Optical waveguide element, optical modulator using same, and optical transmission device

Abstract

The purpose of the present invention is to provide an optical waveguide element capable of preventing a decrease in efficiency of an electrical field applied to an optical waveguide by an electrode while suppressing absorption, by the electrode, of light waves propagating through the optical waveguide and suppressing separation of the electrode.　An optical waveguide element according to the present invention comprises an optical waveguide substrate 1 on which an optical waveguide 10 is formed, a support substrate 3 that supports the optical waveguide substrate 1, and an electrode 2 that applies an electrical field to the optical waveguide 10 and that is provided with an upper electrode M and a base layer m for joining the upper electrode M. The optical waveguide element is characterized in that the base layer m is formed directly on at least the support substrate 3.

Description

Optical waveguide element, optical modulator using the same, and optical transmitter

[0001] The present invention relates to an optical waveguide element, an optical modulator, and an optical transmitter using the same, and more particularly to an optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, a support substrate supporting the optical waveguide substrate, and an electrode for applying an electric field to the optical waveguide, the electrode comprising an upper electrode and a base layer for joining the upper electrode, and an optical modulator and an optical transmitter using the same.

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

[0003] In recent years, high-bandwidth coherent driver modulators (HB-CDMs) have been attracting attention, and the development of optical waveguide elements that can support higher speeds and smaller sizes is expected. For this reason, the width of the optical waveguide must be miniaturized to 1 μm or less, and the electrodes must be positioned closer to the optical waveguide.

[0004] Generally, when placing a metal such as Au on an LN substrate as an electrode, the adhesion between the LN substrate and Au is low, so a film of Au and a metal such as Ti or Nb is formed together as a base layer for the electrode, and an Au electrode layer is then formed on top of that.

[0005] The metal of the underlayer has a higher optical absorption rate than the Au electrode layer, and when the underlayer is placed near the optical waveguide, the light waves propagating through the optical waveguide are absorbed by the underlayer, resulting in a problem of increased optical loss. In particular, as the electrodes and optical waveguides become closer as optical modulators become smaller, the effect of light absorption by the underlayer of the electrodes becomes more pronounced than ever before.

[0006] In Patent Document 1, as shown in FIG. 1, an electrode is composed of a base layer m and an upper electrode (electrode layer) M, and the base layer m is formed on an optical waveguide substrate 1, and the upper electrode M protrudes from the base layer m toward the optical waveguide 10 so that the upper electrode is in direct contact with the optical waveguide substrate.

[0007] In the configuration of FIG. 1 , the base layer m is located away from the optical waveguide 10, and therefore it is expected that the loss of light absorption by the base layer m will be reduced. However, because the base layer m is disposed on the optical waveguide substrate, there is a possibility that the base layer will absorb light distributed in the slab waveguide portion of the optical waveguide substrate 1 (thin portions extending to the left and right of the optical waveguide 10 in FIG. 1 ).

[0008] Furthermore, Patent Document 2 discloses that, as shown in Figure 2, an electrode EL is disposed at a position lower than the surface of a buffer layer BF in order to provide an optical waveguide device with low scattering loss and high electric field efficiency. In Figure 2, an LN substrate (LN) is disposed on a substrate SUB with an intermediate layer IL interposed therebetween, and a buffer layer BF is provided thereon. However, no mention is made of the material of the intermediate layer or the underlayer of the electrode EL. This raises the possibility of problems with the adhesion of the electrode EL, leading to the electrode peeling off.

[0009] PCT / JP2022 / 37614 (filing date: October 7, 2022) JP2021-173791A

[0010] The object of the present invention is to solve the above-mentioned problems and to provide an optical waveguide element that suppresses absorption of light waves propagating through the optical waveguide by the electrodes, suppresses peeling of the electrodes, and prevents a decrease in the efficiency of the electric field applied to the optical waveguide by the electrodes, and further to provide an optical modulator and an optical transmitter that use the optical waveguide element.

[0011] In order to solve the above problems, the optical waveguide element of the present invention and the optical modulator and optical transmitter using the same have the following technical features: (1) An optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, a support substrate that supports the optical waveguide substrate, and an electrode that applies an electric field to the optical waveguide, the electrode comprising an upper electrode and an underlayer for joining the upper electrode, characterized in that the underlayer is formed directly on at least the support substrate.

