Optical modulator, optical transmitter-receiver, and optical transceiver

Abstract

The invention relates to an optical modulator, an optical transmitter-receiver and an optical transceiver. An optical modulator includes: an electro-optical layer including an electro-optical material; and a material layer disposed below the electro-optical layer and having a dielectric constant lower than a dielectric constant of the electro-optical layer. The optical modulator includes: a core layer disposed below the material layer and having a refractive index higher than refractive indexes of the electro-optical layer and the material layer; and an electrode applying an electrical signal to the electro-optical layer. The refractive index of the material layer is 0.85 times or higher than the refractive index of the electro-optical layer.

Description

Technical Field

[0001] The embodiments discussed herein relate to optical modulators, optical transmitter-receivers, and optical transceivers. Background Art

[0002] With the recent increase in the capacity of optical communication systems, optical transmitter-receivers require higher communication speeds and lower power consumption. Specifically, for optical transmitters, efficient optical modulators are extremely needed. For example, a Mach-Zehnder (MZ) optical modulator using a ferroelectric electro-optic material with a high electro-optic coefficient (such as PbLaZrTiO3 (PLZT) or BaTiO3 (BTO)) is one of the devices used to realize a small and efficient optical modulator. For example, PLZT or BTO has an electro-optic coefficient of several hundred pm / V, and the electro-optic coefficient is significantly higher than the electro-optic coefficient of 31 pm / V of a LiNbO3 (lithium niobate: LN) optical modulator. Therefore, for example, the electro-optic coefficients of PLZT and BTO are suitable for materials of small and efficient optical modulators.

[0003] Fig. 22 is a schematic plan view of an example of a conventional light modulator 200 . Fig. 22 The optical modulator 200 shown has an input unit 201, an optical splitter 202, two modulation units 203 arranged in parallel with each other, a multiplexer 204, and an output unit 205. The optical modulator 200 is a Mach-Zehnder optical modulator that modulates the intensity and phase of an optical signal output from the output unit 205 by performing optical phase modulation according to an electrical signal on the signal light from the input unit 201 that has been split and passed through the modulation unit 203. The optical splitter 202 is a coupler that splits the signal light from the input unit 201 and outputs the split signal light to the modulation unit 203.

[0004] The modulation unit 203 has a modulation waveguide 211 and an electrode 212. For example, the modulation waveguide 211 is a waveguide using an electro-optical (EO) material such as PLZT or BTO. The modulation waveguide 211 has a first modulation waveguide 211A and a second modulation waveguide 211B. The electrode 212 has a signal electrode 212A, a first ground electrode 212B, and a second ground electrode 212C. The first modulation unit 203A has a first ground electrode 212B, a signal electrode 212A, and a first modulation waveguide 211A arranged between the first ground electrode 212B and the signal electrode 212A. The second modulation unit 203B has a second ground electrode 212C, a signal electrode 212A, and a second modulation waveguide 211B arranged between the second ground electrode 212C and the signal electrode 212A.

[0005] Fig.23 is along Fig. 22 A schematic cross-sectional view taken along line CC shown in FIG. Fig.23The cross-sectional portion shown is a cross-sectional portion of the first modulation unit 203A of the optical modulator 200. The first modulation unit 203A has a Si substrate (not shown in the figure), a lower cladding layer 221, an EO layer 222, an upper cladding layer 223, a signal electrode 212A, and a first ground electrode 212B. For example, the lower cladding layer 221 is a buffer layer of SiO2 arranged on the Si substrate. For example, the EO layer 222 is a layer of a ferroelectric EO material (such as BTO or PLZT) having a high electro-optic coefficient. The EO layer 222 has been processed to have, for example, a rib-shaped cross-sectional structure so that a signal is guided therethrough, and the EO layer 222 serves as a core layer of the first modulation waveguide 211A. For example, the upper cladding layer 223 is a buffer layer of SiO2 arranged on the EO layer 222 and covering the upper region of the EO layer 222, which surrounds the rib-shaped core 211A1 of the first modulation waveguide 211A. The core 211A1 of the first modulation waveguide 211A is arranged in parallel with and between the first ground electrode 212B and the signal electrode 212A.

[0006] According to the high-frequency electric signal from the signal electrode 212A, an electric field is generated from the signal electrode 212A to the first ground electrode 212B in the first modulation unit 203A, and the optical refractive index of the first modulation waveguide 211A changes according to the electric field. The first modulation unit 203A modulates the phase of the signal light guided through the first modulation waveguide 211A according to the change in the optical refractive index. The first modulation unit 203A outputs the modulated signal light to the multiplexer 204.

[0007] According to the high-frequency electric signal from the signal electrode 212A, an electric field is generated from the signal electrode 212A to the second ground electrode 212C in the second modulation unit 203B, and the light refractive index of the second modulation waveguide 211B changes according to the electric field. The second modulation unit 203B modulates the phase of the signal light guided through the second modulation waveguide 211B according to the change in the light refractive index. The second modulation unit 203B outputs the modulated signal light to the multiplexer 204.

[0008] The multiplexer 204 multiplexes the modulated signal light from the first modulation unit 203A and the modulated signal light from the second modulation unit 203B together, and outputs the multiplexed signal light to the output unit 205. According to the phase states in the first modulation unit 203A and the second modulation unit 203B, interference is generated in the multiplexer 204, and as a result, the signal light whose intensity and phase are modulated is output from the output unit 205.

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 63-049732

[0010] Patent Document 2: Japanese Patent Application Publication No. 2022-032687

[0011] Patent Document 3: U.S. Patent No. 07283689

[0012] Patent Document 4: Japanese Patent Application Publication No. 2007-333756

[0013] Next, the dependence of the propagation characteristics of the electric signal on the dielectric constant in the optical modulator 200 will be described. Fig.24 2 is a diagram illustrating an example of the relationship between a relative dielectric constant, which is a physical property constant determined by the material of the EO layer 222 , and characteristic impedance. Fig.25 is a graph illustrating an example of the relationship between the relative dielectric constant of the EO layer 222 and the electric signal loss. Fig.26 2 is a diagram illustrating an example of the relationship between the relative dielectric constant of the EO layer 222 and the refractive index of the electric signal. The ferroelectric EO material used in the EO layer 222 of the modulation waveguide 211 has a relative dielectric constant of several hundred Å.

[0014] That is, the relative dielectric constant of the EO layer 222, such as PLZT or BTO, is significantly higher than the relative dielectric constant of LN used in the waveguide of a conventional optical modulator. Fig.24 As shown, the characteristic impedance of the optical modulator 200 using the EO layer 222 of PLZT or BTO, for example, becomes lower than the characteristic impedance of the optical modulator using the EO layer of LN. Fig.25 As shown, the electrical signal loss in the optical modulator 200 using the EO layer 222 of PLZT or BTO, for example, becomes higher than the electrical signal loss in the optical modulator using the EO layer of LN. Fig.26 As shown, the electrical signal refractive index of the optical modulator 200 using the EO layer 222 of, for example, PLZT or BTO becomes higher than that of the optical modulator using the EO layer of LN.

[0015] The higher refractive index of the electrical signal of the optical modulator 200 using the EO layer 222 such as PLZT or BTO causes a difference between the propagation speed of light and the propagation speed of the electrical signal to result in a velocity mismatch.

[0016] Specifically, the group refractive index that specifies the propagation speed of light guided via an optical waveguide using an EO material such as BTO or PLZT is, for example, in the range of 2 to 2.5. In contrast, the refractive index of an electrical signal that specifies the propagation speed of an electrical signal applied to an optical waveguide using an EO material such as BTO or PLZT is 3 or more because its relative dielectric constant is very high, reaching several hundred. Therefore, a speed mismatch is generated due to the large difference between the light propagation speed and the electrical signal propagation speed. As a result, for example, the 3 dB band of the optical response of the optical modulator 200 will be severely limited in the optical modulator 200.

[0017] In the case where a velocity mismatch occurs, the influence of the velocity mismatch can be minimized by reducing the waveguide length in the optical modulator 200. However, in this case, because the operating length of the optical modulator 200 is reduced, the half-wavelength voltage Vπ of the phase change π of the optical modulator 200 increases, and the modulation efficiency of the optical modulator 200 decreases. That is, in the case where a velocity mismatch occurs, the effect of improving the modulation efficiency by using a material having a high electro-optic coefficient will not be achieved.

[0018] Therefore, in the case of using an EO material having a high dielectric constant, such as PLZT or BTO, for the core 211A1 of the optical modulator 200, it will be difficult to successfully simultaneously well secure a 3 dB band and achieve high modulation efficiency (low Vπ) for the optical modulator 200.

