Phase shifter and phase shifter array

Abstract

The present disclosure provides a structure for improving heat insulation performance in a thermo-optical phase shifter.　Specifically, provided is a thermo-optical phase shifter (300) formed on a substrate (201), the phase shifter comprising: an optical waveguide (301) that has a core (303) and cladding (305, 307); a heat-insulating grooves (203) that are formed along both lateral surfaces of the optical waveguide and that reach the lower surface of the substrate or reach a portion between the upper surface and the lower surface of the substrate; and a heating mechanism (205) that is formed in a part of the optical waveguide.

Description

Phase shifter and phase shifter array

[0001] The present disclosure relates to phase shifters, and more particularly to thermo-optic phase shifters.

[0002] Waveguide-type optical switches are key devices in optical communication systems, and are widely used in optical communication networks due to their ability to realize flexible optical communication networks. A typical waveguide-type optical switch configuration is one that combines a Mach-Zehnder interferometer (hereinafter referred to as MZI) and a phase shifter, and waveguide-type optical switches with such configurations are being put to practical use in optical communication networks.

[0003] Thermo-optic phase shifters, which utilize temperature-dependent changes in refractive index, are widely used as phase shifters for optical waveguides. However, it is desirable to reduce the manufacturing cost of thermo-optic phase shifters and improve their thermal insulation performance.

[0004] Japanese Patent Application Laid-Open No. 2021-124646

[0005] 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-Based Planar Lightwave Circuit Technology,” J. Lightwave Technol. 19, 371- (2001) A. Sugita, K. Jinguji, N. Takato, K. Katoh, and M. Kawachi, “Bridge suspended silica-waveguide thermo-optic phase shifter and its application to Mach-Zehnder type optical switch”, Trans. IEICE, vol. E73, no. 1, pp. 105 - 109, 1990.

[0006] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a phase shifter that has improved thermal insulation performance and reduced manufacturing costs compared to conventional techniques.

[0007] According to one embodiment of the present disclosure, there is provided a thermo-optical phase shifter formed on a substrate, the thermo-optical phase shifter comprising: an optical waveguide having a core and a cladding; an insulating groove formed along both sides of the optical waveguide, the insulating groove reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a portion of the optical waveguide.

[0008] Furthermore, according to one embodiment of the present disclosure, there is provided a thermo-optic phase shifter formed on a substrate, the thermo-optic phase shifter comprising: an optical waveguide having a core and a cladding; an insulating groove formed along both side surfaces of the optical waveguide, the insulating groove reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a portion of the optical waveguide, wherein the portion below the portion of the optical waveguide where the heating mechanism of the substrate is formed is thinner than other portions.

[0009] Furthermore, according to one embodiment of the present disclosure, there is provided a thermo-optical phase shifter formed on a substrate, the thermo-optical phase shifter comprising: an optical waveguide having a core and a cladding; a heat insulating groove formed along both side surfaces of the optical waveguide, the heat insulating groove reaching the bottom surface of the substrate or between the top and bottom surfaces of the substrate; and a heating mechanism formed in a portion of the optical waveguide, and further comprising a heat dissipation groove formed in a portion of the back surface of the substrate.

[0010] Furthermore, according to one embodiment of the present disclosure, there is provided a thermo-optic phase shifter formed on a substrate, the thermo-optic phase shifter comprising: an optical waveguide having a core and a cladding; a thermal insulating groove formed along both side surfaces of the optical waveguide, the thermal insulating groove reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a part of the optical waveguide, wherein the thermal insulating groove formed along both side surfaces of the optical waveguide reaches between the top and bottom surfaces of the substrate, and the thermo-optic phase shifter is formed so as to satisfy the relationship x1 + 2y1 > y2, where x1 is the width of the thermal insulating groove formed between the optical waveguide and another optical waveguide, y1 is the distance from the bottom end surface of the core to the bottom surface of the thermal insulating groove, and y2 is the distance from the bottom end surface of the core to the back surface of the substrate.

[0011] Furthermore, according to one embodiment of the present disclosure, there is provided a thermo-optic phase shifter formed on a substrate, the thermo-optic phase shifter comprising: an optical waveguide having a core and a cladding; a heat insulating groove formed along both side surfaces of the optical waveguide, the heat insulating groove reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a part of the optical waveguide, wherein the heat dissipation groove is filled with a material having a higher thermal conductivity than the substrate.

