WO2024157313A1 - Phase shifter and optical switch - Google Patents

Abstract

Provided are: a phase shifter capable of maintaining a modulation state even while realizing a reduction in power consumption in comparison to the prior art; and an optical switch that includes the phase shifter. This phase shifter (200) according to the present disclosure is of a waveguide type, the phase shifter including a core (201), a cladding (202) that covers the core (201), and electrical wires (203a, 203b) that are disposed substantially perpendicular to the optical axis direction of the core (201) and in the vicinity of both side surfaces of the core (201), wherein the band-gap energy of the cladding (202) is configured to be greater than the band-gap energies of the core (201) and each of the electrical wires (203a, 203b). Additionally, the optical switch (500) according to the present disclosure includes said phase shifter (200) on at least one of an upper arm (104) and a lower arm (105).

Description

Phase shifter and optical switch

This disclosure relates to a phase shifter and an optical switch.

Waveguide-type optical switches are key devices in optical communication systems, and are widely used within optical communication networks due to their ability to realize flexible optical communication networks. A typical configuration of a waveguide-type optical switch is one that combines a Mach-Zehnder interferometer (hereinafter referred to as MZI) with a phase shifter, and waveguide-type optical switches with this configuration are being put to practical use in optical communication networks.

1 is a top view conceptually showing the structure of a waveguide-type optical switch 100 that combines an MZI and a phase shifter according to the prior art. As shown in Fig. 1, the waveguide-type optical switch 100 includes input waveguides 101a, b, output waveguides 102a, b, a coupler 103a connected to the input waveguides 101a, b, a coupler 103b connected to the output waveguides 102a, b, an upper arm 104 and a lower arm 105 connected to the couplers 103a, b, and a phase shifter 106 connected to the lower arm 105 and modulating the phase of the propagating optical signal. Although not shown here, the waveguide-type optical switch 100 also includes a substrate and a cladding formed on the substrate, and the core region through which the optical signal propagates (input side waveguides 101a, b, output side waveguides 102a, b, upper arm 104, and lower arm 105) has a structure embedded in the cladding.

The waveguide-type optical switch 100 having such a configuration can switch the output waveguide by controlling the phase difference of the optical signals passing through the upper arm 104 and the lower arm 105, which determines the interference condition in the MZI, with the phase shifter 106 (see, for example, Non-Patent Document 1). For example, when the optical path lengths of the upper arm 104 and the lower arm 105 are the same and the phase modulation amount in the phase shifter 106 is 0, the optical signal input from the input side waveguide 101a is output from the output side waveguide 102b (cross state). On the other hand, when the phase of the optical signal changes by π in the phase shifter 106, the interference state changes and the optical signal input from the input side waveguide 101a is output from the output side waveguide 102a (through state).

In general, materials used for optical waveguides can be silicon oxide (SiOx) or silicon (Si). When SiOx is used for both the core and cladding, a dopant is usually added to the SiOx used for the core to increase the refractive index of the core, thereby confining the optical signal to the core. On the other hand, when Si is used for the core, SiOx, which has a lower refractive index than Si, is generally used for the cladding.

Thermo-optic phase shifters are widely used as phase shifters for optical waveguides using SiOx or Si. Thermo-optic phase shifters use a heating mechanism such as a heater to locally heat part of the core, changing the refractive index of the heated area to change the optical path length and thus the phase of the optical signal propagating through the core. This phenomenon in which the refractive index of a material changes depending on the temperature is generally called the thermo-optic effect (in other words, a thermo-optic phase shifter is an element that uses the thermo-optic effect to change the phase of an optical signal).

As mentioned above, thermo-optic phase shifters use the change in refractive index caused by temperature, so in order to maintain the modulation state of the phase shifter, it is necessary to keep the area to be heated at a certain temperature by controlling a heater or the like. This requires that the heater or the like be constantly powered, which poses the problem of high power consumption. There is also the problem that if the power supply is cut off, such as in the case of a disaster when it becomes difficult to transmit electricity, the phase modulation state cannot be maintained (for example, if a thermo-optic phase shifter is used in an optical switch, the switching state of the optical switch cannot be maintained).