[0019] Therefore, an object of one aspect of an embodiment of the present invention is to provide, for example, an optical modulator capable of improving its modulation efficiency while ensuring the operating frequency band of the optical modulator. Summary of the invention

[0020] According to one aspect of the embodiment, an optical modulator includes an electro-optic layer, a material layer, a core layer and an electrode. The electro-optic layer includes an electro-optic material. The material layer is arranged below the electro-optic layer and has a dielectric constant lower than that of the electro-optic layer. The core layer is arranged below the material layer and has a refractive index higher than that of the electro-optic layer and the material layer. The electrode applies an electrical signal to the electro-optic layer. The refractive index of the material layer is 0.85 times or higher than that of the electro-optic layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic plan view of an example of an optical modulator according to the first embodiment;

[0022] Figure 2 is along Figure 1 A schematic cross-sectional view taken along line AA shown in FIG.

[0023] Figure 3 is a diagram illustrating an example of the relationship between the thickness of the EO layer of the optical modulator and the refractive index of the electrical signal;

[0024] Figure 4 is a diagram illustrating an example of the relationship between the thickness of the Si core of the optical modulator and the light group refractive index;

[0025] Figure 5 is a diagram illustrating an example of a relationship between the thickness of a material layer of an optical modulator and a half-wavelength voltage Vπ;

[0026] Figure 6is a diagram illustrating an example of the relationship between the thickness of a material layer of an optical modulator and the light group refractive index;

[0027] Fig. 7A is a diagram illustrating an example of an SOI substrate for a Si core substrate;

[0028] Figure 7B is a diagram illustrating an example of a process of forming a Si thin film in an SOI substrate;

[0029] Figure 7C is a diagram illustrating an example of a Si patterning process;

[0030] Fig.7D is a diagram illustrating an example of a SiO2 film forming process;

[0031] Fig. 7E is a diagram illustrating an example of a Si core substrate that has been subjected to surface planarization polishing;

[0032] Fig. 8A is a diagram illustrating an example of an EO substrate used for an EO thin film substrate;

[0033] Figure 8B is a diagram illustrating an example of a first attachment process;

[0034] Figure 8C is a diagram illustrating an example of a first removal process;

[0035] Fig.8D is a diagram illustrating an example of an EO thin film substrate;

[0036] Fig. 9A is a diagram illustrating an example of a second attachment process;

[0037] Fig. 9B is a diagram illustrating an example of a second removal process;

[0038] Fig. 9C is a diagram illustrating an example of a process of forming an upper cladding layer;

[0039] Fig.9D is a diagram illustrating an example of a metal layer film forming process;

[0040] Fig.9E is a diagram illustrating an example of a metal patterning process;

[0041] Fig.10 is a schematic cross-sectional view of an example of an optical modulator according to a second embodiment;

[0042] Fig.11 is a diagram illustrating an example of a relationship between a relative dielectric constant of an upper cladding layer of an optical modulator and an electric signal loss;

[0043] Fig.12 is a diagram illustrating an example of a relationship between a relative dielectric constant of an upper cladding layer of an optical modulator and a refractive index of an electrical signal;

[0044] Fig.13 is a diagram illustrating an example of the relationship between the refractive index of the upper cladding layer of the optical modulator and the half-wavelength voltage Vπ;

[0045] Fig.14 is a schematic cross-sectional view of an example of an optical modulator according to a third embodiment;

[0046] Fig.15 is a diagram illustrating an example of a relationship between a refractive index of a first material layer of an optical modulator and optical loss;

[0047] Fig.16 is a diagram illustrating an example of a relationship between a dielectric constant of a first material layer of an optical modulator and a half-wavelength voltage Vπ;

[0048] Fig.17 is a schematic plan view illustrating an example of an IQ optical modulator according to a fourth embodiment;

[0049] Fig.18 is along Fig.17 A schematic cross-sectional view taken along line BB shown in FIG.

[0050] Fig.19 is a schematic plan view illustrating an example of a DP-IQ optical modulator according to a fifth embodiment;

[0051] Fig. 20 is a schematic plan view illustrating an example of an optical transmitter-receiver according to a sixth embodiment;

[0052] Fig.21 is a diagram illustrating an example of an optical transceiver;

[0053] Fig. 22 is a schematic plan view of an example of a conventional light modulator;

[0054] Fig.23 is along Fig. 22 A schematic cross-sectional view taken along line CC shown in FIG.

[0055] Fig.24 is a diagram illustrating an example of a relationship between a relative dielectric constant, which is a physical property constant determined by a material of an EO layer, and characteristic impedance;

[0056] Fig.25 is a graph illustrating an example of a relationship between a relative dielectric constant of an EO layer and an electric signal loss; and

[0057] Fig.26 is a diagram illustrating an example of the relationship between the relative dielectric constant of the EO layer and the refractive index of the electric signal. DETAILED DESCRIPTION

[0058] Preferred embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to these embodiments.

[0059] (a) First Embodiment

[0060] Figure 1 1 is a schematic plan view illustrating an example of an optical modulator 1 according to a first embodiment. The optical modulator 1 has an input unit 11, a beam splitter 12, two modulation units 13, a multiplexer 14, and an output unit 15. The optical modulator 1 is a Mach-Zehnder optical modulator that optically modulates the split signal light from the input unit 11 according to an electrical signal. The input unit 11 is a component that is connected to an input waveguide and inputs the signal light from the input waveguide to the optical modulator 1. The output unit 15 is a component that is connected to an output waveguide and outputs the modulated signal light from the optical modulator 1 to the output waveguide. The beam splitter 12 is, for example, a 1×2 MMI coupler that splits the signal light from the input unit 11 and outputs the split signal light to the modulation unit 13, respectively. The multiplexer 14 is, for example, a 2×1 MMI coupler that multiplexes the modulated signal light from the modulation unit 13 together and outputs the multiplexed signal light to the output unit 15.

[0061] The modulation unit 13 has a first modulation unit 13A and a second modulation unit 13B. The modulation unit 13 has a modulation waveguide 21 and an electrode 22. For example, the modulation waveguides 21 are waveguides each having a core made of Si. The modulation waveguide 21 has a first modulation waveguide 21A and a second modulation waveguide 21B. The electrode 22 has a GSG structure having a signal electrode 22A, a first ground electrode 22B, and a second ground electrode 22C.

[0062] The first modulation unit 13A has a first modulation waveguide 21A, a signal electrode 22A, and a first ground electrode 22B, and the first modulation waveguide 21A is arranged between the first ground electrode 22B and the signal electrode 22A. According to the high-frequency electric signal from the signal electrode 22A, an electric field is generated from the signal electrode 22A to the first ground electrode 22B in the first modulation unit 13A, and the light refractive index of the first modulation waveguide 21A changes according to the electric field. The first modulation unit 13A performs phase modulation on the signal light guided via the first modulation waveguide 21A according to the change in the light refractive index. The first modulation unit 13A outputs the signal light that has been modulated to the multiplexer 14.

[0063] The second modulation unit 13B has a second modulation waveguide 21B, a signal electrode 22A, and a second ground electrode 22C, and the second modulation waveguide 21B is arranged between the second ground electrode 22C and the signal electrode 22A. According to the high-frequency electric signal from the signal electrode 22A, an electric field is generated from the signal electrode 22A to the second ground electrode 22C in the second modulation unit 13B, and the light refractive index of the second modulation waveguide 21B changes according to the electric field. The second modulation unit 13B modulates the signal light guided via the second modulation waveguide 21B according to the change in the light refractive index. The second modulation unit 13B outputs the signal light that has been modulated to the multiplexer 14.

[0064] The multiplexer 14 multiplexes the modulated signal light from the first modulation unit 13A and the modulated signal light from the second modulation unit 13B together, and outputs the multiplexed signal light to the output unit 15 .

[0065] Figure 2 is along Figure 1 A schematic cross-sectional view taken along line AA shown in FIG. Figure 2 The section shown is Figure 1 1 is a cross-sectional view of the first modulation unit 13A shown in FIG. The first modulation unit 13A has a Si substrate (not shown in the figure), a lower cladding layer 31, a Si core 35, a material layer 32, an EO layer 33, an upper cladding layer 34, a first ground electrode 22B, and a signal electrode 22A. The lower cladding layer 31 is, for example, a cladding layer of SiO2 stacked on the Si substrate. The Si core 35 is a core layer of an optical waveguide formed in the lower cladding layer 31, and is, for example, a core of the first modulation waveguide 21A. The Si core 35 is arranged to be in contact with the lower side of the material layer 32 below the EO layer 33.

[0066] The material layer 32 is a layer of, for example, Ta2O5 or amorphous Si stacked on the surface of the lower cladding layer 31 and the Si core 35. The EO layer 33 is, for example, an electro-optical layer of an EO material such as BTO stacked on the surface of the material layer 32. The upper cladding layer 34 is, for example, a cladding layer of SiO2 arranged on the area around the Si core 35, the cladding layer covering the surface of the EO layer 33. The Si core 35 is interposed between the first ground electrode 22B and the signal electrode 22A arranged on the EO layer 33. The EO layer 33 and the material layer 32 have a flat plate structure.

