Optical waveguide elements, optical modulators, and optical transmitters

Abstract

This suppresses the occurrence of DC drift due to oxygen deficiency in the substrate. [Solution] The optical waveguide element 1 according to the present invention is characterized by comprising: a substrate 10 made of an electro-optic crystal that forms an optical waveguide 80; an electrode 60 placed on the substrate 10; an electrode underlayer 50 that is in contact with the lower surface of the electrode 60; and an oxygen deficiency prevention layer 40 that is in contact with at least a part of the lower surface of the electrode underlayer 50 and at least a part of the upper surface (substrate upper surface 10a) of the substrate 10, and prevents oxygen deficiency of the substrate 10.

Description

Technical Field

[0001] The present invention relates to an optical waveguide device in which an optical waveguide is formed, an optical modulator including the optical waveguide device, and an optical transmission device including the optical modulator.

Background Art

[0002] In the fields of optical measurement technology and optical communication technology, optical waveguide devices including a substrate on which an optical waveguide is formed are used. For the optical waveguide device of an optical modulator, a substrate made of a material having an electro-optic effect such as lithium niobate (LiNbO3; hereinafter also referred to as LN) is used. In recent years, with the progress of substrate processing technology, it has become possible to make the substrate thinner, and research and development for miniaturization and high density of optical waveguide devices have been progressing.

[0003] Patent Document 1 below discloses an optical waveguide device capable of achieving matching between a modulation signal applied to an electrode and impedance matching by forming a thin portion at a portion where the electrode is located, forming a buffer layer between the substrate and the electrode, and adjusting the thickness of the thin portion.

[0004] Patent Document 2 below includes an optical waveguide formed in the surface of an electro-optic crystal substrate, a buffer layer formed on the optical waveguide, and a drive electrode formed on the buffer layer. As the material of the buffer layer, a mixture of silicon oxide and at least one oxide of one or more elements selected from Group 3 to Group 8, Group 1b, and Group 2b metal elements and semiconductor elements excluding silicon in the periodic table, or a transparent insulator of an oxide of silicon and one or more elements selected from metal elements and semiconductor elements is used, and an optical waveguide device capable of improving DC drift characteristics over a long period is disclosed.

[0005] Patent Document 3 below discloses an optical device capable of reducing optical scattering loss due to roughness on the upper surface and side surfaces of an optical waveguide by forming a rib-type optical waveguide on a substrate and disposing a buffer layer using SiO2 on the entire surface of the substrate including the optical waveguide.

[0006] Patent Document 4, described below, discloses an optical waveguide element having an optical waveguide formed on a substrate, with electrodes arranged on either side of the optical waveguide and a dielectric layer arranged to cover the optical waveguide. This element suppresses light scattering loss due to surface roughness of the optical waveguide and light absorption loss due to electrodes, etc., and also relieves stress caused by the dielectric layer covering the optical waveguide.

[0007] Patent Document 5 discloses an optical element having a substrate formed of lithium niobate crystal and electrodes placed on the substrate, wherein DC drift can be suppressed by using a metal material as the contact metal placed on the contact surface on the electrode side, the standard enthalpy of formation per coordination bond when oxidized being greater than the standard enthalpy of formation per coordination bond of niobium pentoxide. The adhesion between the substrate material (e.g., LN) and the electrode material (e.g., gold) is poor, and electrode delamination can be suppressed by interposing a contact metal layer. By using a metal material that suppresses the removal of oxygen from the substrate as the material constituting the contact metal layer, it is possible to suppress the occurrence of DC drift. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2000-147444 [Patent Document 2] Japanese Patent Application Publication No. 5-257105 [Patent Document 3] Japanese Patent Publication No. 2012-53487 [Patent Document 4] Japanese Patent Publication No. 2024-107822 [Patent Document 5] Japanese Patent Publication No. 2019-174733 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] According to the technology disclosed in Patent Document 1, a buffer layer is formed between the substrate and the electrode, and the electrode is placed on top of the buffer layer. However, since the adhesion between the electrode and the buffer layer is not always high, there is a problem that the electrode is prone to peeling off from the buffer layer.

[0010] According to the technology disclosed in Patent Document 2, DC drift is improved by using a buffer layer placed between the substrate and the electrode, which is a mixture of silicon oxide and a specific oxide, or a transparent insulator made of silicon and an oxide. However, this requires an additional step to manufacture such a specific mixture or oxide, and it is not easy to construct the transparent insulator from a uniform mixture or compound, which may lead to instability in the characteristics of the optical waveguide device.

[0011] According to the technology disclosed in Patent Document 3, for example, as shown in Figure 7, a buffer layer 903 made of SiO2 is arranged over the entire surface of a substrate 901 including rib portions 902 that form a rib-type optical waveguide. Here, an example is shown in which electrodes 904 are arranged so as to sandwich the rib portions 902. However, in this configuration, as schematically shown in Figure 7, when an electric field is applied to the rib-type optical waveguide, the carriers 905 in the buffer layer 903 move in a direction that cancels out the electric field, resulting in a DC drift problem.

[0012] According to the technology disclosed in Patent Document 4, for example, as shown in Figure 8, in a substrate 911 having rib portions 912 that form a rib-type optical waveguide, a dielectric layer 913 is arranged to cover the rib portions 912 and partially cover the surface of electrodes 914 that are arranged to sandwich the rib portions 912. However, in this configuration, as schematically shown in Figure 8, the entire lower surface of the electrode 914 is in direct contact with the substrate 911. As a result, the metal constituting the electrode 914 takes in oxygen from the substrate 911, causing oxygen vacancies in the substrate 911, and when an electric field is applied to the rib-type optical waveguide, the carriers 915 in the substrate 911 may move in a direction that cancels out the electric field, potentially causing DC drift. Also, similar to the technology disclosed in Patent Document 3, when an electric field is applied to the rib-type optical waveguide, the carriers 915 in the dielectric layer 913 may move in a direction that cancels out the electric field, potentially causing DC drift.

