WO2024247268A1 - Optical circuit - Google Patents

Abstract

Disclosed is a novel configuration suitable for an optical circuit that combines a ferroelectric thin film with a silicon photonics (SiPh) circuit, with improved manufacturability and mountability. As an example, the configuration of a LN-Si hybrid optical modulator is disclosed. Electrodes (30a, 30b) to which an electric signal for modulating the refractive index of a LN thin film (12) is applied are formed in a SiPh circuit (100) in a cross-section perpendicular to a substrate (10) surface. Since all the electrodes in the optical circuit are formed during a fabrication process of the SiPh circuit, additional photolithography is not required after attaching the LN thin film. Since an electrode for high frequency and other electrodes such as for biasing can be fabricated at the same height, a fabricated optical modulator chip can be flip-chip mounted in a post-process.

Description

Optical Circuit

The present invention relates to optical circuits, and more specifically to the structure of optical circuits that include ferroelectric thin films.

As IoT and 5G (fifth generation mobile communications system) services become more widespread, there is a demand for even faster optical communications networks that support these services. Research and development is also ongoing for optical modulators, one of the key devices, to achieve higher performance, smaller size, and lower power consumption.

The LN optical modulator is an optical modulator that utilizes the refractive index change caused by the Pockels effect of LiNbO ₃ (hereinafter referred to as LN) crystal, and is one of the core devices for optical communication. It is used as an external modulator to modulate CW light from DFB lasers and the like, and is capable of high-speed modulation exceeding 100 GHz. It is also expected to be used as an optical modulator in optical modules such as the high-bandwidth coherent driver modulator (HB-CDM) proposed by OIF (The Optical Internetworking Forum). In order to achieve smaller size and lower cost, an optical modulator with a thin film LN attached onto a Si waveguide is expected due to its high efficiency and wide bandwidth.

FIG. 1 is a schematic diagram showing the configuration of an LN thin film SiPh hybrid optical modulator (Non-Patent Document 1). The optical modulator 700 has a hybrid configuration of an LN thin film and a SiPh circuit, in which an LN thin film 12 is attached to a silicon photonic (SiPh) circuit in which a SiO ₂ layer and a core layer are formed on a Si substrate 10. A Mach-Zehnder interferometer (MZI) including two interference optical waveguides 21 is formed by the branched core layer. A high-frequency electrode 22 is further formed on the upper surface of the LN thin film 12 so as to sandwich each of the interference waveguides 21. The core, the surrounding SiO 2 cladding, and the LN thin film 12 adjacent to the core function together as an optical waveguide. Light is not only confined by the core, but also seeps into the LN and propagates, functioning as a composite optical waveguide in which the ferroelectric material LN and the Si core are integrated.

By modulating the refractive index of the attached thin-film LN with a high-frequency electrical signal applied to the modulation electrode 22, the interference conditions of the MZI change and the combined optical output of the two interference optical waveguides is turned on and off. By combining thin-film LN with a SiPh circuit, it is expected that optical modules including optical modulators can be significantly miniaturized and made more efficient.

Weigel, Peter O., et al. "Hybrid silicon photonic-lithium niobate electro-optic Mach-Zehnder modulator beyond 100 GHz bandwidth." arXiv preprint arXiv:1803.10365 (2018)

However, the optical modulator with the LN thin film attached to the SiPh circuit shown in FIG. 1 has problems in manufacturability and assembly. The modulation electrode 22, to which an electric signal for refractive index modulation is applied, is formed on the LN thin film 12. Usually, the process of attaching the LN thin film to the SiPh circuit must be performed as an independent process after the photolithography for fabricating the SiPh circuit is completed. As a result, the photolithography process must be performed twice to form the high-frequency electrode 22. This manufacturing process of the optical modulator is complicated and expensive. Furthermore, the height of the PAD relative to the other electrodes fabricated in the SiPh circuit process differs from the height of the PAD relative to the modulation electrode 22. There is also a restriction in that flip-chip mounting for miniaturization cannot be performed in the later process of assembling the fabricated optical modulator chip into an optical module, etc.

