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 in which peeling of an electrode disposed on an optical waveguide substrate is effectively suppressed.　This optical waveguide element comprises: an optical waveguide substrate 1 having an optical waveguide WG; an electrode EL disposed on the optical waveguide substrate; and a functional film BF arranged along the electrode and having a linear expansion coefficient differing from that of the optical waveguide substrate and the electrode. The optical waveguide element is characterized in that a low elasticity material layer LE is provided between the electrode EL and the optical waveguide substrate 1 so as to cross a direction in which the electrode EL extends.

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 in particular to an optical waveguide element including an optical waveguide substrate having an optical waveguide, an electrode disposed on the optical waveguide substrate, and a functional film disposed along the electrode and having a linear expansion coefficient different from those of the optical waveguide substrate and the electrode, and an optical modulator and an optical transmitter using the same.

[0002] As optical communication systems become faster and larger in capacity, the optical modulators used in them are becoming increasingly high-performance and denser. As optical waveguide elements (chips) become smaller, such as in High Bandwidth-Coherent Driver Modulators (HB-CDMs), the spacing between electrodes arranged on optical waveguide substrates with optical waveguides becomes narrower, and more functional films such as buffer layers are arranged adjacent to the electrodes.

[0003] For example, in a small optical waveguide element, a thin plate of lithium niobate (LN) is used, and a rib-type optical waveguide is formed on the surface of the thin plate. ２ A buffer layer such as a buffer layer BF is formed on the optical waveguide WG, and also functions as a cladding for the optical waveguide. In an optical waveguide element that combines multiple Mach-Zehnder optical waveguides, a DC bias electrode is disposed to adjust the phase of the light wave passing through each Mach-Zehnder optical waveguide. FIG. 1 is an enlarged view of a portion of the optical waveguide element, in which the electrode EL, which is electrical wiring for applying a DC voltage to the DC bias electrode, is disposed adjacent to the buffer layer BF that covers the optical waveguide WG. The electrode EL is also disposed on an optical waveguide substrate 1 on which no functional film such as a buffer layer is disposed.

[0004] When the electrode EL is thin, such as when the electrode thickness is less than 3 μm, the electrode will not peel off from the optical waveguide substrate 1. However, when the electrode thickness is 3 μm or more, electrode peeling PL occurs as shown in FIG. 2, causing problems such as deformation of the electrode wiring itself and its proximity to the optical waveguide WG.

[0005] The cause of this electrode peeling will be explained using Figures 3 to 5. Figure 3 is a perspective view of Figure 1, in which an optical waveguide WG is formed on the optical waveguide substrate 1, and a buffer layer BF is formed to cover the optical waveguide. An electrode EL, which is an electrical wiring, is provided so as to be sandwiched between two buffer layers BF.

[0006] The electrode EL is made of Au or the like, and the buffer layer BF is made of SiO ２ Since the linear expansion coefficient of the electrode is larger than that of the optical waveguide substrate, which is LN, when a heat treatment is performed in the manufacturing process of the optical waveguide element, the optical waveguide substrate 1 warps so as to protrude toward the electrode EL, as shown in Fig. 4(a). Fig. 4 shows the optical waveguide substrate 1 and the like as observed from the direction of arrow B in Fig. 3.

[0007] On the other hand, the linear expansion coefficient of the buffer layer BF is smaller than that of the optical waveguide substrate 1. Therefore, during the heat treatment, a warp occurs that is recessed toward the optical waveguide substrate 1, as shown in FIG.

[0008] When observed from the direction of arrow A in Fig. 3, for example, the optical waveguide substrate 1 undergoes wavy deformation during heat treatment, as shown in Fig. 5. In the portion where the electrode EL is disposed, an internal stress a is generated in the upward direction, causing a warp that bulges upward, while in the portion where the buffer layer BF is disposed, an internal stress b is generated in the downward direction, causing a warp that dents downward. In adjacent portions of the optical waveguide substrate 1 where the warp directions are different, large distortion occurs, making it easy for the electrode EL and the like to peel off. Moreover, the electrode EL uses Ti or Nb as the Au underlayer, and the adhesion between the electrode EL and the optical waveguide substrate (LN) 1 is improved by the SiO buffer layer. ２ The adhesion between the electrode EL and the optical waveguide substrate 1 is lower than that between the electrode EL and the optical waveguide substrate 1. Therefore, the peeling of the electrode EL progresses due to the influence of strain. The thicker the electrode EL, the greater the internal stress during the heat treatment, and therefore the more likely the electrode peeling occurs.

