Optical modulator, optical sending-receiving device, and optical transceiver

Abstract

To provide an optical modulator or the like which can improve the efficiency of modulation while securing the operational bandwidth of an optical modulator.SOLUTION: The optical modulator includes: an electro-optical layer including an electro-optical material; and a material layer arranged below the electro-optical layer, the material layer having a lower permittivity than that of the electro-optical layer. The optical modulator includes: a core layer arranged below the material layer, the core layer having a higher index of refraction than that of the electro-optical layer and the material layer; and an electrode for applying an electric signal to the electro-optical layer. The index of refraction is at least 0.85 times larger than the index of refraction of the electro-optical layer.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an optical modulator, an optical transmitter / receiver, and an optical transceiver. [Background technology]

[0002] In recent years, with the increase in capacity of optical communication systems, there is a demand for optical transmitters and receivers with higher communication speeds and lower power consumption. In particular, there is a strong demand for highly efficient optical modulators in optical transmitters. As a means of realizing a small and highly efficient optical modulator, for example, PbLaZrTiO 3 (PLZT) and BaTiO 3 There are Mach-Zehnder (MZ) optical modulators using ferroelectric electro-optical materials such as BTO. For example, the electro-optic coefficient of PLZT and BTO is several hundred pm / V, so that materials such as LiNbO 3 This is significantly higher than the electro-optic coefficient of 31 pm / V of the optical modulator made of (LN: Lithium Niobate). Therefore, the electro-optic coefficients of PLZT, BTO, etc. are suitable as materials for small, highly efficient optical modulators.

[0003] Fig. 22 is a schematic plan view showing an example of a conventional optical modulator 200. The optical modulator 200 shown in Fig. 22 includes an input section 201, a branching section 202, two modulation sections 203 arranged in parallel, a multiplexing section 204, and an output section 205. The optical modulator 200 is a Mach-Zehnder type optical modulator that modulates the intensity and phase of an optical signal output from an output section by optically phase modulating the signal light passing through the modulation section after optical branching from the input section 201 in response to an electrical signal. The branching section 202 is a coupler that optically branches the signal light from the input section 201 and outputs the optically branched signal light to each modulation section 203.

[0004] Each modulation section 203 has a modulation waveguide 211 and an electrode 212. 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 section 203A has a first ground electrode 212B, a signal electrode 212A, and a first modulation waveguide 211A disposed between the first ground electrode 212B and the signal electrode 212A. The second modulation section 203B has a second ground electrode 212C, a signal electrode 212A, and a second modulation waveguide 211B disposed between the second ground electrode 212C and the signal electrode 212A.

[0005] Fig. 23 is a schematic cross-sectional view taken along the line CC shown in Fig. 22. The cross-sectional portion shown in Fig. 23 is a cross-sectional portion of the first modulating section 203A of the optical modulator 200. The first modulating section 203A has a Si substrate (not shown), a lower cladding layer 221, an EO layer 222, an upper cladding layer 223, a signal electrode 212A, and a first ground electrode 212B. The lower cladding layer 221 is a SiO 2 The EO layer 222 is a buffer layer such as SiO 2 . The EO layer 222 is a layer of a ferroelectric EO material having a high electro-optic coefficient, such as BTO or PLZT. The EO layer 222 is processed into, for example, a rib-type cross-sectional structure so that the signal light is guided, and serves as a core layer of the first modulation waveguide 211A. The upper clad layer 223 is disposed on the EO layer 222 and covers an upper portion of the EO layer 222 in the vicinity of the core 211A1 of the rib-type first modulation waveguide 211A. 2 etc. The core 211A1 of the first modulation waveguide 211A is disposed in parallel between the first ground electrode 212B and the signal electrode 212A.

[0006] In the first modulation section 203A, an electric field is generated from the signal electrode 212A to the first ground electrode 212B in response to a high-frequency electric signal from the signal electrode 212A, and the optical refractive index of the first modulation waveguide 211A is changed in response to the electric field. The first modulation section 203A modulates the phase of the signal light guided through the first modulation waveguide 211A in response to the change in the optical refractive index. The first modulation section 203A outputs the modulated signal light to the multiplexing section 204.

[0007] In addition, in the second modulation section 203B, an electric field is generated from the signal electrode 212A to the second ground electrode 212C in response to the high-frequency electric signal from the signal electrode 212A, and the optical refractive index of the second modulation waveguide 211B is changed in response to the electric field. The second modulation section 203B modulates the phase of the signal light guided through the second modulation waveguide 211B in response to the change in the optical refractive index. The second modulation waveguide 203B outputs the modulated signal light to the multiplexing section 204.

[0008] The multiplexer 204 multiplexes the modulated signal light from the first modulator 203A and the modulated signal light from the second modulator 203B, and outputs the multiplexed signal light to the output unit 205. In the multiplexer, interference occurs according to the phase states of the first modulator and the second modulator, and as a result, the output unit outputs a signal light whose intensity and phase are modulated. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 63-049732 [Patent Document 2] Patent Publication No. 2022-032687 [Patent Document 3] US Patent No. 07283689 [Patent Document 4] JP 2007-333756 A Summary of the Invention [Problem to be solved by the invention]

[0010] Next, the dielectric constant dependency of the propagation characteristics of the electric signal in the optical modulator 200 will be described. Fig. 24 is an explanatory diagram showing an example of the relationship between the relative dielectric constant, which is a physical constant determined by the material of the EO layer 222, and the characteristic impedance. Fig. 25 is an explanatory diagram showing an example of the relationship between the relative dielectric constant of the EO layer 222 and the electric signal loss. Fig. 26 is an explanatory diagram showing an example of the relationship between the relative dielectric constant of the EO layer 222 and the electric signal refractive index. The ferroelectric EO material used for the EO layer 222 of the modulation waveguide 211 has a high relative dielectric constant of several hundred.

[0011] That is, the relative dielectric constant of the EO layer 222 of PLZT, BTO, etc. is significantly higher than that of LN used in the waveguide of the conventional optical modulator. As a result, the characteristic impedance of the optical modulator 200 using the EO layer 222 of PLZT, BTO, etc. is lower than that of the optical modulator using the EO layer of LN, as shown in Fig. 24. Also, the electrical signal loss of the optical modulator 200 using the EO layer 222 of PLZT, BTO, etc. is higher than that of the optical modulator using the EO layer of LN, as shown in Fig. 25. Furthermore, the electrical signal refractive index of the optical modulator 200 using the EO layer 222 of PLZT, BTO, etc. is higher than that of the optical modulator using the EO layer of LN, as shown in Fig. 26.

[0012] Among these, an increase in the refractive index of the electric signal in the optical modulator 200 using the EO layer 222 such as PLZT or BTO leads to a velocity mismatch due to a difference between the propagation velocity of light and the propagation velocity of the electric signal.

[0013] Specifically, the group refractive index that determines the propagation speed of light guided through an optical waveguide using an EO material such as BTO or PLZT is, for example, within the range of 2 to 2.5. In contrast, the electrical signal refractive index that determines 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 the relative dielectric constant is very high, at several hundred. Then, a large deviation between the propagation speed of light and the propagation speed of the electrical signal occurs, resulting in a velocity mismatch. As a result, in the optical modulator 200, for example, the 3 dB band of the optical response of the optical modulator 200 is significantly limited.

[0014] Also, when velocity mismatching occurs, it is possible to suppress the effect of velocity mismatching by shortening the waveguide length of the optical modulator 200. However, in this case, the working length of the optical modulator 200 is shortened, and the half-wave voltage Vπ required to change the phase of the optical modulator 200 by π increases, deteriorating the modulation efficiency of the optical modulator 200. In other words, when velocity mismatching occurs, the effect of improving the modulation efficiency by using a material with a high electro-optic coefficient cannot be obtained.

[0015] Therefore, when an EO material having a high dielectric constant such as PLZT or BTO is used for the core 211A1 of the optical modulator 200, it is difficult to ensure both a high 3 dB bandwidth of the optical modulator and high modulation efficiency (low Vπ).

