Optical waveguide element, optical modulator, and optical transmission device

Abstract

Provided is an optical waveguide element, an optical modulator, and an optical transmission device that can be miniaturized while improving electrical characteristics. One embodiment is an optical waveguide element that includes a signal electrode formed so as to extend along an optical waveguide formed on a substrate, and two ground electrodes that sandwich the signal electrode from both sides, the optical waveguide element including: an electrode bending portion that bends the extension direction of the signal electrode and the two ground electrodes; a connection ground electrode that connects the two ground electrodes at a lower portion of the signal electrode in the electrode bending portion; and a buffer layer formed between the signal electrode and the connection ground electrode.

Description

Technical Field

[0001] This invention relates to an optical waveguide element, an optical modulator, and an optical transmission device. Background Technology

[0002] In high-speed / high-capacity optical fiber communication systems, optical modulators are often used, which incorporate optical modulation elements that function as optical waveguides. These waveguides consist of an optical waveguide formed on a substrate and control electrodes that control the light waves propagating within it. Semiconductor optical modulation elements using semiconductor substrates such as InP substrates and LN optical modulation elements using LiNbO3 (hereinafter also referred to as LN (Lithium Niobate)) as the substrate have been practically implemented as optical waveguide elements for optical modulation.

[0003] In optical waveguide elements that perform optical modulation operations, with the miniaturization of the elements, there are optical waveguide elements with electrode foldback structures. A prior art technique is known where, in the electrode foldback structure of such optical waveguide elements, in order to prevent conversion to unwanted propagation modes, an air bridge based on lead bonding is provided between the ground electrodes of the foldback section, thereby reducing the transmission loss and reflection attenuation of the electrical signal.

[0004] [Existing Technical Documents]

[0005] [Patent Literature]

[0006] [Patent Document 1] Chinese Patent Application Publication No. 113985629

[0007] [Patent Document 2] Japanese Patent No. 7504330

[0008] [Patent Document 3] Japanese Patent Application Publication No. 2019-49647

[0009] [Patent Document 4] Japanese Patent Application Publication No. 2022-38756

[0010] [Patent Document 5] Japanese Patent No. 5298849

[0011] [Patent Document 6] Japanese Patent Publication No. 2024-524644 Summary of the Invention

[0012] [The problem the invention aims to solve]

[0013] However, in the prior art, there is a limitation on the miniaturization of components due to the need to ensure the electrode area used for wire bonding. Furthermore, there are issues such as increased light absorption caused by the electrodes, installation risks such as short circuits between electrodes caused by wire bonding, and increased bonding time, making it difficult to miniaturize components while improving electrical characteristics.

[0014] [Technical means to solve the problem]

[0015] One embodiment of this disclosure is an optical waveguide element, which has a signal electrode formed in a portion along the extension direction of an optical waveguide formed on a substrate, and two ground electrodes sandwiching the signal electrode from both sides. The optical waveguide element includes: an electrode bending portion that bends the extension direction of the signal electrode and the two ground electrodes; a connecting ground electrode formed in the electrode bending portion below the signal electrode to connect the two ground electrodes; and a buffer layer formed between the signal electrode and the connecting ground electrode.

[0016] [The effects of the invention]

[0017] According to one embodiment of the present disclosure, components can be miniaturized while improving electrical characteristics. Attached Figure Description

[0018] Figure 1 This is a diagram showing the structure of the optical modulator according to the first embodiment.

[0019] Figure 2 It means used for Figure 1 The diagram shows the structure of the optical modulation element in the optical modulator.

[0020] Figure 3 This is a diagram showing the folded-back structure of the electrodes in an optical modulator.

[0021] Figure 4 yes Figure 3 A detailed view of part A.

[0022] Figure 5 yes Figure 3 VV cross-section diagram.

[0023] Figure 6 This is a diagram showing a first modified example of the folded-back structure of the electrodes of an optical modulator.

[0024] Figure 7 yes Figure 6 Section VII-VII.

[0025] Figure 8 This is a diagram showing a second variation of the folded-back structure of the electrodes of an optical modulator.

[0026] Figure 9 yes Figure 8 IX-IX cross-section.

[0027] Figure 10 This is a diagram showing a third variation of the folded-back structure of the electrodes in an optical modulator.

[0028] Figure 11 yes Figure 10 XI-XI cross-sectional view.

[0029] Figure 12 This is a diagram showing a fourth variation of the folded-back structure of the electrodes in an optical modulator.

[0030] Figure 13 yes Figure 12 Section XIII-XIII.

[0031] Figure 14 This is a diagram showing the fifth variation of the folded-back structure of the electrodes of an optical modulator.

[0032] Figure 15 yes Figure 14 XV-XV cross-section.

[0033] Figure 16 yes Figure 14 XVI-XVI cross-sectional view.

[0034] Figure 17 This is a diagram of the sixth variation of the folded-back structure of the electrodes of an optical modulator.

[0035] Figure 18 yes Figure 17 XVIII-XVIII cross-section.

[0036] Figure 19 This is a diagram showing the structure of the optical transmitting device according to the second embodiment.

