Optical modulator

Abstract

An optical modulator (10) exemplified by the present disclosure comprises: a first set of electrodes (122a-1, 122a-2) that are provided along a first region of a first optical waveguide (114-2), one on each side, and to which differential drive signals are applied; and a second set of electrodes (122b-1, 122b-2) that are provided along a second region of a second optical waveguide (114-1), one on each side, the second region corresponding to a region deviated from the first region in the direction of light propagation, and the differential drive signals being applied to the second set of electrodes. The orientation of an electric field generated in the first region by the first electrode set (122a-1, 122a-2) and the orientation of the electric field generated in the second region by the second electrode set (122b-1, 122b-2) are opposite directions.

Description

Optical Modulator

[0001] The present disclosure relates to optical modulators.

[0002] Mach-Zehnder (MZ) optical modulators that utilize the electro-optic effect, such as thin-film LN optical modulators, are advantageous for broadening the bandwidth of optical modulators, and active technical research is underway. There are many reports that optical modulators that utilize the electro-optic effect are realized as single-phase drive optical modulators.

[0003] On the other hand, for drivers for driving optical modulators, a differential drive design is advantageous in terms of broadband and low power consumption. A method of driving an MZ optical modulator utilizing the electro-optic effect using a differential drive driver is reported in, for example, Patent Document 1.

[0004] In the technology described in Patent Document 1, two systems of differential drive signals S and S bar (hereinafter referred to as S) that are complementary to each other are output from a two-port differential drive driver. - ) are connected to the intermediate interconnect structure (IIS) via three drive signals S - , S, S - Then, the driving signals S - , S, S - In Patent Document 1, the IIS is also referred to as an "RF (radio frequency) substrate" or an "RF carrier."

[0005] US Patent Application Publication No. 2023 / 0305356

[0006] However, in the differential drive configuration described in Patent Document 1, an RF substrate for converting two systems of differential drive signals into three systems of drive signals is provided between the differential drive driver and the LN modulator, which may increase the size of the MZ optical modulator (e.g., the length in the light propagation direction).

[0007] An exemplary object of the present disclosure is to provide an optical modulator that can achieve differential driving, which is more advantageous than single-phase driving, without increasing the size of the optical modulator that utilizes the electro-optic effect.

[0008] Therefore, an optical modulator according to one aspect of the present disclosure comprises an input optical coupler, a first optical waveguide and a second optical waveguide made of a material having an electro-optic effect and guiding each of the two beams of light branched by the input optical coupler, an output optical coupler that combines the output light of the first optical waveguide and the second optical waveguide, a first electrode set provided on both sides along a first region of the first optical waveguide and to which a differential drive signal is applied, and a second electrode set provided on both sides along a second region of the second optical waveguide corresponding to a region shifted from the first region in the propagation direction of the light and to which the differential drive signal is applied, wherein the direction of the electric field generated in the first region by the first electrode set is opposite to the direction of the electric field generated in the second region by the second electrode set.

[0009] 1A and 1B are schematic top views showing a configuration example of an optical modulator according to a first embodiment. (A) is a cross-sectional view taken along the line A-A' in FIG. 1, and (B) is a cross-sectional view taken along the line B-B' in FIG. 1. A diagram showing a comparison example between differential driving (c) of this embodiment, single-phase driving (a), and normal differential driving (b). A schematic top view showing a configuration example of an optical modulator according to a second embodiment. (A) is a cross-sectional view taken along the line A-A' in FIG. 4, (B) is a cross-sectional view taken along the line B-B' in FIG. 4, and (C) is a cross-sectional view taken along the line C-C' in FIG. 4. A schematic top view showing a configuration example of an optical modulator according to a third embodiment. (A) is a cross-sectional view taken along the line A-A' in FIG. 6, (B) is a cross-sectional view taken along the line B-B' in FIG. 6, and (C) is a cross-sectional view taken along the line C-C' in FIG. 6. A schematic top view showing a configuration example of an optical modulator according to a fourth embodiment. FIG. 10 is a schematic top view illustrating a configuration example of an optical modulator according to a fifth embodiment.

[0010] Hereinafter, embodiments will be described in detail with reference to the drawings. However, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims. Furthermore, more detailed descriptions than necessary may be omitted. For example, detailed descriptions of well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0011] Furthermore, in the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. The drawings are schematic, and the dimensional relationships or ratios of elements may differ from reality. The drawings may also include portions in which the dimensional relationships or ratios differ. When numerical values ​​are used in the following description, they are merely examples, and other numerical values ​​may be used in addition or instead.

[0012] First Embodiment Fig. 1 is a schematic top view showing an example of the configuration of an optical modulator 10 according to a first embodiment. Fig. 2(A) is a cross-sectional view taken along the line A-A' in Fig. 1, and Fig. 2(B) is a cross-sectional view taken along the line B-B' in Fig. 1. In Figs. 1, 2(A), and 2(B), the x-axis corresponds to the width direction of the optical modulator 10, and the y-axis corresponds to the height (or thickness) direction of the optical modulator 10. The z-axis corresponds to the length direction of the optical modulator 10, in other words, the propagation direction of the light and electrical signals. This correspondence between the three axes is similar in other embodiments.

