Optical Modulator

Abstract

The present invention provides an optical modulator including a substrate (10), a plurality of electro-optic material layers (20) formed on the substrate, and electrodes (4a, 4b, 5a, 5b) formed on the electro-optic material layers, wherein the electro-optic material layer (20) has a patterned RF section waveguide to which a modulation signal is applied and a patterned DC section waveguide to which a DC bias signal is applied, and in a cross section perpendicular to the light transmission direction, the cross-sectional area of ​​the DC section waveguides (20b1-3) is larger than the cross-sectional area of ​​the RF section waveguides (20a1-3). The optical modulator provided by the present invention can suppress DC drift.

Description

[Technical Field]

[0001] Technical Field The present invention relates to optical modulators used in the fields of optical communication and optical measurement. [Background technology]

[0002] Background technology

[0002] With the widespread use of the Internet, communication traffic has increased significantly, and optical fiber communication has become very important. Optical fiber communication is a communication mode that converts electrical signals into optical signals and transmits the optical signals through optical fibers, and has the characteristics of wide bandwidth, low loss, and strong noise resistance.

[0003]

[0003] Due to these advantages, optical fiber communication has been applied to various products. A direct modulation mode using a semiconductor laser and an external modulation mode using an optical modulator are known as modes for optical modulation devices that convert electrical signals into optical signals. The direct modulation system does not require an optical modulator and is low cost, but has limitations in terms of high-speed modulation. Therefore, the external optical modulation mode is used for high-speed and long-distance applications.

[0004]

[0004] As an optical modulator, a Mach-Zehnder optical modulator has been put to practical use. This optical modulator has an optical waveguide formed near the surface of a lithium niobate single crystal substrate by titanium (Ti) diffusion. High-speed optical modulators with modulation speeds exceeding 40 Gb / s are commercially available. A Mach-Zehnder optical modulator uses an optical waveguide (Mach-Zehnder optical waveguide) with a Mach-Zehnder interferometer structure. A Mach-Zehnder interferometer is a device that splits light emitted from a light source into two beams, causes the two beams to travel different paths, and then recombines the two beams to cause interference. Mach-Zehnder optical modulators using a Mach-Zehnder interferometer are used to generate various modulated light.

[0005]

[0005] When a voltage is applied to a pair of electrodes, a sufficient electric field can be applied to the optical waveguide, and the half-wave voltage Vπ can be reduced. The half-wave voltage Vπ is the difference V1-V2 between the voltage V1 that maximizes the optical output and the voltage V2 that minimizes the optical output, and the drive voltage is proportional to the half-wave voltage Vπ. Therefore, a reduction in the half-wave voltage Vπ causes a reduction in the drive voltage.

[0006]

[0006] However, when a DC bias is applied to such an optical waveguide, there is a problem that the life of the optical modulation element is short due to a large DC drift. DC drift is a change in the optical output operating point over time. Patent Document 1 discloses a technique that can achieve a DC drift suppression effect by preventing the movement of mobile ions by removing at least a part of the waveguide to which a DC voltage is applied.

[0007]

[0007] However, when a DC bias is applied to such an optical waveguide, there is a problem of a short lifespan of the optical modulation element due to a large DC drift. The DC drift is a change in the optical output operating point over time. Patent Document 1 discloses a technique that can achieve the effect of suppressing the DC drift by preventing the movement of mobile ions by removing at least a part of the waveguide to which a DC voltage is applied. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2019 / 069815 Summary of the Invention [Problem to be solved by the invention]

[0009] Summary of the Invention However, the waveguide shape in the optical modulator of Patent Document 1 is not sufficiently effective in suppressing DC drift, and therefore there is a demand for a waveguide shape that can more effectively suppress DC drift. [Means for solving the problem]

[0010]

[0010] The present invention has been achieved in consideration of the above-mentioned problems, and an object of the present invention is to provide an optical modulator having a substrate, a plurality of electro-optical material layers formed on the substrate, and electrodes formed on the electro-optical material layers, wherein the electro-optical material layers have a patterned RF (radio frequency) portion waveguide to which a modulating signal is applied, and a patterned DC (direct current) portion waveguide to which a DC bias signal is applied, and in a cross section perpendicular to the light transmission direction, the cross-sectional area of ​​the DC portion waveguide is larger than the cross-sectional area of ​​the RF portion waveguide.