[0013] According to the technology disclosed in Patent Document 5, in a configuration in which electrodes are placed on a substrate having rib portions that form a rib-type optical waveguide, a contact metal layer made of a metal material that suppresses the removal of oxygen from the substrate is placed between the substrate and the electrodes. However, in this configuration, it is necessary to consider conditions based on the standard enthalpy of formation of niobium pentoxide and adhesion (bonding) with the substrate, which limits the metal material that can be selected as the material for the contact metal layer. Furthermore, Patent Document 5 exemplifies a spacing of 15 μm or 25 μm between electrodes, and if the spacing between electrodes is narrowed (for example, to 10 μm or less) in order to miniaturize and increase the density of the optical waveguide element, the effect of light absorption by the contact metal layer may become significant.

[0014] The present invention has been made in view of the above problems, and aims to provide an optical waveguide element capable of suppressing the occurrence of DC drift due to oxygen deficiency in the substrate, an optical modulator including the optical waveguide element, and an optical transmitting device including the optical modulator. [Means for solving the problem]

[0015] To solve the above problems, the optical waveguide element, optical modulator, and optical transmitting device according to the present invention have the following technical features.

[0016] To achieve the above objective, the optical waveguide element according to the present invention is characterized by comprising: a substrate made of an electro-optic crystal that forms an optical waveguide; an electrode placed on the substrate; an electrode underlayer disposed in contact with the lower surface of the electrode; and an oxygen deficiency prevention layer disposed in contact with at least a part of the lower surface of the electrode underlayer and at least a part of the upper surface of the substrate, for preventing oxygen deficiency of the substrate by the electrode underlayer.

[0017] According to the above configuration, an oxygen deficiency prevention layer is interposed between the electrode underlayer, which is positioned in contact with the lower surface of the electrode, and the substrate, thereby separating the electrode underlayer and the substrate so that they do not come into direct contact in at least a portion of the surface. By placing an oxygen deficiency prevention layer between the electrode underlayer and the substrate in this way, it is possible to prevent oxygen from being taken from the substrate by the electrode underlayer and the electrode positioned above the substrate, thereby suppressing the occurrence of DC drift due to oxygen deficiency in the substrate.

[0018] In the optical waveguide element according to the present invention, the oxygen deficiency prevention layer may be arranged in contact with the entire lower surface of the electrode base layer in the above configuration.

[0019] According to the above configuration, by arranging the oxygen deficiency prevention layer 40 so that the entire lower surface of the electrode underlayer 50 does not come into direct contact with the substrate 10, it is possible to more reliably prevent oxygen from being taken from the substrate by the electrode underlayer and electrodes positioned above the substrate, and to more reliably suppress the occurrence of DC drift due to oxygen deficiency in the substrate.

[0020] In the optical waveguide element according to the present invention, the oxygen deficiency prevention layer may be positioned below the DC electrode to which a DC voltage is applied.

[0021] According to the above configuration, by disposing an oxygen deficiency prevention layer below the DC electrode, the generation of DC drift due to oxygen deficiency in the substrate can be efficiently suppressed.

[0022] In the optical waveguide element according to the present invention, in the above configuration, the oxygen deficiency prevention layer may be disposed below the electrode having a thickness of 1.0 μm or less.

[0023] According to the above configuration, by disposing an oxygen deficiency prevention layer below the electrode having a thickness of 1.0 μm or less disposed in the vicinity of the modulation part (operating part) of the optical waveguide 80, the generation of DC drift due to oxygen deficiency in the substrate can be efficiently suppressed.

[0024] The optical waveguide element according to the present invention, in the above configuration, has a reinforcing substrate joined via a bonding layer below the substrate, the substrate is made of an LN substrate having a thickness of 1.0 μm or less, and a convex portion used as the optical waveguide may be formed on the upper surface of the substrate.

[0025] According to the above configuration, by using an LN thin plate having a convex portion used as an optical waveguide as a substrate, miniaturization and high density of the optical waveguide element can be achieved.

[0026] The optical waveguide element according to the present invention, in the above configuration, the oxygen deficiency prevention layer may not be formed on the surface of the convex portion.

[0027] According to the above configuration, in the convex portion where the optical waveguide is formed and an electric field is applied, it is possible to prevent the generation of DC drift due to carrier movement in the oxygen deficiency prevention layer.

[0028] The optical waveguide element according to the present invention, in the above configuration, the arithmetic mean roughness Ra of the surface of the convex portion may be 5.0 nm or less.

[0029] According to the above configuration, scattering of light waves by the surface of the convex portion can be suppressed.

[0030] In the optical waveguide element according to the present invention, the oxygen deficiency prevention layer may be made of a material with a refractive index of 1.3 or higher and a dielectric constant of 3.0 or higher.

[0031] According to the above configuration, a suitable material can be used as the oxygen deficiency prevention layer.

[0032] In the optical waveguide element according to the present invention, the material constituting the oxygen deficiency prevention layer may be SiO2, and the average atomic ratio of oxygen to silicon may be greater than 1.9.

[0033] According to the above configuration, the oxygen deficiency prevention layer contains sufficient oxygen, preventing oxygen from being removed from the substrate, and thereby suppressing the occurrence of DC drift due to oxygen deficiency in the substrate.

[0034] In the optical waveguide element according to the present invention, the content of the inert gas in the oxygen deficiency prevention layer may be 1.0 to 3.0 atm% in the above configuration.

