Optical waveguide element, optical modulation device using the same, and optical transmission device

Abstract

To provide an optical waveguide element that suppresses an increase in size while also suppressing the degradation of high-frequency characteristics. [Solution] An optical waveguide element comprising an optical waveguide substrate 1 having an electro-optic effect and on which optical waveguides (OW1, OW2) are formed, and a modulation electrode for applying an electric field to the optical waveguide, wherein the optical waveguide and the modulation electrode have folded portions, the modulation electrode uses a capacitively charged electrode CE in the working portion that applies an electric field to the optical waveguide, and a non-capacitively charged electrode NCE other than the capacitively charged electrode is used in the wiring portion, which includes at least the folded portion of the modulation electrode, and a dielectric layer DL is arranged between the signal electrode SIE constituting the modulation electrode and the ground electrode (GE1, GE2) in a part of the wiring portion.

Description

Technical Field

[0001] The present invention relates to an optical waveguide element, an optical modulation device using the same, and an optical transmission device. In particular, the present invention relates to an optical waveguide element having an electro-optic effect, including an optical waveguide substrate on which an optical waveguide is formed, and a modulation electrode for applying an electric field to the optical waveguide, and more particularly to an optical waveguide element in which the optical waveguide and the modulation electrode have a folded portion.

Background Art

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide elements such as optical modulators using an optical waveguide substrate having an electro-optic effect such as lithium niobate (LN) on which an optical waveguide is formed are widely used.

[0003] In order to achieve a wide bandwidth and low power consumption of the driving frequency of an optical waveguide element, it is essential to reduce the driving voltage (Vπ). By increasing the length (electrode length) of the active part (the part where an electric field is applied to the optical waveguide) of an optical waveguide element such as an optical modulator, a reduction in Vπ can be achieved. On the other hand, miniaturization of the optical waveguide element is also required, and as shown in FIG. 1, a method of arranging the optical waveguide OW in a folded manner has been proposed. Also, in accordance with the folding of the optical waveguide OW, the modulation electrode (not shown) also has a folded structure.

[0004] On the other hand, as shown in Patent Document 1, in order to improve the high-frequency characteristics, it has been proposed to adopt a capacitive loading electrode in the active part of the modulation electrode. The capacitive loading electrode structure is composed of a plurality of small segment electrodes such as T-shaped or L-shaped electrodes arranged along the optical waveguide, and a wiring electrode for supplying a modulation signal to these segment electrodes. In the capacitive loading electrode, by adjusting the shape and size of the segment electrodes, etc., the refractive index of the microwave propagating through the electrode can be easily adjusted. Thereby, by adjusting the refractive index of the microwave and the refractive index of the light wave propagating through the optical waveguide, and by matching the propagation speeds of the microwave and the light wave, it becomes possible to achieve a higher modulation efficiency.

[0005] Patent Document 1 discloses a configuration in which a capacitively charged electrode is placed along the optical waveguide, even at the folded portion of the optical waveguide, and an action part is provided. However, when two branch waveguides constituting a Mach-Zehnder type optical waveguide are folded back, the branch waveguide located on the inside of the folded portion and the branch waveguide located on the outside of the folded portion have different lengths. As a result, a phase difference occurs between the two branch waveguides after passing through the folded portion. To resolve this, Patent Document 2 discloses a configuration in which no action part is provided at the folded portion of the branch waveguide.

[0006] Figure 2 is an enlarged view of a part of the optical waveguide element including the folded portion A in Figure 1. Figure 3 shows an example of a cross-sectional view along the dashed line B in Figure 2. The optical waveguides (OW1, OW2) are formed as rib-type optical waveguides on an optical waveguide substrate 1 such as LN. In Figure 3, the optical waveguides are shown as black-filled patterns, but a portion of the surface of the optical waveguide substrate 1 is processed to form convex optical waveguides, and is also part of the optical waveguide substrate 1.

[0007] The optical waveguide substrate 1 is bonded to the holding substrate 3 as a reinforcing member. An intermediate layer 2 is interposed between the optical waveguide substrate 1 and the holding substrate 3.

[0008] In the section where the two branched waveguides (OW1, OW2) are aligned in a straight line (left side of Figure 2), the operational part of the modulation electrode, composed of a signal electrode SIE and ground electrodes (GE1, GE2), is formed. In the operational part, a capacitively charged electrode CE is formed by multiple segment electrodes SE arranged along the optical waveguide (branched waveguide) and wiring electrodes LE connecting them.

