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

Abstract

To provide an optical waveguide element that suppresses the occurrence of drift phenomena caused by voltage drop due to heater electrodes. [Solution] An optical waveguide element comprising a substrate 1 on which an optical waveguide WG is formed, and a plurality of heater electrodes (HT1-HT3) arranged parallel to the optical waveguide in order to adjust the phase of the light wave propagating through the optical waveguide, wherein the plurality of heater electrodes are connected in series by wiring, a phase change occurs in the light wave of the optical waveguide due to the electric field caused by the voltage drop generated at each heater electrode in a part of the optical waveguide, and the plurality of heater electrodes are arranged so as to cancel out at least a part of the electric field.

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 a substrate on which an optical waveguide is formed, and an optical waveguide element in which a plurality of heater electrodes are arranged in parallel with the optical waveguide in order to adjust the phase of light waves propagating through the optical waveguide.

Background Art

[0002] In the fields of optical communication and optical measurement, optical waveguide elements in which optical waveguides are formed on substrates such as lithium niobate (LN) are frequently used. As a bias control method for an optical modulator using LN, there are a method using the electro-optic effect (EO effect) and a method using the thermo-optic effect (TO effect) as disclosed in Patent Documents 1 and 2.

[0003] When using the EO effect, a bias control is performed by applying an electric field to the optical waveguide of LN. Therefore, it has the advantage of low power consumption. However, a drift phenomenon occurs in which the bias point shifts with the passage of time, and thus the bias point fluctuates. Therefore, there is a problem in stability.

[0004] On the other hand, when using the TO effect, as shown in FIG. 1 or 2, a current is passed through a heater electrode (HT) installed near an optical waveguide (WG), and the refractive index of the optical waveguide is changed using the generated Joule heat for control. When using the TO effect, since the drift phenomenon is extremely low, the fluctuation of the bias point is low and the stability is high, which are advantages. However, power consumption for heat generation occurs, and since the thermal change becomes a refractive index change, the response speed of light is affected by the heat capacity.

[0005] Furthermore, if a temperature difference occurs between the left and right sides of the optical waveguide's cross-section, a refractive index distribution will be created within the waveguide, leading to problems such as phase shift and polarization conversion as the light wave propagates. For this reason, there are methods to reduce the temperature distribution using a fine optical waveguide structure, or to prevent a temperature distribution from occurring during heat transfer. These methods include installing heater electrodes HT on both sides of the optical waveguide WG as shown in Figure 1, or installing heater electrodes HT above the optical waveguide WG as shown in Figure 2. In Figure 2, a buffer layer (cladding) 2 is arranged to cover the optical waveguide to suppress the absorption of light waves propagating through the optical waveguide WG by the heater electrodes HT.

[0006] In the method of installing heater electrodes on both sides of an optical waveguide, there are two methods: using the left and right heaters in series connection and using them in parallel connection. As shown in Figure 3, in the case of series connection, a symmetrical arrangement can be achieved by connecting the heater electrodes (HT1, HT2) installed on the left and right sides of the optical waveguide WG with electrode wiring (ELI, EL2, ELO). A buffer layer (cladding) 2 is provided where the wiring EL2 crosses the optical waveguide WG.

[0007] However, at the input terminal (ELI) and output terminal (ELO) of the heater electrode, the voltage drop across the heater electrodes (HT1, HT2) results in different potentials. This causes an electric field EF to be applied to the optical waveguide WG, resulting in the electro-optic effect (EO effect). Consequently, drift occurs due to the EO effect, leading to a problem of poor controllability of the bias point. The potential difference V due to the voltage drop between the input and output terminals can be calculated as V = (R + R) × I, for example, if R is the resistance of each heater electrode (HT1, HT2) and I is the current flowing through the heater electrodes. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International release WO2012 / 161199 [Patent Document 2] Patent No. 4912719 [Overview of the project] [Problems that the invention aims to solve]

[0009] The problem that this invention aims to solve is to provide an optical waveguide element that solves the above-mentioned problems and suppresses the occurrence of drift phenomena caused by voltage drop due to heater electrodes. Furthermore, it aims to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Means for solving the problem]

[0010] To solve the above problems, the optical waveguide element, optical modulation device, and optical transmission device of the present invention have the following technical features. (1) An optical waveguide element comprising a substrate on which an optical waveguide is formed and a plurality of heater electrodes arranged parallel to the optical waveguide in order to adjust the phase of the light waves propagating through the optical waveguide, wherein the plurality of heater electrodes are connected in series by wiring, a phase change occurs in the light waves of the optical waveguide due to the electric field caused by the voltage drop generated at each heater electrode in a part of the optical waveguide, and the plurality of heater electrodes are arranged so as to cancel out at least a part of the electric field.

