Optical modulator element, optical transmitter, and optical transceiver

Abstract

The invention relates to an optical modulator element, an optical transmitter and an optical transceiver. An optical modulator element includes, on a substrate: an optical branching portion and an optical multiplexing portion each including a first material; two optical waveguide arms each connecting the optical branching section and the optical multiplexing section; and electrodes that apply electrical signals to the two optical waveguide arms. Each of the optical waveguides in the two optical waveguide arms includes: a first optical waveguide including a first material; a second optical waveguide comprising a second material having a higher electro-optical effect than the first material; and a transfer unit that performs light transfer between the first optical waveguide and the second optical waveguide. The substrate includes a hollow portion in which all or part of the substrate located below the second optical waveguide in plan view has been removed.

Description

Technical Field

[0001] The embodiments discussed herein relate to optical modulator elements, optical transmitters, and optical transceivers. Background Art

[0002] For example, due to the rapid growth of Internet Protocol (IP) data traffic, there is a demand for greater capacity in optical networks. In addition, in order to improve the spatial accommodation efficiency inside the optical transceiver, it is hoped that the size and integration of the optical transmitter / receiver will be further reduced. The silicon (Si) waveguide used for the optical transmitter / receiver strongly constrains the light, so that the bending radius can be reduced to about 10μm. Therefore, SiPh (silicon photonics) components are beginning to be applied to optical transmitters or optical receivers operating at 64G baud rates.

[0003] Optical devices are also referred to as Si photonic elements (hereinafter referred to as SiPh elements) because the optical circuit is composed of Si waveguides manufactured by using silicon on insulator (SOI) wafers. The optical modulator element and the optical receiver element included in the optical device are connected by optical waveguides. SiPh elements can be manufactured so that a large number of elements can be manufactured at one time by utilizing the process technology and process facilities of Si electrical semiconductor elements and by using, for example, Si wafers with a diameter ranging from 8 inches to 12 inches. In addition, in SiPh elements, light is strongly confined due to the large refractive index of Si of about 3.4, and thus the bending radius of the Si optical waveguide can be reduced to about 10μm. Therefore, the size of the element can be reduced. Therefore, the advantages provided are high economies of scale and low cost.

[0004] Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-270599

[0005] Patent Document 2: Japanese Laid-Open Patent Publication No. 2015-191031

[0006] Patent Document 3: U.S. Patent Application Publication No. 2020 / 0152574

[0007] However, in Si modulators, such as the X polarization modulation unit and the Y polarization modulation unit included in the optical modulator element provided in the conventional optical device, the voltage is applied through the Si layer doped with impurities, rather than by directly applying the voltage to the waveguide using a metal electrode, so that the resistance is higher than the case where only the metal electrode is used. In addition, due to the pN junction structure, the capacity of the optical waveguide used in the Si modulator is large. As a result, the high-frequency loss increases. Therefore, under the assumption that the driver is driven with an actual driving voltage (equal to or less than ±2V or 4V), it is difficult to widen the bandwidth equal to or greater than 50GHz and accelerate the baud rate equal to or greater than 96G.

[0008] Therefore, many reports have been made in academic conferences and the like to attempt to integrate electro-optical materials such as LiNbO3 (hereinafter referred to as LN), which have electro-optical effects capable of accelerating baud rates equal to or greater than 96G on SiPh components. (For example, Mingbo He1 et al., "High-performance hybrid silicon and lithiumniobate Mach-Zehnder modulators for 100Gbit s -1 and beyond (for 100Gbit s -1 High-performance hybrid silicon and lithium niobate Mach-Zehnder modulators (100 Å and above)”, Nat. Photon. 13, 359-364 (2019).

[0009] However, the electric permittivity of Si used for the substrate of the SiPh element is about 12, which is relatively large, so that the refractive index of the high-frequency electric signal traveling through the electrode tends to increase. In particular, as the electrode structure of the modulator, in the case of using a capacitive loading electrode (which increases the available electrode size due to the wide current distribution, which is conducive to reducing high-frequency losses), in addition to the influence of the substrate refractive index, the refractive index of the electric signal is also increased due to the influence of the capacitive loading electrode. Therefore, when compared with the speed of the signal light propagating through the LN waveguide, the speed of the electric signal becomes slightly lower, and bandwidth limitation occurs due to the mismatch of the speed matching. Therefore, it is difficult to achieve a wider bandwidth equal to or greater than 100GHz required for the 200G baud rate.

[0010] Therefore, an object of one aspect of an embodiment of the present invention is to provide an optical modulator element or the like that realizes a wider bandwidth of the modulator by ensuring speed matching between an electrical signal and a signal light. Summary of the invention

[0011] According to aspects of the embodiment, the optical modulator element includes an optical branching portion, an optical multiplexing portion, two optical waveguide arms, and an electrode. Each of the optical branching portion and the optical multiplexing portion includes a first material and is formed on a substrate. Each of the two optical waveguide arms connects the optical branching portion and the optical multiplexing portion and is formed on a substrate. The electrode applies an electrical signal to the two optical waveguide arms and is formed on a substrate. Each of the optical waveguides in the two optical waveguide arms includes a first optical waveguide, a second optical waveguide, and a transition portion. The first optical waveguide includes a first material. The second optical waveguide includes a second material having a higher electro-optical effect than the first material. The transition portion performs optical migration between the first optical waveguide and the second optical waveguide. The substrate includes a hollow portion in which all or part of the substrate located below the second optical waveguide in a plan view has been removed. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a schematic plan view illustrating one example of an optical transmitter / receiver according to the present embodiment;

[0013] Figure 2 is a schematic plan view illustrating one example of a sub-MZM according to the first embodiment;

[0014] Figure 3 is an explanatory diagram illustrating an example of a first migration section included in a sub-MZM;

[0015] Figure 4 is an example along the Figure 3 A cross-sectional schematic diagram of an example of a cross section taken along line AA as illustrated;

[0016] Figure 5 is an example along the Figure 3 A cross-sectional schematic diagram of an example of a cross section taken along line BB is illustrated;

[0017] Figure 6 is an example along the Figure 3 A cross-sectional schematic diagram of an example of a cross section taken along line CC as illustrated;

[0018] Figure 7 is a schematic plan view illustrating one example of a capacitance-loaded electrode included in a sub-MZM;

[0019] Figure 8 is an example along the Figure 2 A cross-sectional schematic diagram of an example of a cross section taken along line AA as illustrated;

[0020] Fig. 9 is an example along the Figure 2 A cross-sectional schematic diagram of an example of a cross section taken along line BB is illustrated;

[0021] Fig.10 is an example along the Figure 2 A cross-sectional schematic diagram of an example of a cross section taken along line CC as illustrated;

[0022] Fig.11 is an explanatory diagram illustrating an example of the relationship between the substrate removal width and the high-frequency refractive index expressed in the sub-MZM;

[0023] Fig.12 is an explanatory diagram illustrating an example of the frequency dependency of the EO characteristics exhibited in the sub-MZM;

[0024] Fig.13 is a cross-sectional schematic diagram illustrating an example of a sub-MZM according to the second embodiment;

[0025] Fig.14 is a cross-sectional schematic diagram illustrating an example of a sub-MZM according to the second embodiment;

[0026] Fig.15 is an explanatory diagram illustrating an example of the relationship between the substrate removal width and the high-frequency refractive index expressed in the sub-MZM;

[0027] Fig.16 is an explanatory diagram illustrating an example of the relationship between the substrate removal rate and the high-frequency refractive index for each substrate removal width;

[0028] Fig.17 is an explanatory diagram illustrating an example of the frequency dependency of the EO characteristics for each substrate removal rate exhibited in the sub-MZM;

[0029] Fig.18 is a schematic plan view illustrating one example of a sub-MZM according to a third embodiment;

[0030] Fig.19 is an example along the Fig.18 A cross-sectional schematic diagram of an example of a cross section taken along line AA as illustrated;

[0031] Fig. 20 is an example along the Fig.18 A cross-sectional schematic diagram of an example of a cross section taken along line BB is illustrated;

[0032] Fig.21 is an example along the Fig.18 A cross-sectional schematic diagram of an example of a cross section taken along line CC as illustrated;

[0033] Fig. 22 is a cross-sectional schematic diagram illustrating an example of a sub-MZM according to a fourth embodiment;

[0034] Fig.23 is a cross-sectional schematic diagram illustrating an example of a sub-MZM according to a fourth embodiment;

[0035] Fig.24 is a cross-sectional schematic diagram illustrating an example of a sub-MZM according to a fifth embodiment;

[0036] Fig.25 is a cross-sectional schematic diagram illustrating an example of a sub-MZM according to a fifth embodiment;

[0037] Fig.26 is a schematic plan view illustrating one example of a modulator according to a sixth embodiment;

[0038] Fig. 27 is a schematic plan view illustrating one example of a sub-MZM according to a seventh embodiment;

[0039] Fig.28 is an example along the Fig. 27 A cross-sectional schematic diagram of an example of a cross section taken along line AA as illustrated;

[0040] Fig.29 is an example along the Fig. 27 A cross-sectional schematic diagram of an example of a cross section taken along line BB is illustrated;

[0041] Fig.30 is a schematic plan view illustrating one example of a modulator according to an eighth embodiment; and

[0042] Fig.31 is a block diagram illustrating one example of an optical transceiver according to the present embodiment. DETAILED DESCRIPTION

[0043] With reference to the accompanying drawings, preferred embodiments of the present invention are described. In addition, the present invention is not limited to these embodiments. In addition, each of the embodiments can be used in any appropriate combination as long as they do not conflict with each other.

[0044] (a) First Embodiment

[0045] Figure 1 is a schematic plan view illustrating one example of the optical transmitter / receiver 1 according to the present embodiment. Figure 1The illustrated optical transmitter / receiver 1 is an optical device of a dual polarization quadrature phase shift keying (DP-QPSK) type having a baud rate of, for example, 200G. The optical transmitter / receiver 1 includes an optical modulator element 2 and an optical receiver element 3. Each of the optical modulator element 2 and the optical receiver element 3 is composed of, for example, a SiPh element. The optical transmitter / receiver 1 includes an optical waveguide 4 for a local oscillator, an optical waveguide 5 for transmission, an optical waveguide 6 for reception, a glass block 7, and a third branch portion 8.