[0067] BTO used as the EO material used in the EO layer 33 has, for example, an electro-optic coefficient of 100 pm / V to 400 pm / V, a relative dielectric constant of 150 or more, and a refractive index of about 2.2, although these values ​​depend on the method of forming it. Ta2O5 used in the material layer 32 has a relative dielectric constant of 28 and a refractive index of about 2.05. The EO layer 33 used in the optical modulator 1 has a thickness of 0.15 μm, the material layer 32 has a thickness of 0.02 μm, the Si core 35 has a width of 0.8 μm, and the interval between the electrodes 22 with the Si core 35 interposed therebetween is 4 μm. The material layer 32 is formed of a material having a lower dielectric constant than that of the EO layer 33. The Si core 35 is formed of silicon having a refractive index higher than that of the EO layer 33 and the material layer 32. The refractive index of the material layer 32 is 0.85 times or more of the refractive index of the EO layer 33.

[0068] Figure 3 is a schematic diagram illustrating an example of the relationship between the thickness of the EO layer 33 of the optical modulator 1 and the refractive index of the electrical signal, and Figure 4 1 is a schematic diagram illustrating an example of the relationship between the thickness of the Si core 35 of the optical modulator 1 and the light group refractive index. Figure 3 As shown, when the thickness of the EO layer 33 in the optical modulator 1 becomes, for example, equal to or less than 0.15 μm, the refractive index of the high-frequency electric signal becomes relatively low, about 3.00 or less. Figure 4 As shown, when the thickness of the Si core 35 in the optical modulator 1 becomes, for example, equal to or greater than 0.07 μm, the group refractive index of the light propagating through the Si core 35 in the optical modulator 1 also becomes high, about 3.00 or higher. Therefore, the conventional problem of the high refractive index of the electric signal is solved, and the propagation speed of the high-frequency electric signal and the propagation speed of the light in the optical modulator 1 become substantially the same, thereby being able to improve the speed mismatch. Since the speed mismatch is improved, even in the operating frequency band of 65 GHz or higher, it is possible to achieve a 3 dB band in which the attenuation amount of the optical response level of the optical modulator 1 is reduced to 3 dB or less.

[0069] In addition, because the speed mismatch can be improved, the length of the optical modulator 1 can be increased to about 4 mm. Therefore, the optical modulator 1 can have a sufficient operating length. The half-wavelength voltage per unit length of the optical modulator 1 having the above-mentioned cross-sectional structure becomes high, about 0.6 Vcm. This combination of higher modulation efficiency and the increased length of the optical modulator 1 obtained due to the improved speed mismatch enables the optical modulator 1 to have a half-wavelength voltage Vπ of 2.0 V or less, thereby having high efficiency.

[0070] Figure 5 is a schematic diagram illustrating an example of the relationship between the thickness of the material layer 32 of the optical modulator 1 and the half-wavelength voltage Vπ, and Figure 6is a schematic diagram illustrating an example of the relationship between the thickness of the material layer 32 and the light group refractive index of the light modulator 1. For ease of description, SiO2, Ta2O5, and Si will be described as examples of the material of the material layer 32. It is assumed here that SiO2 has a refractive index of 1.444, Ta2O5 has a refractive index of 2.05, and Si has a refractive index of 3.45.

[0071] like Figure 5 As shown, the half-wavelength voltage Vπ of the optical modulator 1 tends to increase with the increase in the thickness of the material layer 32. For all of these materials, the increase in the half-wavelength voltage Vπ is at a negligible level until the thickness of the material layer 32 is about 0.02 μm, and the half-wavelength voltage Vπ can be kept low at 1 Vcm or less until the thickness of the material layer 32 is about 0.05 μm. Therefore, the thickness of the material layer 32 is at least desirably set to 0.05 μm or less.

[0072] like Figure 6 As shown, using SiO2 as the material of the material layer 32 is contrary to the purpose of increasing the group refractive index of light to improve the velocity mismatch, because the group refractive index of light tends to decrease with increasing thickness of the material layer 32. It is not preferred to use a material with a low refractive index for the material layer 32, and a material with a refractive index close to the refractive index (2.2) of the EO layer 33 (such as Ta2O5), or a material with a refractive index higher than the refractive index of the EO layer 33 (such as Si containing amorphous Si) is preferred.

[0073] Specifically, using Si as the material of the material layer 32 is preferable in terms of improving the velocity mismatch because the light group refractive index can be increased as the thickness of the material layer 32 increases. Therefore, the refractive index of the material layer 32 is preferably substantially the same as or higher than the refractive index of the EO layer 33, and at least for the material layer 32, a material having a refractive index at least 0.85 times higher than that of the EO layer 33 is preferably used.

[0074] Preferably, the material layer 32 is formed of a material different from that of the EO layer 33 , has an electro-optic coefficient and a relative dielectric constant lower than those of the EO layer 33 , and has a low absorption coefficient for signal light.

[0075] The material for the material layer 32 preferably also functions as an adhesive layer that improves adhesion between the Si core 35 and the EO layer 33, and, for example, Ta 2 O 5 or Si is a suitable material in terms of refractive index and role as an adhesive layer.

[0076] The material for the EO layer 33 of the optical modulator 1 is preferably an electro-optic material having an electro-optic coefficient of about 50 pm / V or more and a relative dielectric constant of about 50 or more, and the material for the material layer 32 is preferably a material having an electro-optic coefficient and a relative dielectric constant lower than those of the EO layer 33 and a low absorption coefficient for signal light. In addition, the material for the material layer 32 is preferably a material having a refractive index of 0.85 times or more of the refractive index of the EO layer 33. The Si core 35 has been described as an example of the core layer of the optical modulator 1, but the core layer may be any core layer having a refractive index higher than that of the EO layer 33 and the material layer 32.

[0077] The process of manufacturing the optical modulator 1 according to the first embodiment will be described below. For example, the optical modulator 1 has: a Si core substrate 40A having a pattern of a Si core portion 35 formed on the surface of the Si core substrate 40A; and an EO thin film substrate 45A having a thin film of an EO layer 33 of BTO or PLZT, for example, formed on the Si substrate and with a SiO2 film interposed between the thin film and the Si substrate. The optical modulator 1 is formed by attaching the Si core substrate 40A and the EO thin film substrate 45A to each other and forming the electrode 22 on the EO layer 33.

[0078] First, a process of manufacturing the Si core substrate 40A will be described. Fig. 7A is a diagram illustrating an example of an SOI substrate 40 used for a Si core substrate 40A. The SOI substrate 40 has a Si substrate 41, a BOX layer 42 stacked on the Si substrate 41, and a Si layer 43 stacked on the BOX layer 42. The Si layer 43 is a Si layer used for silicon photonics and has a thickness of 0.22 μm.

[0079] Figure 7B 4 is a diagram illustrating an example of a process of forming a Si thin film in an SOI substrate 40. Figure 7B As shown, the Si layer 43 in the SOI substrate 40 is formed into a thin film having a thickness of about 0.07 μm by, for example, etching.

[0080] Figure 7C is a diagram illustrating an example of a Si patterning process. Figure 7C As shown, the Si layer 43 which has been made into a Si thin film is patterned to form a Si layer 43A. Fig.7D is a diagram illustrating an example of a SiO2 film forming process. Fig.7D As shown, after patterning, a SiO 2 film 44 is formed on the Si layer 43A and the BOX layer 42 .

[0081] Fig. 7E4A is a diagram illustrating an example of a Si core substrate 40A that has been subjected to surface planarization polishing. The surface of the SiO2 film 44 is polished to a level close to the surface of the Si layer 43A in the SOI substrate 40, thereby exposing the Si layer 43A on the surface, thereby forming the Si core substrate 40A.

[0082] In the structure of the Si core substrate 40A shown as an example in this article, the surface of the Si core substrate 40A has been ideally flattened, and the surface of the Si layer 43A and the surface of the SiO2 film 44 on both sides of the Si layer 43A are on the same plane. However, considering the difference in the amount of material removal during the flattening process, the Si core substrate 40A can be constructed so that the surface of the SiO2 film 44 is at a height lower than the surface height of the Si layer 43A. Even in this case, substantially the same half-wavelength voltage Vπ and optical group refractive index can be obtained. In terms of improving high-frequency characteristics, it is preferred to use a substrate with a high resistivity of 1000Ωcm or more as the Si substrate 41 of the SOI substrate 40.

