Optical waveguide elements and optical modulation devices and optical transmission devices using them

Abstract

This invention provides an optical waveguide element that balances improved electric field efficiency with suppression of thermoelectric effects, an optical modulation device using the same, and an optical transmission device. The optical waveguide element of this invention is made of a ferroelectric material and includes: an optical waveguide substrate (1) having an optical waveguide (WG); and control electrodes (E1, E2) for applying an electric field to the optical waveguide. The optical waveguide element is characterized in that a portion of a convex portion (10) formed on the surface of the optical waveguide substrate is sequentially formed with a buffer layer (BF) and a charge diffusion layer (CD) covering the optical waveguide. The control electrodes are disposed in contact with the optical waveguide substrate.

Description

Technical Field

[0001] This utility model relates to an optical waveguide element and an optical modulation device and an optical transmission device using the same, and more particularly to an optical waveguide element made of ferroelectric material and including an optical waveguide substrate having an optical waveguide and control electrodes for applying an electric field to the optical waveguide, and an optical modulation device and an optical transmission device using the same. Background Technology

[0002] In the fields of optical communication and optical measurement, optical waveguide components using ferroelectric materials such as lithium niobate as substrates are commonly used. In optical waveguide components such as optical modulators, miniaturization or broadband operation is required, and control electrodes are positioned close to the optical waveguide to reduce the driving voltage. However, if the control electrodes are positioned close to the optical waveguide, the absorption and scattering of light waves propagating in the optical waveguide by the metal material constituting the electrodes increases light propagation loss.

[0003] In Patent Document 1 or Patent Document 2, a buffer layer is provided between the optical waveguide and the electrode to suppress the absorption of light waves by the electrode. However, when the buffer layer exists between the optical waveguide and the electrode, the efficiency of the electric field applied to the optical waveguide by the electrode is reduced because the distance between the optical waveguide and the electrode is large and other dielectric materials are present in between.

[0004] Compared to the reduction in electric field efficiency when electrodes are arranged on the upper side of the optical waveguide, as in Patent Document 1 (e.g., Z-cut substrate), the reduction in electric field effect is more significant when electrodes are arranged in a manner that sandwiches the optical waveguide, as in Patent Document 2 (e.g., X-cut substrate).

[0005] On the other hand, in optical waveguide substrates utilizing ferroelectric materials, phenomena such as direct current (DC) drift or temperature drift can generate charges (thermoelectric effect) on the substrate surface. The generation of these charges can lead to the application of an unwanted electric field to the optical waveguide and further complicate optical control based on control electrodes.

[0006] Patent Document 1 or Patent Document 2 discloses an antistatic film (charge diffusion layer) formed between a buffer layer and an electrode, whereby the Si layer is covered by the buffer layer. In this case, since the charge diffusion layer exists between the optical waveguide and the electrode, the distance between them becomes larger, leading to a decrease in electric field efficiency. Furthermore, in Patent Document 1 or Patent Document 2, the presence of the charge diffusion layer between the electrodes prevents the application of a suitable voltage to the electrodes, also contributing to a decrease in the electric field efficiency applied to the optical waveguide.

[0007] [Existing technical documents]

[0008] [Patent Literature]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 10-3064

[0010] [Patent Document 2] Japanese Patent Application Publication No. 2022-189578 Utility Model Content

[0011] [The problem that the utility model aims to solve]

[0012] The problem to be solved by this invention is to provide an optical waveguide element that solves the aforementioned problems while simultaneously improving electric field efficiency and suppressing thermoelectric effects. Furthermore, it provides an optical modulation device and an optical transmission apparatus using this optical waveguide element.

[0013] [Technical means to solve the problem]

[0014] In order to solve the aforementioned problems, the optical waveguide element of this utility model, the optical modulation device and the optical transmission device using it have the following technical features.

