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

Abstract

The present invention provides an optical control element that can suppress the occurrence of thermal degradation phenomena even when a film or the like that which can diffuse lithium is placed near an electro-optic substrate containing lithium. [Solution] In an optical control element having an electro-optic substrate containing lithium and on which an optical waveguide WG is formed, and an electrode 3 for applying an electric field to the optical waveguide, a diffusion layer (transparent conductive film 4) is placed near the electro-optic substrate 1 and on which the lithium contained in the electro-optic substrate can diffuse, and a blocking layer (BL1) is placed between the electro-optic substrate and the diffusion layer to suppress the diffusion of lithium.

Description

[Technical Field]

[0001] The present invention relates to an optical control element, an optical modulation device and an optical transmission device using the same, and more particularly to an optical control element having an electro-optic substrate containing lithium and on which an optical waveguide is formed, and an electrode for applying an electric field to the optical waveguide, and an optical modulation device and an optical transmission device using the same. [Background technology]

[0002] In recent years, novel platforms utilizing thin-film lithium niobate (TFLN) as optical waveguides for optical communications have attracted attention. Furthermore, as shown in Patent Document 1, the development of optical control elements equipped with multifunctional optical circuits by integrating dissimilar materials such as TFLN, Si, and SiN is becoming increasingly active.

[0003] Optical control elements such as lithium niobate (LN) modulators (LN modulators) utilize the Pockels effect, resulting in a refractive index change (electro-optic effect, EO effect) proportional to the electric field applied to the LN. To reduce the driving voltage of LN modulators, a larger electric field is generated by narrowing the spacing between electrodes (modulation electrodes, etc.). The material of typical control electrodes is metal, and narrowing the spacing between control electrodes increases the optical insertion loss, one of the characteristics of optical modulators, because the metal absorbs the light propagating through the optical waveguide. Therefore, when the electrode length is constant, there is an inverse relationship between reducing the driving voltage of the LN modulator and the optical insertion loss.

[0004] Furthermore, in controlling the bias point of an LN modulator, not only the electro-optic effect but also the thermo-optic effect (TO effect) can be used. This utilizes the fact that the refractive index changes with temperature and can be achieved by passing a current through a heater electrode placed near the optical waveguide. In order to efficiently change the refractive index of the optical waveguide, it is necessary to place the heater electrode as close to the optical waveguide as possible, so, as with the EO effect, there is a trade-off between reducing the control current and the optical insertion loss.

[0005] As a method to improve the aforementioned conflict, the use of transparent electrodes has been proposed, as shown in Patent Document 2. As shown in Figure 1, an electrode 3 is formed on an electro-optic substrate 1 that forms an optical waveguide WG, and a transparent conductive film 4 is placed below the electrode. Reference numeral 2 denotes a support substrate that supports the electro-optic substrate 1.

[0006] Non-patent document 1 also shows that a similar structure can improve the reciprocal relationship and enable broadband operation. Furthermore, non-patent document 2 shows that DC drift is suppressed by inserting an indium-tin compound (ITO), a transparent conductive film, between an LN substrate (Ti-diffused LN waveguide) in which an optical waveguide is formed by Ti diffusion and a metal electrode. It has also been reported that inserting ITO increases the optical insertion loss.

[0007] Patent documents 3 and 4 disclose an LN modulator using titanium-doped indium oxide as a transparent conductive film, and disclose that the insertion loss of the Ti-diffused LN waveguide increases by using a transparent conductive film. Furthermore, patent document 4 discloses that the insertion loss changes depending on the heat treatment temperature.

[0008] In recent years, optical modulators have been specified for optical communication applications that integrate a driver, which amplifies the modulation signal applied to the modulator, within the housing. The driver generates a relatively large amount of heat, and integrating the driver increases the temperature inside the housing. As a result, the temperature inside the LN modulator rises, leading to a combination of phenomena such as DC drift, increased drive voltage, and degradation of propagation loss in the optical waveguide. This phenomenon, in which the characteristics of the optical modulator deteriorate due to heating, is called "thermal degradation." Furthermore, when the bias point control of the LN modulator is controlled using the TO effect, thermal degradation is likely to occur near the heater electrode.

