, the leakage light generated at the junction / branch part, especially the leakage light generated between the branch optical waveguides, propagates as unnecessary light in the substrate, and the phenomenon that the unnecessary light recombines with the optical waveguide also occurs. However, this greatly affects the characteristics of the optical waveguide device.

In Patent Document 2, as shown in FIG. 2, the optical waveguide (joining / branching portion) 2 is separated from the multimode waveguide (straight line portion) 23 by using a single mode waveguide 21 as a multimode waveguide 23 by a taper portion 22. A structure including two single mode waveguides (branch waveguides) 24 and 25 arranged as (gap G1) has been proposed. It is also disclosed that the width W1 of the straight portion 23 is set smaller than the width W2 of the interval between the side wall surfaces outside the ends of the two branch waveguides.

However, as shown in FIG. 2, mode conversion of the optical waveguide occurs in the portion where the optical waveguide is divided. In particular, when a thin plate is used, not only the waveguide mode to the

waveguides

24 and 25 but also the conversion to the slab mode using the substrate as the core layer occurs. There is a problem that the operation of the part becomes unstable.

In Patent Document 3, as shown in FIG. 3, the optical waveguide (joint branching portion) 3 is arranged such that the main waveguide 31 and the two branching waveguides 32 are arranged with a predetermined gap (gap G) therebetween. A configuration for forming the mechanical coupling portion 33 is disclosed. With this configuration, excessive loss at the branch portion is reduced.

However, the length of the portion where the narrow gap (gap G) is formed or the portion where the width of the optical waveguide is narrow is long, and the required optical waveguide pattern accuracy is stably reproduced in the optical waveguide (joint branching portion) 3. It is difficult. Further, when applied to a thin plate, similarly to Patent Document 2, conversion to a slab mode using a substrate as a core layer occurs at the end of the optical waveguide, and there is a problem that the operation of the coupling portion becomes unstable. .

International Publication WO2010 / 082673 Japanese Patent No. 3970350 Japanese Patent Laid-Open No. 9-265018

The problem to be solved by the present invention is to solve the above-mentioned problems and to provide an optical waveguide device with small optical insertion loss while realizing miniaturization.

In order to solve the above problems, the optical waveguide device of the present invention has the following technical features.
(1) In an optical waveguide device having an optical waveguide formed on a substrate and a junction portion where the optical waveguide branches or joins at least a part of the optical waveguide, the junction portion includes N optical waveguides. The N optical waveguides are branched to more than N optical waveguides, and the entire N optical waveguides are directly connected to the N optical waveguides and the M optical waveguides. The width W1 is smaller than the entire width W2 of the M optical waveguides, and a discontinuous step is formed on the outer contour of the optical waveguide at the connection position.

(2) In an optical waveguide device having an optical waveguide formed on a substrate and a merging / branching portion where the optical waveguide diverges or merges with at least a part of the optical waveguide, the merging / branching portion joins N optical waveguides. A single optical waveguide, and then the single optical waveguide is branched into M optical waveguides greater than N. The single optical waveguide, the M optical waveguides, At the position where the optical waveguides are directly connected, the width W3 of the one optical waveguide is smaller than the entire width W2 of the M optical waveguides, and a discontinuous step is formed in the outer contour of the optical waveguide at the connection position. It is characterized by being.

(3) In the optical waveguide device according to (2) above, at the position where the N optical waveguides and the one optical waveguide are directly connected, the overall width W1 of the N optical waveguides is It is larger than the width W3 of one optical waveguide, and a discontinuous step is formed on the outer contour of the optical waveguide at the connection position.

(4) In the optical waveguide device according to any one of the above (1) to (3), a branch for removing higher-order mode light is provided upstream of the coupling portion of the optical waveguide in the light propagation direction. An optical waveguide is arranged.

(5) The optical waveguide device according to any one of (1) to (4), wherein the substrate has a thickness of 20 μm or less.

According to the present invention, in an optical waveguide device having an optical waveguide formed on a substrate and a junction portion where the optical waveguide branches or joins at least a part of the optical waveguide, the junction portion includes N optical waveguides. The N optical waveguides are merged and branched into more than N optical waveguides, and the entire N optical waveguides are located at a position where the N optical waveguides and the M optical waveguides are directly connected. The width W1 is smaller than the entire width W2 of the M optical waveguides, and a discontinuous step is formed in the outer contour of the optical waveguide at the connection position. Thus, an optical waveguide device can be obtained in which leakage light leaking out of the optical fiber is suppressed and the excess loss at the junction is small.