International Publication No. 2022/044101

Takashi Goh, Mitsuho Yasu, Kuninori Hattori, Akira Himeno, Masayuki Okuno, and Yasuji Ohmori, "Low Loss and High Extinction Ratio Strictly Nonblocking 16 16 Thermooptic Matrix Switch on 6-in Wafer Using Silica-Bas ed Planar Lightwave Circuit Technology," J. Lightwave Technol. 19, 371- (2001) S. Raghunathan, T. Krishnamohan, K. Parat and K. Saraswat, "Investigation of ballistic current in scaled Floating-gate NAND FLASH and a solution," 2009 IEEE International Electron Devices Meeting (IEDM), pp. 1-4, doi : 10.1109/IEDM.2009.5424216 (2009) K. Yamaguchi, A. Otake, K. Kamiya, Y. Shigeta and K. Shiraishi, "Universal guiding principles for the fabrication of highly scalable MONOS-type memory-atomistic recipes based on designing interface oxygen chemical potential-," 2010 International Electron Devices Meeting, pp. 5.7.1-5.7.4, doi: 10.1109/IEDM.2010.5703305 (2010) B. Ben Bakir et al., "Low-Loss (<1dB) and Polarization-Insensitive Edge Fiber Couplers Fabricated on 200-mm Silicon-on-Insulator Wafers," in IEEE Photonics Technology Letters, vol. 22, no. 11, pp. 739-741, June1, doi: 10.1109/LPT.2010.2044992 (2010).

The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a phase shifter and an optical switch including the phase shifter that can maintain the modulation state while achieving reduced power consumption compared to conventional techniques.

In response to the above-mentioned problems, the present disclosure provides a phase shifter that is a waveguide type phase shifter including a core, a cladding that covers the core, and electrical wiring arranged substantially perpendicular to the optical axis direction of the core and near both side surfaces of the core, and that is configured so that the band gap energy of the cladding is higher than the band gap energy of each of the core and the electrical wiring, and an optical switch that includes the phase shifter in at least one of the upper arm and the lower arm.

FIG. 1 is a top view conceptually showing the structure of a waveguide-type optical switch 100 in which an MZI and a phase shifter are combined according to a conventional technique. 2A and 2B are diagrams conceptually illustrating a structure of a phase shifter 200 according to a first embodiment of the present disclosure, in which FIG. 2A is a top view and FIG. 2B is a cross-sectional view taken along line IIb-IIb. 2A and 2B are diagrams showing band structures of a core 201, a cladding 202, and an electrical wiring 203b in a phase shifter 200 according to a first embodiment of the present disclosure, in which (a) shows a state in which no voltage is applied between the electrical wirings 203a and b, and (b) shows a state in which a voltage is applied between the electrical wirings 203a and b. 2A and 2B are diagrams showing band structures of a core 201, a cladding 202, and an electrical wiring 203b in another form of a phase shifter 200 according to the first embodiment of the present disclosure, where (a) shows a state in which no voltage is applied between the electrical wirings 203a and b, and (b) shows a state in which a voltage is applied between the electrical wirings 203a and b. FIG. 2 is a top view conceptually illustrating a structure of an optical switch 500 using the phase shifter 200 according to the first embodiment of the present disclosure. FIG. 13 is a diagram showing an etching region required when disposing a phase shifter 200 on the optical switch 500 according to the first embodiment of the present disclosure. 2A and 2B are diagrams conceptually illustrating a structure of a phase shifter 700 according to a second embodiment of the present disclosure, in which (a) is a top view and (b) is a cross-sectional view taken along line IIb-IIb. 8A and 8B are diagrams conceptually illustrating the structure of an optical switch 800 according to a second embodiment of the present disclosure, in which (a) is a top view showing the overall structure, (b) is an enlarged view of the connection between an SSC structure 701 of a phase shifter 700 and a lower arm 105, and (c) is a cross-sectional view along the VIIIc-VIIIc cross-sectional line. 9A and 9B are conceptual diagrams showing another structure of the optical switch 800 according to the second embodiment of the present disclosure, in which (a) shows an enlarged view of the connection between the SSC structure 701 of the phase shifter 700 and the lower arm 105, and (b) shows a cross-sectional view along the IXb-IXb cross-sectional line. 10A and 10B are diagrams conceptually illustrating the structure of the connection portion between the phase shifter 700 and the lower arm 105 in another form of the optical switch 800 according to the second embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view along the Xb-Xb cross-sectional line. 10A and 10B are diagrams conceptually illustrating the structure of the connection portion between the phase shifter 700 and the lower arm 105 in another form of the optical switch 800 according to the second embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view along the XIb-XIb cross-sectional line. 10A and 10B are diagrams conceptually illustrating the structure of the connection portion between the phase shifter 700 and the lower arm 105 in another form of the optical switch 800 according to the second embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view along the XIIb-XIIb cross-sectional line.

Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference symbols indicate the same or similar elements, and duplicate descriptions may be omitted. Materials and numerical values are for illustrative purposes and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of an embodiment of the present disclosure.

In the phase shifter according to the present disclosure, the band gap energy of the core material is configured to be lower than the band gap energy of the cladding material. Furthermore, the phase shifter according to the present disclosure includes a mechanism for injecting and removing charge from the outside into the core covered by the cladding. The phase shifter according to the present disclosure configured in this manner is able to control the refractive index of the core by controlling the charge density (electron density) by injecting and removing charge into the core. As a result, it becomes possible to control the phase of the propagating optical signal due to the carrier plasma effect.

(First embodiment)
Hereinafter, a first embodiment of the present disclosure will be described in detail with reference to the drawings. A phase shifter in this embodiment is a phase shifter disposed in a waveguide-type optical switch, and a semiconductor such as Si is used for a core other than the phase shifter of the optical switch.

(Phase shifter configuration)
2 is a diagram conceptually illustrating the structure of a phase shifter 200 according to a first embodiment of the present disclosure, in which (a) is a top view and (b) is a cross-sectional view taken along the line IIb-IIb. As shown in FIG. 2, the phase shifter 200 includes a core 201, a cladding 202 that surrounds the core 201, and electrical wiring 203a, b that is disposed on both sides of the core 201 and is substantially perpendicular to the optical axis direction of the core 201. The electrical wiring 203a, b is connected to an external power source and configured to be able to apply a voltage. Furthermore, in the phase shifter 200 of this embodiment, the band gap energy of the material of the core 201 is configured to be lower than the band gap energy of the material of the cladding 202.

3 is a diagram showing the band structures of the core 201, the cladding 202, and the electric wiring 203b in the phase shifter 200 according to the first embodiment of the present disclosure, in which (a) shows a state in which no voltage is applied from an external power source to the electric wiring 203b, and (b) shows a state in which a voltage is applied from an external power source to the electric wiring 203b. In addition, in FIG. 3, the black areas represent bands occupied by electrons, and the white areas represent empty bands. In addition, in FIG. 3, "e ^- " represents electrons. As described above, in the phase shifter 200, the band gap energy of the material of the core 201 is configured to be lower than the band gap of the material of the cladding 202. Therefore, as shown in FIG. 3(a), in the ground state, the conduction band of the cladding 202 has the highest energy level, and each of the core 201, the cladding 202, and the electric wiring 203b has a band structure in which the valence band is filled with electrons and the conduction band is empty.

On the other hand, when a voltage is applied to the electrical wiring 203b to generate a potential gradient, as shown in FIG. 3B, conduction electrons (carriers) are generated in the electrical wiring 203b. Although the energy level of the carriers is lower than the energy level of the conduction band of the cladding 202, some of the electrons move from the electrical wiring 203b to the core 201 due to the tunnel effect. The movement of electrons between the core 201 and the electrical wiring 203a located on the opposite side of the electrical wiring 203b is blocked by the cladding 202, so the moved electrons remain in the valence band or conduction band of the core 201, and the potential floats. This is similar to the structure and write/erase operation of a floating gate memory (see, for example, Non-Patent Document 2). As a result, charge is injected into the core 201, and the carrier density in the core 201 changes, causing a change in the refractive index due to the carrier plasma effect. As a result, the optical path length felt by the optical signal propagating through the waveguide changes, and accordingly, it becomes possible to function as a self-holding phase shifter.

Considering the operating principle of the waveguide, the core 201 needs to have a higher refractive index than the clad 202. Therefore, in the phase shifter 200, if the core 201 is made of Si, the clad 202 can be made of a material (e.g., SiOx) that has a wider band gap and a lower refractive index than Si, thereby realizing the above-mentioned configuration. In addition, when a semiconductor such as Si is used for the core 201, the same material as the core 201 can be applied to the electrical wiring 203a, b. In such a configuration in which the same material is used for the electrical wiring 203a, b and the core 201, it is possible to omit processes such as film formation of different materials during manufacturing. For example, by using Si for the core 201 and Si for the electrical wiring 203a, b, it is possible to simultaneously form the core 201 and the electrical wiring 203a, b during manufacturing. In addition, as mentioned above, it is desirable for the electrical wiring 203a, b to have high conductivity from the viewpoint of charge injection, so the conductivity of the Si used for the electrical wiring 203a, b may be further improved by doping it with boron (B) or phosphorus (P).