[0011] (2) The optical waveguide element described in (1) above is characterized in that the amount of phase change of the light wave is 0 in the entire optical waveguide in which the plurality of heater electrodes are arranged.

[0012] (3) The optical waveguide element described in (1) above is characterized in that both the input and output sections of the wiring are located on one side of the optical waveguide.

[0013] (4) In the optical waveguide element described in (1) above, the plurality of heater electrodes are arranged on both sides of the optical waveguide.

[0014] (5) In the optical waveguide element described in (1) above, the wiring adjacent to the heater electrode is characterized in that a portion with a larger heat capacity than the heater electrode is formed therein.

[0015] (6) The optical waveguide element described in (1) above is characterized in that a protective film is provided to cover the heater electrode.

[0016] (7) The optical waveguide element described in any of (1) to (6) above is an optical modulation device characterized by comprising an optical fiber housed in a housing that inputs or outputs an optical wave to the optical waveguide.

[0017] (8) The optical modulation device described in (7) above is characterized in that the optical waveguide element is provided with a modulation electrode for modulating an optical wave propagating through the optical waveguide, and the housing has an electronic circuit for amplifying the modulation signal input to the modulation electrode of the optical waveguide element.

[0018] (9) An optical transmitting device characterized by having an optical modulation device as described in (7) above and an electronic circuit that outputs a modulation signal to cause the optical modulation device to perform a modulation operation. [Effects of the Invention]

[0019] The present invention provides an optical waveguide element comprising a substrate on which an optical waveguide is formed, and a plurality of heater electrodes arranged parallel to the optical waveguide to adjust the phase of the light wave propagating through the optical waveguide. The plurality of heater electrodes are connected in series by wiring, and in a portion of the optical waveguide, a phase change occurs in the light wave of the optical waveguide due to the electric field caused by the voltage drop generated at each heater electrode. Since the plurality of heater electrodes are arranged to cancel out at least a portion of the electric field, it is possible to provide an optical waveguide element in which the occurrence of drift phenomena caused by voltage drops due to the heater electrodes is suppressed. Furthermore, by using this optical waveguide element, it is possible to provide optical modulation devices and optical transmission devices with similarly excellent characteristics. [Brief explanation of the drawing]

[0020] [Figure 1] The figure shows an example of a conventional optical waveguide device (with heater electrodes arranged on the left and right sides of the optical waveguide). [Figure 2] The figure shows an example of a conventional optical waveguide device (with a heater electrode arranged above the optical waveguide). [Figure 3] The figure explains the drift phenomenon generated by the voltage drop in the heater electrode. [Figure 4] The figure explains an example of the optical waveguide device of the present invention. [Figure 5] The figure explains the optical waveguide device of FIG. 4 in more detail. [Figure 6] The figure explains an example of the cross-sectional view (a) taken along the dashed-dotted line A-A' and the cross-sectional view (b) taken along the dashed-dotted line B-B' in FIG. 5. [Figure 7] The figure explains another example of the cross-sectional view (a) taken along the dashed-dotted line A-A' and the cross-sectional view (b) taken along the dashed-dotted line B-B' in FIG. 5. [Figure 8] The figure explains another example (example 1) of the optical waveguide device of the present invention. [Figure 9] The figure explains another example (example 2) of the optical waveguide device of the present invention. [Figure 10] The figure explains another example (example 3) of the optical waveguide device of the present invention. [Figure 11] The figure explains another example (example 4) of the optical waveguide device of the present invention. [Figure 12] The figure explains another example (example 1) of the drift phenomenon generated by the voltage drop in the heater electrode. [Figure 13] The figure explains another example (example 2) of the drift phenomenon generated by the voltage drop in the heater electrode. [Figure 14] The figure shows the optical transmission device according to the present invention.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, the present invention will be described in detail using preferred examples. The present invention relates to an optical waveguide element, as shown in Figures 4, 8 to 10, for example, a substrate 1 on which an optical waveguide WG is formed, and a plurality of heater electrodes (HT1, etc.) arranged parallel to the optical waveguide in order to adjust the phase of the light wave propagating through the optical waveguide, wherein the plurality of heater electrodes are connected in series by wiring, and in a part of the optical waveguide, a phase change occurs in the light wave of the optical waveguide due to the electric field caused by the voltage drop generated at each heater electrode, and the plurality of heater electrodes are arranged so as to cancel out at least a part of the electric field. Furthermore, the plurality of heater electrodes are arranged such that the amount of phase change of the light wave is 0 in the entire optical waveguide on which the plurality of heater electrodes are arranged.