[0046] The local oscillator optical waveguide 4 is, for example, a Si waveguide that is optically connected to the local oscillator optical fiber F1 via a glass block 7 by using a butt-joining technique, and propagates the local oscillator light. The transmission optical waveguide 5 is, for example, a Si waveguide that is optically connected to the output side optical fiber F2 via a glass block 7 by using a butt-joining technique, and propagates the transmitted light. The reception optical waveguide 6 is, for example, a Si waveguide that is optically connected to the input side optical fiber F3 via a glass block 7 by using a butt-joining technique, and propagates the received light. The local oscillator light incident from the local oscillator optical waveguide 4 is branched into two at the third branching portion 8, one of the branched local oscillator lights is used as a light source of the optical modulator element 2, and the other of the branched local oscillator lights is used as a local oscillator light of the optical receiver element 3. The branching ratio of the third branching portion 8 is optimally adjusted according to the application.

[0047] The optical receiver element 3 includes a polarization beam splitter (PBS) 11 and a first polarization rotator (PR) 12. The optical receiver element 3 includes a first optical hybrid circuit 13A, a second optical hybrid circuit 13B, and first to fourth photodiode (PD) groups 14A to 14D (14).

[0048] The PBS 11 separates the received light input from the receiving optical waveguide 6 into two orthogonal polarization states, i.e., for example, an X polarization component and a Y polarization component. In addition, the X polarization component is a horizontal polarization component, and the Y polarization component is a vertical polarization component. The PBS 11 outputs the separated X polarization component to the first optical hybrid circuit 13A. In addition, the first PR 12 performs a polarization rotation of 90 degrees on the Y polarization component received from the PBS 11, and outputs the Y polarization component subjected to the polarization rotation to the second optical hybrid circuit 13B.

[0049] The first optical hybrid circuit 13A obtains an optical signal having an I component and a Q component by allowing the local oscillator light to interfere with the X polarization component included in the received light. In addition, the I component is an in-phase axis component, and the Q component is an orthogonal axis component. The first optical hybrid circuit 13A outputs the signal light having the I component included in the X polarization component to the first PD group 14A. The first optical hybrid circuit 13A outputs the signal light having the Q component included in the X polarization component to the second PD group 14B.

[0050] The second optical hybrid circuit 13B obtains a signal light having an I component and a Q component by allowing the local oscillator light to interfere with the Y polarization component that has undergone polarization rotation. The second optical hybrid circuit 13B outputs the signal light having the I component included in the Y polarization component to the third PD group 14C. The second optical hybrid circuit 13B outputs the signal light having the Q component included in the Y polarization component to the fourth PD group 14D.

[0051] The first PD group 14A outputs an electrical signal by performing electrical conversion on the signal light having the I component included in the X polarization component received from the first optical hybrid circuit 13A. The second PD group 14B outputs an electrical signal by performing electrical conversion on the signal light having the Q component included in the X polarization component received from the first optical hybrid circuit 13A.

[0052] The third PD group 14C outputs an electrical signal by performing electrical conversion on the signal light having the I component included in the Y polarization component received from the second optical hybrid circuit 13B. The fourth PD group 14D outputs an electrical signal by performing electrical conversion on the signal light having the Q component included in the Y polarization component received from the second optical hybrid circuit 13B.

[0053] The optical modulator element 2 includes a first branching section 21, an X polarization modulation unit 22, a Y polarization modulation unit 23, a second PR 24, and a polarization beam combiner (PBC) 25. The first branching section 21 branches and outputs the local oscillator light to the X polarization modulation unit 22 and the Y polarization modulation unit 23.

[0054] The X-polarization modulation unit 22 (i.e., the parent MZM disposed on the X-polarization side) includes a second branching section 22A, two child Mach-Zehnder modulators (MZMs) 22B, two parent DC phase shifters 22C, and a first multiplexing section 22D. The second branching section 22A branches and outputs the signal light received from the first branching section 21 to each of the child MZMs 22B. Each of the child MZMs 22B includes a branching section 31, two optical waveguide arms 32, a multiplexing section 33, and an RF electrode 34.

[0055] The branching section 31 included in the X-polarization modulation unit 22 outputs the signal light received from the second branching section 22A to the two optical waveguide arms 32. The multiplexing section 33 multiplexes the signal light segments propagating through the two optical waveguide arms 32, and outputs the multiplexed signal light to the parent DC phase shifter 22C. One of the sub-MZMs 22B included in the X-polarization modulation unit 22 is, for example, a modulation unit that modulates the signal light having the I component included in the X polarization propagating through the two optical waveguide arms 32 according to the high-frequency signal output from the RF electrode 34, and outputs the modulated signal light having the I component to the parent DC phase shifter 22C. In addition, another sub-MZM in the sub-MZM 22B included in the X-polarization modulation unit 22 is, for example, a modulation unit that modulates the signal light having the Q component included in the X-polarization propagating through the two optical waveguide arms 32 according to the high-frequency signal output from the RF electrode 34, and outputs the modulated signal light having the Q component to the parent DC phase shifter 22C.

[0056] Each of the parent DC phase shifters 22C included in the X polarization modulation unit 22 is a phase adjustment unit that adjusts the phase of the signal light modulated with the I component included in the X polarization and received from one of the sub-MZMs 22B according to the driving voltage signal, and adjusts the phase of the signal light modulated with the Q component included in the X polarization and received from the other sub-MZM of the sub-MZM 22B according to the driving voltage signal. As a result of the phase adjustment performed by each of the parent DC phase shifters 22C, the signal light modulated with the I component included in the X polarization and the signal light modulated with the Q component included in the X polarization can be made orthogonal. Both the signal light with the I component included in the X polarization and the signal light with the Q component included in the X polarization that have passed through the parent DC phase shifter 22C are multiplexed in the first multiplexing section 22D, and the multiplexed IQ mixed signal included in the X polarization is output to the PBC 25.

[0057] The Y polarization modulation unit 23 (i.e., the parent MZM on the Y polarization) includes a second branching portion 23A, two sub-MZMs 23B, two parent DC phase shifters 23C, and a first multiplexing portion 23D. The second branching portion 23A branches and outputs the signal light received from the first branching portion 21 to each of the sub-MZMs 23B. Each of the sub-MZMs 23B includes a branching portion 31, two optical waveguide arms 32, a multiplexing portion 33, and an RF electrode 34.

[0058] The branch section 31 included in the Y polarization modulation unit 23 outputs the signal light received from the second branch section 23A to each of the two optical waveguide arms 32. The multiplexing section 33 multiplexes the signal light propagating through each of the two optical waveguide arms 32, and outputs the multiplexed signal light to the parent DC phase shifter 23C. One of the sub-MZMs 23B included in the Y polarization modulation unit 23 is, for example, a phase modulation unit that modulates the signal light having the I component included in the Y polarization propagating through the two optical waveguide arms 32 according to the high-frequency signal output from the RF electrode 34, and outputs the modulated signal light having the I component to the parent DC phase shifter 23C. In addition, another sub-MZM in the sub-MZM 23B included in the Y polarization modulation unit 23 is, for example, a phase modulation unit that modulates the signal light having the Q component included in the Y polarization propagating through the two optical waveguide arms 32 according to the high-frequency signal output from the RF electrode 34, and outputs the modulated signal light having the Q component to the parent DC phase shifter 23C.

[0059] Each of the parent DC phase shifters 23C included in the Y polarization modulation unit 23 is a phase adjustment unit that adjusts the phase of the modulated signal light having the I component included in the Y polarization and having been received from one sub-MZM 23B, and adjusts the phase of the modulated signal light having the Q component included in the Y polarization and having been received from another sub-MZM 23B, according to the driving voltage signal. As a result of the phase adjustment performed by the parent DC phase shifter 22C, the modulated signal light having the I component included in the Y polarization and the modulated signal light having the Q component included in the Y polarization can be made orthogonal. Both the signal light having the I component included in the Y polarization and the signal light having the Q component included in the Y polarization that have passed through the parent DC phase shifter 23C are multiplexed in the first multiplexing section 23D, and the multiplexed IQ mixed signal having the Y polarization is output to the second PR 24.

[0060] The second PR 24 performs polarization rotation of 90 degrees on the IQ mixed signal with Y polarization, and outputs the IQ mixed signal with Y polarization subjected to polarization rotation to the PBC 25. Then, the PBC 25 multiplexes the IQ mixed signal with X polarization received from the X polarization modulation unit 22 and the IQ mixed signal with Y polarization received from the second PR 24 and subjected to polarization rotation, and then outputs the multiplexed IQ mixed signal from the transmission optical waveguide 5.

[0061] Figure 21 is a schematic plan view illustrating one example of the sub-MZM 22B (23B) according to the first embodiment. One of the sub-MZMs 22B included in the X polarization modulation unit 22 will be described. In addition, each of the sub-MZMs 23B included in the Y polarization modulation unit 23 also has the same configuration as that of each of the sub-MZMs 22B included in the X polarization modulation unit 22; therefore, by assigning the same reference numerals to components having the same configuration, repeated descriptions of their configurations and operations will be omitted.

[0062] As described above, each of the sub-MZMs 22B (23B) includes a branch portion 31, two optical waveguide arms 32, a multiplexing portion 33, and an RF electrode 34. The branch portion 31 is, for example, a branch portion made of Si, which branches and outputs the signal light received from the second branch portion 22A (23A) to each of the first Si waveguides 32A included in the two optical waveguide arms 32. The two optical waveguide arms 32 are two arms in which each arm includes two first Si waveguides 32A, two first migration portions 32B, two LN waveguides 32C, two second migration portions 32D, and two second Si waveguides 32E. Each of the first Si waveguides 32A is, for example, a Si waveguide having a channel type. Each of the first Si waveguides 32A is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. Each of the LN waveguides 32C is, for example, an LN waveguide having a ridge type. Each of the second Si waveguides 32E is, for example, a Si waveguide having a channel type. Each of the second Si waveguides 32E is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The first Si waveguide 32A and the second Si waveguide 32E are formed in the same layer, and the LN waveguide 32C is formed in a layer different from the layer in which the first Si waveguide 32A and the second Si waveguide 32E are formed.