[0016] The disclosed technique aims to provide an optical modulator etc. that improves modulation efficiency while ensuring the operating band of the optical modulator. [Means for solving the problem]

[0017] In one embodiment, the optical modulator disclosed in the present application includes an electro-optic layer including an electro-optic material, and a material layer disposed below the electro-optic layer and having a dielectric constant lower than that of the electro-optic layer. The optical modulator further includes a core layer disposed below the material layer and having a refractive index higher than that of the electro-optic layer and the material layer, and an electrode for applying an electric signal to the electro-optic layer. The refractive index of the material layer is 0.85 times or more that of the electro-optic layer. Effect of the Invention

[0018] According to one aspect of the optical modulator etc. disclosed in the present application, an optical modulator etc. that improves modulation efficiency while ensuring the operating band of the optical modulator is provided. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic plan view illustrating an example of an optical modulator according to a first embodiment. [Diagram 2] FIG. 2 is a schematic cross-sectional view taken along line AA shown in FIG. [Diagram 3] FIG. 3 is an explanatory diagram showing an example of the relationship between the thickness of the EO layer of an optical modulator and the refractive index of an electric signal. [Figure 4] FIG. 4 is an explanatory diagram showing an example of the relationship between the thickness of the Si core of the optical modulator and the light group refractive index. [Diagram 5] FIG. 5 is an explanatory diagram showing an example of the relationship between the thickness of a material layer of an optical modulator and the half-wave voltage Vπ. [Figure 6] FIG. 6 is an explanatory diagram showing an example of the relationship between the thickness of a material layer of an optical modulator and the light group refractive index. [Figure 7A] FIG. 7A is an explanatory diagram showing an example of an SOI substrate used for a Si core substrate. [Figure 7B] FIG. 7B is an explanatory diagram showing an example of a step of thinning the Si film of the SOI substrate. [Figure 7C] FIG. 7C is an explanatory diagram showing an example of a Si patterning step. [Figure 7D] FIG. 7D is an explanatory diagram showing an example of a SiO2 film forming step. [Figure 7E] FIG. 7E is an explanatory diagram showing an example of a Si core substrate after surface planarization polishing. [Figure 8A] FIG. 8A is an explanatory diagram showing an example of an EO substrate used for the EO thin film substrate. [Figure 8B] FIG. 8B is an explanatory diagram showing an example of the first bonding step. [Figure 8C] FIG. 8C is an explanatory view showing an example of the first removal step. [Figure 8D] FIG. 8D is an explanatory diagram showing an example of an EO thin film substrate. [Figure 9A] FIG. 9A is an explanatory diagram showing an example of the second bonding step. [Figure 9B] FIG. 9B is an explanatory view showing an example of the second removal step. [Figure 9C] FIG. 9C is an explanatory diagram showing an example of the upper cladding layer forming step. [Figure 9D]FIG. 9D is an explanatory diagram showing an example of a metal film forming step. [Figure 9E] FIG. 9E is an explanatory diagram showing an example of a metal patterning step. [Figure 10] FIG. 10 is a schematic cross-sectional view illustrating an example of an optical modulator according to a second embodiment. [Figure 11] FIG. 11 is an explanatory diagram showing an example of the relationship between the relative dielectric constant of the upper cladding layer of an optical modulator and the electrical signal loss. [Figure 12] FIG. 12 is an explanatory diagram showing an example of the relationship between the relative dielectric constant of the upper cladding layer of an optical modulator and the refractive index of an electric signal. [Figure 13] FIG. 13 is an explanatory diagram showing an example of the relationship between the refractive index of the upper cladding layer of the optical modulator and the half-wave voltage Vπ. [Figure 14] FIG. 14 is a schematic cross-sectional view illustrating an example of an optical modulator according to a third embodiment. [Figure 15] FIG. 15 is an explanatory diagram showing an example of the relationship between the refractive index and optical loss of the first material layer of the optical modulator. [Figure 16] FIG. 16 is an explanatory diagram showing an example of the relationship between the dielectric constant of the first material layer of the optical modulator and the half-wave voltage Vπ. [Figure 17] FIG. 17 is a schematic plan view illustrating an example of an IQ optical modulator according to the fourth embodiment. [Figure 18] FIG. 18 is a schematic cross-sectional view taken along line BB shown in FIG. [Figure 19] FIG. 19 is a schematic plan view illustrating an example of a DP-IQ optical modulator according to a fifth embodiment. [Figure 20] FIG. 20 is a schematic plan view illustrating an example of an optical transceiver according to the sixth embodiment. [Figure 21] FIG. 21 is an explanatory diagram illustrating an example of an optical transceiver. [Figure 22] FIG. 22 is a schematic plan view showing an example of a conventional optical modulator. [Diagram 23] FIG. 23 is a schematic cross-sectional view taken along line CC shown in FIG. [Figure 24]FIG. 24 is an explanatory diagram showing an example of the relationship between the relative dielectric constant, which is a physical constant determined by the material of the EO layer, and the characteristic impedance. [Diagram 25] FIG. 25 is a graph showing an example of the relationship between the relative dielectric constant of an EO layer and electrical signal loss. [Figure 26] FIG. 26 is a diagram illustrating an example of the relationship between the relative dielectric constant of an EO layer and the refractive index of an electric signal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of an optical modulator and the like disclosed in the present application will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiment. EXAMPLES

[0021] FIG. 1 is a schematic plan view showing an example of an optical modulator 1 according to a first embodiment. The optical modulator 1 includes an input section 11, a branching section 12, two modulation sections 13, a multiplexing section 14, and an output section 15. The optical modulator 1 is a Mach-Zehnder type optical modulator that optically modulates the optically branched signal light from the input section 11 in response to an electrical signal. The input section 11 is connected to an input waveguide and is a section that inputs the signal light from the input waveguide to the optical modulator 1. The output section 15 is connected to an output waveguide and is a section that outputs the modulated signal light from the optical modulator 1 to the output waveguide. The branching section 12 is, for example, a 1×2 MMI coupler that optically branches the signal light from the input section 11 and outputs the optically branched signal light to each modulation section 13. The multiplexer 14 multiplexes the modulated signal light from each modulator 13 and outputs the multiplexed signal light to the output unit 15, and is, for example, a 2×1 MMI coupler.

[0022] The modulation section 13 has a first modulation section 13A and a second modulation section 13B. The modulation section 13 has a modulation waveguide 21 and an electrode 22. The modulation waveguide 21 is, for example, a waveguide with a Si core. The modulation waveguide 21 has a first modulation waveguide 21A and a second modulation waveguide 21B. The electrode 22 has a GSG configuration having a signal electrode 22A, a first ground electrode 22B, and a second ground electrode 22C.

[0023] The first modulation section 13A has a first modulation waveguide 21A, a signal electrode 22A, and a first ground electrode 22B, and the first modulation waveguide 21A is disposed between the first ground electrode 22B and the signal electrode 22A. In the first modulation section 13A, an electric field is generated from the signal electrode 22A to the first ground electrode 22B in response to a high-frequency electric signal from the signal electrode 22A, and the optical refractive index of the first modulation waveguide 21A is changed in response to the electric field. The first modulation section 13A phase-modulates the signal light guided through the first modulation waveguide 21A in response to the change in the optical refractive index. The first modulation section 13A outputs the modulated signal light to the multiplexing section 14.

[0024] The second modulation section 13B has a second modulation waveguide 21B, a signal electrode 22A, and a second ground electrode 22C, and the second modulation waveguide 21B is disposed between the second ground electrode 22C and the signal electrode 22A. In the second modulation section 13B, an electric field is generated from the signal electrode 22A to the second ground electrode 22C in response to a high-frequency electric signal from the signal electrode 22A, and the optical refractive index of the second modulation waveguide 21B is changed in response to the electric field. The second modulation section 13B modulates the signal light guided through the second modulation waveguide 21B in response to the change in the optical refractive index. The second modulation section 13B outputs the modulated signal light to the multiplexing section 14.

[0025] The multiplexer 14 multiplexes the modulated signal light from the first modulator 13 A and the modulated signal light from the second modulator 13 B, and outputs the multiplexed signal light to the output unit 15 .

[0026] Fig. 2 is a schematic cross-sectional view taken along the line AA shown in Fig. 1. The cross-sectional portion shown in Fig. 2 is a cross-sectional portion of the first modulation section 13A shown in Fig. 1. The first modulation section 13A has a Si substrate (not shown), a lower clad layer 31, a Si core 35, a material layer 32, an EO layer 33, an upper clad layer 34, a first ground electrode 22B, and a signal electrode 22A. The lower clad layer 31 is a layer made of, for example, SiO 2The lower cladding layer 31 is a cladding layer of the first modulation waveguide 21A. The Si core 35 is a core layer of an optical waveguide formed in the lower cladding layer 31, and is, for example, the core of the first modulation waveguide 21A. The Si core 35 is disposed in contact with the lower side of the material layer 32 located below the EO layer 33.

[0027] A material layer 32 is laminated on the surface of the lower cladding layer 31 and the Si core 35. For example, Ta 2 O 5 The EO layer 33 is an electro-optic layer of an EO material such as BTO that is laminated on the surface of the material layer 32. The upper clad layer 34 is disposed near the Si core 35 and covers the surface of the EO layer 33. The upper clad layer 34 is a layer of, for example, SiO 2 The first ground electrode 22B and the signal electrode 22A are disposed on the EO layer 33, sandwiching the Si core 35. The EO layer 33 and the material layer 32 have a flat plate structure.

