Optical modulator

Abstract

Provided is an optical modulator including a substrate, a plurality of electro-optic material layers formed on the substrate, and an electrode formed on the electro-optic material layers, the electro-optic material layers having an RF portion waveguide to which a modulation signal is applied and patterned and a DC portion waveguide to which a DC bias signal is applied and patterned, the RF portion waveguide being connected to the DC portion waveguide via a connection portion waveguide that is wider than a width of the RF portion waveguide in a cross section perpendicular to a propagation direction of light. The optical modulator according to the present application can suppress DC drift.

Description

Technical Field

[0001] This invention relates to an optical modulator for use in optical communication and optical measurement. Background Technology

[0002] With the widespread adoption of the internet, communication volume has increased dramatically, making fiber optic communication extremely important. Fiber optic communication is a communication method that converts electrical signals into optical signals and transmits them through optical fibers. It features wide bandwidth, low loss, and strong noise immunity.

[0003] Due to these advantages, fiber optic communication is used in a wide variety of products. As a method of converting electrical signals into optical signals using optical modulators, there are known direct modulation methods using semiconductor lasers and external modulation methods using optical modulators. While direct modulation does not require an optical modulator and is low-cost, it has limitations in high-speed modulation. External optical modulation methods are used for high-speed and long-distance applications.

[0004] As an optical modulator, a Mach-Zehnder type optical modulator, in which an optical waveguide is formed near the surface of a lithium niobate single-crystal substrate by Ti (titanium) diffusion, has been put into practical use (see Patent Document 1). A Mach-Zehnder type optical modulator is an optical modulator that uses an optical waveguide with a Mach-Zehnder interferometer structure (Mach-Zehnder optical waveguide). A Mach-Zehnder interferometer is a device that splits light emitted from a light source into two beams, which then overlap again to produce interference after traveling along different paths. Mach-Zehnder type optical modulators using Mach-Zehnder interferometers have been used to generate various modulated lights. High-speed optical modulators exceeding 40 Gb / s have been commercialized, but a major drawback is their total length, which is approximately 10 cm.

[0005] To address this issue, Patent Document 2 discloses a Mach-Zehnder type optical modulator using a c-axis oriented lithium niobate film. Compared to optical modulators using a lithium niobate single-crystal substrate, the optical modulator using a lithium niobate film can achieve a significant reduction in size and driving voltage.

[0006] When a voltage is applied between 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 between V1-V2, which is the voltage V1 that maximizes light output and V2 that minimizes light output. The driving voltage is proportional to the half-wave voltage Vπ. Therefore, reducing the half-wave voltage Vπ means a lower driving voltage.

[0007] In such optical devices, the voltage applied to the optical waveguide changes immediately after the voltage is applied to the electrodes and after a sufficiently long period of time, resulting in a change in the light emitted from the optical modulator. This voltage change applied to the optical waveguide is called DC drift, and it is desirable for the optical modulator to suppress DC drift as much as possible.

[0008] However, when a DC bias is applied to such an optical waveguide, the large DC drift results in a short lifespan for the optical modulation element. DC drift is the change in the optical output operating point over time. Patent Document 3 discloses an optical modulator with a device structure capable of stable control over a long period by reducing DC drift. By removing at least a portion of the waveguide to which the DC voltage is applied, the movement of mobile ions can be prevented, thus achieving a DC drift suppression effect.

[0009] However, the total waveguide length in the optical modulator of Patent Document 1 is about 10 cm, and the suppression effect on DC drift is not sufficient. A waveguide shape that can better suppress DC drift is desired.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 2005-221874

[0013] Patent Document 2: Japanese Patent Application Publication No. 2006-195383

[0014] Patent Document 3: WO2019 / 069815 Summary of the Invention

[0015] The present invention was made in view of the above-mentioned problems, and its object is to provide an optical modulator having a waveguide shape that can better suppress DC drift.

[0016] The optical modulator of the present invention includes: a substrate; a plurality of electro-optic material layers formed on the substrate; and electrodes formed on the electro-optic material layers, wherein the electro-optic material layers have an RF waveguide patterned for applying a modulation signal and a DC waveguide patterned for applying a DC bias signal, and in a cross section perpendicular to the direction of light propagation, the RF waveguide is connected to the DC waveguide via a connecting waveguide wider than the width of the RF waveguide.