[0037] Explanation of icon numbers

[0038] 1: Optical modulator

[0039] 2: Frame

[0040] 3: Optical modulation element

[0041] 4, 5: Signal pins

[0042] 6: Input fiber

[0043] 7: Output fiber

[0044] 8, 9: Bracket

[0045] 10a, 10b, 10c, 11a, 11b: Lenses

[0046] 14: Relay substrate

[0047] 15, 15a, 15b, 15c, 15d: Terminating resistors

[0048] 16: Terminal

[0049] 17: Drive circuit

[0050] 30: Optical substrate

[0051] 30a: Reinforced substrate

[0052] 30b: Lower surface buffer layer

[0053] 30c: LN layer

[0054] 30d: Groove

[0055] 31: Optical waveguide

[0056] 32a, 32b, 32c, 32d: Edges

[0057] 33: Input waveguide

[0058] 34: Branch waveguide

[0059] 35a, 35b: Nested Mach-Zehnder type optical waveguides

[0060] 36a, 36b: Output waveguides

[0061] 37, 37a, 37b, 37c, 37d: Mach-Zehnder type optical waveguides

[0062] 38, 38a, 38b, 38c, 38d: First turnaround area

[0063] 39, 39a, 39b, 39c, 39d: Second turnaround area

[0064] 40: Bias electrode section

[0065] 40a: First bias electrode section

[0066] 40b: Second bias electrode section

[0067] 50, 50a, 50b, 50c, 50d: RF electrode section

[0068] 51, 52, 53: Segmented electrodes

[0069] 54: Low dielectric layer

[0070] 60: Electrode bending section

[0071] 61: Connect the grounding electrode

[0072] 62: Upper surface buffer layer

[0073] 71: Signal electrode

[0074] 72, 73: Grounding electrodes

[0075] 100: Optical Transmitting Device

[0076] 101: Light Source

[0077] 102: Modulation signal generation unit Detailed Implementation

[0078] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, sometimes unnecessary detailed descriptions will be omitted. For example, detailed descriptions of well-known matters or repeated descriptions of substantially the same structures may be omitted.

[0079] [First Implementation Method]

[0080] First, the first embodiment of this disclosure will be described. Figure 1 This refers to the structure of an optical modulator that uses the optical waveguide element, i.e., the optical modulation element, of the first embodiment.

[0081] The optical modulator 1 has a housing 2 and an optical modulation element 3 housed within the housing 2, which generates an optical modulation signal through traveling wave action. The optical modulation element 3, for example, utilizes a nested Mach-Zehnder type coherent communication modulator structure corresponding to polarized wave synthesis modulation to handle multi-value modulation formats such as Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) or Quadrature Amplitude Modulation (QAM). The housing 2, for example, conforms to the industry standard HB-CDM ("Implementation Agreement for the High-Bandwidth Coherent Driver Modulator (HB-CDM) Optical Internetworking Forum (OIF) - HB-CDM-02.0" (July 15, 2021, OIF release)). In addition, the frame 2 is finally fixed with a cover (not shown) that serves as a plate at its opening, thereby making its interior airtight.

[0082] For the optical modulation element 3, it can be a structure in which the sub-component or chip is directly applied to a pluggable module. Alternatively, the optical modulation element 3 can also be a structure installed inside a transceiver. Furthermore, the optical modulation element 3 can be installed in CPO (Co-packaged Optics) or NPO (Near Package Optics). Additionally, the optical modulation element 3 can be installed within IC-TROSA (Integrated Coherent-Transmitter Receiver Optical Sub-Assembly) or COSA (Coherent Optical Sub-Assembly). Finally, the optical modulation element 3 can also be installed in a silicon photonics (SiPh) optoelectronic circuit.

[0083] As the substrate used in the optical modulation element 3, any substrate can be used as long as it is a material on which an optical waveguide is formed on the surface of the substrate. Specifically, as a substrate with electro-optic effect, substrates such as lithium niobate (LN), lithium tantalate (LT), PLZT (lanthanum-modified lead zirconate titanate), or substrates doped with MgO or the like can be used. Alternatively, a film can be formed by vapor-phase growth of materials such as LN on a support substrate such as Si, glass, or sapphire, either directly or with some intermediate layers in between. Furthermore, a substrate formed by thin-film processing of the electro-optic substrate after bonding it to other substrates can also be used. Moreover, substrates made of semiconductor substrates or organic materials such as EO (ethylene oxide) polymers, and quartz substrates used in PLCs (Planar Lightwave Circuits) can also be used. Different types of semiconductor films can also be grown on semiconductor substrates.

[0084] A signal pin 4 is provided in the frame 2, which is used to input a high-frequency electrical signal for modulation of the optical modulation element 3 to the drive circuit 17 mounted on the relay substrate 14. In addition, a signal pin 5 is provided in the frame 2, which is used to input electrical signals for adjusting the operating point of the optical modulation element 3, input power for the operation of the drive circuit 17, and input and output control signals required to operate the drive circuit 17.

[0085] The optical modulator 1 also has an input optical fiber 6 for inputting light into the frame 2 and an output optical fiber 7 for guiding the light modulated by the optical modulator element 3 to the outside of the frame 2 on the same side of the frame 2. In addition, the optical modulator 1 has a polarization synthesizer element 12, which has both beam shifting and polarization synthesizing functions.