[0012] Furthermore, according to one embodiment of the present disclosure, there is provided a thermo-optical phase shifter formed on a substrate, the thermo-optical phase shifter comprising: an optical waveguide having a core and a cladding; a heat insulating groove formed along both side surfaces of the optical waveguide, the heat insulating groove reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a part of the optical waveguide, wherein the cladding and the substrate are the same quartz substrate.

[0013] Furthermore, according to one embodiment of the present disclosure, there is provided a thermo-optic phase shifter formed on a substrate, the thermo-optic phase shifter comprising: an optical waveguide having a core and a cladding; a heat insulating groove formed along both side surfaces of the optical waveguide, the heat insulating groove reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a part of the optical waveguide, wherein the cladding of the optical waveguide is a quartz substrate, and the substrate is made of a material having a lower thermal conductivity than Si.

[0014] Also provided is a phase shifter array comprising a plurality of thermo-optic phase shifters according to the present disclosure.

[0015] In order to suppress heat conduction to adjacent optical waveguides, a thermo-optic phase shifter is provided that has a heat insulating groove for heat insulation and / or a heat dissipation groove for absorbing and dissipating heat, thereby reducing manufacturing costs and improving heat insulating performance.

[0016] Fig. 1 is a top view conceptually showing the structure of a waveguide-type optical switch combining an MZI and a phase shifter; Fig. 2 is a cross-sectional view of a waveguide-type optical switch combining an MZI and a phase shifter according to the prior art; Fig. 3 is a cross-sectional view of a thermo-optic phase shifter according to one embodiment; Fig. 4 is a cross-sectional view of a thermo-optic phase shifter according to another embodiment; Fig. 5 is a cross-sectional view of a thermo-optic phase shifter according to another embodiment; Fig. 6 is a top view showing an array of a plurality of thermo-optic phase shifters.

[0017] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only 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 one embodiment of the present disclosure.

[0018] Before describing the details of various embodiments of the present disclosure, the structure of a waveguide-type optical switch that combines an MZI and a phase shifter will be described, and then a conventional optical switch configuration will be described.

[0019] (Structure of Waveguide-Type Optical Switch) Fig. 1 is a top view conceptually showing the structure of a waveguide-type optical switch 100 that combines an MZI and a phase shifter. 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 a 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 regions through which optical signals propagate (input waveguides 101 a, b, output waveguides 102 a, b, upper arm 104, and lower arm 105) are embedded in the cladding. For convenience, of the two waveguides connecting coupler 103 a and coupler 103 b, the upper side in the XY plane of FIG. 1 is called upper arm 104, and the lower side is called lower arm 105.

[0020] The waveguide-type optical switch 100 having such a configuration can switch the output waveguide by controlling the phase difference between 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 waveguide 101a is output from the output 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 waveguide 101a is output from the output waveguide 102a (through state).

[0021] Generally, materials used for optical waveguides can be silicon oxide (SiOx) or silicon (Si). When SiOx is used for both the core and the 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 within 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.

[0022] 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 a portion of the core, thereby changing the refractive index of the heated area, thereby changing the optical path length and shifting the phase of the optical signal propagating through the core. This phenomenon in which the refractive index of a material changes depending on 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 shift the phase of an optical signal).

[0023] As mentioned above, thermo-optic phase shifters utilize the temperature-dependent change in refractive index, which poses a challenge in terms of power consumption due to heating, such as with a heater. In particular, SiOx has a small thermo-optic constant, so large temperature changes are necessary. Therefore, it is important to efficiently confine the heat generated by a heating mechanism, such as a heater. In addition, it is also important to suppress thermal crosstalk to other waveguides. Taking Figure 1 as an example, if the heat generated by the phase shifter 106 heats not only the lower arm 105 but also the upper arm 104, the refractive index of both waveguides will change, resulting in a smaller change in the optical path length difference between the upper arm 104 and the lower arm 105.

[0024] (Conventional Optical Switch Configuration) Conventionally, to solve the above-mentioned problems, heat flow management has been performed using a Si substrate 201 with high thermal conductivity and thermal insulating grooves 203a, b (hereinafter referred to as 203) as shown in FIG. 2. FIG. 2 shows an optical switch configuration using an MZI with thermal insulating grooves, and is a cross-sectional view of the optical switch shown in FIG. 1 taken along the II-II cross-section line. The heat supplied by the heating mechanism 205, such as a heater, is prevented from spreading by the thermal insulating groove 203, allowing efficient heating of only the lower arm 105. Furthermore, because the Si substrate 201 has a relatively high thermal conductivity, heat reaching the Si substrate spreads throughout the Si substrate, making it possible to prevent heat from flowing around to the upper arm 104 (thermal crosstalk). Furthermore, because the cladding 207a, b, which is closer to the substrate than the core, is also formed of SiOx, which has low thermal conductivity, thickening the undercladding can weaken heat conduction to the substrate, making it possible to maintain a high core temperature even when the amount of heat supplied by the heating mechanism, such as a heater, is the same. This reduces power consumption. However, in order to thicken the undercladding, the undercladding film formation process takes a long time, and the manufacturing cost increases.