[0013] 1, the optical modulator 10 is, for example, an MZ optical modulator, and includes an input optical coupler 112 that branches input light into two, two parallel optical waveguides (arm waveguides) 114-1 and 114-2 that guide each of the two beams of light branched by the input optical coupler 112, and an output optical coupler 116 that combines the output light from the optical waveguides 114-1 and 114-2. In the following description, when the arm waveguides 114-1 and 114-2 are not to be distinguished from each other, they may be abbreviated as "arm waveguide 114."

[0014] Each of the input optical coupler 112, the two arm waveguides 114, and the output optical coupler 116 may be made of a material having an electro-optic effect, such as lithium niobate (LiNbO3), as a non-limiting example.

[0015] For example, as illustrated in the cross-sectional views of Figures 2(A) and 2(B), the optical modulator 10 includes a substrate (e.g., a Si substrate) 101, a lower cladding layer 102 provided on the substrate 101, an LN layer 103 provided on the lower cladding layer 102, and an upper cladding layer 104 provided on the LN layer 103.

[0016] The LN layer 103 may be, for example, an X-cut LN crystal thin film in which the electro-optic effect is prominent in a direction parallel to the substrate surface. The LN layer 103 may also be replaced with, for example, a layer of a polymer-based electro-optic material (EO polymer).

[0017] An input optical coupler 112, two arm waveguides 114, and an output optical coupler 116 are formed on the LN layer 103 by, for example, patterning a waveguide pattern. 2A and 2B exemplarily illustrate that the arm waveguides 114-1 and 114-2 are formed on the LN layer 103.

[0018] The input optical coupler 112, the two arm waveguides 114, and the output optical coupler 116 are surrounded by a lower cladding layer 102 and an upper cladding layer 104 made of a material (e.g., SiO 2 ) with a lower refractive index than the LN layer 103, respectively.

[0019] In other words, the input optical coupler 112, the two arm waveguides 114, and the output optical coupler 116 are each embedded in the cladding material. With this structure, light input to the optical modulator 10 is confined and propagated within the input optical coupler 112, the arm waveguides 114-1 and 114-2, and the output optical coupler 116. The lower cladding layer 102 and the upper cladding layer 104 may have the same or different compositions.

[0020] As shown in FIGS. 1 and 2A, on both sides of one arm waveguide 114-2 in the x-axis direction, for example, signal electrodes 122a-1 and 122a-2 are provided extending in the z-axis direction along part of the arm waveguide 114-2.

[0021] The signal electrodes 122a-1 and 122a-2 are an example of a first electrode set and are made of a conductive material such as aluminum (Al) or gold (Au). Note that the "signal electrodes" are electrodes to which high-frequency signals such as RF signals are applied, and may also be called "high-frequency electrodes" or "traveling-wave electrodes."

[0022] The signal electrode 122a-1 is arranged, for example, on one of the two sides of the arm waveguide 114-2 in the x-axis direction, farther from the arm waveguide 114-1. The signal electrode 122a-2 is arranged, for example, on one of the two sides of the arm waveguide 114-2 in the x-axis direction, nearer to the arm waveguide 114-1.

[0023] In other words, the arm waveguide 114-2 has a first region, a part of which is sandwiched on both sides in the x-axis direction by the signal electrodes 122a-1 and 122a-2 in top view. The first region corresponds to a partial modulation region where light propagating through the arm waveguide 114-2 is modulated.

[0024] A differential drive signal S, which is a high frequency signal such as an RF signal, is applied to one signal electrode 122a-1, and a differential drive signal S, which is complementary to the signal S, is applied to the other signal electrode 122a-2. - will be approved.

[0025] 1 and 2B, signal electrodes 122b-1 and 122b-2 are provided on both sides of the other arm waveguide 114-1 in the x-axis direction, extending in the z-axis direction along a part of the arm waveguide 114-1. The signal electrodes 122b-1 and 122b-2 are also made of a conductive material such as Al or Au.

[0026] The signal electrode 122b-1 is arranged, for example, on one of the two sides of the arm waveguide 114-1 in the x-axis direction, farther from the arm waveguide 114-2. The signal electrode 122b-2 is arranged, for example, on one of the two sides of the arm waveguide 114-1 in the x-axis direction, nearer to the arm waveguide 114-2.

[0027] In other words, the arm waveguide 114-1 has a second region sandwiched between the signal electrodes 122b-1 and 122b-2 on both sides in the x-axis direction in top view. The second region corresponds to a partial modulation region where light propagating through the arm waveguide 114-1 is modulated.

[0028] For example, the second modulation region may be a region shifted in the light propagation direction (z-axis direction) from the first modulation region in the arm waveguide 114-2. The first modulation region and the second modulation region may be regions that do not overlap each other in the light propagation direction (z-axis direction), or may be regions that partially overlap each other. Note that, although the second modulation region is formed after the first modulation region in the light propagation direction in FIG. 1, the first modulation region may be formed after the second modulation region.