[0017] Furthermore, in the optical modulator of the present invention, preferably, the height of the RF section waveguide is the same as the height of the connection section waveguide.

[0018] Furthermore, in the optical modulator of the present invention, preferably, the width of the connecting waveguide is more than 10 times the width of the RF waveguide.

[0019] Furthermore, in the optical modulator of the present invention, preferably, the connecting waveguide is continuous with the DC waveguide of equal width and extends on the substrate.

[0020] Furthermore, in the optical modulator of the present invention, preferably, a protrusion is formed on the upper surface of the DC waveguide on the side opposite to the substrate side, protruding from the upper surface and extending in the same direction as the extension direction of the RF waveguide, wherein the center line of the width direction of the protrusion is located on the same straight line as the center line of the width direction of the RF waveguide.

[0021] Furthermore, in the optical modulator of the present invention, preferably, the end of the protrusion on the waveguide side of the RF section gradually decreases towards the waveguide side of the RF section.

[0022] Furthermore, in the optical modulator of the present invention, preferably, a groove extending in the same direction as the extending direction of the RF waveguide is formed on the upper surface of the DC waveguide on the side opposite to the substrate side, and the center line of the groove in the width direction is on the same straight line as the center line of the RF waveguide in the width direction.

[0023] The optical modulator according to the present invention can effectively suppress DC drift in the optical modulator. Attached Figure Description

[0024] Figures 1(a) and 1(b) are top views of the optical modulator 100 according to the first embodiment of the present invention. Figure 1(a) shows only the optical waveguide, and Figure 1(b) shows the entire optical modulator 100 including the traveling wave electrode.

[0025] Figure 2 This is a schematic cross-sectional view of the optical modulator 100 along the X1-X1', X2-X2' and X3-X3' lines in Figures 1(a) and 1(b).

[0026] Figure 3 This is a schematic cross-sectional view of the waveguide of the optical modulator 200 according to the second embodiment of the present invention.

[0027] Figures 4(a) and 4(b) are schematic cross-sectional views of the connection waveguide of the optical modulator 200 according to the second embodiment of the present invention.

[0028] Figure 5 This is a schematic cross-sectional view of the waveguide of the optical modulator 300 according to the third embodiment of the present invention.

[0029] Figure 6 This is a schematic cross-sectional view of the waveguide of the comparative example optical modulator 400. Detailed Implementation

[0030] Hereinafter, with reference to the accompanying drawings, the manner in which the present invention is carried out will be described in detail.

[0031] Figures 1(a) and 1(b) are top views of the optical modulator (optoelectronic device) 100 according to the first embodiment of the present invention. Figure 1(a) shows only the optical waveguide, and Figure 1(b) shows the entire optical modulator 1 including the traveling wave electrode.

[0032] As shown in Figure 1(b), the optical modulator 1 includes: a Mach-Zehnder optical waveguide 2, having first and second optical waveguides 2a and 2b formed on a substrate 1 and 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 the 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 the DC interaction section 3id of the Mach-Zehnder optical modulator.

[0033] The Mach-Zehnder waveguide 2 is an optical waveguide with a Mach-Zehnder interferometer structure. It has first and second optical waveguides 2a and 2b branching from an input waveguide 2i through a wave-splitting section 2c. The first and second optical waveguides 2a and 2b are combined into an output optical waveguide 2o via a combiner section 2d. The input light Si is split by the wave-splitting section 2c and travels through the first and second optical waveguides 2a and 2b respectively. Then, it is combined at the combiner section 2d and output as modulated light So from the output optical waveguide 2.

[0034] The first and second signal electrodes 4a and 4b are linear electrode patterns overlapping the first and second waveguides 2a and 2b in the top view, and their ends extend to electrode pads disposed near the end face of the substrate 10. That is, one end 4a1 and 4b1 of the first signal electrode 4a and the second signal electrode 4b are led out to the electrode pads disposed near the end face of the substrate 10 to form a signal input port, and the drive circuit 9a is connected to the signal input port. Furthermore, the other ends 4a2 and 4b2 of the first signal electrode 4a and the second signal electrode 4b are led out to the electrode pads disposed near the end face of the substrate 10 and connected to each other via terminating resistor 9b. Thus, the first and second signal electrodes 4a and 4b function as differential coplanar traveling wave electrodes.