[0025] Thermo-optic phase shifters according to various embodiments of the present disclosure will be described below: The thermo-optic phase shifter according to the present disclosure can be combined with an MZI to form the waveguide-type optical switch 100 shown in FIG.

[0026] 3 shows a cross section (cross section taken along line II-II in FIG. 1 ) of an optical switch including a thermo-optic phase shifter 300 according to an embodiment of the present disclosure. The thermo-optic phase shifter 300 includes an optical waveguide 301 having a core 303, an overclad 305, and an underclad 307 (as will be described later, in this embodiment, the substrate 201 is used as the underclad 307), a heating mechanism 205 that heats the optical waveguide 301 to vary the refractive index, and heat insulating grooves 203 formed on both side surfaces of the optical waveguide 301.

[0027] The optical waveguide 301 is formed on the substrate 201 and extends in the propagation direction of the optical signal (for example, the Y direction in FIG. 3 ). The optical waveguide 301 also includes claddings 305 and 307, which have a lower refractive index than the core, surrounding a core 303, which has a relatively high refractive index. This allows the optical signal to be confined within the core 303 and propagate therethrough.

[0028] The core 303 can be formed of, for example, SiO2, as long as the refractive index of the core is higher than the refractive index of the cladding surrounding the core. For example, a silicon photonics waveguide with a Si core can have a similar configuration. Needless to say, a similar configuration can also be used with an optical waveguide based on a compound semiconductor such as InP.

[0029] The claddings 305 and 307 are formed around the core 303. The cladding 305 may be formed to sandwich the core 303 or to surround it. The refractive index of the cladding may be lower than the refractive index of the core.

[0030] The heating mechanism 205 is provided near the optical waveguide 301 and varies the refractive index of the optical waveguide. In Fig. 3, the heating mechanism 205 is arranged in contact with the upper surface of the optical waveguide 301, but it may also be arranged on the side surface of the optical waveguide 301 or so as to surround the optical waveguide 301. The heating mechanism 205 may be arranged in a partial range of the optical waveguide 301 along the extension direction of the optical waveguide 301. Furthermore, the heating mechanism 205 may be installed in direct contact with the optical waveguide 301, or a non-contact method (for example, heating by laser irradiation) may be used.

[0031] In this embodiment, the substrate 201 is used as the underclad 307 of the optical waveguide 301, and the core 303 and the overclad 305 are formed on the substrate 201. A quartz substrate is used for the substrate 201. Using a quartz substrate makes it easy to thicken the underclad 307, enabling stronger heat confinement. In addition, the optical waveguide chip to be manufactured is generally mounted on a mount 309, and it is this mount 309 that dissipates heat.

[0032] The heat insulating grooves 203 are formed on both side surfaces of the optical waveguide 301. The heat insulating grooves 203 do not need to be formed over the entire area of ​​the optical waveguide 301, but only need to be partially formed in at least an area where a heating mechanism 205 for heating the core 303 is installed. In this embodiment, the heat insulating grooves 203 are provided so as to penetrate all the way to the back surface of the substrate 201 used as the underclad 307.

[0033] By forming heat insulating grooves 203 on both sides of the optical waveguide 301 to be heated, thermal interference with other adjacent optical waveguides 311 can be suppressed, thermal crosstalk can be suppressed, and the heating of the core 303 to be heated can be made more efficient, resulting in lower power consumption. Note that the thermo-optic phase shifter shown in this embodiment is effective as a standalone device and is not necessarily limited to an optical switch using an MZI.

[0034] (Embodiment 2) In the above-described embodiment 1, a structure in which the thermal insulating groove 203 penetrates to the back surface of the substrate 201 is illustrated. However, if the substrate 201 is thick, a very deep groove must be formed to allow the thermal insulating groove 203 to penetrate through the substrate 201. Furthermore, in order to efficiently heat the optical waveguide 301 using the thermo-optic phase shifter 300, it is effective to reduce the volume of the object to be heated. Therefore, the thermal insulating groove 203 is generally placed close to the core 303 of the optical waveguide 301. Therefore, high processing accuracy is required to form the thermal insulating groove 203. Increasing the depth of the thermal insulating groove 203 leads to a deterioration in this processing accuracy. Furthermore, since a process with high processing accuracy must be adopted, forming a deep groove requires a long processing time. This problem can be solved by thinning the substrate. However, thinning the substrate also creates another problem: the strength of the substrate is reduced.