[0028] The EO material used for the EO layer 33, for example, BTO, has an electro-optic coefficient of 100 to 400 pm / V, a relative dielectric constant of 150 or more, and a refractive index of about 2.2, although this depends on the formation method. 2 O 5 The dielectric constant of the EO layer 33 is 28, and the refractive index is about 2.05. The thickness of the EO layer 33 used in the optical modulator 1 is 0.15 μm, the thickness of the material layer 32 is 0.02 μm, the width of the Si core 35 is 0.8 μm, the thickness of the Si core 35 is 0.07 μm, and the distance between the electrodes 22 sandwiching the Si core 35 is 4 μm. The material layer 32 is made of a material having a lower dielectric constant than the EO layer 33. The Si core 35 is made of silicon having a higher refractive index than the refractive indexes of the EO layer 33 and the material layer 32. The refractive index of the material layer 32 is 0.85 times or more the refractive index of the EO layer 33.

[0029] FIG. 3 is an explanatory diagram showing an example of the relationship between the thickness of the EO layer 33 of the optical modulator 1 and the refractive index of the electric signal, and FIG. 4 is an explanatory diagram showing an example of the relationship between the thickness of the Si core 35 of the optical modulator 1 and the group refractive index of the light. As shown in FIG. 3, when the thickness of the EO layer 33 of the optical modulator 1 is, for example, 0.15 μm or less, the refractive index of the high-frequency electric signal becomes relatively low, about 3.00 or less. As shown in FIG. 4, when the thickness of the Si core 35 of the optical modulator 1 is, for example, 0.07 μm or more, the group refractive index of the light propagating through the Si core 35 on the optical modulator 1 side also becomes high, about 3.00 or more. As a result, the conventional problem that the refractive index of the electric signal becomes larger is eliminated, and the propagation speed of the high-frequency electric signal and the propagation speed of the light in the optical modulator 1 become almost the same, so that the velocity mismatch can be improved. As a result of the improvement in the velocity matching, the optical modulator 1 can achieve a 3 dB band in which the attenuation of the optical response level of the optical modulator 1 can be suppressed to within 3 dB even in an operating band of 65 GHz or more.

[0030] Moreover, because the velocity mismatch can be improved, it is possible to lengthen the optical modulator to approximately 4 mm. Therefore, the working length of the optical modulator 1 can be sufficiently secured. In the optical modulator 1 having the cross-sectional structure described above, the half-wave voltage per unit length of the optical modulator 1 is high at approximately 0.6 Vcm. By combining this high modulation efficiency with the long optical modulator length achieved by improving the velocity matching, it is possible to obtain a highly efficient optical modulator 1 with a half-wave voltage Vπ of 2.0 V or less.

[0031] 5 is a diagram showing an example of the relationship between the thickness of the material layer 32 of the optical modulator 1 and the half-wave voltage Vπ, and FIG. 6 is a diagram showing an example of the relationship between the thickness of the material layer 32 of the optical modulator 1 and the light group refractive index. For convenience of explanation, the material of the material layer 32 is, for example, SiO 2 , Ta 2 O 5 , Si are exemplified. SiO 2 The refractive index of is 1.444, Ta 2 O 5 The refractive index of is 2.05, and the refractive index of Si is 3.45.

[0032] 5, the half-wave voltage Vπ of the optical modulator 1 tends to increase as the thickness of the material layer 32 increases. For any material, the increase in the half-wave voltage Vπ is negligible up to a thickness of about 0.02 μm for the material layer 32, and the half-wave voltage Vπ can be suppressed to 1 Vcm or less up to a thickness of about 0.05 μm. Therefore, it is desirable to set the thickness of the material layer 32 to at least 0.05 μm or less.

[0033] The material of the material layer 32 is SiO 2 6, as the thickness of the material layer 32 increases, the group refractive index of light tends to decrease, which goes against the purpose of increasing the group refractive index of light from the viewpoint of improving velocity matching. It is not preferable to use a material with a low refractive index for the material layer 32. For example, Ta 2 O 5 A material having a refractive index close to the refractive index (2.2) of the EO layer 33, such as the above, or a material having a refractive index higher than that of the EO layer 33, such as Si including amorphous Si, is preferable.

[0034] In particular, when the material layer 32 is made of Si, the light group refractive index can be increased as the thickness of the material layer 32 is increased, which is preferable from the viewpoint of improving velocity matching. Therefore, the refractive index of the material layer 32 is preferably approximately the same as or higher than that of the EO layer 33, and it is preferable to use a material for the material layer 32 having a refractive index at least 0.85 times higher than that of the EO layer 33.

[0035] The material layer 32 is preferably made of a material different from that of the EO layer 33, and has a lower electro-optic coefficient and a lower dielectric constant than those of the EO layer 33, and a lower absorption coefficient for the signal light.

[0036] The material layer 32 is preferably made of a material that also functions as an adhesive layer to improve adhesion between the Si core 35 and the EO layer 33. For example, Ta 2 O 5 and amorphous silicon are suitable materials not only from the viewpoint of refractive index but also from the viewpoint of use as an adhesive layer.

[0037] The EO layer 33 of the optical modulator 1 may be 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 layer 32 may be a material having a lower electro-optic coefficient and relative dielectric constant than the EO layer 33 and a low absorption coefficient for the signal light. The material layer 32 may be a material having a refractive index that is 0.85 times or more the refractive index of the EO layer 33. Although the Si core 35 is exemplified as the core layer of the optical modulator 1, any core layer having a refractive index higher than those of the EO layer 33 and the material layer 32 may be used.

[0038] Next, a manufacturing process of the optical modulator 1 of the embodiment 1 will be described. The optical modulator 1 is, for example, a Si core substrate 40A on the surface of which a pattern of the Si core 35 is formed, and a SiO 2 The optical modulator 1 is configured by bonding a Si core substrate 40A and the EO thin film substrate 45A together and forming an electrode 22 on the EO layer 33.

[0039] First, the manufacturing process of the Si core substrate 40A will be described. Fig. 7A is an explanatory diagram showing an example of the SOI substrate 40 used for the Si core substrate 40A. The SOI substrate 40 has a Si substrate 41, a BOX layer 42 laminated on the Si substrate 41, and a Si layer 43 laminated on the BOX layer 42. The Si layer 43 is a Si layer with a thickness of 0.22 μm used for silicon photonics.

[0040] 7B is an explanatory diagram showing an example of a Si thinning process of the SOI substrate 40. As shown in FIG. 7B, the Si layer 43 in the SOI substrate 40 is thinned by etching or the like to reduce the thickness of the Si layer 43 to about 0.07 μm.

[0041] FIG. 7C is an explanatory diagram showing an example of a Si patterning step. The Si layer 43 after the Si thin film is patterned to form a Si layer 43A as shown in FIG. 7C. FIG. 7D shows a SiO 2 7D is an explanatory diagram showing an example of a film formation process. After patterning, the Si layer 43A is provided with a SiO 2A film 44 is formed.

[0042] 7E is an explanatory diagram showing an example of a Si core substrate 40A after surface flattening polishing. 2 The film 44 is surface-polished to the vicinity of the surface of the Si layer 43A, and the Si layer 43A is exposed from the surface as shown in FIG. 7E, thereby forming a Si core substrate 40A.

[0043] The surface of the Si core substrate 40A is ideally flattened, and the surface of the Si layer 43A and the SiO 2 However, in consideration of the difference in the amount of loss of each material during planarization, the surface of the SiO 2 The surface height of the film 44 may be lowered. Even in this case, it is possible to obtain substantially the same half-wave voltage Vπ and light group refractive index. In addition, it is preferable to use a substrate with high resistivity of 1000 Ωcm or more for the Si substrate 41 portion of the SOI substrate 40 in order to improve high frequency characteristics.

[0044] Next, a manufacturing process of the EO thin film substrate 45A used in the optical modulator 1 will be described. Fig. 8A is an explanatory diagram showing an example of an EO substrate 45 used in the EO thin film substrate 45A. The EO substrate 45 shown in Fig. 8A has a Si substrate 46, an intermediate layer 47 laminated on the Si substrate 46, and an EO layer 48 laminated on the intermediate layer 47. The EO layer 48 is formed of a ferroelectric EO material such as BTO. The intermediate layer 47 is made of, for example, ZrO 2 , Pt, SrRuO 3 (SRO). If the EO layer 48 is formed directly on the Si sub-substrate 46, the crystal quality of the ferroelectric EO material will deteriorate, so the EO layer 48 is formed on the Si substrate 46 via an intermediate layer 47.

[0045] Fig. 8B is an explanatory diagram showing an example of the first bonding step. A support substrate 49A on which an upper clad layer 49B is laminated is prepared. The EO substrate 45 is turned upside down, and the upper clad layer 49B and the EO layer 48 in the EO substrate 45 are bonded with an adhesive as shown in Fig. 8B. The bonding surface between the EO layer 48 and the upper clad layer 49B may be bonded with a new adhesive layer, and can be appropriately changed.

