Optical circuit

Abstract

An optical circuit (10) includes optical waveguides (14, 16) that are provided with: respective first optical waveguides (141, 161) having a first relative refractive index contrast; and respective second optical waveguides (142, 162) monolithically integrated with the first optical waveguides (141, 161) and having a second relative refractive index contrast lower than the first relative refractive index contrast. An interference region (12) in the optical waveguides (14, 16) where the propagation modes of the optical waveguides (14, 16) interfere with other propagation modes is formed from the second optical waveguides (142, 162). The optical waveguides (14, 16) excluding the interference region (12) are formed from the first optical waveguides (141, 161).

Description

optical circuit

[0001] The present disclosure relates to optical circuits, and more particularly to optical circuits in which optical waveguides are integrated.

[0002] In recent years, circuit elements that combine three primary color lights, R (Red), G (Green), and B (Blue), have been used in the fields of eyeglass-type terminals and projectors. Furthermore, as such a circuit element, an RGB coupler module using a quartz-based planar lightwave circuit (PLC) is disclosed, for example, in Non-Patent Document 1. PLCs create optical waveguides on a planar substrate by patterning and etching using photolithography or the like, and realize various functions by combining multiple basic optical circuits. These functions include, for example, directional couplers, multimode interferometers, and Mach-Zehnder interferometers. Optical waveguides that handle such visible light are typically made of silicon nitride (Si x N y ), and can be made using zirconium (Zr) doped SiO2 for the core.

[0003] Optical waveguides using silicon nitride as a core have recently been attracting attention as a platform for optical circuits that handle visible light. Such optical waveguides are described, for example, in Patent Document 1. The relative refractive index difference of a silicon nitride core optical waveguide is several tens of times that of a known Ge-doped SiO2 core optical waveguide. Therefore, silicon nitride optical waveguides have a high optical confinement strength and are advantageous for miniaturizing circuit area. Here, the relative refractive index difference Δ is expressed using the relative difference between the refractive index n1 of the core and the refractive index n2 of the cladding of the optical waveguide. Specifically, it is expressed by the following formula:

[0004] On the other hand, an optical waveguide using zirconia (Zr)-doped SiO2 as a core has the advantage that the change in effective refractive index due to input of visible light is smaller than that of silicon nitride (Non-Patent Documents 2 and 3).

[0005] WO2010 / 137661

[0006] A. Nakao, R. Morimoto, Y. Kato, Y. Kakinoki, K. Ogawa and T. Katsuyama, “Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays”, Optics Communications 330 pp.45-48 (2014). Y. Fujiwara, J. Sakamoto, T. Hashimoto and K. Watanabe, “Zr-doped Silica-based Planar Lightwave Circuits with High Resistance against Blue Light,” Proc. IDW '21, pp. 242-244 (2021). Y. Fujiwara, J. Sakamoto, S. Katayose, T. Hashimoto and K. Watanabe, “Silica-based planar lightwave circuits with high resistance against blue light for visible-light application,” 2022 Jpn. J. Appl. Phys. 61 SK1021 (2022).

[0007] However, it is known that the refractive index of an optical waveguide using a silicon nitride core is prone to change when light in the ultraviolet to visible wavelength range is input. Such changes in refractive index make it difficult to realize a long-term stable optical circuit for visible light. On the other hand, an optical waveguide using Zr-doped SiO2 for the core has a smaller relative refractive index difference than a silicon nitride core optical waveguide, making it disadvantageous for reducing the circuit area.

[0008] The present disclosure has been made in consideration of the above points, and aims to provide an optical circuit that has stable circuit characteristics for a long period of time even when light with wavelengths from the ultraviolet to visible range is input, and that is advantageous for miniaturization, by selecting materials that have different relative refractive index differences between the portion of the waveguide where refractive index fluctuations are a problem and the other portions.