[0035] The first and second bias electrodes 5a and 5b are independent of the first and second signal electrodes 4a and 4b, so as to apply a DC voltage (DC bias) to the first and second waveguides 2a and 2b. One end 5a1 and 5b1 of the first bias electrode 5a and the second bias electrode 5b are led out to an electrode pad disposed near the end face of the substrate 10 to form a DC bias input port, and the bias circuit 9c is connected to the DC bias port. In this embodiment, the formation regions of the first bias electrode 5a and the second bias electrode 5b are set closer to the output end side of the Mach-Zehnder optical waveguide 2 than the formation regions of the first signal electrode 4a and the second signal electrode 4b. However, they can also be disposed on the input end side.

[0036] As described above, the first and second signal electrodes 4a and 4b constitute an RF waveguide 3a that applies RF signals to the first and second waveguides 2a and 2b, and the first and second bias electrodes 5a and 5b constitute a DC waveguide 3b that applies DC bias voltage to the first and second waveguides 2a and 2b. No signal electrodes or bias electrodes are provided in the intermediate portion between the RF waveguide 3a and the DC waveguide 3b, but a connecting waveguide 3c is provided for connecting the RF waveguide 3a and the DC waveguide 3b.

[0037] Differential signals (modulated signals) with the same absolute value but different signs are input to one end of the first and second signal electrodes 4a and 4b. Since the first waveguide 2a and the second waveguide 2b are made of a material with an electro-optic effect, such as lithium niobate, their refractive indices change according to the electric fields applied to them, such as +Δn and -Δn, respectively, and the phase difference between the pair of optical waveguides changes. The signal light modulated by this phase difference is output from the output waveguide 2o.

[0038] Figure 2 This is a schematic cross-sectional view of the optical modulator 100 along the X1-X1', X2-X2' and X3-X3' lines in Figures 1(a) and 1(b).

[0039] like Figure 2 As shown, the optical modulator 1 of this embodiment has a multilayer structure in which a substrate 10, a waveguide layer (electro-optic material 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 lithium niobate film is formed on the surface of the substrate 1.

[0040] Here, waveguide layer 20 corresponds to the first and second optical waveguides 2a and 2b in Figure 1(a). More specifically, the RF section waveguide 3a of the first optical waveguide 2a in Figure 1(b) corresponds to Figure 2 Waveguide layer 20a located below electrode section 4a 1~3In Figure 1(b), the RF waveguide 3a of the second optical waveguide 2b corresponds to... Figure 2 Waveguide layer 20a located below electrode section 4b 1~3 In Figure 1(b), the DC section waveguide 3b of the first optical waveguide 2a corresponds to... Figure 2 Waveguide layer 20b located below electrode section 5a 1~3 In Figure 1(b), the DC section waveguide 3b of the second optical waveguide 2b corresponds to... Figure 2 Waveguide layer 20b located below electrode section 5b 1~3 In Figure 1(b), the connecting waveguide 3c of the first and second optical waveguides 2a and 2b corresponds to... Figure 2 Medium waveguide layer 20c 1~3 .

[0041] To prevent light propagating in 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 waveguides and the signal electrodes, and the material of the buffer layer can be widely selected as long as it is non-metallic. For example, the buffer layer can be a ceramic layer made of insulating materials such as metal oxides, metal nitrides, and metal carbides. The buffer layer material can be a crystalline material or an amorphous material. As a more preferred embodiment, the buffer layer 30 can be a material with a refractive index lower than that of the waveguide layer 20, such as Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc.

[0042] In electrode layer 4, first and second signal electrodes 4a and 4b, and first and second bias electrodes 5a and 5b are provided. In RF waveguide 3a, the first and second signal electrodes 4a and 4b are opposite to the first and second optical waveguides 2a and 2b via buffer layer 3 to modulate the light traveling in the first and second optical waveguides 2a and 2b. In DC waveguide 3b, the first and second bias electrodes 5a and 5b are opposite to the first and second optical waveguides 2a and 2b via buffer layer 3 to provide DC bias to the light traveling in the first and second optical waveguides 2a and 2b.

