Optical modulator

Abstract

The present invention provides an optical modulator, which includes an optical waveguide (2) formed on a substrate (1) and modulates light waves propagating in the optical waveguide. The optical waveguide has a structure in which a plurality of branch portions (20-22) are connected in multiple stages. Regarding the first branch portion (20) among the plurality of branch portions, the branch waveguides (20a, 20b) branched from the branch point have a relaxation curve with a changing curvature, and in a specified interval from the branch point of the branch waveguide, it is formed by a relaxation curve with a curvature starting from 0. Moreover, the change in the curvature and the change in the width of the optical waveguide are set to be symmetric between the branch waveguides. The second branch portions (21, 22) formed in each branch waveguide are arranged at different positions between the branch waveguides in the direction along the propagation axis of the optical waveguide.

Description

Technical Field

[0001] The present invention relates to an optical modulator, and more particularly to an optical modulator in which an optical waveguide formed on a substrate has a structure in which a plurality of branch portions are connected in a plurality of stages. Background Art

[0002] Optical waveguide components such as optical modulators are widely used in the fields of optical communications and optical measurement. In particular, in nested optical modulators or polarization-combined optical modulators having a structure of multiple Mach-Zehnder optical waveguides, optical modulators with miniaturized component size or excellent optical characteristics or high-frequency characteristics are required.

[0003] In a nested type or polarization wave combining type optical modulator in which multiple modulation units are integrated in parallel, the arrangement of the processing wiring from the connection terminal (pad unit) to the modulation start point in the modulator substrate becomes a problem. Specifically, in order to ensure the processing space for the loop, the size of the substrate is increased, or the length of the processing line is increased, resulting in attenuation of high-frequency signals and the degradation of high-frequency characteristics.

[0004] Furthermore, in order to suppress degradation of optical characteristics such as light loss, the curvature of the optical waveguide needs to be reduced, which increases the size of the substrate and makes it difficult to miniaturize the optical modulator.

[0005] Patent documents 1 to 3 disclose that when a plurality of modulation units are integrated in parallel, the modulation start point of each modulation unit is staggered relative to the propagation direction of the light wave. This can reduce the processing wiring and suppress the increase in the size of the substrate. However, the position of the branching unit provided at the rear section of each branch waveguide is different relative to the branch waveguide branched from the branching unit, so the curvature or bending mode of the branch waveguide is naturally different for each branch waveguide. As a result, it becomes difficult to separate the light wave at the branching unit at a one-to-one branching ratio or to set the loss of the light wave propagating in each branch waveguide to be the same.

[0006] [Prior technical literature]

[0007] [Patent Literature]

[0008] [Patent Document 1] Japanese Patent No. 6233480

[0009] [Patent Document 2] Japanese Patent No. 5233765

[0010] [Patent Document 3] Japanese Patent No. 6220836 Summary of the invention

[0011] [Summary of the Invention]

[0012] [Problems to be solved by the invention]

[0013] The problem to be solved by the present invention is to solve the above problems, and to provide an optical modulator capable of reducing processing wiring, maintaining a branching ratio of 1:1 at a branch portion, and maintaining the loss of light waves propagating in each branch waveguide at the same level.

[0014]

Solution for Solving the Problem

[0015] In order to solve the above problems, the optical modulator of the present invention has the following technical features.

[0016] (1) An optical modulator includes an optical waveguide formed on a substrate and modulates light waves propagating in the optical waveguide. The optical waveguide has a structure connecting a plurality of branch portions in multiple stages. Regarding the first branch portion among the plurality of branch portions, the branch waveguide branched from the branch point has a relaxation curve with a changing curvature, and in a specified interval from the branch point of the branch waveguide, it is formed by a relaxation curve with a curvature starting from 0, and the change in the curvature and the change in the width of the optical waveguide are set to be symmetric between the branch waveguides. The second branch portions formed in each branch waveguide are arranged at different positions between the branch waveguides in the direction along the propagation axis of the optical waveguide.

[0017] (2) In the optical modulator described in (1) above, it is characterized in that the end point of the specified interval is a position where the distance between the branch waveguides becomes equal to or greater than the mode field diameter of the light wave propagating in the branch waveguide.

[0018] (3) In the optical modulator described in (1) or (2) above, it is characterized in that, regarding the first branch portion, the maximum value of the curvature of the branch waveguide with a longer length from the branch point to the second branch portion is smaller.

[0019] (4) In any of the optical modulators described in (1) to (3) above, it is characterized in that the branch waveguide has an arc or a straight line with a constant curvature in the middle of the relaxation curve of the curvature change.

