WO2024252534A1 - Optical modulator - Google Patents

Abstract

Provided is a high-speed optical modulator having excellent optical properties.　The optical modulator comprises two arm optical waveguides connected to a demultiplexer (112), which has a high mesa type optical waveguide structure that is connected to a first high mesa type optical waveguide (111), and a multiplexer (116), which has a high mesa type optical waveguide structure that is connected to a second high mesa type optical waveguide (117). The arm optical waveguides are each provided with a high mesa type optical waveguide (113a, 113b), a rib type optical waveguide (414a, 414b), a high mesa type optical waveguide (115a, 115b), and an electrode (121a, 121b) disposed on the rib type optical waveguide. Each rib type optical waveguide (414a, 414b) includes a lower cladding layer (202), a core layer (204), and an upper cladding layer (205) in this order on a substrate (201). In a cross-section orthogonal to the propagation direction of light, the width of the upper cladding layer is less than the widths of the core layer and the lower cladding layer, and the widths of the core layer and the lower cladding layer include two or more widths, including a first width (W_414n) and a second width (W_414w) which is greater than the first width.

Description

Optical Modulator

This disclosure relates to an optical modulator, and more specifically to an optical modulator using a rib-type optical waveguide.

As optical communication systems become more capacitive, there is a demand for high-speed optical modulators compatible with advanced optical modulation methods. In particular, multilevel optical modulators using digital coherent technology are playing a major role in realizing high-capacity transceivers that exceed 100 Gbps. A multilevel optical modulator splits the input light into two arm optical waveguides, modulates the light propagating through the arm optical waveguides, aligns the phase, and then combines and outputs the resulting interference. The multilevel optical modulator has multiple parallel-stage built-in optical modulators (hereafter referred to as MZ modulators: MZMs) capable of zero-chirp drive. This configuration allows independent signals to be added to the amplitude and phase of the light.

Typical polarization multiplexing IQ optical modulators, which are currently being used in communication networks, have a so-called nested structure, in which each arm of a parent MZM is composed of a child MZM. Two child MZMs are provided in parallel corresponding to each of the X and Y polarization channels, forming an MZM (Quad-parallel MZM) with a total of four child MZMs. The two arms of each child MZM are provided with traveling wave type electrodes to which an RF modulated electrical signal is input to perform a modulation operation on the optical signal propagating in the optical waveguide. In each polarization channel, one of the two such paired child MZMs corresponds to the I channel and the other to the Q channel.

Such a polarization multiplexed IQ optical modulator applies phase modulation to two optical signals propagating within the optical waveguide of the child MZM by inputting an RF modulated electrical signal to one end of a modulation electrode provided along the arm optical waveguide of the child MZM, thereby generating an electro-optic effect (see, for example, Patent Document 1).

FIG. 1 shows an example of a high-speed phase modulator using a Mach-Zehnder interferometer, which corresponds to the child MZM that is the basic unit in a polarization multiplexing IQ optical modulator. First, the configuration of the high-speed phase modulator will be described. In the high-speed phase modulator 100 shown in FIG. 1, one high-mesa type optical waveguide 111 is connected to a 1×2 demultiplexer 112 having a high-mesa type optical waveguide structure. The 1×2 demultiplexer 112 is connected to two high-mesa type optical waveguides 113a and 113b. The high-mesa type optical waveguides 113a and 113b are connected to rib type optical waveguides 114a and 114b, respectively. The rib type optical waveguides 114a and 114b are connected to high-mesa type optical waveguides 115a and 115b, respectively. The two rib type optical waveguides 114a and 114b are separated by a separation groove 118. The high mesa optical waveguides 115a and 115b are connected to a 2x1 multiplexer 116 having a high mesa optical waveguide structure. The 2x1 multiplexer 116 is connected to one high mesa optical waveguide 117.