[0011]

[0011] Furthermore, in the optical modulator of the present invention, preferably, the DC section waveguide comprises a first layer portion formed on the substrate, a first intermediate raised portion formed by raising on the first layer portion, and a first uppermost raised portion formed by raising on the first intermediate raised portion, and the RF section waveguide comprises a second layer portion formed on the substrate, a second intermediate raised portion formed by raising on the second layer portion, and a second uppermost raised portion formed by raising on the second intermediate raised portion, and the height of the uppermost end of the first intermediate raised portion is higher than the height of the uppermost end of the second intermediate raised portion.

[0012] Furthermore, in the optical modulator of the present invention, it is preferable that the first uppermost protrusion of the DC portion waveguide is offset from the center of the first intermediate protrusion when viewed from the cross section.

[0013]

[0013] Furthermore, in the optical modulator of the present invention, multiple DC section waveguides are formed as waveguides on the substrate, and when each is viewed in cross section, the first uppermost protrusion is positioned biased toward the adjacent DC section waveguide.

[0014]

[0014] Furthermore, in the optical modulator of the present invention, preferably, a plurality of DC section waveguides are formed as waveguides on a substrate, and a third layer section is formed on the substrate between adjacent DC section waveguides, and the height of the third layer section located between the adjacent DC section waveguides is higher than the height of the first layer section located on the opposite side of the adjacent DC section waveguides.

[0015] Furthermore, in the optical modulator of the present invention, the plurality of waveguides are preferably one or more pairs of Mach-Zehnder waveguides.

[0016]

[0016] The advantages of the present invention are as follows: The optical modulator provided by the present invention can effectively suppress the DC drift of the optical modulator. [Brief explanation of the drawings]

[0017] BRIEF DESCRIPTION OF THE DRAWINGS [Figure 1a] FIG. 1(a) is a top view of an optical modulator 100 according to an embodiment of the present invention, illustrating only an optical waveguide. [Figure 1b] FIG. 1(b) is a top view of an optical modulator 100 according to an embodiment of the present invention, illustrating the entire optical modulator 100 including traveling wave electrodes. [Figure 2] FIG. 2 is a schematic cross-sectional view of the optical modulator 100 taken along lines X1-X1' and X2-X2' in FIGS. 1(a) and 1(b). [Figure 3] FIG. 3 is a schematic cross-sectional view of a waveguide of an optical modulator 200 according to another embodiment of the present invention. [Figure 4] FIG. 4 is a schematic cross-sectional view of a waveguide of an optical modulator 300 according to another embodiment of the present invention. [Figure 5] FIG. 5 is a schematic cross-sectional view of a waveguide of an optical modulator 400 according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0018] Description of the embodiment

[0018] An embodiment of the present invention will be described below with reference to the drawings.

[0019] 1(a) and 1(b) are top views of an optical modulator 100 (electro-optical device) according to an embodiment of the present invention. Fig. 1(a) illustrates only an optical waveguide, and Fig. 1(b) illustrates the entire optical modulator 100 including traveling wave electrodes.

[0020] 1(a) and 1(b), an optical modulator 100 includes a Mach-Zehnder optical waveguide 2 formed on a substrate 10 and having first and second optical waveguides 2a and 2b arranged parallel to each other, a first signal electrode 4a arranged along the first optical waveguide 2a, a second signal electrode 4b arranged along the second optical waveguide 2b, a first bias electrode 5a arranged along the first optical waveguide 2a, and a second bias electrode 5b arranged along the second optical waveguide 2b. The first signal electrode 4a and the second signal electrode 4b, together with the first optical waveguide 2a and the second optical waveguide 2b, constitute an RF interaction section 3ir of the Mach-Zehnder optical modulator. The first bias electrode 5a and the second bias electrode 5b, together with the first optical waveguide 2a and the second optical waveguide 2b, constitute a DC interaction section 3id of the Mach-Zehnder optical modulator.

[0021]

[0021] The Mach-Zehnder optical waveguide 2 is an optical waveguide having a Mach-Zehnder interferometer structure. The first and second optical waveguides 2a and 2b branch from the input optical waveguide 2i at a branching section 2c, and the first and second optical waveguides 2a and 2b are coupled to the output optical waveguide 2o via a multiplexing section 2d. The input light Si is branched at the branching section 2c, and the branched lights travel through the first and second optical waveguides 2a and 2b, respectively, and are then multiplexed at the multiplexing section 2d to form modulated light So, which is output from the output optical waveguide 2o.

