Phase shifter, phase shift method, and mach-zehnder optical interferometer

Abstract

A phase shifter (100) according to the present invention is provided with: an optical waveguide (101) formed of a core (105) and a cladding (106) surrounding the core (105); a substrate (102) on which the optical waveguide (101) is mounted; a heating means (103) for heating the core (105); and an ultraviolet irradiation means (104) for irradiating the core (105) with ultraviolet rays U. The core (105) contains silicon nitride as the main component, and the density of silicon dangling bonds of the silicon nitride is 1018-1020 cm-3 as expressed in spin density.<sp / >

Description

Phase shifter, phase shifting method, and Mach-Zehnder optical interferometer

[0001] The present invention relates to an optical waveguide phase adjustment mechanism used in optical integrated circuits, and more specifically, to a phase shifter for optical waveguides, a phase shifting method, and a Mach-Zehnder type optical interferometer, which are non-volatile in their state after phase adjustment and capable of phase readjustment. This application claims priority based on Japanese Patent Application No. 2024-161420, filed in Japan on September 18, 2024, the contents of which are incorporated herein by reference.

[0002] With advances in optical integrated circuit technology, large-scale optical circuits such as optical switch matrices for optical communication (Non-Patent Document 1) and optical neural networks (Non-Patent Document 2) are being realized. These optical circuits integrate numerous optical interferometers. The operating point of an optical interferometer is set by applying a phase shift to the optical waveguide that constitutes it.

[0003] The phase shift in an optical interferometer is caused by a change in the refractive index of the core material of the optical waveguide. In conventional optical interferometers using integrated optical circuits, this change in refractive index is generated by the temperature rise when the optical waveguide is heated by an electric heater placed along the optical waveguide, or by the carrier plasma effect when current is injected into an optical waveguide made of semiconductor material.

[0004] In large-scale optical integrated circuits, the phase shifter must be constantly energized to maintain a steady-state phase shift. Therefore, in optical integrated circuits containing many optical interferometers, power consumption becomes very high. From the perspective of reducing power consumption, there is a need for a non-volatile phase shifter that can maintain refractive index changes without energization and further allows for the resetting of that refractive index change.

[0005] Currently, MOS capacitor charge trap type and phase change material loaded type have been proposed as non-volatile phase shifters, but these have problems such as high optical propagation loss and the inability to construct large-scale integrated optical circuits due to material limitations.

[0006] For example, in a MOS capacitor using a silicon waveguide, the metal electrode or polysilicon electrode, which has high light absorption, is close to the optical waveguide, and it is presumed that the optical propagation loss will be a very large value of several tens of dB / cm (Non-Patent Literature 3). Therefore, MOS capacitors using silicon waveguides are not suitable for large-scale integration. In addition, a MOS capacitor structure in which III-V compound semiconductors are integrated on a silicon waveguide is known (Non-Patent Literature 4), but because CMOS process technology cannot be applied, integrating thousands to tens of thousands of devices is extremely difficult. Furthermore, even when using phase change materials such as BTO, the optical propagation loss is very large, about 5 dB / cm, and like III-V compound semiconductors, CMOS process technology cannot be applied, making it unsuitable for large-scale integration (Non-Patent Literature 5).

[0007] K. Suzuki et al., Journal of Lightwave Technology 37, 116-122 (2019).G. Cong et al., Nature Communications 13, 3261 (2022).JF. Song et al., Scientific Reports 6, 22616 (2016).S. Cheung et al., Laser Photonics Review 2024, 2400001 (2024).J. Geler-Kremer et al., Nature Photonics 16, 491-497 (2022).

[0008] The present invention has been made in view of the above circumstances, and aims to provide a phase shifter, a phase shifting method, and a Mach-Zehnder type optical interferometer to which these are applied, which can be manufactured by applying a CMOS process, have low optical propagation loss, can maintain refractive index changes without applying power, and allow the refractive index changes to be reset.

[0009] To solve the above problems, the present invention employs the following means.