[0022] The substrate 1 used in the optical waveguide element of the present invention can be a substrate having an electro-optic effect. Specifically, single-crystal materials such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), which are ferroelectric materials, or 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 can be used in which an electro-optic effect substrate is bonded to another substrate, and then a thin film of the electro-optic effect substrate is fabricated. In addition, semiconductor substrates such as Si, substrates used in SiPh (silicon photonics), and organic material substrates such as EO polymers can also be used. In the optical waveguide element of the present invention, a heater electrode is used to adjust the phase of the light wave propagating through the optical waveguide, and a wide range of materials can be used as long as the refractive index of the optical waveguide changes with temperature changes.

[0023] The substrate 1 on which the optical waveguide is formed can also be one on which the holding substrate is directly bonded or bonded and fixed via an adhesive layer such as resin. For the holding substrate to be directly bonded, it is preferable that it has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but this is not limited to this. 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 is preferably made of a material with a similar coefficient of thermal expansion to the optical waveguide substrate, such as a substrate containing an oxide layer, such as a low dielectric constant substrate of SiO2 or Al2O3, including glass, quartz, fused silica, synthetic silica, Eagle Glass, alkali glass, alkali-free glass, lead glass, Pyrex® glass, soda glass, sapphire, alumina, etc. In addition, it is also possible to use the same LN substrate as the optical waveguide substrate, or a composite substrate in which a silicon film or LN crystal film is formed on the surface of an oxide film formed on a wafer substrate abbreviated as SOI or LNOI. If the refractive index of the holding substrate is higher than that of the optical waveguide substrate, a layer (intermediate layer) with a lower refractive index than that of the optical waveguide substrate, such as SiO2, is provided between the optical waveguide substrate and the holding substrate.

[0024] The Optical Waveguide Working Group (WG) can utilize optical waveguides formed by thermal diffusion of high refractive index materials such as Ti into the optical waveguide substrate, optical waveguides formed by the proton exchange method, and rib-type optical waveguides in which the substrate has convex portions corresponding to the optical waveguide, such as by etching the substrate other than the optical waveguide or forming grooves on both sides of the optical waveguide. Furthermore, it is possible to further increase the refractive index using methods such as thermal diffusion of Ti or the proton exchange method in conjunction with the rib-type optical waveguide. In terms of size, the rib-type optical waveguide has a fine structure with a width and height of about 1 μm or less in order to enhance light confinement.

[0025] A buffer layer (cladding layer) is placed on the upper side of the substrate 1 containing the optical waveguide. This is not only to suppress the scattering of light waves due to surface roughness of the optical waveguide itself, but also to prevent the light waves propagating through the optical waveguide from being absorbed by wiring for heater electrodes or part of control electrodes (modulation electrodes, etc.) or their wiring when such wiring is placed on the upper side of the optical waveguide. Furthermore, the buffer layer (cladding layer) may be formed not only on the above-mentioned portion, but on the entire substrate 1.

[0026] For the buffer layer, a dielectric material with a lower refractive index and higher transparency than the optical waveguide substrate is used. Specifically, materials such as SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, CaF2, and Y2O3 are used, including oxides and fluorides of metal elements from groups 1 to 17 of the periodic table.

[0027] Heater electrodes are made from high-melting-point metals such as Ti, Nb, Ni, Cr, Ta, or W, or their nitrides, or composites of these metals with oxides or nitrides, and are formed on the upper surface of a substrate 1 such as an optical waveguide substrate by sputtering, vapor deposition, or screen printing. Wiring connecting the heater electrodes in series is made from materials with high conductivity and thermal conductivity, such as Au, Cu, or Al.