The present invention was made in consideration of the above-mentioned problems, and provides an optical circuit configuration that combines a ferroelectric thin film and a SiPh circuit with improved manufacturability and mountability.

One embodiment of the present invention is an optical circuit in which a silicon substrate, a cladding layer, and a ferroelectric thin film are arranged in that order, the optical circuit having a core formed within the cladding layer and one or more electrodes formed along the core for applying an electric field to the ferroelectric thin film.

The present invention provides an optical circuit that combines a ferroelectric thin film and a SiPh circuit with improved manufacturability and mountability.

FIG. 1 is a schematic diagram showing the configuration of an LN thin-SiPh hybrid optical modulator. FIG. 1 is a diagram illustrating a schematic configuration of a hybrid optical modulator according to a related art. 1A and 1B are diagrams illustrating the top and side cross-sectional configurations of an optical circuit according to a first embodiment. 11A and 11B are diagrams illustrating the top and side cross-sectional configurations of an optical circuit according to a second embodiment of the present invention. FIG. 1 shows the configuration of a mode converter between a SiN core and a Si core. 11A and 11B are diagrams illustrating the top and side cross-sectional configurations of an optical circuit according to a third embodiment. 11A and 11B are diagrams illustrating the top and side cross-sectional configurations of an optical circuit according to a fourth embodiment. 13A and 13B are diagrams illustrating the top and side cross-sectional configurations of an optical circuit according to a fifth embodiment. 13A and 13B are diagrams illustrating the top and side cross-sectional configurations of an optical circuit according to a sixth embodiment.

The optical circuit disclosed herein provides a novel configuration that combines a ferroelectric thin film and a SiPh circuit, with improved manufacturability and mountability. As an example, a novel configuration suitable for an LN-Si hybrid optical modulator is presented. Electrodes to which an electrical signal that modulates the refractive index of the LN thin film is applied are formed in the SiPh circuit. All electrodes in the optical circuit are formed during the fabrication process of the SiPh circuit, so no additional photolithography is required after attaching the LN thin film. It becomes possible to fabricate high-frequency electrodes and other electrodes such as for bias at the same height, and the fabricated optical modulator chip can be flip-chip mounted in a later process. It improves the manufacturability and mountability of devices that utilize the electro-optical effect of LN, including optical modulators.

Below, we will first clarify the configuration of the optical circuit of the prior art and its problems, using an optical modulator as an example. Next, we will explain in detail the basic configuration and modified examples of the optical circuit of the present disclosure, as well as the manufacturing process. In the following explanation, a hybrid optical modulator refers to a Mach-Zehnder type modulator (MZM) that combines an LN thin film and a SiPh circuit. All drawings of the optical circuit structure described below are schematic diagrams, and the scales in each direction are not consistent. The substrate thickness direction (z direction) is shown significantly enlarged. Please also note that the relative relationships of the height, width, etc. of each element are not necessarily accurately expressed between different elements.

In the following explanation, the optical circuit of the present disclosure will be described as an example in which it is applied to a hybrid optical modulator. However, the configuration of the optical circuit of the present disclosure can also be applied to other optical circuits in which a ferroelectric thin film is attached to a SiPh circuit. Ferroelectric materials are not limited to lithium niobate (LN) and include lithium tantalate (LT) and barium titanate, as long as they are capable of changing the refractive index of the optical waveguide by the electro-optic effect. Furthermore, optical devices that utilize the refractive index modulation of the ferroelectric thin film near the core can include, in addition to optical modulators, polarization controllers and phase shifters that can be used to compensate for polarization mode dispersion.

