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

Abstract

The present application provides an optical waveguide element, an optical modulating device and an optical transmitting apparatus using the optical waveguide element, which suppresses polarization rotation. The optical waveguide element has a substrate (10) and an optical waveguide (2) disposed on the substrate, characterized in that the optical waveguide has an effective refractive index varying portion, which is a portion in which the effective refractive index of the optical waveguide under a fundamental mode A parallel to the polarization plane of an optical wave propagating in the optical waveguide varies with the propagation of the optical wave, and in which the cross-sectional shape of the optical waveguide perpendicular to the propagation direction of the optical wave is a cross-sectional shape in which the effective refractive index of the optical waveguide under the fundamental mode A is higher than the effective refractive index of the optical waveguide under another fundamental mode B perpendicular to the fundamental mode A.

Description

Technical Field

[0001] The present invention relates to optical waveguide elements and optical modulation devices and optical transmission devices using the same, and more particularly to optical waveguide elements having a substrate and an optical waveguide disposed on the substrate, wherein the effective refractive index varies in relation to a specific fundamental mode of light propagating in the optical waveguide. Background Technology

[0002] In the fields of optical measurement and optical communication technologies, optical modulators and other optical waveguide components that utilize substrates with formed optical waveguides are widely used. In recent years, there has been a demand for miniaturization of optical modulators and similar devices, and miniaturization has become one of the solutions. Figure 1 As shown, the following scheme is proposed: a portion of the optical waveguide 2 formed in the optical waveguide element is bent by more than 90 degrees, such as... Figure 1 As shown in (a), the incident and exit directions of light are bent by 90 degrees, or as... Figure 1 As shown in (b), the incident and outgoing directions of light are bent by 180 degrees.

[0003] To bend optical waveguides, enhanced optical confinement is required, as described in Patent Document 1. Figure 2 As shown, a ribbed optical waveguide is used. Figure 2 yes Figure 1 A cross-sectional view at the dashed line A-A' in (a). Waveguide layer 1 is extremely thin, only a few μm thick. Figure 2 In the waveguide layer 1, the waveguide layer 1 is composed of rib-shaped portions 102 protruding from the thin film portion 101.

[0004] Figure 2 Such ribbed waveguides offer advantages such as strong optical confinement, enabling low-loss propagation even in waveguides with small bending radii. However, the waveguide layer is typically made of materials with electro-optic effects, such as lithium niobate (LN), and is widely used based on... Figure 2 Anisotropic materials with different refractive indices along the axes shown (X, Y, or Z axes).

[0005] Therefore, when light waves propagate through an optical waveguide, polarization rotation is easily generated. That is, Figure 2 The TE mode rotates and is transferred to the TM mode. As a result, in addition to the loss caused by converting to a mode orthogonal to the input light, it also leads to various degradations in characteristics such as decreased modulation efficiency, wavelength dispersion, and interpolarization crosstalk.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2017-129834 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] The problem to be solved by the present invention is to provide an optical waveguide element that solves the above-mentioned problems and suppresses polarization rotation. Furthermore, an optical modulation device and an optical transmission apparatus using the optical waveguide element are provided.

[0011] Technical solutions for solving the problem

[0012] In order to solve the above-mentioned problems, the optical waveguide element, the optical modulation device using the same, and the optical transmission device of the present invention have the following technical features.

[0013] (1) An optical waveguide element having a substrate and an optical waveguide disposed on the substrate, characterized in that the optical waveguide has an effective refractive index variation portion, which is a portion in which the effective refractive index of the optical waveguide under a fundamental mode A parallel to the polarization plane of the light wave propagating in the optical waveguide changes with the propagation of the light wave, wherein in the effective refractive index variation portion, the cross-sectional shape of the optical waveguide perpendicular to the propagation direction of the light wave is such that the effective refractive index of the optical waveguide under the fundamental mode A is higher than the effective refractive index of the optical waveguide under other fundamental modes B perpendicular to the fundamental mode A.

[0014] (2) The optical waveguide element according to (1) above is characterized in that, throughout the entire effective refractive index variation section, the high-low relationship of each effective refractive index related to the fundamental mode A and the fundamental mode B is satisfied.