[0009] In order to reduce the stress on the substrate caused by the electrodes, Patent Document 1 proposes disposing resin between the signal electrode and the substrate and between the ground electrode and the substrate along the direction in which the electrodes extend. Specifically, a resin having a width of 1 / 3 or less of the width of the signal electrode and a resin having a width of 1 / 2 or less of the width of the ground electrode are disposed along the direction in which the electrodes extend (the longitudinal direction of the electrodes).

[0010] In HB-CDM and the like, the thickness of the signal electrode and ground electrode adjacent to the optical waveguide is less than 3 μm, and the effect of internal stress on the electrode is relatively small. However, when the electrode is 3 μm or thicker, such as in the electrical wiring of a DC bias electrode, or when the electrode is disposed adjacent to a functional film such as a buffer layer, it is difficult to sufficiently prevent the electrode from peeling off simply by disposing a resin of 1 / 3 or 1 / 2 or less of the width of the electrode on a portion of the underside of the electrode, as in Patent Document 1.

[0011] Japanese Patent Application Laid-Open No. 2020-166053

[0012] The present invention aims to solve the above-mentioned problems by providing an optical waveguide element that effectively prevents peeling of electrodes disposed on an optical waveguide substrate, and also to provide an optical modulator and an optical transmitter that use the optical waveguide element.

[0013] 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 including an optical waveguide substrate having an optical waveguide, an electrode disposed on the optical waveguide substrate, and a functional film disposed along the electrode and having a linear expansion coefficient different from that of the optical waveguide substrate and the electrode, characterized in that a low-elasticity material layer is provided between the electrode and the optical waveguide substrate so as to cross the direction in which the electrode extends.

[0014] (2) In the optical waveguide element described in (1) above, the functional film is a buffer layer that covers the optical waveguide.

[0015] (3) The optical waveguide element according to (1) above, wherein the thickness of the electrode is 3 μm or more.

[0016] (4) In the optical waveguide element described in (1) above, the low-elasticity material layer is made up of a plurality of films, which are discretely arranged along the direction in which the electrodes extend.

[0017] (5) In the optical waveguide element described in (4) above, the interval between the films is set to 500 μm or less.

[0018] (6) In the optical waveguide element described in (1) above, the low-elasticity material layer is exposed from at least one side surface of the electrode.

[0019] (7) In the optical waveguide element described in (1) above, the electrode is a part of a wiring that supplies a DC voltage to a DC electrode that applies a DC voltage to the optical waveguide.

[0020] (8) An optical modulator comprising the optical waveguide element according to (1) 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] (9) The optical modulator according to (8) above is characterized in that a modulation electrode for modulating a light wave propagating through the optical waveguide is disposed on the optical waveguide substrate, and an amplifier circuit for amplifying a modulation signal input to the modulation electrode is provided inside the housing.

[0022] (10) An optical transmitter comprising the optical modulator according to (8) above, a light source for inputting a light wave to the optical modulator, and a signal circuit for inputting a modulated signal to the optical modulator.

[0023] The optical waveguide element of the present invention comprises an optical waveguide substrate having an optical waveguide, an electrode disposed on the optical waveguide substrate, and a functional film disposed along the electrode and having a linear expansion coefficient different from that of the optical waveguide substrate and the electrode, wherein a low-elasticity material layer is provided between the electrode and the optical waveguide substrate and across the direction in which the electrode extends, thereby reducing internal stress generated between the optical waveguide substrate and the electrode and making it possible to provide an optical waveguide element that effectively prevents the electrode from peeling off from the optical waveguide substrate.Furthermore, by using this optical waveguide element, it is possible to provide an optical modulator or an optical transmitter having the above-mentioned effects.