The high mesa type optical waveguides 113a, 113b, the rib type optical waveguides 114a, 114b, and the high mesa type optical waveguides 115a, 115b form an arm optical waveguide that connects the 1x2 demultiplexer 112 and the 2x1 multiplexer 116. In the high-speed phase modulator 100, two high-frequency lines 120a, 120b extending in the light propagation direction (x direction) are formed along the arm optical waveguide. The ends of the high-frequency lines 120a, 120b are connected to termination resistors 150a, 150b. A plurality of slot electrodes 121a, 121b are formed discretely above the rib type optical waveguides 114a, 114b, respectively, and function as an optical modulator that adds a signal to light by the electro-optic effect. The multiple slot electrodes 121a, 121b are connected to the two high-frequency lines 120a, 120b by multiple connection electrodes 122a, 122b, respectively. The high mesa optical waveguides 115a, 115b are respectively loaded with thin-film heaters 130a, 130b on the upper side, and function as thermo-optical phase shifters. The thin-film heaters 130a, 130b are respectively connected to the two pads 131a, 131b by wiring 132a, 132b.

As shown in FIG. 1, the high-speed phase modulator 100 can be divided into a high-mesa optical waveguide region A including a high-mesa optical waveguide 111, a 1×2 splitter 112, and high-mesa optical waveguides 113a and 113b, a high-speed modulation region and rib-type optical waveguide region B including rib-type optical waveguides 114a and 114b, and a high-mesa optical waveguide region C including high-mesa optical waveguides 151 and 115b, a 2×1 multiplexer 116, and a high-mesa optical waveguide 117. At the boundary between the two regions, each optical waveguide has a tapered shape to suppress optical connection loss.

Figure 2 shows a cross section taken along line II-II in Figure 1. The high mesa optical waveguides 113a and 113b will be described with reference to Figure 2. The high mesa optical waveguides 113a and 113b have a structure in which an n-type InP lower cladding layer 202, a p-type InP layer 203, an i-type MQW (Multi Quantum Well) core layer 204, and a high resistance (or non-doped) InP upper cladding layer 205 are stacked on a semi-insulating InP substrate 201. The high resistance InP upper cladding layer 205 is formed by etching the n-type InP upper cladding layer and replacing it by regrowth. By replacing it with the high resistance InP upper cladding layer 205, the propagation loss of light can be reduced. The width W113 of the high-resistance InP upper cladding layer 205 and the i-type MQW core layer 204 in the lateral direction (y direction) perpendicular to the light propagation direction is uniform and is several μm wide (for example, 2 μm wide). The lateral direction (y direction) of the high mesa optical waveguides 113a and 113b is covered with an organic film (BCB: benzocyclobutene) 206, and the large refractive index difference between the i-type MQW core layer 204 and the organic film 206 realizes lateral light confinement.

Figure 3 shows a cross section taken along line III-III in Figure 1. The rib type optical waveguides 114a and 114b will be described with reference to Figure 3. The InP semiconductor layer structure of the rib type optical waveguides 114a and 114b is similar to that of the high mesa type optical waveguides 113a and 113b described with reference to Figure 2, but the upper cladding layer is different. The upper cladding layer of the high mesa type optical waveguides 113a and 113b in Figure 2 is a high resistance (or non-doped) InP upper cladding layer 205, whereas the upper cladding layer of the rib type optical waveguides 114a and 114b in Figure 3 is an n-type InP upper cladding layer 305, which is different. In the rib-type optical waveguides 114a and 114b in FIG. 3, electrical contact is obtained by using an n-type InP upper cladding layer 305 as the upper cladding layer, making it possible to apply an electric field to the i-type MQW core layer 204.

The width (y direction) of the n-type InP upper cladding layer 305 in the lateral direction (y direction) is narrower than the slab width W114 of the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202. In the high-speed modulation region/rib-type optical waveguide region B, the slab width W114 is uniform. As a result, the rib-type optical waveguides 114a and 114b confine light by a lateral confinement effect equivalent to the width of the n-type InP upper cladding layer 305. In the rib-type optical waveguides 114a and 114b, metal slot electrodes 121a and 121b are discretely loaded on the rib-type optical waveguides to perform high-speed phase modulation, and electrical contact is made. As a result, the rib-type optical waveguide region B also serves as a high-speed phase modulation region that performs high-speed phase modulation. By using a rib-type optical waveguide for the arm optical waveguide in the high-speed phase modulation region B, the dark current can be suppressed because there is no sidewall formed by etching the i-type MQW core layer 104 in the region where light is concentrated, and a high-speed phase modulator with excellent long-term stability can be realized.