[0015] (3) The optical waveguide element according to (1) or (2) above is characterized in that a waveguide layer formed of a material having an electro-optic effect is disposed on the substrate, the optical waveguide is a rib-shaped optical waveguide disposed on the waveguide layer, an electrode is formed on the waveguide layer to apply an electric field to the rib-shaped optical waveguide, and the fundamental mode A is a fundamental mode with a higher electro-optic effect based on the electrode for light waves propagating in the optical waveguide.

[0016] (4) The optical waveguide element according to any one of (1) to (3) above, characterized in that the optical waveguide has portions in at least one continuous optical waveguide in which the propagation directions of the light waves differ by more than 90 degrees.

[0017] (5) The optical waveguide element according to any one of (1) to (4) above, characterized in that the cross-sectional shape of the effective refractive index change portion of the optical waveguide is such that the thickness of the optical waveguide in the direction parallel to the fundamental mode B is less than 0.45 times the wavelength of the light wave propagating in the optical waveguide.

[0018] (6) The optical waveguide element according to any one of (1) to (5) above, characterized in that the cross-sectional shape of the effective refractive index change portion of the optical waveguide is such that the thickness of the optical waveguide in the direction parallel to the fundamental mode A is greater than the thickness of the optical waveguide in the direction parallel to the fundamental mode B.

[0019] (7) In any one of (1) to (6) above, the optical waveguide element is characterized in that the refractive index of the substrate is 0.8 times or less than the refractive index of the waveguide layer forming the optical waveguide.

[0020] (8) The optical waveguide element according to any one of (1) to (6) above, characterized in that a low refractive index layer is disposed on the side of the waveguide layer forming the optical waveguide opposite to the substrate, the refractive index of the low refractive index layer being 0.8 times or less than the refractive index of the waveguide layer.

[0021] (9) The optical waveguide element according to any one of (1) to (8) above, characterized in that the waveguide layer forming the optical waveguide is composed of a thin film portion in contact with the substrate and a rib portion protruding from the thin film portion, wherein the thickness of the thin film portion is less than 0.7 times the overall thickness of the waveguide layer.

[0022] (10) The optical waveguide element according to (9) above is characterized in that grooves are formed on both sides of the rib portion in the thin film portion.

[0023] (11) The optical waveguide element according to any one of (1) to (10) above, characterized in that the optical waveguide is a ribbed optical waveguide and the angle between the side of the ribbed optical waveguide and the contact surface between the substrate and the waveguide layer forming the optical waveguide is 50 degrees or more.

[0024] (12) The optical waveguide element according to any one of (1) to (11) above is characterized in that the optical waveguide has a bending portion and the minimum bending radius is 300 μm or less.

[0025] (13) The optical waveguide element according to any one of (1) to (12) above, characterized in that the electrode to which an electric field is applied to the optical waveguide is disposed on the transverse side of the optical waveguide in such a way that it clamps the optical waveguide.

[0026] (14) An optical modulation device, characterized in that the optical modulation device uses an optical waveguide element as described in any one of (1) to (13) above, the optical waveguide element is housed in a housing, and has an optical fiber for inputting or outputting optical waves relative to the optical waveguide.

[0027] (15) The optical modulation device according to (14) above is characterized in that the optical modulation device has an electronic circuit inside the housing that amplifies the modulation signal input to the optical waveguide element.

[0028] (16) An optical transmitting device, characterized in that it comprises: an optical modulation device as described in (14) or (15) above; and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

[0029] Invention Effects

[0030] The present invention provides an optical waveguide element having a substrate and an optical waveguide disposed on the substrate, wherein the optical waveguide has an effective refractive index variation portion, which is the portion in which the effective refractive index of the optical waveguide under the fundamental mode A, which is parallel to the polarization plane of the light wave propagating in the optical waveguide, changes with the propagation of the light wave. In the effective refractive index variation portion, the cross-sectional shape of the optical waveguide perpendicular to the propagation direction of the light wave is such that the effective refractive index of the optical waveguide under the fundamental mode A is higher than the effective refractive index of the optical waveguide under other fundamental modes B perpendicular to the fundamental mode A. Therefore, polarization rotation of the fundamental mode A is suppressed and transferred to the fundamental mode B, thus suppressing polarization rotation. Attached Figure Description

[0031] Figure 1 These are diagrams illustrating examples of bending conventional optical waveguide elements. (a) shows a 90-degree bend, and (b) shows a 180-degree bend.