[0024] 6 is a plan view illustrating a portion of a conventional optical waveguide element. FIG. 7 is a diagram illustrating a state in which an electrode (electrical wiring) has peeled off in the optical waveguide element of FIG. 1. FIG. 8 is a perspective view of the optical waveguide element of FIG. 1. FIG. 9 is a cross-sectional view of a portion of the optical waveguide element of FIG. 3 observed from the direction of arrow B, where (a) shows the electrode EL portion and (b) shows the functional film (buffer layer) BF portion. FIG. 10 is a cross-sectional view of a portion of the optical waveguide element of FIG. 3 observed from the direction of arrow A. FIG. 11 is a plan view illustrating a portion of an optical waveguide element of the present invention. FIG. 6 is a cross-sectional view taken along the dashed-dotted line X-X' in FIG. 6. FIG. 6 is a cross-sectional view taken along the dashed-dotted line Y-Y' in FIG. 6. FIG. 12 is a plan view illustrating a second embodiment of an optical waveguide element of the present invention. FIG. 13 is a cross-sectional view taken along the dashed-dotted line Y1-Y1' in FIG. 9. FIG. 14 is a cross-sectional view taken along the dashed-dotted line Y2-Y2' in FIG. 13. FIG. 14 is a plan view illustrating a third embodiment of an optical waveguide element of the present invention. FIG. 15 is a plan view illustrating an optical transmitting device of the present invention.

[0025] The optical waveguide element according to the present invention, and the optical modulator and optical transmitter using the same, will be described in detail below. The optical waveguide element according to the present invention, as shown in Figures 6 to 8, comprises an optical waveguide substrate 1 having an optical waveguide WG, an electrode EL arranged on the optical waveguide substrate, and a functional film BF arranged along the electrode and having a linear expansion coefficient different from that of the optical waveguide substrate and the electrode, and is characterized in that a low-elasticity material layer LE is provided between the electrode EL and the optical waveguide substrate 1 and across the direction in which the electrode EL extends.

[0026] 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.

[0027] The optical waveguide WG 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 even a rib-type optical waveguide in which the surface of the substrate 1 is convex in the portion corresponding to the optical waveguide, such as 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 using 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.

[0028] The thickness (maximum thickness) of the optical waveguide substrate (thin plate) 1 on which the optical waveguide WG 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 velocity matching between the microwave and light waves of the modulation signal. Furthermore, the height of the rib-type optical waveguide WG (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.

[0029] To enhance the mechanical strength of the optical waveguide substrate 1 having the optical waveguide formed thereon, a holding substrate HS is bonded to the underside of the optical waveguide substrate 1. The optical waveguide substrate 1 and the holding substrate HS are bonded together by direct bonding or via an adhesive layer such as a resin. The holding substrate to be directly bonded preferably has a lower refractive index than the optical waveguide and 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 holding substrate HS 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 holding substrate HS 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 holding substrate HS. The holding substrate is not limited to a substrate formed from a single substrate, but also includes a substrate formed by stacking a plurality of substrates together.

[0030] Electrodes are formed on the optical waveguide substrate 1. The electrodes include a modulation electrode and a DC bias electrode for applying an electric field to the optical waveguide WG. The "electrode" of interest in the optical waveguide element of the present invention is not the modulation electrode or the DC bias electrode, but rather the electrical wiring EL that supplies a DC bias voltage to the DC bias electrode. In the optical waveguide element of the present invention, a low-elasticity material layer is interposed between the electrode and the optical waveguide substrate. Therefore, providing a low-elasticity material layer on the modulation electrode, the electrical wiring for the modulation electrode that supplies a modulation signal to the modulation electrode, or the DC bias electrode itself is undesirable because it can cause propagation loss of the modulation signal and disturb the electric field applied to the optical waveguide. Furthermore, electrical wiring that transmits a received light signal from a light-receiving element for monitoring light waves propagating through the optical waveguide is also included in the "electrode" of the present invention because the presence of a low-elasticity material layer is unlikely to cause propagation loss. Furthermore, a portion of a ground electrode can also be included in the "electrode" of the present invention. Furthermore, the "electrode" to which the present invention is applied is primarily an electrode that extends and has a functional film disposed adjacent to the electrode.