[0015] (1) An optical waveguide element made of a ferroelectric material and comprising: an optical waveguide substrate having an optical waveguide; and a control electrode for applying an electric field to the optical waveguide, wherein the optical waveguide is formed on a portion of a convex portion formed on the surface of the optical waveguide substrate, and a buffer layer and a charge diffusion layer are sequentially formed covering the optical waveguide, and the control electrode is disposed in contact with the optical waveguide substrate.

[0016] (2) The optical waveguide element according to (1), wherein the buffer layer covering the optical waveguide and the charge diffusion layer are configured to cover at least a portion of the control electrode.

[0017] (3) The optical waveguide element according to (1), wherein the control electrode is disposed on another convex portion adjacent to the optical waveguide.

[0018] (4) According to the optical waveguide element described in (1), wherein the control electrode is composed of a thin first electrode portion and a thick second electrode portion.

[0019] (5) The optical waveguide element according to (1), wherein a resin layer is formed covering at least a portion of the charge diffusion layer.

[0020] (6) The optical waveguide element according to (1), wherein the optical waveguide includes a plurality of Mach-Zehnder type optical waveguides, and at least a portion between adjacent Mach-Zehnder type optical waveguides has a portion for exposing the surface of the optical waveguide substrate.

[0021] (7) The optical waveguide element according to (1), wherein the thickness of the charge diffusion layer is set to be in the range of 50 nm or more and 500 nm or less.

[0022] (8) The optical waveguide element according to (1), wherein the thickness of the buffer layer is set to be in the range of 200 nm or more and 3000 nm or less.

[0023] (9) According to the optical waveguide element described in (1), wherein the volume resistivity of the charge diffusion layer is set to 10. 8 Ω·cm or more and 10 14 The range below Ω·cm.

[0024] (10) An optical modulation device, wherein an optical waveguide element according to any one of (1) to (9) is housed within a housing, and includes an optical fiber for inputting or outputting optical waves relative to the optical waveguide.

[0025] (11) The optical modulation device according to (10), wherein the optical waveguide element has a modulation electrode for modulating an optical wave propagating in the optical waveguide, and an electronic circuit for amplifying the modulation signal input to the modulation electrode is provided inside the housing.

[0026] (12) An optical transmitting apparatus, comprising: an optical modulator according to (10); and electronic circuitry for outputting a modulated signal to the optical modulator.

[0027] [Effects of the utility model]

[0028] This invention relates to an optical waveguide element made of ferroelectric material and includes: an optical waveguide substrate having an optical waveguide; and a control electrode for applying an electric field to the optical waveguide. In the optical waveguide element, the optical waveguide is formed as part of a convex portion on the surface of the optical waveguide substrate, and a buffer layer and a charge diffusion layer are sequentially formed covering the optical waveguide. The control electrode is connected to the optical waveguide substrate. Therefore, it is possible to improve the electric field efficiency while suppressing the thermoelectric effect.

[0029] In addition, it is possible to provide an optical modulation device and an optical transmission apparatus that use an optical waveguide element having this feature. Attached Figure Description

[0030] Figure 1 This is a cross-sectional view showing an example of the optical waveguide element of this utility model.

[0031] Figure 2 of (a) Figure 2(b) is a plan view illustrating the shape of the control electrode used in the optical waveguide element of this invention. Figure 2 (a) represents a linear electrode and Figure 2 (b) represents a segmented electrode.

[0032] Figure 3 This is a cross-sectional view showing another example of the optical waveguide element of this utility model.

[0033] Figure 4 (a) ~ Figure 4 (d) is a cross-sectional view illustrating the positional relationship between the electrodes, buffer layer, and charge diffusion layer in the optical waveguide element of this utility model.

[0034] Figure 5 of (a) Figure 5 (b) is a diagram illustrating the state of the lower side of the thick electrode, which serves as the second electrode portion, of the optical waveguide element of this invention.

[0035] Figure 6 This is a diagram illustrating an example of the modulation region and DC bias region in the optical waveguide element of this invention.

[0036] Figure 7 of (a) Figure 7 (b) is a diagram illustrating the stress-relieving structure of the optical waveguide element of this invention.

[0037] Figure 8 This figure shows an example of the optical transmitting device of this utility model.