International Publication No. WO2021/049004A1

The above-mentioned high-speed phase modulator 100 confines light by a lateral confinement effect equivalent to the width of the high-resistance InP upper cladding layer 305, by making the width of the high-resistance InP upper cladding layer 305 of the rib-type optical waveguides 114a and 114b narrower than the slab width W114 of the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 in the high-speed modulation region B. In addition, by making the slab width W114 of the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 sufficiently wide, the effect of unnecessary modes being spread horizontally and removed is obtained. Therefore, it is desirable to make the slab width W114 wide.

However, the high-frequency lines 120a and 120b that propagate high-speed RF modulated electrical signals must be located near the rib-type optical waveguides 114a and 114b, and the slab width W114 of the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 cannot be made sufficiently wide. If the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 exist below the high-frequency lines 120a and 120b, the impedance will be significantly different, causing degradation of the high-frequency characteristics of the high-frequency lines 120a and 120b (for example, increased high-frequency propagation loss, etc.). For these reasons, the slab width W114 is conventionally designed to be about 13 μm.

When the slab width W114 in the rib-type optical waveguides 114a and 114b is 13 μm, if coupling to a higher-order mode occurs due to a misalignment of the mode center of the propagating light at the connection points between the high-mesa optical waveguides 113a and 113b and the rib-type optical waveguides 114a and 114b, the higher-order mode is excited. The excited higher-order mode is not a single-peaked mode field, but a mode field with multiple light intensity peaks, and propagates without radiating, remaining within the i-type MQW core layer 204 of the rib-type optical waveguides 114a and 114b. Furthermore, the higher-order mode recouples at the connection points between the rib-type optical waveguides 114a and 114b and the high-mesa optical waveguides 115a and 115b, causing problems such as the Mach-Zehnder interferometer no longer extinguishing (deterioration of the extinction ratio).

This disclosure was made in consideration of these problems, and its purpose is to provide a high-speed optical modulator with excellent optical characteristics.

In order to achieve such an object, an optical modulator according to an embodiment of the present disclosure includes a first high-mesa optical waveguide, a splitter having a high-mesa optical waveguide structure connected to the first high-mesa optical waveguide, a second high-mesa optical waveguide, a multiplexer having a high-mesa optical waveguide structure connected to the second high-mesa optical waveguide, and two arm optical waveguides connected to the splitter and the multiplexer, each of the two arm optical waveguides including a rib-type optical waveguide and an electrode disposed on the rib-type optical waveguide, the rib-type optical waveguide including a lower cladding layer, a core layer, and an upper cladding layer in that order on a substrate, and in a cross section perpendicular to the light propagation direction, the width of the upper cladding layer is narrower than the widths of the core layer and the lower cladding layer, and the widths of the core layer and the lower cladding layer include two or more different widths including a first width and a second width wider than the first width.

As described above, one embodiment of the present disclosure can provide a high-speed optical modulator with excellent optical characteristics.

FIG. 1 is a top view of a conventional high-speed optical modulator. 2 is a cross-sectional view of the high-speed optical modulator of FIG. 1 taken along line II-II. 3 is a cross-sectional view of the high-speed optical modulator of FIG. 1 taken along line III-III. FIG. 2 is a top view of a high-speed optical modulator according to an embodiment of the present disclosure. 5 is a cross-sectional view of the high-speed optical modulator taken along line VV in FIG. 4. 6 is a cross-sectional view of the high-speed optical modulator taken along line VI-VI of FIG. 4. 7 is a cross-sectional view of the high-speed optical modulator taken along line VII-VII in FIG. 8 is a cross-sectional view of the high-speed optical modulator taken along line VIII-VIII in FIG. 4. FIG. 2 is a top view of a high-speed optical modulator according to an embodiment of the present disclosure.