FIG. 2 is a diagram showing a schematic configuration of a hybrid optical modulator of the prior art. FIG. 2(a) is a top view of the Si substrate surface (x-y surface) of the SiPh circuit, and FIG. 2(b) is a side cross-sectional view (y-z surface) cut perpendicular to the substrate surface and the light traveling direction through line IIb-IIb. FIG. 2(c) is a side cross-sectional view cut perpendicular to the substrate surface and the light traveling direction through line IIc-IIc. In the prior art MZ type hybrid optical modulator 700 shown in FIG. 1, a part including modulation electrodes 22a and 22b and functioning as a refractive index modulation region is shown for one optical waveguide 21 of two interference optical waveguides. Referring to the two side cross-sectional views of FIG. 2(b) and (c), the SiPh circuit in the optical modulator 700 includes a SiO ₂ layer 11 formed on a Si substrate 10 and a rectangular Si core 12.

The SiPh circuit includes, for example, an electrode 23 formed inside SiO ₂ functioning as a cladding layer. The electrode 23 may be a high-frequency electrode through which a modulation signal of the optical modulator is propagated, or may be an electrode for other circuits such as an electric circuit integrated with the optical modulator. The electrode 23 is a schematic diagram showing a configuration different from that of the modulation electrode 22, and the position on the upper surface of the optical circuit 700 does not matter. The position in the height direction on the cross-sectional view is also not limited to one shown in FIG. 2(c). This SiPh circuit can be fabricated by forming each layer on a Si substrate and patterning the core and electrodes by photolithography. The SiPh circuit can also be formed by processing an SOI (Silicon On Insulator) substrate on which a Si layer and a SiO ₂ layer functioning as a core are formed in advance.

The LN thin film 12 is attached on the SiPh circuit. The attachment of the LN thin film is performed by activating the surface layer of the LN thin film and the _SiO2 layer, which are formed in advance to the size of the required area, with an argon beam, and then contacting them and applying pressure and heat. Since the LN thin film 12 is formed by bonding the LN thin film in this way, a physical attachment process other than the Si photolithography process is essential. In order to cause a refractive index change in the LN thin film 12 as a part of the optical waveguide of the light leaking from the Si core 12, for example, gold electrodes 22a and 22b for modulation are formed on the LN thin film 12. At the ends of the electrodes, PADs 24a and 24b for connection are formed. In order to form the modulation electrodes 22a and 22b, a second photolithography process is required after attaching the LN thin film 12, so the manufacturing process of the optical modulator 700 becomes complicated and the cost increases.

Also, referring to (b) and (c) of FIG. 2, the height of the modulation electrodes 22a, 22b is different from the height of the electrode 23 of the SiPh circuit. In terms of the circuit configuration of the optical circuit, if the electrode heights are different, high-density mounting technology using a ball grid array (BGA) cannot be used. In optical modulators and the like that require miniaturization and high-density mounting, the inability to use the powerful BGA technology is a major obstacle to miniaturization of optical modules and optical transmission devices. The optical circuit disclosed herein solves the problems of manufacturability and mountability shown in the above-mentioned optical modulator 700.

[Embodiment 1]
FIG. 3 is a diagram showing the configuration of the top surface and the side cross section of the optical circuit of the first embodiment. As an example of the optical circuit 100 of the present embodiment, the configuration of a hybrid optical modulator is shown. As in FIG. 2, FIG. 3(a) is a top view of the SiPh circuit looking at the Si substrate surface (x-y surface), FIG. 3(b) is a side cross section (x-y surface) cut perpendicularly to the substrate surface and the light traveling direction along line IIIb-IIIb, and FIG. 3(c) is a side cross section (x-y surface) cut perpendicularly to line IIIc-IIIc to the substrate surface and the light traveling direction in the MZ type optical modulator. FIG. 3 shows a schematic diagram of a portion of one optical waveguide 31 of two interference optical waveguides in an MZ type optical modulator, which includes modulation electrodes 30a and 30b and functions as a refractive index modulation region. The configuration of the core 31 of the optical waveguide is the same as the core 21 of the optical circuit 700 of the conventional technology shown in FIG. 2. In the following, the differences from the optical circuit 700 of the conventional technology will be described.