[0035] According to the above configuration, by setting the inert gas content of the oxygen deficiency prevention layer within a specific range, the strength and membrane stress of the oxygen deficiency prevention layer can be adjusted in a well-balanced manner.

[0036] In the optical waveguide element according to the present invention, the thickness of the oxygen deficiency prevention layer may be 10 to 200 nm in the above configuration.

[0037] According to the above configuration, it is possible to stably deposit films that can suppress the occurrence of DC drift due to carriers in the oxygen deficiency prevention layer.

[0038] Furthermore, in order to achieve the above objective, the optical modulator according to the present invention is characterized by comprising the above-mentioned optical waveguide element, a housing for housing the optical waveguide element, an input optical fiber connected to the optical input portion of the optical waveguide element, and an output optical fiber connected to the optical output portion of the optical waveguide element.

[0039] In the above configuration, the optical modulator according to the present invention may include a modulation electrode as the electrode that modulates the light wave propagating through the optical waveguide, and may have a signal amplification circuit inside the housing that amplifies the modulation signal applied to the modulation electrode.

[0040] Furthermore, in order to achieve the above objective, the optical modulator according to the present invention is characterized by comprising the above-mentioned optical modulator, a light source for inputting an optical wave to the optical modulator, and a signal output circuit for outputting the modulation signal. [Effects of the Invention]

[0041] According to the present invention, it is possible to provide an optical waveguide element capable of suppressing the occurrence of DC drift due to oxygen deficiency in the substrate, an optical modulator including the optical waveguide element, and an optical transmitting device including the optical modulator. [Brief explanation of the drawing]

[0042] [Figure 1] This is a plan view showing an example of the overall configuration of the optical waveguide element in the first to third embodiments of the present invention. [Figure 2] This figure shows a cross-section along line AA in Figure 1, relating to an optical waveguide element in the first embodiment of the present invention. [Figure 3] This figure shows a cross-section along line AA in Figure 1, relating to a derivative example of the optical waveguide element in the first embodiment of the present invention. [Figure 4] This figure shows a cross-section along line BB in Figure 1, relating to an optical waveguide element in a second embodiment of the present invention. [Figure 5] This figure shows a cross-section along line BB in Figure 1, relating to an optical waveguide element in a third embodiment of the present invention. [Figure 6] This is a plan view showing the optical modulator and optical transmitter according to the present invention. [Figure 7] This is a diagram illustrating the problems related to the technology disclosed in Patent Document 3. [Figure 8] This is a diagram illustrating the problems related to the technology disclosed in Patent Document 4. [Modes for carrying out the invention]

[0043] Embodiments of the present invention will be described below with reference to the drawings. The drawings referenced herein are not necessarily to an accurate scale with respect to actual dimensions, and some parts are exaggerated or simplified in order to schematically illustrate the configuration according to the present invention. Furthermore, the numerical ranges described herein include upper and lower limits, meaning that any numerical value within that range can be selected.

[0044] As illustrated in each embodiment, the optical waveguide element according to the present invention generally comprises: a substrate made of an electro-optic crystal that forms an optical waveguide; an electrode placed on the substrate; an electrode underlayer positioned in contact with the lower surface of the electrode; and an oxygen deficiency prevention layer positioned in contact with at least a portion of the lower surface of the electrode underlayer and at least a portion of the upper surface of the substrate, which prevents oxygen deficiency of the substrate by the electrode underlayer.

[0045] (First Embodiment) The optical waveguide element in the first embodiment of the present invention will be described below.

[0046] First, the overall configuration of the optical waveguide element 1 in this embodiment will be described with reference to Figure 1. Figure 1 is a plan view showing an example of the overall configuration of the optical waveguide element 1 in this embodiment. Hereafter, the left-right direction in the plan view shown in Figure 1 will be referred to as the longitudinal direction of the optical waveguide element 1, and the up-down direction in the plan view shown in Figure 1 will be referred to as the width direction of the optical waveguide element 1.

[0047] Figure 1 shows an optical waveguide element 1 in which a Mach-Zehnder (MZ) type optical waveguide is formed on a substrate 10 as the optical waveguide 80. However, the optical waveguide 80 according to the present invention is not limited to the MZ type optical waveguide shown in Figure 1, nor is it limited to a Mach-Zehnder type structure. Electrodes 60 (see Figure 2) and the like are appropriately arranged on the upper surface of the substrate 10, but are omitted from the illustration in Figure 1.

[0048] An MZ-type optical waveguide is a waveguide that has at least one branching section 91 and at least one multiplexing section 92 as its basic components. The branching section 91 is the part that branches one optical waveguide 80 into two optical waveguides 80. The multiplexing section 92 is the part that connects the two optical waveguides 80 and combines them into one optical waveguide 80. The optical multiplexing and demultiplexing sections such as the branching section 91 and the multiplexing section 92 may be manufactured by adjusting the shape, size, and refractive index of the parts that make up the optical waveguide 80, and optical couplers or the like may be provided. Note that an MZ-type optical waveguide may have two or more branching sections 91 or two or more multiplexing sections 92.

[0049] The optical waveguide 80 of the optical waveguide element 1 shown in Figure 1 is formed to propagate the input light L11 input from the optical input terminal 80a and output output light L21 and L22 from the two optical output terminals 80b and 80c. The optical waveguide 80 extends longitudinally from the optical input terminal 80a and folds back, branching into two optical waveguides 80 at the branching section 91. The two branched optical waveguides 80 are each further branched into four optical waveguides 80 at the branching section 91, and then further branched into eight optical waveguides 80 at the branching section 91. The portion where the eight optical waveguides 80 extend in parallel (near region R1 in Figure 1) is used as a modulation section (operating section) that modulates the light wave propagating through the optical waveguide 80. The eight parallel-arranged optical waveguides 80 are combined into four optical waveguides 80 and then into two optical waveguides 80 at the multiplexing section 92, and connected to two optical output terminals 80b and 80c, respectively. As a result, the input light L11 input from the optical input terminal 80a is appropriately modulated in the modulation section, and output light L21 and L22 are output from the two optical output terminals 80b and 80c.