[0009] Furthermore, through diligent research by the inventors, it was discovered that when the electrodes near the optical waveguide are made of a transparent conductive film in order to efficiently change the refractive index of the optical waveguide formed in LN, the following phenomenon occurs due to thermal load. After applying a thermal load to an optical control element using an LN (referred to as "thermal history"), the DC drift phenomenon became more likely to occur (increase in DC drift). The drive voltage (Vπ) changes depending on the temperature of the thermal load (thermal history). • Optical insertion loss changes depending on the temperature of the thermal load (thermal history). For this reason, LN modulators using transparent conductive films are difficult to ensure reliability and are not used on a commercial basis.

[0010] One possible cause of the degradation of the optical control element's characteristics due to thermal load (thermal history) is the diffusion of Li contained in the LN substrate into the transparent conductive film. Furthermore, not only the transparent conductive film, but various other films and materials are in contact with the LN substrate. Specifically, these include the support substrate 2 in contact with the electro-optic substrate 1 as shown in Figure 2, the intermediate layer 21 placed between the electro-optic substrate 1 and the support substrate 20 as shown in Figure 3, the buffer layer 5 covering the surface of the electro-optic substrate 1 as shown in Figure 4, and the optical waveguide 6 placed on top of the electro-optic substrate 1 as shown in Figure 5. Furthermore, in the films shown in Figures 2 to 5, it is conceivable that Li diffuses into the film due to thermal load (thermal history), contributing to the occurrence of the DC drift phenomenon. In particular, when the electro-optic substrate is a thin film, such as in TFLN, the Li metal vacancies formed by Li diffusion are more numerous within the thin film substrate to which the electric field is applied, resulting in a more pronounced DC drift phenomenon. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Application No. 2023-216412 (Filing Date: December 22, 2023) [Patent Document 2] Japanese Patent Publication No. 2008-250080 [Patent Document 3] International release WO2006 / 004139A1 [Patent Document 4] Japanese Patent Publication No. 2007-272122 [Non-Patent Literature]

[0012] [Non-Patent Literature 1] Xiangyu Meng, et al.,”High Performance Thin-film Lithium Niobate Modulator Applied ITO Composite Electrode with Modulation Efficiency of 1V·cm”,Cornell University,arXiv:2311.05119,Submitted on 9 Nov. 2023,URL: https: / / arxiv.org / abs / 2311.05119 [Non-Patent Literature 2] C.M.Gee, et al.,”Minimizing dc drift in LiNbO3 waveguide devices”,Applied Physics Letters, Vol. 47, No. 3, pages 221-213(1985) [Summary of the Invention] [Problem to be Solved by the Invention]

[0013] An object of the present invention to solve is to solve the aforementioned problems, and provide an optical control element capable of suppressing the occurrence of thermal degradation even when a film body or the like into which lithium can diffuse is arranged in the vicinity of a lithium-containing electro-optic substrate. A further object of the present invention is to provide an optical modulation device and an optical transmitter using the optical control element. [Means for Solving the Problem]

[0014] To solve the above problems, the optical control element, the optical modulation device and the optical transmitter using the same of the present invention have the following technical features.

[0015] (1) A light control element comprising: an electro-optic substrate that contains lithium and has an optical waveguide formed therein; and an electrode for applying an electric field to the optical waveguide, wherein a diffusion layer which is disposed near the electro-optic substrate and into which lithium contained in the electro-optic substrate can diffuse, and a blocking layer for suppressing diffusion of lithium which is disposed between the electro-optic substrate and the diffusion layer, are provided.

[0016] (2) The light control element according to (1) above, wherein the diffusion layer is a transparent conductive film that constitutes at least a part of the electrode.

[0017] (3) The light control element according to (1) above, wherein the diffusion layer is a support substrate that supports the electro-optic substrate, or an intermediate layer provided between the electro-optic substrate and the support substrate.