[0012] (2) In the optical waveguide element described in (1) above, the upper electrode is disposed so as to extend over a portion of the upper surface of the optical waveguide substrate excluding the optical waveguide.

[0013] (3) In the optical waveguide element described in (2) above, the contact area between the upper electrode and the upper surface of the optical waveguide substrate extends from the end face of the optical waveguide substrate toward the optical waveguide by 0.05 μm or more.

[0014] (4) The optical waveguide element according to (2) above, wherein the end of the optical waveguide substrate is tapered to a smaller thickness, and the angle of the tip is 50 degrees or less.

[0015] (5) The optical waveguide element according to (2) above is characterized in that an underlayer is disposed below the upper electrode extending onto the upper surface of the optical waveguide substrate.

[0016] (6) In the optical waveguide element described in (1) above, the upper electrode is disposed so as to extend over a portion of the lower surface of the optical waveguide substrate excluding the optical waveguide.

[0017] (7) In the optical waveguide element described in (6) above, the contact area between the upper electrode and the lower surface of the optical waveguide substrate extends from the end face of the optical waveguide substrate toward the optical waveguide by 0.05 μm or more.

[0018] (8) The optical waveguide element according to (6) above, wherein the upper electrode is disposed so as to extend onto a part of the optical waveguide substrate excluding the optical waveguide.

[0019] (9) The optical waveguide element according to (1) above, wherein an air layer is formed in the gap between the optical waveguide substrate and the support substrate, at a position closer to the optical waveguide than the position where the underlayer is formed.

[0020] (10) An optical modulator comprising: an optical waveguide element according to any one of (1) to (9) above; a housing for accommodating the optical waveguide element; and an optical fiber for inputting or outputting a light wave to or from the optical waveguide.

[0021] (11) In the optical modulator described in (10) above, the electrode is a modulation electrode for modulating a light wave propagating through the optical waveguide, and an electronic circuit for amplifying a modulation signal input to the modulation electrode is provided inside the housing.

[0022] (12) An optical transmitter comprising the optical modulator according to (10) above, a light source for inputting a light wave to the optical modulator, and an electronic circuit for outputting a modulated signal to the optical modulator.

[0023] The present invention provides an optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, a support substrate that supports the optical waveguide substrate, and an electrode that applies an electric field to the optical waveguide, the electrode comprising an upper electrode and an underlayer for joining the upper electrode. Since the underlayer is formed directly on at least the support substrate, it is possible to suppress absorption by the electrode of light waves propagating through the optical waveguide.

[0024] Furthermore, since the upper electrode is disposed so as to extend over a portion of the upper or lower surface of the optical waveguide substrate excluding the optical waveguide, it is possible to prevent the electrode from peeling off and also to prevent a decrease in the efficiency of the electric field applied to the optical waveguide by the electrode. Furthermore, by using an optical waveguide element having such excellent characteristics, it is possible to provide an optical modulator or an optical transmitter that achieves similar effects.

[0025] 11A and 11B are cross-sectional views showing an example of a conventional optical waveguide element; FIG. 11B are cross-sectional views showing another example of a conventional optical waveguide element; FIG. 11C are cross-sectional views showing a first embodiment of an optical waveguide element of the present invention; FIG. 11D are cross-sectional views showing a second embodiment of an optical waveguide element of the present invention; FIG. 11E are cross-sectional views showing a third embodiment of an optical waveguide element of the present invention; FIG. 11F are cross-sectional views showing a fourth embodiment of an optical waveguide element of the present invention; FIG. 11H are cross-sectional views showing a fifth embodiment of an optical waveguide element of the present invention; FIG. 11H are cross-sectional views showing a sixth embodiment of an optical waveguide element of the present invention; FIG. 11H are cross-sectional views showing a seventh embodiment of an optical waveguide element of the present invention; FIG. 11H are cross-sectional views showing an eighth embodiment of an optical waveguide element of the present invention; FIG. 11H are cross-sectional views showing a simulation model for an optical waveguide element of the present invention; FIG. 11H are diagrams showing a conventional structure used for normalizing the simulation; FIG. 11H are graphs showing simulation results using the model of FIG. 11; FIG. 11H are diagrams showing a manufacturing process for an optical waveguide element of the present invention (part 1); FIG. 11H are diagrams showing a manufacturing process for an optical waveguide element of the present invention (part 2); FIG. 11H are diagrams showing a manufacturing process for an optical waveguide element of the present invention (part 3); FIG. 11H are cross-sectional views showing a ninth embodiment of an optical waveguide element of the present invention; FIG. 11H are diagrams for explaining electric field efficiency in an optical waveguide element of the present invention; and FIG. 11H are cross-sectional views showing a tenth embodiment of an optical waveguide element of the present invention. 1 is a diagram illustrating an example of an optical transmitting device according to the present invention.