[0009] In order to achieve the above object, an optical circuit according to one embodiment of the present disclosure includes an optical waveguide having a first optical waveguide having a first relative refractive index difference and a second optical waveguide having a second relative refractive index difference lower than the first relative refractive index difference, wherein an interference region in the optical waveguide where the propagation mode of the optical waveguide interferes with other propagation modes is constituted by the second optical waveguide, and the optical waveguide excluding the interference region is constituted by the first optical waveguide.

[0010] According to the above-described embodiment, by selecting materials with different relative refractive index differences between the portion of the waveguide where refractive index fluctuations are a problem and the other portions, it is possible to provide an optical circuit that has stable circuit characteristics for a long period of time even when light with wavelengths from the ultraviolet region to the visible region is input, and that is also advantageous for miniaturization.

[0011] 5(c) is a schematic top view illustrating the optical circuit of the present embodiment. (a), (b), and (c) are cross-sectional views of each portion of FIG. 1. (a) and (b) are cross-sectional views of each portion of FIG. 1. (a), (b), and (c) are diagrams illustrating the manufacturing process of the directional coupler of FIG. 1. (a), (b), and (c) are diagrams illustrating the manufacturing process of the directional coupler of FIG. 1, continuing from FIG. 4(c). (a), (b), and (c) are diagrams illustrating the manufacturing process of the directional coupler of FIG. 1, continuing from FIG. 5(c). A diagram illustrating the transmission spectra of the through port and cross port of the directional coupler formed by the optical waveguide of the present embodiment. A diagram illustrating the time dependence of the transmittance of the directional coupler of the optical circuit of the present embodiment. A diagram illustrating the relationship between the optical loss of the directional coupler of the present embodiment and the deviation amount from the design value of the core width. (a), (b), and (c) are cross-sectional views of each portion of the optical circuit of a modified example. (a) and (b) are cross-sectional views of each portion of the optical circuit of the modified example. A diagram illustrating an example of an optical circuit other than a multiplexer.

[0012] An embodiment of the present disclosure will be described below with reference to the drawings. The drawings of this embodiment are intended to explain the configuration, arrangement of each part, functions, actions, effects, and technical concepts of the present disclosure, and are not intended to limit the dimensions, shape, or specific configuration.

[0013] FIG. 1 is a schematic top view illustrating an optical circuit according to the present embodiment. The optical circuit 10 includes optical waveguides 14 and 16. In this embodiment, the optical waveguides 14 and 16 may be, for example, optical waveguides that multiplex G light and B light among optical waveguides that multiplex R, G, and B light. The optical waveguides 14 and 16 form a directional coupler as an optical circuit. The optical waveguide 14 includes an optical waveguide 141 having a core made of silicon nitride and a relative refractive index difference Δ1, and an optical waveguide 142 monolithically integrated with the optical waveguide 141 and having a core made of Zr-doped SiO2, which is SiO2 doped with Zr and has a relative refractive index difference Δ2 lower than Δ1. The optical waveguide 16 includes a waveguide formed of the same material as the optical waveguide 14, that is, an optical waveguide 161 having a core formed of silicon nitride and having a relative refractive index difference Δ1, and an optical waveguide 162 having a core formed of Zr-doped SiO2 and having a relative refractive index difference Δ2, which is monolithically integrated with the optical waveguide 161. Here, "monolithically integrated" means that the optical waveguides 141 and 142 are formed integrally on a single substrate, or that the optical waveguides 161 and 162 are formed integrally on a single substrate.

[0014] As described above, in this embodiment, the optical waveguides 141 and 161 are configured as silicon nitride core optical waveguides with silicon nitride cores. Furthermore, the optical waveguides 142 and 162 are configured as Zr-doped SiO core optical waveguides in which SiO is doped with Zr. Meanwhile, the cladding of the optical waveguides 141, 142, 161, and 162 is configured, for example, of undoped SiO or SiO doped with boron (B) and phosphorus (P). As a result, the relative refractive index difference Δ of the silicon nitride core optical waveguides 141 and 161 is 23%, and the relative refractive index difference Δ of the Zr-doped SiO core optical waveguides 142 and 162 is 1%.