[0035] FIG. 4 shows a cross section (cross section taken along line II-II in FIG. 1 ) of an optical switch including a thermo-optic phase shifter 300 according to an embodiment of the present disclosure. This embodiment has a structure with adiabatic grooves 203 similar to the first embodiment, but the back surface of the substrate 201 is etched to make the portions below the optical waveguides 301 and 311 thinner than the other portions. Unlike conventional etching, etching from the back surface of the substrate eliminates the need for high processing precision and / or long process times. Furthermore, this structure allows the substrate 201 of the thermo-optic phase shifter 300 to be thin while maintaining the strength of the entire substrate 201.

[0036] The structure of this embodiment has the advantage that there is no heat transfer path between the optical waveguide 301 on which the heating mechanism 205 is installed and the adjacent optical waveguide 311, allowing the use of a material with relatively high thermal conductivity, such as silicon, instead of a quartz substrate with low thermal conductivity. However, since changing the material to be processed may require changes in the etching method and type of gas used, if the cladding and substrate have different compositions, it may be necessary to change the processing method during the formation of the insulating groove. For this reason, it is desirable for the cladding and substrate to have the same or similar composition, and it is ideal to use a quartz substrate made of the same SiOx as the cladding.

[0037] In this structure, in particular, the optical waveguides (upper and lower arms of the MZI) of the thermo-optic phase shifter 300 are suspended in the air, which allows the surroundings to be surrounded by air with extremely low thermal conductivity. Non-Patent Document 2 reports that such a structure (also called a suspended bridge shape) efficiently confines the heat supplied from the heating mechanism 205 and has a high effect of low power consumption.

[0038] The main difference between this embodiment and the thermo-optic phase shifter disclosed in Non-Patent Document 2 is the etching process and the accompanying structure. In the conventional thermo-optic phase shifter disclosed in Non-Patent Document 2, the grooves are formed by accessing from the surface of the optical switch where the optical waveguides are formed, whereas in this embodiment, the grooves are formed by accessing from the back surface of the substrate 201. Also, in conventional thermo-optic phase shifters that are accessed from the surface, the etching method leaves a substrate that straddles each optical waveguide at the bottom of the optical switch, but in this embodiment, there is no substrate that straddles the optical waveguides below the optical waveguides.

[0039] The configuration of this embodiment is effective in reducing power consumption and suppressing thermal crosstalk in an optical switch using an MZI, as in embodiment 1. However, as in embodiment 1, the effect is also achieved by using a thermo-optic phase shifter alone, and is not necessarily limited to an optical switch using an MZI.

[0040] (Embodiment 3) In the above-described embodiment 2, the back surface of the substrate 201 is etched to form the insulating groove 203 penetrating the substrate 201, separating the optical waveguide 301 in which the heating mechanism 205 is formed from the substrate 201 of the adjacent optical waveguide 311 (used as the underclad 307). However, when a substrate with low thermal conductivity such as a quartz substrate is used, the amount of heat transferred through the substrate is reduced, so even if both optical waveguides 301 and 311 are connected, the effect is still achieved. This structure can shorten the etching time and improve the strength of the device formed, such as an optical switch.

[0041] The configuration of this embodiment is shown in Figure 5. Compared to the configuration of embodiment 2 (shown in Figure 4), in this embodiment, the heat insulating groove 203 does not penetrate the substrate 201, and a heat conduction path 501 exists between the optical waveguides via the substrate 201. In addition, a heat dissipation groove 503 is provided on the back surface of the substrate 201 to dissipate heat that is transferred to other optical waveguides 311 via the path 501. Furthermore, by filling the heat dissipation groove 503 with a material with high thermal conductivity, such as a thermally conductive paste, and attaching it to a mount 309 made of a metal member, for example, the heat can be dissipated before it interferes with adjacent waveguides.

[0042] To achieve this, it is necessary to design the path that heat takes to bypass the heat insulating groove and reach the adjacent waveguide longer than the path from the heating mechanism 205 to the back surface after etching. Specifically, if the width of the heat insulating groove between adjacent optical waveguides is x1, the distance from the bottom end face of the core to the bottom of the groove is y1, and the distance from the bottom end face of the core to the area filled with a material with high thermal conductivity such as thermally conductive paste, i.e., the back surface of the substrate, is y2, it is desirable to satisfy the relationship of the following formula 1.