Below, the optical modulator according to the embodiment of the present disclosure will be described in detail with reference to the drawings. The same or similar symbols indicate the same or similar elements, and repeated explanations may be omitted. The numerical values and materials in the following description are examples, and the optical modulator according to the present disclosure can be implemented with other numerical values and materials without departing from the gist of the present disclosure.

(Embodiment)
Fig. 4 shows a high-speed optical modulator according to an embodiment of the present disclosure. The high-speed optical modulator 400 shown in Fig. 4 is a high-speed phase modulator using a Mach-Zehnder interferometer, which corresponds to a child MZM that is a basic unit in a polarization multiplexing IQ optical modulator. The high-speed optical modulator 400 has many similarities to the high-speed phase modulator 100 shown in Fig. 1. The following will mainly explain the differences between the high-speed optical modulator 400 and the high-speed phase modulator 100.

The high-speed optical modulator 400 can be divided into a high-mesa optical waveguide region A, a high-speed modulation region/rib-type optical waveguide region B, and a high-mesa optical waveguide region C, similar to the high-speed phase modulator 100 in FIG. 1. The high-mesa optical waveguide region A in the high-speed optical modulator 400 includes the high-mesa optical waveguide 111, the 1×2 splitter 112, and the high-mesa optical waveguides 113a and 113b, similar to the high-speed phase modulator 100. The high-mesa optical waveguide region C in the high-speed optical modulator 400 includes the high-mesa optical waveguides 115a and 115b, the 2×1 multiplexer 116, and the high-mesa optical waveguide 117, similar to the high-speed phase modulator 100.

The high-speed modulation region and rib-type optical waveguide region B of the high-speed optical modulator 400 includes rib-type optical waveguides 414a and 414b connected to the high mesa optical waveguides 113a and 113b and the high mesa optical waveguides 115a and 115b, respectively. At the boundary between the two regions, each optical waveguide has a tapered shape to suppress optical connection loss. A plurality of slot electrodes 121a and 121b are formed discretely above the rib-type optical waveguides 414a and 414b, respectively, and function as an optical modulator that adds a signal to light by the electro-optic effect. The plurality of slot electrodes 121a and 121b are connected to the two high-frequency lines 120a and 120b by a plurality of connection electrodes 122a and 122b, respectively. In order to propagate an RF modulated electrical signal with low loss, the two high-frequency lines 120a, 120b are formed in parallel with the rib-type optical waveguides 414a, 414b above the organic film (e.g., BCB) 206, which is a non-semiconductor material. The multiple connection electrodes 122a, 122b are formed at right angles to the rib-type optical waveguides 414a, 414b.

The layer structure of the InP semiconductor of the rib-type optical waveguides 414a and 414b is similar to the layer structure of the InP semiconductor of the rib-type optical waveguides 114a and 114b described with reference to FIG. 3.

As described above with reference to FIG. 3, the slab width W114 in the lateral direction (y direction) perpendicular to the light propagation direction of the rib-type optical waveguides 114a and 114b in the high-speed modulation region and rib-type optical waveguide region B of the high-speed phase modulator 100 is uniform. In contrast, the high-speed optical modulator 400 according to this embodiment differs from the high-speed phase modulator 100 in that the widths of the rib-type optical waveguides 414a and 414b are not uniform but change discontinuously. As will be described later with reference to FIG. 5 and FIG. 6 to FIG. 8, the widths of the rib-type optical waveguides 414a and 414b change discontinuously between the relatively wide slab width W414w and the narrow slab width W414n. Here, "discontinuously" is intended to mean that the widths of the rib-type optical waveguides 414a and 414b do not change gradually or smoothly between the wide slab width W414w and the narrow slab width W414n along the light guiding direction. That is, "discontinuously" means that there is an abrupt change from the wide slab width W414w to the narrow slab width W414n.