[0063] Each of the first migration portions 32B is an interlayer migration portion in which signal light is optically migrated between the first Si waveguide 32A and the LN waveguide 32C in the corresponding layer. In addition, the first Si waveguide 32A and the LN waveguide 32C are provided in different layers. The RF electrode 34 includes a signal electrode 34A arranged in parallel with the corresponding waveguides included in the two corresponding optical waveguide arms 32, and a ground electrode 34B arranged in parallel with the corresponding waveguides. In the case of inputting a high-frequency signal received from the driver circuit 35, each of the signal electrodes 34A modulates the signal light propagating through the LN waveguide 32C arranged between the ground electrode 34B and the signal electrode 34A.

[0064] Each of the second migration sections 32D is an interlayer migration section in which the modulated signal light is optically migrated between the LN waveguide 32C and the second Si waveguide 32E in the corresponding layer. In addition, the second Si waveguide 32E and the LN waveguide 32C are arranged in different layers. The multiplexing section 33 is a multiplexing section made of Si, for example, which multiplexes the modulated signal light received from each of the second Si waveguides 32E and outputs the multiplexed signal light to each of the parent DC phase shifters 22C (23C).

[0065] Figure 3 2 is an explanatory diagram illustrating an example of a first migration portion 32B included in a sub-MZM 22B. The first migration portion 32B includes a Si substrate 41 and a cladding 42 formed on the Si substrate 41. The cladding 42 is a SiO2 layer having a refractive index lower than that of the Si substrate 41 and a thickness of, for example, 4 μm. The cladding 42 includes a first cladding 42A formed on the Si substrate 41 and a second cladding 42B formed on the first cladding 42A. In addition, similarly, the optical transmitter / receiver 1 also includes the Si substrate 41 and the cladding 42.

[0066] The first cladding 42A is, for example, a SiO2 layer made of SiO2. The second cladding 42B is, for example, a SiO2 layer made of SiO2. Each of the first Si waveguide 32A and the second Si waveguide 32E is a waveguide located in a lower layer arranged in the first cladding 42A on the Si substrate 41. The LN waveguide 32C is a waveguide located in an upper layer formed on the second cladding 42B.

[0067] Figure 4 is an example along the Figure 3 A cross-sectional schematic diagram of one example of a cross section taken along line AA is illustrated. Figure 4 The illustrated first Si waveguide 32A is arranged inside the cladding 42, and the LN waveguide 32C is arranged on the cladding 42. In the first migration portion 32B, the signal light migrates from the first Si waveguide 32A to the LN waveguide 32C as interlayer migration.

[0068] Figure 5 is an example along the Figure 3 A cross-sectional schematic diagram of one example of a cross section taken along line BB is illustrated. Figure 5 The illustrated first Si waveguide 32A is arranged inside the cladding 42, and the LN waveguide 32C is arranged on the cladding 42. The first Si waveguide 32A has a tapered structure in which the waveguide width gradually tapers toward the LN waveguide 32C. In the first migration portion 32B, signal light migrates from the first Si waveguide 32A to the LN waveguide 32C as interlayer migration.

[0069] Figure 6 is an example along the Figure 3A cross-sectional schematic diagram of an example of a cross section taken along line CC is shown. The first Si waveguide 32A has a structure in which the waveguide width is from Figure 3 The cross-sectional portion taken along the line AA shown in the example is directed toward Figure 3 The cross-sectional portion taken along the illustrated line CC gradually narrows, and as shown in FIG. Figure 6 The example ends along Figure 3 The cross-sectional portion taken along the illustrated line CC. In the first transition portion 32B, the signal light confined in the first Si waveguide 32A transitions from the first Si waveguide 32A toward the LN waveguide 32C as interlayer transition.

[0070] In addition, for the convenience of description, Figures 4 to 6 The first migration portion 32B is described, and in addition, the second migration portion 32D also has substantially the same configuration. The second Si waveguide 32E is, for example, a channel-type Si waveguide having a tapered structure in which the waveguide width gradually narrows toward the LN waveguide 32C. In the second migration portion 32D, the signal light confined in the LN waveguide 32C migrates from the LN waveguide 32C toward the second Si waveguide 32E as interlayer migration.

[0071] The first Si waveguide 32A and the second Si waveguide 32E are provided in the same layer. The LN waveguide 32C is provided in a layer different from the layer in which the first Si waveguide 32A and the second Si waveguide 32E are provided.

[0072] Figure 7 2 is a schematic plan view illustrating an example of a capacitance-loaded electrode provided in the sub-MZM 22B. The RF electrode 34 is Figure 7 The illustrated capacitively loaded electrode RF electrode 34 includes a signal electrode 34A and a ground electrode 34B.

[0073] The portion included in the signal electrode 34A and arranged in parallel to the LN waveguide 32C is composed of a plurality of T-shaped tracks. In addition, the portion included in the ground electrode 34B and arranged in parallel to the LN waveguide 32C is also composed of a plurality of T-shaped tracks. The capacitively loaded electrode is composed of a plurality of T-shaped tracks, so that as a result of the wide distribution of current, the available electrode size is increased. This is conducive to reducing high-frequency losses and can contribute to a wider bandwidth. However, in addition to the influence of the refractive index of the Si substrate 41, the refractive index of the electrical signal is also increased by the influence of the capacitively loaded electrode. As a result, when the speed of the electrical signal in the RF electrode 34 is compared with the speed of the signal light propagating through the LN waveguide 32C, the speed of the electrical signal is slightly lower. Therefore, it can be thought that the bandwidth is limited due to the speed mismatch.

[0074] Figure 8 is an example along the Figure 2A cross-sectional schematic diagram of one example of a cross section taken along line AA is illustrated. Figure 8 Each of the two optical waveguide arms 32 illustrated includes a Si substrate 41, a cladding 42 formed on the Si substrate 41, and a first Si waveguide 32A formed in the cladding 42. Each of the two optical waveguide arms 32 includes an LN waveguide 32C formed on the cladding 42 and an RF electrode 34 arranged in parallel with the LN waveguide 32C. The RF electrode 34 includes a signal electrode 34A and a ground electrode 34B, and sandwiches the ridge of the LN waveguide 32C between the signal electrode 34A and the ground electrode 34B.

[0075] Fig. 9 is an example along the Figure 2 A cross-sectional schematic diagram of an example of a cross section taken along line BB is illustrated, and Fig.10 is an example along the Figure 2 The cross-sectional view of one example of the cross-sectional view taken along the illustrated line CC is shown. In addition, the cross-sectional view taken along the line CC is a cross-sectional view of the two optical waveguide arms 32 in the waveguide direction. Fig. 9 and Fig.10 At the illustrated two optical waveguide arms 32, a hollow portion 41A is formed in the Si substrate 41 located at the lower portion of the LN waveguide 32C. In addition, a case where the hollow portion 41A is formed in the Si substrate 41 located at the lower portion of the LN waveguide 32C has been described as an example, but the hollow portion 41A may be formed in the Si substrate 41 located at the lower portion of the LN waveguide 32C and in a portion of the cladding 42 at a position in contact with the Si substrate 41, and appropriate modifications are possible. Each of the hollow portions 41A is a space formed in the Si substrate 41 as a result of removing the portion by dry etching. The opening width W1 of each of the hollow portions 41A corresponds to the substrate removal width.

[0076] In each of the hollow portions 41A, a portion corresponding to the width W1 included in the Si substrate 41 has been removed over the entire lower portion of the LN waveguide 32C by using a method such as dry etching, but the cladding 42 having a thickness of 4 μm remains in the lower portion of the LN waveguide 32C. Therefore, high rigidity and strength sufficient to withstand vibration tests and impact tests are maintained in the LN waveguide 32C. This type of structure is also called a thin film structure, and has high reliability due to a proven track record in the market, such as a tunable Si etalon filter for an external resonator laser, and the like.

[0077] The permittivity of the air in the hollow portion 41A is "1", the permittivity of the SiO2 layer as the cladding 42 is about "4", and the permittivity of Si is "12"; therefore, the permittivity of each of the hollow portion 41A and the cladding 42 is smaller than the permittivity of the Si substrate 41. This means that the refractive index of the high-frequency signal sensed by the high-frequency electrical signal becomes lower, and the speed of the traveling high-frequency signal becomes higher. Therefore, the two optical waveguide arms 32 can match the speed of the signal light fragment propagating through the LN waveguide 32C with the speed of the high-frequency signal.

[0078] Fig.11 2 is an explanatory diagram illustrating an example of the relationship between the substrate removal width and the high-frequency refractive index expressed in the sub-MZM 22B. In addition, for the convenience of description, it is assumed that the substrate removal width is 0 μm in the case where there is no hollow portion 41A of the conventionally used Si substrate 41. In the case where the substrate removal width is 0 μm, as shown in FIG. Fig.11 As illustrated, the high-frequency refractive index of the high-frequency signal is 2.45, while the refractive index of the signal light propagating through the LN waveguide 32C is 2.2. Therefore, the high-frequency refractive index of the high-frequency signal is greater than the refractive index of the light. On the other hand, in the case where the substrate removal width of the hollow portion 41A of the Si substrate 41 according to the present embodiment is, for example, 5 μm to 10 μm included in the range of 3 μm to 12 μm, the high-frequency refractive index of the high-frequency signal is approximately 2.2, and substantially matches the refractive index of the signal light propagating through the LN waveguide 32C.

[0079] Fig.12 2 is an explanatory diagram illustrating an example of the frequency dependency of the EO response exhibited in the sub-MZM 22B. Fig.12 As illustrated, when the substrate removal width of the hollow portion 41A is 10 μm, the EO response to the high-frequency signal can be significantly improved compared to the case where the substrate removal width is 0 μm.

[0080] The sub-MZM 22B (23B) according to the first embodiment includes a hollow portion 41A in the Si substrate 41 located at the lower portion of the LN waveguides 32C included in the respective two optical waveguide arms 32. Therefore, by matching the speed of light propagating through each of the LN waveguides 32C with the speed of the high-frequency signal, a wider bandwidth of the optical modulator element 2 can be achieved.

[0081] Furthermore, a case where the RF electrode 34 provided in the sub-MZM 22B ( 23B) according to the first embodiment is, for example, a capacitance loading type electrode has been described as an example, but a general electrode may be used and appropriate modification is possible.

[0082] The case where the hollow portion 41A provided in the sub-MZM 22B (23B) according to the first embodiment is formed in units of the single-piece LN waveguide 32C included in the two optical waveguide arms 32 has been described as an example. However, the single-piece hollow portion 41A may be formed in units of two LN waveguides 32C, and the embodiment thereof may be described as the second embodiment.