[0009] At the folded section of the optical waveguide, the two branched waveguides (OW1, OW2) are bent so that their optical path lengths are the same. At the folded section of the modulation electrode in Figure 2, the signal electrode SIE and the ground electrodes (GE1, GE2) are bent so that they follow a different path from the optical waveguide. In this way, since the modulation electrodes other than the working part do not require an electric field to be applied to the optical waveguide, a coplanar electrode structure, which is an uncapacitively loaded electrode that does not use segment electrodes, is adopted.

[0010] In a modulation electrode, the working portion is a capacitively charged electrode, while the folded portion is an uncapacitively charged electrode. As a result, the refractive index and impedance of microwaves differ significantly between the two. Generally, the refractive index of microwaves is higher for capacitively charged electrodes than for coplanar electrodes, and the impedance is lower for capacitively charged electrodes than for coplanar electrodes. In the working portion using capacitively charged electrodes, the refractive index of microwaves and light waves is adjusted to be approximately the same. Therefore, in the folded portion, the refractive index of microwaves becomes lower than that of light waves. Consequently, the length of the modulation electrode must be longer than the optical path length of the optical waveguide in the folded portion. As shown in the adjustment section of Figure 2, the modulation electrodes (SIE, GE1, GE2) are configured to be longer than the optical waveguides (OW1, OW2). This causes the size of the optical waveguide element to increase.

[0011] In the adjustment and folding sections of the modulation electrode shown in Figure 2, adjustments such as changing the width of the signal electrode SIE or changing the electrode thickness (height) can be made to increase the microwave refractive index. However, generally, increasing the microwave refractive index increases the impedance of the modulation electrode. This causes impedance mismatch between the working section and the adjustment and folding sections, leading to deterioration of high-frequency characteristics. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] China Patent Publication (CN) No. 113985629 (A) [Patent Document 2] China Utility Model Publication (CN) No. 215986791 (U) [Overview of the project] [Problems that the invention aims to solve]

[0013] The problem that this invention aims to solve is to provide an optical waveguide element that solves the above-mentioned problems, suppresses an increase in the size of the optical waveguide element, and also suppresses the degradation of high-frequency characteristics. Furthermore, it aims to provide an optical modulation device and an optical transmission device using this optical waveguide element. [Means for solving the problem]

[0014] To solve the above problems, the optical waveguide element, optical modulation device using the same, and optical transmission device of the present invention have the following technical features. (1) An optical waveguide element comprising an optical waveguide substrate having an electro-optic effect and an optical waveguide formed thereon, and a modulation electrode for applying an electric field to the optical waveguide, wherein the optical waveguide and the modulation electrode have a folded portion, the modulation electrode uses a capacitively charged electrode in the part that applies an electric field to the optical waveguide, and a non-capacitively charged electrode other than the capacitively charged electrode is used in the wiring portion other than the part that applies an electric field to the optical waveguide, and a dielectric layer is placed between the signal electrode and the ground electrode that constitute the modulation electrode in a part of the wiring portion.

[0015] (2) The optical waveguide element described in (1) above is characterized in that the dielectric constant of the dielectric layer is lower than the dielectric constant of the optical waveguide substrate.

[0016] (3) In the optical waveguide element described in (1) above, the capacitively charged electrode is formed on the optical waveguide substrate in the working portion, and in at least a portion of the non-capacitively charged electrode having a dielectric layer, the optical waveguide substrate is not placed beneath the non-capacitively charged electrode.

[0017] (4) The optical waveguide element described in (1) above is characterized in that the dielectric layer is made of at least one of the following: the material constituting the optical waveguide substrate, the material constituting the buff layer covering the optical waveguide, or an insulating organic material, or a combination thereof.

[0018] (5) In the optical waveguide device according to (1) above, it has a holding substrate for holding the optical waveguide substrate, and an intermediate layer made of a material having a refractive index lower than that of the optical waveguide substrate is provided between the optical waveguide substrate and the holding substrate.

[0019] (6) In the optical waveguide device according to (5) above, at least a part of the wiring portion is characterized in that the lower end of the non-capacitive loading electrode is located at a position lower than the upper surface of the intermediate layer.

[0020] (7) In the optical waveguide device according to (1) above, the modulation electrode has an adjustment portion between the acting portion and the folding-back portion, and in the adjustment portion, the impedance is configured to gradually change.

[0021] (8) The optical waveguide device according to (1) above is housed in a housing, and is an optical modulation device characterized by including an optical fiber for inputting or outputting light waves to or from the optical waveguide.

[0022] (9) In the optical modulation device according to (8) above, it is characterized by having an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide device inside the housing.