[0032] Figure 2 It means Figure 1 The figure shows the cross-sectional view at the dashed line A-A'.

[0033] Figure 3 This is a diagram illustrating an example of the cross-sectional shape of the ribbed optical waveguide of the present invention.

[0034] Figure 4 It is a graph showing the change in effective refractive index relative to the thickness of the waveguide layer in a ribbed optical waveguide.

[0035] Figure 5 This diagram shows an example of a substrate with a low refractive index layer disposed on the upper side of the waveguide layer and the lower side of the waveguide layer formed by two layers.

[0036] Figure 6 This is a diagram illustrating an example of a low-refractive-index film disposed on the upper side of a waveguide layer.

[0037] Figure 7 This diagram illustrates an example of setting the thickness of the thin film portion on the side of the rib in the waveguide layer to be thinner.

[0038] Figure 8 This diagram illustrates the arrangement of optical waveguide patterns and signal electrodes, etc., that can be applied to the optical waveguide elements of this invention.

[0039] Figure 9 This is a diagram of an example integrating multiple Mach-Zehnder type optical waveguides.

[0040] Figure 10 It means Figure 8 The figure is a cross-sectional view at the dashed line B-B' in (a).

[0041] Figure 11 These are diagrams illustrating the bending methods of optical waveguides. (a) is a diagram of bending with a certain curvature, (b) is a diagram consisting of two sections bent with small curvatures and one straight section, and (c) is a diagram consisting of three sections bent with small curvatures and two straight sections.

[0042] Figure 12 This is a top view illustrating the optical modulation device and optical transmission apparatus of the present invention.

[0043] Label Explanation

[0044] 1 Waveguide layer

[0045] 2 Optical waveguide

[0046] 10 substrate

[0047] MD optical modulation device

[0048] OTA optical transmission device Detailed Implementation

[0049] Hereinafter, the optical waveguide element of the present invention will be described in detail using preferred examples.

[0050] like Figure 3 As shown, the optical waveguide element of the present invention is an optical waveguide element having a substrate 10 and an optical waveguide 2 disposed on the substrate. The optical waveguide is characterized by having an effective refractive index variation portion, which is the portion in which the effective refractive index of the optical waveguide under the fundamental mode A, which is parallel to the polarization plane of the light wave propagating in the optical waveguide, changes with the propagation of the light wave. In this effective refractive index variation portion, the cross-sectional shape of the optical waveguide perpendicular to the propagation direction of the light wave is such that the effective refractive index of the optical waveguide under the fundamental mode A is higher than the effective refractive index of the optical waveguide under other fundamental modes B perpendicular to the fundamental mode A.

[0051] As the material for the waveguide layer 1 used in the optical waveguide element of the present invention, a strong dielectric material with electro-optic effect can be used. Specifically, substrates such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), and epitaxial films formed from these materials can be used. Furthermore, various materials such as semiconductor materials and organic materials can also be used as the substrate for the optical waveguide element. In particular, the present invention can be appropriately applied when using anisotropic materials with different refractive indices according to the axis.

[0052] The waveguide layer used in this invention has an extremely thin thickness of about a few μm. Methods include mechanically grinding a crystalline substrate such as LN to achieve thinning, or using an epitaxial film such as LN. In the case of an epitaxial film, for example, as shown in Patent Document 1, the epitaxial film is formed using sputtering, CVD, sol-gel methods, etc., based on the crystal orientation of a single-crystal substrate such as a SiO2 substrate, a sapphire single-crystal substrate, or a single-crystal silicon substrate.

[0053] Because the waveguide layer is thin, a substrate 10 is disposed on the back side of the waveguide layer 1 to improve the mechanical strength of the optical waveguide element. The substrate is preferably made of a material with a lower refractive index than the waveguide layer, such as a SiO2 substrate. Alternatively, it can be a substrate like SOI (Silicon on Insulator) where a low dielectric constant layer is formed on the surface of the waveguide layer side of a high refractive index substrate. The waveguide layer can be directly bonded to the substrate 10, or the substrate 10 can be used as a crystal growth base and have an epitaxial waveguide layer disposed thereon. Furthermore, the substrate 10 can be made of a single unit, or a multilayer substrate can be used as described later. There is also a method of bonding a plate-like body such as an LN substrate via an adhesive layer such as resin. In this case, the adhesive layer and the plate-like body function as the substrate 10.