[0086] The input optical fiber 6 and the output optical fiber 7 are fixed to the frame 2 via brackets 8 and 9, which serve as fixing components. Light input from the input optical fiber 6 is collimated by a lens 11a disposed within the bracket 8, passes through a polarizing synthesizer 12, and then enters the optical modulation element 3 via a lens 10a. This is one example; the input of light to the optical modulation element 3 can also be performed according to existing technology, for example, by guiding the input optical fiber 6 into the frame 2 via the bracket 8, and connecting the end face of the introduced input optical fiber 6 to the end face of the optical substrate 30 (described later) of the optical modulation element 3.

[0087] The two modulated beams output from the optical modulation element 3 are collimated by lenses 10b and 10c respectively, and then combined into a single beam by the polarization combining function of the polarization combining element 12. The combined beam is converged by lens 11b disposed within the bracket 9 and coupled to the output optical fiber 7.

[0088] Within the housing 2 of the optical modulator 1, a relay substrate 14 and a terminator 16 comprising four terminating resistors 15a, 15b, 15c, and 15d having specified impedances are further disposed. Hereinafter, terminating resistors 15a, 15b, 15c, and 15d will be collectively referred to as terminating resistor 15. The electrical connection between the optical modulator 3 and the terminating resistors 15 of the terminator 16 is made, for example, through wire bonding.

[0089] The relay substrate 14 includes a drive circuit 17. The drive circuit 17 amplifies the high-frequency electrical signal input from signal pin 4 and outputs a high-frequency electrical signal, i.e., a drive signal, for modulating the optical modulation element 3. Additionally, the relay substrate 14 relays electrical signals such as those for adjusting the operating point, power supply, and control signals input from signal pin 5 to the optical modulation element 3. The conductor patterns of the relay substrate 14 are connected, for example, to pads (not shown) at one end of the electrodes constituting the optical modulation element 3 via wire bonding. Furthermore, the relay substrate 14... Figure 1 The diagram shows a single substrate, but it can also be constructed by dividing it into multiple substrates as needed. Additionally, the drive circuit 17 can be as follows: Figure 1 It can be mounted on the relay substrate 14, or it can be configured between the relay substrate 14 and the optical modulation element 3. Alternatively, instead of the signal pin 4, the interface for inputting high-frequency electrical signals can also be an FPC (Flexible Printed Circuit) (flexible substrate) configured outside the frame 2.

[0090] Figure 2 It means used for Figure 1 A diagram showing the structure of the optical modulation element 3 in the optical modulator 1. (See diagram for reference.) Figure 2 As shown, optical modulation element 3 is a DP-QPSK modulator, etc.

[0091] The optical modulation element 3 is formed on a main surface of the optical substrate 30. Figure 2 The optical waveguide 31 (shown on the surface) is configured to perform coherent multi-value modulation, for example, exceeding 100 GBaud. The optical substrate 30 is, for example, an X-cut LN substrate with electro-optic effects, which is thin-filmed and processed to a thickness of 20 μm or less (e.g., 2 μm). In addition, the optical waveguide 31 is a convex optical waveguide (e.g., a ribbed optical waveguide or a ridged optical waveguide) formed on the surface of the thin-film optical substrate 30 and composed of protrusions extending in a strip shape.

[0092] The optical substrate 30 is, for example, rectangular, having two sides 32a and 32b extending in the vertical direction and facing each other, and two sides 32c and 32d extending in the horizontal direction and facing each other.

[0093] The optical waveguide 31 includes: an input waveguide 33 that receives input light from the input optical fiber 6 on the upper side of the right side 32b of the optical substrate 30 (arrow pointing to the left of the diagram); and a branch waveguide 34 that branches the input light into two beams with the same amount of light. Additionally, the optical waveguide 31 includes two modulation sections that modulate the individual beams branched by the branch waveguide 34, namely, a nested Mach-Zehnder type optical waveguide 35a and a nested Mach-Zehnder type optical waveguide 35b.

[0094] Nested Mach-Zehnder optical waveguides 35a and 35b each include two Mach-Zehnder optical waveguides 37a and 37b disposed in each of the two waveguide sections constituting a pair of parallel waveguides, as well as Mach-Zehnder optical waveguides 37c and 37d. Hereinafter, Mach-Zehnder optical waveguides 37a, 37b, 37c, and 37d are also collectively referred to as Mach-Zehnder optical waveguide 37. According to the prior art, each Mach-Zehnder optical waveguide 37 includes two parallel waveguides.

[0095] Nested Mach-Zehnder type optical waveguides 35a and 35b fold the propagation direction of light back 180 degrees. After modulation in Mach-Zehnder type optical waveguide 37, light is output from edge 32b of optical substrate 30 to the right of the figure using output waveguides 36a and 36b.

[0096] Nested Mach-Zehnder waveguides 35a and 35b each have a first bias electrode portion 40a, which forms a bias electrode for adjusting the operating point. Mach-Zehnder waveguide 37 has a second bias electrode portion 40b, which forms a bias electrode for adjusting the operating point of each of the four Mach-Zehnder waveguides 37a, 37b, 37c, and 37d.