[0043] The waveguide layer 2 can be made of any electro-optic material without particular limitations, but it is preferably composed of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optical constant and is suitable as a constituent material for optical devices such as optical modulators. Hereinafter, the structure of the present invention when the waveguide layer 2 is a lithium niobate film will be described in detail.

[0044] As substrate 1, there is no particular limitation as long as the refractive index is lower than that of the lithium niobate film. It is preferable to be a substrate that allows the lithium niobate film to be formed into 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 has the property of being easily formed into a c-axis oriented epitaxial film with respect to single crystal substrates of various crystal orientations. The c-axis oriented lithium niobate film has a three-dimensional symmetry. Therefore, it is preferable that the single crystal substrate of the substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a substrate with the c-plane is preferred; in the case of a silicon single crystal substrate, a substrate with the (111) plane is preferred.

[0045] Here, the epitaxial film is a film whose crystal orientation is consistent with that of the substrate or base film relative to the substrate. When the in-plane of the film is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals are aligned in the X, Y, and Z axis directions. For example, the epitaxial film can be verified by first confirming the peak intensity at the orientation position using 2θ-θ X-ray diffraction, and then confirming the poles.

[0046] Specifically, when performing measurements using 2θ-θ X-ray diffraction, the intensity of all peaks outside the target plane must be less than 10% of the maximum peak intensity of the target plane, preferably less than 5%. For example, in a c-axis oriented epitaxial film of lithium niobate, the peak intensity outside the (00L) plane is less than 10% of the maximum peak intensity of the (00L) plane, preferably less than 5%. (00L) is a general term for equivalent planes such as (001) or (002).

[0047] Secondly, poles need to be observed in pole determination. Under the condition of confirming the peak intensity at the first orientation position mentioned above, it only indicates orientation in one direction. Even if the first condition is obtained, the intensity of X-rays at a specific angle position is not high when the in-plane crystal orientation is inconsistent, and poles cannot be observed. Since LiNbO3 is a trigonal crystal structure, there are 3 poles in LiNbO3(014) in a single crystal. In the case of lithium niobate film, it is known that epitaxial growth is performed in a state of so-called bicrystalline state where the crystals are symmetrically combined after rotating 180° around the c-axis. In this case, it becomes a state where 2 of the 3 poles are symmetrically combined, so there are 6 poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate with (100) plane, since the substrate is four-fold symmetrical, 4×3=12 poles are observed. In addition, in this invention, the lithium niobate film epitaxially grown in a bicrystalline state is also included in the epitaxial film.

[0048] The lithium niobate film is composed of Li x NbA y O zA represents an element other than Li, Nb, and O. x is 0.5–1.2, preferably 0.9–1.05. y is 0–0.5. z is 1.5–4, preferably 2.5–303.5. Elements that can be A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, etc., or combinations of two or more.

[0049] As a method for forming lithium niobate films, sputtering, CVD, sol-gel methods, etc., are preferred. If the c-axis of lithium niobate is oriented perpendicular to the main surface of substrate 1, then by applying an electric field parallel to the c-axis, the optical refractive index changes proportionally to the electric field.

[0050] When sapphire is used as the single-crystal substrate, a lithium niobate film can be directly epitaxially grown on the sapphire single-crystal substrate. When silicon is used as the single-crystal substrate, a lithium niobate film is formed by epitaxial growth via a cladding layer (not shown). The cladding layer (not shown) uses a cladding layer with a refractive index lower than that of the lithium niobate film and is suitable for epitaxial growth. Furthermore, as a method for forming a lithium niobate film, a method of thinly grinding and slicing a lithium niobate single-crystal substrate is also known. This method has the advantage of obtaining properties similar to those of a single crystal and is applicable to the present invention.