[0029] The signal electrode 122b-1 is electrically connected to the signal electrode 122a-1 to which one differential drive signal S is applied in the first modulation region, for example. On the other hand, the signal electrode 122b-2 is electrically connected to the signal electrode 122a-1 to which the other differential drive signal S is applied in the first modulation region, for example. - is applied to the signal electrode 122a-2.

[0030] The phrase "electrically connected" between two electrodes means that the two electrodes are in a state of electrical conduction with each other. For example, the two electrodes may be integrated into a single electrode, or individual electrode elements may be connected via one or more conductors (e.g., multilayer wiring and / or wire wiring) without interfering with each other's electrical conduction.

[0031] 1, there are portions where the electrical connection paths intersect (or switch places) between the arm waveguides 114-1 and 114-2, but this does not pose a problem as long as the intersecting portions are electrically insulated from each other. For example, electrical isolation can be achieved by spatially separating the wiring at the intersecting portions using multilayer wiring.

[0032] With the electrode arrangement described above, an electric field is generated between the signal electrodes 122b-1 and 122b-2 in the second modulation region of the arm waveguide 114-1 in the opposite direction to the electric field (or electric field) generated between the signal electrodes 122a-1 and 122a-2 in the first modulation region of the arm waveguide 114-2.

[0033] By reversing the direction of the electric field between the first modulation region and the second modulation region, for example, the sign of the refractive index change due to the electro-optic effect of each arm waveguide 114 is reversed between the first modulation region and the second modulation region, thereby realizing the push-pull operation of the MZ optical modulator 10.

[0034] Further, for each arm waveguide 114, the differential drive signals S and S are applied so that the direction of the electric field is reversed for each modulation region in the z-axis direction. - is applied, a potential difference twice as large as that in the single-phase drive is applied to each arm waveguide 114.

[0035] Therefore, for example, the modulation efficiency in the portion where the signal electrode of the MZ optical modulator 10 is arranged can be improved, and the length (z-axis direction) of the area where the signal electrode is arranged can be reduced. Therefore, it is possible to reduce the optical waveguide loss due to the signal electrode. Furthermore, since an RF substrate such as that described in Patent Document 1 is not required, it is possible to reduce the number of parts of the MZ optical modulator 10 or simplify the structure, and it is also possible to reduce the size of the MZ optical modulator 10.

[0036] 3 shows a comparison example between the differential drive of this embodiment (c), the single-phase drive (a), and the normal differential drive (b). In this comparison example, it is assumed that the optical modulator is driven by a differential drive driver with a supply voltage of 2 Vppd (1 Vpps).

[0037] The subscript "ppd" of the voltage V is an abbreviation for "peak-to-peak differential," and the subscript "pps" is an abbreviation for "peak-to-peak single-end." For single-phase drive (a), it is assumed that one of the differential drive signals is terminated before the input stage to the optical modulator.

[0038] Signal electrode "S" or "S - For ease of comparison, the distance between the signal electrode "S" and the ground electrode "G" in the width direction of the substrate surface is a distance "d". The signal electrode "S" represents an electrode to which a differential drive signal S is applied. - " is the differential drive signal S -3, the reason for making the comparison by making the positional relationship between the signal electrode and the waveguide the same is to make the comparison by making the influence of the increase in optical waveguide loss due to the signal electrode the same.

[0039] The electrode arrangement for single-phase drive (a) is such that, in a cross-sectional view, one signal electrode “S” is provided between two arm waveguides, and one ground electrode “G” is provided at a position away from each of the two arm waveguides.

[0040] The electrode arrangement for normal differential drive (b) is a cross-sectional view of a ground electrode "G" and a signal electrode "S" or "S" on both sides of each of the two arm waveguides. - " and a pair of signal electrodes "S" and "S - In this arrangement, another ground electrode "G" is provided between the electrodes "A" and "B".

[0041] As can be understood from a comparison of (a), (b), and (c) in FIG. 3, in the differential drive (c) of this embodiment, a differential potential difference ΔV=2V can be applied to each of the arm waveguides 114-1 and 114-2, so that the change in electric field ΔE=E + -E - can be doubled (ΔE=2 / d) compared with the single-phase drive (a) and the normal differential drive (b).

[0042] <Embodiment 2> Fig. 4 is a schematic top view showing an example of the configuration of an optical modulator 40 according to embodiment 2. Fig. 5(A) is a cross-sectional view taken along line A-A' in Fig. 4, Fig. 5(B) is a cross-sectional view taken along line B-B' in Fig. 4, and Fig. 5(C) is a cross-sectional view taken along line C-C' in Fig. 4.

[0043] The optical modulator 40 illustrated in FIG. 4 is an MZ optical modulator, and similar to the MZ optical modulator 10 illustrated in FIG. 1, includes an input optical coupler 112, arm waveguides 114-1 and 114-2, and an output optical coupler 116.

[0044] The MZ optical modulator 40 exemplarily outputs differential drive signals S and S - a signal electrode 41 to which one of the differential drive signals S and S is applied (for example, S) - The other (for example, S - ) is applied to a signal electrode 43.