[0031] The electrode disposed on the optical waveguide substrate 1 has a multilayer structure consisting of an upper electrode and an underlayer. The upper electrode is made of a metal such as Au or Cu. The underlayer is used to improve adhesion between the optical waveguide substrate 1 and the upper electrode. The upper electrode is formed to cover the underlayer by electrolytic plating using the underlayer, electroless plating using a resist pattern, gas phase methods such as vapor deposition and sputtering, or a combination of these. The underlayer is made of material such as Ti, Nb, Ni, Cr, or Al, and is formed on the optical waveguide substrate by sputtering, vapor deposition, or the like.

[0032] In the optical waveguide element according to the present invention, a functional film BF is disposed along the electrode EL disposed on the optical waveguide substrate. ２ A buffer layer formed of Al ２ O ３ , MgF ２ , La ２ O ３ , ZnO, HfO ２ , MgO, CaF ２ , Y2O ３ Oxides and fluorides of metal elements in groups 1 to 17 of the periodic table can be used as the buffer layer. In addition, when the linear expansion coefficient of these functional films BF differs from that of the optical waveguide substrate and the adjacent electrodes, it can be said that the present invention is preferably applied.

[0033] The optical waveguide element of the present invention is characterized in that a low-elasticity material layer LE is provided between the electrode EL and the optical waveguide substrate 1, so as to cross the direction in which the electrode EL extends. The low-elasticity material must have a lower modulus of elasticity than the material constituting the electrode, and for example, a resin material can be used. The Young's modulus of resin is about 1 to 2 GPa, which is lower than the Young's modulus of Au used in the electrode (76 to 80 GPa) or the SiO2 used in the buffer layer. ２ This is lower than the Young's modulus (72 to 74 GPa) of

[0034] Examples of resin materials that can be used for the low elasticity material layer LE include thermoplastic resins and thermosetting resins, and examples include polyamide-based resins, melamine-based resins, phenol-based resins, amino-based resins, and epoxy-based resins. Furthermore, photoresists made of thermosetting resins such as permanent resists can also be used as the resin material. The low elasticity material layer can be patterned by a photolithography process.

[0035] FIG. 6 is a plan view illustrating an example of an optical waveguide element of the present invention. FIG. 7 is a cross-sectional view taken along dashed dotted line X-X' in FIG. 6, and FIG. 8 is a cross-sectional view taken along dashed dotted line Y-Y' in FIG. 6. FIGS. 6 and 8 show a case in which a functional film BF such as a buffer layer covering the optical waveguide WG is disposed in the formation region of the optical waveguide WG, sandwiching an electrode EL such as electrical wiring, and a case in which an electrode (thin electrode) TE that applies an electric field to the optical waveguide is disposed adjacent to the optical waveguide WG, and a functional film BF such as a buffer layer is disposed on top of the electrode. In an optical waveguide element to which the present invention is applicable, the functional film BF disposed adjacent to the electrode EL may be a functional film BF that covers the optical waveguide WG, or may be a functional film BF that covers both the optical waveguide WG and the thin electrode TE. Naturally, a functional film BF may be disposed on only one side of the electrode EL.

[0036] The low-elasticity material film LE may be arranged so as to be exposed on both sides of the electrode EL as shown in Figure 6, or may be configured so as to be exposed on only one side of the electrode EL as will be described later. What is important is to arrange the low-elasticity material film LE so as to cross the direction in which the electrode extends in order to reduce the internal stress acting in the direction in which the electrode EL extends. By exposing the low-elasticity material film LE from the electrode EL, it is possible to suppress the transmission of internal stress and to prevent the electrode from peeling off due to internal stress.

