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

Abstract

The present invention provides a phase shifter that can be manufactured using a CMOS process, exhibits low optical propagation loss, can maintain refractive index changes without power supply, and allows for the resetting of those refractive index changes. [Solution] The phase shifter 100 of the present invention comprises an optical waveguide 101 composed 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 U, wherein the core 105 mainly contains silicon nitride, and the density of silicon dangling bonds of silicon nitride is 10 in terms of spin density 18 cm -3 The above 10 20 cm -3 The following applies:

Description

[Technical Field]

[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. [Background technology]

[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 waveguides that constitute 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, non-volatile phase shifters such as MOS capacitor charge trap type and phase change material loaded type have been proposed, 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). [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] K. Suzuki et al., Journal of Lightwave Technology 37, 116-122 (2019). [Non-Patent Document 2] G. Cong et al., Nature Communications 13, 3261 (2022). [Non-Patent Document 3] JF. Song et al., Scientific Reports 6, 22616 (2016). [Non-Patent Document 4] S. Cheung et al., Laser Photonics Review 2024, 2400001 (2024). [Non-Patent Document 5] J. Geler-Kremer et al., Nature Photonics 16, 491-497 (2022). [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The present invention has been made in view of the above circumstances, and an object thereof is to provide a phase shifter, a phase shifting method, and a Mach-Zehnder interferometer applying them, which can be manufactured by applying a CMOS process, have low optical propagation loss, can maintain a refractive index change without energization, and can reset the refractive index change.

Means for Solving the Problems

[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 includes an optical waveguide composed of a core and a cladding surrounding the core, a substrate on which the optical waveguide is mounted, heating means for heating the core, and ultraviolet irradiation means for irradiating ultraviolet rays to the core. The core contains silicon nitride as a main component, and the density of silicon dangling bonds in the silicon nitride is 10 ,

[0011] , -3 , -3 ,

[0013] , ,

[0012] , , 0 , 20 , 18 , , , cm -3 or more and 10 20 cm -3 or less in terms of spin density conversion by ESR measurement.

[0011] (2) In the phase shifter according to (1) above, it is preferable that the spin density is a K 0 spin density corresponding to the electrically neutral silicon dangling bond.

[0012] (3) In the phase shifter according to any one of (1) or (2) above, the ultraviolet irradiation means may be disposed on a side facing one surface of the core outside the cladding, and the heating means may be disposed on a side facing the other surface of the core inside the cladding.

[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. [Effects of the Invention]

[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. [Brief explanation of the drawing]

[0018] [Figure 1] (a) and (b) are cross-sectional views of a phase shifter according to the first embodiment of the present invention. [Figure 2]This figure shows an example configuration of a Mach-Zehnder optical interferometer equipped with the phase shifter of the same embodiment. [Figure 3] (a) and (b) are a top view and a cross-sectional view of a phase shifter according to a second embodiment of the present invention. [Figure 4] This graph shows the change in refractive index of the silicon nitride film used in the waveguide core of a phase shifter due to heating and ultraviolet irradiation. [Figure 5] This graph shows the change in refractive index of the silicon nitride film used in the waveguide core of a phase shifter, as it changes with spin density. [Figure 6] (a) to (f) are graphs showing the relationship between the change in refractive index of the silicon nitride film used in the waveguide core of the phase shifter due to heating and ultraviolet irradiation, and the wavelength of light propagating through the silicon nitride film. [Figure 7] Figure 2 shows a graph illustrating the transmission spectrum of light obtained when the entire interferometer is repeatedly and alternately irradiated with ultraviolet light and one of the phase shifters is heated in a Mach-Zehnder type optical interferometer. [Modes for carrying out the invention]

[0019] Hereinafter, a phase shifter, a phase shifting method, and a Mach-Zehnder optical interferometer according to embodiments to which the present invention is applied will be described in detail with reference to the drawings. Note that, for convenience, the drawings used in the following description may show enlarged versions of key features to make them easier to understand, and the dimensional ratios of each component may not be the same as those in reality. Furthermore, the materials, dimensions, etc., exemplified in the following description are merely examples, and the present invention is not limited to these; it can be implemented with appropriate modifications without altering its essence.

[0020] <First Embodiment> (Phase shifter) Figs. 1(a) and 1(b) are cross-sectional views of the phase shifter 100 according to the first embodiment of the present invention. Fig. 1(b) is a cross-sectional view at the position of the α1-α1 line of the phase shifter 100 in Fig. 1(a). Fig. 1(a) is a cross-sectional view at the position of the β1-β1 line of the phase shifter 100 in Fig. 1(b). 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 cladding 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 multimode. The width W in the case of a single mode needs to fall 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 multimode does not need to fall 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 corresponding density of silicon dangling bonds can be indirectly known. The spin density to be measured is preferably the K 0 spin density corresponding to electrically neutral silicon dangling bonds. The spin density can be measured, for example, using the ESR method.