[0022] The first and second signal electrodes 4a and 4b are linear electrode patterns that overlap the first and second optical waveguides 2a and 2b in a plan view, and both ends of the signal electrodes extend to electrode pads provided near the end surface of the substrate 10. That is, ends 4a1 and 4b1 of the first and second signal electrodes 4a and 4b are extended to electrode pads provided near the end surface of the substrate 10 to form a signal input port, and a driver circuit 9a is connected to the signal input port. Furthermore, the other ends 4a2 and 4b2 of the first and second signal electrodes 4a and 4b are extended to electrode pads provided near the end surface of the substrate 10 and connected to each other via a termination resistor 9. Therefore, the first and second signal electrodes 4a and 4b function as differential coplanar traveling wave electrodes.

[0023]

[0023] The first and second bias electrodes 5a and 5b are independent of the first and second signal electrodes 4a and 4b in order to apply a DC voltage (DC bias) to the first and second waveguides 2a and 2b. Ends 5a1 and 5b1 of the first bias electrode 5a and the second bias electrode 5b are extended to electrode pads provided near the end surface of the substrate 10 to form a DC bias input port, and a bias circuit 9c is connected to the DC bias input port. In this embodiment, the formation regions of the first bias electrode 5a and the second bias electrode 5b are provided closer to the output end of the Mach-Zehnder optical waveguide 2 than the formation regions of the first signal electrode 4a and the second signal electrode 4b, but they can also be provided on the input end side.

[0024] As described above, the first and second signal electrodes 4a and 4b constitute the RF section 3a that applies a radio frequency signal to the first and second waveguides 2a and 2b, and the first and second bias electrodes 5a and 5b constitute the DC bias section that applies a DC bias voltage to the first and second waveguides 2a and 2b. No signal electrode or bias electrode is provided in the intermediate section 3c between the RF section 3a and the DC section 3b.

[0025]

[0025] Differential signals (modulation signals) having the same absolute value (but different polarities) are input to one end of each of the first and second signal electrodes 4a and 4b. Since the first waveguide 2a and the second waveguide 2b are formed of a material having an electro-optic effect (e.g., lithium niobate), the refractive indexes of the first waveguide 2a and the second waveguide 2b change by +Δn and −Δn, respectively, due to the electric field applied to the first waveguide 2a and the second waveguide 2b, and the phase difference between the pair of optical waveguides changes. The signal light modulated by the change in phase difference is output from the output optical waveguide 2o.

[0026]

[0026] FIG. 2 is a schematic cross-sectional view of the optical modulator 100 taken along lines X1-X1' and X2-X2' in FIGS. 1(a) and 1(b).

[0027] 2, the optical modulator 100 according to this embodiment has a multilayer structure in which a substrate 10, a waveguide layer 20, a buffer layer 30, and an electrode layer 40 are stacked in this order. The substrate 10 is, for example, a sapphire substrate, and a waveguide layer 20 made of a lithium niobate film is formed on the surface of the substrate 10. In this case, the waveguide layer 20 corresponds to the first and second optical waveguides 2a and 2b in FIG. 1. More specifically, the RF section 3a of the first optical waveguide 2a in FIG. 1 corresponds to the waveguide layers 20a1-20a3 below the electrode section 4a in FIG. 2, and the RF section 3a of the second optical waveguide 2b in FIG. 1 corresponds to the waveguide layers 20a1-20a3 below the electrode section 4b in FIG. 2. The DC portion 3b of the first optical waveguide 2a in FIG. 1 corresponds to the waveguide layers 20b1-20b3 below the electrode portion 5a in FIG. 2, and the DC portion 3b of the second optical waveguide 2b in FIG. 1 corresponds to the waveguide layers 20b1-20b3 below the electrode portion 5b in FIG. 2. To prevent light propagating through the first and second optical waveguides 2a and 2b from being absorbed by the electrode portions 4a, 4b, 5a, and 5b, a buffer layer 30 is formed at least between the first and second optical waveguides 2a and 2b and the electrode portions 4a, 4b, 5a, and 5b. Therefore, the buffer layer 30 only needs to function as an intermediate layer between the optical waveguide and the signal electrode, and the material of the buffer layer can be selected from a wide range of nonmetallic materials. For example, the buffer layer may be a ceramic layer formed of an insulating material (e.g., metal oxide, metal nitride, or metal carbide). The material of the buffer layer may be a crystalline or amorphous material. In a more preferred embodiment, the buffer layer 30 may be made of a material having a refractive index lower than that of the waveguide layer 20 (for example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc.).