[0051] In this embodiment, such as Figure 2 As shown, in a cross-section perpendicular to the direction of light propagation, the RF waveguide 3a includes: a first layered portion 20a1 formed on a substrate, a first intermediate raised portion 20a2 formed on the first layered portion 20a1, and a first uppermost raised portion 20a3 formed on the first intermediate raised portion 20a2. Here, the first layered portion 20a1 is formed relatively thinly on the substrate 10, and can be formed as a flat plate, but is not limited thereto; it can also be a slightly inclined layer. The first intermediate raised portion 20a2 is raised on the first layered portion 20a1, and for example, it can be raised upwards in a conical shape from the first layered portion 20a1 to a certain height. However, the shape of the first intermediate raised portion 20a2 is not limited thereto; it can be raised only in a conical or columnar shape, or it can be raised in multiple stages. The first uppermost raised portion 20a3 is the uppermost raised section, and can be raised in a columnar shape, for example. The description here uses the terms conical or columnar ridges, but conical or columnar is not limited to cones or cylinders. A ridge can also be a shape whose cross-sectional shape is always trapezoidal, triangular, or rectangular when viewed from the horizontal direction, or it can be a frustum-shaped ridge, etc. The same applies in the following description.

[0052] like Figure 2As shown, the DC waveguide 3b includes: a second layered portion 20b1 formed on a substrate, a second intermediate raised portion 20b2 formed on the second layered portion 20b1, and a second uppermost raised portion 20b3 formed on the second intermediate raised portion 20b2. Here, the second layered portion 20b1 is formed relatively thinly on the substrate 10, and can be formed as a flat plate, but is not limited to this; it can also be a slightly inclined layer. The second intermediate raised portion 20b2 is raised on the second layered portion 20b1, and for example, it can be raised upwards in a conical shape from the second layered portion 20b1 to a certain height. However, the shape of the second intermediate raised portion 20b2 is not limited to this; it can be raised only in a conical or columnar shape, or it can be further raised in multiple stages. The second uppermost raised portion 20b3 is the uppermost raised section, and can be raised in a columnar shape, for example.

[0053] like Figure 2 As shown, the connecting waveguide 3c includes: a third layered portion 20c1 formed on a substrate, a third intermediate raised portion 20c2 formed on the third layered portion 20c1, and a third uppermost raised portion 20c3 formed on the third intermediate raised portion 20c2. Here, the third layered portion 20c1 is formed relatively thinly on the substrate 10, and can be formed as a flat plate, but is not limited thereto; it can also be a slightly inclined layer. The third intermediate raised portion 20c2 is raised on the third layered portion 20c1, and can, for example, be raised upwards in a conical shape from the third layered portion 20c1 to a certain height. However, the shape of the third intermediate raised portion 20c2 is not limited thereto; it can be raised only in a conical or columnar shape, or it can be raised in multiple stages. The third uppermost raised portion 20c3 is the uppermost raised section, and can, for example, be raised in a columnar shape. Furthermore, the connecting waveguide 3c connects the RF waveguide 3a and the DC waveguide 3b. That is, the RF waveguide 3a is connected to the DC waveguide 3b via the connecting waveguide 3c.

[0054] In addition, such as Figure 2 As shown, the distance between the uppermost point of the first uppermost protrusion 20a3 of the RF waveguide 3a and the upper surface of the substrate 10 is the height of the RF waveguide 3a. Similarly, the distance between the uppermost point of the second uppermost protrusion 20b3 of the DC waveguide 3b and the upper surface of the substrate 10 is the height of the DC waveguide 3b. The waveguide layers 20 of both the RF waveguide 3a and the DC waveguide 3b are formed on the substrate 10 with a height approximately h. That is, the height of the DC waveguide 3b is approximately the same as the height of the waveguide layer 20 of the RF waveguide.

[0055] In addition, such as Figure 2As shown, the width of the uppermost end of the first uppermost ridge 20a3 of the RF waveguide 3a is the width of the RF waveguide 3a. The width of the uppermost end of the second uppermost ridge 20b3 of the DC waveguide 3b is the width of the DC waveguide 3b. The width of the uppermost end of the third uppermost ridge 20c3 of the connecting waveguide 3c is the width of the connecting waveguide 3c. The width of the RF waveguide 3a is formed approximately to be a width w1, and the width of the connecting waveguide 3c is formed approximately to be a width w3. Wherein, w3 > w1, and w3 is preferably 10 times or more than w1, more preferably 20 times or more than w1. The width of the DC waveguide 3b is formed approximately to be a width w2. w2 may be the same as or different from w3.