[0018] (4) The light control element according to (1) above, wherein the diffusion layer is a buffer layer provided on the electro-optic substrate.

[0019] (5) The light control element according to (1) above, wherein the diffusion layer is an optical waveguide formed of silicon.

[0020] (6) The light control element according to any one of (1) to (5) above, wherein the blocking layer is a silicon nitride film.

[0021] (7) The light control element according to (6) above, wherein a thickness of the blocking layer is not less than 50 nm and not more than 100 nm.

[0022] (8) An optical modulation device, wherein the light control element according to (1) above is housed in a housing, and the optical modulation device comprises an optical fiber that inputs or outputs an optical wave to or from the light control element.

[0023] (9) The optical modulation device described in (8) above, wherein the electrode comprises at least a modulation electrode for modulating an optical waveguide, and the housing contains an electronic circuit for amplifying a modulation signal input to the modulation electrode.

[0024] (10) An optical transmission device characterized by having an optical modulation device as described in (8) above, a light source that inputs an optical wave to the optical modulation device, and an electronic circuit that outputs a modulation signal to the optical modulation device. [Effects of the Invention]

[0025] The present invention provides an optical control element having an electro-optic substrate containing lithium and on which an optical waveguide is formed, and an electrode for applying an electric field to the optical waveguide, wherein a diffusion layer is disposed near the electro-optic substrate, allowing the lithium contained in the electro-optic substrate to diffuse, and a blocking layer is placed between the electro-optic substrate and the diffusion layer to suppress the diffusion of lithium, thereby suppressing the diffusion of lithium into the diffusion layer and consequently suppressing the occurrence of thermal degradation phenomena. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using this optical control element. [Brief explanation of the drawing]

[0026] [Figure 1] This figure shows a conventional example of a light control element, specifically one in which a transparent electrode (transparent conductive film) is arranged. [Figure 2] This figure shows a conventional example of an optical control element, specifically one that uses a support substrate to support an electro-optical substrate. [Figure 3] This figure shows a conventional example of an optical control element, in which an intermediate layer is provided between the electro-optic substrate and the support substrate. [Figure 4] This figure shows a conventional example of an optical control element, in which a buffer layer is placed on an electro-optic substrate. [Figure 5] This figure shows a conventional example of an optical control element, in which other optical waveguides are arranged on an electro-optic substrate. [Figure 6]This figure shows an example of the optical control element of the present invention, in which a transparent electrode (transparent conductive film) is arranged. [Figure 7] This figure shows an example of an optical control element of the present invention, specifically an example using a support substrate that supports an electro-optical substrate. [Figure 8] This figure shows an example of an optical control element of the present invention, in which an intermediate layer is provided between the electro-optic substrate and the support substrate. [Figure 9] This figure shows an example of an optical control element of the present invention, in which a buffer layer is arranged on an electro-optic substrate. [Figure 10] This figure shows an example of an optical control element of the present invention, in which another optical waveguide is arranged on an electro-optic substrate. [Figure 11] This figure illustrates the manufacturing process of an example of the optical control element of the present invention. [Figure 12] This diagram, following Figure 11, explains the manufacturing process. [Figure 13] This is a plan view illustrating the shape of the electrode used in the optical control element of the present invention. [Figure 14] This is a plan view illustrating other shapes of electrodes used in the optical control element of the present invention. [Figure 15] This diagram illustrates the state in which an electric field is applied to the optical control element of the present invention. [Figure 16] This figure illustrates an example where the optical control element in Figure 15 is replaced with an electrical circuit model. [Figure 17] This figure illustrates the relationship between the thickness of the block layer and the transparent conductive film and the height of the rib-type optical waveguide in the optical control element of the present invention. [Figure 18] This diagram illustrates the positional relationship between a transparent conductive film and a metal electrode placed on it in the optical control element of the present invention. [Figure 19] This figure illustrates an example of an application of the optical control element of the present invention (Figure 12(f)). [Figure 20] These figures illustrate application examples of the optical control element of the present invention (Figures 7 and 9). [Figure 21] This figure illustrates an example of an application of the optical control element (Figure 10) of the present invention. [Figure 22] This figure shows an example of the optical transmission device of the present invention. [Modes for carrying out the invention]