[0010] (1) A phase shifter according to one aspect of the present invention comprises an optical waveguide composed of a core and a cladding surrounding the core, a substrate on which the optical waveguide is mounted, a heating means for heating the core, and an ultraviolet irradiation means for irradiating the core with ultraviolet light, wherein the core mainly contains silicon nitride, and the density of silicon dangling bonds of the silicon nitride is 10 in terms of spin density calculated by ESR measurement. １８ cm －３ The above 10 ２０ cm －３ The following applies:

[0011] (2) In the phase shifter described in (1) above, the spin density corresponds to the electrically neutral silicon dangling bond K ０ Spin density is preferred.

[0012] (3) In the phase shifter described in either (1) or (2) above, the ultraviolet irradiation means may be located outside the cladding on the side facing one surface of the core, and the heating means may be located inside the cladding on the side facing the other surface of the core.

[0013] (4) In the phase shifter described in either (1) or (2) above, the ultraviolet irradiation means may be located outside the cladding on one side of the core, and the heating means may be located outside the cladding in a region that does not obstruct the ultraviolet light irradiated onto the core.

[0014] (5) A phase shift method according to one aspect of the present invention is a phase shift method using a phase shifter described in any one of (1) to (4) above, comprising at least one of a heating step of heating the core and an ultraviolet irradiation step of irradiating the core with ultraviolet light, wherein the energy of the light propagated to the core is 0.64 eV or more and 50% or less of the band gap of the core.

[0015] (6) In the phase shift method described in (5) above, it is preferable that the energy of the ultraviolet light be 85% or more of the band gap of the core, and that the heating temperature by the heating means be 150°C or more and 400°C or less.

[0016] (7) A Mach-Zehnder optical interferometer according to one aspect of the present invention comprises a first optical waveguide and a second optical waveguide equipped with a phase shifter as described in any one of (1) to (4) above, and an optical waveguide coupler that couples the first optical waveguide and the second optical waveguide.

[0017] According to the present invention, it is possible to provide a phase shifter, a phase shifting method, and a Mach-Zehnder type optical interferometer to which these are applied, which can be manufactured by applying a CMOS process, have low optical propagation loss, can maintain refractive index changes without applying power, and allow the refractive index changes to be reset.

[0018] This is a cross-sectional view of a phase shifter according to the first embodiment of the present invention. This is another cross-sectional view of the phase shifter according to the same embodiment. This is a diagram showing an example configuration of a Mach-Zehnder optical interferometer equipped with the phase shifter of the same embodiment. This is a top view of a phase shifter according to the second embodiment of the present invention. This is a cross-sectional view of a phase shifter according to the same embodiment. This is a graph showing the change in refractive index of the silicon nitride film used in the waveguide core constituting the phase shifter due to heating and ultraviolet irradiation. This is a graph showing the change in refractive index of the silicon nitride film used in the waveguide core constituting the phase shifter due to spin density. This is a graph showing the relationship between the change in refractive index of the silicon nitride film used in the waveguide core constituting the phase shifter due to heating and ultraviolet irradiation and the wavelength of light propagating through the silicon nitride film (400 to 420 nm). This is a graph showing the relationship between the same change in refractive index and the wavelength of light propagating through the silicon nitride film (630 to 650 nm). This is a graph showing the relationship between the same change in refractive index and the wavelength of light propagating through the silicon nitride film (940 to 960 nm). This graph shows the relationship between the refractive index change and the wavelength of light propagating through the silicon nitride film (1290-1310 nm). This graph shows the relationship between the refractive index change and the wavelength of light propagating through the silicon nitride film (1530-1550 nm). This graph shows the relationship between the refractive index change and the wavelength of light propagating through the silicon nitride film (1930-1950 nm). This graph shows the transmission spectrum of light obtained when the entire interferometer and one of the phase shifters are alternately and repeatedly irradiated with ultraviolet light and heated, respectively, in the Mach-Zehnder optical interferometer shown in Figure 2.