[0056] Furthermore, it is worth noting that as long as the width w3 of the connecting waveguide 3c is greater than the width w1 of the RF waveguide 3a, it is not necessary for the width w2 of the DC waveguide 3b to be greater than the width w1 of the RF waveguide 3a. The width w2 of the DC waveguide 3b can be greater than the width w1 of the RF waveguide 3a, equal to the width w1 of the RF waveguide 3a, or smaller than the width w1 of the RF waveguide 3a.

[0057] like Figure 2 As shown, in a cross-section perpendicular to the direction of light propagation, the width of the RF waveguide 3a is, for example, 1 μm, and the widths of the DC waveguide 3b and the connecting waveguide 3c are, for example, 20 μm. That is, the RF waveguide 3a is connected to the DC waveguide 3b via the connecting waveguide 3c, which is wider than the RF waveguide 3a. Furthermore, the connecting waveguide 3c extends on the substrate 10, continuous with the DC waveguide 3b, and with a width approximately equal to that of the DC waveguide 3b. Moreover, considering other structures in the Mach-Zehnder waveguide 2, the widths of the DC waveguide 3b and the connecting waveguide 3c are widened to, for example, 40 μm.

[0058] Thus, the widths w2 of the DC waveguide 3b and w3 of the connecting waveguide 3c are set to be greater than the width w1 of the RF waveguide 3a. By setting the widths of the DC waveguide 3b and the connecting waveguide 3c to be larger, DC drift can be reduced.

[0059] The principle by which DC drift can be reduced by increasing the width of the DC section waveguide 3b and the connecting section waveguide 3c is not yet clear. It can be speculated that: since the impedance of the waveguide layer 20 from the bias electrodes 5a and 5b of the DC section waveguide 3b to the ground electrode (not shown) increases, the change in impedance is small even after a long operating time, and the DC drift caused by the change in the internal impedance of the waveguide layer 20 is reduced accordingly.

[0060] Figure 3This is a schematic cross-sectional view of the waveguide of the optical modulator 200 according to the second embodiment of the present invention. The cross-sectional shape of the RF section waveguide 3a of the optical modulator 200 is exactly the same as that of the RF section waveguide 3a of the optical modulator 100, and its description is omitted here.

[0061] like Figure 3 As shown, in a cross-section perpendicular to the direction of light propagation, the DC waveguide 3b of the optical modulator 200, in addition to the second layered portion 20b1, the second intermediate raised portion 20b2, and the second uppermost raised portion 20b3 described in the first embodiment, also includes a protrusion 20b4. Specifically, the protrusion 20b4 protrudes in a conical or columnar shape from the upper surface of the second uppermost raised portion 20b3 on the side opposite to the substrate 10, and extends in the same direction as the extending direction of the RF waveguide 3a. Furthermore, the centerline of the protrusion 20b4 in the width direction of the DC waveguide 3b is on the same straight line as the centerline of the RF waveguide 3a in the width direction.

[0062] According to the second embodiment of the present invention, the optical modulator 200 has a light-guiding protrusion 20b4 provided in the DC waveguide 3b on the cross section perpendicular to the light propagation direction, thereby exhibiting a light-suppressing effect in the DC waveguide 3b and further reducing DC drift.

[0063] Figures 4(a) and 4(b) are schematic cross-sectional views of the connection waveguide 3c of the optical modulator 200 according to the second embodiment of the present invention.

[0064] The connecting waveguide 3c is provided for connecting the RF waveguide 3a and the DC waveguide 3b. Since the DC waveguide 3b of the optical modulator 200 according to the second embodiment has a protrusion 20b4, in order to smoothly connect the RF waveguide 3a and the DC waveguide 3b, the upper surface of the connecting waveguide 3c is shaped to gradually decrease in height from the end of the protrusion 20b4 on the RF waveguide 3a side toward the RF waveguide 3a side. The upper surface of the connecting waveguide 3c can be linearly decreasing in height from the end of the protrusion 20b4 on the RF waveguide 3a side toward the RF waveguide 3a side, as shown in FIG. 4(a), or it can be curvedly decreasing in height from the end of the protrusion 20b4 on the RF waveguide 3a side toward the RF waveguide 3a side, as shown in FIG. 4(b).