[0046] In addition, it is preferable to planarize the bonding surface between the upper cladding layer 49B and the EO layer 48 in the EO substrate 45 by, for example, lapping or chemical mechanical polishing (CMP). In addition, it is preferable to perform surface cleaning and surface activation treatment before bonding. After surface activation, the upper cladding layer 49B and the EO layer 48 are bonded together, and bonding may be performed in a vacuum atmosphere as necessary, and pressure may be applied at an appropriate temperature during bonding.

[0047] Fig. 8C is an explanatory diagram showing an example of the first removal step. Next, the Si substrate 46, the intermediate layer 47, and a part of the EO layer 48 in the EO substrate 45 bonded to the upper cladding layer 49B are removed to form an EO layer 48A having a predetermined thickness, for example, 0.15 µm, as shown in Fig. 8C. Note that the removal method is preferably, for example, lapping or chemical mechanical polishing (CMP).

[0048] FIG. 8D is an explanatory diagram showing an example of a thin-film EO substrate 45A. As shown in FIG. 8D, the EO thin-film substrate 45A is completed by forming a material layer 32 on the EO layer 48A. As described above, the material layer 32 is preferably a material whose refractive index is equal to or higher than that of the EO layer 48, for example, Ta oxide or Si film. The material layer 32 can ensure adhesion between the Si core substrate 40A and the EO thin-film substrate 45A. As described later, the EO layer and the material layer 32 for improving adhesion as shown in FIG. 8D are further bonded to another substrate (FIG. 7D) on which a Si core waveguide is formed to form a modulator waveguide. In such a bonding method, it is possible to bond a good-quality EO crystal film formed on a separate substrate on which crystal growth is easy to perform to a different substrate, so that it is possible to relatively easily create a structure in which an arbitrary substrate material and an EO material are combined, compared to direct film formation by epitaxial growth.

[0049] Next, a method for manufacturing the optical modulator 1 by bonding between the Si core substrate 40A and the EO thin film substrate 45A will be described. FIG. 9A is an explanatory diagram showing an example of the second bonding step. As shown in FIG. 9A, the EO thin film substrate 45A is inverted, and the material layer 32 of the inverted EO thin film substrate 45A is bonded to the Si layer 43A of the Si core substrate 40A. For convenience of explanation, if the sizes of the EO thin film substrate 45A and the Si core substrate 40A are the same, the wafers may be bonded together as they are. However, if the sizes of the two are different, for example, the EO thin film substrate 45A may be diced into pieces at a predetermined size, and the plurality of pieces of the EO thin film substrate 45A may be die-bonded onto the Si core substrate 40A, and other appropriate modifications may be made.

[0050] In addition, in the process of bonding the wafer of the EO thin film substrate 45A and the wafer of the Si core substrate 40A, the surface of the Si core substrate 40A where the Si core 35 is exposed is bonded to the surface of the material layer 32 of the EO thin film substrate 45A. Note that, before bonding, for example, flattening treatment, surface cleaning, and surface activation treatment may be performed on the surface of the Si core substrate 40A and the surface of the EO thin film substrate 45A. Furthermore, during bonding, bonding may be performed in a vacuum or at an appropriate temperature and pressure, and can be appropriately changed.

[0051] 9B is an explanatory diagram showing an example of the second removal step. After the Si core substrate 40A and the EO thin film substrate 45A are bonded together, as shown in FIG. 9B, the support substrate 49A on the opposite side of the EO layer 33 of the EO thin film substrate 45A is removed. To remove the support substrate 49A, for example, XeF 2 It is preferable to use a process that selectively removes only Si from other materials by etching with a gas.

[0052] Fig. 9C is an explanatory diagram showing an example of the upper cladding layer forming step. A part of the upper cladding layer 49B shown in Fig. 9B is etched to form the upper cladding layer 34 on the EO layer 33. A part of the upper cladding layer 49B can be removed so that the electrode 22 can be disposed on the EO layer 33. The etching is performed by, for example, RIE (Reactive Ion Etching), ion milling, or the like.

[0053] 9D is an explanatory diagram showing an example of a metal layer forming process. After removing a part of the upper cladding layer 34, as shown in FIG. 9D, a metal layer 22D to be used as the electrode 22 is formed on the surface of the EO layer 33 and the upper cladding layer 34. The metal layer 22D has, for example, a Ti film 22D1 as a contact metal with the EO layer 33, and an Au film 22D2 formed on the Ti film 22D1. The Ti film 22D1 / Au film 22D2 may be formed by a method such as deposition, and a plating process may be further performed when a thick film is applied with the Au film 22D2. Instead of the Au film 22D2, for example, a film of Al, Ag, Cu, etc. may be used, and may be changed as appropriate.

[0054] Fig. 9E is an explanatory diagram showing an example of a metal patterning process. After forming the metal layer 22D shown in Fig. 9D, the upper cladding layer 34 has a part of the metal layer 22D removed to form an electrode pattern 22E sandwiching the Si core 35 so that an electric field can be applied near the Si core 35, as shown in Fig. 9E. As a result, the optical modulator 1 is completed as shown in Fig. 1.

[0055] The optical modulator 1 of the first embodiment includes an EO layer 33 containing an electro-optical material, and a material layer 32 disposed below the EO layer 33 and having a dielectric constant lower than that of the EO layer 33. The optical modulator 1 further includes, for example, a Si core layer 35 disposed below the material layer 32 and having a refractive index higher than those of the EO layer 33 and the material layer 32, for example, a refractive index of 3.45, and an electrode 22 for applying an electric signal to the EO layer 33. The refractive index of the material layer 32 is set to 0.85 times or more that of the EO layer 33. That is, 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, is increased by applying the Si core 35 having a high refractive index, thereby bringing the refractive index of the high-frequency electric signal closer to the group refractive index of the light. As a result, the propagation speed of the high-frequency electric signal and the propagation speed of the light in the optical modulator 1 become almost the same, and therefore the velocity mismatch between the propagation speed of the electric signal and the propagation speed of the light can be improved. This makes it possible to avoid a situation in which the operating band of the optical modulator 1 is limited. Moreover, since the optical modulator 1 improves the velocity mismatch, it is possible to lengthen the interaction length and suppress the half-wave voltage Vπ, thereby improving the modulation efficiency of the optical modulator 1.

[0056] In the optical modulator 1, the Si core 35 made of a material with a high refractive index of 3.45 is used, making it possible to increase the group refractive index of light propagating through the Si core 35. In addition, although the refractive index of the high-frequency electrical signal is increased by using the EO layer 33 with a high dielectric constant, the group refractive index of light is increased to bring the group refractive index of light closer to the refractive index of the high-frequency electrical signal, thereby mitigating the velocity mismatch. As a result, the optical modulator 1 does not need to have a short action length, but can have a long action length, so that the operating band of the optical modulator 1 can be secured while improving the modulation efficiency.

[0057] In addition, the optical modulator 1 has the material layer 32 disposed between the EO layer 33 and the Si core 35, so that the adhesion between the Si core 35 and the EO layer 33 can be ensured. In the first place, since the EO layer 33 is a crystalline material and is formed by epitaxial growth, it is difficult to form the EO layer 33 on the Si core 35 and the lower clad layer 31. Therefore, the material layer 32 is disposed between the Si core 35 and the lower clad layer 31 and the EO layer 33. In addition, since the refractive index of the material layer 32 is set to be 0.85 times or more the refractive index of the EO layer 33, it is possible to maintain or increase the group refractive index of light compared to the case where the material layer 32 is not present. As a result, it is possible to realize a stable structure of the optical modulator 1 by increasing the adhesion between the Si core 35 and the EO layer 33 without impairing the effect of improving the speed matching between the propagation speed of the high-frequency electrical signal and the propagation speed of light in the optical modulator 1.

[0058] The electro-optic coefficient of the EO layer 33 is higher than that of the material layer 32. As a result, a low half-wave voltage Vπ can be realized.

[0059] The dielectric constant of the EO layer 33 having a high electro-optic coefficient is higher than 50, and the electro-optic coefficient of the EO layer 33 is higher than 50 pm / V, while the dielectric constant of the material layer 32 is 50 or less, and the electro-optic coefficient of the material layer 32 is 50 pm / V or less. As a result, the refractive index of the high frequency electric signal of the EO layer 33 can be kept relatively low, and the group refractive index of light can be increased by the Si core 35.

[0060] EO layer 33 is BaTiO 3 (BTO), PbLaZrTiO 3 (PLZT), PbZrTiO 3 The material layer 32 includes at least one of the following materials: Ta 2 O 5 and Si. The high electro-optic coefficient of these materials allows the half-wave voltage Vπ to be low.