[0023] (10) An optical transmission device characterized by having the optical modulation device according to (8) above and an electronic circuit for outputting a modulation signal for causing the optical modulation device to perform a modulation operation.

Advantages of the Invention

[0024] The present invention relates to an optical waveguide element having an electro-optical effect and including an optical waveguide substrate on which an optical waveguide is formed and a modulation electrode for applying an electric field to the optical waveguide. The optical waveguide and the modulation electrode include a folded portion. The modulation electrode uses a capacitive loading electrode for an acting portion that applies an electric field to the optical waveguide, and a non-capacitive loading electrode other than the capacitive loading electrode is used for at least a wiring portion including the folded portion of the modulation electrode outside the acting portion. A dielectric layer is disposed between a signal electrode and a ground electrode that constitute the modulation electrode in a part of the wiring portion, so that the microwave refractive index of the wiring portion including the folded portion can be configured to be higher, and miniaturization of the optical waveguide element can be realized. Further, the impedance of the wiring portion can be further reduced, impedance matching with the acting portion can be achieved, and deterioration of high-frequency characteristics can be suppressed. Furthermore, it is also possible to provide an optical modulation device and an optical transmission device using the optical waveguide element having these advantages.

Brief Description of the Drawings

[0025] [Figure 1] It is a plan view showing an example of an optical waveguide element having a folded optical waveguide. [Figure 2] It is a plan view enlarging a portion of the dotted line frame A in FIG. 1 and showing an example of a conventional optical waveguide element. [Figure 3] It is a view showing an example of a cross section on the dashed-dotted line B in FIG. 2. [Figure 4] It is a plan view explaining an outline of the optical waveguide element of the present invention. [Figure 5] It is a view showing an example of a cross section on the dashed-dotted line B in FIG. 4. [Figure 6] It is a view showing an example of a cross section on the dashed-dotted line C in FIG. 4. [Figure 7] It is a view showing an example of a cross section on the dashed-dotted line D in FIG. 4. [Figure 8] It is a cross-sectional view showing a plurality of structural patterns of a coplanar electrode, showing Structures 1 to 3. [Figure 9] It is a cross-sectional view showing a plurality of structural patterns of a coplanar electrode, showing Structures 4 to 6. [Figure 10] This is a cross-sectional view showing multiple structural patterns of the coplanar electrode, specifically structures 7-9. [Figure 11] This is a cross-sectional view illustrating a model for evaluating the effect of the depth of electrodes penetrating the intermediate layer. [Figure 12] This graph shows the changes in microwave refractive index and impedance in the model shown in Figure 11. [Figure 13] This is a plan view showing a first embodiment of the optical waveguide element of the present invention. [Figure 14] This is a plan view showing a second embodiment of the optical waveguide element of the present invention. [Figure 15] This is a plan view showing a third embodiment of the optical waveguide element of the present invention. [Figure 16] This is a plan view showing a fourth embodiment of the optical waveguide element of the present invention. [Figure 17] This is a plan view showing a fifth embodiment of the optical waveguide element of the present invention. [Figure 18] This is a plan view showing a sixth embodiment of the optical waveguide element of the present invention. [Figure 19] This is a plan view showing a seventh embodiment of the optical waveguide element of the present invention. [Figure 20] This is a plan view showing an eighth embodiment of the optical waveguide element of the present invention. [Figure 21] This is a plan view showing a ninth embodiment of the optical waveguide element of the present invention. [Figure 22] This is a plan view showing a tenth embodiment of the optical waveguide element of the present invention. [Figure 23] This is a plan view showing an eleventh embodiment of the optical waveguide element of the present invention. [Figure 24] This is a plan view showing a twelfth embodiment of the optical waveguide element of the present invention. [Figure 25] This is a cross-sectional view (Cross Section 1) showing an example of an application of the optical waveguide element of the present invention. [Figure 26] This is a cross-sectional view (Cross Section 2) showing an example of an application of the optical waveguide element of the present invention. [Figure 27]This is a cross-sectional view (Cross Section 3) showing an example of an application of the optical waveguide element of the present invention. [Figure 28] This is a cross-sectional view (Cross Section 4) showing an example of an application of the optical waveguide element of the present invention. [Figure 29] This is a cross-sectional view (Cross Section 5) showing an example of an application of the optical waveguide element of the present invention. [Figure 30] This is a plan view illustrating the optical modulation device and optical transmission device of the present invention. [Modes for carrying out the invention]