[0054] The rib-shaped protrusions constituting the optical waveguide can be formed by dry etching or wet etching of the waveguide layer. Alternatively, to increase the refractive index of the ribs, a method can be used to thermally diffuse high-refractive-index materials such as Ti at the rib locations.

[0055] Although Figure 2 Not shown in the text, but described later, the electrodes that apply an electric field to the optical waveguide can be formed on a substrate or waveguide layer using a metal such as Au through electroplating. Depending on the requirements, substrate electrodes such as Ti or Au can also be formed and stacked on them using electroplating. The patterning of the substrate electrodes can be achieved using patterning methods employing photoresist or similar agents.

[0056] The optical waveguide element of the present invention is characterized in that the optical waveguide has an effective refractive index variation section, which is the portion in which the effective refractive index of the optical waveguide under the fundamental mode A, which is parallel to the polarization plane of the light wave propagating in the optical waveguide, changes with the propagation of the light wave. Furthermore, in this effective refractive index variation section, the cross-sectional shape of the optical waveguide perpendicular to the propagation direction of the light wave is set such that the effective refractive index of the optical waveguide under the fundamental mode A is higher than the effective refractive index of the optical waveguide under other fundamental modes B perpendicular to the fundamental mode A.

[0057] The meaning of "polarization plane of light propagating in an optical waveguide" will be explained. In optical waveguide elements, a specific polarization plane of light is generally utilized. This is because, depending on the shape and structure of the optical waveguide itself, and also depending on the state of the electric field applied to the optical waveguide, there are cases where the refractive index differs depending on the polarization plane of the light wave. Therefore, light waves with specific polarization planes are generally selected and utilized. In this invention, the focus is on light waves having this selected specific polarization plane, and the "polarization plane" of this light wave is represented as described above.

[0058] The so-called "basic schema" is equivalent to Figure 2 The "TE mode" and "TM mode" are shown. When one "fundamental mode A" is the TE mode, the other "fundamental mode B" is the TM mode. Furthermore, when using a conductive layer 1 with an electro-optic effect, such as LN, when... Figure 3 When an electric field is applied along the Z-axis, and the polarization plane of the selected light wave is along the Z-axis using an X-cut LN, the TE mode becomes the "fundamental mode A". In this case, the TE mode is also the fundamental mode with a higher electro-optic effect.

[0059] The term "effective refractive index variation section," in relation to "fundamental mode A," refers to the portion of the optical waveguide where the effective refractive index changes. Specifically, it refers to the portion where the propagation direction of light changes in anisotropic materials where the refractive index differs along the direction of light propagation, or the portion where the shape of the optical waveguide changes due to refractive index variations along its shape during propagation. For example, when an X-cut LN is used for waveguide layer 1, from the direction of… Figure 3 The range of the optical waveguide 2 in the Y direction, from the part where the angle changes from the Y direction to the Z direction to the final return to the Y direction, corresponds to this "effective refractive index change section". Additionally, in Figure 3 The same applies to the case where the width W of the optical waveguide 2 gradually changes in the direction of light wave propagation, forming a so-called cone shape.

[0060] In the optical waveguide element of the present invention, in the effective refractive index change section, since the effective refractive index of the optical waveguide set to fundamental mode A is higher than that of the optical waveguide set to fundamental mode B, the polarization rotation from fundamental mode A to fundamental mode B is suppressed. Of course, it is desirable to maintain the high-low relationship between the refractive indices of fundamental modes A and B throughout the entire effective refractive index change section. However, even if a reversal of the refractive indices occurs in a part of the effective refractive index change section, as long as the rotation from fundamental mode A to fundamental mode B is suppressed when the light wave is finally emitted from one end of the effective refractive index change section, this is permitted as part of the present invention.

[0061] like Figure 3As shown, the cross-sectional shape of the effective refractive index variation section of the optical waveguide is adjusted by setting the width W and height (t1 or t1-t2) of the ribbed optical waveguide. Furthermore, the high refractive index material diffused within waveguide layer 1 can be locally varied as needed.