[0050] The optical waveguide 80 may be provided with a spot size conversion unit (SSC) or a grading unit at its ends, namely the optical input terminal 80a and the optical output terminals 80b and 80c, which change the cross-sectional diameter of the optical wave. The cross-sectional diameter of the optical wave can be changed. The configuration of the spot size conversion unit or grading unit is not particularly limited and can be realized using existing technologies.

[0051] The cross-sectional structure of the optical waveguide element 1 in this embodiment will now be described. Figure 2 shows the optical waveguide element 1 in this embodiment, and is a cross-section along line AA in Figure 1. Figure 2 shows a cross-section perpendicular to the longitudinal direction of the optical waveguide element 1 (a cross-section perpendicular to the optical propagation direction in the optical waveguide 80). The left-right direction in the cross-sectional diagram shown in Figure 2 corresponds to the width direction of the optical waveguide element 1. Hereafter, the up-down direction in the cross-sectional diagram shown in Figure 2 will be referred to as the height direction of the optical waveguide element 1.

[0052] The substrate 10 of the optical waveguide element 1 is made of a material that exhibits an electro-optic effect. Examples of materials exhibiting an electro-optic effect include lithium niobate (LN), lithium tantalate (LT), and lead lanthanum zirconate titanate (PLZT). These materials may also be doped with MgO or the like. Furthermore, vapor-grown films of these materials, or composite substrates formed by bonding these materials to different substrates, may also be used.

[0053] The thickness of the substrate 10 is preferably, for example, 1.0 μm or less. The thickness of the substrate 10 refers to the height from the bottom surface of the substrate 10 to the flat top surface (substrate top surface 10a) where the convex portion 15 is not formed. By making the substrate 10 a thin plate with a thickness of 1.0 μm or less, the driving voltage can be reduced and the size can be reduced.

[0054] An optical waveguide 80 is formed on the substrate 10 of the optical waveguide element 1. A convex portion 15 is formed on the substrate 10, protruding from the flat upper surface 10a of the substrate. The convex portion 15 is provided in the portion corresponding to the optical waveguide 80, forming a convex optical waveguide, which is the path through which light waves propagate.

[0055] The method for forming the convex optical waveguide is not particularly limited. For example, the substrate 10 may be etched to form the convex portion 15 (rib portion), or grooves may be formed on both sides of the optical waveguide 80 to form the convex portion 15 (ridge portion). In addition, the refractive index may be further increased by diffusing Ti or the like onto the surface of the substrate 10 using a thermal diffusion method or a proton exchange method, in accordance with the convex optical waveguide. The size of the convex portion 15 is not particularly limited, but the width and height can be about 1.0 μm, similar to a normal convex optical waveguide.

[0056] To increase the mechanical strength of the thinned substrate 10, a reinforcing substrate (support substrate) 20 may be placed beneath the substrate 10, as shown in Figure 2. Note that the lower part of the reinforcing substrate 20 is omitted from the illustration in Figure 2. The thickness of the reinforcing substrate 20 is not particularly limited, but can be, for example, about 0.2 to 1.0 mm. The reinforcing substrate 20 may be bonded to the substrate 10 via a bonding layer (intermediate layer) 30, as shown in Figure 2, or it may be directly bonded to the substrate 10. The material of the reinforcing substrate 20 is not particularly limited, but for example, Si, glass, quartz, fused silica, synthetic silica, alkali glass, alkali-free glass, lead glass, borosilicate glass, soda glass, sapphire, alumina, etc. can be used.

[0057] An electrode 60 is placed on the substrate 10. In this embodiment, as shown in Figure 2, an oxygen deficiency prevention layer 40 is formed in accordance with the position of the electrode 60. The oxygen deficiency prevention layer 40 can be formed on the upper surface (substrate upper surface 10a) of the substrate 10 by sputtering or the like.

[0058] The oxygen deficiency prevention layer 40 is placed between the substrate 10 and the electrode underlayer 50. The oxygen deficiency prevention layer 40 is positioned so that its lower surface is in contact with the substrate 10 and its upper surface is in contact with the electrode underlayer 50. The oxygen deficiency prevention layer 40 interposed between the substrate 10 and the electrode underlayer 50 has the role of preventing oxygen from being taken from the substrate 10 by the electrode underlayer 50 (and furthermore, by the electrode 60 on its upper surface).

[0059] The material for the oxygen deficiency prevention layer 40 can be a dielectric material with a lower refractive index and higher transparency than the material for the substrate 10 (e.g., LN) and the material for the electrode 60 (e.g., gold (Au)). Preferably, the refractive index of the oxygen deficiency prevention layer 40 is 1.3 or higher, and the dielectric constant of the oxygen deficiency prevention layer 40 is 3 or higher. Furthermore, it is preferable to select a material that exhibits low light absorption in the wavelength band of the light waves propagating through the optical waveguide 80. Specifically, it is preferable to use oxides, fluorides, or nitrides of metal elements from groups 1 to 17 of the periodic table as the material for the oxygen deficiency prevention layer 40, such as SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, CaF2, Y2O3, etc.