[0020]

Effect of the Invention

[0021] The optical modulator of the present invention includes an optical waveguide formed on a substrate and modulates the light wave propagating in the optical waveguide. Among them, the optical waveguide has a structure connecting multiple branch portions in multiple stages. Regarding the first branch portion among the multiple branch portions, the branch waveguide branched from the branch point is composed of a relaxation curve with a changing curvature and an arc or a straight line with a constant curvature. In a specified section of the branch waveguide starting from the branch point, it is formed by a relaxation curve with a curvature starting from 0, and the change in the curvature and the change in the width of the optical waveguide are set to be symmetric between the branch waveguides. The second branch portions formed in each branch waveguide are arranged at different positions between the branch waveguides in the direction along the propagation axis of the optical waveguide. Therefore, an optical modulator can be provided, which can reduce processing wiring, maintain the branch ratio of the branch portions at one-to-one, and moreover, can maintain the losses of the light waves propagating in each branch waveguide at the same level. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 FIG. is a diagram showing a first embodiment of the optical modulator of the present invention.

[0023] Figure 2 FIG. is a diagram for explaining the structure of the branch portion of the optical modulator of the present invention.

[0024] Figure 3 FIG. is a diagram showing the change in the curvature of the branch waveguide of the branch portion.

[0025] Figure 4 FIG. is a diagram for explaining the symmetric region of the branch portion.

[0026] Figure 5 FIG. is a diagram showing a second embodiment of the optical modulator of the present invention.

[0027] Figure 6 FIG. is a diagram showing a third embodiment of the optical modulator of the present invention.

[0028] Figure 7 FIG. is a diagram showing a fourth embodiment of the optical modulator of the present invention.

[0029] Figure 8 FIG. is a diagram showing a fifth embodiment of the optical modulator of the present invention.

[0030] REFERENCE SIGNS LIST

[0031] 1 Substrate

[0032] 2 Optical waveguide

[0033] 20 - 22 Branch portions

[0034] 20a, 20b Branch waveguides

[0035] 31, 32 Modulation electrodes DETAILED DESCRIPTION OF THE INVENTION

[0036] Hereinafter, a preferred example is used to describe the optical modulator of the present invention in detail.

[0037] As Figures 1 to 4 shown, the optical modulator of the present invention includes an optical waveguide 2 formed on a substrate 1, which modulates the light wave propagating in the optical waveguide. It is characterized in that the optical waveguide has a structure connecting a plurality of branch portions (20-22) in multiple segments. Regarding the first branch portion 20 among the plurality of branch portions, the branch waveguides (20a, 20b) branched from the branch point have a relaxation curve with a changing curvature, and in a specified interval starting from the branch point of the branch waveguide, it is formed by a relaxation curve with a curvature starting from 0, and the change in the curvature and the change in the width of the optical waveguide are set to be symmetric between the branch waveguides. The second branch portions (21, 22) formed in each branch waveguide are arranged at different positions between the branch waveguides in the direction of the propagation axis of the optical waveguide ( Figure 2 the direction of the arrow X).

[0038] Figure 1 FIG. is a top view showing a first embodiment of the optical modulator of the present invention. An optical waveguide 2 and modulation electrodes (31, 32) are formed on a substrate 1. L represents the light wave incident on the optical waveguide 2. In Figure 1 it, the ground electrode and the bias electrode are omitted. Moreover, in Figure 1 it, the electrode configuration when the substrate 1 is of the X-cut type is illustrated, but of course, a Z-cut type substrate can also be used.

[0039] The substrate 1 can be a substrate having an electro-optic effect such as lithium niobate (LN), InP, or Si. Moreover, like a planar light circuit (PLC), a substrate such as quartz or an Si substrate that can increase the refractive index difference between the optical waveguide and the substrate can be used for a part of the substrate other than the part where an electric field is applied by the modulation electrode or the bias electrode to form branch portions and the like.

[0040] In the waveguide structure, various structures such as a rib waveguide or an embedded waveguide structure can be used in addition to the LN diffusion waveguide.

[0041] The optical modulator of the present invention is characterized in that when other branch portions (21, 22) are arranged in the subsequent stage of the branch waveguides (20a, 20b) branched by the Figure 1 branch portion 20, the positions of the branch portion 21 and the branch portion 22 are staggeredly arranged with respect to the propagation direction of the light L ( Figure 1 the lateral direction).

[0042] In order to ensure the configuration of the branch portions (21, 22) in such a subsequent stage, at the branch portion 20, not only the lengths of the branch waveguides 20a and 20b are changed, but also the shape of the optical waveguide of the branch portion is studied. It is essential to maintain the branching ratio at the branch portion at one-to-one and keep the losses of the light waves propagating in each branch waveguide at the same level.

[0043] As a specific structure of the branch portion, as Figure 2 shown, a easement curve with a changing curvature is used for the branch waveguide.