[0027] The optical waveguide element of the present invention will be described in detail below using preferred examples. As shown in Figures 6 to 10, the optical control element of the present invention is an optical control element having an electro-optic substrate 1 containing lithium and on which an optical waveguide WG is formed, and an electrode 3 for applying an electric field to the optical waveguide, characterized in that a diffusion layer (4, 2, 21, 5, 6) is arranged near the electro-optic substrate 1 and can diffuse the lithium contained in the electro-optic substrate, and a blocking layer (BL1 to 5) is arranged between the electro-optic substrate and the diffusion layer to suppress the diffusion of lithium.

[0028] Examples of "lithium-containing electro-optical substrates" used in this invention include lithium niobate and lithium tantalate. Furthermore, materials such as Au are used as electrodes, and materials such as Ti and Cr are used as base electrodes when forming the electrodes.

[0029] Figure 6 illustrates an example where the transparent conductive film (transparent electrode) 4 corresponds to the diffusion layer. In this invention, a transparent conductive film (transparent electrode) refers to a material that is conductive and more transparent than metal in the wavelength range of the light wave used. Materials for transparent conductive films include ITO, ZnO, and SnO2. When a transparent conductive film is directly formed on an LN substrate using these materials, diffusion of Li has been observed in all transparent conductive films made of ITO, ZnO, and SnO2 when a thermal load (e.g., 600°C for 1 hour in an atmospheric environment) is applied. The diffusion of Li into the transparent conductive film causes a "thermal degradation phenomenon" that degrades DC drift, drive voltage, and propagation loss in the optical waveguide. Si is transparent in the infrared region and has low electrical resistance, making it suitable for use as a transparent conductive film. As will be described later, the Li diffusion phenomenon also occurs in Si.

[0030] In the optical control element of the present invention, a film (block layer) BL1 that suppresses the diffusion of Li is placed between the electro-optic substrate 1 and the transparent conductive film 4, which is a diffusion layer. By placing the block layer, the diffusion of Li contained in the electro-optic substrate 1 into the transparent conductive film 4 due to thermal load is suppressed.

[0031] The following are some of the requirements for the block membrane BL1 in Figure 6. • To be optically transparent. • The refractive index is lower than that of an electro-optic substrate (LN substrate). • The diffusion of Li can be suppressed even with a thin film. • High dielectric constant

[0032] "Optical transparency" is a required condition because an optical waveguide WG is formed on the electro-optic substrate 1, and the block layer is also placed close to the optical waveguide. This is necessary to suppress the optical insertion loss of light waves propagating through the optical waveguide. In particular, it is required that the substrate does not absorb light waves.

[0033] The requirement that "the refractive index is lower than that of the electro-optic substrate" dictates that the area where the block film BL1 is loaded onto the electro-optic substrate 1 (TFLN) should not function as an optical waveguide. If this area were to function as an optical waveguide, it could potentially interact (optically couple) with the original optical waveguide WG, leading to degradation of the optical control element's characteristics and increased design complexity.

[0034] The requirement that "Li diffusion be suppressed even with a thin film" stems from the fact that inserting the block film BL1 separates electrode 3 from the electro-optic substrate 1 (optical waveguide WG), weakening the strength of the electric field formed by the electrode and reducing the efficiency of changing the refractive index of the optical waveguide. Therefore, the thickness of the block layer must be as thin as possible. Naturally, even with a thin film, it is necessary to suppress Li diffusion.

[0035] The reason why a high dielectric constant is important is that, as will be explained later, it affects the efficiency of changing the refractive index of the optical waveguide using the EO effect, and this efficiency decreases if the dielectric constant of the block film is low.