The optical circuit 100 differs from the conventional configuration in that the modulation electrodes 30a, 30b are formed in the _SiO2 cladding area in a plane perpendicular to the substrate of the SiPh circuit, and are formed on the LN thin film outside the SiPh circuit. Here, the PADs 34a, 34b connected to the modulation electrodes 30a, 30b do not contribute to the refractive index modulation of the LN thin film and are not included in the refractive index modulation area. The modulation electrodes 30a, 30b have PADs 34a, 34b at both ends through vias (VIA), and the PADs are connected to an electric circuit that supplies an electric signal, an electric signal source, etc. by some means. They may be wire-bonded or connected to other electric elements and driver circuits by BGA. Also, they may not have PADs and may be directly connected from an electric circuit integrated on the same Si substrate as the optical modulator. In the optical modulator, the modulation electrodes are high-frequency electrodes to which high-frequency electric signals are applied.

In FIG. 3, for the purpose of showing the input point of the modulation signal, PADs 34a and 34b are shown as being generally adjacent to the LN thin film 12. The modulation electrodes 30a and 30b may be extended, and the PADs may be formed at positions away from the region of the LN thin film 12. Please note that the positions of PADs 34a and 34b in FIG. 3 are just an example.

Since the modulation electrodes 30a, 30b are formed in the SiO2 _clad region in a plane perpendicular to the substrate of the SiPh circuit, the modulation electrodes are fabricated in the same photolithography process as all other electrodes of the SiPh circuit. No additional photolithography process is required after the attachment process of the LN thin film 12 as in the conventional technology. Furthermore, it is possible to form the PADs of all electrodes, including the modulation electrodes, to the same height. Therefore, flip chip mounting using BGA or bumps is possible.

The optical circuit of the present invention can therefore be implemented as an optical circuit in which a silicon substrate 10, a cladding layer 11, and a ferroelectric thin film 12 are arranged in that order, with a core 31 formed within the cladding layer, and one or more electrodes 30a, 30b formed along the core for applying an electric field to the ferroelectric thin film.

3(b) and 3(c), the modulating electrodes 30a, 30b are shown to have the same bottom height as the Si core 31 in the substrate thickness direction (z direction), but are not limited to this. As long as the modulating electrodes 30a, 30b are formed in the area of the _SiO2 layer 11 in a plane perpendicular to the substrate of the SiPh circuit, the modulating electrodes are fabricated in the same photolithography process as all other electrodes of the SiPh circuit. Electrodes other than the modulating electrodes may be fabricated at different heights.

[Embodiment 2]
FIG. 4 is a diagram showing the top and side cross-sectional configurations of the optical circuit of the second embodiment. As an example of the optical circuit 200 of this embodiment, the configuration of a hybrid optical modulator is shown. FIG. 4(a) is a top view of the Si substrate surface (xy plane) of the SiPh circuit, FIG. 4(b) is a side cross-sectional view (xy plane) cut along line IVb-IVb, and FIG. 4(c) is a side cross-sectional view (xy plane) cut along line IVc-IVc perpendicular to the substrate surface and the light traveling direction. FIG. 4 shows a schematic diagram of a portion of one optical waveguide 32 of two interference optical waveguides in an MZ type optical modulator 200, which includes modulation electrodes 30a and 30b and functions as a refractive index modulation region. The differences from the optical circuit 100 of the first embodiment are as follows.

In the optical circuit 100 of the first embodiment described above, the optical waveguide in which the refractive index is modulated is composed of the Si core 31, the surrounding SiO ₂ clad, and the LN thin film 12 adjacent to the core in the refractive index modulation region, forming a composite waveguide. The Si core 31 also constitutes a continuous optical waveguide together with the surrounding SiO ₂ clad through the inside and outside of the refractive index modulation region having a hybrid configuration with the LN thin film. On the other hand, in the optical circuit 200 of this embodiment, the optical waveguide in which the refractive index is changed by an electric signal is composed of a silicon nitride (SiN) core 32 that is closer to the LN thin film 12 than the Si core. Outside the refractive index modulation region having a hybrid configuration with the LN thin film, the optical waveguide is composed of the Si core 31, which is separate from the SiN core. In the thickness direction (z direction) of the substrate, the SiN core 32 can be formed at a higher position than the Si core 31 and closer to the LN thin film 12. Therefore, the refractive index modulation of the LN thin film 12 by an electric signal can be efficiently realized. The SiN core 32 and the Si core 31 having different heights can be optically connected by the mode converter 40 formed near the boundary of the LN thin film 12 in FIG.