[0065] By smoothly connecting the protrusion 20b4 formed in the DC waveguide 3b to the RF waveguide 3a, the connecting waveguide 3c exhibits a light suppression effect, thereby further reducing DC drift.

[0066] Figure 5This is a schematic cross-sectional view of the waveguide of the optical modulator 300 according to the third embodiment of the present invention. The cross-sectional shape of the RF section waveguide 3a of the optical modulator 300 is exactly the same as that of the RF section waveguide 3a of the optical modulators 100 and 200, and is omitted here.

[0067] like Figure 5 As shown, in a cross-section perpendicular to the direction of light propagation, a groove 20b5 is formed on the upper surface of the second uppermost protrusion 20b3 in the DC waveguide 3b of the optical modulator 300. Specifically, the groove 20b5 is formed on the upper surface of the second uppermost protrusion 20b3 on the side opposite to the substrate 10, extending in the same direction as the extending direction of the RF waveguide 3a. Furthermore, the centerline of the groove 20b5 in the width direction of the DC waveguide 3b is on the same straight line as the centerline of the RF waveguide 3a in the width direction. Figure 5 In the embodiment shown, two slots 20b5 are formed in each of the first and second optical waveguides 2a and 2b, respectively. However, the slots 20b5 are not limited to this and may be one or more.

[0068] According to the third embodiment of the present invention, the optical modulator 300 has a light-suppressing effect in the DC section waveguide 3b by providing a slot 20b5 for guiding light in a cross section perpendicular to the light propagation direction, thereby further reducing DC drift.

[0069] [Example]

[0070] Accelerated tests were conducted on the DC drift of the optical modulator for the comparative and exemplary cases. In the accelerated tests, a DC bias voltage was continuously applied at a high temperature of 100°C, and the duration (in hours) when the DC drift reached 50% was recorded.

[0071] Figure 6 This is a schematic cross-sectional view of the waveguide of a comparative example optical modulator 400. The optical modulator 400 has... Figure 6 The optical modulator shown has a cross-sectional structure.

[0072] In the optical modulator 400 of Comparative Examples 1 to 6, the DC section waveguide 3b and the connecting section waveguide 3c have the same cross-sectional shape as the RF section waveguide 3a, wherein the width w1 of the RF section waveguide 3a is 1.2 μm, and the width w2 of the DC section waveguide 3b and the width w3 of the connecting section waveguide 3c are both 1.2 μm.

[0073] The optical modulators in Examples 1-3 are characterized by having Figure 2 The optical modulator shown has a cross-sectional structure. The width w1 of the RF waveguide 3a is 1.2 μm, the width w2 of the DC waveguide 3b and the width w3 of the connecting waveguide 3c are both 20 μm.

[0074] The materials used in all embodiments and comparative examples are the same. Table 1 shows the evaluation results of DC drift for each embodiment and comparative example.

[0075] [Table 1]

[0076] Comparative Example 1 1.2 18.6 Comparative Example 2 1.2 17.8 Comparative Example 3 1.2 10.7

[0077] Comparative Example 5 1.2 15.6 Comparative Example 6 1.2 88.9 Example 1 20 119 Example 2 20 168 Example 3 20 185

[0078] Table 1 shows that in most of Comparative Examples 1-6, the DC drift reached 50% (the ratio of DC drift to DC bias) within 20 hours. However, in Examples 1-3, where the widths of the DC waveguide 3b and the connecting waveguide 3c were increased, the DC drift only reached 50% after at least 119 hours. Furthermore, the elapsed time for each of Examples 1-3 was longer than that for Comparative Examples 1-6. Therefore, it is evident that by setting a larger width for the DC waveguide 3b and the connecting waveguide 3c, DC drift can be reduced.

[0079] Furthermore, it is worth noting that as long as the width of the connecting waveguide 3c is greater than the width of the RF waveguide 3a, it is not necessary for the width of the DC waveguide 3b to be greater than that of the RF waveguide 3a in order to realize the present invention.