[0019] Hereinafter, a phase shifter, a phase shift method, and a Mach-Zehnder interferometer according to an embodiment to which the present invention is applied will be described in detail with reference to the drawings. Note that the drawings used in the following description may show the characteristic portions enlarged for the sake of easy understanding of the characteristics, and the dimensional ratios of the respective components are not necessarily the same as the actual ones. Also, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be appropriately modified and implemented within the range that does not change the gist thereof.

[0020] <First Embodiment> (Phase Shifter) FIGS. 1A and 1B are cross-sectional views of a phase shifter 100 according to the first embodiment of the present invention. FIG. 1B is a cross-sectional view at the position of the α １ -α １ line of the phase shifter 100 in FIG. 1A. FIG. 1A is a cross-sectional view at the position of the β １ -β １ line of the phase shifter 100 in FIG. 1B. The phase shifter 100 mainly includes an optical waveguide 101, a substrate 102, heating means 103, and ultraviolet irradiation means 104.

[0021] The optical waveguide 101 is mounted on one surface 102a side of the substrate 102, and includes a core 105 that propagates light and a clad 106 that surrounds the core 105 so that the propagating light does not leak out.

[0022] The core 105 extends in the direction D (here, the Y direction) in which light propagates. The length L of the core 105 in the direction D and the width (core diameter) W of the cross-section orthogonal to the direction D are determined according to the wavelength, amplitude, etc. of the light propagating through the core 105. The width W of the cross-section of the core 105 differs between the case where the optical waveguide 101 is a single mode and the case where it is a multi-mode. The width W in the case of a single mode needs to be within the range of about 0.5 times to 2 times the wavelength of the propagating light in the core 105, but the width in the case of a multi-mode does not need to be within this range.

[0023] The material of the core 105 mainly contains silicon nitride having silicon dangling bonds within a predetermined density range. Although it is difficult to directly measure the density of silicon dangling bonds, since it has a linear correlation with the spin density, by measuring the spin density, the density of the corresponding silicon dangling bonds can be indirectly known. The spin density to be measured is preferably the spin density corresponding to electrically neutral silicon dangling bonds, K ０ The spin density can be measured, for example, using the ESR method.

[0024] The density of silicon dangling bonds in the core 105, in terms of the spin density of silicon nitride, is 1×10 １８ cm －３ or more and 1×10 ２０ cm －３ or less. By adjusting the density of silicon dangling bonds so as to be within this spin density range, the refractive index change of silicon nitride when heated and irradiated with ultraviolet light, which will be described later, can be within the range of 1×10 －４ or more and 1×10 －２ or less, which is required in practical applications. When a wavelength filter is constructed using such silicon nitride, sufficient wavelength adjustment accuracy can be obtained in practical applications.

[0025] In FIG. 1B, the cladding 106 is configured to sandwich the core 105 with two claddings (upper cladding 106A and lower cladding 106B). The upper cladding 106A and the lower cladding 106B are preferably made of the same material and may be integrated. Note that an integral cladding 106 may be configured to surround the core 105.

[0026] The thickness (minimum thickness) T of the claddings 106A and 106B is preferably such that the influence of absorption and scattering of the propagating light by peripheral structures such as a heater can be ignored, and is preferably about 1.9 times or more the wavelength of the propagating light in the cladding 106.

[0027] The material of the cladding 106 has a refractive index lower than that of the core 105. For example, when forming the cladding 106 based on a CMOS process, silicon oxide, silicon oxynitride, or the like can be used as the material of the cladding 106.

[0028] The heating means (heater) 103 is arranged around the core 105 and is configured to heat the core 105 at a predetermined timing. The heating means 103 is not particularly limited, and for example, an electric heater including a heating element and a power source can be used. When the heating element is arranged around the core 105 and a voltage is applied using the power source, the core 105 can be heated by the heat generated from this heating element. As the material of the heating element, a metal-based material such as TiN or Ta may be used, or a conductive semiconductor material such as silicon doped with impurities may be used.

[0029] The ultraviolet irradiation means 104 is arranged around the core 105 and is configured to irradiate the core 105 with ultraviolet light U at a predetermined timing. The ultraviolet irradiation means 104 is not particularly limited, and for example, an ultraviolet lamp can be used.