[0026] The optical waveguide element of the present invention will be described in detail below using preferred examples. As shown in Figures 4 and 5 to 7, the present invention relates to an optical waveguide element comprising an optical waveguide substrate 1 having an electro-optic effect and on which optical waveguides (OW1, OW2) are formed, and a modulation electrode for applying an electric field to the optical waveguide, wherein the optical waveguide and the modulation electrode have folded portions, the modulation electrode uses a capacitively charged electrode CE in the working portion that applies an electric field to the optical waveguide, and a non-capacitively charged electrode NCE other than the capacitively charged electrode is used in the wiring portion other than the working portion, at least including the folded portion of the modulation electrode, and a dielectric layer DL is arranged between the signal electrode SIE constituting the modulation electrode and the ground electrode (GE1, GE2) in a part of the wiring portion.

[0027] The optical waveguide substrate 1 used in the optical waveguide element of the present invention is a substrate having an electro-optic effect. Specifically, substrates such as lithium niobate (LN), lithium tantalate (LT), PLZT (lead zirconate lanthanum), and magnesium-doped substrates of these substrate materials can be used. Vapor-grown films made of these materials can also be used. Furthermore, the optical waveguide element of the present invention can be applied to heterogeneous structures in which LN is bonded to SiPh, and can also be used in structures in which the optical waveguide is formed of LN, Si, SiN, etc.

[0028] On the surface of the optical waveguide substrate 1 having an electro-optical effect, protrusions are formed as optical waveguides, constituting rib-type optical waveguides (OW1, OW2). Electrodes EL are formed so as to sandwich the rib-type optical waveguides (OW1, OW2) in order to apply an electric field based on a modulation signal or DC bias to the optical waveguides. To achieve velocity matching between the microwave and optical waves of the modulation signal, the thickness of the optical waveguide substrate 1 can be set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less. To increase the mechanical strength of such a thin substrate 1, it is bonded and fixed to a holding substrate 3 either by direct bonding or via a low refractive index intermediate layer 2 such as SiO2 or resin on the underside of the substrate 1. As the holding substrate 3 for direct bonding, a material with a lower refractive index than the optical waveguide substrate 1 and a similar coefficient of thermal expansion to the optical waveguide substrate, such as alkali-free glass or a substrate containing an oxide layer of quartz, is preferably used. Composite substrates, such as those with a silicon oxide layer formed on a silicon substrate (abbreviated as SOI or LNOI), or those with a silicon oxide layer formed on an LN substrate, are also available. When using such composite substrates, it is possible to omit the intermediate layer.

[0029] As a method for forming optical waveguides (OW1, OW2), as shown in Figure 5 or 6, it is possible to use a rib-type optical waveguide in which the portion corresponding to the optical waveguide is a protrusion on the substrate, such as by etching the optical waveguide substrate 1 or forming grooves on both sides of the optical waveguide. When using the thin plate substrate described above, the height of the rib-type optical waveguide is set to 90% or less, more preferably 80% or less, and even more preferably 70% or less, of the thickness of the optical waveguide substrate 1. The width of the rib-type optical waveguide is set to 40% to 150% of the thickness of the optical waveguide substrate 1, which allows single-mode propagation of the optical mode field. It is also possible to form a vapor-deposited film on the holding substrate 3 and process the film into the shape of an optical waveguide.

[0030] In the optical waveguide element to which the present invention is applied, as shown in Figures 4 and 5, a dielectric layer DL is provided in a portion of the folded wiring section (adjustment section or folded section) between the working section of the modulation electrode and the next working section. This makes it possible to set a higher refractive index for microwaves in the wiring section, and as a result, it becomes possible to set a shorter length for the wiring section. By shortening the length of the wiring section, miniaturization of the optical waveguide element can be achieved.

[0031] Furthermore, by using a dielectric layer DL, it becomes possible to lower the impedance of the wiring section and set a smaller impedance difference between the capacitively charged electrode CE of the working section and the coplanar electrode (uncapacitively charged electrode NCE) of the wiring section (adjustment section and folded section), thereby improving high-frequency characteristics.

[0032] The dielectric materials used for the dielectric layer (DL) include SiO2, Al2O3, LiNbO3, LiTaO3, high-purity Si, TiO2, Ta2O5, Nb2O5, SiN, SiON, and resins (insulating organic materials). As for resins, it is also possible to leave a portion of the photoresist used in the manufacturing process of optical waveguide elements without removing it and use it as a "permanent resist."