[0083] Next, a manufacturing process of the EO film substrate 45A used for the optical modulator 1 will be described. Fig. 8A is a diagram illustrating an example of an EO substrate 45 used for an EO film substrate 45A. Fig. 8A The EO substrate 45 shown has a Si substrate 46, an intermediate layer 47 stacked on the Si substrate 46, and an EO layer 48 stacked on the intermediate layer 47. The EO layer 48 is formed of a ferroelectric EO material such as BTO. For example, the intermediate layer 47 is a layer of any one of ZrO2, Pt, and SrRuO3 (SRO). Forming the EO layer 48 directly on the Si substrate 46 deteriorates the quality of the crystal in the ferroelectric EO material, so the EO layer 48 is formed on the Si substrate 46 via the intermediate layer 47.

[0084] Figure 8B 49A is a diagram illustrating an example of the first attachment process. A support substrate 49A to be laminated with a cover layer 49B is prepared. The EO substrate 45 is vertically flipped, as shown in FIG. Figure 8B As shown, the upper cladding layer 49B and the EO layer 48 in the EO substrate 45 are attached to each other with an adhesive. This attachment can be modified as needed, and the bonding surfaces between the EO layer 48 and the upper cladding layer 49B can be bonded to each other with a new adhesive layer.

[0085] The bonding surface between the upper cladding layer 49B and the EO layer 48 in the EO substrate 45 is desirably flattened by, for example, grinding polishing or chemical mechanical polishing (CMP). Surface cleaning and surface activation are preferably performed before bonding. The upper cladding layer 49B and the EO layer 48 are attached to each other after surface activation, but bonding may be performed under a vacuum atmosphere as needed, or pressure may be applied at an appropriate temperature when bonding.

[0086] Figure 8C is a diagram illustrating an example of the first removal process. Figure 8C As shown, the Si substrate 46, the intermediate layer 47, and a portion of the EO layer 48 in the EO substrate 45 attached to the upper cladding layer 49B are removed so that the EO layer 48A having a predetermined thickness (e.g., 0.15 μm) is formed. For example, abrasive polishing or chemical mechanical polishing (CMP) is desirably used as a method for removing the above components.

[0087] Fig.8D 4 is a diagram illustrating an example of an EO film substrate 45A. Fig.8D As shown, the EO thin film substrate 45A is completed by forming a material layer 32 on the EO layer 48A. As already described, the material of the material layer 32 is preferably a material having a refractive index substantially the same as or higher than that of the EO layer 48, and preferred examples of the material include tantalum oxide and Si film. The material layer 32 provides adhesion between the Si core substrate 40A and the EO thin film substrate 45A. As described later, by Fig.8D The EO layer 48A and the material layer 32 for improving adhesion shown in FIG. 3 are further attached to another substrate ( Fig.7D ), forming a modulated waveguide. In this attachment method, a high-quality EO crystal film formed on a substrate on which crystals are easily grown can be attached to a different substrate individually, and thus a structure having any combination of substrate material and EO material can be made relatively easily compared to directly forming a film by epitaxial growth.

[0088] A method of manufacturing the optical modulator 1 by performing bonding between the Si core substrate 40A and the EO thin film substrate 45A will be described below. Fig. 9A is a diagram illustrating an example of the second attachment process. Fig. 9A As shown, the EO film substrate 45A is flipped, and the material layer 32 of the flipped EO film substrate 45A and the Si layer 43A of the Si core substrate 40A are attached to each other. For ease of description, if the size of the EO film substrate 45A is the same as that of the Si core substrate 40A, the wafers can be attached to each other as they are. However, this attachment can be appropriately modified if their sizes are different from each other, for example, the EO film substrate 45A can be cut into small pieces of a predetermined size, and multiple pieces of the EO film substrate 45A can be attached to the Si core substrate 40A by die bonding.

[0089] In the process of bonding the wafer of the EO thin film substrate 45A and the wafer of the Si core substrate 40A together, the exposed surface of the Si core substrate 40A is attached to the surface of the material layer 32 of the EO thin film substrate 45A, and the exposed surface is where the Si core portion 35 is exposed. Before attachment, the surface of the Si core substrate 40A and the surface of the EO thin film substrate 45A may be subjected to, for example, planarization, surface cleaning and / or surface activation. In addition, the bonding may be modified as needed and may be performed under vacuum or at an appropriate temperature and an appropriate pressure as needed.

[0090] Fig. 9B 4A is a diagram illustrating an example of the second removal process. After the Si core substrate 40A and the EO thin film substrate 45A have been attached to each other, as shown in FIG. Fig. 9B As shown, the support substrate 49A on the side of the EO thin film substrate 45A opposite to the side where the EO layer 33 is located is removed. For example, a process for selectively removing only Si from other materials by etching using XeF2 gas is preferably used for removing the support substrate 49A.

[0091] Fig. 9C 1 is a diagram illustrating an example of a process for forming an upper cladding layer. Fig. 9B A portion of the upper cladding layer 49B is shown to form the upper cladding layer 34 on the EO layer 33. A portion of the upper cladding layer 49B can be removed so that the electrode 22 can be disposed on the EO layer 33. Etching is performed by, for example, reactive ion etching (RIE) or ion milling.

[0092] Fig.9D 34 is a diagram illustrating an example of a metal layer film forming process. Fig.9D As shown, the metal layer 22D used as the electrode 22 is formed on the surface of the EO layer 33 and the upper cladding layer 34, as shown in FIG. Fig.9D As shown. The metal layer 22D has, for example, a Ti film 22D1 serving as a contact metal in contact with the EO layer 33 and an Au film 22D2 formed on the Ti film 22D1. The Ti film 22D1 and the Au film 22D2 can be formed by a method such as vapor deposition, and in the case where the Au film 22D2 is made into a thick film, an additional plating process can be performed. The Au film 22D2 can be appropriately modified and can be changed to a film of, for example, Al, Ag, or Cu.

[0093] Fig.9E is a diagram illustrating an example of a metal patterning process. Fig.9EAs shown, the electrode pattern 22E is formed so that the Si core 35 is interposed therebetween, so that by removing a portion of the metal layer 22D, an electric field can be applied to a region near the Si core 35, which is as shown in FIG. Fig.9D The area of ​​the upper cladding layer 34 on which the metal layer 22D is formed is shown. As a result, the Figure 1 The light modulator 1 is shown.

[0094] The optical modulator 1 according to the first embodiment has: an EO layer 33 including an electro-optical material; and a material layer 32 arranged below the EO layer 33 and having a lower dielectric constant than that of the EO layer 33. In addition, the optical modulator 1 has, for example: a Si core 35 arranged below the material layer 32 and having a higher refractive index (for example, a refractive index of 3.45) than that of the EO layer 33 and the material layer 32; and an electrode 22 for applying an electric signal to the EO layer 33. The refractive index of the material layer 32 is 0.85 times or more of the refractive index of the EO layer 33. That is, by applying the Si core 35 having a high refractive index to increase the group refractive index, the refractive index of the high-frequency electric signal and the group refractive index of the light are made closer to each other, and the increase is related to the refractive index of the electric signal, which tends to be relatively high due to the influence of the EO layer 33 having a high dielectric constant. As a result, the propagation speed of the high-frequency electric signal and the propagation speed of the light become substantially the same in the optical modulator 1, so that the speed mismatch between the propagation speed of the electric signal and the propagation speed of the light can be improved. Therefore, the operation band of the optical modulator 1 can be prevented from being limited. More importantly, since the speed mismatch in the optical modulator 1 is improved, the operation length can be increased, the half-wavelength voltage Vπ can be reduced, and the modulation efficiency of the optical modulator 1 can be improved.

[0095] The use of the Si core 35 made of a material having a high refractive index of 3.45 in the optical modulator 1 can increase the group refractive index of light propagating through the Si core 35. In addition, the use of the EO layer 33 having a high dielectric constant increases the refractive index of the high-frequency electrical signal, but increasing the group refractive index of the light makes the group refractive index of the light closer to the refractive index of the high-frequency electrical signal and alleviates the speed mismatch. As a result, the operating length of the optical modulator 1 does not need to be reduced but can be increased, and the operating frequency band of the optical modulator 1 can be ensured while improving the modulation efficiency.

[0096] Since the material layer 32 is arranged between the EO layer 33 and the Si core 35 in the optical modulator 1, adhesion between the Si core 35 and the EO layer 33 can be obtained. Since the EO layer 33 is a crystalline material, it is difficult to form the EO layer 33 on the Si core 35 and the lower cladding layer 31 by epitaxial growth. Therefore, the material layer 32 is arranged between: the Si core 35 and the lower cladding layer 31; and the EO layer 33. In addition, since the refractive index of the material layer 32 is set to 0.85 times or more of the refractive index of the EO layer 33, the group refractive index of light can be maintained or increased compared with the case where there is no material layer 32. As a result, by increasing the adhesion between the Si core 35 and the EO layer 33, a stable structure of the optical modulator 1 can be achieved without deteriorating the effect of improving the speed matching between the propagation speed of the high-frequency electric signal and the propagation speed of light in the optical modulator 1.