[0061] The thickness of the EO layer 33 is within a range of 0.1 to 0.3 μm, the thickness of the Si core 35 is within a range of 0.05 to 0.10 μm, and the thickness of the material layer 32 is 0.005 to 0.05 μm or less. As a result, the refractive index of the high-frequency electrical signal of the EO layer 33 can be kept relatively low, while the group refractive index of light can be increased by the Si core 35.

[0062] For convenience of explanation, the EO layer 33, the material layer 32, and the upper cladding layer 34 of the optical modulator 1 are each made of a single material. However, for example, a plurality of materials whose dielectric constants, refractive indexes, and electro-optic coefficients fall within the ranges considered to be preferable for each layer may be combined, and appropriate modifications can be made.

[0063] A core thickness conversion section may be provided at a coupling section connecting between the input waveguide connected to the input section 11 of the optical modulator 1 and the Si core 35, or at a coupling section connecting between the output waveguide connected to the output section 15 of the optical modulator 1 and the Si core 35. The core thickness conversion section is structured to convert the thickness of the Si core 35 from, for example, 0.05 to 0.10 μm to 0.15 μm or more. The core thickness conversion section is a tapered structure in which the core width from the input waveguide to the Si core 35 gradually changes, or a tapered structure in which the core width from the Si core 35 to the output waveguide gradually changes. As a result, the coupling loss at the coupling section can be suppressed.

[0064] Further, the Si core 35 is exemplified as a channel type waveguide, but may be, for example, a ridge type waveguide, and can be appropriately changed. EXAMPLES

[0065] Fig. 10 is a schematic cross-sectional view of the optical modulator 1 of the second embodiment. The same components as those of the optical modulator 1 of the first embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The cross-sectional portion shown in Fig. 10 is a cross-sectional portion of a first modulation section 13A in the optical modulator 1. The optical modulator 1 of the second embodiment differs from the optical modulator 1 of the first embodiment in that the material of the upper cladding layer 34A and the material of the lower cladding layer 31 are different from each other.

[0066] FIG. 11 is an explanatory diagram showing an example of the relationship between the dielectric constant of the upper cladding layer 34A of the optical modulator 1 and the electric signal loss, and FIG. 12 is an explanatory diagram showing an example of the relationship between the dielectric constant of the upper cladding layer 34A of the optical modulator 1 and the electric signal refractive index. When the dielectric constant of the material of the upper cladding layer 34A is increased, the electric signal loss (high frequency loss) increases as shown in FIG. 11, similarly to the case where the dielectric constant of the EO layer 33 is increased. Furthermore, when the dielectric constant of the material of the upper cladding layer 34A is increased, the electric signal refractive index (high frequency refractive index) also increases as shown in FIG. 12. Therefore, the material of the upper cladding layer 34A is selected from SiO 2 In order to suppress the electrical signal loss and the electrical signal refractive index to 10% or less, respectively, compared to the case where a material with a relative dielectric constant of 15 or less is preferably used.

[0067] 13 is an explanatory diagram showing an example of the relationship between the refractive index of the upper cladding layer 34A of the optical modulator 1 and the half-wave voltage Vπ. If the refractive index of the upper cladding layer 34A is too high, the light distribution spreads too much from the EO layer 33 to the upper cladding layer 34A, which leads to a deterioration in the optical modulation efficiency in the EO layer 33 (an increase in the half-wave voltage Vπ), as shown in FIG. 13. Therefore, it is preferable to adopt a material having a refractive index of 1.8 or less as the material for the upper cladding layer 34A. Therefore, SiO 2 The upper cladding layer 34A other than the above may be, for example, MgF 2 , MgO, Al 2 O 3 , Y 2 O 3 etc. or a resin with a refractive index of 1.8 or less is preferred.

[0068] In the optical modulator 1 of the second embodiment, the material of the upper cladding layer 34A disposed on the EO layer 33 is made of a material having a relative dielectric constant of 15 or less and a refractive index of 1.8 or less. As a result, the modulation efficiency of the optical modulator 1 can be improved by suppressing the half-wave voltage while suppressing the electric signal loss and the electric signal refractive index. EXAMPLES

[0069] 14 is a schematic cross-sectional view showing an example of the optical modulator 1 of the third embodiment. The same components as those of the optical modulator 1 of the first embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The optical modulator 1 of the third embodiment differs from the optical modulator 1 of the first embodiment in that a first material layer 36 is disposed between the EO layer 33 and the upper cladding layer 34A and the electrode 22.

[0070] From the viewpoint of modulation efficiency, a structure in which the electrode 22 is in direct contact with the EO layer 33 as in the optical modulator 1 of the first embodiment is preferable, but from the viewpoint of manufacturing errors, for example, it is possible that a layer deposited on the EO layer 33 cannot be completely removed and remains under the electrode 22. To deal with such a situation, a first material layer 36 is intentionally disposed between the electrode 22 and the EO layer 33 to ensure a stable layer structure.

[0071] 15 is an explanatory diagram showing an example of the relationship between the refractive index of the first material layer 36 of the optical modulator 1 and optical loss. By adopting a material having a lower refractive index than that of the EO layer 33 for the first material layer 36, it is possible to suppress the seepage of the light distribution from the EO layer 33 to the electrode 22, and therefore it is possible to suppress optical loss due to optical absorption in the electrode 22. The effect of suppressing optical loss depends on the refractive index of the first material layer 36, and as shown in FIG. 15, when the refractive index of the EO layer 33 is around 2.2 and the refractive index of the first material layer 36 is 1.8 or less, optical loss can be reduced. Therefore, from the viewpoint of suppressing optical loss, it is preferable to use SiO 2 having a refractive index of 1.8 or less for the first material layer 36. 2 , MgF 2 , Al 2 O 3 Materials such as MgO are preferred.

[0072] FIG. 16 is an explanatory diagram showing an example of the relationship between the dielectric constant of the first material layer 36 of the optical modulator 1 and the half-wave voltage Vπ. In the optical modulator 1 of the third embodiment, it is also possible that a high-frequency electric signal is applied to the first material layer 36 disposed between the electrode 22 and the EO layer 33. In particular, when a material having a low dielectric constant is disposed between the electrode 22 and the EO layer 33 as the first material layer 36, the voltage applied to the first material layer 36 becomes relatively high, and the electric field applied to the EO layer 33 becomes weak. As a result, the half-wave voltage Vπ increases as shown in FIG. 16. Therefore, in order to suppress the increase in the half-wave voltage Vπ, it is preferable to use a material having a relatively high dielectric constant of 8 or more for the first material layer 36. For example, MgO, HfO 2 , Ta 2 O 5 , ZrO 2 and Nb 2 O 5 Materials such as the above are preferred.

[0073] That is, the first material layer 36 of the optical modulator 1 of the third embodiment is made of at least 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.

[0074] In the optical modulator 1 of the third embodiment, a first material layer 36 is disposed between the electrode 22 and the EO layer 33, and the refractive index of the first material layer 36 is set to 1.8 or less. As a result, the first material layer 36 can suppress the seepage of the light distribution from the EO layer 33 to the electrode 22, thereby suppressing the light loss.

[0075] In the optical modulator 1, a first material layer 36 is disposed between the electrode 22 and the EO layer 33, and the relative dielectric constant of the first material layer 36 is set to at least 8. As a result, the electric field applied to the EO layer 33 also becomes stronger, making it possible to suppress an increase in the half-wave voltage Vπ. EXAMPLES

[0076] FIG. 17 is a schematic plan view showing an example of the configuration of an IQ optical modulator 100 of the fourth embodiment. The same components as those of the optical modulator 1 of the first embodiment are given the same reference numerals, and the description of the overlapping components and operations is omitted. The IQ optical modulator 100 shown in FIG. 17 has a first branching section 61, an I (Inphase) component optical modulator 1A1, a Q (Quadrature) component optical modulator 1A2, two first DCPS (Direct Current Phase Shifter) 51, and a first multiplexing section 62. One of the first DCPS 51 is a phase adjustment element that adjusts the phase of the I component signal light from the I component optical modulator 1A1. The other first DCPS 51 is a phase adjustment element that adjusts the phase of the I component signal light from the Q component optical modulator 1A2.

[0077] The I component optical modulator 1A1 phase-modulates the I component optical signal. The Q component optical modulator 1A2 phase-modulates the Q component optical signal. The I component optical modulator 1A1 has a branching unit 12, two modulation units 13A and 13B (13), two second DCPSs 52, and a multiplexing unit 14. The second DCPSs 52 are phase adjustment elements that adjust the phase of the I component signal light from the modulation unit 13. The Q component optical modulator 1A2 has a branching unit 12, two modulation units 13A and 13B (13), two second DCPSs 52, and a multiplexing unit 14. The second DCPSs 52 are phase adjustment elements that adjust the phase of the Q component signal light from the modulation unit 13.