[0097] Four Mach-Zehnder type optical waveguides 37a, 37b, 37c, and 37d each have: first return regions 38a, 38b, 38c, and 38d where the light propagation direction is reversed by 180 degrees; second return regions 39a, 39b, 39c, and 39d; and radio frequency (RF) electrode sections 50a, 50b, 50c, and 50d. Hereinafter, first return regions 38a, 38b, 38c, and 38d are collectively referred to as first return region 38. Second return regions 39a, 39b, 39c, and 39d are collectively referred to as second return regions 39. RF electrode section 50a, RF electrode section 50b, RF electrode section 50c, and RF electrode section 50d are collectively referred to as RF electrode section 50.

[0098] The Mach-Zehnder type optical waveguide 37, after reversing the direction of light propagation by 180 degrees in the first reversal region 38 (shown from left to right in the diagram), and after reversing the direction of light propagation by 180 degrees in the second reversal region 39 (shown from right to left in the diagram), is connected to the output waveguides 36a and 36b.

[0099] The RF electrode section 50 is located between the first foldback region 38 and the second foldback region 39, and modulates the light wave propagating in the Mach-Zehnder type optical waveguide 37 according to the high-frequency electrical signal output from the drive circuit 17. Specifically, in the RF electrode section 50, signal electrodes that modulate the four Mach-Zehnder type optical waveguides 37a, 37b, 37c, and 37d are formed between two parallel waveguides along the extension direction. In addition, according to the prior art, in each signal electrode, two ground electrodes are formed on the main surface of the optical substrate 30, sandwiching these signal electrodes at a distance offset from each other and along the extension direction of the two parallel waveguides.

[0100] Therefore, in the RF electrode section 50, each signal electrode, together with the two ground electrodes, forms a line with a specified impedance distribution constant. Consequently, the four drive signals output from the drive circuit 17 are input to the signal electrodes corresponding to the four Mach-Zehnder type optical waveguides 37a, 37b, 37c, and 37d, respectively, and propagate as traveling waves within each signal electrode. After the drive signals propagating in the signal electrodes modulate the light waves propagating in the corresponding Mach-Zehnder type optical waveguide 37, they are terminated by the corresponding terminating resistor 15.

[0101] The signal electrode that modulates the Mach-Zehnder type optical waveguide 37, and the two ground electrodes corresponding to the signal electrode, are folded back in the RF electrode section 50 in a manner that corresponds to the folding back of the Mach-Zehnder type optical waveguide 37 along the extension direction of the Mach-Zehnder type optical waveguide 37.

[0102] Figure 3 This is a diagram showing the folded-back structure of the electrodes of optical modulator 1. More specifically, Figure 3 The example illustrates the foldback structure of the signal electrode and the two ground electrodes related to the modulation operation of the RF electrode section 50. Figure 4 yes Figure 3 A detailed view of part A. Figure 5 yes Figure 3 VV cross-section diagram.

[0103] like Figure 3 As shown, the signal electrode 71 extending from the RF electrode section 50 beyond the first return region 38 and the second return region 39, and the two ground electrodes 72 and 73, which are parallel to the signal electrode 71 and are located at a certain distance from each other, extend in a U-shape in the electrode bending section 60 with their directions reversed by 180 degrees.

[0104] In the first foldback region 38 and the second foldback region 39, the distance from the foldback start position of the Mach-Zehnder type optical waveguide 37 near the end of the RF electrode portion 50 to the outermost periphery of the Mach-Zehnder type optical waveguide 37 that bulges due to the foldback is set to 500 μm or less. The electrode bend portion 60 does not overlap with the outermost periphery of the Mach-Zehnder type optical waveguide 37, but is further positioned on the outer side.

[0105] Figure 4 yes Figure 3 This is a detailed partial view of section A, showing the electrode structure of the Mach-Zehnder type optical waveguide 37 in the RF electrode section 50. Figure 4 In the diagram, light enters from the left and exits from the right.

[0106] The optical modulation element 3, serving as an optical waveguide element, has a signal electrode 71, a ground electrode 72, and a ground electrode 73 in the RF electrode section 50 for controlling the light wave propagating in the Mach-Zehnder type optical waveguide 37. The signal electrode 71 is disposed between two parallel waveguides in the Mach-Zehnder type optical waveguide 37 along the extension direction of the waveguide. The two ground electrodes 72 and 73 are respectively disposed along the extension direction of the waveguide at positions facing the signal electrode 71, sandwiching the two parallel waveguides in the Mach-Zehnder type optical waveguide 37.

[0107] The signal electrode 71, ground electrode 72, and ground electrode 73 are made of gold (Au), for example. To improve the adhesion with the optical substrate 30, chromium (Cr), titanium (Ti), nickel (Ni), niobium (Nb), or other base metals may also be used.

