WO2025248614A1 - Optical modulator - Google Patents

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

This disclosure relates to an optical modulator.

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 have been many reports of optical modulators that utilize the electro-optic effect being realized as single-phase drive optical modulators.

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

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 and output from a two-port differential driver are converted into three systems of drive signals S ^- , S, and S ^- in an intermediate interconnect structure (IIS). Then, the drive signals S ^- , S, and S ^- are provided to three parallel arm waveguides, respectively. Note that in Patent Document 1, the IIS is also referred to as an "RF (radio frequency) substrate" or "RF carrier."

US Patent Application Publication No. 2023/0305356

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

One exemplary objective of the present disclosure is to provide an optical modulator that can achieve differential drive, which is more advantageous than single-phase drive, without increasing the size of the optical modulator that utilizes the electro-optic effect.

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, which guide each of the two beams of light branched by the input optical coupler; an output optical coupler which combines the output light from 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.

1 is a schematic top view illustrating a configuration example of an optical modulator according to a first embodiment. 1, (A) is a cross-sectional view taken along line AA' in FIG. 1, and (B) is a cross-sectional view taken along line BB' in FIG. 10A and 10B are diagrams illustrating comparative examples of the differential drive of the present embodiment (c), single-phase drive (a), and normal differential drive (b). FIG. 10 is a schematic top view illustrating a configuration example of an optical modulator according to a second embodiment. 4A is a cross-sectional view taken along the line AA' in FIG. 4, FIG. 4B is a cross-sectional view taken along the line BB' in FIG. 4, and FIG. 4C is a cross-sectional view taken along the line CC' in FIG. FIG. 10 is a schematic top view illustrating a configuration example of an optical modulator according to a third embodiment. 6A is a cross-sectional view taken along line AA' in FIG. 6, (B) is a cross-sectional view taken along line BB' in FIG. 6, and (C) is a cross-sectional view taken along line CC' in FIG. FIG. 10 is a schematic top view illustrating 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.

Embodiments will now 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 matters that are already well-known or redundant descriptions of substantially identical configurations may be omitted.

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 each element may differ from reality. Even between drawings, there may be parts where the dimensional relationships or ratios differ. When numerical values are given in the following description, they are merely examples, and other numerical values may be used in addition or instead.

<Embodiment 1>
FIG. 1 is a schematic top view showing an example of the configuration of an optical modulator 10 according to the 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.

As illustrated in FIG. 1, the optical modulator 10 is, for example, an MZ optical modulator, and includes an input optical coupler 112 that branches the input light into two, two parallel optical waveguides (arm waveguides) 114-1 and 114-2 that guide each of the two branches of light 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."

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 (LiNbO ₃ ), as a non-limiting example.

For example, as illustrated in the cross-sectional views of Figures 2(A) and 2(B), the optical modulator 10 comprises 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.

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

The input optical coupler 112, two arm waveguides 114, and output optical coupler 116 are each formed on the LN layer 103, for example, by patterning a waveguide pattern. For example, Figures 2(A) and 2(B) show arm waveguides 114-1 and 114-2 formed on the LN layer 103.

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 (eg, SiO ₂ ) with a lower refractive index than the LN layer 103 , respectively.

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, respectively. The lower cladding layer 102 and the upper cladding layer 104 may have the same or different compositions.

As shown in Figures 1 and 2(A), signal electrodes 122a-1 and 122a-2 are provided on both sides of one arm waveguide 114-2 in the x-axis direction, extending in the z-axis direction along a portion of the arm waveguide 114-2.

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 a "signal electrode" is an electrode to which a high-frequency signal such as an RF signal is applied, and may also be referred to as a "high-frequency electrode" or "traveling wave electrode."

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

In other words, arm waveguide 114-2 has a first region in which a portion is sandwiched on both sides in the x-axis direction by signal electrodes 122a-1 and 122a-2 when viewed from above. The first region corresponds to a partial modulation region where light propagating through arm waveguide 114-2 is modulated.

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.

Furthermore, as shown in Figures 1 and 2(B), 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 portion 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.

Signal electrode 122b-1 is arranged, for example, on one of the two sides of arm waveguide 114-1 in the x-axis direction, farther from arm waveguide 114-2. Signal electrode 122b-2 is arranged, for example, on one of the two sides of arm waveguide 114-1 in the x-axis direction, closer to arm waveguide 114-2.

In other words, arm waveguide 114-1 has a second region in which a portion is sandwiched on both sides in the x-axis direction by signal electrodes 122b-1 and 122b-2 when viewed from above. The second region corresponds to a partial modulation region where light propagating through arm waveguide 114-1 is modulated.