[0078] The first branching unit 61 optically branches the signal light from the input waveguide and outputs the branched signal light to each optical modulator 1 (1A1, 1A2). The I component optical modulator 1A1 outputs the phase-modulated I component signal light from the multiplexing unit 14 in the optical modulator 1A1 to the first DCPS 51. The first DCPS 51 phase-shifts the phase-modulated I component signal light and outputs the phase-shifted I component signal light to the first multiplexing unit 62.

[0079] Furthermore, the Q component optical modulator 1A2 outputs the phase-modulated signal light of the Q component from the multiplexing section 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 Q component signal light to the first multiplexing section 62. The first multiplexing section 62 multiplexes the I component signal light and the Q component signal light, and outputs the multiplexed IQ component signal light to the output waveguide.

[0080] Fig. 18 is a schematic cross-sectional view taken along the line BB shown in Fig. 17. The cross-sectional portion shown in Fig. 18 is a cross-sectional portion of the optical modulator 1A1 for the I component. The cross-sectional portion of the modulation section 13 of the optical modulator 1A1 for the I component shown in Fig. 18 has a Si substrate 41, a lower cladding layer 31, a Si core 35, a material layer 32, an EO layer 33, and an upper cladding layer 34.

[0081] A cross-sectional portion of the second DCPS 52 of the optical modulator 1A1 for the I component includes a Si substrate 41, a lower cladding layer 31, a Si core substrate 35, an upper cladding layer 34, a thermo-optic heater 52B, and a metal wiring 52A. The thermo-optic heater 52B is an electrical resistor such as TiN arranged in the vicinity of the Si core 35, which is the modulation waveguide 21A. The metal wiring 52A is electrically connected to the thermo-optic heater 52B and is a wiring that supplies a current to the thermo-optic heater 52B. The thermo-optic heater 52B generates heat by passing a current from the metal wiring 52A to the thermo-optic heater 52B. In the second DCPS 52, the refractive index of silicon in the first modulation waveguide 21A changes due to the heat of the thermo-optic heater 52B, and the phase of the light passing through the first modulation waveguide 21 is shifted.

[0082] Ferroelectric EO material is used only for the cross-section of the modulation section 13, and the branching section 12 and multiplexing section 14 are constructed using silicon photonics technology, making it possible to arrange the branching / multiplexing section and DCPS in a small area. As a result, it is possible to realize an optical modulator element that is compact, capable of high-efficiency and high-speed operation, and has high integration and functionality.

[0083] The branching section 12 and the multiplexing section 14 for constituting the optical modulator 1 are also configured using a hybrid waveguide of a high dielectric EO material and a Si core 35. However, when the optical modulator 1 is mounted on a silicon photonics element, the branching section 12 and the multiplexing section 14 for constituting the optical modulator 1 are also configured using a silicon photonics waveguide, making it possible to make the most of the characteristics of silicon photonics and reduce the size. In addition, by forming a part of the two waveguides constituting the optical modulator 1 on the silicon photonics element side, it is possible to realize a phase shifter for appropriately adjusting the phase of the optical modulator 1 by a heater formed on the waveguide, and a small and low-power DCPS can be realized. The DCPS using the heater can be used not only for phase adjustment of the optical modulator 1, but also for phase adjustment between the I channel and the Q channel of the IQ optical modulator 100.

[0084] The Si core 35 of the optical modulator 1A has a core thickness of, for example, 0.07 μm, and the Si waveguide 35A connecting to the branching section 12 and the multiplexing section 14 has a core thickness of, for example, 0.22 μm and a core width of, for example, 0.5 μm. Therefore, conversion waveguides for low-loss connection are arranged at the optical coupling sites between the Si core 35 and the Si waveguide 35A connecting to the branching section 12, and between the Si core 35A connecting to the multiplexing section 14 and the Si waveguide 35A connecting to the multiplexing section 14, since the core thicknesses are different.

[0085] The IQ optical modulator 100 of the fourth embodiment includes an I-component optical modulator 1A1(1) and a Q-component optical modulator 1A2(1). The optical modulator 1 includes an EO layer 33 including an electro-optic material, and a material layer 32 disposed below the EO layer 33 and having a lower dielectric constant than the EO layer 33. The optical modulator 1 further includes a Si core layer 35, for example, disposed 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 electric signal to the EO layer 33. The refractive index of the material layer 32 is set to 0.85 times or more the refractive index of the EO layer 33. That is, the refractive index of the electric signal of the EO layer 33 is kept relatively low, and the group refractive index of the light is increased by the Si core 35, thereby bringing the refractive index of the electric signal closer to the group refractive index of the light. As a result, the propagation speed of the electrical signal and the propagation speed of light in the optical modulator 1 become almost the same, so that the velocity mismatch in the IQ optical modulator 100 can be improved. Therefore, it is possible to avoid a situation in which the operating band of the IQ optical modulator 100 is limited. Moreover, since the IQ optical modulator 100 improves the velocity mismatch while maintaining a long operating length, the half-wave voltage Vπ can be suppressed, and the modulation efficiency of the IQ optical modulator 100 can be improved.

[0086] Although the IQ optical modulator 100 of the fourth embodiment has the optical modulator 1 of the first embodiment built therein, the optical modulator 1 of the second or third embodiment may be used, and modifications can be made as appropriate. EXAMPLES

[0087] Fig. 19 is a schematic plan view showing an example of the configuration of a DP-IQ optical modulator 110 according to a fifth embodiment. The same components as those of the IQ optical modulator 100 shown in Fig. 17 are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The DP-IQ optical modulator 110 shown in Fig. 19 is configured by arranging four optical modulators 1 in parallel. The DP-IQ optical modulator 110 includes a second branching section 63, an IQ optical modulator 100A (100) for the X-polarized component, an IQ optical modulator 100B (100) for the Y-polarized component, a PR (Polarization Rotator) 64, and a PBC (Polarization Beam Combiner) 65.

[0088] The second branching unit 63 optically branches the input light from the input waveguide, and outputs the branched signal light to each 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.

[0089] The IQ optical modulator 100 includes a first branching section 61, an I component optical modulator 1A1, a Q component optical modulator 1A2, two first DCPSs 51, and a first multiplexing section 62.

[0090] The first branching unit 61 optically branches the signal light from the second branching unit 63, and outputs the branched signal light to each of the optical modulators 1A1 and 1A2. The I-component optical modulator 1A1 outputs the phase-modulated I-component signal light from the multiplexing unit 14 in the optical modulator 1A1 to the first DCPS 51. The first DCPS 51 phase-shifts the phase-modulated I-component signal light, and outputs the phase-shifted I-component signal light to the first multiplexing unit 62.

[0091] Furthermore, the Q component optical modulator 1A2 outputs the phase-modulated signal light of the Q component from the multiplexing unit 14 in the optical modulator 1A2 to the first DCPS 51. The first DCPS 51 phase-shifts the phase-modulated signal light of the Q component, and outputs the phase-shifted Q component signal light to the first multiplexing unit 62. The first multiplexing unit 62 multiplexes the I component signal light and the Q component signal light, and outputs the multiplexed IQ component signal light to the PBC 65.

[0092] The first multiplexing unit 62 in the IQ optical modulator 100A of the X polarization component multiplexes the I component signal light of the X polarization component from the multiplexing unit 14 in the I component optical modulator 1A1 and the Q component signal light of the X polarization component from the multiplexing unit 14 in the Q component optical modulator 1A2. The first multiplexing unit 62 outputs the IQ component signal light of the X polarization component to the PBC 65.

[0093] The first multiplexing unit 62 in the Y-polarized IQ optical modulator 100B multiplexes the I-component signal light of the Y-polarized component from the multiplexing unit 14 in the I-component optical modulator 1A1 and the Q-component signal light of the Y-polarized component from the multiplexing unit 14 in the Q-component optical modulator 1A2. The first multiplexing unit 62 outputs the IQ-component signal light of the Y-polarized component to the PR 64. The PR 64 performs polarization rotation on the IQ-component signal light of the Y-polarized component, and outputs the IQ-component signal light of the Y-polarized component after the polarization rotation to the PBC 65. The PBC 65 multiplexes the IQ-component signal light of the X-polarized component and the IQ-component signal light of the Y-polarized component after the polarization rotation, and outputs the multiplexed XY-polarized component signal light to the output waveguide as transmission light.

[0094] Since ferroelectric EO material is used only for the cross-sectional portion of the modulation section 13, the second branching section 63 for the input / output of the optical modulator 1, the first branching section 61 between I and Q, the first multiplexing section 62, the PR 64 and PBC 65 for polarization splitting and combining, etc. are configured using silicon photonics technology. This makes it possible to arrange them in a small area. As a result, it is possible to realize an optical modulator element that is small, capable of high-efficiency and high-speed operation, and has high integration and functionality.