[0083] (b) Second Embodiment

[0084] Fig.13 is a cross-sectional schematic diagram illustrating one example of the sub-MZM 22B1 ( 23B1 ) according to the second embodiment. Fig.13 The cross-sectional schematic diagram shown is along Figure 2 The cross section corresponds to the cross section taken along the illustrated line BB. In addition, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter / receiver 1 according to the first embodiment, repeated descriptions of their configurations and operations will be omitted. The sub-MZM 22B1 (23B1) according to the second embodiment is different from the sub-MZM 22B (23B) according to the first embodiment in that a hollow portion 41B located at the lower portion of two LN waveguides 32C in units of two LN waveguides 32C is formed in the Si substrate 41.

[0085] By performing dry etching, a Si substrate 41 located at the lower portion of the two LN waveguides 32C included in the two optical waveguide arms 32 is formed. Fig.13 The illustrated hollow portion 41B has an opening width W2 that is wider than the opening width W1 of the hollow portion 41A according to the first embodiment.

[0086] Fig.14 is a schematic cross-sectional view of one example of the sub-MZM 22B1 ( 23B1 ) according to the second embodiment. Fig.14 The cross-sectional schematic diagram shown is along Figure 2 The cross section corresponds to the cross section taken along line CC illustrated. Fig.14 The illustrated hollow portions 41B are formed in the Si substrate 41 at predetermined intervals in the waveguide direction of the LN waveguide 32C.

[0087] In the Si substrate 41 located at the lower part of the two LN waveguides 32C, a hollow part 41B and a residual part 41C are included. Each of the hollow parts 41B has an opening width W2 of the two optical waveguide arms 32 in the width direction and an opening width La of the two optical waveguide arms 32 in the waveguide direction. The opening width W2 of each of the hollow parts 41B is, for example, 80 μm. Each of the residual parts 41C has a width Ls in the waveguide direction of the residual part 41C located between the hollow parts 41B. The thickness of each of the residual parts 41C located at the lower part of the two LN waveguides 32C is, for example, 4 μm.

[0088] Including the plurality of residual portions 41C in the lower portions of the two LN waveguides 32C enables the LN waveguide 32C to ensure high rigidity and strength sufficient to withstand, for example, a vibration test and an impact test.

[0089] In the case where the opening width W2 of each of the hollow portions 41B is wide and the width is equal to or greater than 80 μm, for the case of the structure described in the first embodiment in which the Si substrate 41 is removed in the entire waveguide direction (=structure in which the substrate removal rate is 100%), the refractive index of the high-frequency signal becomes equal to or less than 2.0. Therefore, it is conceivable that a speed mismatch occurs as a result of the speed of the high-frequency signal being too high with respect to the refractive index of 2.2 of light.

[0090] On the other hand, for a structure in which only a portion of the Si substrate 41 is removed in the direction along the LN waveguide 32C, the refractive index of the high-frequency signal is adjusted by changing the removal rate of the Si substrate 41, and an appropriate removal rate is obtained. Therefore, the speed of the signal light fragment propagating through the LN waveguide 32C and the speed of the high-frequency signal can be matched.

[0091] Fig.15 : is an explanatory diagram illustrating an example of the relationship between the substrate removal width W2 and the high-frequency refractive index expressed in the sub-MZM 22B. In the case where the opening width W2 of the hollow portion 41B is 40 μm to 80 μm, if the substrate removal rate is called 100% by removing the area corresponding to all the lower portions of the two LN waveguides 32C of the Si substrate 41, the refractive index of the high-frequency signal becomes 1.9, and the refractive index of the signal light propagating through the LN waveguide 32C becomes 2.2.

[0092] Fig.16 is an explanatory diagram illustrating an example of the relationship between the substrate removal rate and the high-frequency refractive index for each substrate removal width, and Fig.172 is an explanatory diagram illustrating an example of the frequency dependency of the EO characteristics for each substrate removal rate exhibited in the sub-MZM 22B1 (23B1). As described above in the second embodiment, when the substrate removal rate is assumed to be 60% by arranging the hollow portion 41B having La of 30 μm and the residual portion 41C having Ls of 20 μm located at the lower portion of the two LN waveguides 32C at a predetermined interval, as shown in FIG. Fig.16 As illustrated, the refractive index of the high-frequency signal becomes about 2.2. Therefore, the refractive index of the signal light propagating through the LN waveguide 32C and the refractive index of the high-frequency signal are substantially matched.

[0093] Moreover, in the case where the opening width W2 of the hollow portion 41B is set to 40 μm to 80 μm, the bandwidth of the EO response can be widened in the same manner as described in the first embodiment. In addition, assuming that the arm interval is 40 μm and the substrate removal rate defined by La / (La+Ls) is 100%, even if the removal width of the Si substrate 41 in the region corresponding to the lower portion of the LN waveguide 32C included in the arm is changed, the refractive index of the high-frequency signal does not change when the removal width is equal to or greater than 40 μm. Therefore, it is preferable that the width of the Si substrate 41 located in the region corresponding to the lower portion of the LN waveguide 32C included in the arm and to be removed is set to be equal to or greater than the width of the arm interval, and it is preferable that, as Fig.16 As illustrated, the substrate removal rate defined by La / (La+Ls) is 40% (0.4) to 80% (0.8).

[0094] Moreover, the opening width W2, the opening width La, and the width Ls are not limited to the above-mentioned widths, and as long as the EO bandwidth can be widened by making the refractive index of the high-frequency signal and the refractive index of the signal light substantially match, appropriate modifications are possible. In addition, a case has been described as an example in which, according to the second embodiment, the hollow portions 41B and the residual portions 41C are arranged at regular intervals in the Si substrate 41 located in the region corresponding to the lower portions of the two LN waveguides 32C. However, the hollow portions 41B and the residual portions 41C do not always need to be arranged at regular intervals, as long as the EO bandwidth can be widened by making the refractive index of the high-frequency signal and the refractive index of the signal light substantially match, and appropriate modifications are possible.

[0095] The case where, in the sub-MZM 22B1 (23B1) according to the second embodiment, the hollow portion 41B and the residual portion 41C are formed by removing a portion of the Si substrate 41 located in the region corresponding to the lower portions of the two LN waveguides 32C has been described as an example. However, the hollow portion and the residual portion may be formed by removing a portion of the Si substrate 41 located in the region corresponding to the lower portion of the single-piece LN waveguide 32C included in the sub-MZM 22B (23B) according to the first embodiment, and appropriate modifications are possible.

[0096] In the LN waveguide 32C included in the sub-MZM 22B (23B) according to the first embodiment and the sub-MZM 22B1 (23B1) according to the second embodiment, the Pockels coefficient indicating the electro-optic effect is low, i.e., about 30 pm / V. Therefore, in order to improve the modulation efficiency so that the driver circuit is driven by the actual amplitude voltage, as Figure 2 As illustrated, the length of the modulator arm needs to be 10 mm or longer. Therefore, it is impossible to utilize the economy of scale that is an advantage of SiPh components. Therefore, an embodiment that solves this situation will be described below as a third embodiment.

[0097] (c) Third Embodiment

[0098] Fig.18 2 is a cross-sectional schematic diagram illustrating an example of a sub-MZM 22B2 (23B2) according to the third embodiment. In addition, for ease of description, by assigning the same reference numerals to components having the same configuration as the components in the optical transmitter / receiver 1 according to the second embodiment, repeated descriptions of their configurations and operations will be omitted. In addition, the sub-MZM 23B2 also has the same configuration as that of the sub-MZM 22B2; therefore, by assigning the same reference numerals to components having the same configuration as the components in the sub-MZM 22B2, repeated descriptions of their configurations and operations will be omitted. The sub-MZM 22B2 (23B2) according to the third embodiment is different from the sub-MZM 22B1 (23B1) according to the second embodiment in that two optical waveguide arms 32 are formed to have a folded structure.

[0099] The sub-MZM 22B2 includes a branch portion 31, two optical waveguide arms 32, a multiplexing portion 33, and an RF electrode 34. The branch portion 31 is a branch portion made of, for example, Si, which branches and outputs the signal light received from the second branch portion 22A (23A) to each of the first Si waveguides 32A included in the corresponding two optical waveguide arms 32. The two optical waveguide arms 32 respectively include two first Si waveguides 32A, two first migration portions 32B, and two first LN waveguides 32C1. The two optical waveguide arms 32 respectively include two third migration portions 51A, two third Si waveguides 51B, two folded waveguides 51C, two fourth Si waveguides 51D, and two fourth migration portions 51E. The two optical waveguide arms 32 respectively include two second LN waveguides 32C2, two second migration portions 32D, and two second S1 waveguides 32E.

[0100] The first Si waveguide 32A is a Si waveguide having, for example, a channel type. The first Si waveguide 32A is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The first LN waveguide 32C1 is an LN waveguide having, for example, a ridge type. The third Si waveguide 51B is a Si waveguide having, for example, a channel type connected to the folded waveguide 51C. The third Si waveguide 51B is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The folded waveguide 51C is a Si waveguide having a channel type that is formed into a U-shaped folded structure with a small bending radius because the folded waveguide 51C strongly confines light.

[0101] The fourth Si waveguide 51D is a Si waveguide having, for example, a channel type connected to the folded waveguide 51C. The fourth Si waveguide 51D is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The second LN waveguide 32C2 is an LN waveguide having, for example, a ridge type. The second Si waveguide 32E is a Si waveguide having, for example, a channel type. The second Si waveguide 32E is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape.

[0102] The first migration portion 32B is an interlayer migration portion in which signal light is optically migrated between the first Si waveguide 32A and the first LN waveguide 32C1 in the corresponding layer. The RF electrode 34 includes a signal electrode 34A arranged in parallel with the corresponding waveguides included in the two corresponding optical waveguide arms 32, and a ground electrode 34B arranged in parallel with the corresponding waveguides included in the two corresponding optical waveguide arms 32. In the case of inputting a high-frequency signal received from the driver circuit 35A, each of the signal electrodes 34A modulates the signal light propagating through the first LN waveguide 32C1 arranged between the ground electrode 34B and the signal electrode 34A.