[0097] The material of the EO layer 33 has an electro-optic coefficient higher than that of the material layer 32. As a result, a low half-wavelength voltage Vπ can be achieved.

[0098] The EO layer 33 having a high electro-optic coefficient has a relative dielectric constant higher than 50, the electro-optic coefficient of the EO layer 33 is higher than 50 pm / V, and the material layer 32 has a relative dielectric constant of 50 or lower and an electro-optic coefficient of 50 pm / V or lower. As a result, the refractive index of the EO layer 33 to high-frequency electrical signals can be relatively low, and the group refractive index of light can be increased by means of the Si core 35.

[0099] The EO layer 33 includes a material containing at least any one of BaTiO3 (BTO), PbLaZrTiO3 (PLZT), and PbZrTiO3 (PZT). The material layer 32 includes a material containing at least any one of Ta2O5 and Si. The high electro-optic coefficients of these materials enable the half-wavelength voltage Vπ to be reduced.

[0100] The EO layer 33 has a thickness in the range of 0.1 μm to 0.3 μm, the Si core 35 has a thickness in the range of 0.05 μm to 0.10 μm, and the material layer 32 has a thickness of 0.005 μm to 0.05 μm or less. As a result, the refractive index of the high-frequency electric signal can be relatively low, and the group refractive index of light can be increased by means of the Si core 35.

[0101] For ease of description, a case where the EO layer 33, the material layer 32, and the upper cladding layer 34 in the optical modulator 1 are each formed of a single material has been described as an example. However, this example can be modified appropriately, for example, for each of these layers, a combination of a plurality of materials having a dielectric constant, a refractive index, and a high electro-optic coefficient within the ranges respectively considered to be preferred for these layers can be used.

[0102] The joining portion where the input waveguide of the input unit 11 connected to the optical modulator 1 is connected to the Si core 35, or the joining portion where the output waveguide of the output unit 15 connected to the optical modulator 1 is connected to the Si core 35 may include a core thickness transformation portion. For example, the core thickness transformation portion is a structure for transforming the thickness of the Si core 35 from a thickness of 0.05 μm to 0.10 μm to a thickness of 0.15 μm or more. The core thickness transformation portion has a tapered structure in which the core width gradually changes from the input waveguide to the Si core 35, or a tapered structure in which the core width gradually changes from the Si core 35 to the output waveguide. As a result, the coupling loss at the joining portion can be reduced.

[0103] A channel waveguide has been described as an example of the Si core 35 , but this may be modified appropriately, and, for example, a ridge waveguide may be employed instead.

[0104] (b) Second Embodiment

[0105] Fig.10 2 is a schematic cross-sectional view of the optical modulator 1 according to the second embodiment. By assigning the same reference numerals to the same components as those of the optical modulator 1 according to the first embodiment, description of the same components and their operations will be omitted. Fig.10 The portion at the cross section shown is the portion at the cross section of the first modulation unit 13A in the optical modulator 1. The optical modulator 1 according to the second embodiment is different from the optical modulator 1 according to the first embodiment in that the upper cladding layer 34A and the lower cladding layer 31 in the optical modulator 1 according to the second embodiment are made of materials different from each other.

[0106] Fig.11 is a diagram illustrating an example of the relationship between the relative dielectric constant of the upper cladding layer 34A and the electric signal loss in the optical modulator 1, and Fig.12 1 is a diagram illustrating an example of the relationship between the relative dielectric constant of the upper cladding layer 34A and the refractive index of the electrical signal in the optical modulator 1. Fig.11 Similar to the increase in the relative dielectric constant of the EO layer 33 shown, increasing the relative dielectric constant of the material of the upper cladding layer 34A will increase the electrical signal loss (high frequency loss). Fig.12 As shown, increasing the relative dielectric constant of the material of the upper cladding layer 34A increases the electrical signal refractive index (high frequency refractive index). Therefore, it is preferred to use a material with a relative dielectric constant of 15 or less as the material of the upper cladding layer 34A to keep the electrical signal loss and the electrical signal refractive index each at 10% or less of the electrical signal loss and the electrical signal refractive index when SiO2 is used as the material.

[0107] Fig.133 is a diagram illustrating an example of the relationship between the refractive index of the upper cladding layer 34A and the half-wavelength voltage Vπ in the optical modulator 1. When the refractive index of the upper cladding layer 34A is too high, the light distribution is diffused too much from the EO layer 33 to the upper cladding layer 34A, thereby reducing the light modulation efficiency in the EO layer 33 (the half-wavelength voltage Vπ increases), as shown in FIG. Fig.13 As shown. Therefore, it is preferable to use a material with a refractive index of 1.8 or less as the material of the upper cladding layer 34A. Therefore, the material of the upper cladding layer 34A is preferably, for example, MgF2, MgO, Al2O3, Y2O3, or a resin with a refractive index of 1.8 or less, which is a material other than SiO2.

[0108] The material of the upper cladding layer 34A arranged on the EO layer 33 in the optical modulator 1 according to the second embodiment has a relative permittivity of 15 or less and a refractive index of 1.8 or less. As a result, the half-wavelength voltage can be reduced, and the modulation efficiency of the optical modulator 1 can be improved while reducing the electrical signal loss and the electrical signal refractive index.

[0109] (c) Third Embodiment

[0110] Fig.14 is a schematic cross-sectional view of an example of an optical modulator 1 according to a third embodiment. By assigning the same reference numerals to the same components as those of the optical modulator 1 according to the first embodiment, descriptions of the same components and operations thereof will be omitted. The optical modulator 1 according to the third embodiment is different from the optical modulator 1 according to the first embodiment in that the optical modulator 1 according to the third embodiment has a first material layer 36 arranged between: the EO layer 33; and the upper cladding layer 34A and the electrode 22.

[0111] In terms of modulation efficiency, a structure having the electrode 22 in direct contact with the EO layer 33 like the optical modulator 1 according to the first embodiment is ideal, but in terms of manufacturing errors, for example, a layer deposited on the EO layer 33 may remain under the electrode 22 and not be completely removed. In order to solve this situation, intentionally arranging the first material layer 36 between the electrode 22 and the EO layer 33 can obtain a stable layer structure.

[0112] Fig.15 is a schematic diagram illustrating an example of the relationship between the refractive index of the first material layer 36 and the optical loss in the optical modulator 1. Using a material having a refractive index lower than that of the EO layer 33 as the first material layer 36 prevents the light distribution from extending from the EO layer 33 to the electrode 22, thereby being able to reduce the optical loss caused by the light absorption in the electrode 22. This effect of reducing the optical loss depends on the refractive index of the first material layer 36, such as Fig.15As shown, when the first material layer 36 has a refractive index of 1.8 or less and the EO layer 33 has a refractive index of about 2.2, light loss can be reduced. Therefore, in terms of reducing light loss, it is preferred to use a material with a refractive index of 1.8 or less (such as SiO2, MgF2, Al2O3 or MgO) as the material of the first material layer 36.

[0113] Fig.16 is a diagram illustrating an example of the relationship between the dielectric constant of the first material layer 36 and the half-wavelength voltage Vπ in the optical modulator 1. In the optical modulator 1 according to the third embodiment, a high-frequency electric signal can also be applied to the first material layer 36 arranged between the electrode 22 and the EO layer 33. Specifically, in the case where a material having a low dielectric constant is arranged as the first material layer 36 between the electrode 22 and the EO layer 33, the voltage applied to the first material layer 36 becomes relatively high, and thus the electric field applied to the EO layer 33 is weakened. As a result, as Fig.16 As shown, the half-wavelength voltage Vπ increases. Therefore, it is preferable to use a material having a relatively high dielectric constant of 8 or more for the first material layer 36 to minimize the increase in the half-wavelength voltage Vπ, for example, a material such as MgO, HfO2, Ta2O5, ZrO2 or Nb2O5 is preferably used.

[0114] That is, the first material layer 36 in the optical modulator 1 according to the third embodiment is formed of at least any one of a material having a refractive index of 1.8 or less and a material having a relative dielectric constant of 8 or more.

[0115] In the optical modulator 1 according to the third embodiment, the first material layer 36 is arranged between the electrode 22 and the EO layer 33 and has a refractive index of 1.8 or less. As a result, the first material layer 36 can reduce light distribution extending from the EO layer 33 to the electrode 22 and reduce light loss.

[0116] The first material layer 36 is disposed between the electrode 22 and the EO layer 33 in the optical modulator 1 and has a relative dielectric constant of at least 8 or more. As a result, the electric field applied to the EO layer 33 is strengthened, thereby minimizing the increase in the half-wavelength voltage Vπ.