[0095] The branching section 12 and the multiplexing section 14 for constituting the optical modulator 1 are also configured using a hybrid waveguide of a high dielectric EO material and a Si core 35. However, when the optical modulator 1 is mounted on a silicon photonics element, the branching section 12 and the multiplexing section 14 for constituting the optical modulator 1 are also configured using a silicon photonics waveguide, making it possible to make the most of the characteristics of silicon photonics and reduce the size. By forming a part of the two waveguides constituting the optical modulator 1 on the silicon photonics element side, it is possible to realize a DCPS for appropriately adjusting the phase of the optical modulator 1 by a heater formed on the waveguide, and a small-sized and low-power-consumption phase shifter can be realized. The DCPS using the heater can be used not only for phase adjustment of the optical modulator 1, but also for phase adjustment between the I channel and the Q channel of the IQ optical modulator 100.

[0096] The DP-IQ optical modulator 110 of the fifth embodiment includes an X-polarized IQ optical modulator 100A and a Y-polarized IQ optical modulator 100B. The optical modulator 1 includes an EO layer 33 containing an EO material, and a material layer 32 disposed below the EO layer 33 and having a lower dielectric constant than the EO layer 33. The optical modulator 1 further includes a Si core layer 35 disposed 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 electric signal to the EO layer 33. The refractive index of the material layer 32 is set to 0.85 times or more the refractive index of the EO layer 33. That is, the refractive index of the electric signal of the EO layer 33 is kept relatively low, and the group refractive index of the light is increased by applying the Si core 35, thereby bringing the refractive index of the electric signal closer to the group refractive index of the light. As a result, the propagation speed of the electrical signal and the propagation speed of light in the optical modulator 1 become almost the same, and the velocity mismatch in the DP-IQ optical modulator 110 can be improved. This makes it possible to avoid a situation in which the operating band of the DP-IQ optical modulator 110 is limited. Moreover, since the DP-IQ optical modulator 110 improves the velocity mismatch while maintaining a long operating length, the half-wave voltage Vπ can be suppressed, and the modulation efficiency of the DP-IQ optical modulator 110 can be improved.

[0097] In a structure in which an optical modulator 1 using a ferroelectric EO material is mounted on a silicon photonics element, optical coupling between the optical modulator 1 and the Si waveguide 35A of silicon photonics can be achieved by connecting the Si core 25 and the Si waveguide 35A. Since the optimal width and thickness of the Si core 35 in the optical modulator 1 differs from the core width or core thickness of the Si waveguide 35A used in general silicon photonics, a conversion waveguide for connection with low loss may be inserted, and appropriate changes can be made. EXAMPLES

[0098] Fig. 20 is a schematic plan view showing an example of the configuration of an optical transceiver 120 according to a sixth embodiment. The same components as those of the DP-IQ optical modulator 110 shown in Fig. 19 are denoted by the same reference numerals, and the description of the overlapping configurations and operations will be omitted. The optical transceiver 120 shown in Fig. 20 has an optical modulator element 120A including the DP-IQ optical modulator 110, and an optical receiver element 120B that receives a DP-QAM signal. The optical transceiver 120 integrates the optical modulator element 120A and the optical receiver element 120B 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 branching unit 67, a receiving light input unit 72, a PBS (Polarization Beam Splitter) 68, a PR (Polarization Rotator) 69, a first optical hybrid circuit 70A (70), and a second optical hybrid circuit 70B (70). The optical receiver element 120B has four first light receiving elements 71A (71) and four second light receiving elements 71B (71).

[0099] The third branching unit 67 branches the light from a light source (not shown) from the input waveguide, outputs one of the branched lights to the second branching unit 63 in the DP-IQ optical modulator 110 as an input light source of the modulator, and outputs the other to each hybrid circuit 70 as a local light of the receiver. The receiving light input unit 72 inputs the receiving light from an optical fiber (not shown). The PBS 68 splits the 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 rotates the Y-polarized receiving light by 90 degrees, and outputs the Y-polarized receiving light after the polarization rotation to the second optical hybrid circuit 70B.

[0100] The first optical hybrid circuit 70A causes the X-polarized component of the received light to interfere with the local light to obtain I-component and Q-component optical signals. The first optical hybrid circuit 70A outputs the I-component optical signal of the X-polarized component to the first light receiving element 71A1. The first optical hybrid circuit 70A outputs the Q-component optical signal of the X-polarized component to the first light receiving element 71A2.

[0101] The second optical hybrid circuit 70B causes the Y-polarized component of the received light to interfere with the local light to obtain I- and Q-component optical signals. The second optical hybrid circuit 70B outputs the I-component optical signal of the Y-polarized component to the second light receiving element 71B1. The second optical hybrid circuit 70B outputs the Q-component optical signal of the Y-polarized component to the second light receiving element 71B2.

[0102] The first light receiving element 71A1 is, for example, a PD (Photo Detector) that converts an optical signal of an I component of an X polarization component from the first optical hybrid circuit 70A into an electric signal of the I component after the electric conversion. The first light receiving element 71A2 converts an optical signal of a Q component of an X polarization component from the first optical hybrid circuit 70A into an electric signal of the Q component after the electric conversion. The second light receiving element 71B1 is, for example, a PD that converts an optical signal of an I component of a Y polarization component from the second optical hybrid circuit 70B into an electric signal of the I component after the electric conversion. The second light receiving element 71B2 converts an optical signal of a Q component of a Y polarization component from the second optical hybrid circuit 70B into an electric signal of the Q component after the electric conversion.

[0103] The optical transceiver 120 of the sixth embodiment can be miniaturized because the optical modulator element 120A including the DP-IQ optical modulator 110 and the optical receiver element 120B are mounted using silicon photonics technology.

[0104] The optical modulator element 120A has a DP-IQ optical modulator 110 incorporating a plurality of optical modulators 1. The optical modulator 1 has an EO layer 33 containing an EO material, and a material layer 32 disposed below the EO layer 33 and having a lower dielectric constant than the EO layer 33. The optical modulator 1 further has, for example, a Si core layer 35 disposed 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 electric signal to the EO layer 33. The refractive index of the material layer 32 is set to be 0.85 times or more the refractive index of the EO layer 33. In other words, the refractive index of the electric signal of the EO layer 33 is kept relatively low, and the group refractive index of light is increased by applying the Si core 35, thereby bringing the refractive index of the electric signal closer to the group refractive index of light. As a result, the propagation speed of the electrical signal and the propagation speed of light in the optical modulator 1 become almost the same, and the velocity mismatch in the DP-IQ optical modulator 110 can be improved. This makes it possible to avoid a situation in which the operating band of the DP-IQ optical modulator 110 is limited. Moreover, since the DP-IQ optical modulator 110 improves the velocity mismatch while maintaining a long operating length, the half-wave voltage Vπ can be suppressed, and the modulation efficiency of the DP-IQ optical modulator 110 can be improved.

[0105] Next, an optical transceiver 130 employing the optical modulator 1 of this embodiment will be described. FIG. 21 is an explanatory diagram showing an example of the optical transceiver 130. The optical transceiver 130 shown in FIG. 21 includes an LD 131, a transceiver module 132, and a DSP (Digital Signal Processor) 133. The LD 131 is, for example, a light source that emits laser light. The transceiver module 132 includes an optical transceiver 120, a driver circuit (DRV) 121, and a TIA (Transimpedance Amplifier) ​​122. The optical transceiver 120 includes an optical modulator element 120A and an optical receiver element 120B. The optical modulator element 120A is, for example, a DP-IQ optical modulator 110A or the like. The DSP 133 controls the entire optical transceiver 120. The DSP 133 is an electric component that performs digital signal processing, such as IQ modulation processing of a transmission signal and demodulation processing of a reception signal.

[0106] The DSP 133 executes processing such as encoding of transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the driver circuit 121. The driver circuit 121 drives the optical modulator element 120A in response to the electrical signal from the DSP 133.

[0107] The optical receiver element 120B converts the signal light into an electric signal. The TIA 122 amplifies the electric signal after the electric conversion, and outputs the amplified electric signal to the DSP 133. The DSP 133 performs processing such as decoding of the electric signal obtained from the TIA 122 to obtain received data.

[0108] For convenience of explanation, the optical transceiver 130 has been illustrated as incorporating the optical modulator element 120A and the optical receiver element 120B, but the optical transceiver 130 may also be applied to an optical transmitter incorporating only the optical modulator element 120A.

[0109] In the optical transceiver 130, the optical modulator 1 in the optical modulator element 120A is highly efficient, so the output amplitude of the driver circuit 121 can be suppressed, enabling low power consumption operation, and the high speed operation enables large capacity optical signal transmission. As a result, a large capacity and low power consumption optical communication module can be realized.

[0110] Furthermore, each component of each unit shown in the figure does not necessarily have to be physically configured as shown in the figure. In other words, the specific form of distribution and integration of each unit is not limited to that shown in the figure, and all or part of them can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc.