[0036] Silicon nitride (SiN) is preferred as the material for the block layer. SiO2 is a material that satisfies some of the conditions required for the block film described above. In practice, we prepared a substrate on which either a SiN or SiO2 film was formed, and then an ITO film was formed on top of that as a transparent conductive film. We then investigated the state of Li diffusion when a thermal load (600°C for 1 hour in an atmospheric environment) was applied. The SiN and SiO2 film thicknesses were tested at 50 nm and 100 nm.

[0037] As a result, it was confirmed that SiN effectively suppressed Li diffusion even at a thickness of 50 nm, but Li diffusion was observed within the ITO film even at a thickness of 100 nm with SiO2. From this, it can be understood that SiN is preferable as a blocking film, while SiO2 is unsuitable as a blocking film. Furthermore, when using SiN as the block film, setting its thickness to 50 nm or more can sufficiently suppress the diffusion of Li. Also, since a thinner block film is preferable from the viewpoint of suppressing an increase in the driving voltage, it is preferable to set it to 100 nm or less.

[0038] Figure 7 illustrates an example where the support substrate 2 supporting the electro-optic substrate 1 corresponds to a diffusion layer. Figure 8 illustrates an example in which an intermediate layer 21 is placed between the electro-optic substrate 1 and the support substrate 20, and the intermediate layer 21 corresponds to a diffusion layer. The following describes an example in which SiO2 is used for the support substrate 2 and the intermediate layer 21, but the present invention is not limited to this, and can be similarly applied when using a material in which Li can diffuse.

[0039] Figures 7 and 8 show the configuration of an optical control element including a TFLN (electro-optical substrate). As shown in Figure 7, the TFLN is attached to an SiO2 wafer which is the support substrate 2, or as shown in Figure 8, the TFLN is attached to a thermally oxidized Si wafer (which has a Si support substrate 20 and an SiO2 intermediate layer 21). In both Figure 7 and Figure 8, LN and SiO2 are in contact, and when a TFLN modulator utilizing the electro-optical effect is made using such a TFLN wafer, a DC drift phenomenon occurs.

[0040] When TFLN and SiO2 are directly joined, Li ions diffuse into the SiO2 due to thermal history. Additionally, Li metal vacancies are formed in the LN, generating charges corresponding to the Li ions. When a voltage is applied to the electrodes, creating an electric field in the TFLN optical waveguide, over time, the Li ions and Li metal vacancies move due to the electric field, forming electric field lines between them. Since these electric field lines are in the opposite direction to the electric field formed by the electrodes, the electric field in the optical waveguide attenuates over time. This is the DC drift phenomenon.

[0041] To suppress this DC drift phenomenon, a SiN film (block layer BL2-BL3) with a thickness of approximately 50 nm is inserted between the SiO2 wafer (support substrate 2) and the TFLN (electro-optic substrate) in Figure 7, and between the thermally oxidized Si wafer (SiO2 intermediate layer 21) and the TFLN (electro-optic substrate) in Figure 8. As mentioned above, SiN is highly effective in suppressing Li diffusion, and a thickness of 50 nm or more is sufficient to achieve this effect. Furthermore, the requirements for the block layer (BL2-BL3) are that it should not affect the strength of the electric field applied to the optical waveguide WG, and it is preferable that its dielectric constant be lower than that of the electro-optic substrate. There are no particular restrictions on the upper limit of the layer thickness.

[0042] Figure 9 illustrates an example where buffer layer 5 corresponds to a diffusion layer. In this method, a buffer layer 5, which is an SiO2 film, is inserted between the electro-optic substrate (TFLN substrate) 1 on which an optical waveguide is formed and the electrode 3 to suppress the absorption of light waves propagating through the optical waveguide by the electrode. As shown in Figure 4, when the buffer layer is formed in contact with LN, Li diffuses into the SiO2, making DC drift more likely to occur. Therefore, in Figure 9, a SiN film (block layer BL4) is inserted between the LN (electro-optic substrate 1) and the SiO2 (buffer layer 5) to suppress Li diffusion. Note that the amount of Li diffusion from SiO2 is negligible compared to transparent conductive films. Therefore, although DC drift occurs, there is no increase in driving voltage or optical insertion loss. Furthermore, the conditions required for block layer BL4 in Figure 9 are basically the same as the conditions required for block layer BL1 in Figure 6 described above.