In addition, since the M optical waveguides (branch optical waveguides) can secure the width of the single mode waveguide from the branch end, the optical confinement is strong and the branch angle between the optical waveguides (branch optical waveguides) is increased. It is possible to reduce the radius of curvature of the branched optical waveguides, and the like, and a wide angle between the branched optical waveguides can be realized, and the optical waveguide device can be downsized.

Further, in an optical waveguide device having an optical waveguide formed on a substrate and a junction portion where the optical waveguide branches or joins at least a part of the optical waveguide, the junction portion is joined by N optical waveguides. A single optical waveguide, and then the single optical waveguide is branched into M optical waveguides greater than N. The single optical waveguide and the M optical waveguides are connected to each other. At the directly connected position, the width W3 of the one optical waveguide is smaller than the entire width W2 of the M optical waveguides, and a discontinuous step is formed on the outer contour of the optical waveguide at the connected position. Therefore, similarly to the above-described invention, it is possible to suppress excessive loss at the junction and reduce the size of the optical waveguide device.

It is a figure explaining the junction part disclosed by patent document 1. FIG. It is a figure explaining the junction part disclosed by patent document 2. FIG. It is a figure explaining the junction part disclosed by patent document 3. FIG. It is a figure explaining the structure of the optical waveguide device of this invention. (A) is a plan view of the entire optical waveguide device, (b) is an enlarged view of a region indicated by symbol A in (a), and (c) is an enlarged view of a region indicated by symbol B in (b). It is a figure which expands the connection part of the junction part in the optical waveguide device of this invention. It is a figure explaining the electric field distribution of the light wave in the optical waveguide device of this invention. It is a figure explaining the application example of the optical waveguide device of this invention, and is an example which provided the high-order mode light removal means in the front | former stage of the junction part. It is a figure explaining the example which joins two optical waveguides and branches to three optical waveguides. (A) is a figure which shows the whole Mach-Zehnder type | mold optical waveguide, (b) is an enlarged view of the junction C in (a). It is a figure explaining the example which provided one optical waveguide (connection part) between two optical waveguides and three optical waveguides. It is a figure explaining the result of having analyzed by the light propagation method (BPM) in the prior art example (a) and this invention (b) using the application example of FIG.

Hereinafter, the present invention will be described in detail using preferred examples.
As shown in FIGS. 4 to 6, the optical waveguide device of the present invention has an optical waveguide 5 formed on a substrate 4 and an optical waveguide having a junction part where the optical waveguide branches or joins at least a part of the optical waveguide. In the device, the merging / branching portion is configured such that N optical waveguides 51 merge and branch to M optical waveguides (53, 54) that are larger than N, and the N optical waveguides and At the position where the M optical waveguides are directly connected, the overall width W1 of the N optical waveguides is smaller than the overall width W2 of the M optical waveguides, and the outer contour of the optical waveguide at the connection position. Is characterized in that discontinuous steps are formed.

As a substrate used for the optical waveguide device of the present invention, for example, a material having an electro-optic effect such as lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate) can be used. is there. Further, a quartz substrate used for a planar lightwave circuit (PLC), or a material such as a polymer or a semiconductor can be used.

In addition, the present invention can be particularly preferably applied to an optical waveguide device using a thin plate having a substrate thickness of 20 μm or less. This is because the thin plate itself functions as a slab type optical waveguide, and confinement by the optical waveguide is weakened.

The optical waveguide used in the present invention is formed by a method of forming a portion higher than the refractive index of the substrate on the surface of the substrate, such as a thermal diffusion method using Ti or the proton exchange method, or forming a ridge or rib on the substrate. In addition, a method of forming a convex optical waveguide or the like can be employed.

Although not shown in FIG. 4, a control electrode can be formed along the optical waveguide 5. Control electrodes include various types of electrodes depending on the function or purpose, such as a signal electrode and a ground electrode for applying a modulation signal, a bias electrode for applying a DC voltage, and a switching electrode for switching an optical path. It has been known. The control electrode can be formed by depositing a metal film such as Ti / Au on the substrate by a vapor deposition method or a sputtering method, and patterning it using a photolithographic technique, or a gold plating method. Further, if necessary, a buffer layer such as a dielectric SiO ₂ may be provided on the surface of the substrate after the optical waveguide is formed to suppress the absorption or scattering of light waves by the electrode formed on the upper side of the optical waveguide.