[0030] In FIG. 1B, the case where the ultraviolet irradiation means 104 is arranged outside the cladding 106 on one side (upper side), that is, the side facing one surface of the core 105, and the heating means 103 is arranged inside the cladding 105 on the other side (lower side), that is, the side facing the other surface of the core 105, is illustrated. The phase shifter 100 is configured such that the heating means 103 does not interfere with the ultraviolet irradiation by the ultraviolet irradiation means 104. The heating means 103 is spaced apart from the core 105 to such an extent that it does not affect the propagation of light. Here, the case where the width of the heating means 103 is larger than the width of the core 105 in the plan view of FIG. 1A is illustrated, but it may be equal to or less than the width of the core 105. Here, the case where one heating means 103 and one ultraviolet irradiation means 104 are arranged respectively is illustrated, but two or more of each may be arranged.

[0031] The phase shifter 100 can be manufactured by a CMOS process. Specifically, films of the heating means 103, lower cladding 106B, core 105, and upper cladding 106A are formed sequentially on the substrate 102. Each of these films can be formed using a known film deposition method such as plasma CVD. Subsequently, the silicon nitride film, which is the material for the core 105, is subjected to a pretreatment by repeatedly alternating ultraviolet irradiation and heating multiple times. This pretreatment is an essential process to stably obtain a non-volatile refractive index change of silicon nitride.

[0032] (Phase Shift Method) The phase shift method using the phase shifter 100 comprises at least one of a heating step of heating the core 105 and an ultraviolet irradiation step of irradiating the core 105 with ultraviolet light. By performing heat treatment or ultraviolet irradiation on the core 105, the refractive index of the silicon nitride constituting the core 105 is changed, and a phase shift can be performed. When heat treatment is performed, the refractive index decreases, and when ultraviolet irradiation is performed, the refractive index increases.

[0033] The refractive index change in silicon nitride is due to changes in the charged state of the dangling bond caused by ultraviolet irradiation and heat treatment. Heat treatment causes the dangling bond to reach a stable charged state, resulting in a decrease in refractive index. Ultraviolet irradiation neutralizes the dangling bond, resulting in an increase in refractive index. Since each state is maintained for a long period at room temperature, a non-volatile refractive index change can be achieved.

[0034] The energy of the light propagated to core 105 must be 0.64 eV or higher, and less than or equal to 50% of the bandgap energy of core 105. This is because the energy level of the silicon dangling bond is located near the middle of the bandgap, and introducing light with a higher energy than this would change the charge state of the dangling bond.

[0035] The propagation loss of optical waveguides using such silicon nitride materials is low, less than 1 dB / cm, around 1550 nm (0.8 eV), which is the optical communication wavelength band, making them fully applicable to large-scale integration on chips on the order of centimeters.

[0036] Furthermore, the ultraviolet energy used to neutralize dangling bonds is generally considered to be 85% or more of the material's bandgap energy. It has also been found that heating temperatures above 150°C are effective in returning dangling bonds to a stable charged state. The upper limit of the heating temperature is approximately 400°C, based on the backend processes of typical CMOS processes.

[0037] (Mach-Zehnder Optical Interferometer) Figure 2 shows an example configuration of a Mach-Zehnder optical interferometer 300 equipped with a phase shifter 100. The Mach-Zehnder optical interferometer 300 consists of a first optical waveguide 301 and a second optical waveguide 302 equipped with a phase shifter 100, and an optical waveguide coupler 303 (optical demultiplexer 303A, optical multiplexer 303B) that couples the first optical waveguide 301 and the second optical waveguide 302. The optical demultiplexer 303A is connected to the input waveguide, and the optical multiplexer 303B is connected to the output waveguide. As described above, the transmission spectrum can be freely shifted by irradiating the core 105 with ultraviolet light and heating it.