In addition, from the viewpoint of utilizing the tunneling effect, it is desirable that the width of the cladding 202 (the length in the direction in which the electrons move) is thin, for example, 1 μm or less. On the other hand, since it is desirable that the injected charge (moved electrons) does not move to the other electrical wiring (electrical wiring 203a in the example shown in FIG. 3) during charge injection, the width of the cladding 202 between the core 201 and electrical wiring 203b may be configured to be thinner than the width of the cladding 202 between the core 201 and electrical wiring 203a.

In the above explanation, it is desirable that the material of the electrical wiring 203a, b is the same as that of the core 201 or a material with a high band gap energy. However, as shown in FIG. 4, it is also possible to form crystal defects (e.g., defects caused by dangling bonds or impurity addition) in the core 201 and form a defect level in the forbidden band between the valence band and conduction band of the core 201 by the crystal defects, thereby retaining electric charges at the defect level. For example, it is known that defects in SiN surrounded by SiOx can trap and retain electric charges, and it has been reported that electric charges can be injected and removed by applying an electric field (see, for example, Non-Patent Document 3). Therefore, by using SiN for the core 201 in the phase shifter 200 and using SiOx as a cladding, it is also possible to trap electric charges in the defects in the SiN and use it as a phase shifter. Even in such a form, if the electric field is set to 0 (constant potential), the electric charges remain in the SiN core, so it can function as a self-retaining phase shifter in the same way.

(Configuration of optical switch)
Fig. 5 is a top view conceptually illustrating a structure of an optical switch 500 using the phase shifter 200 according to the first embodiment of the present disclosure. As shown in Fig. 5, the optical switch 500 has a configuration in which the phase shifter of the waveguide-type optical switch 100 according to the conventional technology shown in Fig. 1 is the above-mentioned phase shifter 200. Here, the couplers 103a and 103b can be, for example, directional couplers (DC).

In the optical switch 500 having such a configuration, an optical signal input from either one of the input waveguides 101a, b is distributed by the coupler 103a to both the upper arm 104 and the lower arm 105 and propagates through each. The optical signal is then rejoined by the coupler 103b and output from the two output waveguides 102a, b. If there is a difference in the optical path length of each of the upper arm 104 and the lower arm 105, the interference state when the signals are joined at the coupler 103b changes, and the intensity ratio of the output light changes.

In the optical switch 500 according to this embodiment, the above-mentioned phase shifter 200 is disposed in at least one of the upper arm 104 or the lower arm 105. This phase shifter 200 performs phase modulation of the optical signal using the carrier plasma effect, making it possible to control the output intensity and output ratio of the optical signal output from the output side waveguides 102a, b. For example, when the optical path lengths of the upper arm 104 and the lower arm 105 are the same, if the phase modulation amount in the phase shifter 200 is 0, it becomes a cross state, and if the phase modulation amount in the phase shifter 200 is π, it changes to a through state. In other cases, it takes an intermediate state between the through state and the cross state according to the phase modulation amount in the phase shifter 200, and is distributed and output from both the output side waveguides 102a, b at a ratio corresponding to the phase modulation amount. In other words, the optical switch 500 can also function as a coupler/splitter that changes the branching ratio of the output optical signal and a variable optical attenuator (VOA) that controls the output intensity.

In FIG. 5, the phase shifter 200 is depicted as being disposed in the lower arm 105, but this is for illustrative purposes only, and the phase shifter 200 may be disposed in the upper arm 104, or in both the upper arm 104 and the lower arm 105. From the viewpoint of controlling the interference state of the optical signal, it is sufficient to dispose the phase shifter 200 in either the upper arm 104 or the lower arm 105, or to control only one of the phase shifters 200 installed in both the upper arm 104 and the lower arm 105. On the other hand, in order to achieve an effect such as suppressing the polarization characteristics so that each path has different characteristics, it is necessary to control both the upper arm 104 and the lower arm 105.