[0045] For example, when viewed from above, one signal electrode 41 is electrically connected to T-shaped sub-electrodes 411 and 413 and L-shaped sub-electrodes 412 and 414. For example, when viewed from above, the other signal electrode 43 is electrically connected to T-shaped sub-electrodes 431 and 433 and L-shaped sub-electrodes 432 and 434.

[0046] 4, one arm waveguide 114-1 has, for example, a first modulation region sandwiched on both sides in the x-axis direction by T-shaped sub-electrodes 411 and 431. The sub-electrode 411 is an example of a first sub-electrode, and is arranged, for example, on one of the sides in the x-axis direction of the arm waveguide 114-1 closer to the arm waveguide 114-2.

[0047] The sub-electrode 431 is an example of a second sub-electrode, and is arranged, for example, on one of the two sides of the arm waveguide 114-1 in the x-axis direction, farther from the arm waveguide 114-2. The sub-electrodes 411 and 431 are an example of a first electrode set to which a differential drive signal is applied.

[0048] The other arm waveguide 114-2 has, for example, a second modulation region sandwiched on both sides in the x-axis direction by L-shaped sub-electrodes 412 and 432. The second modulation region may be, for example, a region shifted in the light propagation direction (z-axis direction) from the first modulation region in the arm waveguide 114-1. The first modulation region and the second modulation region may be regions that do not overlap each other in the light propagation direction (z-axis direction), or may be regions that partially overlap each other.

[0049] The sub-electrode 412 is an example of a third sub-electrode and is arranged, for example, on one of the two sides of the arm waveguide 114-2 in the x-axis direction, closer to the arm waveguide 114-1. The sub-electrode 432 is an example of a fourth sub-electrode and is arranged, for example, on one of the two sides of the arm waveguide 114-2 in the x-axis direction, farther from the arm waveguide 114-1. The sub-electrodes 412 and 432 are an example of a second electrode set to which a differential drive signal is applied.

[0050] Furthermore, one arm waveguide 114-1 has a third modulation region sandwiched on both sides in the x-axis direction by T-shaped sub-electrodes 413 and 433. The third modulation region may be, for example, a region shifted in the light propagation direction (z-axis direction) from the second modulation region in the arm waveguide 114-2.

[0051] The other arm waveguide 114-2 has a fourth modulation region sandwiched on both sides in the x-axis direction by L-shaped sub-electrodes 414 and 434. The fourth modulation region may be, for example, a region shifted in the light propagation direction (z-axis direction) from the third modulation region in the arm waveguide 114-1.

[0052] The second modulation region and the third modulation region may be regions that do not overlap each other in the light propagation direction (z-axis direction), or may be regions that partially overlap each other. Similarly, the third modulation region and the fourth modulation region may be regions that do not overlap each other in the light propagation direction (z-axis direction), or may be regions that partially overlap each other.

[0053] The electrode arrangement of the third modulation region may be equivalent to the electrode arrangement of the first modulation region, and the electrode arrangement of the fourth modulation region may be equivalent to the electrode arrangement of the second modulation region. In other words, the MZ optical modulator 40 may have a configuration in which the electrode arrangement of the first modulation region and the electrode arrangement of the second modulation region are alternately repeated along the light propagation direction between the arm waveguides 114.

[0054] In this way, the first to fourth modulation regions are alternately arranged along the light propagation direction (z-axis direction) between the arm waveguides 114-1 and 114-2 in the top view of Fig. 4. In other words, the MZ optical modulator 40 illustrated in Fig. 4 has a configuration in which partial modulation regions partially (or discontinuously) sandwiched between electrodes in each of the two arm waveguides 114 are alternately switched along the light propagation direction (z-axis direction) between both arm waveguides 114.

[0055] The spacing (pitch) of each modulation region in the light propagation direction, in other words, the pitch between the sub-electrodes in the z-axis direction, can be designed, for example, so that the lengths of the first to fourth modulation regions are sufficiently small relative to the maximum wavelength of the modulated electrical signal (for example, 1 / 20 or less). This allows, for example, the length of the sub-electrode that applies the modulated electrical signal to the arm waveguide in the light propagation direction to be a length that is not affected by frequency dependency. As a non-limiting example, the length of the portion of each sub-electrode that forms each modulation region along the arm waveguide 114 can be approximately 10 micrometers (μm) to 100 μm.

[0056] Although FIG. 4 illustrates an example in which two sets of partial modulation regions that alternate between both arm waveguides 114 are repeatedly provided along the light propagation direction (a total of four modulation regions), three or more sets may be repeatedly provided along the light propagation direction.

[0057] For example, assume that the length of the portion of each sub-electrode along the arm waveguide 114 is about 10 μm to 100 μm as described above, and the length in the z-axis direction of the MZ optical modulator 40 is about several millimeters (mm). In this case, a large number of modulation regions, for example, on the order of a dozen to several tens of regions, can be alternately arranged between the arm waveguides 114 along the light propagation direction (z-axis direction).