[0038] Explanation of icon numbers

[0039] 1: Optical waveguide substrate

[0040] 2: Intermediate layer

[0041] 3: Maintain substrate

[0042] BF: Buffer layer

[0043] CD: Charge diffusion layer

[0044] E1: First electrode section (thin electrode)

[0045] E2: Second electrode section (thick electrode)

[0046] RL: Resin layer

[0047] WG: Optical Waveguide

[0048] F: Optical fiber

[0049] CA: Frame

[0050] MD: Optical Modulation Device

[0051] DRV: Drive Circuit

[0052] DSP: Digital Signal Processor

[0053] OTA: Optical Transmission Device Detailed Implementation

[0054] The optical waveguide element of this invention will now be described in detail using preferred embodiments.

[0055] like Figure 1 , Figures 3-5 (b) and Figure 7 of (a) Figure 7 As shown in (b), the optical waveguide element of this invention is made of ferroelectric material and includes: an optical waveguide substrate 1 having an optical waveguide (WG); and control electrodes (E1, E2) for applying an electric field to the optical waveguide. The optical waveguide element is characterized in that a portion of the convex portion (10) formed on the surface of the optical waveguide substrate is formed with a buffer layer (BF) and a charge diffusion layer (CD) covering the optical waveguide in sequence, and the control electrodes are connected to the optical waveguide substrate.

[0056] The optical waveguide substrate 1 used in the optical waveguide element of this invention is made of a ferroelectric substrate. Specifically, substrates such as lithium niobate (LN) or lithium tantalate (LT) can be used, or substrates doped with magnesium can be used. Furthermore, lead lanthanum zirconate titanate (PLZT), barium titanate (BTO), and lead zirconate titanate (PZT (Lead Zirconate Titanate), Pb(Zr,Ti)O3) can also be used. Additionally, vapor-grown films obtained from these materials can also be used. The term "optical waveguide substrate" in this invention refers not only to a thin plate-like body made by grinding an LN substrate, but also includes films formed by vapor-grown methods.

[0057] To improve the mechanical strength of the optical waveguide substrate 1, a holding substrate 3 is disposed on the lower side of the optical waveguide substrate 1, separated by an intermediate layer 2. The holding substrate 3 can also be made of materials such as silicon substrate, glass, quartz, or silicon photonics (SiPh) circuits.

[0058] In addition, in the intermediate layer 2, dielectric materials with low dielectric constants and refractive indices, such as SiO2, Al2O3, and Ta2O5, are used.

[0059] To improve the overall mechanical strength of the substrate, including the optical waveguide substrate, the thickness of the substrate 3 is set to approximately 300 μm to 1000 μm. Furthermore, to suppress light absorption by the substrate 3, the intermediate layer 2 is made of materials such as SiO2, and its thickness is set to 2 μm to 10 μm.

[0060] Ridge-shaped or rib-shaped protrusions are formed on the surface of the optical waveguide substrate 1, and an optical waveguide is formed on a portion of the convex portion (10). For purposes such as matching the speed of the microwave and the light wave of the modulation signal, the thickness of the optical waveguide substrate 1 is set to 1 μm or less. In addition, by setting the height or width of the optical waveguide (WG) provided on the optical waveguide substrate 1 to 1 μm or less, the light confinement intensity can be increased, the curvature of the optical waveguide can be increased (the bending radius is small), and therefore it is suitable for miniaturization of optical waveguide components. Furthermore, if the size of the optical waveguide (the height or width of the optical waveguide WG) is reduced, the driving voltage of the modulation signal can be reduced because the electrodes can be arranged in a way that is close to the optical waveguide. It is also possible to shorten the length of the active portion of the optical waveguide involved in the modulation operation or DC bias, thus contributing to further miniaturization.

