JP6260631B2 - Optical waveguide device - Google Patents

Description

  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 technical fields such as optical communication and optical measurement, optical waveguide devices such as optical modulators and optical switches are often used. In an optical waveguide device, an optical waveguide is formed on a substrate having an electro-optic effect such as lithium niobate or a substrate such as a semiconductor substrate.

  As the pattern shape of the optical waveguide, for example, a branching section where the optical waveguide branches into a plurality of parts, or a combining part where the plurality of optical waveguides join, such as a Mach-Zehnder type optical waveguide, is used. In particular, in order to realize large-capacity and high-speed transmission, optical modulators compatible with multilevel modulation formats and polarization multiplexing systems are widely used together with 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 portion and the merging portion of the optical waveguide, which are 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, light waves propagating from the optical waveguide into the substrate at the bent portion or coupling portion of the optical waveguide leak out, and the optical insertion loss of the optical waveguide device increases.

  In order to reduce the size of the coupling / branching portion, an optical waveguide structure as disclosed 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 due to insufficient optical confinement of the optical waveguide occurs at a bent portion near the branch start portion (branch end). Moreover, since the extending direction of the optical waveguide and the width of the optical waveguide simultaneously change at the connection portion, a large structural change occurs with respect to light propagation, and loss due to light mode conversion is 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 easily occurs, excessive loss increases, and wavelength dependence of the optical branching ratio occurs.

  In addition, in order to reduce the driving voltage of the optical waveguide device and to increase the bandwidth, 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 JP-A-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 part where the optical waveguide branches or joins at least a part of the optical waveguide, the junction part is a single optical waveguide, The optical waveguide is configured to be branched into more than one M optical waveguides, and the one optical waveguide includes a portion extending in a tapered shape before being directly connected to the M optical waveguides. At the position where the optical waveguide and the M optical waveguides are directly connected, the one optical waveguide and the M optical waveguides are parallel to each other, and the entire width W1 of the one optical waveguide is calculated as follows. It is smaller than the entire width W2 of the M optical waveguides, the center of the width W1 coincides with the center of the width W2 at the connection position, and a discontinuous step is formed in the outer contour of the optical waveguide. The M optical waveguides are formed from the connection position. The section of the first bent part up to including the minimum radius of curvature is set such that the width of the optical waveguide is the same.

(2) In the optical waveguide device according to (1), the relationship between the width W1 and the width W2 is set so that W1 / W2 is in a range of 0.5 to 0.9. Features.

(3) In the optical waveguide device according to (1) or (2), the one optical waveguide is a single-mode optical waveguide before the tapered portion.

(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 disposed upstream of the coupling portion of the optical waveguide in the light propagation direction. An optical waveguide is arranged.

(5) In the optical waveguide device according to any one of (1) to (4), 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 one optical waveguide. The optical waveguide is configured to be branched into more than one M optical waveguides, and the one optical waveguide includes a portion extending in a tapered shape before being directly connected to the M optical waveguides. At the position where the optical waveguides and the M optical waveguides are directly connected, the entire width W1 of the one optical waveguide is smaller than the entire width W2 of the M optical waveguides. Since the center of the width W1 and the center of the width W2 coincide with each other and a discontinuous step is formed in the outer contour of the optical waveguide, leakage leaks from the branch portions of the M optical waveguides. Suppresses light and excess loss at the junction Less light waveguide device is obtained.

  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.

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.
In the following, an example in which the number of N is one or more will be described, but the present invention can be more suitably applied particularly when the number of N is one.

  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. It is also possible to use materials such as quartz substrates, polymers, and semiconductors used in planar lightwave circuits (PLC).

  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 can be formed by a method of forming a portion higher than the refractive index of the substrate on the substrate surface, such as a thermal diffusion method using Ti or a proton exchange method, or by 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 according to function and 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. Furthermore, if necessary, a buffer layer such as dielectric SiO ₂ may be provided on the surface of the substrate after the optical waveguide is formed to suppress absorption and scattering of light waves by the electrodes formed on the upper side of the optical waveguide.

  The feature of the optical waveguide device of the present invention lies in the shape of the coupling portion of the optical waveguide shown in FIGS. 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 the present invention, the optical waveguides before and after the junction are axisymmetric, and the number of the optical waveguides at the junction is 1 before branching and 2 after branching, as shown in FIG. A Y branch (1 × 2 branch) of the book is illustrated. 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 that time, in order to reduce the loss at the bent portion on the two branch side, the optical waveguide width of the minimum radius of curvature portion of the bent portion is also shown in FIG. It is preferable to prepare the same optical waveguide width. 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 too wide, a large structural change occurs with respect to light propagation, and loss due to light mode conversion also occurs. In 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 When the total width of the optical waveguide is W2, W1 <W2. As a result, the matching of the optical electric field distribution before and after the connection of the coupling / branching portion becomes high, and the wavelength dependence of the loss and 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, the inventors have found that excessively reducing the width of the waveguide to reduce the excess loss has completed the present invention.

  By improving the light confinement at the coupling / branching portion, it is possible to suppress the generation of the leakage light L0 as shown in FIG. As a result, in the Mach-Zehnder type optical waveguide, the element size required for the junction and bending is reduced, and an optical waveguide device having excellent 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 portion (51-54), the branched optical waveguide (55, 56) is provided. 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 deviation 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. 8B shows a joining / branching portion (2 × 3 branches) having two optical waveguides to be joined 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 63 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. It is preferable that the width is set smaller than the entire width W2 of the waveguide, and a discontinuous step is formed on 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 portion according to the present invention, the coupling / branching portion provided with the branching optical waveguide (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 the junction part which concerns on this invention, and has a shape like 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 ratio W1 / W2 of the width of the optical waveguide 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 the range of 0.6-0.8.

  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 the coupling to unnecessary light and the mode conversion at the connection portion are reduced and the loss and branching ratio are more stable. . Also, with respect to the bending waveguide section, it is possible to use an optical waveguide width with high optical confinement of the optical waveguide from the connection 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, 50-74 Optical waveguide 6, 7 Joint branch

Claims (5)

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 portion is configured such that one optical waveguide branches into more than one M optical waveguides,
The one optical waveguide has a portion that expands in a tapered shape before being directly connected to the M optical waveguides,
At the position where the one optical waveguide and the M optical waveguides are directly connected, the one optical waveguide and the M optical waveguides are parallel to each other, and the entire width of the one optical waveguide W1 is smaller than the entire width W2 of the M optical waveguides. At the connection position, the center of the width W1 and the center of the width W2 coincide with each other, and the outer contour of the optical waveguide is not present. A continuous step is formed,
The M optical waveguides are set so that the widths of the optical waveguides are the same in the section from the connection position to the minimum bending radius of the first bent portion. device.   2. The optical waveguide device according to claim 1, wherein the relationship between the width W1 and the width W2 is set so that W1 / W2 is in a range of 0.5 to 0.9. Waveguide device.   3. The optical waveguide device according to claim 1, wherein the single optical waveguide is a single mode optical waveguide before the tapered portion.   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.

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