[0058] Furthermore, while FIG. 4 shows an example in which partial modulation regions are alternately arranged in the order of the arm waveguide 114-1 and the arm waveguide 114-2 with respect to the light propagation direction, the partial modulation regions may alternatively be arranged in the order of the arm waveguide 114-2 and the arm waveguide 114-1, conversely.

[0059] Furthermore, the shape of the sub-electrodes (reference numerals omitted) partially (or intermittently) arranged on both sides of each of the arm waveguides 114-1 and 114-2 along the light propagation direction is not limited to a T-shape or an L-shape. For example, it is sufficient for each of the sub-electrodes to be electrically connected to the signal electrode 41 or 43 and have electrode portions that partially extend along both sides of each arm waveguide 114.

[0060] In the electrode arrangement described above, a differential drive signal S is applied to the signal electrode 41, and a differential drive signal S is applied to the signal electrode 43. - When this is applied, an electric field is applied to both arm waveguides 114 such that the direction of the x-axis component is reversed for each modulation region.

[0061] Therefore, similarly to the first embodiment, push-pull operation is realized, and it is possible to apply a potential difference twice as large as that in single-phase driving to each arm waveguide 114. Furthermore, compared to the first embodiment, it is possible to narrow the pitch between the sub-electrodes in the light propagation direction (in other words, to increase the sub-electrode arrangement density in the light propagation direction).

[0062] By narrowing the pitch between the sub-electrodes in the light propagation direction, it is easy to increase the number of modulation regions per arm waveguide 114, which can be expected to improve the modulation efficiency of the MZ optical modulator 40 and reduce the size of the MZ optical modulator 40 (e.g., shorten the length in the z-axis direction).

[0063] <Example of Wiring of Sub-Electrodes> Next, with reference to Figures 5(A) to 5(C), an example of electrical connection (wiring) in cross section between each of the sub-electrodes illustrated in Figure 4 and the signal electrode 41 or 43 will be described. As illustrated in Figures 5(A) to 5(C), each of the sub-electrodes and the signal electrode 41 or 43 may be electrically connected by multilayer wiring.

[0064] For example, each of the signal electrodes 41 and 43 is disposed on the upper surface of the upper cladding layer 104 at a position spaced apart in the x-axis direction from each of the arm waveguides 114-1 and 114-2.

[0065] As illustrated in FIG. 5A, the electrode portions extending in the z-axis direction of the T-shaped sub-electrodes 411 and 431 that form the first modulation region are arranged on both sides of the arm waveguide 114-1 in the width direction on the LN layer 103.

[0066] 5A, each of the electrode portions is connected to the signal electrodes 41 and 43 arranged on the upper surface of the upper clad layer 104 through an electrode portion (or wiring) formed in a crank shape so as to pass above the arm waveguide 114. Note that the "crank shape" is merely an example, and an electrode portion (or wiring) having a shape different from the "crank shape" may be applied to the inter-electrode connection. This also applies to the following explanation.

[0067] On the other hand, as illustrated in FIG. 5B, of the L-shaped sub-electrodes 412 and 432 that form the second modulation region, the electrode portion of the sub-electrode 412 that extends in the z-axis direction is arranged on the LN layer 103 on the side closer to the arm waveguide 114-1 of both sides of the arm waveguide 114-2 in the width direction.

[0068] Furthermore, as illustrated in FIG. 5C, of ​​the L-shaped sub-electrodes 412 and 432 that form the second modulation region, the electrode portion of the sub-electrode 432 that extends in the z-axis direction is arranged on the LN layer 103 on the far side of the arm waveguide 114-2 in the width direction, away from the arm waveguide 114-1.

[0069] The electrode portions of the sub-electrodes 412 and 432 arranged on the LN layer 103 are connected to the signal electrodes 41 and 43, respectively, through electrode portions (or wiring) formed in a crank shape so as to pass above the arm waveguide 114 in a cross-sectional view.

[0070] In a cross-sectional view, the wiring between the signal electrodes 41 and 43 and the individual sub-electrodes may all be at the same height in the y-axis above the arm waveguide 114, or some or all of them may be at different heights. Furthermore, the wiring in the portion that exceeds the arm waveguide 114 in the y-axis direction may be designed to be spaced apart from the arm waveguide 114 at a distance in the y-axis direction that prevents optical loss due to light absorption by the sub-electrode, for example, a distance of about 2 μm. These points also apply to the third embodiment described below.

[0071] Furthermore, the connection (or wiring) form between the T-shaped sub-electrodes 413 and 431 that form the third modulation region and the signal electrodes 41 and 43 may be understood to be equivalent to the connection form of the first modulation region illustrated in Figure 5 (A).

[0072] Similarly, the connection (or wiring) form between the L-shaped sub-electrodes 414 and 434 that form the fourth modulation area and the signal electrodes 41 and 43 may be understood to be equivalent to the connection form of the second modulation area illustrated in Figures 5(B) and 5(C).

[0073] As described above, by using multilayer wiring for the electrical connection between the signal electrode 41 or 43 and one or more sub-electrodes, it becomes easy to provide appropriate wiring that spatially avoids, for example, physical interference with the individual arm waveguides 114 and / or unintended contact between the wirings. Therefore, it also becomes easier to adjust the pitch between the sub-electrodes in the z-axis direction, for example.