[0015] The optical waveguides 14 and 16 constitute a multiplexer. That is, the optical waveguide 14 includes a multiplexing region 12 where the propagation mode of the optical waveguide 14 interferes with another propagation mode (the propagation mode of the optical waveguide 16). Similarly, the optical waveguide 14 includes a multiplexing region 12 where the propagation mode of the optical waveguide 16 interferes with another propagation mode (the propagation mode of the optical waveguide 14). A "propagation mode" is a condition for the existence of a standing wave with a node near the interface between the core and cladding of the optical waveguide. The propagation modes of the optical waveguides 14 and 16 interfere with each other in the multiplexing region 12, causing multiplexing.

[0016] The distance between the optical waveguides 142 and 162 that form the multiplexing region (gap width of the directional coupler) is 1 μm, and the length of the optical waveguides 142 and 162 in the waveguide direction (coupling length) is 250 μm.

[0017] The optical waveguide 14 has an input terminal INa and an output terminal OUTa. The optical waveguide 16 has an input terminal INb and an output terminal OUTb. Light a with wavelength λa is input from the input terminal INa. Light b with wavelength λb is input from the input terminal INb. Light a and light b are guided through the optical waveguides 141 and 161, respectively, and are partially multiplexed in the multiplexing region 12. The multiplexed light is output from the output terminal OUTb. The light that is not multiplexed is output from the output terminal OUTa. A path that inputs from the input terminal INa and outputs from the output terminal OUTa, and a path that inputs from the input terminal INb and outputs from the output terminal OUTb are called a through port, and a path that inputs from the input terminal INa and outputs from the output terminal OUTb, and a path that inputs from the input terminal INb and outputs from the output terminal OUTa are called a cross port.

[0018] At least one of the light a and light b input from the input terminals INa and INb to the optical circuit 10 has a wavelength of 200 nm or more and 700 nm or less. In this embodiment, the wavelength of the light a is 445 nm, and the wavelength of the light b is 560 nm.

[0019] The optical waveguide 141 having a silicon nitride core has a bent portion 145. In this embodiment, the radius of curvature r of the bent portion 145 is 100 μm. The radius of curvature is the radius r of an imaginary circle whose arc follows the bent portion 145, as shown in FIG.

[0020] In this embodiment, the areas other than the area where the propagation modes interfere are configured with optical waveguides 141, 161 having a higher relative refractive index difference. As a result, even if a bent portion 145 is present in the optical waveguide 141, the radius of curvature of this portion can be reduced, making it possible to miniaturize the circuit. On the other hand, in this embodiment, the multiplexing region 12 where the propagation modes interfere is configured with optical waveguides 142, 162 using a material with a lower relative refractive index difference. As a result, this embodiment makes it possible to select a material for the multiplexing region 12 by prioritizing suppression of deterioration of circuit characteristics over circuit miniaturization. As a result, an optical circuit can be obtained that achieves both miniaturization and long-term operational stability at a high level.

[0021] The cores of the optical waveguides 141, 142, 161, and 162 of this embodiment are not limited to these configurations. The core of the optical waveguide 141 may contain at least one of aluminum nitride and aluminum oxide in addition to silicon nitride. Furthermore, the core of the optical waveguide 16 may contain SiO2 doped with Zr, SiO2 doped with hafnium (HF), or even aluminum oxide.

[0022] Figures 2(a), 2(b), 2(c), 3(a), and 3(b) are all cross-sectional views illustrating the cross sections of the optical waveguides 14 and 16. The optical waveguides 14 and 16 have the same cross-sectional structure. Figure 2(a) is a cross-sectional view taken along arrows A and A in Figure 1, showing a cross section of a silicon nitride core optical waveguide. Figure 2(b) is a cross-sectional view taken along arrows B and B, showing the connection portion between a silicon nitride core optical waveguide and a Zr-doped SiO2 core optical waveguide. Figure 2(c) is a cross-sectional view taken along arrows C and C, showing a Zr-doped SiO2 core optical waveguide. Figure 3(a) is a cross-sectional view taken along arrows D and D. Figure 3(b) is a cross-sectional view taken along arrows E and E. 2(a) to 3(b), the optical waveguide 14 is constructed by laminating a lower cladding 24, a silicon nitride core 163, and an upper cladding 26 in this order on a substrate 22. The optical waveguide 16 is constructed by laminating a lower cladding 24, a Zr-doped SiO2 core 164, and an upper cladding 26 in this order on a substrate 22. The lower cladding is made of undoped SiO2, and the upper cladding is made of SiO2 doped with B and P.