FIG. 5 is a diagram showing the configuration of a mode converter between a SiN core and a Si core. FIG. 5(a) is a top view of the Si substrate surface (xy surface) of the optical circuit 200, showing only two types of cores 32 and 31 near the mode converter 40. FIG. 5(b) is a side cross-sectional view (xz surface) of the mode converter 40, cut along the substrate surface and the light traveling direction through the center line (Vb-Vb line) of the core. Referring to FIG. 5(a), the mode converter 40 has a first taper at the tip of the SiN core 32, in which the core width gradually narrows, and a second taper at the tip of the Si core 31, in which the core width gradually widens. Referring to FIG. 5(b), two taper parts overlap above and below within the region of the mode converter 40 of the SiO ₂ cladding layer 11, and efficiently optically couples optical waveguides of different heights. The mode converter is not limited to the one shown in FIG. 5, and can be realized by other configurations as long as it is capable of converting the mode of light.

In FIG. 4, the mode converter 40 is shown near the end of the LN thin film 12, but it may be located within the LN thin film 12 region or outside the LN region 12 region. The position of the mode converter 40 is not important as long as the SiN core 32 can produce a sufficient electro-optic effect (e.g., a change in refractive index) with the adjacent modulation electrodes 30a, 30b.

The optical circuit of the present invention can therefore be implemented as a core that includes a first core 32 of silicon nitride formed on the side of the ferroelectric thin film 12 in the thickness direction (z direction) of the substrate in the region where the ferroelectric thin film 12 is formed on the substrate surface (x-y plane), and a second core 31 of silicon formed on the side of the substrate closer to the first core in the thickness direction, and the first core and second core are optically coupled.

According to the optical circuit 200 of the second embodiment, an additional photolithography process is not required after the process of attaching the LN thin film 12, and it is possible to form the PADs of all electrodes, including the modulation electrodes, to the same height. The SiN core in the refractive index modulation region where the LN thin film is attached can also be formed within the series of manufacturing processes for the SiPh circuit. Since the SiN core 32 and the LN thin film 12 can be placed close to each other, applying the configuration of the optical circuit 200 to an optical modulator can efficiently generate refractive index modulation.

4(b) and (c), the modulating electrodes 30a, 30b are shown as being located between the SiN core 32 and the Si core 31 in the substrate thickness direction (z direction), but are not limited thereto. As long as the modulating electrodes 30a, 30b are formed in the region of the SiO2 _layer 11 in a plane perpendicular to the substrate of the SiPh circuit, the modulating electrodes are fabricated in the same photolithography process as all other electrodes of the SiPh circuit. Electrodes other than the modulating electrodes may be fabricated at different heights.

[Embodiment 3]
FIG. 6 is a diagram showing the configuration of the top and side cross sections of the optical circuit of the third embodiment. As an example of the optical circuit 300 of this embodiment, the configuration of a hybrid optical modulator is shown. FIG. 6(a) is a top view of the SiPh circuit looking at the Si substrate surface (xy plane), FIG. 6(b) is a side cross section (xy plane) cut perpendicularly to the substrate surface and the light traveling direction along the line VIb-VIb, and FIG. 6(c) is a side cross section (xy plane) cut perpendicularly to the substrate surface and the light traveling direction along the line VIc-VIc in the MZ type optical modulator 300. FIG. 6 shows a schematic diagram of a portion of one optical waveguide 31 of two interference optical waveguides, which includes modulation electrodes 30a and 30b and functions as a refractive index modulation region. The differences from the optical circuit 100 of the first embodiment are as follows.