[0028]

[0028] The first and second signal electrodes 4a and 4b and the first and second bias electrodes 5a and 5b are provided on the electrode layer 40. In the RF section 3a, in order to modulate the light traveling in the first and second optical waveguides 2a and 2b, the first and second signal electrodes 4a and 4b face the first and second optical waveguides 2a and 2b via the buffer layer 30. In the DC section 3b, in order to apply a DC bias voltage to the light traveling in the first and second optical waveguides 2a and 2b, the first and second bias electrodes 5a and 5b face the first and second optical waveguides 2a and 2b via the buffer layer 30.

[0029]

[0029] As long as the waveguide layer 20 is formed of an electro-optic material, the type of the waveguide layer 20 is not particularly limited, but it is preferably formed of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optic constant and is therefore suitable as a constituent material for electro-optic devices (e.g., optical modulators). Below, a detailed description will be given of the configuration of the present invention when the waveguide layer 20 is formed of a lithium niobate film.

[0030]

[0030] As long as the substrate 10 has a refractive index lower than that of the lithium niobate film, the type of substrate 10 is not particularly limited. Preferably, the substrate 10 is a substrate on which the lithium niobate film can be formed as an epitaxial film, preferably a sapphire single crystal substrate or a silicon single crystal substrate. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film can be easily formed as a c-axis oriented epitaxial film on single crystal substrates with various crystal orientations. Since the c-axis oriented lithium niobate film has three-fold symmetry, it is preferable that the underlying single crystal substrate have the same symmetry. Therefore, the single crystal sapphire substrate preferably has a c-surface, and the single crystal silicon substrate preferably has a (111) surface.

[0031]

[0031] As used herein, the term "epitaxial film" refers to a film whose crystal orientation is aligned with respect to the underlying substrate or film. If the film surface is the XY plane and the film thickness direction is the Z axis, the crystals are aligned along the X, Y, and Z axes. For example, the presence of an epitaxial film can be confirmed by first measuring the peak intensity at the orientation position by 2θ-θ X-ray diffraction and then identifying the pole points.

[0032]

[0032] In detail, first, in 2θ-θ X-ray diffraction measurement, the total peak intensity excluding the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity on the target plane. For example, in a c-axis oriented epitaxial lithium niobate film, the peak intensity excluding the (00L) plane is 10% or less, preferably 5% or less, of the maximum peak intensity on the (00L) plane. (00L) is a general term that collectively refers to (001), (002), and other equivalent planes.

[0033] Next, poles must be observed in the measurement. Under the condition that peak intensity is confirmed at the first orientation position, only orientation in a single direction is indicated. Even if the first condition is met, if the in-plane crystal orientation is nonuniform, the X-ray intensity does not increase at a specific angle, and poles cannot be observed. Because LiNbO3 has a trigonal crystal system, single-crystal LiNbO3 (014) has three poles. It is known that lithium niobate films grow epitaxially in a so-called twin crystal state, in which crystals rotated 180 degrees around the c-axis are symmetrically bonded. In this case, three poles are symmetrically bonded to form six poles. When a lithium niobate film is formed on a single-crystal silicon substrate with a (100) plane, the substrate has four-fold symmetry, so 4 × 3 = 12 poles are observed. In this invention, lithium niobate films epitaxially grown in a twin crystal state are also considered epitaxial films.

[0034]

[0034] The composition of the lithium niobate film is LixNbAyOz. A represents an element other than Li, Nb, and O. The number x ranges from 0.5 to 1.2, preferably from 0.9 to 1.05. The number y ranges from 0 to 0.5. The number z ranges from 1.5 to 4, preferably from 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, either alone or in combination.