[0108] The signal electrode 71 has multiple segmented electrodes 51, which are arranged closer to the Mach-Zehnder type optical waveguide 37 than the signal electrode 71 and are divided along the extension direction of the Mach-Zehnder type optical waveguide 37. The ground electrode 72 has multiple segmented electrodes 52, which are arranged closer to the Mach-Zehnder type optical waveguide 37 than the ground electrode 72 and are divided along the extension direction of the Mach-Zehnder type optical waveguide 37. The ground electrode 73 has multiple segmented electrodes 53, which are arranged closer to the Mach-Zehnder type optical waveguide 37 than the ground electrode 73 and are divided along the extension direction of the Mach-Zehnder type optical waveguide 37. A low dielectric layer 54 with a lower relative permittivity than the optical substrate 30 is disposed between the lower portion of each of the signal electrode 71, ground electrode 72, and ground electrode 73 and the optical substrate 30. The relative permittivity of the low dielectric layer 54 is preferably 1 or more and 10 or less. The low dielectric layer 54 may be composed of, for example, low dielectric resins such as SiO2, BCB (benzocyclobutene), or silicon nitride.

[0109] In the optical modulation element 3, an electric field is applied to the Mach-Zehnder optical waveguide 37 by segmented electrodes 51, 52, or 53 positioned close to it, causing an electro-optic effect to act on the waveguide, thereby performing optical modulation. That is, segmented electrodes 51, 52, and 53 are examples of the active electrodes.

[0110] The signal electrode 71, ground electrode 72, and ground electrode 73, extending along the Mach-Zehnder type optical waveguide 37, are formed in a U-shape at the electrode bend 60, which is located further outward than the first fold-back region 38 or the second fold-back region 39 of the Mach-Zehnder type optical waveguide 37, thus reversing their extension direction. Therefore, the signal electrode 71, ground electrode 72, and ground electrode 73 are arranged along the extension direction of the Mach-Zehnder type optical waveguide 37 after being folded back using the first fold-back region 38 or the second fold-back region 39. Furthermore, the structure of the operating electrodes exemplifies a single structure, but a differential structure is also possible.

[0111] At the electrode bend 60, a connecting ground electrode 61 is formed at the lower part of the signal electrode 71 to connect the two ground electrodes 72 and 73. For example, as shown in the example, multiple connecting ground electrodes 61 are formed at equal intervals in the extending direction of the two ground electrodes 72 and 73.

[0112] In the optical modulation element 3, two ground electrodes 72 and 73 in the electrode bend 60 are connected by a connecting ground electrode 61, thereby suppressing the conversion to unwanted propagation modes in the electrode bend 60 and improving electrical characteristics. Furthermore, as shown in the example, when multiple connecting ground electrodes 61 are formed, phase shift can be mitigated more reliably.

[0113] like Figure 5 As shown, the optical substrate 30 is configured such that a lower surface buffer layer 30b, which serves as a substrate, is formed on the reinforcing substrate 30a, and an LN layer 30c is formed on the lower surface buffer layer 30b.

[0114] The reinforced substrate 30a is a substrate formed from materials such as Si, glass, crystal, fused silica, synthetic silica, alkali glass, alkali-free glass, lead glass, borosilicate glass, sodium glass, sapphire, and alumina.

[0115] The lower surface buffer layer 30b is a thin film formed of a dielectric material with a lower refractive index and higher transparency compared to the LN layer 30c. The dielectric material used for the lower surface buffer layer 30b can be, for example, SiO2 or Al2O3, SiN, MgF2, La2O3, ZnO, HfO2, MgO, CaF2, Y2O3, etc., and oxides, fluorides, and nitrides of metal elements from groups 1 to 17 of the periodic table can be used.

[0116] The LN layer 30c is a layer used to form the rib-shaped optical waveguide 31. It is formed with a thickness of less than 1.0 μm and is bonded to the reinforcing substrate 30a via the lower surface buffer layer 30b. The LN layer 30c is, for example, a single crystal material such as LN or LT with electro-optic effect, and may also be doped with MgO or the like.

[0117] A grounding electrode 61 is formed on the LN layer 30c. That is, the grounding electrode 61 and the segmented electrodes 51, 52, and 53, formed in the RF electrode section 50 in a manner that sandwiches a rib-shaped Mach-Zehnder waveguide 37, are formed on the same layer. The thickness of the grounding electrode 61 and the segmented electrodes 51, 52, and 53 formed at this time is less than 1 μm. Furthermore, the grounding electrode 61 and the segmented electrodes 51, 52, and 53 are formed of the same metal (e.g., gold (Au)) as the signal electrode 71, ground electrode 72, and ground electrode 73.

[0118] In the formation of the ground electrode 61, for example, electron beam (EB) lithography can be applied. When forming the ground electrode 61 using EB lithography, the linewidth of the ground electrode 61 can be set to a typical minimum value of 2 μm in EB lithography. Since the typical mask linewidth used in photolithography is 4 μm and the typical lead width in wire bonding is 20 μm, the linewidth of the ground electrode 61 can be shortened to less than 20 μm, and further shortened to 4 μm, when forming the ground electrode 61 using EB lithography. Thus, in the optical modulation element 3, by making the linewidth of the ground electrode 61 thinner, the number of reflection points required for impedance matching can be reduced, thereby mitigating impedance mismatch.