In the optical circuit 100 of the first embodiment described above, the LN thin film 12 is configured only in the refractive index modulation region where the MZI including the two interference optical waveguides 21 is configured. The ends of the modulation electrodes 30a, 30b to which an electrical signal is applied are provided with PADs to which bonding wires or the like from an electrical signal source are connected, so it is essential to align the LN thin film so that it does not cover the PADs. Furthermore, when multiple optical modulators are fabricated on one wafer, a process of aligning multiple LN thin films and attaching each one is required.

The optical circuit 300 of this embodiment differs from the first and second embodiments in that the PADs 35a, 35b of the modulating electrodes 30a, 30b are configured on the back surface of the Si substrate opposite the LN thin film 12. The PADs 35a, 35b are formed on the side of the Si substrate 10 opposite the side on which the LN thin film 12 is attached. The modulating electrodes 30a, 30b and the PADs 35a, 35b are connected by through-silicon vias (TSVs) 36a, 36b, respectively.

Since the external electric signal source and the PADs 35a and 35b can be connected to the surface of the Si substrate opposite to the LN thin film 12, there is no need to limit the position of the LN thin film 12, and the accuracy of the attachment position may be low. For multiple optical modulator chips, one LN thin film can be attached to the entire surface of the _SiO2 layer 11.

6(b) and 6(c), the modulating electrodes 30a and 30b are shown as having the same bottom height as the Si core 31 in the substrate thickness direction (z direction), but are not limited to this configuration. If the modulating electrodes 30a and 30b are configured in the region of the _SiO2 layer 11 in a plane perpendicular to the substrate of the SiPh circuit, the modulating electrodes can be fabricated in the same photolithography process as all other electrodes of the SiPh circuit. Electrodes other than the modulating electrodes may be fabricated at different heights.

[Embodiment 4]
FIG. 7 is a diagram showing the configuration of the top surface and the side cross section of the optical circuit of the fourth embodiment. As an example of the optical circuit 400 of this embodiment, the configuration of a hybrid optical modulator is shown. FIG. 7(a) is a top view of the SiPh circuit looking at the Si substrate surface (xy surface), FIG. 7(b) is a side cross section (xy surface) cut along the line VIIb-VIIb, and FIG. 7(c) is a side cross section (xy surface) cut along the line VIIc-VIIc perpendicular to the substrate surface and the light traveling direction. FIG. 6 shows a schematic diagram of a portion of one of two interference optical waveguides in an MZ type optical modulator 400, which includes modulation electrodes 30a and 30b and functions as a refractive index modulation region. The differences between the optical circuits 100, 200, and 300 of the above-mentioned embodiments and the optical circuit 400 are as follows.

In the optical modulator using the optical circuit 400 of this embodiment, the core is composed of a SiN 32 core in the refractive index modulation region and a Si core 31 outside the refractive index modulation region, similar to the optical modulator 200 of embodiment 2. It also has a mode converter 40 between the SiN core and the Si core shown in FIG. 5. The modulation electrodes 33a, 33b differ from the electrodes of the other embodiments described above in that they extend in the x direction along the core and have a wall-like shape standing in the z direction. The TSV can be considered to be continuously formed in the x direction. In other words, the high-frequency electrodes as modulation electrodes are wall-like electrodes 33a, 33b that are continuously formed and penetrate from the height of the core 32 to the surface opposite the cladding layer 11 of the substrate in the thickness direction of the substrate 10.

As mentioned above, FIG. 7 is a schematic diagram, and the scales in each direction are not the same. The substrate thickness direction (z direction) is shown significantly enlarged, and the cross sections (y-z planes) of the modulation electrodes 30a and 30b in the first to third embodiments are actually rectangular with a horizontal length in the y direction. On the other hand, the cross sections (y-z planes) of the modulation electrodes 33a and 33b in the fourth embodiment shown in FIG. 7(b) can be rectangular with a vertical length in the z direction as well as a horizontal length in the y direction, allowing the electrode shape to be changed significantly. It should be noted that the characteristic impedance as a transmission path for high-frequency electrical signals can also be calculated with different parameter values. The same characteristic impedance can be obtained with electrodes 33a and 33b that are narrower in the y direction parallel to the substrate surface than the modulation electrodes in the first to third embodiments. Therefore, by narrowing the width of the electrodes, the wall-shaped modulation electrodes 33a and 33b in the fourth embodiment shown in FIG. 7(b) are more suitable for high-density integration.