[0033] Regarding the film thickness and width of the dielectric layer, firstly, when using resin (permanent resist), the film thickness is in the range of 0.25 to 15 μm, preferably 0.5 μm to 12.5 μm, and more preferably 0.75 μm to 10 μm. Nextly, when using SiO2, the film thickness is in the range of 0.005 to 10 μm, preferably 0.01 to 8 μm, and more preferably 0.02 to 7 μm.

[0034] The width of the dielectric layer can be defined as the ratio of the width of the dielectric layer to the electrode spacing (for example, the distance between a signal electrode and the adjacent ground electrode). Ideally, the dielectric layer should be formed with the same width as the electrode spacing, in which case the ratio is 100%. Of course, it is not limited to this, and can be set appropriately within the range of 50% to 100%.

[0035] The dielectric layer may use only one of the materials described above, or it may use a combination of multiple materials. Here, when a material is selected from the various materials constituting the optical waveguide element and used as the dielectric layer DL of the present invention, the microwave refractive index and impedance of each electrode structure were calculated by simulation for several structural patterns shown in Figures 8 to 10.

[0036] Structure 1 in Figure 8 is a conventional electrode structure in which an optical waveguide substrate 1 (LN) is provided on a holding substrate (Si) 3 via an intermediate layer (SiO2) 2, and modulation electrodes (material Au, signal electrode SIE and ground electrodes (GE1, GE2)) are arranged on top of it. The thickness of each material and the width, height, and spacing of the electrodes are as follows. (Simulation conditions) ·Holding substrate: Si • Intermediate layer: SiO2, 2.0 μm thick • Optical waveguide substrate: LN with thicknesses of 0.3, 0.5, and 0.7 μm • Electrodes: Made of Au, with a height of 15.0 μm, a signal electrode width of 10.0 μm, and a distance of 10.0 μm between the signal electrode and the ground electrode. • Buffer layer: SiO2, 1.0 μm thick • Permanent resist (insulating organic material): 8.0 μm thick

[0037] Structure 2 in Figure 8 is obtained by removing the optical waveguide substrate 1 from Structure 1. Structure 3 in Figure 8 shows electrodes placed on the intermediate layer 2, with an optical waveguide substrate 1 remaining between the electrodes.

[0038] Structure 4 in Figure 9 is a structure in which a buffer layer (SiO2) is placed on the optical waveguide substrate 1 of Structure 3. The thickness of the buffer layer BF is 1.0 μm.

[0039] Structure 5 in Figure 9 is the same as Structure 1, but with a permanent resist PR layered on the optical waveguide substrate 1 and between the electrodes. The thickness of the permanent resist PR is 8.0 μm.

[0040] Structure 6 in Figure 9 is a configuration in which a permanent resist PR (8.0 μm thick) is placed on top of the buffer layer BF of structure 4.

[0041] Structure 7 in Figure 10 shows the electrodes of structure 6 embedded in the intermediate layer 2 and the holding substrate 3. The amount of electrode embedded in the holding substrate 3 is 0.0 to 2.0 μm.

[0042] Structure 8 in Figure 10 is a permanent resist PR placed on the optical waveguide substrate 1 of Structure 3. The thickness of the permanent resist PR is 8.0 μm.

[0043] Structure 9 in Figure 10 is a permanent resist placed on top of the intermediate layer 2 of structure 2, between the electrodes. The thickness of the permanent resist PR is 8.0 μm.

[0044] Table 1 shows the microwave refractive index (nm) and impedance Z (Ω) for the various structural patterns described in Figures 8 to 10. The values ​​for each structure are shown as a ratio relative to structure No. 1.

[0045] [Table 1]

[0046] Since Structure 1 is a conventional example, a preferred structural pattern is one in which the ratio of microwave refractive indices in nm relative to Structure 1 is greater than 1.0, and the ratio of impedance Z relative to Structure 1 is less than 1.0. For this reason, Configuration 2 is undesirable. Furthermore, Configurations 3 to 9 are preferable as dielectric layer configurations of the present invention because the numerical values ​​of refractive index and impedance are improved.

[0047] Next, to evaluate the effect of embedding electrodes (SIE, GE1, GE2) in the intermediate layer 2 and the holding substrate 3, as shown in structure 7 of Figure 10, the changes in microwave refractive index nm and impedance Z were simulated as the amount of electrode embedding h in the intermediate layer 2 was varied, assuming an intermediate layer (SiO2) thickness H of 2.0 μm, as shown in Figure 11. The holding substrate 3 was made of Si, the thickness of the optical waveguide substrate 1 was 0.5 μm, the thickness of the buffer layer (SiO2) BF was 1.0 μm, and the thickness of the permanent resist PR was 8.0 μm. Furthermore, the width of the signal electrode was 15.0 μm, the thickness (height) of the electrode was 10.0 μm, and the distance between the signal electrode and the ground electrode was 10.0 μm.