[0023] 2B, the core of the optical waveguide 16 switches between a silicon nitride core 163 and a Zr-doped SiO core 164 before and after the multiplexing region 12. The silicon nitride core 163 is stacked at a higher position than the Zr-doped SiO core 164. The length (width) W2 of the Zr-doped SiO core 164 in the direction perpendicular to the waveguide direction is 1 μm, and the width W1 of the silicon nitride core 163 is 1 μm.

[0024] (Method of manufacturing optical waveguide) Figures 4(a) to 6(c) are diagrams for explaining the manufacturing process of the optical waveguides 14 and 16. Since the manufacturing methods of the optical waveguides 14 and 16 are similar, only the manufacturing process of the optical waveguide 16 will be illustrated and explained. This explanation will particularly explain the process of manufacturing the cross-sectional shape shown in Figure 2(b). Note that "up" in the following explanation refers to the direction of gravity.

[0025] As shown in FIG. 4A, in this embodiment, a substrate 22 is first prepared. Then, a lower cladding 24 is formed on the substrate 22 (FIG. 4B), and a silicon nitride core layer 461 is formed on the lower cladding 24 (FIG. 4C). Next, similar to known processes for fabricating silica-based optical waveguides, the silicon nitride core layer 461 is patterned by photolithography or electron beam lithography and dry etching. Specifically, a resist pattern 51 is formed on the silicon nitride core layer 461 (FIG. 5A), and the silicon nitride core 163 is fabricated by dry etching (FIG. 5B). During the dry etching to form the silicon nitride core 163, the lower cladding 24 is overetched by 0.9 μm to adjust the heights of the Zr-doped SiO core 164 and silicon nitride core 163 from the substrate 22, which will be formed later. Such height adjustment is performed to align the optical axis centers at the connection portion between the silicon nitride core 163 and the Zr-doped SiO2 core 164, which have different heights.

[0026] After forming the silicon nitride core 163, a Zr-doped SiO2 core layer 562 is formed on the silicon nitride core 163 (FIG. 6(a)). Next, a resist pattern 61 for forming the Zr-doped SiO2 core is formed, and the Zr-doped SiO2 core 164 is formed by dry etching (FIG. 6(b)). Next, the upper clad 26 is formed on the Zr-doped SiO2 core 164, completing the optical waveguides 14 and 16 (FIG. 6(c)). The silicon nitride core layer 461, the Zr-doped SiO2 core layer 562, the upper clad 26, and the lower clad 24 can be formed by chemical vapor deposition (CVD), sputtering, flame hydrolysis deposition (FHD), sol-gel, or the like.

[0027] In the optical waveguides 14 and 16 manufactured as described above, the lower cladding 24 has a height (thickness) of 15 μm, and the upper cladding 26 has a thickness of 20 μm. The Zr-doped SiO core layer 562 has a thickness of 2 μm, and the silicon nitride core layer 461 has a thickness of 0.2 μm. The doping amounts of B and P in the upper cladding 26 are adjusted to match the refractive index of the undoped SiO in the lower cladding 24.

[0028] (Circuit Characteristics) Next, the circuit characteristics of the optical circuit 10 of this embodiment will be described. FIG. 7 is a diagram showing the transmission spectra of the through port and cross port of a directional coupler formed by the optical waveguides 14 and 16. The horizontal axis of FIG. 7 represents the wavelength of the transmitted light, and the vertical axis represents the transmittance corresponding to the wavelength. Curve S in FIG. 7 represents the transmission spectrum of the through port, and curve C represents the transmission spectrum of the cross port. As shown in FIG. 7, the through port has a high transmittance at a wavelength of 560 nm, and the cross port has a high transmittance at a wavelength of 445 nm. This directional coupler achieves the operation of multiplexing two lights with wavelengths of 445 nm and 560 nm.