[0117] (d) Fourth Embodiment

[0118] Fig.17 1 is a schematic plan view of a configuration example of an IQ optical modulator 100 according to a fourth embodiment. By assigning the same reference numerals to the same components as those of the optical modulator 1 according to the first embodiment, description of the same components and their operations will be omitted. Fig.17The IQ optical modulator 100 shown has a first optical splitter 61, an optical modulator 1A1 for an in-phase (I) component, an optical modulator 1A2 for a quadrature (Q) component, two first direct current phase shifters (DCPS) 51, and a first multiplexer 62. One of the first DCPSs 51 is a phase adjustment element that adjusts the phase of the signal light of the I component from the optical modulator 1A1 of the I component. The other of the first DCPSs 51 is a phase adjustment element that adjusts the phase of the signal light of the Q component from the optical modulator 1A2 of the Q component.

[0119] The optical modulator 1A1 of the I component performs phase modulation of the optical signal of the I component. The optical modulator 1A2 of the Q component performs phase modulation of the optical signal of the Q component. The optical modulator 1A1 of the I component has a beam splitter 12, two modulation units 13A and 13B (13), two second DCPSs 52, and a multiplexer 14. Each of the second DCPSs 52 is a phase adjustment element that adjusts the phase of the signal light of the I component from the modulation unit 13. The optical modulator 1A2 of the Q component has a beam splitter 12, two modulation units 13A and 13B (13), two second DCPSs 52, and a multiplexer 14. Each of the second DCPSs 52 is a phase adjustment element that adjusts the phase of the signal light of the Q component from the modulation unit 13.

[0120] The first optical splitter 61 splits the signal light from the input waveguide and outputs the split signal light to the optical modulators 1A1 and 1A2 (1) respectively. The optical modulator 1A1 of the I component outputs the signal light of the I component from the multiplexer 14 in the optical modulator 1A1, which has been phase modulated, to the first DCPS 51. The first DCPS 51 shifts the phase modulated signal light of the I component and outputs the phase shifted signal light of the I component to the first multiplexer 62.

[0121] The optical modulator 1A2 of the Q component outputs the signal light of the Q component which has been phase modulated from the multiplexer 14 in the optical modulator 1A2 to the first DCPS 51. The first DCPS 51 shifts the phase of the phase modulated signal light of the Q component, and outputs the phase-shifted signal light of the Q component to the first multiplexer 62. The first multiplexer 62 multiplexes the signal light of the I component and the signal light of the Q component together, and outputs the multiplexed signal light of the I component and the Q component to the output waveguide.

[0122] Fig.18 is along Fig.17 Schematic cross-section along line BB. Fig.18 The portion at the cross section shown is the portion at the cross section of the optical modulator 1A1 of the I component. Fig.18As shown, the portion of the modulation unit 13 at the cross section of the optical modulator 1A1 of the I component has a Si substrate 41 , a lower cladding layer 31 , a thin Si core 35 , a material layer 32 , an EO layer 33 , and an upper cladding layer 34 .

[0123] The portion of the second DCPS 52 at the cross section of the optical modulator 1A1 of the I component has a Si substrate 41, a lower cladding layer 31, a thin Si core 35, an upper cladding layer 34, a thermo-optical heater 52B, and a metal wiring 52A. The thermo-optical heater 52B is a resistor, such as TiN, arranged at a position near the thin Si core 35 as the first modulation waveguide 21A. The metal wiring 52A is a wiring that is electrically connected to the thermo-optical heater 52B and supplies current to the thermo-optical heater 52B. Heat is generated in the thermo-optical heater 52B by the current flowing from the metal wiring 52A to the thermo-optical heater 52B. In the second DCPS 52, the light passing through the first modulation waveguide 21A is phase-shifted by changing the refractive index of silicon in the first modulation waveguide 21A, and the change is derived from the heat in the thermo-optical heater 52B.

[0124] Ferroelectric EO material is used only for the portion of the modulation unit 13 located at the cross section, and the optical splitter 12 and the multiplexer 14 are formed using silicon photonics technology, so the optical splitter and the multiplexer and the DCPS can be arranged in a small area. The optical modulator element that can be realized is small in size, can operate efficiently and at high speed, and is highly integrated and powerful.

[0125] A case has been described as an example in which the optical splitter 12 and the multiplexer 14 included in the optical modulator 1 are also formed using a hybrid waveguide containing a ferroelectric EO material and a thin Si core 35. However, in the case where the optical modulator 1 is mounted on a silicon photonic element, forming the optical splitter 12 and the multiplexer 14 included in the optical modulator 1 using a silicon photonic waveguide can be reduced in size by utilizing the characteristics of silicon photonics. In addition, forming portions of the two waveguides included in the optical modulator 1 at the silicon photonic element makes it possible to realize a DCPS with a small size and low power consumption by realizing a phase shifter for appropriately adjusting the phase in the optical modulator 1 with the aid of a heater formed on the waveguide. The DCPS using a heater can be used not only for phase adjustment in the optical modulator 1, but also for phase adjustment between the I channel and the Q channel in the IQ optical modulator 100.

[0126] The thin Si core 35 in the optical modulator 1A1 (1A2) has a core thickness of, for example, 0.07 μm, and the Si waveguide 35A connecting the optical splitter 12 and the multiplexer 14 to each other has a core thickness of, for example, 0.22 μm, and a core width of, for example, 0.5 μm. Therefore, at the optical junction between the Si waveguide 35A connected to the optical splitter 12 and the thin Si core 35, and at the optical junction between the Si waveguide 35A connected to the multiplexer 14 and the thin Si core 35, a connection conversion waveguide is arranged to achieve low connection loss because of the difference in core thickness therebetween.

[0127] The IQ optical modulator 100 according to the fourth embodiment has an optical modulator 1A1 (1) of an I component and an optical modulator 1A2 (1) of a Q component built therein. The optical modulator 1 has an EO layer 33 including an electro-optical material and a material layer 32 arranged below the EO layer 33 and having a lower dielectric constant than the EO layer 33. In addition, for example, the optical modulator 1 has: a thin Si core 35 arranged below the material layer 32 and having a higher refractive index (for example, a refractive index of 3.45) than the refractive indexes of the EO layer 33 and the material layer 32; and an electrode 22 for applying an electrical signal to the EO layer 33. The refractive index of the material layer 32 is 0.85 times or more of the refractive index of the EO layer 33. That is, the refractive index of the electrical signal is made relatively low, and the group refractive index of the light is increased by means of the thin Si core 35, so that the refractive index of the electrical signal and the group refractive index of the light are closer to each other. As a result, the propagation speed of the electric signal and the propagation speed of light in the optical modulator 1 become substantially the same, thereby improving the speed mismatch in the IQ optical modulator 100. Therefore, it is possible to prevent the operating band of the IQ optical modulator 100 from being limited. More importantly, while keeping the operating length long, the speed mismatch in the IQ optical modulator 100 is improved, thereby reducing the half-wavelength voltage Vπ, and thus improving the modulation efficiency of the IQ optical modulator 100.

[0128] The case in which the optical modulator 1 according to the first embodiment is built into the IQ optical modulator 100 according to the fourth embodiment has been described as an example, but this example can be modified appropriately and the optical modulator 1 according to the second embodiment or the third embodiment can be used instead.

[0129] (e) Fifth Embodiment

[0130] Fig.19 1 is a schematic plan view of a configuration example of a DP-IQ optical modulator 110 according to the fifth embodiment. Fig.17 The same components of the illustrated IQ optical modulator 100 are assigned the same reference numerals, and description of the same components and their operations will be omitted. Fig.19The DP-IQ optical modulator 110 shown is formed by arranging four optical modulators 1 in parallel with each other. The DP-IQ optical modulator 110 has a second beam splitter 63, an IQ optical modulator 100A (100) for an X polarization component, an IQ optical modulator 100B (100) for a Y polarization component, a polarization rotator (PR) 64, and a polarization beam combiner (PBC) 65.

[0131] The second optical splitter 63 splits the input light from the input waveguide and outputs the split signal lights to the IQ optical modulator 100. The IQ optical modulator 100A (100) for the X polarization component has an optical modulator 1A1 for the I component of the X polarization component and an optical modulator 1A2 for the Q component of the X polarization component.

[0132] Each of the IQ optical modulators 100 has a first optical splitter 61 , an optical modulator 1A1 for an I component, an optical modulator 1A2 for a Q component, two first DCPSs 51 , and a first multiplexer 62 .

[0133] The first optical splitter 61 splits the signal light from the second optical splitter 63, and outputs the split signal light to the optical modulators 1A1 and 1A2, respectively. The optical modulator 1A1 of the I component outputs the signal light of the I component from the multiplexer 14 in the optical modulator 1A1, which has been phase modulated, to the first DCPS 51. The first DCPS 51 shifts the phase modulated signal light of the I component, and outputs the phase shifted signal light of the I component to the first multiplexer 62.