[0060] When SiO2 is used as the material for the oxygen deficiency prevention layer 40, it is preferable to make the average atomic ratio O / Si greater than 1.9 (O / Si > 1.9). This ensures that the oxygen deficiency prevention layer 40 contains sufficient oxygen, preventing it from removing oxygen from the substrate 10 and suppressing the occurrence of DC drift due to oxygen deficiency in the substrate 10. The average atomic ratio in the oxygen deficiency prevention layer 40 can be detected by Rutherford backscattering analysis (RBS analysis).

[0061] The content of inert gas (e.g., argon) in the oxygen deficiency prevention layer 40 is preferably 1.0 to 3.0 atm%. The content of inert gas in the oxygen deficiency prevention layer 40 can be detected by Rutherford backscattering analysis (RBS analysis), similar to the average atomic ratio.

[0062] If there is a large amount of inert gas in the oxygen deficiency prevention layer 40, the density of the oxygen deficiency prevention layer 40 will be low, and sufficient strength may not be obtained. On the other hand, if there is a small amount of inert gas in the oxygen deficiency prevention layer 40, the density of the oxygen deficiency prevention layer 40 will be high, and the film stress of the oxygen deficiency prevention layer 40 will have a greater influence on the substrate 10, making delamination from the substrate 10 more likely. Therefore, by controlling the inert gas content in the oxygen deficiency prevention layer 40 to fall within the above range, the oxygen deficiency prevention layer 40 can be made physically stable. The inert gas content in the oxygen deficiency prevention layer 40 can be controlled to fall within the above range by appropriately adjusting the pressure of the inert gas used during film deposition by sputtering or the like, and the film deposition rate.

[0063] The thickness of the oxygen deficiency prevention layer 40 is preferably set within a specific range for the following reasons. If the oxygen deficiency prevention layer 40 is too thick, carriers in the oxygen deficiency prevention layer 40 may move due to the electric field, potentially causing DC drift. Also, if the oxygen deficiency prevention layer 40 is too thick, the film stress of the oxygen deficiency prevention layer 40 will have a greater influence on the substrate 10, making delamination from the substrate 10 more likely. Therefore, the thickness of the oxygen deficiency prevention layer 40 is preferably 200 nm or less, and more preferably 100 nm or less. On the other hand, since the oxygen deficiency prevention layer 40 is deposited by sputtering or the like, if the thickness is made too thin, it becomes difficult to control the film thickness. From the viewpoint of process stability, the thickness of the oxygen deficiency prevention layer 40 is preferably 10 nm or more, and more preferably 20 nm or more. That is, the thickness of the oxygen deficiency prevention layer 40 is preferably 10 to 200 nm, and more preferably 20 to 100 nm.

[0064] The electrode base layer 50 is positioned between the substrate 10 and the electrode base layer 50 such that its lower surface is in contact with the substrate 10 and its upper surface is in contact with the electrode base layer 50. The electrode base layer 50 acts as an adhesive layer between the oxygen deficiency prevention layer 40 and the electrode 60, enhancing the adhesion (bonding) of the electrode 60 and preventing the electrode 60 from peeling off.

[0065] The material of the electrode underlayer 50 is preferably selected considering its adhesion to the substrate 10 and the oxygen deficiency prevention layer 40. For example, Nb, Ti, Al, Mn, Cr, Ni, Pt, SiN, etc., can be used. The method of forming the electrode underlayer 50 and its thickness are not particularly limited. For example, the thickness of the electrode underlayer 50 may be about the same as or less than that of the oxygen deficiency prevention layer 40.

[0066] The electrode 60 is used to modulate the light wave propagating through the optical waveguide 80 and is positioned near the optical waveguide 80. In this embodiment, as shown in Figure 2, the case in which the electrode 60 is positioned on either side of the optical waveguide 80 (X-cut substrate) is illustrated, but it is also applicable to the case in which the electrode 60 is positioned above the optical waveguide 80 (Z-cut substrate). Furthermore, although this specification mainly illustrates and explains a single-electrode structure, it is also applicable to a differential electrode structure. The electrode 60 includes a modulation electrode that applies a modulation signal to the optical waveguide 80 and a DC electrode that applies a DC bias voltage.

[0067] The material of electrode 60 is not particularly limited as long as it is a metallic material with low resistance and excellent impedance characteristics, and can be Au, Ag, Cu, etc. The method of forming electrode 60 is also not particularly limited, and can be sputtering, vapor deposition, plating, etc., according to conventional methods.

[0068] In this embodiment, the optical waveguide element 1 has an oxygen deficiency prevention layer 40 formed on the upper surface (substrate upper surface 10a) of the substrate 10 on which the convex portion 15 is formed, and an electrode 60 is formed on the upper surface of the oxygen deficiency prevention layer 40 via an electrode underlayment layer 50. In other words, the optical waveguide element 1 in this embodiment has an oxygen deficiency prevention layer 40 arranged on the substrate upper surface 10a, an electrode underlayment layer 50 arranged on the upper surface of the oxygen deficiency prevention layer 40, and an electrode 60 arranged on the upper surface of the electrode underlayment layer 50.

[0069] The oxygen deficiency prevention layer 40 is positioned between the substrate 10 and the electrode underlayer 50. In this embodiment, the oxygen deficiency prevention layer 40 is positioned in contact with the entire lower surface of the electrode underlayer 50, and the upper surface of the substrate 10 and the lower surface of the electrode underlayer 50 are separated by the oxygen deficiency prevention layer 40 and do not come into direct contact with each other.

[0070] The oxygen deficiency prevention layer 40 is made of a material that does not substantially remove oxygen from the substrate 10, and by being interposed between the substrate 10 and the electrode underlayer 50 and the electrode 60, it plays a role in preventing the electrode underlayer 50 and the electrode 60 from removing oxygen from the substrate 10. As a result, the oxygen deficiency prevention layer 40 prevents the electrode underlayer 50 and the electrode 60, which are positioned above the substrate 10, from removing oxygen from the substrate 10, thereby suppressing the occurrence of DC drift due to oxygen deficiency in the substrate 10.