The characteristics of the optical waveguide device of the present invention lie in the shape of the coupling portion of the optical waveguide shown in FIGS. 4 (a) to 4 (c). FIG. 4A shows an example of a so-called nested optical waveguide in which a sub Mach-Zehnder type waveguide is incorporated in a branching waveguide of the main Mach-Zehnder type waveguide. A portion surrounded by a dotted line indicates a branching portion or a merging portion of the optical waveguide. FIG. 4B is an enlarged view of the dotted area A in FIG. 4A, and FIG. 4C is an enlarged view of the dotted area B in FIG. 4B.

In the optical waveguide device according to one aspect of the present invention, the optical waveguides before and after the junction are axisymmetric, and the number of optical waveguides at the junction is one before branching as shown in FIG. The latter illustrates two Y branches (1 × 2 branches). As will be described later, the configuration of the branching unit according to the optical waveguide device of the present invention is such that N optical waveguides merge and branch into more than N optical waveguides. Can be applied. Further, in the present invention, the relationship between the number of optical waveguides in the connecting portion and the width of the optical waveguide is important, and the effect of the present invention can be obtained even when the input / output direction of the coupling / branching portion is switched.

As shown in FIG. 4B, the width of the optical waveguide is usually set so that the single optical waveguide 51 on the input side is a single mode optical waveguide. In order to reduce the discontinuity of the electric field at the connection portion (connection position in FIG. 4C) from the width, the optical waveguide is tapered and thickened as indicated by reference numeral 52. In the connecting portion on the branch optical waveguide (53, 54) side, the branch optical waveguide (53, 54) is connected with a gap in order to reduce the influence of pattern accuracy.

At this time, since the loss at the bent portion on the two branch side is reduced, the optical waveguide width of the minimum curvature radius portion of the bent portion is also shown in FIG. The same optical waveguide width may be prepared. As a result, the angle between the branched optical waveguides can be increased, and an optical waveguide device that is smaller and that suppresses light emission due to bending can be provided.

If the taper-shaped optical waveguide 52 is made too wide, a large structural change occurs with respect to the propagation of light, and loss due to light mode conversion also occurs. In one aspect of the present invention, as shown in FIG. 4C and FIG. 5, regarding the relationship of the width of the optical waveguide at the connection position, the optical waveguide width W1 on the side where the number is small before and after the start of branching is set to the number of optical waveguides. When the total width of the optical waveguide on the larger side is W2, W1 <W2. Thereby, the matching of the optical electric field distribution before and after the connection of the coupling / branching portion is increased, and the wavelength dependence of the loss or the branching ratio can be reduced. The electric field distribution of the light wave propagating in FIGS. 5 and 6 is schematically shown by reference numerals L1 to L5.

In the junction / branch portion according to the present invention, the entire width W1 of the optical waveguide to be merged and the entire width W2 of the optical waveguide to be branched are discontinuous in the outer contour of the optical waveguide at W1 <W2. A step is formed. Conventionally, such a step has been considered to cause light wave scattering and cause excessive loss.However, as a result of extensive research, the present inventors have conducted divergent guidance even if such a step exists. As a result, it has been found that securing a wider waveguide width reduces the excess loss, and the present invention has been completed.

By improving the light confinement at the coupling / branching portion, it is possible to suppress the generation of the leaked light L0 as shown in FIG. As a result, in the Mach-Zehnder type optical waveguide, the element size required for the junction or bending is reduced, and an optical waveguide device excellent in characteristics such as insertion loss and On / Off extinction ratio can be obtained while being small. .

FIG. 7 shows another embodiment of the optical waveguide device according to the present invention. On the upstream side (optical waveguide 50) in the light propagation direction of the coupling portions (51 to 54), there are branched optical waveguides (55, 56). This is an example of arrangement. By providing the branched optical waveguides (55, 56), it is possible to prevent higher-order mode light from entering the coupling / branching portion. By adopting such a structure, even when the electric field distribution of light deviates from the center of the optical waveguide due to, for example, fiber connection displacement or bending, variation in the wavelength dependence of the branching ratio can be suppressed. it can. In particular, it is more effective when the waveguide substrate is a thin plate of 20 μm or less. Naturally, the shape shown in FIG.

FIG. 8 (b) shows a merging / branching portion (2 × 3 branches) having two optical waveguides to be merged and three optical waveguides to be branched. For example, as shown in FIG. 8A, the two optical waveguides (61, 62) to be joined are two branch optical waveguides of the Mach-Zehnder type optical waveguide, and the center of the three optical waveguides to be branched is The output optical waveguide 63 and the two optical waveguides (64, 65) sandwiching the output optical waveguide can also be used as a leakage light optical waveguide that guides the leakage light at the junction.