It does not matter if the positional accuracy in attaching the LN thin film 12 is low, and one LN thin film can be attached to the entire surface of the SiO ₂ layer 11 for a plurality of optical modulator chips.

[Embodiment 5]
FIG. 8 is a diagram showing the configuration of the top surface and the side cross section of the optical circuit of the fifth embodiment. As an example of the optical circuit 500 of the present embodiment, the configuration of a hybrid optical modulator is shown. FIG. 8(a) is a top view of the SiPh circuit looking at the Si substrate surface (xy surface), FIG. 8(b) is a side cross section (xy surface) cut perpendicularly to the substrate surface and the light traveling direction along the line VIIIb-VIIIb, and FIG. 8(c) is a side cross section (xy surface) cut perpendicularly to the line VIIIc-VIIIc. FIG. 8 shows a schematic diagram of a portion of one optical waveguide 31 of two MZ type interference optical waveguides in an MZ type optical modulator, which includes modulation electrodes 37a and 37b and functions as a refractive index modulation region.

The configuration of the optical circuit 500 of this embodiment is the same as that of the optical circuit 300 of embodiment 3 shown in FIG. 6 in terms of the configuration of the optical waveguide and the modulation electrodes. The difference from the configuration of embodiment 3 is that the modulation electrodes 37a and 37b are made of Si with impurities added by ion implantation, rather than metals such as aluminum, copper, or gold. By using ion-implanted Si as the material for the high-frequency electrodes, the high-frequency electrodes can be formed in the same layer as the Si core 31. According to the configuration of the optical circuit 500 of this embodiment, the modulation electrodes 37a and 37b can be formed directly next to the Si core 31 at the same height, so that the electric field tends to concentrate inside the LN thin film 12. If the configuration of the optical circuit 500 is applied to an optical modulator, efficient refractive index modulation becomes possible.

In the optical circuit 500 in Fig. 8, an example is shown in which the PADs 35a, 35b are formed on the back side of the Si substrate, and the modulation electrodes 37a, 37b made of Si doped with impurities are combined. However, the present invention can also be applied to the case in which the PADs 34a, 34b are formed on the top surface of the _SiO2 layer 11 as in the first and second embodiments shown above. In this case, however, the LN thin film needs to be attached only to the top side of the modulation electrodes.

[Embodiment 6]
9 is a diagram showing the top and side cross-sectional configurations of the optical circuit of the sixth embodiment. As an example, the diagram shows the configuration of a hybrid optical modulator 600. FIG. 9(a) is a top view of the SiPh circuit looking at the Si substrate surface (xy plane), FIG. 9(b) is a side cross-sectional view (xy plane) cut perpendicularly to the substrate surface and the light traveling direction along line IXb-IXb, and FIG. 9(c) is a side cross-sectional view (xy plane) cut perpendicularly to line IXc-IXc and line IXc in the MZ type optical modulator 600. FIG. 9 shows a schematic diagram of a portion of one optical waveguide of two interference optical waveguides in the MZ type optical modulator 600, which includes a modulation electrode and functions as a refractive index modulation region.

The optical circuit 600, like the optical circuit 200 of the second embodiment, has an optical waveguide structure in which the SiN core 32 and the Si core 31 are coupled by the mode conversion section 40. The SiN core 32 and the LN thin film 12 can be brought into close proximity, so that, like the optical circuit 200, refractive index modulation can be efficiently generated.

The optical circuit 600 of this embodiment differs from the optical circuit of embodiment 2 in that the positions of the modulation electrodes 30a, 30b in the substrate thickness direction (z direction) are at the same height as the SiN core 32. In addition, PADs 35a, 35b to which bonding wires from an electrical signal source are connected for applying an electrical signal to the modulation electrodes are formed on the back surface of the Si substrate 10 on the side opposite the LN thin film 12, as in embodiments 3 to 5. The modulation electrodes 30a, 30b and the PADs 35a, 35b are connected by TSVs 36a, 36b, respectively.