[0134] The optical modulator 1A2 of the Q component outputs the signal light of the Q component which has been phase modulated from the multiplexer 14 in the optical modulator 1A2 to the first DCPS 51. The first DCPS 51 shifts the phase of the phase modulated signal light of the Q component, and outputs the phase shifted signal light of the Q component to the first multiplexer 62. The first multiplexer 62 multiplexes the signal light of the I component and the signal light of the Q component together, and outputs the multiplexed signal light of the I component and the Q component to the PBC 65.

[0135] The first multiplexer 62 in the IQ optical modulator 100A for the X polarization component multiplexes the signal light of the I component of the X polarization component from the multiplexer 14 in the optical modulator 1A1 for the I component and the signal light of the Q component of the X polarization component from the multiplexer 14 in the optical modulator 1A2 for the Q component. The first multiplexer 62 outputs the signal lights of the I and Q components of the X polarization component to the PBC 65.

[0136] The first multiplexer 62 in the IQ optical modulator 100B for Y polarization multiplexes the signal light of the I component of the Y polarization component from the multiplexer 14 in the optical modulator 1A1 of the I component and the signal light of the Q component of the Y polarization component from the multiplexer 14 in the optical modulator 1A2 of the Q component. The first multiplexer 62 outputs the signal lights of the I and Q components of the Y polarization component to the PR 64. The PR 64 polarization-rotates the signal lights of the I and Q components of the Y polarization component, and outputs the polarization-rotated signal lights of the I and Q components of the Y polarization component to the PBC 65. The PBC 65 multiplexes the signal lights of the I and Q components of the X polarization component and the polarization-rotated signal lights of the I and Q components of the Y polarization component, and outputs the multiplexed signal lights of the X and Y polarization components to the output waveguide as transmission light.

[0137] Since the ferroelectric EO material is used only for the portion at the cross section of the modulation unit 13, the second optical splitter 63 for inputting and outputting to and from the optical modulator 1, the first optical splitter 61 and the first multiplexer 62 for the I component and the Q component, and the PR 64 and the PBC 65 for polarization separation and combination are formed using silicon photonics technology. This enables arrangement in a small area. Therefore, the optical modulator element that can be realized is small in size, can operate efficiently and at high speed, and is highly integrated and powerful.

[0138] A case has been described as an example in which the optical splitter 12 and the multiplexer 14 included in the optical modulator 1 are also formed using a hybrid waveguide including a ferroelectric EO material and a Si core 35. However, in the case where the optical modulator 1 is mounted on a silicon photonic element, forming the optical splitter 12 and the multiplexer 14 included in the optical modulator 1 using a silicon photonic waveguide makes it possible to reduce the size by utilizing the characteristics of silicon photonics. Forming portions of the two waveguides included in the optical modulator 1 at the silicon photonic element makes it possible to implement a DCPS with the aid of a heater formed on the waveguide, the DCPS being used to appropriately adjust the phase in the optical modulator 1, thereby realizing a small-sized and low-power phase shifter. The DCPS using a heater can be used not only for phase adjustment in the optical modulator 1, but also for phase adjustment between the I channel and the Q channel in the IQ optical modulator 100.

[0139] The DP-IQ optical modulator 110 according to the fifth embodiment has an IQ optical modulator 100A for X polarization and an IQ optical modulator 100B for Y polarization built in. The optical modulator 1 has an EO layer 33 including an electro-optical material and a material layer 32 arranged below the EO layer 33 and having a lower dielectric constant than the EO layer 33. In addition, for example, the optical modulator 1 has: a Si core 35 arranged below the material layer 32 and having a higher refractive index (for example, a refractive index of 3.45) than the refractive indexes of the EO layer 33 and the material layer 32; and an electrode 22 for applying an electrical signal to the EO layer 33. The refractive index of the material layer 32 is 0.85 times or more of the refractive index of the EO layer 33. That is, the refractive index of the electrical signal is made relatively low and the group refractive index of the light is increased by applying the Si core 35 so that the refractive index of the electrical signal and the group refractive index of the light are closer to each other. As a result, the propagation speed of the electric signal and the propagation speed of light in the optical modulator 1 become substantially the same, thereby being able to improve the speed mismatch in the DP-IQ optical modulator 110. Therefore, it is possible to prevent the operation band of the DP-IQ optical modulator 110 from being limited. More importantly, while keeping the operation length long, the speed mismatch can be improved in the DP-IQ optical modulator 110, thereby being able to reduce the half-wavelength voltage Vπ, and the modulation efficiency of the DP-IQ optical modulator 110 can be improved.

[0140] In the structure having the optical modulator 1 mounted on the silicon photonic element, the optical modulator 1 using the ferroelectric EO material, the optical modulator 1 and the thin Si waveguide 35A of the silicon photonics can be optically connected to each other through the connection between the Si core 35 and the thin Si waveguide 35A. The appropriate width and thickness of the Si core 35 in the optical modulator 1 are different from the core width and core thickness of the thin Si waveguide 35A used in general silicon photonics, so this connection can be appropriately modified, so that a conversion waveguide for connection with less loss can be inserted.

[0141] (f) Sixth Embodiment

[0142] Fig. 20 1 is a schematic plan view of a configuration example of an optical transmitter-receiver 120 according to the sixth embodiment. Fig.19 The same components of the illustrated DP-IQ optical modulator 110 are assigned the same reference numerals, and description of the same components and their operations will be omitted. Fig. 20The optical transmitter-receiver 120 shown has an optical modulator element 120A including a DP-IQ optical modulator 110, and an optical receiver element 120B that receives a DP-QAM signal. The optical transmitter-receiver 120 has the optical modulator element 120A and the optical receiver element 120B integrated into the optical transmitter-receiver 120 by using silicon photonics technology. The optical modulator element 120A is, for example, the DP-IQ optical modulator 110. The optical receiver element 120B has a third optical splitter 67, a received light input unit 72, a polarization beam splitter (PBS) 68, a polarization rotator (PR) 69, a first optical hybrid circuit 70A (70), and a second optical hybrid circuit 70B (70). The optical receiver element 120B has four first optical receiver elements 71A (71) and four second optical receiver elements 71B (71).

[0143] The third optical splitter 67 splits the light entering from the light source (not shown in the figure) via the input waveguide, outputs one of the split lights to the second optical splitter 63 in the DP-IQ optical modulator 110, which is used as the input light source of the modulator, and outputs the other of the split lights to each hybrid circuit 70 as the local oscillator light of the receiver. The receiving light input unit 72 inputs the receiving light from the optical fiber (not shown in the figure). The PBS 68 splits the input receiving light from the receiving light input unit 72 into X-polarized receiving light and Y-polarized receiving light, outputs the X-polarized receiving light to the first optical hybrid circuit 70A, and outputs the Y-polarized receiving light to the PR 69. The PR 69 performs a 90-degree polarization rotation on the Y-polarized receiving light, and outputs the Y-polarized receiving light that has been polarized rotated to the second optical hybrid circuit 70B.

[0144] The first optical hybrid circuit 70A obtains optical signals of the I component and the Q component by interfering the local oscillator light with the X polarization component of the received light. The first optical hybrid circuit 70A outputs the optical signal of the I component, which is derived from the X polarization component, to the first optical receiver element 71A1. The first optical hybrid circuit 70A outputs the optical signal of the Q component, which is derived from the X polarization component, to the first optical receiver element 71A2.

[0145] The second optical hybrid circuit 70B obtains optical signals of the I component and the Q component by interfering the local oscillator light with the Y polarization component of the received light. The second optical hybrid circuit 70B outputs the optical signal of the I component, which is derived from the Y polarization component, to the second optical receiver element 71B1. The second optical hybrid circuit 70B outputs the optical signal of the Q component, which is derived from the Y polarization component, to the second optical receiver element 71B2.

[0146] The first optical receiver element 71A1 is, for example, a photodetector (PD), which performs electrical conversion on the optical signal of the I component of the X polarization component from the first optical hybrid circuit 70A, and outputs the electrical signal of the I component obtained by the electrical conversion. The first optical receiver element 71A2 performs electrical conversion on the optical signal of the Q component of the X polarization component from the first optical hybrid circuit 70A, and outputs the electrical signal of the Q component obtained by the electrical conversion. The second optical receiver element 71B1 is, for example, a PD, which performs electrical conversion on the optical signal of the I component of the Y polarization component from the second optical hybrid circuit 70B, and outputs the electrical signal of the I component obtained by the electrical conversion. The second optical receiver element 71B2 performs electrical conversion on the optical signal of the Q component of the Y polarization component from the second optical hybrid circuit 70B, and outputs the electrical signal of the Q component obtained by the electrical conversion.

[0147] The optical transmitter-receiver 120 according to the sixth embodiment has the optical modulator element 120A and the optical receiver element 120B including the DP-IQ optical modulator 110 mounted thereon by silicon photonics technology, and thus can be reduced in size.