[0024] The density of silicon dangling bonds in the core 105 is 1×10 18 cm -3 or more and 1×10 20 cm -3The following is achieved by adjusting the density of the silicon dangling bond to fall within this spin density range, thereby reducing the refractive index change of silicon nitride when heated and irradiated with ultraviolet light, as described later, to a practically obtainable 1 × 10⁻¹⁶. -4 The above is 1 x 10 -2 The following range is possible. When a wavelength filter is constructed using such silicon nitride, sufficient wavelength tuning accuracy for practical use can be obtained.

[0025] In Figure 1(b), the cladding 106 is configured to sandwich the core 105 between two cladding layers (upper cladding 106A and lower cladding 106B). The upper cladding 106A and lower cladding 106B are preferably made of the same material and may be integrated. The integrated cladding 106 may also surround the core 105.

[0026] The thickness (minimum thickness) T of cladding 106A and 106B is preferably such that the effects of absorption and scattering of propagating light by surrounding structures such as heaters can be ignored, and is preferably about 1.9 times or more the wavelength of the propagating light within cladding 106.

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

[0028] The heating means (heater) 103 is positioned around the core 105 and configured to heat the core 105 at predetermined timings. The heating means 103 is not particularly limited, but for example, an electric heater equipped with a heating element and a power supply can be used. When the heating element is positioned around the core 105 and a voltage is applied using the power supply, the heat generated from the heating element can heat the core 105. As the material of the heating element, metallic materials such as TiN and Ta may be used, or conductive semiconductor materials such as silicon doped with impurities may be used.

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

[0030] Figure 1(b) illustrates a case where the ultraviolet irradiation means 104 is located outside the cladding 106 on one side (upper side), i.e., the side facing one surface, of the core 105, and the heating means 103 is located inside the cladding 105 on the other side (lower side), i.e., the side facing the other surface. The phase shifter 100 is configured such that the heating means 103 does not obstruct the ultraviolet irradiation by the ultraviolet irradiation means 104. The heating means 103 is spaced away from the core 105 to the extent that it does not affect the propagation of light. Here, in the plan view of Figure 1(a), the width of the heating means 103 is shown to be greater than the width of the core 105, but it may be less than or equal to the width of the core 105. Here, the case where one heating means 103 and one ultraviolet irradiation means 104 are arranged is shown, 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 known film deposition methods 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 in silicon nitride.

[0032] (Phase shifting method) The phase shifting 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 can be changed, thereby performing a phase shift. 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 propagating to core 105 must be 0.64 eV or higher, and less than 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 will 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 energy of the ultraviolet light used to neutralize the dangling bond 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 the dangling bond to a stable charged state. The upper limit of the heating temperature is approximately 400°C, based on the backend processes of a typical CMOS process.

[0037] (Mach-Zehnder type 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 transmitted spectrum can be freely shifted by irradiating and heating the core 105 with ultraviolet light.

[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 3(a) and 3(b) are cross-sectional views of a phase shifter 200 according to a second embodiment of the present invention. Figure 3(b) is a cross-sectional view of the phase shifter 200 in Figure 3(a) at the position of the α2-α2 line. Figure 3(a) is a top view of the phase shifter 200 in Figure 3(b) as seen from the ultraviolet irradiation means 104 side. 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 in 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-U5 and heat treatment A1-A5. The spin density of this silicon nitride film is 1.2 x 10⁻¹⁶. 19 cm -3For 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 treatment, 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 neutralized dangling bond density. 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 1 × 10⁻⁶ spin density 18 ~1 × 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 If 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 6(a) to 6(f) 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] Significant changes in non-volatile refractive index have 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 greater energy will change the charge state of the dangling bond.

[0047] (Examples) A Mach-Zehnder optical interferometer, as shown in Figure 2, was constructed. Light was propagated by alternating UV irradiation of both phase shifters and heater heating of one phase shifter, and the resulting transmission spectra were measured. UV irradiation was performed using 4.88 eV and 6.7 eV UV light emitted from a low-pressure mercury lamp. The core heating temperature was 180-190°C. Figure 7 shows a graph of the measurement results. The horizontal axis of the graph represents the wavelength of the propagated light, and the vertical axis represents 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. [Explanation of Symbols]

[0049] 100, 200... Phase shifter 101...Optical waveguide 102... Circuit board 102a...One side of the circuit board 103...Heating means 104...Ultraviolet irradiation means 105 core 106...Clad 106A... Upper cladding 106B...Lower cladding 300... Mach-Zehnder type optical interferometer D... Direction of light propagation U...Ultraviolet rays T-Clad 106 thickness 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, The system comprises an ultraviolet irradiation means for irradiating the core with ultraviolet light, The core contains silicon nitride as its main component, and the density of the silicon dangling bond 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 ultraviolet irradiation means is positioned outside the cladding, on the side facing one surface of the core. The phase shifter according to either claim 1 or 2, characterized in that the heating means is arranged inside the cladding on the side facing the other side of the core.

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

5. A phase shifting method using a phase shifter according to claim 1 or 2, A heating step for heating the core, The process includes at least one of the following: an ultraviolet irradiation step of irradiating the core with ultraviolet light, The phase shift method according to either claim 1 or 2, characterized in that 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 energy of the ultraviolet light is set to 85% or more of the band gap of the core. The phase shift method according to claim 5, characterized in that the heating temperature by the heating means is 150°C or higher and 400°C or lower.

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

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