[0028] The optical waveguide element of the present invention is characterized by the arrangement of heater electrodes (HT1 to HT3), as shown in Figure 4, which adjust the direction and magnitude of the electric fields (EF1, EF2) generated between opposing electrodes (wirings) across the optical waveguide by the voltage drop caused by the heater electrodes. In this case, in a portion of the optical waveguide, a phase change occurs in the light wave of the optical waveguide due to the electric field caused by the voltage drop generated at each heater electrode, and the multiple heater electrodes are arranged so that at least a portion of the electric field cancels out. Furthermore, the entire optical waveguide in which the heater electrodes (HT1 to HT3) are arranged is set so that the amount of phase change of the light wave is substantially zero. These measures suppress the occurrence of drift phenomena.

[0029] In this invention, "zero phase change of light wave" means that when the phase changes due to the electric field generated by each heater electrode are integrated along the optical waveguide, the phase change is substantially zero (a state in which no phase change occurs). Even if it is not substantially zero, if the phase change is within the same range as the phase change when a dither signal for bias control (within 5% of Vπ) is applied, as a drift phenomenon when used as an optical waveguide element (optical modulation device), it is included in the concept of this invention. Furthermore, in this invention, even if the phase change is not substantially zero, a certain effect can be obtained by arranging the heater electrodes so as to partially cancel out the effects of the generated electric field.

[0030] Let's explain the optical waveguide element in Figure 4 in more detail. The heater electrodes (HT1~HT3) in Figure 4 are connected in series in the following order: wiring input (ELI) → heater electrode (HT1) → wiring (EL2) → heater electrode (HT2) → wiring (EL3) → heater electrode (HT3) → wiring output (ELO). The wiring (ELI, EL2, EL3, ELO) has high conductivity, and voltage drops in each wiring are negligible or nonexistent. Furthermore, a buffer layer 2 is provided between the wiring (EL2 and EL3) and the optical waveguide WG. This buffer layer 2 is intended to prevent light absorption by the metal constituting the wiring, and is made of a transparent material with a lower refractive index than the optical waveguide forming material. Furthermore, although the buffer layer 2 is formed only between the wiring (EL2 and EL3) and the optical waveguide WG in this example, it may also be formed in other areas such as under the heater electrodes, or on the entire substrate 1, as mentioned above.

[0031] In Figure 4, each heater electrode (HT1 to HT3) is set to have the same "resistance per unit length" (hereinafter referred to as "wire resistivity"), and has a resistance proportional to its length. For example, heater electrodes (HT1) and (HT3) have a resistance of R, and heater electrode (HT2) has a resistance of 2R. In other words, near the input section (ELI) of the wiring, a voltage drop equivalent to a resistance of 2R occurs, which is the sum of the resistance R of heater electrode (HT1) and the resistance R of the left half of heater electrode (HT2). The electric field caused by this voltage drop forms an electric field EF1 that flows from the wiring (ELI) to the heater electrode (HT2), as shown in Figure 4.

[0032] Next, near the output section (ELO) of the wiring, a voltage drop equivalent to a resistance of 2R occurs, which is the sum of the resistance R of the right half of the heater electrode (HT2) and the resistance R of the heater electrode (HT3). As shown in Figure 4, the electric field caused by this voltage drop forms an electric field EF2 directed from the heater electrode (HT2) towards the wiring (ELO). Here, the voltage drop near the wiring (ELI) and near the wiring (ELO) has the same potential difference, and the directions of the electric fields are opposite to each other. Therefore, the phase changes caused by the electric fields formed by the heater electrodes cancel each other out, and the amount of phase change becomes virtually zero.

[0033] Here, the strength and direction of the electric field were explained focusing on the vicinity of the wiring (ELI, ELO). However, if the heater electrodes have a uniform linear resistivity, are arranged symmetrically on either side of the optical waveguide WG, and are equidistant from the optical waveguide, then an electric field with continuously changing strength is generated between opposing heater electrodes (e.g., HT1 and HT2, HT2 and HT3) even in the middle of the heater electrode. However, in the left and right halves of the heater electrode (HT2) in Figure 4, the electric field strength distribution is completely symmetrical, and its direction is opposite. Therefore, even if we consider the change in the electric field in the middle of the heater electrode, the above results remain unchanged, and the amount of phase change is effectively zero.