The second modulation region may be, for example, a region offset 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 with each other in the light propagation direction (z-axis direction), or may be regions that partially overlap. 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 also be formed after the second modulation region.

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-2 to which the other differential drive signal ^S is applied in the first modulation region, for example.

Note that when two electrodes are "electrically connected," it is sufficient 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 impeding mutual conduction.

In addition, in Figure 1, there are portions where the electrical connection paths intersect (or switch) between arm waveguides 114-1 and 114-2, but this does not pose a problem if 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.

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

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 push-pull operation of the MZ optical modulator 10.

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

Therefore, for example, it is possible to increase the modulation efficiency in the portion of the MZ optical modulator 10 where the signal electrodes are arranged, and reduce the length (z-axis direction) of the area where the signal electrodes are arranged. This makes it possible to reduce optical waveguide loss due to the signal electrodes. Furthermore, because an RF substrate such as that described in Patent Document 1 is not required, it is possible to reduce the number of parts in the MZ optical modulator 10 or simplify the structure, and it is also possible to reduce the size of the MZ optical modulator 10.

Figure 3 shows a comparison example between the differential drive of this embodiment (c) and single-phase drive (a) and 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).

The subscript "ppd" for 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.

For ease of comparison, the distance in the substrate surface width direction between the signal electrode "S" or "S ^- " and the ground electrode "G" is "d". The signal electrode "S" represents the electrode to which the differential drive signal S is applied, and the signal electrode "S ^- " represents the electrode to which the differential drive signal S ^- is applied. Note that in Figure 3, the positional relationship between the signal electrode and the waveguide is made the same for comparison, so that the effect of the signal electrode on the increase in optical waveguide loss is made the same.

In the single-phase drive (a) electrode arrangement, in cross-sectional view, one signal electrode "S" is provided between the two arm waveguides, and one ground electrode "G" is provided at a distance from each of the two arm waveguides.

The electrode arrangement for normal differential drive (b) is such that, in a cross-sectional view, a pair of a ground electrode "G" and a signal electrode "S" or " ^S- " is provided on both sides of each of the two arm waveguides, and another ground electrode "G" is provided between the signal electrodes "S" and " ^S- ".

As can be seen from a comparison of (a), (b), and (c) in Figures 3A and 3B, 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 the change in electric field ΔE = E ⁺ -E ^- can be doubled (ΔE = 2/d) compared to the single-phase drive (a) and the normal differential drive (b).

<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 CC' in Fig. 4.

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

Moreover, the MZ optical modulator 40 illustratively includes a signal electrode 41 to which one of the differential drive signals S and S ⁻ (for example, S) is applied, and a signal electrode 43 to which the other of the differential drive signals S and S ⁻ (for example, S ⁻ ) is applied.

One signal electrode 41 is electrically connected to, for example, T-shaped sub-electrodes 411 and 413 and L-shaped sub-electrodes 412 and 414 when viewed from above. The other signal electrode 43 is electrically connected to, for example, T-shaped sub-electrodes 431 and 433 and L-shaped sub-electrodes 432 and 434 when viewed from above.

As shown in FIG. 4, one arm waveguide 114-1 has a first modulation region sandwiched between T-shaped sub-electrodes 411 and 431 on both sides in the x-axis direction. Sub-electrode 411 is an example of a first sub-electrode, and is arranged, for example, on the side of arm waveguide 114-1 on both sides in the x-axis direction closer to arm waveguide 114-2.

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

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, for example, be a region shifted in the light propagation direction (z-axis direction) from the first modulation region in arm waveguide 114-1. The first modulation region and second modulation region may be regions that do not overlap with each other in the light propagation direction (z-axis direction), or may be regions that partially overlap with each other.

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

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, for example, be a region offset in the light propagation direction (z-axis direction) from the second modulation region in arm waveguide 114-2.

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, for example, be a region offset in the light propagation direction (z-axis direction) from the third modulation region in arm waveguide 114-1.

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

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.

In this way, the first to fourth modulation regions are arranged alternately 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 intermittently) sandwiched between electrodes in each of the two arm waveguides 114 alternate along the light propagation direction (z-axis direction) between the two arm waveguides 114.

The spacing (pitch) between the modulation regions 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.

Note that Figure 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), but three or more sets may also be repeatedly provided along the light propagation direction.

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

Furthermore, while Figure 4 shows an example in which partial modulation regions are alternately arranged in the order of arm waveguide 114-1 and arm waveguide 114-2 in the light propagation direction, the partial modulation regions may alternatively be arranged in the order of arm waveguide 114-2 and arm waveguide 114-1.