Furthermore, when the core 201 of the phase shifter 200 and the cores in other regions, for example the upper arm 104 and the lower arm 105, are made of the same material, the core 201 in the phase shifter 200 must be covered with the cladding 202 in the optical axis direction of the optical signal. However, in this case, reflected light in the optical circuit can lead to problems such as increased loss. Therefore, in such a case, it is desirable to form the interface between the cladding 202 and the core 201 at an angle that is not perpendicular to the optical axis direction of the optical signal. In addition, when passing through the cladding 202 inserted in the optical axis direction of the optical signal, confinement is weakened due to the absence of the core 201 with a high refractive index, which causes loss due to diffusion. Therefore, it is desirable for the inserted cladding 202 to be thin. For example, it is desirable to keep the thickness to less than the wavelength at which the effect of diffusion is small.

(Phase shifter arrangement method)
When forming the optical switch 500 so as to embed the optical waveguide in a dielectric film formed on a wafer, it is necessary to form a pattern in a multi-layer structure in order to make an electrical connection (connection between the electrical wiring 203a, b and an external power source outside the wafer) in a direction perpendicular to the wafer surface. However, when forming a pattern in a multi-layer structure, processing becomes difficult and an increase in the number of steps may occur, such as the need to form a pattern by etching or the like, form a film on the pattern, and further flatten the pattern. Such an increase in the number of steps causes an increase in manufacturing costs and a prolongation of the manufacturing LT, so it is desirable to form the electrical connection on the same plane parallel to the wafer surface.

On the other hand, since the electrical wirings 203a and 203b also have resistance, a voltage drop occurs when a current flows. From this perspective, in order to improve the efficiency of the above-mentioned tunnel effect, it is desirable to electrically connect the external power source to the electrical wirings 203a and 203b as close to the phase shifter 200 as possible. From this perspective, the arrangement of the phase shifter 200 relative to the optical switch 500 according to this embodiment includes exposing the electrical wirings 203a and 203b in the same plane as the core by etching the regions 601a and 601b where the electrical wirings 203a and 203b exist and where no core (e.g., the upper arm 104 or the lower arm 105) exists, as shown in FIG. 6. By using such an arrangement method, it is possible to manufacture an optical switch 500 that can inject and remove charges with high efficiency while suppressing increases in manufacturing costs and prolonged manufacturing LT.

For example, as shown in FIG. 6(a), an example of a region near the phase shifter 200 and without a core is region 601a located at the center of the upper arm 104 and the lower arm 105. By forming electrical wiring 203a up to this region 601a and etching region 601a, it is possible to expose electrical wiring 203a. If region 601a is then filled with, for example, a metal, it becomes possible to electrically connect to an external power source.

Depending on the design, the area corresponding to region 601a may be narrow and etching may not be possible. In such a case, as shown in FIG. 6(b), the electrical wiring 203a may be formed across the side where the phase shifter 200 is not arranged (the upper arm 104 side in FIG. 6(b)), and the etching area may be made in region 601b located outside the upper arm 104, with the same effect being obtained. In such a case, it is desirable to minimize the effect that the electrical wiring 203a has on the optical signal propagating through the upper arm 104.

Second Embodiment
Hereinafter, a first embodiment of the present disclosure will be described in detail with reference to the drawings. A phase shifter in this embodiment is a phase shifter disposed in a waveguide-type optical switch, and SiOx is used for a core other than the phase shifter of the optical switch.

(Phase shifter configuration)
In general, an optical waveguide using SiOx as a core has a characteristic that the loss in propagation and the connection loss with an optical fiber are small. However, SiOx has a high band gap energy, and in a waveguide using SiOx as a core, SiOx is often used for the cladding as well. For this reason, when SiOx is used for the core of the above-mentioned phase shifter 200, it is difficult to keep electrons in the core 201 and confine the charge. Therefore, in the optical switch 500 according to the present disclosure, even if SiOx is used for the core other than the phase shifter 200 of the optical switch 500 (for example, the upper arm 104 and the lower arm 105, etc.), it is desirable to use a material with a low band gap energy such as Si or SiN for the core 201 in the phase shifter 200.

However, in optical waveguide circuits, when the refractive index of the core is partially changed, loss due to mode coupling occurs at the boundary (transition region) where the refractive index changes. Therefore, when a different material is used only for the core 201 of the phase shifter 200 described above, the boundary between the core 201 and the core in which SiOx is used becomes a transition region accompanied by a change in refractive index. Therefore, it is desirable to configure the transition region so that the mode changes adiabatically. From this perspective, in addition to the configuration of the phase shifter 200, the phase shifter according to this embodiment further includes a spot-size converter structure (hereinafter referred to as SSC) in the part where the core 201 becomes the transition region.