[0034] Furthermore, the amount of phase change is not simply determined by the electric field strength E (V / m), but is proportional to the product E × L of the optical waveguide length L (m) to which the electric field is applied. In Figure 4, the electric field strength is symmetrical on both sides of the heater electrode (HT2) and the lengths are the same, so it is simplified. However, when using heater electrodes with different linear resistivity, or when the lengths of each heater electrode are different, these factors must also be considered when evaluating the amount of phase change. In addition, a potential difference is generated between the wiring (EL11) and wiring (EL31) and between wiring (EL32) and wiring (EL51) in Figure 5 due to the voltage drop across the heater electrodes. Therefore, when the heater electrodes are short, it is necessary to consider the state of the electric field between these wirings (value of E × L) when setting the length of the wiring, etc.

[0035] Next, the heater electrodes placed near the actual optical waveguide are not always continuous along the optical waveguide as shown in Figure 4, but are often used in combination with short heater electrodes (HT1~HT4) as shown in Figure 5. This is to localize the region of the optical waveguide heated by the heater electrodes and to achieve a temperature change in the optical waveguide (response speed due to heating / cooling) that follows the speed of the change in current (energizing / cutting) of the heater electrodes. Note that while the heater electrode (HT2) in Figure 4 is used, in Figure 5 it is configured as two separate heater electrodes (HT2, HT3). To improve the response speed, it is preferable to divide the heater electrode into smaller ones, but if, for example, the gap between the input and output sections of the wiring (terminal gap) is small, for example, 20 μm or less, or 10 μm or less, it may be formed as a single continuous heater electrode, as shown in Figure 4 (HT2).

[0036] In Figure 5, two types of wiring are used: wiring with high conductivity and large heat capacity (volume) (ELI0, EL20, EL30, EL40, EL50), and wiring (ELI1, EL21) which is arranged to partially overlap the heater electrode (HT1), as shown in Figure 6(a), a cross-sectional view of the dashed line A-A' in Figure 5, and has high conductivity but a smaller heat capacity (volume) compared to the wiring (ELI0, EL20, EL30, EL40, EL50).

[0037] While it is possible to construct both types of wiring shown in Figure 5 using wiring with high heat capacity, the wiring is microfabricated, and placement accuracy is extremely important. For this reason, using thin (small volume) wiring for connections to the heater electrodes near the optical waveguide results in better yield. Furthermore, if a wire with a large heat capacity is placed on top of the heater electrode, the heat generated by the heater electrode escapes to the wire, heating the optical waveguide and requiring a larger power (Pπ).

[0038] Furthermore, wiring with a large heat capacity (such as thick electrodes like ELI0) acts as a heat sink, dissipating heat when the heater electrode is disconnected. As mentioned above, increasing the heat capacity of the wiring increases the heating power (Pπ), but the heating rate can be improved by increasing the current. On the other hand, heat dissipation does not improve unless the thermal diffusion is good, so improving the reaction rate requires measures to improve thermal diffusion. In this sense, the role of the wiring as a heat sink is important.

[0039] In Figure 6, the heater electrodes are exposed. However, some materials used for heater electrodes change their electrical resistance due to oxidation, and this change in electrical resistance makes temperature control difficult. Therefore, as shown in Figure 7, it is possible to form a protective film (antioxidant film) PC made of SiN, SiO2, Al2O3, benzoxylobutene (BCB), etc., to cover the heater electrodes (HT1, HT2, HT3, HT4). In this case, an opening OP is formed in the protective film PC to connect the thin wiring (EL21) and the thick wiring (EL20). It is also necessary to note that the amount of heat escaping to the wiring (EL20) changes depending on the area of ​​this opening. Furthermore, as shown in Figure 7(b), the presence of a protective film between the heater electrode and the optical waveguide reduces the opportunities for the heater electrode to be directly exposed to the outside air, thereby suppressing chemical changes when the resistive member is heated, and thus improving the stability of heater control.

[0040] The following are some of the key features regarding the arrangement of the heater electrodes in the optical waveguide element of the present invention. (1) Both the input and output sections of the wiring are located on one side of the optical waveguide WG. The wiring required to reverse the direction of the electric field generated by the voltage drop across the heater electrode midway through the optical waveguide is a "C" shaped wiring as shown below. First, starting from the input section of the wiring, the wiring extends in one direction along the optical waveguide, crosses the optical waveguide midway, extends in the opposite direction along the optical waveguide, and returns to a position opposite the input section across the optical waveguide. This is called the "first half of the wiring." Next, from the end of the first half of the wiring, the wiring further extends in the opposite direction along the optical waveguide, crosses the optical waveguide midway, extends in the aforementioned one direction along the optical waveguide, and returns to the output section of the wiring located near the input section. This is called the "second half of the wiring." Thus, it is necessary to create a "C" shaped wiring configuration with the first half of the wiring and the second half of the wiring, and for this reason, the input and output sections of the wiring are located on one side of the optical waveguide.