[0026] The optical waveguide element of the present invention, and the optical modulator and optical transmitter using the same will be described in detail below using preferred examples. Cross-sectional views showing examples of the optical waveguide element of the present invention are shown in Figures 3 to 10, 17, and 19. The optical waveguide element of the present invention comprises an optical waveguide substrate 1 on which an optical waveguide 10 is formed, a support substrate 3 that supports the optical waveguide substrate 1, and an electrode 2 that applies an electric field to the optical waveguide 10, the electrode 2 comprising an upper electrode M and an underlayer m for bonding the upper electrode M, characterized in that the underlayer m is formed directly on at least the support substrate 3.

[0027] The substrate (optical waveguide substrate) 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect. Specifically, substrates such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), as well as substrates made of these substrate materials doped with MgO or the like, can be used. These materials can also be used to form films using vapor phase growth methods such as sputtering, evaporation, or CVD. Furthermore, a substrate made by bonding a substrate having an electro-optic effect to another substrate and then thin-film processing the electro-optic substrate can also be used. Furthermore, semiconductor substrates and substrates made of organic materials such as EO polymers can also be used.

[0028] The optical waveguide 10 can be an optical waveguide in which a high refractive index material such as Ti is thermally diffused into a substrate (optical waveguide substrate) 1, an optical waveguide formed by proton exchange, or a rib-type optical waveguide 10 in which the portion of the substrate corresponding to the optical waveguide is convex, as shown in Figure 3, by etching the substrate 1 other than the optical waveguide or by forming grooves on both sides of the optical waveguide. Furthermore, in accordance with the rib-type optical waveguide, it is also possible to further increase the refractive index by diffusing Ti or the like onto the substrate surface by thermal diffusion or proton exchange. The size of the rib-type optical waveguide is a finely structured optical waveguide with a width and height of about 1 μm to enhance light confinement.

[0029] The thickness (maximum thickness) of the optical waveguide substrate (thin plate) 1 on which the optical waveguide 10 is formed is set to 10 μm or less, more preferably 5 μm or less, and even more preferably 1 μm or less, in order to achieve speed matching between the microwave and light waves of the modulation signal. Furthermore, the height of the rib-type optical waveguide 10 (the height of the portion protruding from the slab waveguide) is set to 80% or less of the maximum thickness of the optical waveguide substrate, specifically 4 μm or less, more preferably 3 μm or less, and even more preferably 0.8 μm or less or 0.4 μm or less.

[0030] To increase the mechanical strength of the optical waveguide substrate 1 having the optical waveguide formed thereon, a support substrate 3 is bonded to the underside of the optical waveguide substrate 1. The optical waveguide substrate 1 and the support substrate 3 are bonded together by direct bonding or via an adhesive layer such as a resin. The support substrate to be directly bonded preferably has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but is not limited thereto. In the case of direct bonding, an intermediate layer such as a metal oxide or metal may be included in the bonding portion. Furthermore, the support substrate 3 is preferably made of a material having a thermal expansion coefficient similar to that of the optical waveguide substrate 1, such as a substrate containing an oxide layer of quartz or glass. Furthermore, it is also possible to use the same LN substrate as the optical waveguide substrate 1, or a composite substrate formed with a silicon oxide layer on a silicon substrate, abbreviated as SOI or LNOI, or a composite substrate formed with a silicon oxide layer on an LN substrate. If the refractive index of the support substrate 3 is higher than that of the optical waveguide substrate 1, a layer with a lower refractive index than the optical waveguide substrate 1 is provided between the optical waveguide substrate 1 and the support substrate 3. As will be described later, the support substrate according to the present invention is not limited to a substrate formed from a single substrate, but also includes a substrate formed by stacking a plurality of substrates together.