[0061] The buffer layer (BF) covering the optical waveguide can utilize transparent materials such as SiO2. The thickness of the buffer layer is set in the range of 200 nm to 3000 nm. When it is above 200 nm, it can suppress light scattering loss caused by the roughness of the side surface of the optical waveguide or the light absorption caused by the charge diffusion layer. In addition, if the thickness of the buffer layer is too thick, the film stress increases, which can also become a cause of stress-induced drift.

[0062] Charge diffusion layers (CDs) can utilize metal nitrides such as TiN, AlN, and SiN; or metal oxides such as In2O3, ZnO, SnO2, and CdO; transparent conductive films such as ITO, AZO, ISO, and IZO; and metal doped films such as Al-doped ZnO, Ga-doped ZnO, and F-doped SnO.

[0063] The thickness of the charge diffusion layer (CD) is set to be in the range of 50 nm or more and 500 nm or less. A certain degree of charge diffusion effect can be obtained when the thickness is 50 nm or more. On the other hand, if the thickness of the charge diffusion layer is too thick, the film stress will increase, which may lead to the generation of cracks in the film itself or to the introduction of stress onto the optical waveguide substrate 1.

[0064] The volume resistivity of the charge diffusion layer is set to 10. 8 Ω·cm or more and 10 14 The range of Ω·cm or less enables the application of an electrode-based electric field to the optical waveguide even when it is configured in contact with a portion of the control electrode, and also allows for efficient diffusion of the charge generated on the surface of the optical waveguide substrate due to the thermoelectric effect.

[0065] As described above, the charge diffusion layer is constructed such that it efficiently releases charge by using conductive materials such as metal oxides or metal nitrides in a portion. The ratio of oxygen or nitrogen to the metal is set to be between 0.3 and 2.5. This is because if it is a metal film monomer, reliability issues arise due to short circuits between electrodes or oxidation over time. Furthermore, if it is a film with a high nitrogen or oxygen content, the resistivity is high (it is an insulating film) and it does not function as a conductive film.

[0066] If we show an example of the physical properties of SiN used as a charge diffusion layer, it would be a refractive index of 1.9–2.3 (wavelength 1550 nm) and a resistivity of 10. 8 Ω·cm or more and 10 14 Below Ω·cm, the N / Si ratio is 1.2 to 1.4.

[0067] The control electrode used in the optical waveguide element of this invention utilizes a metal with high conductivity, such as Au. The control electrode consists of a first electrode portion (E1) arranged close to the optical waveguide (WG) and applying an electric field to the waveguide, and a second electrode portion (E2) for wiring to supply power to the first electrode portion. The first electrode portion is a thin electrode with a thickness of 0.2 μm to 3.0 μm. Conversely, the second electrode portion is a thick electrode with a thickness of 3 μm to 100 μm.

[0068] Since the second electrode portion (E2) is formed on the top of the charge diffusion layer such as SiN, it is preferable to assemble a close-packed layer such as Nb, Ti, Ni, Cr, Al, etc., and form an electrode layer (Au) on it.

[0069] The control electrode has a modulation electrode (traveling wave electrode) for propagating a modulation signal as microwaves and a DC electrode for applying a DC bias voltage. In the first electrode portion (E1) of the modulation electrode, not only can... Figure 2 As shown in (a), a linear electrode (E1) along the optical waveguide can also be used, such as Figure 2 A segmented electrode (E1) with a capacitor loading type as shown in (b).

[0070] The optical waveguide element of this utility model has at least the following two technical features.

[0071] (1) The control electrodes (E1, E2) are connected to the optical waveguide substrate 1.

[0072] (2) A portion of the convex portion (10) formed on the surface of the optical waveguide substrate is formed to form a buffer layer (BF) and a charge diffusion layer (CD) covering the optical waveguide.

[0073] By directly placing the control electrodes (E1, E2) on the optical waveguide substrate 1, the distance between the optical waveguide and the electrodes can be reduced, thus suppressing the decrease in electric field efficiency. In particular, by constructing the control electrodes from a thin first electrode portion (E1) and a thick second electrode portion (E2), and by placing the first electrode portion (E1) close to the optical waveguide, the electric field can be concentrated, thereby expecting an improvement in electric field efficiency.