[0080] Although the present invention has been specifically described above in conjunction with the accompanying drawings and embodiments, it is to be understood that the above description does not limit the invention in any way. For example, in the description of the optical modulators 100-300 above, the cross-sectional shape of a pair of Mach-Zehnder optical waveguides was used as an example. However, it is not limited to this, and it may also be one or more optical waveguides having the above-described cross-sectional shape.

[0081] Those skilled in the art can make modifications and variations to the present invention as needed without departing from the essential spirit and scope of the invention, and all such modifications and variations fall within the scope of the present invention.

[0082] Symbol Explanation

[0083] 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… Wavelength splitter; 2d… Wavelength combiner; 2i… Input optical waveguide; 2o… Output optical waveguide; RF waveguide… 3a; DC waveguide… 3b; Connector waveguide… 3c; First signal electrode… 4a; Second signal electrode… 4b; A bias electrode…5a; a second bias electrode…5b; a first layered portion…20a1; a first intermediate raised portion…20a2; a first uppermost raised portion…20a3; a second layered portion…20b1; a second intermediate raised portion…20b2; a second uppermost raised portion…20b3; a third layered portion…20c1; a third intermediate raised portion…20c2; a third uppermost raised portion…20c3; a protrusion…20b4; a groove…20b5.

Claims

1. An optical modulator, characterized in that, have: substrate; Multiple electro-optic material layers formed on the substrate; as well as Electrodes formed on the electro-optic material layer, The electro-optic material layer has an RF waveguide that applies a modulation signal and is patterned, and a DC waveguide that applies a DC bias signal and is patterned. In a cross-section perpendicular to the direction of light propagation, the RF waveguide is connected to the DC waveguide via a connecting waveguide that is wider than the RF waveguide. The width of the RF waveguide is the width of the uppermost end of the first uppermost bulge of the RF waveguide. The width of the DC waveguide is the width of the uppermost end of the second uppermost bulge of the DC waveguide. The width of the connecting waveguide is the width of the uppermost end of the third uppermost ridge of the connecting waveguide.

2. The optical modulator according to claim 1, characterized in that, The height of the RF waveguide is the same as the height of the connecting waveguide.

3. The optical modulator according to claim 1, characterized in that, The width of the connecting waveguide is more than 10 times the width of the RF waveguide.

4. The optical modulator according to claim 2, characterized in that, The width of the connecting waveguide is more than 10 times the width of the RF waveguide.

5. The optical modulator according to any one of claims 1 to 4, characterized in that, The connecting waveguide is continuous with the DC waveguide at the same width and extends on the substrate.

6. The optical modulator according to any one of claims 1 to 4, characterized in that, On the upper surface of the DC waveguide on the side opposite to the substrate side, a protrusion is formed that protrudes from the upper surface and extends in the same direction as the extending direction of the RF waveguide. The centerline of the protrusion in the width direction is on the same straight line as the centerline of the waveguide in the width direction of the RF section.

7. The optical modulator according to claim 5, characterized in that, On the upper surface of the DC waveguide on the side opposite to the substrate side, a protrusion is formed that protrudes from the upper surface and extends in the same direction as the extending direction of the RF waveguide. The centerline of the protrusion in the width direction is on the same straight line as the centerline of the waveguide in the width direction of the RF section.

8. The optical modulator according to claim 6, characterized in that, The height of the upper surface of the connecting waveguide on the side opposite to the substrate side gradually decreases from the end of the protrusion on the RF waveguide side toward the RF waveguide side.

9. The optical modulator according to claim 7, characterized in that, The height of the upper surface of the connecting waveguide on the side opposite to the substrate side gradually decreases from the end of the protrusion on the RF waveguide side toward the RF waveguide side.

10. The optical modulator according to any one of claims 1 to 4, characterized in that, On the upper surface of the DC waveguide opposite to the substrate side, a groove extending in the same direction as the extension direction of the RF waveguide is formed. The centerline of the slot in the width direction is on the same straight line as the centerline of the RF waveguide in the width direction.

11. The optical modulator according to claim 5, characterized in that, On the upper surface of the DC waveguide opposite to the substrate side, a groove extending in the same direction as the extension direction of the RF waveguide is formed. The centerline of the slot in the width direction is on the same straight line as the centerline of the RF waveguide in the width direction.

Images