Figure 5 shows a cross section taken along the cross section line V-V in Figure 4. Near the boundary between high mesa optical waveguide region A, including the connection positions with high mesa optical waveguides 113a, 113b, and high-speed modulation region/rib type optical waveguide region B, rib type optical waveguides 414a, 414b have a wide slab width W414w. As can be seen from Figure 4, near the boundary between high mesa optical waveguide region B, including the connection positions with high mesa optical waveguides 115a, 115b, and high mesa optical waveguide region C, rib type optical waveguides 414a, 414b have a wide slab width W414w. In the portion of the rib-type optical waveguide 414a, 414b having a wide slab width W414w, the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 are located in the range from the separation groove 118 to below the high-frequency lines 120a, 120b.

At the position of the cross-sectional line V-V in FIG. 4, the width (y direction) of the high-resistance n-type InP upper cladding layer 205 of the rib-type optical waveguides 414a, 414b is 2 μm. The wide slab width W414w of the i-type MQW core layer 204, p-type InP layer 203, and n-type InP lower cladding layer 202 is 30 μm. The length of the wide slab width W414w in the light propagation direction (x direction) is 100 μm. The thickness of the i-type MQW core layer 204 is 0.5 μm. The fundamental mode of light has a unimodal mode field with maximum intensity at the center of the 2 μm width of the high-resistance n-type InP upper cladding layer 205.

Figures 6, 7, and 8 show cross sections taken along the lines VI-VI, VII-VII, and VIII-VIII in Figure 4, respectively. Between the portions of the rib-type optical waveguides 414a and 414b having the wide slab width W414w on the high mesa optical waveguide region A side and the high mesa optical waveguide region C side, the rib-type optical waveguides 414a and 414b have a slab width W414n that is narrower than the wide slab width W414w.

As shown in FIG. 6, the portions of the rib-type optical waveguides 414a, 414b that are not loaded with the slot electrodes 121a, 121b have a slab width W414n that is narrower than the wide slab width W414w.

The width (y direction) of the high-resistance n-type InP upper cladding layer 205 of the rib-type optical waveguides 414a, 414b at the position of the cross-sectional line VI-VI in FIG. 4 is 2 μm. The narrow slab width W414n of the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 is 13 μm, which is narrower than the wide width 414 (30 μm) at the position of the cross-sectional line V-V in FIG. 4 (FIG. 5). The thickness of the i-type MQW core layer 204 is 0.5 μm. The fundamental mode of light has a unimodal mode field with maximum intensity at the center of the 2 μm width of the high-resistance n-type InP upper cladding layer 205.

As shown in FIG. 7, the portions of the rib-type optical waveguides 414a, 414b on which the slot electrodes 121a, 121b are loaded have a slab width W414n that is narrower than the wide slab width W414w.

Also, as shown in FIG. 8, at the positions where the connection electrodes 122a, 122b that connect the slot electrodes 121a, 121b and the high-frequency lines 120a, 120b are arranged, the portions of the rib-type optical waveguides 414a, 414b have a slab width W414n that is narrower than the wide slab width W414w.

At the positions of the cross-sectional lines VII-VII and VIII-VIII in FIG. 4, the width (y direction) of the n-type InP upper cladding layer 305 of the rib-type optical waveguides 414a, 414b is 2 μm. The narrow slab width W414n of the i-type MQW core layer 204, the p-type InP layer 203, and the n-type InP lower cladding layer 202 is 13 μm, which is narrower than the wide width 414 (30 μm) at the position of the cross-sectional line V-V in FIG. 4 (FIG. 5). The thickness of the i-type MQW core layer 204 is 0.5 μm. The fundamental mode of light has a unimodal mode field with maximum intensity at the center of the 2 μm width of the n-type InP upper cladding layer 305.