Furthermore, the shape of the sub-electrodes (reference numerals omitted) arranged partially (or intermittently) along the light propagation direction on both sides of each of the arm waveguides 114-1 and 114-2 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 an electrode portion that extends partially along both sides of the respective arm waveguide 114.

In the electrode arrangement described above, when 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, 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.

Therefore, similar to embodiment 1, push-pull operation is achieved, and it is possible to apply twice the potential difference to each arm waveguide 114 compared to single-phase drive. Furthermore, compared to embodiment 1, the pitch between the sub-electrodes in the light propagation direction can be narrowed (in other words, the sub-electrode arrangement density in the light propagation direction can be increased).

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).

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

For example, each of the signal electrodes 41 and 43 is positioned on the upper surface of the upper cladding layer 104 at a distance in the x-axis direction from each of the arm waveguides 114-1 and 114-2.

As shown in Figure 5(A), 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.

Each of these electrode portions is connected to 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 in the cross-sectional view of Figure 5(A). Note that the "crank shape" is merely an example, and electrode portions (or wiring) of shapes other than the "crank shape" may be used for inter-electrode connections. This also applies to the following explanations.

On the other hand, as shown in Figure 5(B), 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 positioned on the LN layer 103 on either side of the arm waveguide 114-2 in the width direction, closer to the arm waveguide 114-1.

Furthermore, as shown in FIG. 5(C), 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 disposed on the LN layer 103 on either side of the arm waveguide 114-2 in the width direction, farther from the arm waveguide 114-1.

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 crank-shaped electrode portions (or wiring) that pass above the arm waveguide 114 in cross-sectional view.

In addition, 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 in a cross-sectional view, or some or all of them may be at different heights. Furthermore, the wiring that extends beyond the arm waveguide 114 in the y-axis direction may be designed to maintain a distance from the arm waveguide 114 in the y-axis direction that prevents optical loss due to light absorption by the sub-electrodes, for example, a spacing of approximately 2 μm. These points also apply to embodiment 3, which will be described later.

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

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 can be understood to be equivalent to the connection form of the second modulation area illustrated in Figures 5(B) and 5(C).

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

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

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 cladding layer 104 (in other words, without passing through other layers) to electrically connect each sub-electrode to the signal electrode 41 or 43.

Even in such single-layer wiring, the wiring portion that extends beyond the arm waveguide 114 in the y-axis direction can be designed to maintain a distance from the arm waveguide 114 in the y-axis direction that prevents optical loss due to light absorption by the sub-electrode, for example, a spacing of approximately 2 μm.

<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.

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 embodiment 2, 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.

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

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

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.

In other words, in embodiment 3, the sub-electrode arrangements of the second modulation region and the fourth modulation region are replaced with T-shaped sub-electrode arrangements from the L-shaped sub-electrode arrangements of embodiment 2 (Figure 4). Therefore, in the MZ optical modulator 60, each of the first to fourth modulation regions has a T-shaped sub-electrode arrangement.

As in embodiment 2, the first to fourth modulation regions may be regions that are offset 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.

As shown in FIG. 6, in the second modulation region, two T-shaped sub-electrodes 612 and 632 are arranged with their x-axis directions oriented in a staggered manner. Similarly, in the fourth modulation region, two T-shaped sub-electrodes 614 and 634 are arranged with their x-axis directions oriented in a staggered manner.

This staggered arrangement reduces the pitch between the sub-electrodes in the light propagation direction compared to embodiment 2, which uses an L-shaped sub-electrode arrangement. In other words, the sub-electrode arrangement density in the light propagation direction can be increased.

On the other hand, the length of each of the sub-electrode portions that sandwich 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.

For example, in the example of embodiment 2 where single-layer wiring is used to connect the L-shaped sub-electrodes, overlap between the sub-electrodes in the z-axis direction is not permitted, and there is a limit to how narrow the pitch between the sub-electrodes can be. This can limit 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.

In contrast, in an example in which multilayer wiring, as described below in Figures 7(A) to 7(C), is applied to the connection configuration of individual T-shaped sub-electrodes arranged in a staggered combination as described above, overlap between sub-electrodes in the z-axis direction is permitted, making it easier to narrow the pitch between sub-electrodes. Therefore, it becomes easier to elongate the individual sub-electrode portions that sandwich each arm waveguide 114 on both sides in the x-axis direction in the light propagation direction.

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 modulation efficiency by lengthening the modulation region, compared to the sub-electrode arrangement of embodiment 2 (Figure 4).