[0074] When a buffer layer (BF) and a charge diffusion layer (CD) are formed on the optical waveguide substrate 1, such as Figure 1 As shown, a first electrode portion (E1) is grounded to the optical waveguide substrate 1, and a buffer layer or charge diffusion layer is disposed to cover the first electrode portion (E1).

[0075] In this invention, in the active portion of the optical waveguide where the electric field of the control electrodes acts, the charge diffusion layer (CD) does not necessarily need to be electrically connected to the control electrodes (E1, E2). While this does not preclude electrical connection, to avoid reducing the electric field effect, the charge diffusion layer (CD) is configured not to be electrically connected to the vicinity of the front end of the first electrode portion (E1) (the portion close to the optical waveguide). Specifically, the charge diffusion layer (CD) is disposed above the first electrode portion, separated by a buffer layer (BF).

[0076] Even when a charge diffusion layer is disposed on the first electrode portion (E1) with a buffer layer in between, by making the thickness of the buffer layer thin within the stated numerical range, the charge moving in the charge diffusion layer is gradually released to the first electrode portion, thus achieving the function of the charge diffusion layer.

[0077] Furthermore, by configuring the charge diffusion layer in between the buffer layer and the electrode, the electric field generated by the electrode can be made larger compared to the case where the charge diffusion layer is in direct contact with the electrode. In particular, by eliminating the case where the charge diffusion layer is in direct contact with the front end of the first electrode portion where the electric field is concentrated, the concentration effect of the electric field can be further improved.

[0078] The invention will be described in part as a feature of the present invention, which is that "a buffer layer (BF) and a charge diffusion layer (CD) covering the optical waveguide are formed sequentially on a portion of the convex portion (10) formed on the surface of the optical waveguide substrate".

[0079] By forming a convex portion on the surface of an optical waveguide substrate, such as the Z-axis surface of an X-cut LN substrate, different charges are generated on the side of the convex portion. Therefore, by forming a charge diffusion layer that covers the convex portion serving as the optical waveguide, the generated charges can be diffused, suppressing the generation of thermoelectric effects.

[0080] On the other hand, for the portion of the convex portion on the surface of the optical waveguide substrate 1 that is not used as an optical waveguide (WG), charges may also be generated on the side of the convex portion. Therefore, it is preferable to configure it to be covered by a control electrode (especially the first electrode portion (E1)) or a charge diffusion layer (CD). In addition, the charge diffusion layer may be disposed directly on the side of the convex portion or disposed in the form of a buffer layer.

[0081] As mentioned above, the charge diffusion layer and the electrode may not be directly connected. This is because the charge is released from the charge diffusion layer to the electrode via the buffer layer.

[0082] From the perspective of suppressing charging, it is effective to construct a thinner buffer layer and a thicker charge diffusion layer within the stated numerical range.

[0083] The charge is not only released to the electrode through the buffer layer, but also electrically connected to a part of the control electrode of the active part or an electrode (wiring part) outside the active part, thereby enabling the charge to be released directly to the electrode.

[0084] Since the performance of the charge diffusion layer is affected by changes in resistance, materials that are not easily oxidized are preferred. SiN has very strong barrier properties against moisture and other environmental factors, and also exhibits excellent heat resistance and mechanical strength.

[0085] In addition, such as Figure 3 As shown, oxidation can be further suppressed by covering the charge diffusion layer (CD) with the resin layer RL. In particular, it is preferable to provide the resin layer RL at the DC electrode.

[0086] Next, use Figure 4 (a) ~ Figure 4 Section (d) explains the relationship between the configuration positions of the buffer layer (BF) and the charge diffusion layer (CD) and the control electrodes (E1, E2).

[0087] Figure 4 (a) The charge diffusion layer (CD) is configured to cover only the side surface of the optical waveguide (WG). This can counteract the potential on the side surfaces (+Z plane, -Z plane) of the convex portion of the X-cut LN substrate. Of course, by connecting the first electrode portion (E1) as a thin electrode to the optical waveguide substrate 1, the reduction in electric field efficiency can be suppressed.