[0103] The third migration portion 51A is an interlayer migration portion in which the signal light is optically migrated between the first LN waveguide 32C1 and the third Si waveguide 51B in the corresponding layer. The fourth migration portion 51E is an interlayer migration portion in which the signal light is optically migrated between the fourth Si waveguide 51D and the second LN waveguide 32C2 in the corresponding layer. In the case where a high-frequency signal received from the driver circuit 35A is input, the signal electrode 34A modulates the signal light propagating through the second LN waveguide 32C2 arranged between the ground electrode 34B and the signal electrode 34A.

[0104] The second migration section 32D is an interlayer migration section in which the signal light is optically migrated between the second LN waveguide 32C2 and the second Si waveguide 32E in the corresponding layer. The multiplexing section 33 is a multiplexing section made of, for example, Si, which multiplexes the signal light received from each of the second Si waveguides 32E and outputs the multiplexed signal light to the parent DC phase shifter 22C (23C).

[0105] The two optical waveguide arms 32 couple a portion between the first LN waveguide 32C1 and the second LN waveguide 32C2 by the folded waveguide 51C, so that the longitudinal dimension can be reduced to ½ compared with the dimension of the LN waveguide 32C according to the first embodiment.

[0106] The sub-MZM 22B2 includes a first heater electrode 52A disposed near the third Si waveguide 51B, a second heater electrode 52B disposed near the fourth Si waveguide 51D, and an electrode wiring 53. The electrode wiring 53 is a metal wiring that injects current into the first heater electrode 52A and injects current into the second heater electrode 52B.

[0107] Fig.19 is an example along the Fig.18 A cross-sectional schematic diagram of one example of a cross section taken along line AA is illustrated. Fig.19 Each of the illustrated two optical waveguide arms 32 includes a Si substrate 41, a cladding 42 formed on the Si substrate 41, a first Si waveguide 32A formed in the cladding 42, and a second Si waveguide 32E formed in the cladding 42. The first Si waveguide 32A and the second Si waveguide 32E are formed in the same layer.

[0108] Each of the two optical waveguide arms 32 includes a first LN waveguide 32C1 formed on a cladding 42, a second LN waveguide 32C2 formed on the cladding 42, and an RF electrode 34 arranged in parallel with the first LN waveguide 32C1 and the second LN waveguide 32C2. The first LN waveguide 32C1 and the second LN waveguide 32C2 are formed in the same layer. Each of the first LN waveguides 32C1 is arranged in parallel between an associated signal electrode 34A and an associated ground electrode 34B. Each of the second LN waveguides 32C2 is arranged in parallel between an associated signal electrode 34A and an associated ground electrode 34B. The first LN waveguide 32C1 and the second LN waveguide 32C2 are formed in a layer different from a layer in which the first Si waveguide 32A and the second Si waveguide 32E are arranged.

[0109] Fig. 20 is an example along the Fig.18 A cross-sectional schematic diagram of one example of a cross section taken along line BB is illustrated. A hollow portion 41D and a residual portion 41E are included in the Si substrate 41 located at the lower portion of the first LN waveguide 32C1 and the second LN waveguide 32C2. Including a plurality of residual portions 41E in the lower portion of each of the first LN waveguide 32C1 and the second LN waveguide 32C2 enables the first LN waveguide 32C1 and the second LN waveguide 32C2 to ensure high rigidity and strength sufficient to withstand, for example, a vibration test and an impact test.

[0110] The permittivity of the air in the hollow portion 41D is "1", the permittivity of the SiO2 layer as the cladding 42 is about "4", and the permittivity of Si is "12"; therefore, the permittivity of each of the hollow portion 41D and the cladding 42 is smaller than the permittivity of the Si substrate 41. This means that the refractive index of the high-frequency signal sensed by the high-frequency electrical signal becomes lower, and the speed of the traveling high-frequency signal becomes higher. Therefore, the speed of the signal light fragment propagating through both the first LN waveguide 32C1 and the second LN waveguide 32C2 can be made to match the speed of the high-frequency signal.

[0111] The signal electrode 34A includes a first signal electrode arranged near the first LN waveguide 32C1, a second signal electrode arranged near the second LN waveguide 32C2, and a U-shaped signal electrode electrically connecting a portion between the first signal electrode and the second signal electrode. Each of the ground electrodes 34B includes a first ground electrode arranged near the first LN waveguide 32C1, a second ground electrode arranged near the second LN waveguide 32C2, and a U-shaped ground electrode electrically connecting a portion between the first ground electrode and the second ground electrode.

[0112] The sub-MZM 22B2 is connected to a driver circuit 35A having a differential drive type. The driver circuit 35A is connected to a first signal electrode included in the signal electrode 34A and a second signal electrode included in the signal electrode 34A. The driver circuit 35A allows a high-frequency signal to propagate through both the first signal electrode and the first LN waveguide 32C1 in the same electric field direction, and allows a high-frequency signal to propagate through both the second signal electrode and the second LN waveguide 32C2 in the same electric field direction, so that a situation in which the modulation phase is canceled can be avoided.

[0113] In the case where a driver circuit of a commercially available open collector type is used as the driver circuit 35A of the differential drive type and a DC voltage is applied to each of the first LN waveguide 32C1 and the second LN waveguide 32C2, it is conceivable that a DC drift occurs in the operating point voltage of the LN modulator. However, in the sub-MZM 22B2 according to the third embodiment, a phase shifter including a heater electrode 52 that compensates for the DC drift is provided. The first phase shifter includes a third Si waveguide 51B and a first heater electrode 52A formed at a lower portion of the third Si waveguide 51B. In addition, the second phase shifter includes a fourth Si waveguide 51D and a second heater electrode 52B formed at a lower portion of the fourth Si waveguide 51D.

[0114] Fig.21 is an example along the Fig.18 A cross-sectional schematic diagram of an example of a cross section taken along line CC is illustrated. Fig.21 The illustrated cross-sectional portion is a cross-sectional portion of the first phase shifter. In addition, for ease of description, the second phase shifter also has the same configuration as that of the first phase shifter; therefore, by assigning the same reference numerals to components having the same configuration, repeated descriptions of their configurations and operations will be omitted. Fig.21 The illustrated first phase shifter includes a Si substrate 41 , a cladding layer 42 , a third Si waveguide 51B formed in the cladding layer 42 , and a first heater electrode 52A formed in the cladding layer 42 and formed at a lower portion of the third Si waveguide 51B.

[0115] The first phase shifter heats the first heater electrode 52A by causing current to flow into the first heater electrode 52A, and heats the third Si waveguide 51B by the heat generated by the first heater electrode 52A. As a result of heating the third Si waveguide 51B, the refractive index of the third Si waveguide 51B increases, so that the first phase shifter adjusts the phase of the signal light propagating through the third Si waveguide 51B. As a result of the phase of the signal light being adjusted, the operating point voltage can be controlled by an external automatic bias control (ABC) circuit, so that DC drift can be compensated. The Si waveguide itself does not generate DC drift, making it relatively easy to ensure the life of the DC drift. In addition, the refractive index change caused by temperature in Si is large, so that the electric power consumed in the heater can be suppressed. In addition, the heater electrode 52 can be formed by the manufacturing process used in SiPh.

[0116] (d) Fourth Embodiment

[0117] Fig. 22 2 is a cross-sectional schematic diagram illustrating an example of a sub-MZM 22B3 (23B3) according to the fourth embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter / receiver 1 according to the first embodiment, repeated descriptions of their configurations and operations will be omitted. The optical transmitter / receiver 1 according to the fourth embodiment includes an optical modulator element 2 formed by using a 2.5-dimensional packaging method and an optical receiver element 3 formed by using a 2.5-dimensional packaging method. The optical transmitter / receiver 1 according to the fourth embodiment is different from the optical transmitter / receiver 1 according to the first embodiment in that a sub-MZM 22B3 (23B3) is arranged instead of the sub-MZM 22B (23B). Fig. 22 The illustrated cross-sectional portion is a cross-sectional portion of the first migration portion 32B included in the sub-MZM 22B3 ( 23B3 ), the second migration portion 32D included in the sub-MZM 22B3 ( 23B3 ), and the PD 14 .

[0118] The first migration portion 32B included in the sub-MZM 22B3 includes a Si substrate 41, a cladding 42, a first Si waveguide 32A formed in the cladding 42, a first LN waveguide 32C1 formed on the cladding 42, a signal electrode 34A, and a ground electrode 34B. The second migration portion 32D included in the sub-MZM 22B3 includes a Si substrate 41, a cladding 42, a second Si waveguide 32E formed in the cladding 42, a second LN waveguide 32C2 formed on the cladding 42, a signal electrode 34A, and a ground electrode 34B.

[0119] The sub-MZM 22B3 includes a through-Si via (TSV) 34D that passes through a portion between the top surface of the cladding 42 and the top surface of the Si substrate 41 and electrically connects a portion between the signal electrode 34A arranged on the top surface of the cladding 42 and an electrode pad arranged on the top surface of the Si substrate 41.

[0120] The PD 14 includes a Si substrate 41, a clad 42, a Si layer 62 formed in the clad 42, a Ge layer 63 formed in the clad 42 and in contact with the Si layer 62, and through holes 64. Each of the through holes 64 passes through a portion between the Si layer 62 and a pad electrode between the clad 42 and the Si substrate 41, and electrically connects the Si layer 62 to a portion between the electrode pads located between the clad 42. The PD 14 includes through holes 65, each of which passes through a portion between an electrode pad located on the top surface of the Si substrate 41 and an electrode pad located between the clad 42 and the Si substrate 41, and electrically connects an electrode pad on the Si substrate 41 to a portion between the electrode pads located between the clad 42 and the Si substrate 41.

[0121] It is preferable to reduce the surface roughness of the top surface of the cladding 42 in which the first LN waveguide 32C1 and the second LN waveguide 32C2 are integrally mounted by chemical mechanical polishing (CMP). In addition, it is assumed that the distance in the height direction between the first LN waveguide 32C1 included in the first migration portion 32B and the first Si waveguide 32A is within, for example, 1500 nm. It is assumed that the distance in the height direction between the second LN waveguide 32C2 included in the second migration portion 32D and the second Si waveguide 32E is, for example, 1500 nm.