[0038] Figure 2 shows a Mach-Zehnder optical interferometer 300 with an asymmetric structure in which the lengths of the first optical waveguide 301 and the second optical waveguide 302 are different. However, a symmetric structure in which the lengths of the first optical waveguide 301 and the second optical waveguide 302 are the same may also be used. The transmission spectrum obtained from a symmetric Mach-Zehnder optical interferometer is flat over a wide wavelength range, and the intensity of the light can be arbitrarily changed by operating a phase shifter, and the light output can also be switched. Figure 2 illustrates a 1x1 configuration Mach-Zehnder optical interferometer, but this may also be a 2x2 configuration or a 1x2 configuration Mach-Zehnder optical interferometer.

[0039] As described above, in the phase shifter 100 of this embodiment, the core 105 portion has a spin density equivalent to 10 １８ cm －３ The above 10 ２０ cm －３The material primarily contains silicon nitride having silicon dangling bonds corresponding to the following. Therefore, by heating and UV irradiation, the refractive index can be changed at a predetermined timing, and the changed state can be maintained for a long period of time. Furthermore, since no power is required for the refractive index change, power consumption can be reduced. In addition, since the phase shifter 100 of this embodiment is manufactured from a material to which a CMOS process can be applied, thousands to tens of thousands of devices can be integrated. Moreover, in the phase shifter 100 of this embodiment, there are no optical loss materials such as metal electrodes near the core 105, so optical propagation loss can be prevented.

[0040] <Second Embodiment> Figures 3A and 3B are a top view and a cross-sectional view of the phase shifter 200 according to the second embodiment of the present invention. Figure 3B shows the α of the phase shifter 200 in Figure 3A. ２ -α ２ This is a cross-sectional view at the position of the line. Figure 3A is a top view of the phase shifter 200 of Figure 3B, as seen from the side of the ultraviolet irradiation means 104. In the phase shifter 200, the ultraviolet irradiation means 104 is located outside the cladding 105, on one side (upper side) of the core 105, and the heating means 103 is located outside the cladding 105, in a region that does not obstruct the ultraviolet U irradiated onto the core 105. The other configurations are the same as those of the phase shifter 100 of the first embodiment, have the same functions, and the same reference numerals are used for corresponding components. In this embodiment, since the heating means 103 is located outside the cladding 105, it can be formed more easily than when it is located inside the cladding 105.

[0041] Figure 4 shows the results of measuring the change in refractive index of a silicon nitride film used in the waveguide core of a phase shifter, after alternating UV irradiation U1 to U5 and heat treatment A1 to A5. The spin density of this silicon nitride film is 1.2 x 10⁻¹⁶. １９ cm －３ For the ultraviolet irradiation of the silicon nitride film, ultraviolet light with energies of 4.88 eV and 6.7 eV emitted from a low-pressure mercury lamp was used, and the heating temperature was set to 300°C. The horizontal axis of the graph shows the content and order of the processing, and the vertical axis shows the refractive index.

[0042] The refractive index of silicon nitride changes with UV irradiation and heat treatment. UV irradiation increases the refractive index, which is thought to be due to the neutralization of the silicon dangling bonds within the silicon nitride. Conversely, heat treatment decreases the refractive index, which is thought to be due to the silicon dangling bonds within the silicon nitride becoming stable. It has been confirmed that this altered refractive index can be maintained for at least one month.

[0043] Figure 5 shows K ０ This shows the refractive index measurements of silicon nitride films with different spin densities. The horizontal axis of the graph is K. ０ The graph shows the spin density, and the vertical axis represents the refractive index.

[0044] K is an indicator of the density of neutralized dangling bonds. ０ A linear correlation has also been found between the ESR measurement results of spin density and the change in refractive index, with a coefficient of approximately 1 × 10⁻⁶. －２２ cm －３ It is of the order of magnitude. Therefore, the minimum required refractive index change is 1 × 10⁻⁶. －４ cm －３ , the maximum refractive index change is 1 × 10 －２ cm －３ Therefore, to obtain these, K in ESR measurement ０ Spin density of 1 x 10 １８ ~1 x 10 ２０ cm ３ A silicon dangling bond equivalent to the specified value is required. The minimum refractive index change is 1 × 10⁻⁶. －４ , and a maximum value of 1 × 10 －２ When a wavelength filter is constructed using this material, it will be approximately 1 × 10⁻⁶ －４ , and 1 × 10 －２ This quantity provides the wavelength adjustment accuracy and is a practically sufficient value.