[0031] The electrode 2 that applies an electric field to the optical waveguide 10 is composed of an upper electrode M and an underlayer m. The upper electrode M is made of a metal such as Au or Cu. The underlayer m is used to improve adhesion between the support substrate 3 and the electrode 2 (electrode layer M1). The upper electrode M is formed so as to cover the underlayer by electrolytic plating using the underlayer, electroless plating using a resist pattern, a gas phase method such as vapor deposition or sputtering, or a combination of these. Because the underlayer m is formed directly on at least the support substrate 3, it has good adhesion and does not peel off from the electrode.

[0032] The material of the underlayer m must have a bond energy of oxygen to the metal, such as Ti, Nb, Ni, Cr, or Al, greater than that of the material used for the upper electrode M. In particular, the bond energy of oxygen to the metal must be 300 kJ (mol-O) or less. －１ The above metals are preferable because they bond strongly with oxygen in the LN substrate. The film formation method can be sputtering, vapor deposition, or the like.

[0033] The optical waveguide element of the present invention is characterized in that the underlayer m is formed directly on the support substrate 3 as shown in FIG. 3. In this case, the support substrate 3 is made of, for example, SiO 2 , which has a lower dielectric constant than the optical waveguide substrate 1. ２ The optical waveguide substrate is LN, the underlayer is made of Ti or Nb, and the upper electrode is made of Au.

[0034] Since no underlayer m is disposed on the optical waveguide substrate 1, it is possible to suppress an increase in propagation loss by, for example, absorbing light waves propagating through the slab waveguide of the optical waveguide substrate 1. The underlayer m can be formed not only on the support substrate 3 adjacent to the optical waveguide substrate 1 as shown in Fig. 3, but also at a distance from the optical waveguide substrate 1 as shown in Fig. 4. This makes it possible to further suppress absorption loss in the electrodes.

[0035] Furthermore, as shown in FIG. 5, by disposing a base layer m on a portion of the lower side of the upper electrode M and further disposing the base layer m away from the optical waveguide substrate 1, not only can the absorption loss of the electrode be suppressed, but the upper electrode M is brought closer to the optical waveguide 10, thereby increasing the electric field efficiency and enabling a lower driving voltage to be achieved in optical waveguide elements such as optical modulators.

[0036] 6 to 10 are characterized in that the upper electrode M is disposed so as to extend over at least one of a portion of the upper surface or a portion of the lower surface of the optical waveguide substrate 1 excluding the optical waveguide 10. In Fig. 6, the upper electrode M is disposed so as to cover a portion of the upper surface of the optical waveguide substrate 1. With this configuration, the upper electrode M is made of, for example, Au, and therefore does not easily absorb the light waves propagating through the slab waveguide, thereby suppressing electrode absorption loss. Moreover, since the upper electrode is disposed in close proximity to the optical waveguide 10, a lower driving voltage is also possible.

[0037] 7, a portion of the support substrate 3 is removed by wet etching to form a recess 30. Specifically, the support substrate 3 is wet-etched to form a step structure (trapezoidal portion), and the bottom portion of the step structure is over-etched to make the depth of the recess 30 deeper. Because a gap is formed between a portion of the lower surface of the optical waveguide substrate 1 and the support substrate 3, a portion of the upper electrode M is disposed so as to wrap around the lower surface of the optical waveguide substrate. Furthermore, the upper electrode M can be disposed not only on the lower surface of the optical waveguide substrate 1, but also on the upper surface of the optical waveguide substrate 1, as in FIG. 6.

[0038] In Figure 7, the upper electrode M does not fill a portion of the recess 30, forming an air layer (cavity) G. Since the refractive index of this air layer G is lower than that of the support substrate (low-dielectric-constant substrate) 3, the presence of the air layer G partially increases the light confinement strength. As a result, it has the effect of suppressing the expansion of the optical mode field diameter that expands from the optical waveguide 10 toward the surrounding slab waveguide, and has the effect of suppressing the light wave from expanding and reaching the underlying layer. In other words, the air layer G is preferably formed in a gap between the optical waveguide substrate 1 and the support substrate 3, at a position closer to the optical waveguide (10) than the position where the underlying layer m is formed.

[0039] 8 shows a case where the support substrate 3 is etched into a step structure (trapezoidal portion), and the upper electrode M is disposed so as to sink into the lower surface side of the optical waveguide substrate 1. In the case of FIG. 8, an air layer G is provided as in FIG. 7.