[0035]

[0035] It is desirable to form a lithium niobate film by a film formation method such as sputtering, CVD, or sol-gel. When the c-axis of the lithium niobate film is oriented perpendicular to the main surface of the single-crystal substrate, an electric field is applied parallel to the c-axis, thereby changing the optical refractive index in proportion to the strength of the electric field. When the single-crystal substrate is sapphire, the lithium niobate film is formed directly on the sapphire single-crystal substrate by epitaxial growth. When the single-crystal substrate is silicon, the lithium niobate film is formed by epitaxial growth on a cladding layer (not shown) formed on the substrate. The cladding layer (not shown) is formed of a material that has a lower refractive index than the lithium niobate film and should be suitable for epitaxial growth. Known methods for forming lithium niobate films include thinly polishing or slicing a lithium niobate single-crystal substrate. This method has the advantage of being able to obtain the same properties as a single crystal and is applicable to the present invention.

[0036] 2, in a cross section perpendicular to the light transmission direction, the cross-sectional area of ​​the waveguide layer 20 of the DC section 3b is larger than the cross-sectional area of ​​the waveguide layer 20 of the RF section 3a. By configuring the cross-sectional area of ​​the waveguide layer 20 of the DC section 3b to be larger, it is possible to reduce DC drift.

[0037]

[0037] The principle of reducing DC drift by increasing the cross-sectional area of ​​the waveguide layer 20 of the DC section 3b is not clear, but can be inferred as follows: Since the impedance of the waveguide layer 20 from the bias electrodes 5a and 5b of the DC section 3b to the ground electrode (not shown) increases, even after a long operation time, the change in impedance is small, and therefore the DC drift caused by the change in impedance in the waveguide layer 20 is reduced accordingly.

[0038] 2, the waveguide layer 20 of the DC section 3b includes a first layer portion 20b1 formed on the substrate 10, a first intermediate protrusion portion 20b2 formed by protruding from the first layer portion 20b1, and a first uppermost protrusion portion 20b3 formed by protruding from the first intermediate protrusion portion 20b2. In this specification, the first layer portion 20b1 is formed thinly on the substrate 10 and may be, for example, but is not limited to, formed in a flat shape or may be a slightly inclined layer. The first intermediate protrusion portion 20b2 is formed by protruding from the first layer portion 20b1. For example, the first intermediate protrusion portion 20b2 may first protrude upward from the first layer portion 20b1 in a trapezoidal shape and then further protrude upward in a columnar shape. However, the shape of the first intermediate ridge 20b2 is not limited thereto, and the first intermediate ridge 20b2 may be trapezoidal or columnar, or may be raised in more stages. The first uppermost ridge 20b3 is the uppermost ridge and may be trapezoidal or columnar, for example. Although trapezoidal or columnar ridges are described herein, the trapezoidal or columnar shapes are not limited to conical or cylindrical shapes. The ridges may always have a shape with a horizontal cross section such as a trapezoid, triangle, or rectangle. Furthermore, the ridges may have shapes such as a frustum. The same applies to the following description.

[0039] The waveguide layer 20 of the RF section 3a includes a second layer portion 20a1 formed on the substrate 10, a second intermediate protrusion portion 20a2 formed by protruding from the second layer portion 20a1, and a second uppermost protrusion portion 20a3 formed by protruding from the second intermediate protrusion portion 20a2. In this specification, the second layer portion 20a1 is formed thinly on the substrate 10 and may be formed, for example, in a flat shape, but is not limited thereto, or may be a slightly inclined layer. The second intermediate protrusion portion 20a2 is formed by protruding from the second layer portion 20a1. For example, the second intermediate protrusion portion 20a2 may first protrude upward from the second layer portion 20a1 in a trapezoidal shape, and the inclination angle of the second intermediate protrusion portion 20a2 may be smaller than the inclination angle of the first intermediate protrusion portion 20b2. That is, the second intermediate protrusion 20a2 protrudes upward in a manner that is gentler (flatter) than the inclination angle of the first intermediate protrusion 20b2. However, the shape of the second intermediate protrusion 20a2 is not limited thereto, and the inclined side of the second intermediate protrusion 20a2 may be formed in a curved shape. The second uppermost protrusion 20a3 is the uppermost protrusion, and may protrude in a trapezoidal or columnar shape, for example.