[0071] In this embodiment, the widths of the stacked oxygen deficiency prevention layer 40, electrode base layer 50, and electrode 60 are all set to be the same. Specifically, as shown in Figure 2, the oxygen deficiency prevention layer 40 is formed so that its side surface 40a coincides with the side surface 50a of the electrode base layer 50 and the side surface 60a of the electrode 60, and no film is formed between the electrodes 60.

[0072] In this embodiment, based on the finding that carriers in the film covering the surface 15a of the convex portion 15 and the upper surface 10a of the substrate between the electrodes 60 are one of the causes of DC drift (see, for example, Figures 7 and 8), as shown in Figure 2, the upper surface 10a of the substrate between the electrodes 60, including the surface 15a of the convex portion 15, is not covered with a specific material and is exposed to air. Note that the surface 15a of the convex portion 15 refers to the upper surface and side surface of the convex portion 15 that protrudes from the upper surface 10a of the substrate.

[0073] When the surface of the convex portion 15 is not covered with a specific material and is exposed to air, the arithmetic mean roughness Ra of the surface 15a of the convex portion 15 is preferably 5.0 nm or less, and more preferably 3.0 nm or less, in order to suppress light wave scattering by the surface 15a of the convex portion 15. The arithmetic mean roughness Ra of the convex portion 15 can be measured and calculated using an atomic force microscope (AFM). By reducing the roughness of the surface 15a of the convex portion 15 in this way, light wave scattering by the surface 15a of the convex portion 15 can be suppressed. As a result, without forming a light scattering suppression layer covering the surface 15a of the convex portion 15, it is possible to suppress the occurrence of DC drift due to oxygen deficiency in the substrate 10 while suppressing light wave scattering.

[0074] On the other hand, as shown in the derivative example in Figure 3, the light scattering suppression layer 100 may be formed to cover the surface 15a of the convex portion 15. The light scattering suppression layer 100 has the function of suppressing the scattering of light waves by the surface 15a of the convex portion 15. The light scattering suppression layer 100 may be made of the same material as the oxygen deficiency prevention layer 40, for example, or it may be a film that does not cause carrier movement, such as a photosensitive insulating film (permanent film). Furthermore, a material that does not cause carrier movement may be filled between the electrodes 60.

[0075] As shown in Figure 3, the light scattering suppression layer 100 is preferably formed to cover only the surface 15a of the convex portion 15, and it is preferable that the oxygen deficiency prevention layer 40 positioned below the electrode 60 and the light scattering suppression layer 100 covering the surface 15a of the convex portion 15 are separated in the width direction and not connected. By limiting the placement of the light scattering suppression layer 100 to only the surface 15a of the convex portion 15 in this way, even when a material that allows carriers to move to the light scattering suppression layer 100 is used, the occurrence of DC drift can be minimized while appropriately and effectively suppressing the scattering of light waves by the surface 15a of the convex portion 15.

[0076] In Figure 2, the oxygen deficiency prevention layer 40 is positioned below all of the illustrated electrodes 60, but the oxygen deficiency prevention layer 40 may be selectively positioned depending on the attributes of the electrodes 60.

[0077] The oxygen deficiency prevention layer 40 has the effect of suppressing the occurrence of DC drift due to oxygen deficiency in the substrate 10, and it is preferable to place it at least below the DC electrode to which a DC voltage such as a DC bias voltage is applied. For this reason, the oxygen deficiency prevention layer 40 may be placed only below the DC electrode. In this case, the oxygen deficiency prevention layer 40 does not need to be placed below the modulation electrode to which the modulation signal is applied to the optical waveguide 80, or the oxygen deficiency prevention layer 40 may also be placed below the modulation electrode.

[0078] Furthermore, in order to prevent oxygen deficiency in the substrate 10 at the modulation section (operating section) of the optical waveguide 80, the oxygen deficiency prevention layer 40 may be placed only below the electrodes 60 located near the modulation section (operating section). Specifically, the thickness of the electrodes 60 located near the modulation section (operating section) of the optical waveguide 80 is 1.0 μm or less, and the oxygen deficiency prevention layer 40 may be placed only below the electrodes 60 with a thickness of 1.0 μm or less. In this case, the oxygen deficiency prevention layer 40 does not need to be placed below electrodes 60 with a thickness exceeding 1.0 μm, or the oxygen deficiency prevention layer 40 may also be placed below electrodes 60 with a thickness exceeding 1.0 μm.

[0079] (Second Embodiment) A second embodiment of the present invention will now be described. The optical waveguide element 1 in the second embodiment differs from that of the first embodiment described above in that the width of the oxygen deficiency prevention layer 40 is set to be greater than the width of the electrode underlayer 50 and the electrode 60. Components having the same function as those in the embodiments described above are described in a simplified or omitted manner and are denoted by the same reference numerals.

[0080] Figure 4 shows the optical waveguide element 1 in this embodiment, and is a cross-sectional view along the line BB in Figure 1.

[0081] The widths of the stacked oxygen deficiency prevention layer 40, electrode base layer 50, and electrode 60 do not necessarily have to be the same. As shown in Figure 4, the width of the oxygen deficiency prevention layer 40 may be set to be greater than the widths of the electrode base layer 50 and electrode 60. Specifically, as shown in Figure 4, the oxygen deficiency prevention layer 40 is formed such that its side surface 40a is located closer to the convex portion 15 that forms the optical waveguide 80 than the side surface 50a of the electrode base layer 50 and the side surface 60a of the electrode 60. The surface 15a of the convex portion 15 may be in contact with air, or a light scattering suppression layer 100 may be formed on the surface 15a of the convex portion 15, as shown in Figure 3.