[0040] 9, unlike Fig. 8, the gap between the optical waveguide substrate 1 and the support substrate 3 is filled with the upper electrode M, and no air layer is formed. Increasing the area of ​​the upper electrode M in contact with the optical waveguide substrate 1 makes it possible to reduce the driving voltage. Furthermore, increasing the area of ​​the upper electrode M in contact with the low dielectric constant substrate, which is the support substrate 3, contributes to reducing the driving voltage.

[0041] As shown in Figures 7 to 9, when the support substrate 3 has a stepped structure, it becomes difficult to arrange the upper electrode M on the upper surface of the optical waveguide substrate 1 as the distance between the optical waveguide substrate 1 and the lower step of the step of the support substrate 3 increases. For this reason, as shown in Figure 10, the end of the optical waveguide substrate 1 is tapered, and the angle θ2 of the tip is set to 50 degrees or less. It is preferably 35 degrees or less, and more preferably 20 degrees or less. By forming it in a tapered shape, it becomes easy to protrude the upper electrode above the optical waveguide substrate 1 without being affected by the thickness of the optical waveguide substrate 1 (the thickness of the slab waveguide portion) or the thickness of the electrode 2. The angle θ1 of the side surface of the optical waveguide 10 (trapezoidal portion) is set to approximately 50 degrees or more and 80 degrees or less.

[0042] Next, using the model shown in Fig. 11, a simulation was performed to evaluate the influence of the overhang UL of the upper electrode M onto the upper surface and the overhang DL of the upper electrode M onto the lower surface of the optical waveguide substrate 1. Fig. 13 is a graph showing the simulation results.

[0043] The following conditions were set as boundary conditions for the simulation. Under the conditions of each overhang amount (UL, DL), the gap Gap between the upper electrodes was set to 4.0 to 4.8 μm, and the relational expression (y=a×exp(b×x), a and b are coefficients) between the electrode absorption loss (y=Loss) and the drive voltage Vπ (x=VπL) was calculated from the simulation (coefficients a and b were found). From the relational expression, the electrode absorption loss (y) was compared when the drive voltage was the same value (x=1.0). Note that the comparison was made when the gap amount (Gap) of the upper electrode was set so that VπL=1.0. Height of the optical waveguide 10: 0.1 to 1.0 μm Thickness of the slab waveguide of the optical waveguide substrate 1: 0.1 to 1.0 μm Material of the optical waveguide substrate 1: LN Material of the support substrate 3: SiO ２ Gap between the optical waveguide substrate 1 and the support substrate 3: 0.1 to 0.4 μm

[0044] The amount of protrusion (upper Au amount) UL of the upper electrode M from the upper surface of the optical waveguide substrate 1 was changed from 0 μm to 0.5 μm in 0.05 μm increments. Next, for each upper Au amount UL, the amount of protrusion (lower Au amount) DL of the upper electrode M from the lower surface of the optical waveguide substrate 1 was changed from 0 μm to 0.5 μm in 0.05 μm increments.

[0045] A simulation was performed using the conventional structure of FIG. 12 to calculate VπL and Loss, and the vertical axis of the graph in FIG. 13 was normalized with this calculated Loss (electrode absorption loss) set to 1.0. Therefore, the graph in FIG. 13 shows Loss (electrode absorption loss) due to changes in the lower Au amount. Referring to the graph, it is easy to see that in both cases, Loss decreases as the upper Au amount UL or the lower Au amount DL increases. In both the upper Au amount UL and the lower Au amount DL, a tendency for Loss to decrease is observed at 0.05 μm or more, and the effect of reducing Loss becomes significant at a thickness of preferably 0.15 μm or more, more preferably 0.25 μm or more.

[0046] 14 to 16 are diagrams illustrating an example of a manufacturing process for an optical waveguide element of the present invention. (a) A support substrate 3 and an optical waveguide substrate 1 are bonded. (b) A photoresist PR1 is patterned on the upper surface of the optical waveguide substrate 1. (c) The outer shape of the optical waveguide substrate (the portion including the slab waveguide) is formed by etching. (d) A photoresist PR2 is placed to match the optical waveguide. The photoresist PR2 may be formed by forming a new resist film or by processing the photoresist PR1.