[0044] As an example of the easement curve, a clothoid or a cubic function with a constantly changing curvature is preferred. In addition, even a sine wave curve or a curve with a smoothly changing curvature change amount is also acceptable. However, as will be described later, in order to obtain an easement curve with equal curvature changes near the branch portion and connected to a curve with a different curvature, it is more convenient to use a cubic function or a clothoid.

[0045] Figure 3 Focusing on Figure 2 one of the branch waveguides, the change in curvature is shown.

[0046] Figure 3 The left end of the straight line of

[0047] In Figure 2 (or Figure 3 ) corresponds to the branch point of the branch portion. Starting from a curvature of 0, the curvature gradually increases, passes through a maximum value (curvature kmax), and then gradually decreases to a curvature of 0. Then, the branch waveguide bends in the opposite direction. At this time, the curvature change is that the curvature gradually increases, passes through a maximum value (curvature k’max), and then gradually decreases to a curvature of 0. The portion with the curvature change uses an easement curve. Figure 3 In

[0048] At the change point where the curvature increases and then decreases in

[0049] (or

[0050] As Figure 2As shown, the length of the lower branch waveguide from the branch point to the branch section of the rear section is longer than that of the upper branch waveguide. Therefore, the optical loss of the light wave propagating in the branch waveguide tends to increase. However, the maximum curvature (k2max or k2’max) of the lower branch waveguide is set to a value smaller than the maximum curvature (k1max or k1’max) of the upper branch waveguide. As a result, the bending of the branch waveguide on the longer side becomes gentle, so that the excess loss caused by bending can be reduced. As a result, the intensity of the light wave entering the branch section of the rear section from each branch waveguide can also be kept substantially constant between the branch waveguides.

[0051] The maximum curvature k1max (k2max) of the branch waveguide in the front half and the maximum curvature k1’max (k2’max) of the branch waveguide in the rear half may be the same or different. The same case can make the bent part more compact.

[0052] In addition, although Figure 2 not shown explicitly, as Figure 3 shown, a straight line (curvature 0) can be inserted at the part where the curvature becomes 0. The straight line part can be arranged between the transition curves, but it can also be configured to be arranged after the curvature 0 at the right end of the Figure 3 straight line to adjust the position of the branch section of the rear section (or the starting point of the action of the modulation electrode) (shifted backward).

[0053] Moreover, as a feature of the optical modulator of the present invention, as Figure 4 shown, in a specified section starting from the branch point of the branch section, it is formed by a transition curve with a curvature starting from 0, and the change in the curvature and the change in the width of the optical waveguide are set to be symmetric between the branch waveguides (20a, 20b). As a result, the light wave at the branch section can be branched with a branching ratio of one to one.

[0054] As Figure 3 shown, the symmetric region can be set within the range of the transition curve (the part where the curvature increases) (symmetric region 1), but it can also be set within the range up to the circular arc part with a constant curvature (curvature kmax) after the transition curve (symmetric region 2).

[0055] As Figure 4 shown, the terminal of the specified section of the symmetric region should be set to the part where the branch waveguides (20a, 20b) are separated by a specified distance d or more. In this way, the unstable region where there is a possibility that the light wave propagating in one branch waveguide transfers to the other branch waveguide is set as the symmetric region, and the stable part that does not transfer is excluded from the symmetric region, thereby ensuring a branching ratio of one to one. The distance d is preferably set to be 1 times or more of the mode field diameter of the light wave propagating in the branch waveguide.

[0056] Figure 5 2 is a diagram showing a second embodiment of the optical modulator of the present invention. Specifically, the branching portion using a relaxation curve is provided not only on the branching side (dashed line A) of the nested optical waveguide 2, but also on the combining side (dashed line B). Furthermore, by alternately exchanging the lengths of the branch waveguides on the branching side and the combining side, the optical path lengths between the branches of the main Mach-Zehnder structure of the nested optical waveguide can be made consistent.

[0057] Figure 6 1 is a diagram showing a third embodiment of the optical modulator of the present invention. In the polarization wave combining type optical modulator, a branching portion using a relaxation curve is adopted at the branching portion (dashed line A). After modulation, one of the two light waves emitted from the substrate 1 has its polarization plane rotated by a wavelength plate, and a polarization beam splitter is used as a polarization combiner (PBC) to combine the two light waves in a state where the polarization planes are orthogonal to each other.

[0058] Figure 7 yes Figure 6 The fourth embodiment of the optical modulator of the present invention is shown in the figure. The main feature is that the branching part using the relaxation curve is adopted not only in the first branching part 20 but also in the second branching part (21, 22). With this structure, not only the structure of the optical waveguide can be miniaturized, but also a structure can be realized in which the branching ratio is maintained at one to one and the light propagation loss is suppressed.