[0082] In this embodiment, the oxygen deficiency prevention layer 40 is formed to cover a wider area of ​​the substrate surface 10a compared to the first embodiment described above. By increasing the width of the oxygen deficiency prevention layer 40, which is positioned between the substrate 10 and the electrode underlayer 50 and electrode 60, an alignment tolerance is ensured so that even if the alignment of the electrode 60 is misaligned, it remains in a position that does not remove oxygen from the substrate 10. This makes it possible to more reliably prevent oxygen from being removed from the substrate 10 by the electrode underlayer 50 and electrode 60, and to more reliably suppress the occurrence of DC drift due to oxygen deficiency in the substrate 10.

[0083] (Third embodiment) A third embodiment of the present invention will now be described. The optical waveguide element 1 in the third embodiment differs from that of the first and second embodiments described above in that the width of the oxygen deficiency prevention layer 40 is smaller, and a part of the lower surface of the electrode underlayer 50 is in direct contact with the substrate 10. Components having the same function as those in the embodiments described above are described in a simplified or omitted manner and are denoted by the same reference numerals.

[0084] Figure 5 shows the optical waveguide element 1 in this embodiment, and is a cross-sectional view along line BB in Figure 1.

[0085] The electrode 60 is configured to apply an electric field to the optical waveguide 80 formed in the convex portion 15. The substrate 10 between the electrodes 60 on either side of the convex portion 15, or the substrate 10 near it, becomes an electric field path. It is preferable to reliably suppress oxygen deficiency in the substrate 10 in this electric field path, while the oxygen deficiency prevention layer 40 does not necessarily need to be placed in the substrate 10 at locations away from the electric field path. In other words, in this embodiment, the oxygen deficiency prevention layer 40 is placed only in locations where it has the effect of suppressing the occurrence of DC drift, while allowing a part of the lower surface of the electrode underlayer 50 to come into contact with the substrate 10.

[0086] The oxygen deficiency prevention layer 40 can be positioned biased toward the convex portion 15 where the optical waveguide 80 is formed. For example, as shown in Figure 5, one end (side surface 40b) located away from the convex portion 15 may be positioned inward in the width direction of the electrode base layer 50. The widthwise distance D (see Figure 5) between the side surface 40b of the oxygen deficiency prevention layer 40 located away from the convex portion 15 and the side surface 60a of the electrode 60 is preferably 5.0 μm or more. This prevents oxygen from being taken from the substrate 10 by the electrode base layer 50 and the electrode 60, thereby suppressing the occurrence of DC drift due to oxygen deficiency in the substrate 10. The surface 15a of the convex portion 15 may be in contact with air, or a light scattering suppression layer 100 may be formed on the surface 15a of the convex portion 15 as shown in Figure 3.

[0087] When the side surface 40b of the oxygen deficiency prevention layer 40 is positioned inward in the width direction of the electrode base layer 50, the electrode base layer 50 has a surface that contacts the upper surface of the oxygen deficiency prevention layer 40, and also has a portion that directly contacts the substrate 10 (contact surface 70 between the electrode base layer 50 and the substrate 10) at a location away from the convex portion 15. The contact surface 70 between the electrode base layer 50 and the substrate 10 is positioned at a distance of 5.0 μm or more from the side surface 60a of the electrode 60. Note that there is no optical waveguide 80 on the left side of Figure 5, and the contact surface 70 between the electrode base layer 50 and the substrate 10 is wider for the electrode 60 on the left side of Figure 5 than for the other electrodes 60.

[0088] The underside of the electrode base layer 50 is not flat but has an uneven surface. The uneven surface of the electrode base layer 50 has the advantage of increasing the contact area with each layer on the underside of the electrode base layer 50 (substrate 10 and oxygen deficiency prevention layer 40), which improves adhesion through an anchoring effect.

[0089] The optical modulator and optical transmitter according to the present invention will be described below. The present invention can provide an optical modulator and optical transmitter utilizing the optical waveguide element 1 in each of the embodiments described above.

[0090] Figure 6 is a plan view showing the optical modulator 300 and optical transmitter 400 according to the present invention. The optical modulator 300 shown in Figure 6 includes an optical waveguide element 1, a housing 301, an input optical fiber 302, and an output optical fiber 303. Here, as an example, we will describe the case in which the optical waveguide element 1 in this embodiment is applied to a broadband coherent driver modulator (HB-CDM).

[0091] In the optical modulator 300, the optical waveguide element 1 is housed in a housing 301. An input optical fiber 302 is connected to the optical input section of the optical waveguide element 1, including the optical input terminal 80a, and an output optical fiber 303 is connected to the optical output section, including the optical output terminals 80b and 80c. By connecting the optical waveguide element 1 inside the housing 301 to the outside of the housing 301 with optical fibers, a compact optical modulator 300 can be provided. A spatial optical system may be interposed between the optical input section and the input optical fiber 302, and between the optical output section and the output optical fiber 303. Furthermore, the optical waveguide element 1 described above has two optical output terminals 80b and 80c. In this case, as shown in Figure 6, the optical modulator 300 may have a polarization combining section 304, which is configured to polarize and combine the light output from the two optical output terminals 80b and 80c and guide it to the output optical fiber 303.

[0092] As shown in Figure 6, the optical transmitter 400 can be configured by connecting a signal output circuit 401, which generates an electrical signal So (modulation signal) that is a high-frequency signal for performing modulation operation, and a signal amplification circuit 402, which amplifies the electrical signal So to generate an amplified signal S (modulation signal). The signal output circuit 401 and the signal amplification circuit 402 may be located outside the housing 301 of the optical modulator 300, but by placing them inside the housing 301, efficient transmission of the modulation signal and miniaturization of the optical transmitter 400 can be achieved.