[0048] The simulation results are shown in the graph in Figure 12. Figure 12 shows the results as a ratio relative to the state with no burial depth. Referring to Figure 12, as the burial depth h of the electrode increases, the microwave refractive index nm increases, and the impedance Z gradually decreases. From these results, it can be said that burying the electrode deeper in the intermediate layer is a preferable approach.

[0049] The optical waveguide element of the present invention can be provided in various forms by changing the shape of the folded portion of the electrode, the shape of the adjustment portion and folded portion of the electrode, and the structure of the dielectric layer. Figures 13 to 24 show some of these forms.

[0050] Figure 13 is a plan view illustrating Embodiment 1 of the optical waveguide element of the present invention. The working portion is a capacitively charged electrode, and the folded portion is an uncapacitively charged electrode. The ground electrode GE2 connecting the two working portions is configured as a single common GND electrode. Furthermore, a dielectric layer DL is regulated between the signal electrode SIE and the ground electrodes (GE1, GE2) at the folded portion of the electrode. This configuration suppresses the degradation of high-frequency characteristics, enables miniaturization and low Vπ, reduces impedance differences, and is also expected to suppress signal crosstalk between the upper and lower working parts.

[0051] Figure 14 is a plan view illustrating Embodiment 2 of the optical waveguide element of the present invention. The main feature is that, unlike Embodiment 1 in Figure 13, the ground electrode GE2 is provided separately in two working parts, and the ground electrode GE2 is bent using the folded portion. This configuration suppresses degradation of high-frequency characteristics, enabling miniaturization, lower Vπ, and reduced impedance differences. It also allows for improvement of high-frequency dips (radiation losses to the substrate).

[0052] Figure 15 is a plan view illustrating Embodiment 3 of the optical waveguide element of the present invention. In addition to providing a dielectric layer DL between the electrodes, adjustment parts are provided on the electrodes. This configuration allows the arrival times of optical and electrical signals moving from one working part to the other to be the same, thereby improving high-frequency characteristics.

[0053] Figure 16 is a plan view illustrating Embodiment 4 of the optical waveguide element of the present invention. In the electrode adjustment section, the width of the ground electrodes (GGE1, GE2) is widened, the change in electrode spacing is tapered, and the spacing between the signal electrode and the ground electrode is narrowed in the folded section. This configuration allows for impedance matching between the working section and the folded section by gradually changing the electrode spacing in a tapered shape. Furthermore, since the arrival times of optical and electrical signals can be made the same, high-frequency characteristics are also improved. In addition, while it is difficult to narrow the electrode spacing when the electrodes are thick, forming a dielectric layer DL achieves an impedance equivalent to that of narrowing the electrode spacing. Therefore, by providing a dielectric layer and narrowing the electrode spacing in the folded section, it is possible to achieve a higher refractive index and lower impedance.

[0054] Figure 17 is a plan view illustrating Embodiment 5 of the optical waveguide element of the present invention. The structure has a widened signal electrode SIE in the adjustment section and a narrowed distance between the signal electrode and the ground electrode. This configuration allows for impedance matching between the working section and the folded section because the electrode spacing changes in a tapered shape. Furthermore, the wider width of the signal electrode (SIE) allows for a greater electrode thickness. Thus, increasing the electrode width allows for a greater electrode thickness, improving high-frequency characteristics.

[0055] Figure 18 is a plan view illustrating Embodiment 6 of the optical waveguide element of the present invention. The structure is such that the electrode spacing is narrowed by narrowing the width of the signal electrode SIE in the adjustment section to a extent that does not increase propagation loss, and widening the widths of the ground electrodes GE1 and GE2. This configuration allows for impedance matching between the working section and the folded section because the electrode spacing changes in a tapered shape. Furthermore, it reduces the phase difference of electrical signals caused by bending.

[0056] Figure 19 is a plan view illustrating Embodiment 7 of the optical waveguide element of the present invention. In the adjustment section of the modulation electrode, the electrode is extended in the vertical direction to form a delay circuit. This configuration makes it possible to further miniaturize the chip in the left-right (horizontal) direction of the drawing.

[0057] Figure 20 is a plan view illustrating Embodiment 8 of the optical waveguide element of the present invention. The shape of the modulation electrode in the folded portion is extended in the vertical direction. This configuration makes it possible to miniaturize the optical waveguide element itself (chip) in the left-right direction (horizontal direction) of the drawing.