[0062] When waveguide layer 1 uses a material with electro-optic effect, the electrodes that apply an electric field to the ribbed optical waveguide form part of the waveguide layer. The "fundamental mode A" of the present invention is the fundamental mode with a higher electro-optic effect based on the electrodes for light waves propagating in the optical waveguide. For example, when waveguide layer 1 is an X-cut LN, it is the TE mode, and when it is a Z-cut LN, it becomes the TM mode.

[0063] Figure 3 Waveguide layer 1 is an X-cut LN or LT crystal. The XYZ arrows in the diagram represent the crystal axes, with the X-axis positioned vertically and the Z-axis positioned horizontally. In LN crystals, the refractive index along the Z-axis is 2.138, and the refractive index along the X or Y axis is 2.211. Furthermore, the axis with the highest electro-optic effect is the Z-axis, i.e., [missing information]. Figure 3 The electrodes that apply an electric field to the optical waveguide are arranged on the left and right sides, sandwiching the optical waveguide (rib portion 102). The fundamental mode in the Z-axis direction is... Figure 2 That TE mode is the base mode.

[0064] In contrast, Figure 3 The axis direction with the lowest electro-optic effect is the X-axis direction, and the fundamental mode formed here becomes Figure 2 Such a TM mode. The effective refractive index of the TE mode or TM mode is affected by... Figure 3 The influence of the height (height of the waveguide layer) t1 or width W of the ribbed optical waveguide. Therefore, for example, by controlling Figure 3 The waveguide layer thickness (t1) of such a ribbed waveguide can suppress the conversion from TE mode to TM mode.

[0065] Figure 4 This graph illustrates the variation of the effective refractive index with respect to the thickness of the waveguide layer. The substrate (lower cladding) uses SiO2 (refractive index n2 = 1.44), waveguide layer 1 uses X-cut lithium niobate (LN) (refractive index n1 = 2.138 (Z-axis) / 2.211 (X, Y-axis)), and the upper cladding uses air (refractive index n3 = 1), forming a three-layer planar waveguide. Furthermore, Figure 4 The dispersion curves of each fundamental mode of the TE and TM modes are shown (wavelength of light λ = 1.55 μm).

[0066] Figure 4The horizontal axis represents the waveguide layer thickness t1, and the vertical axis represents the effective refractive index of each propagation axis of the light wave. As the propagation axes of the light wave, propagation along the Y-axis (Y propagation), the Z-axis (Z propagation), and the mid-Y-Z axis (45° propagation) are shown, along with the effective refractive index in the TE mode for each propagation axis. Additionally, the effective refractive index in the TM mode is also shown. Figure 4 .

[0067] Generally speaking, as the thickness t1 of the waveguide layer decreases, the effective refractive index also decreases. Figure 4 The characteristic feature is that the effective refractive indices of the TE and TM modes reverse around t1 = 0.7 μm. In particular, the TE and TM modes exhibit a region of reversed effective refractive index relationship during Y propagation and 45° propagation.

[0068] Electromagnetic waves can couple from a high-speed (low effective refractive index) mode to a low-speed (high effective refractive index) mode, but cannot couple from a slow mode to a high-speed mode. Therefore, in regions thicker than 0.7 μm, the effective refractive index of the TM mode is higher than that of the TE mode, depending on the propagation direction. In this case, it means easier to switch from the TE mode to the TM mode. That is, the polarization of the TE mode, which is the fundamental mode, rotates.

[0069] In contrast, if the region is thinner than 0.7 μm, the effective refractive index of the TE mode is higher than that of the TM mode in both propagation directions, therefore the TE mode cannot couple with the TM mode. When input to an optical modulator using an X-cut LN, the TE mode, where the electro-optic effect is stronger, is generally used. Therefore, if the waveguide layer is thinner than 0.7 μm and has a width of optical waveguide with sufficient optical confinement strength, unwanted polarization rotation from the TE mode to the TM mode can be suppressed.

[0070] exist Figure 4 The example illustrates the use of an X-cut LN in the waveguide layer, but for devices operating in TE mode, polarization rotation can be suppressed by controlling the thickness of the waveguide layer in the same way, regardless of the material of the waveguide layer.