[0093] Alternatively, the optical transmitter 400 may be equipped with a light source 403, and configured to input the light (input light L1) emitted by the light source 403 to the optical input terminal 80a of the optical waveguide element 1. This allows the light output from the light source 403 to be modulated by the optical modulator 300, and the modulated light (output light L2) to be output from the optical transmitter 400.

[0094] The optical waveguide element 1 in this embodiment is applicable to various devices related to optical measurement technology and optical communication technology. The optical waveguide element 1 in this embodiment may be mounted inside a transceiver, or it may be mounted in a pluggable module. The pluggable module has an electrical interface that can be inserted into and removed from an optical transmission device, and an optical interface that can be connected to an optical fiber connector, enabling the optical transmission device to implement high-performance transceiver functionality.

[0095] The optical waveguide element 1 in this embodiment may be mounted on packaged modules such as CPO (Co-Packaged Optics) or NPO (Near Package Optics), or on subassemblies such as IC-TROSA (Integrated Coherent Transmit-Receive Optical Sub-Assembly) or COSA (Coherent Optical Sub-Assembly). Furthermore, it can be incorporated into optical circuits utilizing silicon photonics technology. The optical waveguide element 1 in this embodiment achieves the effect of suppressing the occurrence of DC drift and can provide excellent operational stability in various devices.

[0096] The embodiments described above are provided to facilitate understanding of the present invention and do not limit it. The components disclosed in the embodiments described above are intended to include all design modifications and equivalents that fall within the technical scope of the present invention. Furthermore, technical ideas obtained by appropriately combining the concepts illustrated in each embodiment are also encompassed within the present invention. [Explanation of Symbols]

[0097] 1 Optical waveguide element 10, 901, 911 circuit boards 10a Top surface of board 15 Convex part 15a surface 20 Reinforcement substrate (support substrate) 30 Bonding layer (middle layer) 40. Oxygen deficiency prevention layer 40a, 40b, 50a, 60a Side view 50 Electrode base layer 60, 904, 914 electrodes 70 Contact surface 80 Optical waveguide 80a Optical input terminal 80b, 80c optical output end 91 Branching point 92 Wave-combining section 100 Light scattering suppression layer 300 Optical Modulators 301 cabinet 302 Input optical fiber 303 Output optical fiber 304 Polarization Synthesis Unit 400 Optical Transmitter 401 Signal Output Circuit 402 Signal Amplifier Circuit 403 Light source 902, 912 Rib section 903 Buffer Layer 905, 915 Carrier 913 Dielectric layer L1, L11 Input Light L2, L21, L22 Output Light So electrical signals (modulated signals) S Amplified signal (modulated signal)

Claims

1. A substrate made of electro-optic crystals that forms an optical waveguide, The electrodes placed on the substrate, The electrode substrate, which is positioned in contact with the lower surface of the electrode, An oxygen deficiency prevention layer is disposed in contact with at least a portion of the lower surface of the electrode underlayer and at least a portion of the upper surface of the substrate, and prevents oxygen deficiency of the substrate by the electrode underlayer. An optical waveguide element characterized by comprising the following features.

2. The optical waveguide element according to claim 1, characterized in that the oxygen deficiency prevention layer is arranged in contact with the entire lower surface of the electrode substrate layer.

3. The optical waveguide element according to claim 1 or 2, characterized in that the oxygen deficiency prevention layer is located below the DC electrode to which a DC voltage is applied.

4. The optical waveguide element according to claim 1 or 2, characterized in that the oxygen deficiency prevention layer is disposed below the electrode with a thickness of 1.0 μm or less.

5. The substrate has a reinforcing substrate bonded to the lower part of the substrate via a bonding layer. The substrate is made of an LN substrate with a thickness of 1.0 μm or less. The optical waveguide element according to claim 1 or 2, characterized in that a convex portion used as an optical waveguide is formed on the upper surface of the substrate.

6. The optical waveguide element according to claim 5, characterized in that the oxygen deficiency prevention layer is not formed on the surface of the convex portion.

7. The optical waveguide element according to claim 5, characterized in that the arithmetic mean roughness Ra of the surface of the convex portion is 5.0 nm or less.

8. The optical waveguide element according to claim 1 or 2, characterized in that the oxygen deficiency prevention layer is made of a material with a refractive index of 1.3 or higher and a dielectric constant of 3.0 or higher.

9. The material constituting the oxygen deficiency prevention layer is SiO ２ The optical waveguide element according to claim 1 or 2, characterized in that the average atomic ratio of oxygen to silicon is greater than 1.

9.

10. The optical waveguide element according to claim 1 or 2, characterized in that the content of the inert gas in the oxygen deficiency prevention layer is 1.0 to 3.0 atm%.

11. The optical waveguide element according to claim 1 or 2, characterized in that the thickness of the oxygen deficiency prevention layer is 10 to 200 nm.

12. An optical modulator comprising: an optical waveguide element according to claim 1 or 2; a housing for housing the optical waveguide element; an input optical fiber connected to the optical input portion of the optical waveguide element; and an output optical fiber connected to the optical output portion of the optical waveguide element.

13. The optical modulator according to claim 12, characterized in that the electrode comprises a modulation electrode that modulates a light wave propagating through the optical waveguide, and the housing has a signal amplification circuit that amplifies the modulation signal applied to the modulation electrode.

14. The optical modulator according to claim 13, A light source that inputs light waves to the aforementioned optical modulator, An optical transmitting device characterized by having a signal output circuit that outputs the modulated signal.

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