[0148] The optical modulator element 120A has a DP-IQ optical modulator 110 having a plurality of optical modulators 1 built therein. Each of the optical modulators 1 has: an EO layer 33 containing an EO material; and a material layer 32 arranged below the EO layer 33 and having a lower dielectric constant than that of the EO layer 33. In addition, for example, each of the optical modulators 1 has: a Si core 35 arranged below the material layer 32 and having a higher refractive index (for example, a refractive index of 3.45) than that of the EO layer 33 and the material layer 32; and an electrode 22 for applying an electrical signal to the EO layer 33. The refractive index of the material layer 32 is 0.85 times or more of the refractive index of the EO layer 33. That is, by making the refractive index of the electrical signal relatively low and increasing the group refractive index of light by applying the Si core 35, the refractive index of the electrical signal and the group refractive index of light are made closer to each other. As a result, the propagation speed of the electric signal and the propagation speed of light in the optical modulator 1 become substantially the same, thereby improving the speed mismatch in the DP-IQ optical modulator 110. Therefore, it is possible to prevent the operation band of the DP-IQ optical modulator 110 from being limited. More importantly, while keeping the operation length long, the speed mismatch is improved in the DP-IQ optical modulator 110, thereby reducing the half-wavelength voltage Vπ, and thus the modulation efficiency of the DP-IQ optical modulator 110 can be improved.

[0149] The following description is about an optical transceiver 130 in which the optical modulator 1 according to the embodiment is employed. Fig.21 is a diagram illustrating an example of the optical transceiver 130 . Fig.21The optical transceiver 130 shown has an LD 131, a transmitter-receiver module 132, and a digital signal processor (DSP) 133. The LD 131 is, for example, a light source that emits laser light. The transmitter-receiver module 132 has an optical transmitter-receiver 120, a driver circuit (DRV) 121, and a transimpedance amplifier (TIA) 122. The optical transmitter-receiver 120 has an optical modulator element 120A and an optical receiver element 120B. For example, the optical modulator element 120A is a DP-IQ optical modulator 110A. The DSP 133 controls the entire optical transmitter-receiver 120. The DSP 133 is an electronic component that performs digital signal processing such as IQ modulation processing of a transmission signal and demodulation processing of a reception signal.

[0150] For example, the DSP 133 performs processing such as encoding of transmission data, generates an electrical signal containing the transmission data, and outputs the generated electrical signal to the driver circuit 121. The driver circuit 121 drives the optical modulator element 120A according to the electrical signal from the DSP 133.

[0151] The optical receiver element 120B performs electrical conversion of the signal light. The TIA 122 amplifies the electrical signal obtained by the electrical conversion, and outputs the amplified electrical signal to the DSP 133. The DSP 133 obtains reception data by performing processing such as decoding the electrical signal obtained from the TIA 122.

[0152] For convenience of description, a case in which the optical modulator element 120A and the optical receiver element 120B are built in the optical transceiver 130 has been described as an example, but the present embodiment is also applicable to an optical transmitting and receiving device in which only the optical modulator element 120A is built.

[0153] Since the optical modulator 1 in the optical modulator element 120A is highly efficient, the optical transceiver 130 can operate at low power consumption by reducing the output amplitude of the driver circuit 121, and since the optical transceiver 130 operates at high speed, large-capacity optical signal transmission is enabled. As a result, a high-capacity and low-power optical communication module can be realized.

[0154] The components of each unit shown in the drawings may not be physically configured as shown in the drawings. That is, the specific forms of separation and integration of each unit are not limited to those shown in the drawings, for example, all or part of each unit may be configured to be functionally or physically separated or integrated in any unit according to various loads and usage conditions.

[0155] In addition, all or any part of the various processing functions implemented by each device may be implemented on a central processing unit (CPU) (or a microcomputer such as a microprocessing unit (MPU) or a microcontroller unit (MCU)). In addition, all or any part of the various processing functions may of course be implemented on a program analyzed and executed by the CPU (or a microcomputer such as an MPU or an MCU), or implemented on hardware through wired logic.

[0156] According to one embodiment of the optical modulator disclosed by the present application, for example, an optical modulator capable of improving the modulation efficiency thereof while ensuring the operating frequency band of the optical modulator is provided.

Claims

1. An optical modulator, comprising: an electro-optic layer, the electro-optic layer comprising an electro-optic material; a material layer disposed below the electro-optic layer and having a dielectric constant lower than that of the electro-optic layer; a core layer, which is arranged below the material layer and has a refractive index higher than the refractive indexes of the electro-optical layer and the material layer; as well as an electrode that applies an electrical signal to the electro-optical layer, Wherein, the refractive index of the material layer is 0.85 times or higher than the refractive index of the electro-optical layer.

2. The optical modulator according to claim 1, wherein The electro-optic layer has an electro-optic coefficient higher than that of the material layer.

3. The optical modulator according to claim 1, wherein: The electro-optic layer has a relative dielectric constant higher than 50, and the electro-optic layer has an electro-optic coefficient higher than 50 pm / V, and The material layer has a relative dielectric constant equal to or less than 50, and the material layer has an electro-optic coefficient equal to or less than 50 pm / V.

4. The optical modulator according to claim 1, wherein: The electro-optic layer includes BaTiO3 (BTO), PbLaZrTiO3 (PLZT) or PbZrTiO3 (PZT) or any combination thereof.

5. The optical modulator according to claim 1, wherein: The material layer includes Ta2O5 or Si or a combination thereof.

6. The optical modulator according to claim 1, wherein: The electro-optic layer has a thickness in the range of 0.1 μm to 0.3 μm, the core layer has a thickness in the range of 0.05 μm to 0.10 μm, and the material layer has a thickness of 0.005 μm to 0.05 μm or less.

7. The optical modulator according to claim 1, wherein: An input waveguide connected to the input port of the optical modulator is connected to a joining portion of the core layer, or an output waveguide connected to the output port of the optical modulator is connected to a joining portion of the core layer and has a core thickness converter that converts the thickness of the core layer from 0.05 μm to 0.10 μm to 0.15 μm or greater.

8. The optical modulator according to claim 1, further comprising: A cladding layer is disposed on at least a portion of the electro-optical layer, the portion being located on the core layer, wherein the cladding layer has a relative dielectric constant of 15 or less and a refractive index of 1.8 or less.

9. The optical modulator according to claim 1, further comprising: a cladding layer, the cladding layer being arranged on at least a portion of the electro-optical layer, the portion being located on the core layer, Wherein, the coating layer includes SiO2, MgF2, MgO, Al2O3 or resin or any combination thereof, and the resin has a refractive index of 1.8 or less.

10. The optical modulator according to claim 1, further comprising: a first material layer, the first material layer being arranged between the electrode and the electro-optical layer, Wherein, the first material layer has a refractive index of 1.8 or less.

11. The optical modulator according to claim 10, wherein: The first material layer includes SiO2, MgF2, Al2O3 or MgO or any combination thereof.

12. The optical modulator according to claim 1, further comprising: a first material layer, the first material layer being arranged between the electrode and the electro-optical layer, Wherein, the first material layer has a relative dielectric constant of at least 8 or higher.

13. The optical modulator according to claim 12, wherein: The first material layer includes MgO, HfO2, Ta2O5, ZrO2 or Nb2O5 or any combination thereof.

14. The optical modulator according to claim 1, wherein: The optical modulator has two optical waveguides serving as the core layer, an electrode for applying an electrical signal to each of the two optical waveguides, a splitter for splitting the light into the two optical waveguides, and a multiplexer for multiplexing the light from the two optical waveguides together, and the optical modulator modulates the light guided via the two optical waveguides according to the electrical signal.

15. An optical transmitter-receiver, the optical transmitter-receiver comprising: a light modulator element that modulates the guided light according to an electrical signal; as well as a light receiving element that converts the received light into an electrical signal, in, The light modulator element comprises: an electro-optic layer, the electro-optic layer comprising an electro-optic material; a material layer disposed below the electro-optic layer and having a dielectric constant lower than that of the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than those of the electro-optical layer and the material layer; and an electrode that applies an electrical signal to the electro-optical layer, Wherein, the refractive index of the material layer is 0.85 times or higher than the refractive index of the electro-optical layer.

16. The optical transmitter-receiver according to claim 15, further comprising: A driver that drives the light modulation element; as well as An amplifier amplifies the electrical signal from the light receiving element.

17. An optical transceiver, comprising: a light modulator element that modulates the guided light according to an electrical signal; a light receiver element that converts received reception light into an electrical signal; as well as a signal processor that generates the electrical signal to the light modulator element and obtains the electrical signal from the light receiver element, in, The light modulator element comprises: an electro-optic layer, the electro-optic layer comprising an electro-optic material; a material layer disposed below the electro-optic layer and having a dielectric constant lower than that of the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than those of the electro-optical layer and the material layer; and an electrode that applies an electrical signal to the electro-optical layer, Wherein, the refractive index of the material layer is 0.85 times or higher than the refractive index of the electro-optical layer.

Images