[0119] After forming the layer connecting the ground electrode 61, an upper surface buffer layer 62 is formed on the LN layer 30c with a specified film thickness. The upper surface buffer layer 62 is a thin film formed of a dielectric material with a low refractive index and high transparency compared to the LN layer 30c. The dielectric material used for the upper surface buffer layer 62 can be, for example, SiO2 or Al2O3, SiN, MgF2, La2O3, ZnO, HfO2, MgO, CaF2, Y2O3, etc., and oxides, fluorides, and nitrides of metal elements from groups 1 to 17 of the periodic table can be used. Alternatively, the upper surface buffer layer 62 can be a permanent photoresist, or a photoresist based on a thermosetting resin can be used.

[0120] Then, through photolithography and other processes, signal electrode 71, ground electrode 72, and ground electrode 73 are formed on the upper surface buffer layer 62. Ground electrode 61 is connected to ground electrode 72 and ground electrode 73 in a vertically connected manner that penetrates the upper surface buffer layer 62. Alternatively, ground electrode 61 is not vertically connected to signal electrode 71, but is instead formed by sandwiching the upper surface buffer layer 62 between ground electrode 61 and signal electrode 71.

[0121] Thus, in the electrode bend 60, the two ground electrodes 72 and 73 are connected via a connecting ground electrode 61 formed on a lower layer than the signal electrode 71, ground electrode 72, and ground electrode 73. Therefore, the connection between the ground electrodes 72 and 73 in the electrode bend 60 can be performed without using wire bonding. That is, the time required for wire bonding can be reduced. Furthermore, since it is not necessary to ensure the gap between the electrodes used for wire bonding, the optical modulation element 3 can be miniaturized while improving electrical characteristics. Additionally, when the two ground electrodes 72 and 73 are connected via the connecting ground electrode 61 formed on the lower layer, it is easier to form a bridge on the two ground electrodes 72 and 73 using wire bonding, and since the connecting ground electrode 61 is formed on the lower layer, it is less prone to breakage. For example, in conventional wire bonding, stress is applied to the electrodes and the substrate below them during bonding, thus posing a risk of breakage. In contrast, in this disclosure, since a grounding electrode 61 is formed under the signal electrode 71 and connected to the underside of the electrode, no stress is applied, and the connection can be made without the risk of damage even when the second-stage (signal electrode 71) electrode is thin (e.g., less than 10 μm).

[0122] [First Variation]

[0123] Here, a first modified example of the foldback structure of the electrodes of the optical modulator 1 will be described. Figure 6 This is a diagram showing a first modified example of the folded-back structure of the electrodes of the optical modulator 1. Figure 7 yes Figure 6 Section VII-VII.

[0124] like Figure 6 and Figure 7 As shown, in the first modified example, the grounding electrode 61 is formed on the entire surface of the electrode bend 60. Thus, when the grounding electrode 61 is formed on the entire surface of the electrode bend 60, the grounding area where the grounding electrodes 72 and 73 connect to the grounding electrode 61 increases, improving process stability.

[0125] [Second variation]

[0126] Here, a second modified example of the foldback structure of the electrodes of the optical modulator 1 will be described. Figure 8 This is a diagram showing a second modified example of the folded-back structure of the electrodes of the optical modulator 1. Figure 9 yes Figure 8 IX-IX cross-section.

[0127] like Figure 8 and Figure 9As shown, in the second modification, the upper surface buffer layer 62 is not sandwiched between the grounding electrode 72, the grounding electrode 73 and the connecting grounding electrode 61. Thus, in the second modification, poor connection between the connecting grounding electrode 61 and the grounding electrodes 72 and 73 can be suppressed, further improving process stability compared to the first modification.

[0128] [Third variation]

[0129] Here, a third variation of the foldback structure of the electrodes of the optical modulator 1 will be described. Figure 10 This is a diagram showing a third variation of the foldback structure of the electrodes of the optical modulator 1. Figure 11 yes Figure 10 XI-XI cross-sectional view.

[0130] like Figure 10 and Figure 11 As shown, in the third modified example, an LN layer 30c with a groove 30d is formed in the electrode bending portion 60 at a position corresponding to the lower part of the signal electrode 71. Thus, by creating the groove 30d in the LN layer 30c, the loss of high-frequency signals propagating in the signal electrode 71 can be reduced.

[0131] [Fourth variation]

[0132] Here, a fourth variation of the foldback structure of the electrodes of the optical modulator 1 will be described. Figure 12 This is a diagram showing a fourth variation of the foldback structure of the electrodes of the optical modulator 1. Figure 13 yes Figure 12 Section XIII-XIII.

[0133] like Figure 12 and Figure 13 As shown, in the fourth variation, a portion of the ground electrode 72 and the ground electrode 73 is configured to cover the upper portions of both ends of the upper surface buffer layer 62 formed between the ground electrode 72 and the ground electrode 73 from above. By forming the ground electrode 72 and the ground electrode 73 in this way, the peeling of the upper surface buffer layer 62 from the upper surface of the optical modulation element 3 can be suppressed.

[0134] [Fifth Variation]

[0135] Here, a fifth variation of the foldback structure of the electrodes of the optical modulator 1 will be described. Figure 14 This is a diagram showing a fifth variation of the foldback structure of the electrodes of the optical modulator 1. Figure 15 yes Figure 14 XV-XV cross-section. Figure 16 yes Figure 14 XVI-XVI cross-sectional view.