[0122] Fig.23 : is a schematic plan view illustrating one example of a sub-MZM 22B3 (23B3) according to the fourth embodiment. In the Si substrate 41 located at the lower portion of the first LN waveguide 32C1 and the second LN waveguide 32C2, a hollow portion 41F and a residual portion 41G are included. Including a plurality of residual portions 41G in the lower portion of the first LN waveguide 32C1 and the second LN waveguide 32C2 enables the first LN waveguide 32C1 and the second LN waveguide 32C2 to ensure high rigidity and strength sufficient to withstand, for example, a vibration test and an impact test.

[0123] The permittivity of the air in the hollow portion 41F is "1", the permittivity of the SiO2 layer as the cladding 42 is about "4", and the permittivity of Si is "12"; therefore, the permittivity of each of the hollow portion 41F and the cladding 42 is smaller than the permittivity of the Si substrate 41. This means that the refractive index of the high-frequency signal sensed by the high-frequency electrical signal becomes lower, and the speed of the traveling high-frequency signal becomes higher. Therefore, the speed of the signal light fragment propagating through the first LN waveguide 32C1 and the second LN waveguide 32C2 can be made to match the speed of the high-frequency signal.

[0124] However, the SiPh wafer has a laminated structure composed of multiple layers including a metal wiring layer, an insulating layer, etc., and the distance between the Si waveguide and the top surface is equal to or greater than 7000 nm. Therefore, regarding the distance between the first migration portion 32B made of Si and the top surface of the cladding layer 42, the CMP process cannot be performed within 1500 nm.

[0125] The SiPh wafer is temporarily bonded to a different Si substrate used as a handling substrate, and the Si substrate (not illustrated) located on the SiPh wafer side is removed from the back surface by using a grinding and polishing process. At this time, the PD 14 including the Ge layer 63 is vertically inverted to have a Fig. 22 The illustrated vertical structural relationship. The first Si waveguide 32A included in the first migration portion 32B, the second Si waveguide 32E included in the second migration portion 32D, and the Si layer 62 are all located in the Si layer of the SOI corresponding to the source of the SiPh wafer and are located at the same height, and the Si layer 62 is adjacent to the Ge layer 63 included in the PD 14, has been doped and has conductive properties.

[0126] Here, the through hole 64 electrically connected to the Si layer 62 included in the PD 14 is buried in the Si substrate 41, so that it is necessary to pull the electrode up to the top surface of the Si substrate 41 through the TSV 65. When manufacturing the TSV 65, it is necessary to generate a hole in the Si substrate 41 by dry etching, so that when manufacturing the TSV, it is possible to form a hole in the Si substrate 41 by dry etching. Fig.23 The partial hollow portion 41F is formed in the illustrated Si substrate 41. Therefore, the hollow portion 41F formed in the Si substrate 41 does not significantly increase the cost.

[0127] The optical transmitter / receiver 1 according to the fourth embodiment includes the optical modulator element 2 formed by using the 2.5-dimensional packaging method and the optical receiver element 3 formed by using the 2.5-dimensional packaging method. The optical modulator element 2 and the optical receiver element 3 formed by using the 2.5-dimensional packaging method can be realized.

[0128] (e) Fifth Embodiment

[0129] Fig.24 1 is a cross-sectional schematic diagram illustrating one example of a sub-MZM 22B4 (23B4) according to the fifth embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter / receiver 1 according to the first embodiment, repeated descriptions of their configurations and operations will be omitted. The optical transmitter / receiver 1 according to the fifth embodiment is different from the optical transmitter / receiver 1 according to the first embodiment in that a sub-MZM 22B4 (23B4) and a PD 141 are arranged instead of the sub-MZM 22B (23B) and the PD 14. Fig.24 The illustrated cross-sectional portions are those of the first migration portion 32B included in the sub-MZM 22B4 ( 23B4 ), the second migration portion 32D included in the sub-MZM 22B4 ( 23B4 ), and the PD 141 .

[0130] The first migration portion 32B included in the sub-MZM 22B4 includes a Si substrate 41, a cladding 42, a first Si waveguide 32A formed in the cladding 42, and a hollow portion 42C formed in the cladding 42. The first migration portion 32B includes a first LN waveguide 32C1 formed in the hollow portion 42C, a signal electrode 34A, and a ground electrode 34B. The hollow portion 42C is formed by digging a portion of the cladding 42 into a groove shape by dry etching. The second migration portion 32D included in the sub-MZM 22B4 includes a Si substrate 41, a cladding 42, a second Si waveguide 32E formed in the cladding 42, and a hollow portion 42C. The second migration portion 32D includes a second LN waveguide 32C2 formed in the hollow portion 42C, a signal electrode 34A, and a ground electrode 34B.

[0131] PD 141 includes Si substrate 41, cladding 42, Si layer 62A formed in cladding 42, Ge layer 63A formed in cladding 42 and formed on Si layer 62A. PD 14 includes through holes 64A, each of which passes through a portion between Si layer 62A and an electrode pad located on the top surface of cladding 42, and electrically connects Si layer 62A and the electrode pad located on cladding 42. PD 141 includes TSV 61D, which passes through a portion between electrode pad located on the top surface of Si substrate 41 and electrode pad located on the top surface of cladding 42, and electrically connects electrode pad located on Si substrate 41 and electrode pad located on cladding 42.

[0132] The height distance between the first LN waveguide 32C1 included in the first transition portion 32B and the first Si waveguide 32A is set within 1500 nm, for example. The height distance between the second LN waveguide 32C2 included in the second transition portion 32D and the second Si waveguide 32E is set within 1500 nm, for example.

[0133] Fig.25 : is a schematic plan view illustrating one example of the sub-MZM 22B4 according to the fifth embodiment. In the Si substrate 41 located at the lower part of the first LN waveguide 32C1 and the second LN waveguide 32C2, a hollow part 41H and a residual part 41J are included. Including a plurality of residual parts 41J in the lower part of the first LN waveguide 32C1 and the second LN waveguide 32C2 enables the first LN waveguide 32C1 and the second LN waveguide 32C2 to ensure high rigidity and strength sufficient to withstand, for example, a vibration test and an impact test.

[0134] In the sub-MZM 22B4 according to the fifth embodiment, compared with the sub-MZM 22B3 according to the fourth embodiment, it is not necessary to perform the CMP process on all regions of the top surface of the clad layer 42. The distance in the height direction between the first LN waveguide 32C1 and the first Si waveguide 32A included in the first migration portion 32B and the distance in the height direction between the second LN waveguide 32C2 and the second Si waveguide 32E included in the second migration portion 32D can be set to be within 1500 nm, and the distance between the Si waveguide layer and the top surface determined according to the multi-layer laminated structure composed of the metal wiring layer, the insulating layer, etc. formed in the SiPh wafer can also be set to 7000 nm or more in a compatible manner.

[0135] In addition, in the sub-MZM 22B4 according to the fifth embodiment, there is no need to temporarily bond the SiPh wafer to a different Si substrate used as a handling substrate, as compared to the sub-MZM 22B3 according to the fourth embodiment. In this case, the PD 141 including the Ge layer 63A is not vertically inverted, thereby having Fig.24 Here, the first Si waveguide 32A, the second Si waveguide 32E, and the Si layer 62A are all formed in the Si layer of the SOI corresponding to the source of the SiPh wafer and are located at the same height.

[0136] In addition, the electrode provided in the PD 141 can use a through hole 64A, which is manufactured in a normal SiPh wafer and electrically connected to a doped Si layer 62A having conductive properties located in the PD 141 without any change, and is connected to a transimpedance amplifier (TIA). On the other hand, the electrode provided in the sub-MZM 22B4 is connected to the driver circuit 35A via a connection electrode 34D extending upward from a signal electrode 34A located on the bottom surface of the hollow portion 42C to the top surface. In addition, a TSV 61D that passes through the Si substrate 41 for 2.5-dimensional packaging is used to connect to an interposer wiring substrate (not illustrated) located on the back surface side of the Si substrate 41. However, when manufacturing the TSV 61D, it is necessary to generate a hole in the Si substrate 41 by dry etching, and when the TSV is formed by dry etching, it is possible to simultaneously generate a TSV as a substrate. Fig.25 The illustrated Si substrate 41 has a hollow portion 41H in a local portion. Therefore, the formation of the hollow portion 41H does not significantly increase the cost.

[0137] (f) Sixth Embodiment

[0138] Fig.26 1 is a schematic plan view illustrating an example of a modulator according to a sixth embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter / receiver 1 according to the third embodiment, repeated descriptions of their configurations and operations will be omitted. The modulator includes an X polarization modulation unit 22 composed of two single-type MZMs operating at a 200G baud rate, and a Y polarization modulation unit 23 composed of two single-type MZMs. The modulator is an MZM using a dual polarization quadrature amplitude modulation (DP-QAM) method.

[0139] Fig.26 The illustrated X polarization modulation unit 22 is an IQ modulator including a second branch 22A, two sub-MZMs 22B5 and a single parent DC phase shifter 22C. The Y polarization modulation unit 23 is an IQ modulator including a second branch 23A, two sub-MZMs 23B5 and a single parent DC phase shifter 23C.

[0140] The modulator is connected to a driver circuit 35A which inputs a high-frequency signal to each of the RF electrode 34 included in the sub-MZM 22B5 and the RF electrode 34 included in the sub-MZM 23B5 and is formed in a differential driving type for DP-QAM.

[0141] In the modulator according to the sixth embodiment, by applying the modulator to an MZM having a DP-QAM type, the speeds of the signal light propagating through the first LN waveguide 32C1 and the second LN waveguide 32C2 are allowed to match, thereby achieving a wide bandwidth. In addition, the two optical waveguide arms 32 included in the sub-MZM 22B5 (23B5) have a folded structure, so that the size of the optical transmitter / receiver 1 can be reduced by reducing the size of the modulator in the longitudinal direction.

[0142] In addition, a case has been described as an example in which the two optical waveguide arms 32 included in the sub-MZM 22B2 (23B2) according to the third embodiment are formed to have a single folded structure to reduce the size of the modulator in the longitudinal direction. However, this example is not limited to this, and an embodiment in which a further reduction in size is achieved will be described below as a seventh embodiment. In addition, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter / receiver 1 according to the third embodiment, repeated descriptions of their configurations and operations will be omitted.

[0143] (g) Seventh Embodiment

[0144] Fig. 27 is a schematic cross-sectional view illustrating one example of the sub-MZM 22B6 (23B6) according to the seventh embodiment. The sub-MZM 22B2 according to the third embodiment is different from the sub-MZM 22B6 according to the seventh embodiment in that two optical waveguide arms 32 are configured to have a double-folded structure.