[0029] FIG. 8 compares the time dependence of the transmittance of the directional coupler of the optical circuit of this embodiment with that of a directional coupler composed only of silicon nitride core optical waveguides (hereinafter referred to as the "sample circuit"). The transmitted light shown in FIG. 8 is an example in which visible light (blue light) is input. The horizontal axis of FIG. 8 represents the input time of the blue light, and the vertical axis represents the optical loss. In the figure, SiN_C represents data on the crossport of the sample circuit, and SiN_S represents data on the throughport of the sample circuit. In the figure, Emb_C represents data on the crossport of the directional coupler of this embodiment, and Emb_S represents data on the throughport of the directional coupler of this embodiment. The optical loss indicated by Emb_C is for a wavelength of 445 nm, and the optical loss indicated by Emb_S is for a wavelength of 560 nm. The plot in FIG. 8 sets the optical loss at light input time 0 as 0. The data for SiN_C, SiN_S, Emb_C, and Emb_S were obtained by inputting blue light with a wavelength of 445 nm and an optical intensity of 90 mW into a directional coupler to degrade the optical waveguide, and then measuring the transmission spectra of the same circuit at wavelengths of 445 nm and 560 nm at regular intervals.

[0030] As is clear from Figure 8, the optical loss of the directional coupler of this embodiment remains close to zero even after 200 hours, whereas the optical loss of the directional coupler of the sample circuit increases over time. This is thought to be due to the input of high-power visible light changing the effective refractive index of the silicon nitride optical waveguide in the multiplexing region, resulting in degradation of circuit characteristics. This demonstrates that this embodiment provides a stable optical circuit over the long term, even when high-power visible light is input. The directional coupler used as the sample circuit, which is composed of a silicon nitride core waveguide, has a core width of 0.4 μm, a core thickness of 0.2 μm, a gap width of 0.2 μm, a coupling length of 80 μm, and a relative refractive index difference of Δ23%.

[0031] 1, the present embodiment shows that the optical waveguide 141 has a bent portion 145, and by using a silicon nitride core optical waveguide as the optical waveguide 141, the bend radius is 100 μm. If the optical waveguide 141 were a Zr-doped SiO2 core optical waveguide, the bend radius would be 1 mm. This clearly shows that the present embodiment can make the optical circuit more compact than known configurations.

[0032] In addition to the above effects, this embodiment can also improve the dimensional tolerance of the directional coupler. Figure 9 shows the relationship between the optical loss at a wavelength of 445 nm at the cross port of the directional coupler of this embodiment and the deviation of the core width from the design value. The horizontal axis of Figure 9 represents the deviation of the core width, and the vertical axis represents the optical loss. The dashed curve in the figure represents the optical loss of a sample circuit of a directional coupler constructed solely with a silicon nitride core optical waveguide. The solid curve represents the optical loss of the directional coupler of this embodiment. The deviation is set to zero when the deviation is zero. Figure 9 clearly shows that the directional coupler of this embodiment is less susceptible to an increase in optical loss relative to the deviation of the core width than the sample circuit. This is because silicon nitride has a higher refractive index than Zr-doped SiO2, resulting in a larger change in the effective refractive index of the optical waveguide relative to a change in core width. This embodiment's improved tolerance to deviations in core width enables optical circuits to be realized with a high yield.

[0033] (Modification) This embodiment is not limited to the configuration described above, and may further include an undoped SiO layer between the optical waveguides 14, 16 and the upper cladding. That is, the optical waveguides 14, 16 of this embodiment have a Zr-doped SiO core waveguide in which refractive index changes are unlikely to occur when visible light is input, thereby achieving the effects of stabilizing circuit characteristics and improving tolerance. Furthermore, in this embodiment, the silicon nitride core and the Zr-doped SiO core may be further covered with an undoped SiO layer.