[0111] Furthermore, the various processing functions performed by each device may be executed in whole or in part on a CPU (Central Processing Unit) (or a microcomputer such as an MPU (Micro Processing Unit) or an MCU (Micro Controller Unit)). It goes without saying that the various processing functions may be executed in whole or in part on a program analyzed and executed by a CPU (or a microcomputer such as an MPU or an MCU), or on hardware using wired logic.

[0112] The following supplementary notes are further disclosed regarding the above-described embodiments including the present example.

[0113] (Appendix 1) An electro-optic layer including an electro-optic material; a layer of material disposed below the electro-optic layer and having a low dielectric constant relative to the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than the refractive indexes of the electro-optic layer and the material layer; an electrode for applying an electric signal to the electro-optic layer; The refractive index of the material layer is An optical modulator having a refractive index of at least 0.85 times that of the electro-optic layer.

[0114] (Appendix 2) The electro-optic coefficient of the electro-optic layer is 2. The optical modulator of claim 1, wherein the electro-optic coefficient is higher than that of the material layer.

[0115] (Note 3) The relative dielectric constant of the electro-optic layer is higher than 50, and the electro-optic coefficient of the electro-optic layer is higher than 50 pm / V; 2. The optical modulator according to claim 1, wherein the material layer has a relative dielectric constant of 50 or less and an electro-optic coefficient of 50 pm / V or less.

[0116] (Additional Note 4) The electro-optical layer is BaTiO 3(BTO), PbLaZrTiO 3 (PLZT), PbZrTiO 3 2. The optical modulator according to claim 1, comprising at least one of:

[0117] (Additional Note 5) The material layer is Ta 2 O 5 2. The optical modulator according to claim 1, comprising at least one of:

[0118] (Appendix 6) The optical modulator according to appendix 1, characterized in that the electro-optic layer has a thickness in the range of 0.1 to 0.3 μm, the core layer has a thickness in the range of 0.05 to 0.10 μm, and the material layer has a thickness of 0.005 to 0.05 μm or less.

[0119] (Appendix 7) A coupling portion connecting between an input waveguide connected to an input portion of the optical modulator and the core layer, or a coupling portion connecting between an output waveguide connected to an output portion of the optical modulator and the core layer, 2. The optical modulator according to claim 1, further comprising a core thickness conversion section that converts the thickness of the core layer from 0.05 to 0.10 μm to 0.15 μm or more.

[0120] (Additional Note 8) The electro-optic layer further includes a clad layer disposed on at least the electro-optic layer on the core layer, 2. The optical modulator according to claim 1, wherein the cladding layer has a relative dielectric constant of 15 or less and a refractive index of the cladding layer of 1.8 or less.

[0121] (Additional Note 9) The electro-optic layer further includes a clad layer disposed on at least the electro-optic layer on the core layer, The cladding layer is SiO 2 , MgF 2 , MgO, Al 2 O 3 and a resin having a refractive index of 1.8 or less.

[0122] (Supplementary Note 10) Further comprising a first material layer disposed between the electrode and the electro-optic layer; 2. The optical modulator according to claim 1, wherein the first material layer has a refractive index of 1.8 or less.

[0123] (Additional Note 11) The first material layer is SiO 2 , MgF 2 , Al 2 O 3 11. The optical modulator according to claim 10, comprising at least one of MgO and MgO.

[0124] (Supplementary Note 12) Further comprising a first material layer disposed between the electrode and the electro-optical layer; 2. The optical modulator according to claim 1, wherein the first material layer has a relative dielectric constant of at least 8 or more.

[0125] (Additional Note 13) The first material layer is MgO, HfO 2 , Ta 2 O 5 , ZrO 2 and Nb 2 O 5 13. The optical modulator according to claim 12, comprising at least one of the following:

[0126] (Supplementary Note 14) The optical modulator comprises: 2. The optical modulator according to claim 1, comprising: two optical waveguides that are the core layer; electrodes that apply an electric signal to each optical waveguide; a branching section that branches light into two optical waveguides; and a combining section that combines light from the two optical waveguides; and wherein the optical modulator modulates light guided through the optical waveguides in response to the electric signal.

[0127] (Appendix 15) An optical modulator element that modulates guided light in response to an electrical signal; an optical receiver element for converting received light into an electrical signal; The optical modulator element includes: an electro-optic layer comprising an electro-optic material; a layer of material disposed below the electro-optic layer and having a low dielectric constant relative to the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than the refractive indexes of the electro-optic layer and the material layer; an electrode for applying an electric signal to the electro-optic layer; The refractive index of the material layer is An optical transceiver having a refractive index that is 0.85 times or more the refractive index of the electro-optical layer.

[0128] (Appendix 16) A driver circuit for driving the optical modulator element; 16. The optical transceiver according to claim 15, further comprising an amplifier circuit for amplifying an electrical signal from the optical receiver element.

[0129] (Appendix 17) An optical modulator element that modulates guided light in response to an electrical signal; an optical receiver element that converts received light into an electrical signal; a signal processing unit that generates an electrical signal to the optical modulator element and acquires the electrical signal from the optical receiver element; The optical modulator element includes: an electro-optic layer comprising an electro-optic material; a layer of material disposed below the electro-optic layer and having a low dielectric constant relative to the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than the refractive indexes of the electro-optic layer and the material layer; an electrode for applying an electric signal to the electro-optic layer; The refractive index of the material layer is An optical transceiver having a refractive index that is 0.85 times or more the refractive index of the electro-optic layer. [Explanation of symbols]

[0130] 1 Optical Modulator 22 electrodes 32 material layer 33 EO layer 34 Upper cladding layer 35 Si core 36 First material layer

Claims

1. an electro-optic layer comprising an electro-optic material; a layer of material disposed below the electro-optic layer and having a low dielectric constant relative to the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than the refractive indexes of the electro-optic layer and the material layer; an electrode for applying an electric signal to the electro-optic layer; The refractive index of the material layer is An optical modulator having a refractive index of at least 0.85 times that of the electro-optic layer.

2. The electro-optic coefficient of the electro-optic layer is 2. The optical modulator of claim 1, wherein the electro-optic coefficient is high compared to the electro-optic coefficient of the material layer.

3. the dielectric constant of the electro-optic layer is greater than 50 and the electro-optic coefficient of the electro-optic layer is greater than 50 pm / V; 2. The optical modulator according to claim 1, wherein the material layer has a relative dielectric constant of 50 or less and an electro-optic coefficient of 50 pm / V or less.

4. The electro-optic layer is BaTiO ３ (BTO), PbLaZrTiO ３ (PLZT), PbZrTiO ３ 2. The optical modulator according to claim 1, further comprising at least one of: (PZT).

5. The electro-optic layer further includes a clad layer disposed on at least the electro-optic layer on the core layer, 2. The optical modulator according to claim 1, wherein the relative dielectric constant of said cladding layer is 15 or less, and the refractive index of said cladding layer is 1.8 or less.

6. The electro-optic layer further includes a clad layer disposed on at least the electro-optic layer on the core layer, The cladding layer is SiO ２ , MgF ２ , MgO, Al ２ O ３ 2. The optical modulator according to claim 1, further comprising at least one of a resin having a refractive index of 1.8 or less and a resin having a refractive index of 1.8 or less.

7. a first material layer disposed between the electrode and the electro-optic layer; 2. The optical modulator according to claim 1, wherein the refractive index of the first material layer is 1.8 or less.

8. a first material layer disposed between the electrode and the electro-optic layer; 2. The optical modulator according to claim 1, wherein the first material layer has a relative dielectric constant of at least 8.

9. an optical modulator element that modulates guided light in response to an electrical signal; an optical receiver element for converting received light into an electrical signal; The optical modulator element includes: an electro-optic layer comprising an electro-optic material; a layer of material disposed below the electro-optic layer and having a low dielectric constant relative to the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than the refractive indexes of the electro-optic layer and the material layer; an electrode for applying an electric signal to the electro-optic layer; The refractive index of the material layer is An optical transceiver, characterized in that the refractive index is 0.85 times or more the refractive index of the electro-optical layer.

10. an optical modulator element that modulates guided light in response to an electrical signal; an optical receiver element that converts received light into an electrical signal; a signal processing unit that generates an electrical signal to the optical modulator element and acquires the electrical signal from the optical receiver element; The optical modulator element includes: an electro-optic layer comprising an electro-optic material; a layer of material disposed below the electro-optic layer and having a low dielectric constant relative to the electro-optic layer; a core layer disposed below the material layer and having a refractive index higher than the refractive indexes of the electro-optic layer and the material layer; an electrode for applying an electric signal to the electro-optic layer; The refractive index of the material layer is An optical transceiver, characterized in that the refractive index is 0.85 times or more the refractive index of the electro-optic layer.

Images