[0043] Figure 10 illustrates an example where another optical waveguide 6 placed on the electro-optic substrate 1 corresponds to a diffusion layer. As shown in Figure 5, optical control elements have been developed in which an optical waveguide made of Si is directly loaded onto a TFLN, or in which a Si optical waveguide and a TFLN are joined via SiO2. Due to thermal load, the diffusion phenomenon of Li also occurs within Si. Furthermore, as mentioned above, it is difficult to suppress Li diffusion with SiO2. Therefore, as shown in Figure 10, by inserting the SiN film, which is the block layer BL5, between the Si or SiO2 constituting the other optical waveguide 6 and the electro-optic substrate TFLN, the thermal degradation phenomenon can be suppressed.

[0044] By forming patterned amorphous Si(a-Si) on TFLN, it is possible to fabricate TFLN waveguides loaded with Si optical waveguides. Although these Si optical waveguides also undergo thermal degradation due to heat load, this degradation can be suppressed by inserting a SiN film between the Si and the TFLN. In the case of Figure 10, the requirements for the block layer are that if interaction such as the transfer of light waves between the optical waveguide formed on the electro-optic substrate 1 and other optical waveguides 6 is expected, then a thinner block layer is preferable. However, in areas where interaction is not expected, there are no particular restrictions on the thickness of the block layer.

[0045] Next, an example of a light control element using a transparent conductive film will be described using Figures 11 and 12 to explain the fabrication procedure. As shown in Figure 11(a), an Xcut-TFLN substrate 1 with a thickness of approximately 0.4 μm is prepared, and a SiN film 7 with a thickness of approximately 0.3 to 0.6 μm is formed by reactive sputtering. Since the heat resistance temperature of TFLN is said to be around 600°C, it is necessary to form the film at a relatively low temperature. The TFLN substrate 1 is placed on a support substrate 2.

[0046] Next, as shown in Figure 11(b), a pattern MP of a 30 nm thick Cr film is formed on the SiN film using lithography. Using the lift-off method rather than the wet etching method results in less roughness at the edges of the Cr film.

[0047] As shown in Figure 11(c), the pattern (MP) of the Cr film is transferred to the SiN film 7 by dry etching. A mixed gas of CHF3 and O2 can be used for dry etching. After removing the remaining Cr film with a commercially available Cr etching solution (main component: diammonium cerium(IV) nitrate), heat treatment was performed at 400°C in air.

[0048] As shown in Figure 12(d), after forming a resist pattern using lithography, SiN(BL), ITO(4), Cr(3'), and Au(3) films were deposited in that order. The SiN and ITO films were formed by reactive sputtering, while the Cr and Au films were deposited by vacuum deposition. The thicknesses of each film were 50 nm, 200 nm, 50 nm, and 300 nm.

[0049] As shown in Figure 12(d), after film deposition, the resist (RP) was dissolved to form a multilayer film pattern (SiN(BL), ITO(4), Cr(3'), Au(3)) on the TFLN(1) as shown in Figure 12(e).

[0050] Subsequently, electrodes (30) were formed according to their intended use, as shown in Figure 12(f). The electrodes (30) were made of Au, and Ti was used as the base electrode (30'). In the optical control element shown in Figure 12(f), the vicinity of SiN(7) within TFLN(1) functions as an optical waveguide.

[0051] Various shapes can be adopted for the electrode 3 (transparent conductive film 4) that is close to the optical waveguide and the electrode (30) that is thicker in height. One example is shown in the plan view in Figures 13 and 14. In Figure 13, both electrode 3 and electrode 30 are arranged linearly along the optical waveguide WG. In contrast, in Figure 14, electrode 3 is an assembly of T-shaped segment electrodes. Furthermore, the electrode 3 can be configured not only as shown in Figure 12(f), where a metal electrode 3 is placed on top of the transparent conductive film 4, but also, for example, as shown in Figure 18, by removing a portion of the electrode 3 so that the transparent conductive film 4 is exposed near the optical waveguide.