[0041] (2) In the "first half wiring" and the "second half wiring" described above, each must have at least one heater electrode incorporated into it. Since an electric field is generated by a potential difference caused by the voltage drop across the heater electrode, one or more heater electrodes are installed in both the first and second halves of the wiring. To make the amount of phase change applied to the optical waveguide the same in the first and second halves of the wiring, for example, as shown in Figures 8 to 11, the resistance value and length (length along the optical waveguide) of the heater electrodes placed in the "first half of the wiring" (to the left of line C in the diagram) and the "second half of the wiring" (to the right of line C in the diagram), as well as the length of the wiring on either side of the optical waveguide, are adjusted, with the input and output positions of the wiring (dotted-dotted line C) as the boundary. It is preferable to configure the wiring so that the sum of the product E × L of the electric field strength E in the heater electrode and wiring and the length L of the portion of the optical waveguide to which the electric field is applied is the same in the first half of the wiring and the second half of the wiring.

[0042] Naturally, using films with the same linear resistivity for all heater electrodes simplifies the design and makes manufacturing easier. Furthermore, if the length of the heater electrode is shorter than the length of the wiring, the effect of the electric field on the heater electrode can be ignored.

[0043] Embodiments of the optical waveguide element of the present invention, other than those described in Figures 4 and 5, will be explained using Figures 8 to 11. However, the present invention is not limited to these embodiments. In addition, in Figures 8 to 11, a single type of wiring is shown as in Figure 4 in order to simplify the drawings, but it goes without saying that wiring of different thicknesses may be used as in Figure 5. Figure 8 shows eight heater electrodes (HT1-HT8) with the same resistance R and the same length arranged in series. The heater electrodes and wiring are arranged symmetrically around the dashed line C, and the strength of the electric field generated on the left side (first half of the wiring) and the right side (second half of the wiring) of the dashed line C, as well as the length of the optical waveguide to which the electric field is applied, are set to be the same. The direction of the electric field is reversed on the left and right sides of the diagram.

[0044] In Figure 9, the heater electrodes are constructed from films with the same linear resistivity, and the length of the heater electrodes (HT1, HT2) is twice that of the other heater electrodes (HT3-HT6). All heater electrodes (HT3-HT6) have the same resistance value R, and the resistance value of heater electrodes (HT1, HT2) is 2R. In this way, it is also possible to divide a single heater electrode into multiple smaller heater electrodes and arrange them accordingly. Naturally, the sum of the phase changes caused by the electric field EF1 generated to the left of the dashed line C and the phase changes caused by the electric fields EF2 and EF3 generated to the right of line C is equal to each other.

[0045] Figure 10 shows an example where electric fields (EF1, EF2) are generated between the heater electrodes (HT1, HT2) and the wiring EL. In addition to this example, in the wiring to the right of the dashed line C, the illustrated heater electrode (HT2) may be removed and placed on the wiring EL side (the wiring at the bottom of the figure).

[0046] In Figure 11, the heater electrodes (HT1, HT3) have a resistance value of R, and the heater electrodes (HT2, HT4) have a resistance value of 2R. Even in this configuration, it is possible to achieve the same phase change in the electric fields EF1 and EF2. Furthermore, the same result can be obtained by swapping the positions of heater electrodes (HT3) and (HT4) in Figure 11. In Figures 8-11, the wiring EL connecting the heater electrodes is positioned further away from the optical waveguide than the heater electrodes. This reduces the strength of the electric field in the optical waveguide due to the wiring EL, allowing the design to be simplified by considering only the electric field from the heater electrodes.

[0047] Figures 12 and 13 show examples of optical waveguide elements that cannot be used in the present invention. Figure 12 shows an example where the input (ELI) and output (ELO) of the wiring are located on opposite sides of the optical waveguide. In this case, the electric fields EF1 and EF2 are in the same direction and do not act in a direction that cancels out each other's phase changes, so the drift phenomenon cannot be suppressed.