[0136] like Figure 14 , Figure 15 and Figure 16 As shown, in the fifth modification, the grounding electrode 61 is formed in an arc shape along the folded-back portion of the grounding electrode 72 and the grounding electrode 73. Furthermore, in the fifth modification, the grounding electrode 61 connecting the grounding electrode 72 and the grounding electrode 73 is formed radially at equal intervals, orthogonal to the extending direction of the signal electrode 71.

[0137] If the electrode width of the grounding electrode 61, which is orthogonal to the lower layer of the signal electrode 71, is increased, the impedance decreases, causing impedance mismatch. Therefore, in the fifth modification, by making the grounding electrodes 61 connecting the ground electrodes 72 and 73 radially spaced at equal intervals, the electrode width of the grounding electrodes 61 located below the signal electrode 71 can be shortened, thereby suppressing impedance mismatch. In addition, since the grounding electrodes 61 are formed in a U-shape along the fold-back portion of the ground electrodes 72 and 73, the grounding area connected to the grounding electrodes 72 and 73 to the grounding electrodes 61 is increased, improving process stability.

[0138] [Sixth Variation]

[0139] Here, a sixth variation of the foldback structure of the electrodes of the optical modulator 1 will be described. Figure 17 This is a diagram showing a sixth variation of the foldback structure of the electrodes of the optical modulator 1. Figure 18 yes Figure 17 XVIII-XVIII cross-section.

[0140] like Figure 17 and Figure 18 As shown, in the sixth modification, unlike the fifth modification, the grounding electrode 61 is not formed in an arc shape along the folded-back portion of the grounding electrodes 72 and 73, but is instead formed in a rectangular shape. The grounding electrode 61 can be formed in a rectangular shape as described above.

[0141] [Second Implementation]

[0142] Next, a second embodiment of this disclosure will be described. This embodiment is an optical transmitting apparatus equipped with an optical modulator 1 of the first embodiment or a variation thereof. Figure 19 This is a diagram showing the structure of the optical transmitting device according to the second embodiment.

[0143] like Figure 19As shown, the optical transmitting device 100 includes an optical modulator 1, a light source 101, and a modulation signal generation unit 102. The modulation signal generation unit 102 is an electronic circuit that generates a high-frequency signal (modulation signal) for causing the optical modulator 1 to perform modulation operations. For example, based on transmission data supplied from an external source, the modulation signal generation unit 102 generates four modulation signals for input to the optical modulation element 3 included in the optical modulator 1, and inputs them to the signal pin 4 of the optical modulator 1. As a result, the optical modulator 1 modulates the light from the light source 101 incident from the input optical fiber 6, and outputs the modulated light via the output optical fiber 7.

[0144] In the optical transmitting device 100 with the above structure, the use of the optical modulator 1 enables miniaturization of components while improving electrical characteristics.

[0145] [Other Implementation Methods]

[0146] Various modifications of the first and second embodiments can be arbitrarily combined to form an optical waveguide element (e.g., optical modulation element 3). For example, they can be combined in all other modifications for application.

[0147] Furthermore, the optical waveguide element in this disclosure is not limited to the optical modulation element 3 that uses nested Mach-Zehnder waveguides for optical modulation, but can be any optical waveguide element that uses an optical waveguide formed with arbitrary patterns to achieve arbitrary functions. For example, the optical waveguide element can be composed of a single Mach-Zehnder waveguide (e.g., a structure of single Mach-Zehnder waveguides arranged in parallel), or it can be an element that realizes functions such as optical switching by including directional coupler waveguides and / or Y-branch waveguides.

[0148] Furthermore, the electrode structure of the optical waveguide element in this disclosure is typically coplanar, with the signal electrode and ground electrode in the active section, folded-back section, etc., all formed on the same plane. However, other structures are also possible (e.g., all segments or all coplanar). Regarding the electrode structure of the optical waveguide element in this disclosure, since the folded-back section of the optical waveguide is further forward than the folded-back section of the electrode, the electrical circuit (electrode length) is longer than the optical waveguide. Therefore, if the active section is typically speed-matched with the optical waveguide, the effective refractive index of the high-frequency signal of the electrode is lower than the effective refractive index (group refractive index) of the light in the optical waveguide.

[0149] Furthermore, the present invention is not limited to the structure of the described embodiments and its alternative structures, and can be implemented in various forms without departing from its spirit.

[0150] [Structure supported by the described embodiments]

[0151] The embodiments and variations support the following structures.

[0152] (Structure 1) An optical waveguide element having a signal electrode formed in a portion along the extension direction of an optical waveguide formed on a substrate, and two ground electrodes sandwiching the signal electrode from both sides, the optical waveguide element comprising: an electrode bending portion that bends the extension direction of the signal electrode and the two ground electrodes; a connecting ground electrode formed in the electrode bending portion at the lower part of the signal electrode to connect the two ground electrodes; and a buffer layer formed between the signal electrode and the connecting ground electrode.

[0153] Accordingly, connections between ground electrodes at locations where electrodes are folded back can be made without the use of wire bonding. Therefore, it is unnecessary to ensure a gap between electrodes used for wire bonding, thereby improving electrical characteristics while miniaturizing the component.