[0052] Next, we will explain the conditions (dielectric constant, thickness) of the block layer placed between the electro-optic substrate and the transparent conductive film using a model. Figure 15 is a simple model in which a block layer BL is placed between an electro-optic substrate (LN) 1 and a transparent conductive film 4. Figure 16 shows the electrical circuit for the part indicated by the dotted line EF in Figure 15.

[0053] The voltage V in the electro-optic substrate 1, including the optical waveguide, when a voltage V is applied to the two transparent conductive films 4 in Figure 15. LN The relationship is expressed by the following formula: V LN / V=(C P / C LN ) / (2+C P / C LN ) Here, C P is the capacitance of the block layer, and C LN is the capacitance of the electro-optic substrate. Here, V LN / V is maximized under the condition that the value of C P is increased. In other words, increasing the dielectric constant of the block layer or reducing the thickness of the block layer is effective. Therefore, as shown in FIG. 15, when the block layer BL is arranged in the path of electric lines of force EF to which an electric field from the electrode is applied, it is preferable to increase the dielectric constant of the material forming the block layer and reduce the thickness of the block layer.

[0054] Next, a method for further suppressing optical insertion loss using a transparent conductive film will be described with reference to FIG. 17 or FIG. 18. FIG. 17 shows that the total thickness H of the block layer BL and the transparent conductive film 4 arranged thereon TCO is made thicker than the height (H rib ) of the optical waveguide (rib-type optical waveguide) WG. This makes it possible to distance the metal electrode 3 arranged on the upper side of the transparent conductive film 4 away from the optical waveguide WG, thereby suppressing absorption of propagating light by the electrode 3. Specifically, the film thickness (H TFLN ) of TFLN is about 0.4 to 0.5 μm, and the rib height is about 0.2 to 0.25 μm. Considering a margin approximately twice the rib height, H TCO > 0.5 μm. At this time, even if the relative position between the optical waveguide and the electrode 3 is deviated, optical loss hardly increases, so the alignment margin is increased.

[0055] In FIG. 18, after the step of FIG. 12(e), the metal electrodes (3, 3') near the optical waveguide are retreated by wet etching of Au and Cr, thereby suppressing absorption by the metal electrodes without increasing the driving voltage.

[0056] Application examples of the various light control elements described above will be described below. In Figure 19, instead of leaving only the optical waveguide portion of the SiN film (7) in Figure 11(c), a thin layer of the SiN film outside the optical waveguide is left and used as a block layer (BL). This makes it possible to eliminate the process of separately forming the block layer shown in Figure 12(d).

[0057] Figure 20 shows a combination of the optical control elements in Figures 7 and 9. When an SiO2 film exists as a support substrate 2 or buffer layer 5 sandwiching the electro-optic substrate 1, it is also possible to place block layers (BL2, BL4) on the upper and lower surfaces in contact with the electro-optic substrate 1.

[0058] Figure 21 shows an application example of the optical control element of Figure 10, in which the Si optical waveguide 6' is formed within the SiO2 constituting the support substrate 2. Even in such cases, thermal degradation can be suppressed by placing a SiN block layer (BL5') between the electro-optic substrate 1 and the support substrate 2.

[0059] Next, we will describe examples of applying the optical control element of the present invention to optical modulation devices and optical transmission devices. In the following description, we will use an example of a high-bandwidth-coherent driver modulator (HB-CDM), but the present invention is not limited to this and can also be applied to optical phase modulators, optical modulators with polarization synthesis functions, optical waveguide elements integrating more or fewer Mach-Zehnder type optical waveguides, bonding devices with optical waveguide elements made of other materials such as silicon, and devices for sensor applications.