[0074] The connection forms illustrated in Figures 5(A) to 5(C) are examples of connection forms using multi-layer wiring, but the electrical connection (wiring) between each sub-electrode and signal electrode 41 or 43 illustrated in Figure 4 in cross-sectional view may also be a connection form using single-layer wiring.

[0075] For example, in Figures 5(A) to 5(C), the signal electrodes 41 and 43 may be provided on the LN layer 103 on which the sub-electrodes are provided, and in this case, wiring may be provided within the upper clad layer 104 (in other words, without passing through other layers) to electrically connect each sub-electrode to the signal electrode 41 or 43.

[0076] Even in such single-layer wiring, the wiring portion that exceeds the arm waveguide 114 in the y-axis direction can be designed to have a distance, for example, a gap of about 2 μm in the y-axis direction, from the arm waveguide 114 such that no optical loss occurs due to light absorption by the sub-electrode.

[0077] <Embodiment 3> Fig. 6 is a schematic top view showing a configuration example of an optical modulator 60 according to embodiment 3. Fig. 7(A) is a cross-sectional view taken along line A-A' in Fig. 6, Fig. 7(B) is a cross-sectional view taken along line B-B' in Fig. 6, and Fig. 7(C) is a cross-sectional view taken along line C-C' in Fig. 6.

[0078] The optical modulator 60 illustrated in FIG. 6 is an MZ optical modulator, and corresponds to a configuration in which, in the configuration of the MZ optical modulator 40 illustrated in FIG. 4 of the second embodiment, each of the L-shaped sub-electrodes for the arm waveguide 114-2 is replaced with a T-shaped sub-electrode, and the pitch between the sub-electrodes in the light propagation direction is narrower than in the example of FIG. 4.

[0079] For example, when viewed from above, the signal electrode 41 is electrically connected to T-shaped sub-electrodes 411 and 413 and T-shaped sub-electrodes 612 and 614. When viewed from above, the other signal electrode 43 is electrically connected to T-shaped sub-electrodes 431 and 433 and T-shaped sub-electrodes 632 and 634.

[0080] 6, one arm waveguide 114-1 has, for example, a first modulation region sandwiched on both sides in the x-axis direction by T-shaped sub-electrodes 411 and 431. The other arm waveguide 114-2 has, for example, a second modulation region sandwiched on both sides in the x-axis direction by T-shaped sub-electrodes 612 and 632.

[0081] Similarly, one arm waveguide 114-1 has a third modulation region sandwiched on both sides in the x-axis direction by T-shaped sub-electrodes 413 and 433. The other arm waveguide 114-2 has a fourth modulation region sandwiched on both sides in the x-axis direction by T-shaped sub-electrodes 614 and 634.

[0082] In other words, in the third embodiment, the sub-electrode arrangements of the second and fourth modulation regions are replaced with T-shaped sub-electrode arrangements from the L-shaped sub-electrode arrangements of the second embodiment (FIG. 4). Therefore, in the MZ optical modulator 60, each of the first to fourth modulation regions has a T-shaped sub-electrode arrangement.

[0083] As in the second embodiment, the first to fourth modulation regions may be regions that are shifted from one another in the light propagation direction (z-axis direction). The first to fourth modulation regions may be regions that do not overlap one another in the light propagation direction, or may be regions that partially overlap one another.

[0084] 6, in the second modulation region, two T-shaped sub-electrodes 612 and 632 are arranged with their x-axis directions staggered. Similarly, in the fourth modulation region, two T-shaped sub-electrodes 614 and 634 are arranged with their x-axis directions staggered.

[0085] This staggered arrangement reduces the pitch between the sub-electrodes in the light propagation direction compared to the second embodiment, which employs an L-shaped sub-electrode arrangement. In other words, the sub-electrode arrangement density in the light propagation direction can be increased.

[0086] On the other hand, the length of each sub-electrode portion that sandwiches each arm waveguide 114 on both sides in the x-axis direction, in other words, the length of each modulation region where light propagating through each arm waveguide 114 is modulated, can be easily increased in the light propagation direction.

[0087] For example, in the example of embodiment 2 in which single-layer wiring is applied to the connection of the L-shaped sub-electrodes, overlapping between the sub-electrodes in the z-axis direction is not permitted, and therefore there is a limit to how narrow the pitch between the sub-electrodes can be made, which can restrict the length in the light propagation direction of the individual sub-electrode portions that sandwich each of the arm waveguides 114 on both sides in the x-axis direction.

[0088] 7A to 7C, the sub-electrodes are connected in a staggered arrangement, and overlapping between the sub-electrodes in the z-axis direction is permitted, making it easier to narrow the pitch between the sub-electrodes. This makes it easier to lengthen the length of the individual sub-electrode portions that sandwich each arm waveguide 114 on both sides in the x-axis direction in the light propagation direction.

[0089] As described above, the sub-electrode arrangement of embodiment 3 can contribute to further reducing the size of the MZ optical modulator 60 (e.g., shortening the length in the light propagation direction) and further improving the modulation efficiency by lengthening the modulation region, compared to the sub-electrode arrangement of embodiment 2 (Figure 4).