[0037] The electrodes EL to which the low-elasticity material film LE of the present invention is applied are mainly those having a width W1 in the range of 2 to 100 μm and a thickness T1 in the range of 3 to 50 μm. In particular, when the thickness of the electrode is 3 μm or more, it is preferable to apply the present invention to alleviate internal stress. Furthermore, when the electrode EL has a length of 500 μm or more, the present invention is applied to alleviate internal stress.

[0038] The length L1 of the low-elasticity material layer LE in the direction in which the electrode EL extends is 5 μm to 10 μm, and the interval (period) L2 at which the multiple low-elasticity material films LE are discretely arranged is L1 to L1 x 10, specifically, preferably 5 to 100 μm. The low-elasticity material films LE are arranged so that L2 is at least 500 μm or less. The thickness T3 of the low-elasticity material layer LE is 0.1 to 4 μm, and a thickness of 1 μm or more is effective in alleviating internal stress in the electrode EL.

[0039] The thickness of the functional film BF adjacent to the electrode EL is SiO ２ In this case, it is preferable to apply the present invention to those with a width of 0.05 to 2 μm. Furthermore, the width D3 of the functional film BF also depends on the distance D1 between the functional film BF and the electrode EL, but if it is more than twice D1, the internal stress of the functional film BF becomes high, so it is preferable to apply the present invention. Furthermore, if the distance D1 between the functional film BF and the electrode EL is 200 μm or less, the functional film BF is more susceptible to the influence of distortion.

[0040] In the above explanation, we have used an example in which the linear expansion coefficient of the functional film BF is lower than that of the electrode EL, but it goes without saying that the present invention can also be applied when the linear expansion coefficient of the functional film BF is higher than that of the electrode EL.

[0041] When the length (L1) of the low-elasticity material film LE is smaller than the width (W1) of the electrode EL (L1≦W1), it is possible to sufficiently ensure adhesion between the electrode EL and the optical waveguide substrate 1 by setting the ratio W1 / L1 to 0.5 to 1. Furthermore, by setting the ratio W1 / L1 to less than 0.5, adhesion becomes stronger, but the distortion suppression effect becomes smaller.

[0042] When the length (L1) of the low-elasticity material film LE is greater than the width (W1) of the electrode EL (L1≧W1), the internal stress between the electrode EL and the optical waveguide substrate 1 can be sufficiently suppressed by setting the ratio W1 / L1 to 1 to 10. Moreover, distortion can be sufficiently suppressed while ensuring adhesion between the electrode EL and the optical waveguide substrate 1. Furthermore, by setting the ratio W1 / L1 to 10 or more, the distortion suppression effect is sufficient, but the adhesion between the electrode EL and the optical waveguide substrate 1 becomes insufficient.

[0043] Next, an example in which the low-elasticity material layer LE is exposed from only one side of the electrode EL will be described with reference to Figures 9 to 11. Figure 9 is a plan view of the optical waveguide element, and Figure 10 is a cross-sectional view taken along the dashed line Y1-Y1' in Figure 9. Also, Figure 11 is a cross-sectional view taken along the dashed line Y2-Y2' in Figure 9.

[0044] The low-elasticity material film LE is arranged so as to be exposed from at least one side surface of the electrode EL. When the low-elasticity material film LE is arranged discretely, the protruding sides of the low-elasticity material film LE may be alternately repeated as shown in Figure 9, or the low-elasticity material film LE may be exposed on only one side. However, if the low-elasticity material film LE is exposed on only one side of the electrode EL, internal stress may accumulate on the non-exposed side, and therefore it is preferable to set it so that at least one low-elasticity material film LE is exposed on one side within a length of 500 μm in the extending direction of the electrode EL.

[0045] 10 and 11, setting the width W4 of the portion under the electrode EL where the low-elasticity material layer LE is not disposed to be equal to or less than half the width W1 of the electrode EL is highly effective in suppressing internal stress. Also, the protrusion amount W3 of the low-elasticity material layer LE protruding from the electrode EL may be 0 or more, and is preferably set to be equal to or less than half the width W1 of the electrode EL.