According to the configuration of the optical circuit 600 of this embodiment, the modulating electrodes 30a, 30b are formed at the same height directly beside the SiN core, which makes it easier for the electric field to concentrate inside the LN thin film 12. If the configuration of the optical circuit 600 is applied to an optical modulator, efficient refractive index modulation becomes possible.
Since the external electric signal source and the PADs 35a and 35b can be connected on the opposite side of the LN thin film 12, there is no need to limit the position of the LN thin film 12, and the accuracy of the attachment position may be low. For multiple optical modulator chips, one LN thin film can be attached to the entire surface of the _SiO2 layer 11.

In the optical circuit 600 of Fig. 9, an example is shown in which PADs 35a, 35b are formed on the back side of the Si substrate, and the positions of the modulating electrodes 30a, 30b in the substrate thickness direction are at the same height as the SiN core 32. However, the present invention can also be applied to the case in which the PADs 34a, 34b are formed on the upper surface of the _SiO2 layer 11 in the above-mentioned embodiment 2. In this case, however, the LN thin film needs to be attached only to the upper side of the modulating electrode. In addition, the optical circuit 600 of this embodiment can be combined with the modulating electrode 3 made of Si to which impurities are added by ion implantation or the like in embodiment 5.

In the above-mentioned embodiments, a hybrid optical modulator combining an LN thin film and a SiPh circuit has been described as an example, but instead of the LN thin film, a ferroelectric such as lithium tantalate (LT) or barium titanate can be used. Furthermore, optical devices that utilize the electro-optical effect of a ferroelectric thin film are not limited to optical modulators. For example, a polarization controller that arbitrarily controls the polarization state of light, or a phase shifter that arbitrarily changes the phase state of light can be realized. When used as a polarization controller or phase shifter, it should be noted that a low-frequency control signal, a constant-value DC signal, or the like can be applied to the electrodes that apply an electric field to the above-mentioned ferroelectric thin film, rather than a modulation signal. In addition, when the polarization controller is operated as a polarization scrambler, the signal may be on the order of several GHz.

As described above in various embodiments, the optical circuit of the present invention can improve the manufacturability and mountability of optical circuits that combine ferroelectric thin films and SiPh circuits.

This invention can be used in optical communications.

Claims (8)

An optical circuit comprising a silicon substrate, a cladding layer, and a ferroelectric thin film, arranged in that order,
a core formed within the cladding layer;
and one or more electrodes formed along the core for applying an electric field to the ferroelectric thin film. The core is
a first core of silicon nitride formed on the side of the ferroelectric thin film in a thickness direction of the substrate in a region in a substrate surface where the ferroelectric thin film is formed; and
a second core of silicon formed on a side of the substrate relative to the first core in the thickness direction;
The optical circuit according to claim 1 , wherein the first core and the second core are optically coupled. The optical circuit of claim 2, wherein the one or more electrodes are formed at the same height as the first core in the thickness direction. The optical circuit according to any one of claims 1 to 3, wherein the one or more electrodes are wall-shaped electrodes that are formed continuously in the thickness direction of the substrate, penetrating from the height of the core to the surface of the substrate opposite the cladding layer. An optical circuit according to any one of claims 1 to 3, wherein the one or more electrodes are made of silicon doped with impurities. An optical circuit according to any one of claims 1 to 3, in which a PAD is formed on the opposite side of the substrate from the cladding layer via a through silicon via (TSV) at the end of the electrode. An optical circuit according to any one of claims 1 to 3, in which a PAD is formed on the upper surface of the cladding layer through a via at the end of the electrode. The cladding layer is made of _SiO2 ,
the ferroelectric thin film is any one of lithium niobate (LN), lithium tantalate (LT), and barium titanate;
4. The optical circuit according to claim 1, wherein the one or more electrodes are supplied with an electric signal that causes an electro-optic effect in the ferroelectric thin film.

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