[0145] The two optical waveguide arms 32 include two first Si waveguides 32A, two first migration portions 32B, and two first LN waveguides 32C11. The two optical waveguide arms 32 include two third migration portions 51A1, two third Si waveguides 51B1, two folded waveguides 51C1, two fourth Si waveguides 51D1, two fourth migration portions 51E1, and two second LN waveguides 32C12. The two optical waveguide arms 32 include two fifth migration portions 51G1, two fifth Si waveguides 51F1, two folded waveguides 51H1, two sixth Si waveguides 51J1, two sixth migration portions 51K1, and two third LN waveguides 32C13. The two optical waveguide arms 32 include two second migration portions 32D and two second Si waveguides 32E.

[0146] The first Si waveguide 32A is a Si waveguide having, for example, a channel type. The first Si waveguide 32A is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The first LN waveguide 32C11 is an LN waveguide having, for example, a ridge type. The third Si waveguide 51B1 is a Si waveguide formed in, for example, a channel type and connected to the folded waveguide 51C1. The third Si waveguide 51B1 is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The folded waveguide 51C1 is a Si waveguide formed in a channel type formed in a U-shaped folded structure with a small bending radius because the third Si waveguide 51B1 strongly confines light.

[0147] The fourth Si waveguide 51D1 is a Si waveguide formed in, for example, a channel type and connected to the folded waveguide 51C1. The fourth Si waveguide 51D1 is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The second LN waveguide 32C12 is an LN waveguide formed in, for example, a ridge type.

[0148] The fifth Si waveguide 51F1 is a Si waveguide formed in, for example, a channel type and connected to the folded waveguide 51H1. The fifth Si waveguide 51F1 is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The folded waveguide 51H1 is a Si waveguide formed in a channel type formed in a U-shaped folded structure with a small bending radius because the fifth Si waveguide 51F1 strongly confines light.

[0149] The sixth Si waveguide 51J1 is a Si waveguide formed in, for example, a channel type and connected to the folded waveguide 51H1. The sixth Si waveguide 51J1 is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape. The third LN waveguide 32C13 is an LN waveguide formed in, for example, a ridge type. The second Si waveguide 32E is a Si waveguide formed in, for example, a channel type. The second Si waveguide 32E is a tapered waveguide in which one end of the waveguide width is formed to have a tapered shape.

[0150] The first migration portion 32B is an interlayer migration portion in which signal light is optically migrated between the first Si waveguide 32A and the first LN waveguide 32C11 in the corresponding layer. The RF electrode 34 includes a signal electrode 34A arranged in parallel with the corresponding waveguides included in the two optical waveguide arms 32, and a ground electrode 34B arranged in parallel with the corresponding waveguides. In the case of inputting a high-frequency signal received from the driver circuit 35A, each of the signal electrodes 34A modulates the signal light propagating through the associated first LN waveguide 32C11 arranged between the associated ground electrode 34B and the signal electrode 34A.

[0151] The third migration portion 51A1 is an interlayer migration portion in which signal light is optically migrated between the first LN waveguide 32C11 and the third Si waveguide 51B1 in the corresponding layer. The fourth migration portion 51E1 is an interlayer migration portion in which signal light is optically migrated between the fourth Si waveguide 51D1 and the second LN waveguide 32C12 in the corresponding layer. In the case where a high-frequency signal received from the driver circuit 35A is input, each of the signal electrodes 34A modulates the signal light propagating through the associated second LN waveguide 32C12 arranged between the associated ground electrode 34B and the signal electrode 34A.

[0152] The fifth migration portion 51G1 is an interlayer migration portion in which signal light is optically migrated between the second LN waveguide 32C12 and the fifth Si waveguide 51F1 in the corresponding layer. The sixth migration portion 51K1 is an interlayer migration portion in which signal light is optically migrated between the sixth Si waveguide 51J1 and the third LN waveguide 32C13 in the corresponding layer. In the case where a high-frequency signal received from the driver circuit 35A is input, each of the signal electrodes 34A modulates the signal light propagating through the associated third LN waveguide 32C13 arranged between the associated ground electrode 34B and the signal electrode 34A.

[0153] The second migration section 32D is an interlayer migration section in which the signal light is optically migrated between the third LN waveguide 32C13 and the second Si waveguide 32E in the corresponding layer. The multiplexing section 33 is a multiplexing section made of, for example, Si, which multiplexes the signal light received from each of the second Si waveguides 32E and outputs the multiplexed signal light to the parent DC phase shifter 22C (23C).

[0154] The two optical waveguide arms 32 respectively connect a portion between the first LN waveguide 32C11 and the second LN waveguide 32C12 by using a folded waveguide 51C1, and respectively connect a portion between the second LN waveguide 32C12 and the third LN waveguide 32C13 by using a folded waveguide 51H1. Compared with the LN waveguide 32C according to the first embodiment, the size of the modulator in the longitudinal direction can be reduced to 1 / 3.

[0155] The sub-MZM 22B6 includes a first heater electrode 52A1 arranged in the corresponding fifth Si waveguide 51F1, a second heater electrode 52B1 arranged in the corresponding sixth Si waveguide 51J1, and an electrode wiring 53A. The electrode wiring 53A is a metal wiring that injects current into the first heater electrode 52A1 and injects current into the second heater electrode 52B1.

[0156] Fig.28 is an example along the Fig. 27 A cross-sectional schematic diagram of one example of a cross section taken along line AA is illustrated. Fig.28Each of the illustrated two optical waveguide arms 32 includes a Si substrate 41, a cladding 42 formed on the Si substrate 41, and a first Si waveguide 32A formed in the cladding 42. Each of the two optical waveguide arms 32 includes a fifth Si waveguide 51F1 formed in the cladding 42, and a sixth Si waveguide 51J1 formed in the cladding 42. The first Si waveguide 32A, the third Si waveguide 51B1, the fourth Si waveguide 51D1, the fifth Si waveguide 51F1, the sixth Si waveguide 51J1, and the second Si waveguide 32E are formed in the same layer.

[0157] Each of the two optical waveguide arms 32 includes a first LN waveguide 32C11 formed on the cladding 42, a second LN waveguide 32C12 formed on the cladding 42, and a third LN waveguide 32C13 formed on the cladding 42. Each of the two optical waveguide arms 32 includes a first LN waveguide 32C11, a second LN waveguide 32C12, and a third LN waveguide 32C13 arranged in parallel to the RF electrode 34. The first LN waveguide 32C11, the second LN waveguide 32C12, and the third LN waveguide 32C13 are formed in the same layer. Each of the first LN waveguides 32C11 is arranged in parallel between the associated signal electrode 34A and the associated ground electrode 34B. Each of the second LN waveguides 32C12 is arranged in parallel between the associated signal electrode 34A and the associated ground electrode 34B. Each of the third LN waveguides 32C13 is arranged in parallel between the associated signal electrode 34A and the associated ground electrode 34B.

[0158] Fig.29 is an example along the Fig. 27 A cross-sectional schematic diagram of an example of a cross section taken along line BB is illustrated. A hollow portion 41K and a residual portion 41L are included in the Si substrate 41 located at the lower portion of the first LN waveguide 32C11, the second LN waveguide 32C12, and the third LN waveguide 32C13. A plurality of residual portions 41L are included in the lower portion of the first LN waveguide 32C11, the second LN waveguide 32C12, and the third LN waveguide 32C13. Therefore, the first LN waveguide 32C11, the second LN waveguide 32C12, and the third LN waveguide 32C13 can ensure high rigidity and strength sufficient to withstand, for example, a vibration test and an impact test.

[0159] The permittivity of air in each of the hollow portions 41K is "1", the permittivity of the SiO2 layer as the cladding 42 is about "4", and the permittivity of Si is "12", so that the permittivity of each of the hollow portion 41K and the cladding 42 is smaller than the permittivity of the Si substrate 41. This means that the refractive index of the high-frequency signal sensed by the high-frequency electrical signal becomes lower, and the speed of the traveling high-frequency signal becomes higher. Therefore, the speed of the signal light fragment propagating through the first LN waveguide 32C11, the second LN waveguide 32C12, and the third LN waveguide 32C13 can be matched with the speed of the high-frequency signal.

[0160] Each of the signal electrodes 34A includes a first signal electrode arranged near the associated first LN waveguide 32C11, a second signal electrode arranged near the associated second LN waveguide 32C12, and a third signal electrode arranged near the associated third LN waveguide 32C13. Each of the ground electrodes 34B includes a first ground electrode arranged near the associated first LN waveguide 32C11, a second ground electrode arranged near the associated second LN waveguide 32C12, and a third ground electrode arranged near the associated third LN waveguide 32C13. The sub-MZM 22B6 is connected to a driver circuit 35A formed in a differential drive type. The driver circuit is connected to the first signal electrode, the second signal electrode, and the third signal electrode. The driver circuit 35A applies a high-frequency signal to the first signal electrode and the first LN waveguide 32C11 in the same electric field direction, to the second signal electrode and the second LN waveguide 32C12 in the same electric field direction, and to the third signal electrode and the third LN waveguide 32C13 in the same electric field direction. Therefore, a situation where the modulation phase is cancelled can be avoided.

[0161] In the sub-MZM 22B6, a phase shifter including a heater electrode 52 and compensating for DC drift is arranged. The third phase shifter includes a fifth Si waveguide 51F1, and a first heater electrode 52A1 formed at a lower portion of the corresponding fifth Si waveguide 51F1. In addition, the fourth phase shifter includes a sixth Si waveguide 511, and a second heater electrode 52B1 formed at a lower portion of the corresponding sixth Si waveguide 511.

[0162] The third phase shifter heats the first heater electrode 52A1 by causing a current to flow into the first heater electrode 52A1, and heats the fifth Si waveguide 51F1 by the heat generated by the first heater electrode 52A1. As a result of heating the fifth Si waveguide 51F1, the refractive index of the fifth Si waveguide 51F1 increases, so that the third phase shifter adjusts the phase of the signal light propagating through the fifth Si waveguide 51F1. As a result of the phase of the signal light being adjusted, the operating point voltage can be controlled by an external automatic bias control (ABC) circuit, so that DC drift can be compensated.