FIG. 7 is a conceptual diagram showing the structure of a phase shifter 700 according to a second embodiment of the present disclosure, where (a) shows a top view and (b) shows a cross-sectional view along the IIb-IIb cross-sectional line. As described above and as shown in FIG. 7, in the phase shifter 700, the tip of the core 201 in the configuration of the phase shifter 200 further includes an SSC structure 701. The region in which this SSC structure 701 is formed corresponds to the transition region described above.

In general, when optical elements are connected to each other, it is important to match the mode fields of the light propagating in the optical elements in order to reduce loss at the connection point. When two optical elements are butt-connected, the coupling efficiency of the propagating optical signal is determined by the overlap integral of the mode fields of both. When the core is Si, the mode field diameter (Mode Field Diameter: hereinafter referred to as MFD) is about 300 nm, while in the case of SiOx, it is several μm, so that a large coupling loss occurs due to the mismatch of this MFD. However, in the phase shifter 700 according to this embodiment, the SSC structure 701 having a tapered structure is formed, so that the mode changes adiabatically. More specifically, it is configured so that the confinement efficiency changes with the change in diameter, and the MFD changes accordingly. Therefore, the mismatch of MFD at the connection point with the core using SiOx is eliminated, making it possible to suppress loss due to mode coupling.

In addition, in FIG. 7, the SSC structure 701 is depicted as a tapered structure in which the diameter of the core 201 changes continuously, but this is for illustrative purposes only, and the structure of the SSC structure 701 may be of any shape as long as it is a structure that changes the mode adiabatically.

(Configuration of optical switch)
8 is a diagram conceptually illustrating the structure of an optical switch 800 according to a second embodiment of the present disclosure, in which (a) is a top view showing the entire structure, (b) is an enlarged view of a connection between an SSC structure 701 of a phase shifter 700 and a lower arm 105, and (c) is a cross-sectional view taken along the line VIIIc-VIIIc. As shown in Fig. 8, the optical switch 800 has a configuration in which the phase shifter of the conventional waveguide-type optical switch 100 shown in Fig. 1 is the above-mentioned phase shifter 700. Here, for example, Si can be applied to the core 201 of the phase shifter 700, and SiOx can be applied to the lower arm 105.

In FIG. 8, the SSC structure 701 has a tapered structure in which the tip of the core 201 is tapered, and the SSC structure 701 and the tip of the lower arm 105 made of SiOx are depicted as being arranged to butt against each other. The core 201 and the lower arm 105 are covered with an undercladding layer 801 and an overcladding layer 802, and the relative refractive index difference between the lower arm 105 and each cladding layer is set to be smaller than the relative refractive index difference between the core 201 and the SSC structure 701 of the phase shifter 700 and each cladding layer. In this embodiment, the lower arm 105 is configured so that the core 201 has a larger core cross-sectional area and MFD than Si. In such a case, the optical signal propagating through the core 201 is weakly confined as it approaches the tip in the SSC structure 701, and therefore the MFD becomes larger. This eliminates the mismatch in MFD with the lower arm 105, and reduces the coupling loss.

In addition, in FIG. 8, the tips of the SSC structure 701 and the lower arm 105 are depicted as being butted together, but as shown in FIG. 9, the SSC 701 structure may be covered by the lower arm 105. In such a structure, the optical signal that is no longer able to be contained as it approaches the tip of the SSC structure 701 leaks out to the surrounding lower arm 105. The leaked optical signal adiabatically transitions to the lower arm 105, making this optical transition process adiabatic and making it possible to more efficiently suppress the loss of optical energy.

Furthermore, in the examples shown in Figures 8 and 9, the thickness of the core 201 and the thickness of the lower arm 105 are depicted as being different, and therefore the respective height centers are depicted as not coinciding. However, as shown in Figure 10, by aligning these height centers, it is possible to further suppress the coupling loss at the connection between the two.