[0090] 7A to 7C show examples in which the T-shaped sub-electrodes shown in FIG. 6 are electrically connected to the signal electrodes 41 and 43 by multilayer wiring. The use of multilayer wiring makes it easy to narrow the pitch between the sub-electrodes as described above. Note that FIG. 7A is equivalent to the example of sub-electrode wiring for the first modulation region shown in FIG. 5A.

[0091] For example, as shown in FIG. 7B, the electrode portions extending in the z-axis direction of the T-shaped sub-electrodes 612 and 632 that form the second modulation region are arranged on both sides of the arm waveguide 114-2 in the width direction on the LN layer 103.

[0092] As illustrated in FIG. 7B, the electrode portion of the sub-electrode 612 arranged on the LN layer 103 is connected to the signal electrode 41 through an electrode portion (or wiring) formed in a crank shape so as to pass above the arm waveguide 114-2 in a cross-sectional view.

[0093] Similarly, as illustrated in FIG. 7(C), the electrode portion of the sub-electrode 632 arranged on the LN layer 103 is connected to the signal electrode 43 through an electrode portion (or wiring) formed in a crank shape so as to pass above each of the two arm waveguides 114 in a cross-sectional view.

[0094] In addition, the connection (or wiring) form between the T-shaped sub-electrodes 413 and 431 that form the third modulation area and the signal electrodes 41 and 43 may also be understood to be equivalent to the connection form of the first modulation area illustrated in Figure 7 (A).

[0095] Similarly, the connection (or wiring) form between the T-shaped sub-electrodes 614 and 634 forming the fourth modulation region and the signal electrodes 41 and 43 may be understood to be equivalent to the connection form of the second modulation region illustrated in Figures 7(B) and 7(C).

[0096] In the above-described second and third embodiments, it has been described that single-layer wiring or multi-layer wiring can be applied to the electrical connection between the signal electrodes 41 and 43 and the sub-electrodes. Alternatively, wire wiring by, for example, wire bonding may be applied to the electrical connection between the signal electrodes 41 and 43 and the sub-electrodes. An example of this is shown in FIG. 8 .

[0097] 8 is a schematic top view showing an example of the configuration of an optical modulator 80 according to the fourth embodiment. The optical modulator 80 shown in FIG. 8 is an MZ optical modulator, and similar to the first to third embodiments, includes an input optical coupler 112, arm waveguides 114-1 and 114-2, and an output optical coupler 116. The MZ optical modulator 80 also includes, for example, a signal electrode 41 to which a differential drive signal S is applied, and an output optical coupler 116 to which a differential drive signal S is applied. - and a signal electrode 43 to which a signal is applied.

[0098] For example, in top view, linear sub-electrodes 811 to 814 are electrically connected to one signal electrode 41 by wire wiring 850. For example, in top view, linear sub-electrodes 831 to 834 are electrically connected to the other signal electrode 43 by wire wiring 870.

[0099] 8, one arm waveguide 114-1 has, for example, a first modulation region sandwiched on both sides in the x-axis direction by sub-electrodes 811 and 831. The other arm waveguide 114-2 has, for example, a second modulation region sandwiched on both sides in the x-axis direction by sub-electrodes 812 and 832.

[0100] Similarly, one arm waveguide 114-1 has a third modulation region sandwiched on both sides in the x-axis direction by sub-electrodes 813 and 833. The other arm waveguide 114-2 has a fourth modulation region sandwiched on both sides in the x-axis direction by sub-electrodes 814 and 834.

[0101] As in the second embodiment, the first to fourth modulation regions may be regions that are shifted from one another in the light propagation direction (z-axis direction). The first to fourth modulation regions may be regions that do not overlap one another in the light propagation direction, or may be regions that partially overlap one another.

[0102] Therefore, for example, differential drive signals S and S are applied to signal electrodes 41 and 43, respectively. - is given, the direction of the electric field in the x-axis direction alternately reverses between the two arm waveguides 114 for different modulation regions in the z-axis direction, as in the first to third embodiments.

[0103] Therefore, a potential difference twice as large as that in single-phase driving can be applied to each arm waveguide 114, thereby increasing the modulation efficiency of the MZ optical modulator 80. Furthermore, as in the first to third embodiments, an RF substrate such as that described in Patent Document 1 is not required, so it is possible to reduce the number of parts of the MZ optical modulator 80 or simplify the structure, and it is also possible to reduce the size of the MZ optical modulator 80.

[0104] Furthermore, in the fourth embodiment, since it is not necessary to use a multilayer wiring process, for example, a desired sub-electrode arrangement can be easily realized, which can contribute to reducing the manufacturing cost of the MZ optical modulator 80.

[0105] Furthermore, in the fourth embodiment, for example, electrode portions or wiring that cross above each individual arm waveguide 114 in the upper cladding layer 104 can be eliminated, so that loss of light propagating through each individual arm waveguide 114 due to crossing wiring can be avoided or suppressed.