[0043]

[0044] The heat dissipation groove 503 functions well if it is formed locally in a portion of the extension direction of the optical waveguides 301 and 311. As an example, when a heating mechanism 205 such as a heater is used, the heat dissipation groove 503 may have a length equal to the length of the heating mechanism 205 (Y direction in FIG. 5 ).

[0045] (Embodiment 4) In the embodiments of the present disclosure, particularly in embodiments 2 and 3, it is necessary to etch the back surface of the substrate 201. This etching range can be set roughly if it is made wider, allowing for a larger tolerance in processing accuracy. As a result, a process with a high processing speed can be adopted. For example, as in the etching method proposed by us in Patent Document 1, an etching method using a nozzle 4-4 that performs local etching and moves and sweeps, or processing using a dicing blade with an adjusted excavation depth, is possible. In these exemplary methods, the wafer processing area is a straight line or line segment with a certain width. Therefore, it is desirable that the thermo-optic phase shifter to be processed is arranged on a straight line within the wafer.

[0046] An example of the arrangement of phase shifters in an optical switch 600 having multiple channels is shown in Figure 6. By arranging the phase shifters in a linear region 601 in this way, it is possible to improve the processing speed and minimize the influence on surrounding waveguides, etc.

[0047] In the first to fourth embodiments, the substrate is shown as quartz, but any material with a thermal conductivity similar to that of the cladding or lower than that of the common substrate, Si, will be effective. For example, the present invention will be effective even when a sapphire substrate is used.

[0048] Although a SiO2 core waveguide has been described as an example, a similar configuration can be achieved with a Si core silicon photonics waveguide. That is, a similar configuration can be achieved by using a substrate with a lower refractive index than the core material as an undercladding. It goes without saying that a similar configuration can also be achieved with a waveguide made of a compound semiconductor such as InP.

[0049] Additional Considerations The foregoing description of embodiments of the present invention has been presented for purposes of illustration and is not intended to be exhaustive or to be limited to the precise form disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0050] Finally, the language used herein has been selected primarily for readability and instructional purposes, and may not have been selected to delineate or limit the subject matter of the invention. Accordingly, it is intended that the scope of the invention be limited not by this detailed description, but rather by the appended claims. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the claims.

[0051] REFERENCE SIGNS LIST 100 Waveguide-type optical switch 101a, b Input waveguide 102a, b Output waveguide 103a, b Coupler 104 Upper arm 105 Lower arm 106 Phase shifter 201 Substrate 203a, b Heat insulating groove 205 Heating mechanism 207 Cladding 300 Thermo-optic phase shifter 301, 311 Optical waveguide 303 Core 305 Overclad 307 Underclad 309 Mount 503 Heat dissipation groove 600 Optical switch 601 Arrangement area of ​​phase shifter

Claims

1. A thermo-optic phase shifter formed on a substrate, comprising: an optical waveguide having a core and a cladding; adiabatic grooves formed along both sides of the optical waveguide, the adiabatic grooves reaching the bottom surface of the substrate or reaching between the top and bottom surfaces of the substrate; and a heating mechanism formed in a part of the optical waveguide.

2. The thermo-optic phase shifter according to claim 1, wherein a portion of said substrate below the portion of said optical waveguide on which said heating mechanism is formed is thinner than other portions.

3. The thermo-optic phase shifter according to claim 1, further comprising a heat dissipation groove formed in a portion of the rear surface of said substrate.

4. A thermo-optic phase shifter as described in claim 3, wherein the heat insulating grooves formed along both side surfaces of the optical waveguide reach between the upper and lower surfaces of the substrate, and are formed so as to satisfy the relationship x1 + 2y1 > y2, where x1 is the width of the heat insulating groove formed between the optical waveguide and another optical waveguide, y1 is the distance from the lower end surface of the core to the lower surface of the heat insulating groove, and y2 is the distance from the lower end surface of the core to the back surface of the substrate.

5. A thermo-optic phase shifter as claimed in claim 3, wherein said heat dissipation grooves are filled with a material having a higher thermal conductivity than said substrate.

6. The thermo-optic phase shifter of claim 1, wherein said cladding and said substrate are the same quartz substrate.

7. A thermo-optic phase shifter according to claim 2, wherein the cladding of the optical waveguide is a quartz substrate, the substrate being made of a material having a lower thermal conductivity than Si.

8. A phase shifter array comprising a plurality of thermo-optic phase shifters according to any one of claims 1 to 7.

Images