[0071] Depend on Figure 4 It is known that the effective refractive index of the TE mode in Z-propagation is always higher than that of the TM mode. That is, when the light wave propagates only in the Z-axis direction, polarization rotation of the TE mode does not occur. However, as with Z-propagation and Y-propagation, in a continuous optical waveguide, when the direction of light wave propagation changes, polarization rotation occurs in the region where the effective refractive index is reversed. Figure 1 As shown, the present invention can be more suitable for optical waveguide elements with a bending angle of more than 90 degrees.

[0072] Furthermore, the wavelength λ of light is not limited to λ = 1.55 μm. Even in its vicinity, such as C-band (1.52–1.57 μm) and L-band (1.57–1.61 μm), the dispersion curves show approximately the same tendency when the waveguide substrate thickness is normalized relative to the wavelength. Moreover, Figure 4 The thickness (t1) of the 0.7 μm waveguide layer, when referenced to the wavelength of light λ = 1.55 μm, is approximately 0.45 times. That is, it can be understood as the thickness of the optical waveguide in the direction parallel to the fundamental mode (TM mode) with lower electro-optic effect. Figure 3 The t1 is preferably less than 0.45 times the wavelength of the light wave propagating in the optical waveguide.

[0073] Preferably, for Figure 3 Regarding the cross-sectional shape of the optical waveguide shown, the thickness (width W) of the waveguide parallel to the direction with higher electro-optic effect (TE mode, left-right direction in the attached figure) is greater than the thickness (height t1) of the waveguide parallel to the direction with lower electro-optic effect (TM mode, top-bottom direction in the attached figure). If the width (W) of the rib portion is widened, the effective refractive index of the TE mode becomes relatively higher, and the difference in effective refractive index between modes becomes larger, thus more effectively suppressing the occurrence of polarization rotation.

[0074] Figure 3 The optical waveguide 2 has a substrate 10 serving as a cladding layer disposed below the waveguide layer (core) 2. The refractive index n2 of the substrate 10 is preferably 0.8 times or less than the refractive index n1 of the waveguide layer. This is because the smaller the refractive index of the cladding layer, the lower the effective refractive index of the TM mode, and the greater the difference in effective refractive index between modes.

[0075] like Figure 5 As shown, when a low-refractive-index layer 11 is configured as an upper cladding layer on the upper side of waveguide layer 1, the refractive index n3 of this low-refractive-index layer 11 is preferably 0.8 times or less than the refractive index n1 of waveguide layer 1. Furthermore, even when the upper cladding layer 11 is covered with a material transparent to the wavelength of light (SiO2, resin, etc.), or when a material with a refractive index (n2) different from that in the embodiment (n2 < 1.7) is used in the lower cladding layer, the relationship of the dispersion curves shows a similarity to... Figure 4 Broadly the same tendency. Additionally, such as Figure 5 As shown, the lower cladding layer may not be a single substrate, but rather an intermediate layer 12 between the waveguide layer 1 and the substrate 13. In this case, the substrate can be made of a material with a higher refractive index than the waveguide layer. For example, a combination of SiO2 for the intermediate layer and Si for the substrate can be used.

[0076] In addition, such as Figure 6As shown, a thin film 14 can also be used to cover the surface for purposes such as smoothing the waveguide surface. SiO2 or similar materials can be used as the material for the thin film 14.

[0077] like Figure 3 As shown, regarding the cross-sectional shape of the optical waveguide, as an example, the waveguide layer 1 is composed of a thin film portion (plate portion) 101 that contacts the substrate 10 and a rib portion 102 protruding from the film portion. The thickness (t2) of the film portion is preferably 0.7 times or less than the overall thickness (t1) of the waveguide layer (1). Depending on the situation, the film portion may be omitted. The thinner the film portion 101, the stronger the light confinement of the rib portion, which can suppress light leakage at the bend and also suppress light absorption at the electrode action portion.

[0078] Moreover, such as Figure 7 As shown, grooves 15 are preferably formed on both sides of the rib portion 102 in the film portion 101. This also has the effect of enhancing the light confinement of the rib portion.

[0079] Figure 3 The angle θ formed by the side of the ribbed waveguide relative to the contact surface between the substrate 10 and the waveguide layer 1 is preferably 50 degrees or more, avoiding the 45 degrees that easily leads to polarization rotation. More ideally, the angle θ is closer to 90 degrees. This is because when the angle θ is close to 90 degrees, polarization rotation at the bend of the waveguide is less likely to occur.