[0048] Figure 13 shows an example where the lengths of the heater electrodes (HT1 to HT6) are all the same, but the resistance of the heater electrodes (HT1 and HT2) is twice that of the other heater electrodes (HT3 to HT6). The voltage drop (sum of resistances) on both sides of the dashed line C is the same, but because the lengths of the optical waveguide sections to which the electric field is applied are different, the amount of phase change is not the same on both sides, and as a result, the drift phenomenon cannot be suppressed.

[0049] Figure 14 shows an example of an optical transmission device. In recent years, optical modulation devices that integrate driver ICs and optical waveguide elements in the same housing, such as high-bandwidth-coherent driver modulators (HB-CDMs), have attracted attention, and there is a growing need for configurations suitable for miniaturization, such as the optical waveguide element of the present invention.

[0050] In the optical modulation device of the present invention, an optical waveguide element is arranged in a housing CA made of metal or the like. Input light L1, such as from a semiconductor laser LD, is input to the optical waveguide WG formed in the optical waveguide element within the housing via an optical fiber or other optical component such as a lens. On the other hand, the light wave emitted from the optical waveguide element is input to another optical fiber F and becomes output light L2. When outputting light, polarization combining means and optical components OB such as lenses are used as needed. Modulation electrodes (not shown) are formed on the substrate 1 of the optical waveguide element. In addition, reinforcing members are arranged on the substrate of the input / output section of the optical waveguide element as needed to increase mechanical strength. In the diagram, the symbol HT indicates the region where the heater electrode is located.

[0051] In an optical modulation device, a driver circuit element DRV that generates an electrical signal S applied to the modulation electrode of the optical waveguide element is arranged adjacent to the optical waveguide element, and the optical waveguide element and the driver circuit element DRV are housed in the same housing CA.

[0052] Furthermore, it is possible to configure the device as an optical transmitter by adding a signal generator DSP (Digital Signal Processing Unit) that generates the modulation signal So input to the driver circuit element DRV. It is also possible to integrate the enclosure CA and the signal generator DSP into a single chassis. [Industrial applicability]

[0053] As described above, the present invention makes it possible to provide an optical waveguide element that suppresses the occurrence of drift phenomena caused by voltage drop due to heater electrodes. Furthermore, it is also possible to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Explanation of Symbols]

[0054] 1 circuit board 2. Buffer layer (cladding layer) WG optical waveguide HT1~HT6 Heater electrodes EL2~EL51 Wiring ELI wiring input section ELO wiring output section EF1~EF4 Electric field

Claims

1. In an optical waveguide element comprising a substrate on which an optical waveguide is formed, and a plurality of heater electrodes arranged parallel to the optical waveguide in order to adjust the phase of the light waves propagating through the optical waveguide, The aforementioned multiple heater electrodes are connected in series by wiring. An optical waveguide element characterized in that, in a portion of the optical waveguide, a phase change occurs in the optical wave of the optical waveguide due to the electric field caused by the voltage drop generated at each heater electrode, and the plurality of heater electrodes are arranged so as to cancel out at least a portion of the electric field.

2. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that the amount of phase change of the light wave is zero throughout the entire optical waveguide in which the plurality of heater electrodes are arranged.

3. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that both the input and output sections of the wiring are located on one side of the optical waveguide.

4. In the optical waveguide element according to claim 1, The optical waveguide element is characterized in that the plurality of heater electrodes are arranged on both sides of the optical waveguide.

5. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that a portion of the wiring adjacent to the heater electrode has a larger heat capacity than the heater electrode.

6. In the optical waveguide element according to claim 1, An optical waveguide element characterized by having a protective film covering the heater electrode.

7. An optical modulation device characterized in that the optical waveguide element according to any one of claims 1 to 6 is housed in a housing and comprises an optical fiber that inputs or outputs an optical wave to the optical waveguide.

8. An optical modulation device according to claim 7, characterized in that the optical waveguide element comprises a modulation electrode for modulating a light wave propagating through the optical waveguide, and the housing contains an electronic circuit for amplifying a modulation signal input to the modulation electrode of the optical waveguide element.

9. An optical transmitting device characterized by comprising an optical modulation device according to claim 7 and an electronic circuit that outputs a modulation signal for causing the optical modulation device to perform a modulation operation.

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