[0040] 2, the height L1 of the top end of the first intermediate ridge 20b2 in the DC section 3b is higher than the height L2 of the top end of the second intermediate ridge 20a2 in the RF section 3a. That is, the bottom end of the first uppermost ridge 20b3 is located above the bottom end of the second uppermost ridge 20a3. As a result, the cross-sectional area of ​​the first intermediate ridge 20b2 is larger than the cross-sectional area of ​​the second intermediate ridge 20a2, which increases the cross-sectional area of ​​the waveguide layer 20 in the DC section 3b and reduces DC drift.

[0041] 3 is a schematic cross-sectional view of a waveguide of an optical modulator 200 according to another embodiment of the present invention. The cross-sectional shape of the RF section 3a of the optical modulator 200 is exactly the same as the cross-sectional shape of the RF section 3a of the optical modulator 100, and therefore, description thereof will be omitted in this specification.

[0042]

[0042] Referring to FIG. 3, in the waveguide layer 20 of the DC section 3b of the optical modulator 200, in a cross section perpendicular to the light transmission direction, the first uppermost protrusion 20b3 is b2 In the example of FIG. 3, both of the pair of first uppermost ridges 20b3 below the pair of bias electrodes 5a and 5b of the DC section 3b are configured to be offset from the center. Alternatively, one of the pair of first uppermost ridges 20b3 below the pair of bias electrodes 5a and 5b of the DC section 3b may be configured to be offset from the center. Furthermore, both of the two first uppermost ridges 20b3 may be, for example, close to the inside or outside of the pair of bias electrodes 5a and 5b of the DC section 3b.

[0043]

[0043] Furthermore, from the viewpoint of increasing the impedance of the waveguide layer 20 from the bias electrodes 5a and 5b to the ground electrode (not shown, for example, installed outside the bias electrodes 5a and 5b), it is preferable to position the first uppermost ridge 20b3 closer to the adjacent DC section waveguide, that is, to shift both of the two first uppermost ridges 20b3 inside the pair of bias electrodes 5a and 5b of the DC section 3b.

[0044]

[0044] In an optical modulator 200 according to another embodiment of the present invention, the impedance of the waveguide layer 20 from a pair of bias electrodes 5a and 5b of the DC section 3b to a ground electrode (not shown) is increased, thereby suppressing changes in the impedance of this portion of the waveguide layer 20 and reducing DC drift.

[0045] 4 is a schematic cross-sectional view of a waveguide of an optical modulator 300 according to another embodiment of the present invention. The cross-sectional shape of the RF section 3a of the optical modulator 300 is exactly the same as the cross-sectional shape of the RF section 3a of each of the optical modulators 100 and 200, and therefore a description thereof will be omitted in this specification.

[0046]

[0046] As shown in Figure 4, the difference between optical modulator 300 and optical modulator 200 is that the third layer portion 20c1 formed on the substrate 10 is formed between the waveguides of adjacent DC sections 3b, and the height of the third layer portion 20c1 installed between the waveguides of adjacent DC sections 3b is higher than the height of the first layer portion 20b1 installed on the opposite side of the waveguides of adjacent DC sections 3b.

[0047]

[0047] In detail, the third layer portion 20c1 is formed between the pair of first intermediate protuberances 20b2 of the DC portion 3b, and the height of the third layer portion 20c1 is higher than the height of the first layer portion 20b1 located on both sides of the pair of first intermediate protuberances 20b2. When a bias voltage is applied to the pair of bias electrodes 5a and 5b in a differential manner, the thickness of the electro-optic material layer between the waveguide layer 20 under the pair of bias electrodes 5a and 5b increases, so that the impedance at this position increases. Therefore, even after a long operating time, the impedance at this position has a small change compared to the initial impedance, thereby reducing the DC drift. [Example]

[0048] Example

[0048] In the comparative example and the example, an accelerated test of DC drift of the optical modulator was carried out. In the accelerated test, a DC bias was continuously applied at a high temperature of 100°C, and when the DC offset reached 50%, the duration was recorded.

[0049]

[0049] Fig. 5 is a schematic cross-sectional view of a waveguide of an optical modulator 400 according to a comparative example. The cross-sectional shape of the waveguide layer 20 of the optical modulator 400 has a shape in which a raised portion 20d2 is formed on a layer portion 20d1. The optical modulators of comparative examples A and B are optical modulators having the cross-sectional structure illustrated in Fig. 5.