[0080] Figure 8 These are examples of optical waveguide components with various patterns. Figure 8 In (a), signal electrode S and ground electrodes (G1, G2) are shown as electrodes. Figure 8 In (b) to (d), only the signal electrodes are shown. Additionally, the arrow Lin indicates the incident light wave, and the hollow arrow indicates the input electrical signal.

[0081] Figure 8 (a) is a diagram showing the optical waveguide wiring bent at 90 degrees, with one of the input and output components positioned on the side. Of course, the same operation can be achieved even if the light wave is input from a direction opposite to that indicated by Lin. Figure 8 (b) is a diagram showing the input and output configured on the same end face by turning back 180 degrees. Figure 8 (c) is a diagram showing the folded-back branch waveguide section of a Mach-Zehnder type optical waveguide. Figure 8 In (c), two signal electrodes are used, but either one can be used. Furthermore, Figure 8 Example (d) is an example of multiple stages of reversal. Thus, the present invention can be appropriately applied when the direction of light wave travels changes midway, particularly in regions where the directions differ by more than 90 degrees within an optical waveguide.

[0082] In addition, not limited to Figure 8 The optical waveguide element shown, which incorporates a Mach-Zehnder interferometer (MZI interferometer), can also be applied to phase modulators without an MZI interferometer. Furthermore, it can also be applied to... Figure 9 The optical modulator shown is an integrated MZI interferometer capable of multi-level modulation.

[0083] Figure 10 express Figure 8 The sectional view at the dashed line B-B' in (a). Figure 3 , Figure 8 In this design, since X-cut (waveguide layer) is primarily used, the electrodes (signal electrode S, ground electrodes G1, G2) that apply an electric field to the ribbed waveguide 2 are positioned on the side of the ribbed waveguide in a manner that sandwiches the ribbed waveguide. In LN, LT, etc., this invention is not limited to X-cut; Y-cut can also be applied. Furthermore, even with Z-cut, the direction of the electric field applied to the waveguide (Z-axis direction) simply becomes... Figure 3 In the vertical direction, this invention can also suppress polarization rotation. However, in the case of Z-cutting, since the optical waveguide only changes from the X-axis to the Y-axis, polarization rotation will not occur due to the different directions of light travel. Of course, in order to prevent polarization rotation even with Z-cutting, the cross-sectional shape of the optical waveguide needs to be set to optimally determine the refractive index between the TE mode and the TM mode.

[0084] Figure 11 This is an example illustrating the bending method of optical waveguide 2. For example... Figure 11 As shown in (a), the waveguide is generally bent with the same curve (radius r), but preferably the minimum bending radius is 300 μm or less. More preferably, it is 200 μm or less, and it can also be 100 μm or less. Therefore, even with a limited substrate (chip) width, the direction of travel of the waveguide can be changed in any direction. In particular, as... Figure 11 As shown in (b) and (c), by setting the range of the curved portion (dashed lines C, C', D, D', D”) to a minimum and using more straight portions, the layout design of the optical waveguide becomes easier.

[0085] The optical waveguide element of the present invention has a modulation electrode disposed on the substrate 10 (waveguide layer 1) for modulating the light wave propagating in the optical waveguide, and as shown in the figure. Figure 12 It is housed within the casing 8 as shown. Furthermore, by providing an optical fiber (F) that inputs and outputs light waves relative to the optical waveguide, an optical modulation device MD can be constructed. The optical fiber not only... Figure 12 It can be positioned on the outside of the housing 8, or it can be inserted into the housing for configuration and fixation through the through hole in the side wall of the housing.

[0086] An optical transmitting device (OTA) can be constructed by connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal that causes the optical modulator MD to modulate the signal to the optical modulator MD. Since the modulation signal applied to the optical waveguide element needs to be amplified, a drive circuit (DRV) is used. The drive circuit (DRV) and the digital signal processor (DSP) can be disposed outside the housing 8, or they can be disposed inside the housing 8. In particular, by disposing of the drive circuit (DRV) inside the housing, the propagation loss of the modulation signal from the drive circuit can be further reduced.