[0058] Figure 21 is a plan view illustrating Embodiment 9 of the optical waveguide element of the present invention. Below the uncapacitively charged electrode NCE in the folded portion, the optical waveguide substrate 1 is removed, and the intermediate layer 2 is exposed. This configuration allows for low impedance at the electrode folding section and a high microwave refractive index (nm), thus enabling miniaturization of the optical waveguide element.

[0059] Figure 22 is a plan view illustrating Embodiment 10 of the optical waveguide element of the present invention. Below the non-capacitively charged electrode (NCE) that constitutes the electrode adjustment section and the folded section, the optical waveguide substrate 1 is removed, and the intermediate layer 2 is exposed. This configuration allows for a lower impedance in the electrodes between working sections compared to Example 9. Furthermore, the structure allows for a higher microwave refractive index in nm, enabling miniaturization.

[0060] Figure 23 is a plan view illustrating Embodiment 11 of the optical waveguide element of the present invention. The structure is such that there is no optical waveguide substrate 1 below the non-capacitively charged electrode NCE in the adjustment section and the folded section, and the two branch waveguides (OW1 and OW2) are set so as not to intersect each other. When folding the optical waveguide, the inner branch waveguide OW2 is formed with a meandering delay section so that its optical path length is the same as that of branch waveguide OW1. Furthermore, in order to apply the appropriate positive and negative electric fields to the folded optical waveguide, the optical waveguide in the next operating section is a polarization-reversed optical waveguide (OW1(POR), OW2(POR)). This configuration can be applied to structures without intersections in the optical waveguide, and it is possible to suppress propagation losses caused by the intersection of optical waveguides. Furthermore, the presence of the dielectric layer DL allows the length of the adjustment section to be set shorter than in the case without the dielectric layer. The electrode spacing of the adjustment section changes in a tapered shape.

[0061] Figure 24 is a plan view illustrating Embodiment 12 of the optical waveguide element of the present invention. The structure lacks an optical waveguide substrate 1 below the non-capacitively charged electrode NCE of the adjustment and folding sections, and the modulation electrode is "GS + S - It has a differential electrode of type G. Accordingly, the arrangement of the segment electrode SE is adjusted to be different in the two working parts. This configuration can be applied to differential electrode structures without intersections in the optical waveguide, and the Vπ reduction effect of the differential electrode can also be expected.

[0062] Next, to illustrate various embodiments of the dielectric layer DL, different cross-sectional shapes of the dielectric layer along the dashed line D in Figure 4 are shown in Figures 25 to 29. In the cross-sectional view 1 shown in Figure 25, only permanent resist PR is placed between the electrodes of the uncapacitated electrode.

[0063] In the cross-sectional view 2 shown in Figure 26, there is no optical waveguide substrate 1 below the non-capacitively charged electrode, and the permanent resist PR is placed on top of the intermediate layer 2.

[0064] In the cross-sectional view 3 shown in Figure 27, the optical waveguide substrate 1 remains between the electrodes of the non-capacitively charged electrode, and the permanent resist PR is placed on top of it.

[0065] In the cross-sectional view 4 shown in Figure 28, there is no optical waveguide substrate 1 below the non-capacitively charged electrode. However, a buffer layer BF is placed to cover the optical waveguide, and the optical waveguide substrate 1 and buffer layer BF remain between the electrodes, with a permanent resist PR placed on top of them.

[0066] The cross-sectional view 5 shown in Figure 29 shows that a dielectric layer (permanent resist PR) is arranged to cover the non-capacitively charged electrode.

[0067] As described above, various embodiments of optical waveguide elements have been explained, but it goes without saying that the present invention is not limited to these, and can also be applied to more complex optical waveguide elements that combine these various embodiments.

[0068] Furthermore, in the case of X-plates such as LNs, the refractive index (nm) of the electrical signal also changes at positions perpendicular to the modulation signal. Therefore, in conventional X-plate optical modulators, it is necessary to adjust the electrode spacing in the Z direction relative to the X direction, but this adjustment results in a slight impedance shift. In the present invention, when there is no electro-optic crystal in the folded portion, since it is not an anisotropic material, electrical signals can be transmitted with the refractive index nm and impedance matched.