[0106] <Embodiment 5> In the above-described embodiments 1 to 4, a ground electrode (G) extending in the z-axis direction along the signal electrodes 41 and 43 may be provided on the upper clad layer 104 away from each of the signal electrodes 41 and 43 in the x-axis direction.

[0107] Taking the configuration of the MZ optical modulator 40 shown in FIG. 4 of the second embodiment as an example, for example, as shown in FIG. 9, ground electrodes 45 and 47 may be provided symmetrically in the x-axis direction on the upper clad layer 104 at positions spaced apart in the x-axis direction from the signal electrodes 41 and 43, respectively.

[0108] By providing the ground electrodes 45 and 47, it is possible to stabilize the electric field generated between the sub-electrodes in the individual modulation regions formed alternately in the light propagation direction between both arm waveguides 114. Therefore, for example, when a plurality of MZ optical modulator blocks each consisting of an input optical coupler 112, two arm waveguides 114, and an output optical coupler 116 are arranged in parallel in the x-axis direction corresponding to a plurality of channels or as an IQ modulator, it is possible to suppress crosstalk that may occur between the MZ optical modulator blocks, and it is possible to improve the optical modulation efficiency compared to when the ground electrodes 45 and 47 are not provided.

[0109] <Additional Notes> In the above-described first to fifth embodiments, for example, the narrower the pitch between the sub-electrodes in the light propagation direction (in other words, the higher the density of the sub-electrodes in the light propagation direction), the lower the impedance as a high-frequency line may be due to the capacitance of the sub-electrodes.

[0110] Furthermore, as the capacitance of the sub-electrode increases, the speed of the differential drive signal, which is a high-frequency electrical signal, may also decrease, which may result in a loss of speed matching between the differential drive signal and the light that is modulated in the modulation region of the high-frequency arm waveguide 114.

[0111] Therefore, the arrangement density of the sub-electrodes in the light propagation direction can be determined based on the electrical and optical velocity matching condition. For example, the arrangement density of the sub-electrodes may be determined based on the impedance required for the high-frequency line and the electrical velocity of the differential drive signal within a range that satisfies the velocity matching condition.

[0112] The term "connect" used in this disclosure may be read as "coupled." "Connected" or "coupled" may be understood to mean any direct or indirect "connection" or "coupling" between two or more elements. For example, the term may also be understood to include an indirect "connection" or "coupling" where one or more intermediate elements are interposed between two elements that are "connected" or "coupled" to each other.

[0113] Any reference to an element followed by a designation such as "first...," "second...," etc. does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

[0114] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0115] The present disclosure is useful, for example, in optical communication technology.

[0116] 10, 40, 60, 80 Optical modulator 41, 43 Signal electrode 45, 47 Ground electrode 101 Substrate 102 Lower cladding layer 103 LN layer 104 Upper cladding layer 112 Input optical coupler 114-1, 114-2 Optical waveguide (arm waveguide) 116 Output optical coupler 122a-1, 122a-2, 122b-1, 122b-2 Signal electrodes 411 to 414, 431 to 434, 612, 614, 632, 634, 811 to 814, 831 to 834 Sub-electrodes 850, 870 Wire wiring

Claims

1. An optical modulator comprising: an input optical coupler; first and second optical waveguides made of a material having an electro-optic effect, each guiding a respective light beam split into two by the input optical coupler; an output optical coupler that combines the output light beams of the first and second optical waveguides; a first set of electrodes provided on both sides along a first region of the first optical waveguide, and to which a differential drive signal is applied; and a second set of electrodes provided on both sides along a second region of the second optical waveguide, corresponding to a region shifted from the first region in the propagation direction of the light, and to which the differential drive signal is applied, wherein the direction of the electric field generated in the first region by the first electrode set is opposite to the direction of the electric field generated in the second region by the second electrode set.

2. An optical modulator as claimed in claim 1, further comprising: a first signal electrode to which one of the differential drive signals is applied; and a second signal electrode to which the other of the differential drive signals is applied, wherein the first electrode set comprises: a first sub-electrode provided on one of both sides of the first region closer to the second optical waveguide and electrically connected to the first signal electrode; a second sub-electrode provided on one of both sides of the first region farther from the second optical waveguide and electrically connected to the second signal electrode; and the second electrode set comprises: a third sub-electrode provided on one of both sides of the second region farther from the first optical waveguide and electrically connected to the first signal electrode; and a fourth sub-electrode provided on one of both sides of the second region closer to the first optical waveguide and electrically connected to the second signal electrode.

3. The optical modulator of claim 2, wherein one or more of the first sub-electrode, the second sub-electrode, the third sub-electrode, and the fourth sub-electrode have a T-shape or an L-shape when viewed from above the optical modulator.

4. The optical modulator according to claim 2, wherein the electrical connection is a connection by multilayer wiring or wire wiring.

5. The optical modulator according to claim 2, further comprising a first ground electrode for said first signal electrode and a second ground electrode for said second signal electrode.

6. The optical modulator according to claim 1, wherein the spacing between the first electrode set and the second electrode set in the propagation direction of the light is determined based on a velocity matching condition between the differential drive signal and the light.

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