[0087] Industrial applicability

[0088] As described above, according to the present invention, it is possible to provide an optical waveguide element that suppresses polarization rotation. Furthermore, it is also possible to provide an optical modulation device and an optical transmission device using the optical waveguide element.

Claims

1. An optical waveguide element comprising a substrate and an optical waveguide disposed on the substrate, characterized in that, A waveguide layer formed of a material with electro-optic effect is disposed on the substrate, and the optical waveguide is a rib-shaped optical waveguide disposed on the waveguide layer. The waveguide layer is an X-cut lithium niobate crystal or an X-cut lithium tantalate crystal. Light waves with a specific polarization plane propagate in this optical waveguide. The optical waveguide has an effective refractive index variation section. Let the fundamental mode of the TE mode and the TM mode, which are the fundamental modes of the light wave propagating in the optical waveguide and are parallel to the polarization plane, be the fundamental mode A. The effective refractive index variation section is the part of the effective refractive index of the optical waveguide under the fundamental mode A that changes with the propagation direction of the light wave. In the effective refractive index variation section, the cross-sectional shape of the optical waveguide perpendicular to the propagation direction of the light wave is as follows: the effective refractive index of the optical waveguide under the fundamental mode A is higher than the effective refractive index of the optical waveguide under the fundamental mode of the TM mode perpendicular to the fundamental mode A, i.e., the fundamental mode B.

2. The optical waveguide element according to claim 1, characterized in that, Throughout the entire effective refractive index variation region, the high-low relationship of each effective refractive index related to the fundamental mode A and the fundamental mode B is satisfied.

3. The optical waveguide element according to claim 1 or 2, characterized in that, Electrodes are formed on the waveguide layer to apply an electric field to the ribbed optical waveguide.

4. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide has portions within at least one continuous optical waveguide where the propagation directions of the light waves differ by more than 90 degrees.

5. The optical waveguide element according to claim 1 or 2, characterized in that, The cross-sectional shape of the effective refractive index change section of the optical waveguide is such that the thickness of the optical waveguide in the direction parallel to the fundamental mode B is less than 0.45 times the wavelength of the light wave propagating in the optical waveguide.

6. The optical waveguide element according to claim 1 or 2, characterized in that, The cross-sectional shape of the effective refractive index change section of the optical waveguide is such that the thickness of the optical waveguide in the direction parallel to the fundamental mode A is greater than the thickness of the optical waveguide in the direction parallel to the fundamental mode B.

7. The optical waveguide element according to claim 1 or 2, characterized in that, The refractive index of the substrate is less than 0.8 times that of the waveguide layer forming the optical waveguide.

8. The optical waveguide element according to claim 1 or 2, characterized in that, A low-refractive-index layer is disposed on the side of the waveguide layer that forms the optical waveguide opposite to the substrate. The refractive index of the low-refractive-index layer is less than 0.8 times that of the refractive index of the waveguide layer.

9. The optical waveguide element according to claim 1 or 2, characterized in that, The waveguide layer forming the optical waveguide consists of a thin film portion in contact with the substrate and a rib portion protruding from the thin film portion. The thickness of the thin film portion is less than 0.7 times the overall thickness of the waveguide layer.

10. The optical waveguide element according to claim 9, characterized in that, Grooves are formed on both sides of the rib portion in the membrane portion.

11. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide is a ribbed optical waveguide, and the angle between the side of the ribbed optical waveguide and the contact surface between the substrate and the waveguide layer forming the optical waveguide is 50 degrees or more.

12. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide has a bend with a minimum bending radius of less than 300 μm.

13. The optical waveguide element according to claim 3, characterized in that, The electrode that applies an electric field to the optical waveguide is positioned on the transverse side of the optical waveguide in a manner that clamps the optical waveguide.

14. An optical modulation device, characterized in that, The optical modulation device uses the optical waveguide element as described in any one of claims 1 to 13, the optical waveguide element is housed in a housing, and has an optical fiber for inputting or outputting optical waves relative to the optical waveguide.

15. The optical modulation device according to claim 14, characterized in that, The optical modulator has an electronic circuit inside the housing that amplifies the modulation signal input to the optical waveguide element.

16. An optical transmitting device, characterized in that, have: The optical modulation device according to claim 14 or 15; and The electronic circuit outputs a modulation signal that causes the optical modulator to perform modulation.

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