Figures 7(A) to 7(C) show an example in which each T-shaped sub-electrode shown in Figure 6 is electrically connected to signal electrode 41 or 43 using multi-layer wiring. By using multi-layer wiring, it is easy to narrow the pitch between the sub-electrodes as described above. Note that Figure 7(A) is equivalent to the sub-electrode wiring example for the first modulation region shown in Figure 5(A).

For example, as shown in Figure 7(B), 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.

As shown in Figure 7(B), 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.

Similarly, as illustrated in Figure 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.

The connection (or wiring) between the T-shaped sub-electrodes 413 and 431 that form the third modulation region and the signal electrodes 41 and 43 may also be understood to be equivalent to the connection configuration of the first modulation region illustrated in Figure 7(A).

Similarly, the connection (or wiring) form between the T-shaped sub-electrodes 614 and 634 that form 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).

<Fourth Embodiment>
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 .

Fig. 8 is a schematic top view showing an example of the configuration of an optical modulator 80 according to embodiment 4. The optical modulator 80 shown in Fig. 8 is an MZ optical modulator, and similar to the above-described embodiments 1 to 3, 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 a signal electrode 43 to which a differential drive signal S ^- is applied.

For example, when viewed from above, linear sub-electrodes 811 to 814 are electrically connected to one signal electrode 41 by wire wiring 850. For example, when viewed from above, linear sub-electrodes 831 to 834 are electrically connected to the other signal electrode 43 by wire wiring 870.

As shown in FIG. 8, one arm waveguide 114-1 has 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 a second modulation region sandwiched on both sides in the x-axis direction by sub-electrodes 812 and 832.

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.

As in embodiment 2, the first to fourth modulation regions may be regions that are offset 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.

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

As a result, 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 with embodiments 1 to 3, an RF substrate such as that described in Patent Document 1 is not required, which makes it possible to reduce the number of parts in the MZ optical modulator 80 or simplify the structure, and also reduce the size of the MZ optical modulator 80.

Furthermore, in embodiment 4, since it is not necessary to use a multilayer wiring process, for example, the desired sub-electrode arrangement can be easily achieved, which can contribute to reducing the manufacturing costs of the MZ optical modulator 80.

Furthermore, in embodiment 4, for example, it is possible to eliminate the need for electrode portions or wiring that cross above each individual arm waveguide 114 within the upper cladding layer 104, thereby avoiding or suppressing losses in light propagating through each individual arm waveguide 114 due to crossing wiring.

<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.

Using the configuration of the MZ optical modulator 40 shown in Figure 4 of embodiment 2 as an example, as shown in Figure 9, ground electrodes 45 and 47 may be provided symmetrically in the x-axis direction on the upper cladding layer 104 at positions spaced apart in the x-axis direction from the signal electrodes 41 and 43, respectively.

By providing the ground electrodes 45 and 47, the electric field generated between the sub-electrodes in the individual modulation regions formed alternately in the light propagation direction between the two arm waveguides 114 can be stabilized. Therefore, for example, when multiple MZ optical modulator blocks 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 to accommodate multiple channels or as an IQ modulator, crosstalk that can occur between the MZ optical modulator blocks can be suppressed, and optical modulation efficiency can be improved compared to when the ground electrodes 45 and 47 are not provided.

<Additional information>
In the above-described embodiments 1 to 5, 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 can be due to the capacitance of the sub-electrodes.

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 being modulated in the modulation region of the high-frequency arm waveguide 114.

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

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 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.

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 method to distinguish between two or more elements. For example, 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.

Although the present disclosure has been described in detail above, it will be clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the content 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, which are 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.

This disclosure is useful, for example, in optical communication technology.

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 (6)

an input optical coupler;
a first optical waveguide and a second optical waveguide, each made of a material having an electro-optic effect, for guiding each of the two beams of light branched by the input optical coupler;
an output optical coupler that couples the output light from the first optical waveguide and the output light from the second optical waveguide;
a first set of electrodes disposed on opposite sides of a first region of the first optical waveguide and adapted to receive a differential drive signal;
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;
An optical modulator, wherein a direction of an electric field generated in the first region by the first electrode set is opposite to a direction of an electric field generated in the second region by the second electrode set. a first signal electrode to which one of the differential drive signals is applied;
a second signal electrode to which the other of the differential drive signals is applied;
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;
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;
2. The optical modulator according to claim 1, further comprising: 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. 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. The optical modulator of claim 2, wherein the electrical connection is a connection using multilayer wiring or wire wiring. The optical modulator of claim 2, further comprising a first ground electrode for the first signal electrode and a second ground electrode for the second signal electrode. The optical modulator of 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.