[0088] exist Figure 4 In (b), the charge diffusion layer (CD) is configured to cover the side of the optical waveguide (WG) and extend slightly laterally (in the direction of the thin electrode). Thus, when forming the charge diffusion layer, there is a margin for offset in left-right alignment relative to the optical waveguide, which can mitigate the need for processing precision.

[0089] exist Figure 4 (a) and Figure 4 In case (b), in the working part shown in the figure, the charge diffusion layer (CD) is not connected to the electrodes (E1, E2), but by configuring it to be connected to the ground electrode (wiring part) in a part other than the working part, the charge can be released efficiently.

[0090] exist Figure 4 In (c), as shown in the dashed box A, the charge diffusion layer (CD) is connected to the first electrode portion (E1), which serves as a thin electrode. As a result, the charge generated when a potential difference is created on the side surfaces (+Z plane, -Z plane) of the convex portion of the X-cut LN substrate can be released to the thick electrode (second electrode portion E2) via the thin electrode (first electrode portion E1).

[0091] exist Figure 4 In (d), as shown in dashed box B, the charge diffusion layer (CD) is connected to the second electrode portion (E2), which serves as the thick electrode. More specifically, the thick electrode (second electrode portion E2) is configured to cover the buffer layer (BF) and the charge diffusion layer (CD) disposed on the thin electrode (first electrode portion E1). Therefore, the charge diffusion layer (CD) becomes a structure embedded between the electrodes. In this way, by increasing the connection width between the charge diffusion layer and the thick electrode, a margin is provided relative to the left-right alignment offset when the charge diffusion layer (CD) is formed, which can mitigate the processing accuracy issue. In addition, the electrical connection between the charge diffusion layer (CD) and the electrode can be made more reliable.

[0092] like Figure 4 (a) ~ Figure 4 As in (d), by covering the other convex portions (convex portions that do not form an optical waveguide) 11 adjacent to the optical waveguide (WG) with the first electrode portion (E1), the charge generated on the sides (+Z plane, -Z plane) of the convex portions can be released. Additionally, as... Figure 4 (c) and Figure 4 In the same way as (d), the charge can also be effectively released by covering the first electrode portion (E1) with a charge diffusion layer (CD).

[0093] Next, as Figure 5 As shown in (a), the case in which a charge diffusion layer (CD) is arranged in the radio frequency (RF) modulation section including the signal electrode E2 (signal) constituting the modulation electrode and the ground electrode E2 (Ground, GND) is described.

[0094] Of course, in the RF modulation section, just like in the DC modulation section which includes a DC bias electrode, charging generated in the RF modulation section can be suppressed by configuring a charge diffusion layer.

[0095] However, since the charge diffusion layer is placed in the RF modulation section, there is a possibility that the dielectric loss will increase and the high-frequency characteristics will deteriorate. Therefore, from the viewpoint of improving high-frequency characteristics, it is preferable to narrow the placement area of ​​the charge diffusion layer (CD) or not to place the charge diffusion layer (CD).

[0096] Furthermore, such as Figure 5 As shown in (b), by not configuring a charge diffusion layer in the portion of the second electrode portion (E2) that serves as a thick electrode that is connected to the optical waveguide substrate 1, the formation of the shielding electric field disappears, and the electric field from the signal electrode E2 (Signal) to the ground electrode E2 (GND) can be applied efficiently.

[0097] Figure 6 This is a plan view of the optical waveguide substrate 1, showing an example of an optical waveguide (WG). The input light Lin is branched into four parallel Mach-Zehnder type optical waveguides, and then partially combined to be emitted as output light (Lout1, Lout2).

[0098] exist Figure 6 In the diagram, the part consisting of the modulation electrodes (RF modulation section) is shown in dashed box RF, and the part consisting of the DC bias electrodes (DC modulation section) is shown in dashed box DC.