10 is a conceptual diagram showing the structure of the connection between the phase shifter 700 and the lower arm 105 in another form of the optical switch 800 according to the second embodiment of the present disclosure, where (a) shows a top view and (b) shows a cross-sectional view along the Xb-Xb cross-sectional line. In the example shown in FIG. 10, the core 201 of the phase shifter 700 is configured to be covered with an undercladding layer 1001 and an overcladding layer 1002. Here, the same material as that of the lower arm 105 is applied to each of the undercladding layer 1001 and the overcladding layer. The core 201 and the lower arm 105 are butted together with their height centers aligned, and further, the undercladding layer 1001, the overcladding layer 1002, and the lower arm 105 are configured to be covered with the undercladding layer 801 and the overcladding layer 802. Thus, in the example shown in FIG. 10, the phase shifter 700 and the lower arm 105 have a configuration in which the waveguides having a "dual structure" are butt-connected.

In FIG. 10, the overcladding layer 1002 is depicted as covering parts of the core 201 other than the SSC structure 701, but as shown in FIG. 11, the overcladding layer 1002 may cover only the area corresponding to the SSC structure 701. In the example shown in FIG. 11, the process of forming or patterning a SiOx layer on the SSC structure 701 during manufacturing is not required, which has the advantage of reducing factors that deteriorate the circuit on the phase shifter 700 side.

Also, as shown in FIG. 12, not only the overcladding layer 1002 but also the overcladding layer 802 may be formed only in the region corresponding to the SSC structure 701. In the example shown in FIG. 12, in the portion of the core 201 that is not covered by the overcladding layer 1002 and the overcladding layer 802, air plays the role of a cladding, so that the optical signal is confined within the core 201.

The thickness of the undercladding layer 801 and the overcladding layer 802 need only be such that the mode field of the optical signal is sufficiently contained therein. For example, when the undercladding layer 801 and the overcladding layer 802 are made of SiOx, the thickness of each layer may be approximately several tens of μm.

In the phase shifter and optical switch disclosed herein, there is no upper limit to the cross-sectional size of each core, and it is also possible to make it a multi-mode optical waveguide that propagates multiple modes of light for the wavelength of the optical signal used. Also, by reducing the cross-sectional size of the core, it is possible to make it a single-mode optical waveguide that propagates only the lowest mode. Note that when the optical signal is single-mode, there are generally two methods for connecting cores together: adiabatic coupling and butt coupling. In the above explanation, the connection between the phase shifter and the optical switch has been described as being in the form of butt coupling, but this is not limited to this, and it may be adiabatic coupling or a combination of both.

Furthermore, the phase shifter and optical switch according to the present disclosure can be manufactured by techniques used in existing optical circuit manufacturing methods. For example, a deposition method such as the flame volume deposition method can be used to form the SiOx layer, and a deposition method such as the sputtering method can be used to form the Si layer.

Integration methods for optical circuits combining elements with different MFDs generally include hybrid integration, which combines separate substrates, and monolithic integration, which uses a single common substrate. In the case of hybrid integration, a process for accurately aligning the SSC structure 701 and the lower arm 105 (also called an alignment process) is required, which can increase manufacturing costs. In particular, when Si is used for the core 201, as described above, the core is very thin, measuring several hundred nm, so there is a high requirement for alignment accuracy, and there is a problem that a high-precision alignment process requires very high costs. On the other hand, monolithic integration is a manufacturing method in which different materials are integrated on the same substrate, and therefore the problems caused by such alignment processes are eliminated. From this perspective, it is desirable that the optical switch 800 according to this embodiment is formed by monolithic integration.

As described above, the phase shifter and optical switch disclosed herein are characterized by using the carrier plasma effect to control the refractive index by injecting and removing electric charges. Phase shifters and optical switches with such characteristics do not require constant power supply, unlike conventional techniques for controlling the refractive index using heaters or the like. Therefore, they are expected to be applied to optical communication systems as phase shifters and optical switches that can reduce power consumption compared to conventional techniques.

Claims (5)

A waveguide type phase shifter,
The core,
A cladding covering the core;
Electric wiring arranged substantially perpendicular to the optical axis direction of the core and in the vicinity of both side surfaces of the core;
Equipped with
A phase shifter configured such that the bandgap energy of the cladding is higher than the bandgap energy of each of the core and the electrical wiring. The phase shifter of claim 1, wherein the core further comprises a spot size conversion structure at its tip. The phase shifter of claim 1, wherein the core and the electrical wiring are semiconductors and made of the same material. The phase shifter of claim 1, wherein the core has crystal defects. A Mach-Zehnder interference type optical switch, comprising a phase shifter according to claims 1 to 4 on at least one of an upper arm and a lower arm.

Images