[0050]

[0050] The optical modulators of Examples 1A and 1B are optical modulators having the cross-sectional structure illustrated in Figure 2. The optical modulators of Examples 2A and 2B are optical modulators having the cross-sectional structure illustrated in Figure 3. The optical modulators of Examples 3A and 3B are optical modulators having the cross-sectional structure illustrated in Figure 4. The materials used for the components of each Example and Comparative Example are the same.

[0051]

[0051] Table 1 shows the evaluation results of DC drift for each of the examples and comparative examples.

[0052] [Table 1]

[0053]

[0052] Table 1 shows that in Comparative Examples A and B, the DC drift reaches 50% (the ratio of the DC drift to the DC bias) after more than 40 hours. In contrast, in Examples 1A and 1B, in which the cross-sectional area of ​​the waveguide layer in the DC section is increased, the DC drift reaches 50% after about 90 hours. Furthermore, in Examples 2A and 2B, in which the top ridge is offset from the center, the DC drift reaches 50% after about 120 hours, further suppressing the DC drift compared to Examples 1A and 1B. Furthermore, in Examples 3A and 3B, in which a third (intermediate) layer section is provided between a pair of DC section waveguides, the DC drift reaches 50% after about 169 hours, further suppressing the DC drift.

[0054]

[0053] The present invention has been described above in detail with reference to the drawings and embodiments, but the above description is not intended to limit the present invention in any way. For example, in the above description of the optical modulators 100 to 300, the cross-sectional shape of a pair of Mach-Zehnder optical waveguides is taken as an example. However, the present invention is not limited to this example, and there may be one or more optical waveguides having the above-mentioned cross-sectional shapes. Those skilled in the art may modify and change the present invention as needed without departing from the essential spirit and scope of the present invention, and these modifications and changes fall within the scope of the present invention. [Explanation of symbols]

[0055]

[0054] Explanation of reference symbols 100, 200, 300, 400 Optical Modulator 10 Substrate 20 Waveguide layer 30 buffer layer 40 electrode layer 2a First optical waveguide 2b Second optical waveguide 2c Branch 2d combining section 2i input optical waveguide 2o output optical waveguide 3a RF section 3b DC section 4a First signal electrode 4b Second signal electrode 5a First bias electrode 5b Second bias electrode 20b1 first layer portion 20b2 first intermediate protuberance 20b3 First uppermost ridge 20a1 second layer 20a2 second intermediate protuberance 20a3 Second highest ridge 20c1 Third layer

Claims

1. A substrate; a plurality of electro-optic material layers formed on the substrate; an electrode formed on the electro-optical material layer, the electro-optical material layer has a patterned RF portion waveguide to which a modulation signal is applied and a patterned DC portion waveguide to which a DC bias signal is applied; In a cross section perpendicular to the light transmission direction, the cross-sectional area of ​​the DC section waveguide is larger than the cross-sectional area of ​​the RF section waveguide, the DC section waveguide comprises a first layer section formed on the substrate, a first intermediate protrusion section formed by protruding from the first layer section, and a first uppermost protrusion section formed by protruding from the first intermediate protrusion section; the RF section waveguide comprises a second layer section formed on the substrate, a second intermediate protrusion section formed by protruding from the second layer section, and a second uppermost protrusion section formed by protruding from the second intermediate protrusion section; The height of the uppermost end of the first intermediate protrusion is higher than the height of the uppermost end of the second intermediate protrusion. Optical modulator.

2. 2. The optical modulator according to claim 1, wherein the first uppermost ridge of the DC section waveguide is offset from the center of the first intermediate ridge when viewed from the cross section.

3. 3. The optical modulator according to claim 1, wherein a plurality of the DC section waveguides are formed as waveguides on the substrate, and when each of the DC section waveguides is viewed from the cross section, the first uppermost protrusion is positioned to one side toward the adjacent DC section waveguide.

4. a plurality of the DC section waveguides are formed as waveguides on the substrate, a third layer portion formed on the substrate between adjacent DC portion waveguides; a height of the third layer portion located between the adjacent DC section waveguides is greater than a height of the first layer portion located on the opposite side of the adjacent DC section waveguides; 4. The optical modulator according to claim 1.

5. 5. The optical modulator according to claim 3, wherein the plurality of waveguides are one or more pairs of Mach-Zehnder waveguides.

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