[0099] In each functional unit shown in the RF modulation unit and DC modulation unit, the size of the region where the charge diffusion layer is formed is studied. Furthermore, since electrodes are also disposed in each functional unit, the area of ​​the region where the charge diffusion layer is formed, relative to the area of ​​the functional unit excluding the area occupied by the electrodes, is also studied. More specifically, locally it is the area of ​​the portion sandwiched by opposing electrodes, and overall it is the sum of the areas sandwiched by each electrode.

[0100] In the DC modulation section, the area occupied by the charge diffusion layer is set to 70% or more, preferably 80% or more, and more preferably 100% (the entire surface). This improves charge diffusion efficiency over a wide area of ​​the DC modulation section. Since drift is prone to occur in the DC modulation section, a wide-area arrangement is preferred.

[0101] Conversely, in the RF modulation section, the high-frequency characteristics are reduced if a charge diffusion layer is present. Therefore, in the RF modulation section, the arrangement of the charge diffusion layer is preferably minimized. Furthermore, considering... Figure 6 The surface of the optical waveguide substrate 1, which includes an RF modulation section or a DC modulation section (excluding the electrode configuration section), as shown, has its area occupied by the charge diffusion layer suppressed to 80% or less of the total element area, preferably 70% or less, and more preferably 50% or less, from the viewpoint of suppressing the generation of film stress based on the charge diffusion layer.

[0102] Next, regarding such Figure 6 The optical waveguide (WG) shown includes multiple Mach-Zehnder type optical waveguides, and a method for mitigating the internal stress generated between the individual Mach-Zehnder type optical waveguides due to films such as electrodes or charge diffusion layers is explained.

[0103] Figure 7 of (a) Figure 7 (b) is Figure 6 The cross-sectional view at the dotted line X-X' in the figure. Symbols E20 and E21 are the signal electrodes that constitute the modulation electrodes.

[0104] exist Figure 7 In (a), a portion of the surface of the optical waveguide substrate 1 is exposed by setting a gap S1 between two ground electrodes G located between adjacent Mach-Zehnder type optical waveguides.

[0105] In addition, Figure 7 In (b), a gap S2 is provided in a portion of the buffer layer (BF) or charge diffusion layer (CD) located between adjacent Mach-Zehnder type optical waveguides, so that a portion of the surface of the optical waveguide substrate 1 is exposed.

[0106] Thus, by providing a portion of the surface of the optical waveguide substrate exposed between at least a portion of adjacent Mach-Zehnder type optical waveguides, it is possible to cut off the internal stress generated by the film.

[0107] The size of gap S1 or gap S2 is 3 μm or more, preferably 5 μm or more, and more preferably 10 μm or more.

[0108] In addition, it is effective in Figure 6 In the region where the charge diffusion layer is formed, such as the DC modulation section, the surface of the optical waveguide substrate, which is longer than the optical waveguide (WG) located in the active section, is exposed.

[0109] Thin buffer layers or resin layers that do not affect membrane stress can also be disposed on the exposed portion of the optical waveguide substrate 1.

[0110] Next, examples of applying the optical waveguide element of this invention to optical modulation devices or optical transmission devices such as high-bandwidth coherent driver modulators (HB-CDM) will be described. Hereinafter, an example using an HB-CDM will be described, but this invention is not limited thereto; it can also be applied to optical phase modulators, optical modulators with polarization wave synthesis capabilities, optical waveguide elements integrating more or fewer Mach-Zehnder waveguides, bonding devices with optical waveguide elements made of other materials such as silicon, and devices for sensor applications, etc.

[0111] like Figure 8 As shown, the optical waveguide element is housed within the frame CA. Furthermore, by providing an optical fiber (F) that is relative to the input / output optical wave of the optical waveguide (WG), an optical modulation device MD can be constructed.

[0112] exist Figure 8 In this configuration, the optical fiber F is introduced into the frame through a through-hole in the side wall of the frame CA, directly bonding the optical component or substrate to the optical fiber. However, this is not a limitation; the optical fiber can also be optically coupled to the optical waveguide element via an optical block or lens barrel, including an optical lens optically coupled to the optical waveguide element, a polarization wave combiner, etc. Furthermore, to ensure stable bonding with the optical fiber or optical block, reinforcing members can be overlapped and arranged along the end face of the optical waveguide substrate.