[0060] As shown in Figure 22, the optical control element has an optical waveguide WG formed on an electro-optic substrate 1 such as a TFLN. The electro-optic substrate is also provided with control electrodes (not shown), such as a modulation electrode and a DC bias electrode, which modulate the light waves propagating through the optical waveguide. The optical control element is housed in a housing CA. Furthermore, by providing an optical fiber (F) for inputting and outputting light waves to the optical waveguide, an optical modulation device MD can be constructed.

[0061] In Figure 22, the optical fiber F is optically coupled to the optical waveguide WG within the optical waveguide element using an optical block equipped with an optical lens, a lens barrel, or a polarization multiplexer OB. However, the optical fiber may be introduced into the housing through a through-hole penetrating the side wall of the housing, and the optical fiber may be directly joined to an optical component or substrate, or an optical fiber with a lens function at its end may be optically coupled to the optical waveguide within the optical waveguide element. Furthermore, to ensure stable bonding with the optical fiber and optical block, reinforcing members (not shown) can be placed in layers along the end face of the electro-optical substrate.

[0062] An optical transmitter (OTA) can be configured by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So to the optical modulation device MD to perform modulation operations on the optical modulation device MD. In order to obtain the modulation signal S to be applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. For this reason, in Figure 22, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and the digital signal processor DSP can be placed outside the housing CA, but they can also be placed inside the housing CA. In particular, by placing the driver circuit DRV inside the housing, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.

[0063] The input light L1 to the optical modulation device MD may be supplied from outside the optical transmitter OTA, but as shown in Figure 22, a semiconductor laser (LD) can also be used as the light source. The output light L2 modulated by the optical modulation device MD is output externally via an optical fiber F. [Industrial applicability]

[0064] As described above, the present invention makes it possible to provide an optical control element that can suppress the occurrence of thermal degradation phenomena even when a film or the like that which can diffuse lithium is placed near an electro-optic substrate containing lithium. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using the optical control element. [Explanation of symbols]

[0065] 1 Electro-optic substrate 2. Support substrate 3 electrodes 3' Cr membrane 4 Transparent conductive film (transparent electrode) 5 Buffer Layer 6 Si waveguide 7 SiN film 30 electrodes 30' Underlay electrode WG optical waveguide BL, BL1-5 block layers F Optical Fiber LD light source CA cabinet MD Optical Modulation Device DRV driver circuit DSP (Digital Signal Processor) OTA Optical Transmitter

Claims

1. An electro-optic substrate containing lithium and on which an optical waveguide is formed, In an optical control element having an electrode for applying an electric field to the optical waveguide, A diffusion layer is disposed near the electro-optic substrate, and lithium contained in the electro-optic substrate can diffuse therein. An optical control element characterized by having a blocking layer that suppresses lithium diffusion placed between the electro-optic substrate and the diffusion layer.

2. The optical control element according to claim 1, characterized in that the diffusion layer is a transparent conductive film that constitutes at least a part of the electrode.

3. The optical control element according to claim 1, characterized in that the diffusion layer is a support substrate that supports the electro-optic substrate or an intermediate layer provided between the electro-optic substrate and the support substrate.

4. An optical control element according to claim 1, characterized in that the diffusion layer is a buffer layer provided on the electro-optic substrate.

5. An optical control element according to claim 1, characterized in that the diffusion layer is an optical waveguide formed of silicon.

6. An optical control element according to any one of claims 1 to 5, characterized in that the block layer is a silicon nitride film.

7. The optical control element according to claim 6, characterized in that the thickness of the block layer is 50 nm or more and 100 nm or less.

8. The optical control element described in claim 1 is housed within a housing, An optical modulation device characterized by comprising an optical fiber that inputs or outputs an optical wave to the optical control element.

9. In the optical modulation device according to claim 8, The electrode comprises at least a modulation electrode for modulating the light wave propagating through the optical waveguide, An optical modulation device characterized by having an electronic circuit inside the housing that amplifies the modulation signal input to the modulation electrode.

10. The optical modulation device according to claim 8, A light source that inputs light waves to the optical modulation device, An optical transmitting device characterized by having an electronic circuit that outputs a modulation signal to the optical modulation device.

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