[0045] Figures 6A to 6F show the refractive index measurements of silicon nitride films subjected to ultraviolet irradiation and heat treatment. Ultraviolet irradiation was performed using ultraviolet light with energies of 4.88 eV and 6.7 eV emitted from a low-pressure mercury lamp. Heat treatment was performed at 300°C. The horizontal axis of the graphs represents the wavelength of the propagating light, and the vertical axis represents the refractive index.

[0046] A significant change in non-volatile refractive index has been observed in the wavelength range of 400 nm to 1950 nm (energy range of 3.1 eV to 0.064 eV). However, when constructing and propagating an optical waveguide, the energy of the light must be less than half the bandgap energy of the core material. This is because the energy level of the silicon dangling bond is located near the middle of the bandgap, and introducing light with a higher energy will change the charge state of the dangling bond.

[0047] (Example) A Mach-Zehnder optical interferometer, as shown in Figure 2, was fabricated. The transmission spectrum obtained was measured when light was propagated while alternatingly irradiating both phase shifters with ultraviolet light and heating one phase shifter with a heater. The ultraviolet irradiation was performed using ultraviolet light with energies of 4.88 eV and 6.7 eV emitted from a low-pressure mercury lamp. The core heating temperature by the heater was 180 to 190°C. Figure 7 is a graph showing the measurement results. The horizontal axis of the graph shows the wavelength of the propagated light, and the vertical axis shows the transmittance of the propagated light.

[0048] The transmission spectra obtained by UV irradiation and heater heating alternate between two transmittance states, suggesting that the phase shift can be changed to a non-volatile state.

[0049] 100, 200... Phase shifter 101... Optical waveguide 102... Substrate 102a... One side of the substrate 103... Heating means 104... Ultraviolet irradiation means 105... Core 106... Cladding 106A... Upper cladding 106B... Lower cladding 300... Mach-Zehnder type optical interferometer D... Direction of light propagation U... Ultraviolet light T... Thickness of cladding 106 W... Width of the core cross-section

Claims

1. An optical waveguide comprising a core and a cladding surrounding the core; a substrate on which the optical waveguide is mounted; a heating means for heating the core; and an ultraviolet irradiation means for irradiating the core with ultraviolet light, wherein the core mainly contains silicon nitride, and the density of the silicon dangling bonds of the silicon nitride is 10 in terms of spin density. １８ cm －３ The above 10 ２０ cm －３ A phase shifter characterized by the following:

2. The spin density corresponds to the electrically neutral silicon dangling bond K ０ The phase shifter according to claim 1, characterized in that it is a spin density.

3. The phase shifter according to either claim 1 or 2, characterized in that the ultraviolet irradiation means is located outside the cladding on a side facing one surface of the core, and the heating means is located inside the cladding on a side facing the other surface of the core.

4. The phase shifter according to either claim 1 or 2, characterized in that the ultraviolet irradiation means is located outside the cladding and on one side of the core, and the heating means is located outside the cladding and in a region that does not obstruct the ultraviolet light irradiated onto the core.

5. A phase shift method using a phase shifter according to claim 1 or 2, comprising at least one of a heating step of heating the core and an ultraviolet irradiation step of irradiating the core with ultraviolet light, wherein the energy of the light propagated to the core is 0.64 eV or more and is 50% or less of the band gap of the core.

6. The phase shift method according to claim 5, characterized in that the energy of the ultraviolet light is 85% or more of the band gap of the core, and the heating temperature by the heating means is 150°C or more and 400°C or less.

7. A Mach-Zehnder optical interferometer comprising a first optical waveguide and a second optical waveguide equipped with a phase shifter according to claim 1 or 2, and an optical waveguide coupler that couples the first optical waveguide and the second optical waveguide.

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