[0163] The fourth phase shifter heats the second heater electrode 52B1 by causing a current to flow into the second heater electrode 52B1, and heats the sixth Si waveguide 51J1 by the heat generated by the second heater electrode 52B1. As a result of heating the sixth Si waveguide 51J1, the refractive index of the sixth Si waveguide 51J1 increases, so that the fourth phase shifter adjusts the phase of the signal light propagating through the sixth Si waveguide 51J1. As a result of the phase of the signal light being adjusted, the operating point voltage can be controlled by an external automatic bias control (ABC) circuit, so that DC drift can be compensated.

[0164] The two optical waveguide arms 32 according to the seventh embodiment respectively connect a portion between the first LN waveguide 32C11 and the second LN waveguide 32C12 by using a folded waveguide 51C1, and respectively connect a portion between the second LN waveguide 32C12 and the third LN waveguide 32C13 by using a folded waveguide 51H1. Therefore, the size of the modulator in the longitudinal direction can be reduced to 1 / 3 compared with the LN waveguide 32C according to the first embodiment.

[0165] (h) Eighth Embodiment

[0166] Fig.30 1 is a schematic plan view illustrating an example of a modulator according to an eighth embodiment. The modulator includes an X polarization modulation unit 22 composed of two single-type MZMs operating at a baud rate of 200G, and a Y polarization modulation unit 23 composed of two single-type MZMs. The modulator according to the sixth embodiment is different from the modulator according to the eighth embodiment in that an MZM of a dual polarization quadrature phase shift keying (DP-QPSK) type is arranged instead of an MZM of a DP-QAM type.

[0167] Fig.30 The illustrated X polarization modulation unit 22 is one of the IQ modulators including the second branch 22A, two sub-MZMs 22B7, and a single parent DC phase shifter 22C. The Y polarization modulation unit 23 is another IQ modulator of the IQ modulator including the second branch 23A, two sub-MZMs 23B7, and a single parent DC phase shifter 23C.

[0168] The modulator is connected to a driver circuit 35A which is formed in a differential driving type used for DP-QPSK and inputs a high-frequency signal to both the RF electrode 34 included in the sub-MZM 22B7 and the RF electrode 34 included in the sub-MZM 23B7.

[0169] In the modulator according to the eighth embodiment, by applying the modulator to an MZM having a DP-QPSK type, the speeds of the signal light propagating through the first LN waveguide 32C11, the second LN waveguide 32C12, and the third LN waveguide 32C13 are allowed to match, thereby achieving a wide bandwidth. In addition, the two optical waveguide arms 32 included in the sub-MZM 22B7 (23B7) have a folded structure, so that the size of the optical transmitter / receiver 1 can be reduced by reducing the size of the modulator in the longitudinal direction.

[0170] A case has been described as an example in which LN (LiNbO3) is used as the electro-optical material of the optical modulator element 2. For example, a perovskite-type oxide material may also be used, and appropriate modifications are possible. Examples of the perovskite-type oxide material used include (Pb)(Zr,Ti)O3 (hereinafter, represented by PZT), (Pb,La)(Zr,Ti)O3 (hereinafter, represented by PLZT), BaTiO3 (hereinafter, represented by BTO), (Sr,Ba)TiO3 (hereinafter, represented by SBT), and LiNbO3 (hereinafter, represented by LN). However, a perovskite-type oxide material having another electro-optical effect may be used.

[0171] In addition, in the present embodiment, the case of using the LN waveguide 32C has been described as an example, but any material can be used as long as a material having a lower dielectric constant than a Si capacitor and also having an electro-optical effect higher than that of Si is used, and appropriate modifications are possible.

[0172] (i) Ninth Embodiment

[0173] Hereinafter, an optical transceiver 100 using the optical transmitter / receiver 1 according to the present embodiment will be described. Fig.31 is a block diagram illustrating one example of the optical transceiver 100 according to the present embodiment. Fig.31 The illustrated optical transceiver 100 includes an optical transmitter / receiver 110 and a digital signal processor (DSP) 120. The optical transmitter / receiver 110 includes an optical modulator element 111 in an optical module 130, a driver circuit 112, an optical receiver element 113 in the optical module 130, and a transimpedance amplifier 114. The DSP 120 performs overall control of the optical transmitter / receiver 110. The DSP 120 is an electronic component that performs IQ modulation processing on a transmission signal and demodulation processing on a reception signal, as well as performs digital signal processing.

[0174] The DSP 120 performs processing such as encoding transmission data, generating an electrical signal including the transmission data, and then outputs the generated electrical signal to the driver circuit 112. The driver circuit 112 drives the optical modulator element 111 according to the electrical signal received from the DSP 120. The optical modulator element 111 optically modulates the signal light. The optical modulator element 111 is the optical modulator element in the first to eighth embodiments described above.

[0175] The optical receiver element 113 performs electrical conversion on the signal light. The TIA 114 amplifies the electrical signal subjected to the electrical conversion, and outputs the amplified electrical signal to the DSP 120. The DSP 120 obtains the received data by performing a decoding process or the like on the electrical signal acquired from the TIA 114. The optical transmitter / receiver 1 is composed of both the optical modulator element 111 and the optical receiver element 113. However, this embodiment is not limited to the optical transmitter / receiver 1, and an optical transmitter including only the optical modulator element 111 as a built-in unit may also be applicable.

[0176] According to an aspect of one embodiment, the wider bandwidth of the modulator is achieved by ensuring a speed match between the electrical signal and the signal light.

Claims

1. A light modulator element, comprising: an optical branching section and an optical multiplexing section, each of the optical branching section and the optical multiplexing section including a first material and formed on a substrate; two optical waveguide arms, each of which connects the optical branching section and the optical multiplexing section and is formed on the substrate; as well as an electrode that applies an electrical signal to the two optical waveguide arms and is formed on the substrate, wherein: Each of the optical waveguides in the two optical waveguide arms comprises: a first optical waveguide, the first optical waveguide comprising the first material; a second optical waveguide comprising a second material having a higher electro-optic effect than the first material; and a migration section that performs light migration between the first optical waveguide and the second optical waveguide, and The substrate includes a hollow portion in which all or part of the substrate located below the second optical waveguide in a plan view has been removed.

2. The light modulator element according to claim 1, wherein The electrodes are capacitively loaded electrodes.

3. The light modulator element according to claim 1, wherein The first material includes Si, and the second material includes LiNbO 3 .

4. The light modulator element according to claim 1, wherein The hollow portion is a hollow portion formed by removing all or part of the substrate located below the two optical waveguide arms.

5. The light modulator element according to claim 1, wherein The substrate located at the lower portion of the second optical waveguide comprises: the hollow portion; and The remaining part other than the hollow part, and The ratio of the hollow portion in the region of the substrate located at the lower portion of the second optical waveguide is within a range of 40% to 80%.

6. The light modulator element according to claim 1, wherein The opening width of the hollow portion is at least in the range of 3 μm to 12 μm.

7. The light modulator element according to claim 6, wherein: The opening width of the hollow portion is in the range of 5 μm to 10 μm included in the range of 3 μm to 12 μm.

8. The light modulator element according to claim 1, wherein The second optical waveguide comprises: The first straight waveguide; a second linear waveguide; and A folded waveguide connects the first straight waveguide and the second straight waveguide, and the folded waveguide includes the first material.

9. The light modulator element according to claim 8, wherein: The folded waveguide: comprising a heater electrode arranged in the vicinity of the folded waveguide and heating the folded waveguide, and The phase of the signal light propagating through the folded waveguide is adjusted according to the injection of current into the heater electrode.

10. The optical modulator element according to claim 1, further comprising a cladding layer formed on the substrate, wherein: The light modulator element comprises: the first optical waveguide, the first optical waveguide being formed in the cladding; the second optical waveguide, the second optical waveguide being formed on the cladding; and The electrode is formed on the cladding, wherein the light modulator element further includes a through hole that passes through the front and rear sides of both the substrate and the cladding and is electrically connected to the electrode.

11. The optical modulator element according to claim 1, further comprising a cladding layer formed on the substrate, the optical modulator element comprising: the first optical waveguide, the first optical waveguide being formed in the cladding; the second optical waveguide formed in the hollow portion included in the cladding; as well as The electrode is formed in the hollow portion included in the cladding, wherein the light modulator element further includes a through hole that passes through the front and rear sides of both the substrate and the cladding and is electrically connected to the electrode.

12. The light modulator element according to claim 1, wherein The second optical waveguide comprises: The first straight waveguide; a second linear waveguide; A third straight waveguide; a first folded waveguide connecting the first straight waveguide and the second straight waveguide; and a second folded waveguide, the second folded waveguide connecting the second straight waveguide and the third straight waveguide, and Each of the first folded waveguide and the second folded waveguide includes the first material.

13. An optical transmitter including an optical modulator element, the optical modulator element including: an optical branching section and an optical multiplexing section located on a substrate, each of the optical branching section and the optical multiplexing section including a first material; two optical waveguide arms, each of the two optical waveguide arms connecting the optical branching section and the optical multiplexing section; and electrodes that apply electrical signals to the two optical waveguide arms, wherein Each of the optical waveguides in the two optical waveguide arms comprises: a first optical waveguide, the first optical waveguide comprising the first material; a second optical waveguide, the second optical waveguide comprising a second material, the second material having a higher electro-optic effect than the first material; as well as a migration section that performs light migration between the first optical waveguide and the second optical waveguide, and The substrate includes a hollow portion in which all or part of the substrate located below the second optical waveguide in a plan view has been removed.

14. An optical transceiver, comprising: a light modulator element, the light modulator element comprising a light modulator; a light receiver element, the light receiver element comprising a light receiver; as well as a processor that performs signal processing on the optical modulator and the optical receiver, wherein The light modulator element includes: an optical branching section and an optical multiplexing section, each of the optical branching section and the optical multiplexing section comprising a first material; two optical waveguide arms, each of the two optical waveguide arms connecting the optical branching section and the optical multiplexing section; and electrodes, the electrodes applying electrical signals to the two optical waveguide arms, Each of the optical waveguides in the two optical waveguide arms comprises: a first optical waveguide, the first optical waveguide comprising the first material; a second optical waveguide comprising a second material having a higher electro-optic effect than the first material; and a migration section that performs light migration between the first optical waveguide and the second optical waveguide, and The substrate includes a hollow portion in which all or part of the substrate located below the second optical waveguide in a plan view has been removed.

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