As described above, in this embodiment, the rib-type optical waveguides 414a and 414b have a wide slab width W414w at two locations near the boundary between the high-speed modulation region/rib-type optical waveguide region B, which includes the connection positions with the high-mesa optical waveguides 113a, 113b, 115a, and 115b, and the high-mesa optical waveguide regions A and C. The reason for this is explained below.

At the two connection positions between the high mesa optical waveguides 113a, 113b, 115a, 115b and the rib optical waveguides 414a, 414b, higher-order modes are excited due to misalignment of the mode field center, etc., but by widening the width 414n of the rib optical waveguides 414a, 414b near the connection positions, the excited higher-order modes remain in the core layer and are not guided. Therefore, it is effective to widen the width of the rib optical waveguides 414a, 414b near the boundary between the high-speed modulation region/rib optical waveguide region B, which includes the two connection positions, and the high mesa optical waveguide regions A and C, as in this embodiment.

In addition, since the slot electrodes 121a and 121b are not arranged near the boundary side between the high mesa type optical waveguide regions A and C in the high-speed modulation region and rib type optical waveguide region B, an area where the connection electrodes 122a and 122b are not arranged can be secured. In other words, the i-type MQW core layer 204, p-type InP layer 203, and n-type InP lower cladding layer 202 are not arranged directly below the connection electrodes 122a and 122b. Since high frequency waves also flow in the connection electrodes 122a and 122b, the deterioration of high frequency characteristics is minimized by not providing semiconductors such as the i-type MQW core layer 204, p-type InP layer 203, and n-type InP lower cladding layer 202 directly below the connection electrodes 122a and 122b.

(Modifications)
As can be understood from the above, by widening the width of the rib-type optical waveguides 414a, 414b between the multiple connection electrodes 122a, 122b arranged in the direction of light propagation, avoiding the positions where the connection electrodes 122a, 122b are installed, it is also effective to configure the structure to increase the removal rate of higher modes and limit the deterioration of high-frequency characteristics.

Figure 9 shows a high-speed optical modulator 900 according to a modified embodiment of the high-speed optical modulator 400 of Figure 4. The high-speed modulation region and rib-type optical waveguide region B of the high-speed optical modulator 900 shown in Figure 9 includes rib-type optical waveguides 914a and 914b connected to the high-mesa optical waveguides 113a and 113b and the high-mesa optical waveguides 115a and 115b, respectively. As with the high-speed optical modulator 400 of Figure 4, each optical waveguide has a tapered shape at the boundary between the two regions to suppress optical connection loss. A plurality of slot electrodes 121a and 121b are formed discretely above the rib-type optical waveguides 914a and 914b, respectively, and function as an optical modulator that adds a signal to light by the electro-optic effect. The multiple slot electrodes 121a, 121b are connected to the two high-frequency lines 120a, 120b by multiple connection electrodes 122a, 122b, respectively. The two high-frequency lines 120a, 120b are formed parallel to the rib-type optical waveguides 914a, 914b above the organic film (e.g., BCB) 206, which is a non-semiconductor material, in order to propagate the RF modulated electrical signal with low loss. The multiple connection electrodes 122a, 122b are formed perpendicular to the rib-type optical waveguides 414a, 414b.

The layer structure of the InP semiconductor of the rib-type optical waveguides 414a and 414b is similar to the layer structure of the InP semiconductor of the rib-type optical waveguides 114a and 114b described with reference to FIG. 3.

In the high-speed optical modulator 900 shown in FIG. 9, the rib-type optical waveguides 914a, 914b have a wide width not only near the boundary between the high-mesa optical waveguide region A, which includes the connection positions with the high-mesa optical waveguides 113a, 113b, and the high-speed modulation region/rib-type optical waveguide region B, but also between the multiple connection electrodes 122a, 122b arranged in the light propagation direction within the high-speed modulation region/rib-type optical waveguide region B.