[0047] (e) After forming the optical waveguide 10, the side surface of the optical waveguide 10 is covered with a protective film 20, the photoresist PR2 is removed, and then the protective film 20 is also removed. (f) The state after the photoresist PR2 and other materials have been removed. (g) A base layer m and a portion M1 of the upper electrode are laminated over the entire surface. (h) Photoresist PR3 covers the area other than where the upper electrode M will be formed.

[0048] (i) The upper electrode M is formed by plating according to the pattern of the photoresist PR3. At this time, if necessary, the upper electrode M is grown so that it penetrates into the underside of the optical waveguide substrate 1. By adjusting the processing steps in this plating method, it is possible to control the growth of the electrode layer, for example, to determine whether or not an air layer G is formed and to adjust its size. (j) The photoresist PR3 is removed. (k) The thin upper electrode M1 disposed on the base layer m is removed. (l) The unnecessary base layer m is removed to complete the optical waveguide element. Optical waveguide elements are formed by forming multiple optical waveguide elements on a single wafer substrate, and then cutting each optical waveguide element into chips for use. The explanation of the present invention focuses on optical waveguide elements formed into chips.

[0049] 17, an underlayer m1 is provided on a portion of the optical waveguide substrate 1. Providing an underlayer on the optical waveguide substrate 1 is not ideal because it increases the likelihood of absorption of light waves propagating through the slab waveguide. However, the underlayer m1 makes it possible to improve the adhesion of the upper electrode M. Furthermore, since there is also an underlayer m on the support substrate 3 side, it is preferable to keep the underlayer m1 on the optical waveguide substrate to a necessary minimum.

[0050] 18 shows how the electric field distribution changes depending on the relative positions of the optical waveguide substrate and the electrodes. In Fig. 18(a), the optical waveguide substrate 1 and the electrodes (M, m) are positioned a short distance apart. Because the support substrate 3 is a low-dielectric-constant material, the electric field is concentrated mainly on the optical waveguide substrate 1. However, in the area where the optical waveguide substrate and the electrode are spaced apart, the support substrate 3 has a higher dielectric constant than air, so the electric field mainly passes through the support substrate 3.

[0051] In contrast, when an electrode (upper electrode M) is in contact with the optical waveguide substrate 1 as shown in Figure 18(b), the electric field generated by the electrode directly penetrates the optical waveguide substrate 1, resulting in a higher electric field efficiency than in Figure 18(a). Moreover, by arranging the upper electrode M to extend beyond the optical waveguide substrate 1, the distance between the electrodes sandwiching the optical waveguide 10 is narrowed, making it possible to further increase the electric field efficiency. The symbol L in Figure 18 indicates the cross section of a light wave propagating through the optical waveguide 10.

[0052] Figure 19 shows an example of a support substrate formed by stacking multiple substrates. The support substrate is composed of a low dielectric constant layer 30 such as SiO2 and a reinforcing substrate 31 for increasing mechanical strength. A Si substrate or the like can be used as the reinforcing substrate. In the present invention, a composite substrate (30, 31) such as that shown in Figure 19 can also be used as one of the "support substrates."

[0053] Next, examples of application of the optical waveguide element of the present invention to an optical modulator or optical transmitter will be described. While the following description will use an example of HB-CDM, the present invention is not limited to this, and can also be applied to optical phase modulators, optical modulators with polarization combining functions, optical modulators integrating more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide substrates made of other materials such as silicon, devices for sensor applications, and the like.

[0054] As shown in FIG. 20 , the optical waveguide element includes an optical waveguide 10 formed on an optical waveguide substrate 1 and an electrode (not shown), such as a modulation electrode, that modulates the light wave propagating through the optical waveguide 10. The substrate 1 is housed within a housing CA. Furthermore, an optical modulator MD can be configured by providing an optical fiber (F) that inputs and outputs light waves to the optical waveguide. In FIG. 20 , the optical fiber (F) is introduced into the housing CA through a through-hole penetrating the sidewall, and the optical waveguide substrate 1 and the optical fiber are directly bonded. Alternatively, the light wave L1 incident from the optical fiber F and the light wave L2 emitted from the optical fiber F can be optically coupled to the optical waveguide 10 within the optical waveguide substrate 1 via an optical block equipped with an optical lens, a lens barrel, a polarization multiplexer, or the like. Furthermore, to ensure stable bonding with the optical fiber or optical block, a reinforcing member 4 can be placed on top of the optical waveguide substrate 1 along the end face of the substrate 1.