[0046] FIG. 12 shows a case where multiple electrical wirings (EL1-EL4) are arranged to supply DC bias voltages to multiple DC bias electrodes (DC1-DC4). For example, electrode TE corresponds to multiple DC bias electrodes (DC1-DC4), and four representative electrical wirings (EL1-EL4) are shown as being connected to electrode TE. For example, the spacing D4 between adjacent electrical wirings is 20-150 μm, and the functional films BF adjacent to these are densely arranged, resulting in significant distortion of the optical waveguide substrate 1. Furthermore, the lengths of the electrical wirings (EL1-EL4) along electrode TE are different, resulting in particularly large distortion of the optical waveguide substrate 1. For this reason, the placement of a low-elasticity material layer LE is extremely effective in suppressing electrode peeling due to distortion.

[0047] Next, examples of application of the optical waveguide element of the present invention to an optical modulator or an optical transmitter will be described. While the following describes an optical waveguide element having a nested optical waveguide, the present invention is not limited to this, and can also be applied to HB-CDM, optical phase modulators, optical modulators with polarization combining functions, optical modulators integrating more or fewer Mach-Zehnder optical waveguides, bonding devices with optical waveguide substrates made of other materials such as silicon, devices for sensor applications, and the like.

[0048] As shown in FIG. 13 , the optical waveguide element includes an optical waveguide WG formed on an optical waveguide substrate 1 and electrodes (not shown), such as a modulation electrode, that modulate the light waves propagating through the optical waveguide WG. 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 and from the optical waveguide. In FIG. 13 , 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 using an optical component OP. 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 WG in 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 optical components such as an optical fiber or an optical block, an auxiliary member SS is placed on the optical waveguide substrate 1 along the end face of the substrate 1.

[0049] 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. 13, 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 the modulation signal degradation is minimal, a DRV is not necessary, and the optical modulator MD can be directly modulated by the DSP.

[0050] 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 within the optical transmitter OTA using a semiconductor laser as a light source. The output light L2 modulated by the optical modulator MD is output to the outside via an optical fiber F.

[0051] As described above, the present invention makes it possible to provide an optical waveguide element that effectively prevents peeling of electrodes disposed on an optical waveguide substrate, and also to provide an optical modulator or an optical transmitter that uses the optical waveguide element.

[0052] 1 Optical waveguide substrate (thin plate, film body) HS Holding substrate WG Optical waveguide EL Electrode (electrical wiring, etc.) LE Low elasticity material layer BF Functional film (buffer layer, etc.) SS Auxiliary substrate 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 having an optical waveguide, an electrode disposed on the optical waveguide substrate, and a functional film disposed along the electrode and having a linear expansion coefficient different from that of the optical waveguide substrate and the electrode, characterized in that a low-elasticity material layer is provided between the electrode and the optical waveguide substrate and across the direction in which the electrode extends.

2. The optical waveguide element according to claim 1, wherein the functional film is a buffer layer that covers the optical waveguide.

3. The optical waveguide element according to claim 1, wherein the thickness of said electrode is 3 μm or more.

4. An optical waveguide element according to claim 1, wherein the low-elasticity material layer is composed of a plurality of films, which are discretely arranged along the direction in which the electrodes extend.

5. An optical waveguide element according to claim 4, wherein the interval between the films is set to 500 μm or less.

6. The optical waveguide element according to claim 1, wherein the low-elasticity material layer is exposed from at least one side of the electrode.

7. An optical waveguide element according to claim 1, wherein said electrode is part of a wiring that supplies a DC voltage to a DC electrode that applies a DC voltage to said optical waveguide.

8. An optical modulator comprising: the optical waveguide element according to claim 1; a housing for accommodating said optical waveguide element; and an optical fiber for inputting or outputting a light wave to said optical waveguide.

9. An optical modulator according to claim 8, wherein a modulation electrode for modulating the light wave propagating through the optical waveguide is disposed on the optical waveguide substrate, and an amplifier circuit for amplifying the modulation signal input to the modulation electrode is provided inside the housing.

10. An optical transmitter comprising: the optical modulator according to claim 8; a light source for inputting a light wave to said optical modulator; and a signal circuit for inputting a modulated signal to said optical modulator.

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