[0059] In addition, if Figure 8 As shown in the fifth embodiment of the optical modulator of the present invention, a three-branch structure of an optical waveguide may be adopted in the front section of the branching section (20 to 22), and a structure for removing the swinging portion (higher-order mode light) of the light wave propagating in the optical waveguide may be installed. Thus, the branching ratio of the branching section can be stably maintained at one-to-one. The three-branch structure in the front section of the branching section 20 is a structure for removing the swinging portion of the light wave generated when the light wave input from the outside is coupled to the optical waveguide 2 of the substrate 1. On the other hand, the three-branch structure in the front section of the branching section (21, 22) is a structure for removing the swinging portion of the light wave generated at the branching section (branching point or curved portion of the branch waveguide).

[0060] The optical waveguide formed on the substrate 1 is not limited to a structure formed by diffusion of Ti on the LN substrate, but may be a ridge type optical waveguide having a groove formed along the optical waveguide. Furthermore, it is a well-known technology for those skilled in the art to adopt a thin plate structure with a thickness of the substrate set to 20 μm or less and to achieve speed matching between light waves and microwaves (modulation signals), and it is self-evident that such a structure may also be incorporated into the optical modulator of the present invention.

[0061] The propagation direction of the light wave propagating in the optical waveguide of substrate 1 ( Figure 2 The X-axis) and its vertical direction ( Figure 2When the refractive index on the Y-axis has anisotropy, in order to obtain a one-to-one branching ratio, a symmetric structure is required such that the central axis of the branching start point coincides with the propagation direction (X-axis). Also, by using an S-shaped curve formed by combining easement curves and circular arcs with the same curvature change but different bending directions, the direction of the end point of the curve can be set to the propagation direction (X-axis direction). As a result, regarding the direction of the subsequent branched section to be connected, it can also be made to coincide with the X-axis direction, and it is easy to form a structure with a one-to-one branching ratio even in a multi-stage branch.

[0062]

Industrial Applicability

[0063] As described above, according to the present invention, it is possible to provide an optical modulator that can reduce processing wiring while maintaining a one-to-one branching ratio at the branched section, and can maintain the loss of light waves propagating in each branched waveguide at the same level.

Claims

1. An optical modulator includes an optical waveguide formed on a substrate and modulates light waves propagating in the optical waveguide, characterized in that, The optical waveguide has a structure that connects multiple branch portions to multiple segments. Regarding the first branch portion among the multiple branch portions, it has a first branch waveguide and a second branch waveguide that branch from a branch point. The first branch waveguide has a front half relaxation curve and a rear half relaxation curve. The front half relaxation curve is a relaxation curve in which the curvature continuously changes from a curvature of 0 to a first maximum curvature k1max, and then the curvature continuously changes from the first maximum curvature k1max to a curvature of 0. The rear half relaxation curve bends in a direction opposite to that of the front half relaxation curve, and is a relaxation curve in which the curvature continuously changes from a curvature of 0 to a second maximum curvature k1’max, and then the curvature continuously changes from the second maximum curvature k1’max to a curvature of 0. The second branch waveguide has a front half relaxation curve and a rear half relaxation curve. The front half relaxation curve is a relaxation curve in which the curvature continuously changes from a curvature of 0 to a third maximum curvature k2max, and then the curvature continuously changes from the third maximum curvature k2max to a curvature of 0. The rear half relaxation curve bends in a direction opposite to that of the front half relaxation curve, and is a relaxation curve in which the curvature continuously changes from a curvature of 0 to a fourth maximum curvature k2’max, and then the curvature continuously changes from the fourth maximum curvature k2’max to a curvature of 0. An arc having a constant curvature equal to the maximum curvature is disposed at least in part of a portion having any one of the first maximum curvature to the fourth maximum curvature. The terminals of the rear half relaxation curve of the first branch waveguide and the terminals of the rear half relaxation curve of the second branch waveguide are disposed at different positions in the direction along the transmission axis of the optical waveguide. In a specified interval from the branch point of the branch waveguide, between the first branch waveguide and the second branch waveguide, the change in the curvature and the change in the width of the optical waveguide are set to be symmetric with respect to the central axis of the branch start point, which is the same direction as the transmission axis. Moreover, the end point of the specified interval is set to a position where the distance between the first branch waveguide and the second branch waveguide is equal to or greater than the mode field diameter of the optical wave propagating in the branch waveguide. The second branch portions formed in each branch waveguide are disposed at different positions between the first branch waveguide and the second branch waveguide in the direction along the propagation axis.

2. The optical modulator according to claim 1, characterized in that, Regarding the first branch portion, the maximum curvature of the branch waveguide with a longer length from the branch point to the second branch portion is smaller.

3. The optical modulator according to claim 1 or 2, characterized in that, In the first branch waveguide or the second branch waveguide, there is a straight line between the front half relaxation curve and the rear half relaxation curve.

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