[0055] An optical transmitter OTA can be configured by connecting an electronic circuit (digital signal processor, DSP) that outputs a modulation signal S0 that causes the optical modulator MD to perform modulation operations to the optical modulator MD. To obtain the modulation signal S to be applied to the optical waveguide element, the modulation signal S0 output from the digital signal processor DSP can be amplified. For this reason, in FIG. 20, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and digital signal processor DSP can be located outside the housing CA, but they can also be located inside the housing CA. In particular, locating the driver circuit DRV inside the housing can further reduce the propagation loss of the modulation signal from the driver circuit. If degradation of the modulation signal is minimal, a DRV is not necessary, and the optical modulator MD can be directly modulated by the DSP.

[0056] The input light L1 to the optical modulator MD may be supplied from outside the optical transmitter OTA, but it may also be incorporated integrally into the optical transmitter OTA using a semiconductor laser (not shown) as a light source. The output light L2 modulated by the optical modulator MD is output to the outside via an optical fiber F.

[0057] As described above, according to the present invention, it is possible to provide an optical waveguide element that suppresses absorption of light waves propagating through the optical waveguide by the electrodes, suppresses peeling of the electrodes, and prevents a decrease in the efficiency of the electric field applied to the optical waveguide by the electrodes.Furthermore, it is possible to provide an optical modulator and an optical transmitter using the optical waveguide element.

[0058] REFERENCE SIGNS LIST 1 Optical waveguide substrate (thin plate, film) 2 Electrode 3 Support substrate 4 Reinforcing member 10 Optical waveguide m, m1 Underlayer M Upper electrode F Optical fiber CA Housing MD Optical modulator DRV Driver circuit DSP Digital signal processor OTA Optical transmitter

Claims

1. An optical waveguide element comprising an optical waveguide substrate on which an optical waveguide is formed, a support substrate supporting the optical waveguide substrate, and an electrode for applying an electric field to the optical waveguide, the electrode comprising an upper electrode and an underlayer for joining the upper electrode, wherein the underlayer is formed directly on at least the support substrate.

2. An optical waveguide element according to claim 1, wherein the upper electrode is disposed so as to extend over a portion of the upper surface of the optical waveguide substrate.

3. An optical waveguide element according to claim 2, characterized in that the contact area between the upper electrode and the upper surface of the optical waveguide substrate extends from the end face of the optical waveguide substrate toward the optical waveguide by 0.05 μm or more.

4. An optical waveguide element according to claim 2, wherein the end of the optical waveguide substrate is tapered in thickness and the angle of the tip is 50 degrees or less.

5. An optical waveguide element according to claim 2, characterized in that an underlayer is disposed below said upper electrode extending onto the upper surface of said optical waveguide substrate.

6. An optical waveguide element according to claim 1, wherein the upper electrode is disposed so as to extend over a portion of the lower surface of the optical waveguide substrate.

7. An optical waveguide element as claimed in claim 6, characterized in that the contact area between the upper electrode and the lower surface of the optical waveguide substrate extends from the end face of the optical waveguide substrate towards the optical waveguide by 0.05 μm or more.

8. An optical waveguide element according to claim 6, characterized in that the upper electrode is disposed so as to extend onto a portion of the optical waveguide substrate excluding the optical waveguide.

9. An optical waveguide element according to claim 1, characterized in that an air layer is formed in the gap between the optical waveguide substrate and the support substrate, at a position closer to the optical waveguide than the position where the undercoat layer is formed.

10. An optical modulator comprising: an optical waveguide element according to any one of claims 1 to 9; a housing for accommodating said optical waveguide element; and an optical fiber for inputting or outputting a light wave to said optical waveguide.

11. An optical modulator as claimed in claim 10, wherein the electrode is a modulation electrode for modulating an optical wave propagating through the optical waveguide, and the optical modulator further comprises an electronic circuit inside the housing for amplifying a modulation signal input to the modulation electrode.

12. An optical transmitter comprising: an optical modulator according to claim 10; a light source for inputting an optical wave to said optical modulator; and an electronic circuit for outputting a modulated signal to said optical modulator.

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