[0034] FIGS. 10(a) to 11(b) are cross-sectional views illustrating the modified example. The top view of the modified example is omitted because it is similar to FIG. 1. However, FIG. 10(a) is a cross-sectional view taken along arrows A-A in FIG. 1, FIG. 10(b) is a cross-sectional view taken along arrows B-B, and FIG. 10(c) is a cross-sectional view taken along arrows C-C in FIG. 1. FIG. 11(a) is a cross-sectional view taken along arrows D-D in FIG. 1, and FIG. 11(b) is a cross-sectional view taken along arrows E-E in FIG. 1. As shown in FIGS. 10(a) to 11(b), the optical circuit of the modified example includes a non-doped SiO layer 101 between the silicon nitride core 163, the Zr-doped SiO core 164, and the upper cladding 26. This modified example suppresses changes in the refractive index of the upper cladding 26 due to propagation light leaking from the silicon nitride core 163 and the Zr-doped SiO core 164.

[0035] The above-described embodiment has been described using an example of a multiplexer configured as a directional coupler. However, the configuration of the multiplexer is not limited thereto, and may be, for example, a multi-mode interferometer (MMI), an arrayed waveguide grating (AWG) optical multiplexer / demultiplexer, a Mach-Zehnder interferometer (MZI), etc. This embodiment is effective for any optical circuit that uses interference between two or more waveguide modes.

[0036] Furthermore, the optical circuit of this embodiment is not limited to a multiplexer, as long as one propagation mode interferes with another propagation mode. The optical circuit of this embodiment can be used in an optical demultiplexer, an optical modulator, an optical switch, etc., and can be similarly effective. When used as an optical modulator or an optical switch, a heater may be further installed.

[0037] 12 shows an optical modulator using a Mach-Zehnder interferometer, which is another example of an optical circuit. Interference of propagation modes occurs in a branching section 121 and a multiplexing section 122 of the Mach-Zehnder interferometer, and this embodiment can be applied to the branching section 121 and the multiplexing section 122.

[0038] REFERENCE SIGNS LIST 10 Optical circuit 12 Wave-combining region 14, 16, 141, 142, 161, 162 Optical waveguide 22 Substrate 24 Lower clad 26 Upper clad 51, 61 Resist pattern 101 Non-doped SiO2 layer 121 Wave-demultiplexing portion 122 Wave-combining portion 145 Bent portion 163 Silicon nitride core 164 Zr-doped SiO2 core

Claims

1. An optical circuit comprising an optical waveguide comprising a first optical waveguide having a first relative refractive index difference and a second optical waveguide having a second relative refractive index difference lower than the first relative refractive index difference, wherein an interference region in the optical waveguide where the propagation mode of the optical waveguide interferes with another propagation mode is constituted by the second optical waveguide, and the optical waveguide excluding the interference region is constituted by the first optical waveguide.

2. The optical circuit of claim 1, wherein the core of said first optical waveguide comprises at least one of silicon nitride, aluminum nitride, and aluminum oxide.

3. The optical circuit of claim 1, wherein the core of said second optical waveguide comprises at least one of zirconia, hafnium-doped silicon dioxide, and aluminum oxide.

4. The optical circuit according to claim 1, wherein the second optical waveguide has a core made of undoped silicon dioxide and a cladding made of silicon dioxide containing at least one of fluorine and boron.

5. The optical circuit of claim 1, further comprising an upper cladding above said first optical waveguide and said second optical waveguide, and including an undoped silicon dioxide layer between said first optical waveguide, said second optical waveguide and said upper cladding.

6. The optical circuit according to claim 1, wherein the interference region constitutes a multiplexer.

7. The optical circuit according to claim 1, wherein light having a wavelength of 200 nm or more and 700 nm or less is input to at least one of said first optical waveguide and said second optical waveguide.

8. The optical circuit according to claim 1, wherein said first optical waveguide and said second optical waveguide are monolithically formed.

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