In the case of FIG. 8 as well, the entire width W1 of the optical waveguide on the merging side is made smaller than the entire width W2 of the optical waveguide on the branch side, similarly to the case of the 1 × 2 branch Y branch. It becomes possible to suppress excessive loss.

Further, as shown in FIG. 9, two (N) optical waveguides (71, 72) merge to form one optical waveguide 73, and then, the one optical waveguide 73 is merged at the joining / branching portion. It is configured to branch into three (M) optical waveguides, which is larger than N. In order to suppress the excessive loss, the width W3 of the one optical waveguide is set to the M light guides at a position where the one optical waveguide 73 and the two (M) optical waveguides are directly connected. The width may be set smaller than the entire width W2 of the waveguide, and a discontinuous step may be formed in the outer contour of the optical waveguide at the connection position.

Further, in the coupling / branching portion of FIG. 9, the N optical waveguides as a whole are also at a position where the two (N) optical waveguides (71, 72) and the one optical waveguide 73 are directly connected. The width W1 is larger than the width W3 of the single optical waveguide, and an excessive loss can be suppressed by forming a discontinuous step on the outer contour of the optical waveguide at the connection position.

In order to confirm the effect of the coupling / branching part according to the present invention, the coupling / branching part provided with the branched optical waveguides (55, 56) as shown in FIG. 7 was analyzed by the light propagation method (BPM). The simulation result is shown in FIG. FIG. 10A shows a conventional example, where the width of the optical waveguide at the connection position of the junction is W1 = W2, and the outer contour of the optical waveguide is continuously connected.

On the other hand, FIG. 10 (b) is a joining / branching part according to the present invention, and has a shape as shown in FIG. 10A and 10B, it can be easily understood that leakage light is generated from the branch ends between the branched optical waveguides in the conventional example.

Furthermore, the optical confinement effect when the optical waveguide width ratio W1 / W2 was changed was examined from the analysis result by the light propagation method. A certain amount of leakage light suppression effect was confirmed when W1 / W2 was in the range of 0.5 to 0.9. More preferably, it was in the range of 0.6 to 0.8.

By setting the ratio W1 / W2 of the width of the optical waveguide within this range, it is possible to obtain an optical waveguide device in which coupling to unnecessary light or mode conversion at the connection portion is reduced and the loss or branching ratio is more stable. . Also, with respect to the bending waveguide section, it is possible to use an optical waveguide width in which optical confinement of the optical waveguide is high from the connecting section to the minimum bending radius section, so that the propagation loss is smaller and the radiation loss is suppressed. Recombination with the optical waveguide can be prevented.

According to the present invention, it is possible to provide an optical waveguide device with small optical insertion loss while realizing miniaturization.

4

Substrate

5, 50 to 74

Optical waveguide

6, 7 Joint branch

Claims

In an optical waveguide device having an optical waveguide formed on a substrate, and a junction part where the optical waveguide branches or merges in at least a part of the optical waveguide,
The merging / branching portion is configured such that N optical waveguides merge and branch into M optical waveguides greater than N,
At the position where the N optical waveguides and the M optical waveguides are directly connected, the overall width W1 of the N optical waveguides is smaller than the overall width W2 of the M optical waveguides. An optical waveguide device, wherein a discontinuous step is formed in the outer contour of the optical waveguide at a position.
In an optical waveguide device having an optical waveguide formed on a substrate, and a junction part where the optical waveguide branches or merges in at least a part of the optical waveguide,
The coupling / branching unit is configured such that N optical waveguides merge to form one optical waveguide, and then the one optical waveguide branches into M optical waveguides greater than N.
At the position where the one optical waveguide and the M optical waveguides are directly connected, the width W3 of the one optical waveguide is smaller than the entire width W2 of the M optical waveguides, An optical waveguide device characterized in that a discontinuous step is formed in the outer contour of the optical waveguide.
3. The optical waveguide device according to claim 2, wherein an overall width W1 of the N optical waveguides at the position where the N optical waveguides and the one optical waveguide are directly connected is the one optical waveguide device. An optical waveguide device having a width greater than the width W3 of the waveguide and having a discontinuous step formed on the outer contour of the optical waveguide at the connection position.
4. The optical waveguide device according to claim 1, wherein a branching optical waveguide for removing higher-order mode light is disposed upstream of the coupling portion of the optical waveguide in the light propagation direction. An optical waveguide device characterized by comprising:
5. The optical waveguide device according to claim 1, wherein the substrate has a thickness of 20 μm or less.