As described above, in the high-speed optical modulator of the present disclosure, a portion of the rib-type optical waveguide in the high-speed modulation region and rib-type optical waveguide region B is made wide (e.g., 30 μm), and the remaining portion is made narrow (e.g., 13 μm). This is equivalent to a configuration in which slab waveguides of different widths are connected in series. The shape and effective refractive index of the higher-order mode when the core width in the optical waveguide is 13 μm differs from that of the higher-order mode when the core width is 30 μm, so mode mismatch occurs at the connection point of the slab waveguide, resulting in loss. On the other hand, the fundamental mode, which is the main mode, is confined to the upper cladding width of about 2 μm, so it propagates without feeling the width of the slab waveguide, such as 13 μm or 30 μm, and is therefore less affected. As a result, only the higher-order mode is removed, and deterioration of the extinction ratio is suppressed.

As shown in Figure 5, if the i-type MQW core layer 204, p-type InP layer 203, and n-type InP lower cladding layer 202 are formed below the high-frequency lines 120a, 120b parallel to the rib-type optical waveguides 414a, 414b, the high-frequency characteristics will deteriorate. However, in the optical modulator 400 of this embodiment, the length of the rib-type optical waveguides 414a, 414b with the wide slab width W414w in the light propagation direction (x direction) is short (for example, 100 μm), and the deterioration of the high-frequency characteristics is also limited.

In the embodiment of the present disclosure, an example is shown in which the rib-type optical waveguides 414a, 414b have two different widths (W414w and W414n), but it goes without saying that the same effect can be obtained even if the rib-type optical waveguides 414a, 414b have three or more different widths.

According to one embodiment of the present disclosure, it is possible to provide a high-speed optical modulator with excellent optical characteristics.

100 High-speed phase modulator 111 High mesa type optical waveguide 112 1×2 demultiplexer 113a, 113b High mesa type optical waveguide 114a, 114b, 414a, 414b, 914a, 914b Rib type optical waveguide 115a, 115b High mesa type optical waveguide 116 2×1 multiplexer 117 High mesa type optical waveguide 118 Separation groove 120a, 120b High frequency line 121a, 121b Slot electrode 122a, 122b Connection electrode 130a, 130b Thin film heater 131a, 131b Pad 132a, 132b Wiring 150a, 150b Termination resistor 201 Semi-insulating InP substrate 202 n-type InP lower cladding layer 203 p-type InP layer 204 i-type MQW core layer 205 high-resistance InP upper cladding layer 206 organic film (BCB)
305 High resistance InP upper cladding layer 400, 900 High speed optical modulator

Claims (3)

A first high mesa type optical waveguide;
a demultiplexer having a high mesa type optical waveguide structure connected to the first high mesa type optical waveguide;
A second high mesa type optical waveguide;
a multiplexer having a high mesa type optical waveguide structure connected to the second high mesa type optical waveguide;
two arm optical waveguides connected to the demultiplexer and the multiplexer;
An optical modulator comprising:
Each of the two arm optical waveguides has
A rib-type optical waveguide;
an electrode disposed on the rib-type optical waveguide;
Equipped with
The rib type optical waveguide is
a lower clad layer, a core layer, and an upper clad layer, in that order, on a substrate;
In a cross section perpendicular to the direction of light propagation, the width of the upper clad layer is narrower than the widths of the core layer and the lower clad layer,
An optical modulator, wherein the widths of the core layer and the lower cladding layer include two or more different widths including a first width and a second width wider than the first width. a high-frequency line arranged in parallel to the rib-type optical waveguide,
2. The optical modulator according to claim 1, wherein in a cross section perpendicular to the propagation direction of the light, the core layer having the second width and the lower cladding layer are disposed between the high-frequency line and the substrate. Each of the two arm optical waveguides has
a third high mesa type optical waveguide connected to the demultiplexer and the rib type optical waveguide;
a fourth high mesa type optical waveguide connected to the rib type optical waveguide and the multiplexer; and
Further equipped with
3. The optical modulator according to claim 1, wherein the core layer and the lower cladding layer having the second width are arranged near a connection position between the rib type optical waveguide and the third high mesa type optical waveguide and near a connection position between the rib type optical waveguide and the fourth high mesa type optical waveguide.

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