[0113] By connecting the electronic circuitry (digital signal processor DSP) that outputs the modulation signal So to the optical modulator MD to perform modulation, an optical transmitting device OTA can be constructed. To obtain the modulation signal S applied to the optical waveguide element, the modulation signal So output from the digital signal processor DSP needs to be amplified. Therefore, in Figure 8 In this configuration, a driver circuit (DRV) is used to amplify the modulated signal. The driver circuit (DRV) or the digital signal processor (DSP) can be configured either outside or inside the housing (CA). In particular, configuring the driver circuit (DRV) inside the housing further reduces propagation losses of the modulated signal from the driver circuit.

[0114] Regarding the optical modulation device, the state of assembly in the frame has been described, but the optical waveguide elements or parts that are sub-component products before being assembled in the frame are also included in the optical modulation device of this utility model.

[0115] Furthermore, the modulation electrode of the optical waveguide element exemplifies a single modulation signal, but is not limited thereto. This invention can also be applied to differential electrode structures that use differential signals.

[0116] Furthermore, it is shown that the first electrode portion or the second electrode portion can be formed into an electrode layer, but it can also be composed of a combination of multiple electrode layers.

[0117] [Industry availability]

[0118] As described above, according to this invention, an optical waveguide element that balances improved electric field efficiency with suppression of thermoelectric effects can be provided. Furthermore, an optical modulation device and an optical transmission apparatus using this optical waveguide element can be provided.

Claims

1. An optical waveguide element, made of a ferroelectric material, comprising: Optical waveguide substrate, having an optical waveguide; and control electrodes to apply an electric field to the optical waveguide, the optical waveguide element being characterized in that, The optical waveguide is formed as part of the protrusion on the surface of the optical waveguide substrate. A buffer layer and a charge diffusion layer are sequentially formed to cover the optical waveguide. The control electrode is connected to the optical waveguide substrate.

2. The optical waveguide element according to claim 1, characterized in that, The buffer layer covering the optical waveguide and the charge diffusion layer are configured to cover at least a portion of the control electrode.

3. The optical waveguide element according to claim 1, characterized in that, The control electrode is disposed on another convex portion adjacent to the optical waveguide.

4. The optical waveguide element according to claim 1, characterized in that, The control electrode consists of a thin first electrode portion and a thick second electrode portion.

5. The optical waveguide element according to claim 1, characterized in that, A resin layer is formed that covers at least a portion of the charge diffusion layer.

6. The optical waveguide element according to claim 1, characterized in that, The optical waveguide includes multiple Mach-Zehnder type optical waveguides. At least a portion between adjacent Mach-Zehnder type optical waveguides has a portion that exposes the surface of the optical waveguide substrate.

7. The optical waveguide element according to claim 1, characterized in that, The thickness of the charge diffusion layer is set to be in the range of 50 nm or more and 500 nm or less.

8. The optical waveguide element according to claim 1, characterized in that, The thickness of the buffer layer is set to be in the range of 200 nm or more and 3000 nm or less.

9. The optical waveguide element according to claim 1, characterized in that, The volume resistivity of the charge diffusion layer is set to 10. 8 Ω·cm or more and 10 14 The range below Ω·cm.

10. An optical modulation device, characterized in that, The optical waveguide element as described in any one of claims 1 to 9 is housed within the frame, and This includes optical fibers that are input to or output optical waves relative to the optical waveguide.

11. The optical modulation device according to claim 10, characterized in that, The optical waveguide element has modulation electrodes for modulating light waves propagating in the optical waveguide. The frame contains electronic circuitry that amplifies the modulation signal input to the modulation electrode.

12. An optical transmitting device, characterized in that, It has: the optical modulation device as described in claim 10; and The electronic circuit outputs a modulation signal to the optical modulation device.

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