[0069] Next, we will describe examples of applying the optical waveguide element of the present invention to optical modulation devices and optical transmitting devices. Figure 30 illustrates an optical modulation device incorporating an optical waveguide element using a folded optical waveguide, but the present invention is not limited to this and can also be applied to optical phase modulators, optical modulators with polarization combining functions, optical waveguide elements integrating more Mach-Zehnder type optical waveguides, bonding devices with optical waveguide elements made of other materials such as silicon, and devices for sensor applications. Furthermore, it goes without saying that it can also be applied to high-bandwidth-coherent driver modulators (HB-CDM).

[0070] As shown in Figure 30, the optical waveguide element has an optical waveguide OW formed on an optical waveguide substrate 1 and a modulation electrode (not shown) that modulates the light wave propagating through the optical waveguide, and is housed in a housing CA. Furthermore, by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide, an optical modulation device MD can be constructed. The optical fiber F is optically coupled to the optical waveguide OW in the optical waveguide element using an optical block or lens barrel equipped with an optical lens. However, the optical fiber may be introduced into the housing through a through-hole penetrating the side wall of the housing, and the optical fiber may be directly joined to an optical component or substrate, or an optical fiber with a lens function at the end of the optical fiber may be optically coupled to the optical waveguide in the optical waveguide element. L1 represents incident light, and L2 represents outgoing light.

[0071] An optical transmitter (OTA) can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So to the optical modulation device MD to perform modulation operations on the optical modulation device MD. In order to obtain the modulation signal S to be applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. For this reason, in Figure 30, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and the digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit. [Industrial applicability]

[0072] As described above, the present invention makes it possible to provide an optical waveguide element that suppresses an increase in the size of the optical waveguide element while also suppressing the degradation of high-frequency characteristics. Furthermore, it makes it possible to provide an optical modulation device and an optical transmission device using the optical waveguide element. [Explanation of Symbols]

[0073] 1 Optical waveguide substrate OW,OW1~2 Optical waveguide CE capacitively loaded electrode NCE non-capacitively loaded electrode SE segment electrode LE wiring electrodes SIE signal electrode GE1~2 Ground electrode DL Dielectrics F Optical Fiber CA cabinet MD Optical Modulation Device DRV driver circuit DSP (Digital Signal Processor) OTA Optical Transmitter

Claims

1. An optical waveguide element comprising an optical waveguide substrate having an electro-optic effect and an optical waveguide formed thereon, and a modulation electrode for applying an electric field to the optical waveguide, The optical waveguide and the modulation electrode are provided with a folded portion. The modulation electrode uses a capacitively charged electrode in the part that applies an electric field to the optical waveguide, and uses a non-capacitively charged electrode other than the capacitively charged electrode in the wiring section, at least including the folded portion of the modulation electrode, outside of the part that applies the electric field. An optical waveguide element characterized in that a dielectric layer is placed between the signal electrode and the ground electrode constituting the modulation electrode in a part of the wiring section.

2. An optical waveguide element according to claim 1, characterized in that the dielectric constant of the dielectric layer is lower than the dielectric constant of the optical waveguide substrate.

3. An optical waveguide element according to claim 1, characterized in that in the working portion, the capacitively charged electrode is formed on the optical waveguide substrate, and in at least a portion of the non-capacitively charged electrode having a dielectric layer, the optical waveguide substrate is not disposed beneath the non-capacitively charged electrode.

4. An optical waveguide element according to claim 1, characterized in that the dielectric layer is made of at least one of the materials constituting the optical waveguide substrate, the materials constituting the buff layer covering the optical waveguide, or an insulating organic material, or a combination thereof.

5. An optical waveguide element according to claim 1, characterized in that it has a holding substrate for holding the optical waveguide substrate, and an intermediate layer made of a material with a lower refractive index than the optical waveguide substrate is provided between the optical waveguide substrate and the holding substrate.

6. An optical waveguide element according to claim 5, characterized in that at least a portion of the wiring portion has a lower end of the uncapacitively charged electrode located lower than the upper surface of the intermediate layer.

7. An optical waveguide element according to claim 1, characterized in that the modulation electrode has an adjustment section between the working section and the folded section, and the impedance in the adjustment section is configured to change gradually.

8. The optical modulation device is characterized in that the optical waveguide element described in claim 1 is housed within a housing and comprises an optical fiber that inputs or outputs an optical wave to the optical waveguide.

9. An optical modulation device according to claim 8, characterized in that it has an electronic circuit inside the housing that amplifies the modulation signal input to the modulation electrode of the optical waveguide element.

10. An optical transmitting device characterized by comprising an optical modulation device according to claim 8, and an electronic circuit that outputs a modulation signal for causing the optical modulation device to perform a modulation operation.

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