[0154] (Structure 2) The optical waveguide element according to Structure 1, wherein the bent portion of the electrode is formed on the substrate in a region where the optical waveguide is not formed.

[0155] Therefore, the electrodes can be bent while avoiding the impact on the optical waveguide.

[0156] (Structure 3) The optical waveguide element according to Structure 2, wherein the bent portion of the electrode is formed in a region further outward than the region where the optical waveguide is bent.

[0157] Therefore, the electrodes can be bent while shortening the optical path length of the optical waveguide.

[0158] (Structure 4) An optical waveguide element according to any one of Structures 1 to 3, wherein the grounding electrode is formed on the layer on which the optical waveguide is formed.

[0159] Accordingly, an optical waveguide can be formed in the layer between the substrate and the grounding electrode.

[0160] (Structure 5) An optical waveguide element according to any one of structures 1 to 4, wherein a plurality of the grounding electrodes are formed in the electrode bend.

[0161] Therefore, by connecting multiple grounding electrodes, phase shift can be mitigated more reliably.

[0162] (Structure 6) An optical waveguide element according to any one of Structures 1 to 5, wherein, on the substrate, the signal electrode and the two ground electrodes have an active electrode in a region along the optical waveguide that causes an electro-optic effect to act on the optical waveguide, and the connecting ground electrode and the active electrode are formed in the same layer on the substrate.

[0163] Accordingly, the grounding electrode and the working electrode can be formed in the same process.

[0164] (Structure 7) The optical waveguide element according to Structure 6, wherein the thickness of the connecting ground electrode and the working electrode is less than 1 μm.

[0165] Accordingly, a thin film with a thickness of less than 1 μm can be formed connecting the grounding electrode and the working electrode.

[0166] (Structure 8) An optical waveguide element according to any one of Structures 1 to 7, wherein the electrode bend is U-shaped such that the extension directions of the signal electrode and the two ground electrodes are reversed by 180 degrees.

[0167] Accordingly, the extension directions of the signal electrode and the two ground electrodes can be reversed by 180 degrees in a U-shape.

[0168] (Structure 9) An optical waveguide element according to any one of Structures 1 to 8, wherein the electrode width of the grounding electrode is less than 20 μm.

[0169] Accordingly, the linewidth can be refined to below 20 μm based on the general wire width of the lead bonding, thereby reducing impedance mismatch.

[0170] (Structure 10) An optical modulator, comprising: an optical waveguide element according to any one of Structures 1 to 9 as an optical modulation element; a housing for receiving the optical waveguide element; an optical fiber for inputting light to the optical waveguide element; and an optical fiber for guiding light output from the optical waveguide element to the outside of the housing.

[0171] Therefore, it is possible to realize an optical modulator that can improve electrical characteristics while miniaturizing components.

[0172] (Structure 11) An optical transmitting device, comprising: an optical modulator according to Structure 10; and electronic circuitry for generating an electrical signal for causing the optical waveguide element to perform optical modulation.

[0173] Accordingly, it is possible to realize an optical transmitting device that can improve electrical characteristics while miniaturizing components.

Claims

1. An optical waveguide element comprising a signal electrode formed in a portion along the extension direction of an optical waveguide formed on a substrate, and two ground electrodes sandwiching the signal electrode from both sides, the optical waveguide element comprising: The electrode bending portion bends the extension direction of the signal electrode and the two ground electrodes; Connect the grounding electrode, which is formed at the lower part of the signal electrode at the bent portion of the electrode, and connect the two grounding electrodes. as well as A buffer layer is formed between the signal electrode and the grounding electrode.

2. The optical waveguide element according to claim 1, wherein, The bent portion of the electrode is formed on the substrate in a region where the optical waveguide is not formed.

3. The optical waveguide element according to claim 2, wherein, The bent portion of the electrode is formed in a region that is further outward than the bent region of the optical waveguide.

4. The optical waveguide element according to claim 1, wherein, The grounding electrode is formed on the layer on which the optical waveguide is formed.

5. The optical waveguide element according to claim 1, wherein, Multiple grounding electrodes are formed at the bent portion of the electrode.

6. The optical waveguide element according to claim 1, wherein, On the substrate, the signal electrode and the two ground electrodes have functional electrodes along the region of the optical waveguide that enable the electro-optic effect to act on the optical waveguide. The grounding electrode and the active electrode are formed on the same layer on the substrate.

7. The optical waveguide element according to claim 6, wherein, The thickness of the grounding electrode and the working electrode is less than 1 μm.

8. The optical waveguide element according to claim 1, wherein, The U-shaped bend of the electrode reverses the extension directions of the signal electrode and the two ground electrodes by 180 degrees.

9. The optical waveguide element according to claim 1, wherein, The width of the grounding electrode is less than 20 μm.

10. An optical modulator, comprising: As an optical modulation element, the optical waveguide element as described in any one of claims 1 to 9; The frame houses the optical waveguide element; Optical fiber, inputting light into the optical waveguide element; as well as The optical fiber guides the light output from the optical waveguide element to the outside of the frame.

11. An optical transmitting device, comprising: The optical modulator as described in claim 10; as well as The electronic circuit generates an electrical signal to cause the optical waveguide element to perform optical modulation.

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